Role of Seed Banks in Supporting Ecosystem and Biodiversity Conservation and Restoration

Abstract

The world is witnessing massive land degradation caused by climate change and various anthropogenic activities. There has been a significant increase in habitat restoration efforts, with demand for seeds to restore these degraded ecosystems in some cases outstripping supply. Traditionally, seeds for restoration activities have mainly been sourced through collections from the wild, but with the growing seed demand, this is increasingly becoming unsustainable. In order to ensure responsible restoration practice, restoration practitioners need to explore other options of economical, ethical and sustainable sourcing of seeds. Ex situ seed banks can leverage their technical and infrastructural capacity to play a greater and more direct role in supporting biodiversity and ecosystem conservation and restoration, particularly through the supply of quality ecologically and genetically suitable seed. In this paper, we review whether ex situ seed banks possess the capacity and competence for supporting habitat restoration and the challenges they are likely to face in these efforts. The review focuses on seed collecting, field-based seed bulking, seed handling and storage, seed quality control as well as experience and capacity in facilitating germplasm exchange. The availability of high-quality germplasm collections of documented provenance and with broad genetic diversity is arguably the greatest resource and asset that seed banks have in supporting habitat restoration.

1. Introduction

Due to the massive land degradation being witnessed globally, caused by both climate change and various anthropogenic activities, which is increasingly threatening the survival of human kind, there is an increased awareness of the need to arrest this trend. Ecological restoration is now being given sufficient attention and is a global priority. The current decade (2021–2030) has been designated by the UN General Assembly as The UN Decade on Ecosystem Restoration. The importance of restoring degraded habitats is recognized by the Convention of Biological Diversity (CBD), which aimed to restore 15% of degraded ecosystems by 2020 [1]. However, these targets were not achieved and the post-2020 Global Biodiversity Framework has been developed, which aims to restore 30% of degraded habitats by 2030 [2]. Projects aimed at restoring terrestrial ecosystems are increasing both in number and scale, with a resultant sharp rise in demand for planting materials to support them. Currently, several large-scale seed-based restoration projects are underway around the world [3]. Traditionally, seeds for restoration activities have mainly been sourced from collections from the wild, but with the growing seed demand, this is increasingly becoming unsustainable [4]. Wild sourcing of seed can easily result in further loss of biodiversity and habitat, especially if unregulated, thereby making seed supply unpredictable from year to year [5,6]. In addition, climatic and environmental factors also greatly affect seed production and can contribute to unpredictable seed supply through their effect on phenology and masting. Due to these and other factors, some large-scale restoration projects have faced challenges in accessing sufficient-quality seeds of well-adapted native species.

In order to ensure responsible restoration practice, restoration practitioners need to explore other options of economical, ethical and sustainable sourcing of seeds such as field-based seed production. Seed production areas (SPA) have been established as a strategy to ease pressure on wild populations and enhance reliable seed supply [7]. Seed sourcing can sustainably be undertaken within the framework of existing biodiversity conservation programs and facilities. In some cases, this may involve restoration using species-rich plantings [8]. On the other hand, many large-scale afforestation projects have largely focused on the use of tree species in restoration, e.g., [9,10], and in such cases species diversity is rarely given priority [11]. This narrow deployment of genetic resources disregards thousands of useful restoration species and their associated intraspecific diversity, the majority of which is conserved in the world’s seed banks. This paper reviews the role of ex situ seed banks in supporting restoration activities. In particular, it attempts to answer two questions: (1) do seed banks have the necessary technical, infrastructural and operational capacity to undertake restoration seed supply? and (2) what challenges are seed banks likely to face in the mobilization of sufficient quantities of high-quality and genetically appropriate restoration seeds? This paper considers restoration as the process of supporting the recovery of habitats that are degraded or destroyed by human- or non-human-mediated activities [12]. Depending on the goal of the restoration activity, this might encompass either re-establishing the suite of native species originally present in the site, or simply re-vegetating with any species. The review focuses on the key elements that are necessary for a successful restoration seed supply for orthodox seeded species (

Table 1. Key elements of a successful restoration seed supply program.

Element	Description
Conducive policy and regulatory framework *	Phytosanitary regulations and germplasm exchange policies.
Technical capabilities *	Human resource capacity, both technical and managerial, and the operational procedures for seed production and seed handling.
Physical capacity *	Sufficiency of the available infrastructural capacity for seed production, handling and storage. This includes seed bulking fields, irrigation and seed handling equipment.
Reliable quality assurance mechanism *	Existing seed quality control and assurance processes, including seed certification schemes.
Access to sufficient resources and reliable financing mechanisms	Capacity to access working capital to fund operational and capital costs necessary for restoration seed supply.
Sufficient demand	Existence of stable and predictable demand for restoration seed and buyers’ willingness to pay for the seed.
Seed availability	Determines the capacity of seed banks to assemble collections of wild plants. Seed availability varies widely among species, populations, seasons and in response to various environmental and anthropogenic factors.

* This paper reviews the capacity available in seed banks in relation to these key elements.

). In the case of recalcitrant and other non-orthodox seeded species which cannot be stored in seed banks, these are conserved using other approaches such as field gene banks, in vitro conservation and cryo-preservation. Although these species also play a role in restoration, they are not covered here due to their conservation approaches which are outside the focus of this review.

This review covers agricultural and forestry seed banks as well as those conserving wild plant species. There is an emerging trend whereby seed banks are increasingly being bestowed with the broad mandate of conserving agricultural, forestry and wild species. In Africa, this mainly began with the Millennium Seed Bank Partnership, in which national seed banks partnered with the Royal Botanic Gardens, Kew, to collect wild species which were banked in these seed banks, with copies being sent to the Millennium Seed Bank. This review also notes that there are seed banks engaged solely in the conservation of wild plants, and these may be more inclined to support restoration as compared to agricultural and forestry seed banks. It is important to note that, in addition to wild species, some agricultural species, particularly forage species, are also being used in restoration of degraded habitats as they provide a source of livelihood, particularly in the case of pastoral communities (Figure 1) [3]. This review therefore notes that, although agricultural, forestry and wild plant seed banks may have distinct differences and varying resources and capacities, they all have a role to play in restoration. This indicates that even purely agricultural seed banks may also have a role in restoration, depending on the species conserved and their potential use in restoration.

2. Importance of Seed Banks in Genetic Resource Conservation and Supporting Ecosystem Restoration

Due to the numerous threats that biodiversity continues to face, the value of ex situ conservation is increasingly being recognized [13,14]. Seed banks are a pragmatic and effective approach to the conservation of plant genetic resources (PGR). Currently, there are more than 1750 seed banks conserving some of the world’s most important PGR [14]. In addition to playing a complementary role to in situ biodiversity conservation efforts, ex situ seed banks can directly support such efforts by contributing to habitat restoration through provision of germplasm, data, expertise and specialist facilities [3,15,16,17,18,19]. Vital and unique skill sets and expertise in diverse disciplines that are of direct or indirect relevance to habitat restoration are often available in ex situ seed banks. Personnel working in seed banks can, for example, play a useful role in providing scientific expertise to guide in the reintroduction of species [20] as well as in situ biodiversity conservation. The concept of a restoration seed bank whose mandate includes not just routine germplasm conservation but also undertaking seed production to support restoration has been proposed [17]. Seed banks are particularly important in the restoration of species that are extinct in their natural habitats [18]. In addition, seed bank collections are of great value in enhancing species recovery and reintroduction of rare, threatened, extinct and protected species [16], thereby contributing directly to in situ conservation-oriented restoration [21].

Seed banks are especially important in supplying plant material of native and wild species which are not commercially available and for which seed growers may be reluctant to produce due to uncertain market demands. In order to ensure profitability, commercial nurseries and independent seed collectors often deal with a limited set of market-preferred species. This makes it difficult for a market-based seed supply system to meet the seed and intraspecific diversity needs of a restoration program, particularly where the goal is to enhance species diversity [22]. Seed banks can partner with development agencies to enhance the availability of this wide diversity for restoration. While seed banks can supply small quantities of seed for seed companies and restoration practitioners to carry out seed multiplication, they also have an option to increase the amount of seeds themselves. This latter option depends on the technical and infrastructural capacity of seed banks as well as the availability of good climate to support the seed increase of target restoration species. However, although the potential of seed banks in supplying restoration planting materials is increasingly being recognized, it remains grossly unexplored. Merritt and Dixon [17] argue that if seed banks are to effectively contribute to restoration activities through seed supply, they need to strengthen and adapt their capacity, as well as refocus their strategies. Strategic investment in research and capacity for seed production is required if the increasing seed demands are to be met by seed banks.

3. Existing and Potential Technical and Infrastructural Capacity

Seed production, either for restoration or agriculture, is a highly specialized activity that requires a wide range of capacities. The key competencies and capacity that are essential for implementing a successful and effective restoration seed mobilization program are often available in seed banks. These include availability of wide species diversity and its associated data and information, and technical and infrastructural capacity in diverse areas of seed handling, seed testing and storage (

Table 2. Technical and physical capacity for ecosystem restoration seed supply available in seed banks and existing challenges.

Seed Supply Aspect	Seed Bank Capacity	Challenge
Availability of genetically appropriate germplasm	Genetically diverse, geo-referenced, multi-provenance germplasm collections available.	Collection gaps and incomplete documentation exist; risk of genetic material having altered genetic make-up during seed bank operations and subsequent grow out events.
Selection of adapted restoration material	Possesses capacity to match germplasm source and planting site due to availability of various types of data and information.	Potential restoration value of some native species and ecotypes held in seed banks is unknown; incomplete documentation.
Seed sourcing	In some cases, it might be relatively easy to find information on species phenology, distribution patterns and optimum collecting time of native species due to established networks.	Seed shattering nature and poor understanding of the phenology of some native species leads to challenges in accurate timing of seed maturity.
Seed processing	Various types of seed processing equipment exist.	Seed processing of native species is more complex and challenging due to large morphological diversity of seed and seed-bearing structures.
Seed drying	Low temperature ultra-drying facilities exist.	Space in drying facilities is usually limited, although this can be overcome through ambient drying, as happens in some seed banks.
Storage	Both temperature-controlled and non-controlled storage facilities are available in some seed banks.	Inadequate knowledge on seed storage behavior of some native species, although models are increasingly being developed to predict seed longevity [23,24].
Seed quality assurance and control	Experience in routine seed viability testing of diverse species.	Seed viability testing and dormancy-breaking treatment protocols are lacking for some native species; low seed viability due to long storage periods, poor seed handling or poor storability.
Facilitating germplasm exchange	Wide experience in facilitating germplasm exchange including meeting relevant regulatory requirements.	Reluctance to share germplasm by some national seed banks; Proliferation of germplasm protectionist policies and tendencies nationally; strict phytosanitary regulations that hinder germplasm exchange.

).

3.1. Access to Wide Genetic Diversity of Restoration Species

During restoration, it is important to ensure that the intraspecific diversity of a species or natural population is adequately represented. In some cases, multiple ecotypes of a species or population are curated in a seed bank, a practice that helps to capture broad genetic variability [16,25]. This rich diversity is arguably the greatest resource and asset that seed banks have in supporting habitat restoration. However, this is not always the case, as some taxa are represented by just a single or a few accessions which may not cover the full geographical range of the taxa. Compared to living collections, seed banks have a greater representation of interspecific diversity, thus making them more suited for biodiversity restoration and supporting in situ conservation [26]. The use of wide functional and taxonomic diversity during restoration is of fundamental importance in establishing healthy, resilient, productive, stable and sustainable ecosystems [27]. This is important in ensuring evolutionary and adaptive potential as well as fitness [28,29]. The availability of multi-provenance collections in seed banks is of great value in light of the many sometimes conflicting recommendations on the appropriate source of restoration seed in terms of provenance.

Although it has often been argued that locally sourced germplasm is most suitable for restoration [30], there is emerging evidence suggesting that, due to climate change, such genetic resources are likely to lose local adaptation leading to reduced fitness [31]. For example, this has been observed in terms of shifts in species range as a result of climate change, in some cases leading to extinctions [32,33]. In situations where local populations are unable to adapt due to climate change, genetic materials sourced from distant provenances may be more suitable. In order to increase the adaptation to different climatic conditions and enhance ecosystem resilience, many studies have recommended the use of germplasm obtained from many sources [31,34,35,36,37]. Others have suggested that sourcing seeds from areas with the same ecological and environmental conditions is a more suitable seed sourcing approach [38]. In cases where climate change has outpaced local adaptation, ex situ banked seeds collected from a particular site may not be suitable for restoring the same site in the future [39,40]. This adaptational lag in banked seeds needs to be considered when sourcing seeds for restoration planting. Whichever of these seed sourcing approaches is used, seed bank collections will be particularly valuable as t, in some cases, they hold wide diversity in terms of provenance and biogeography of collection sites, which are important in determining functional diversity and helping keep pace with climate change adaptations. In addition to adaptability, other aspects of plant growth such as seed germination and longevity have been found to be impacted by provenances, with a negative correlation between seed germination and annual rainfall being reported [41]. Seeds from hotter and drier climates remain viable longer during storage [42,43], thus providing greater conservation and restoration value.

The broad functional trait variation available in seed banks not only helps predict and determine a species’ performance [44], but also has an impact on their capacity to deliver a variety of ecosystem services based on their functional diversity [45]. Ecological restoration strategies that help establish species and functionally diverse habitats help to provide multiple ecosystem services [46]. Additionally, such approaches play an important role in recovering and enriching landscape-level plant diversity.

3.2. Availability of Information and Data

As compared to some plant nurseries, seed banks hold important information, such as provenance and taxonomic identity, which is vital in guiding both expert and non-expert consumers engaged in restoration planting [47]. In order to restore ecosystem functionality and ensure that it is both self-sustaining and self-regulating, it is important to have an understanding of the plant species diversity originally present in the target restoration site [48]. Where this set of information is not available, seed banks can use their eco-geographical data to provide insights on spatial patterns of species occurrences, although these data are likely to be incomplete due to collection gaps occasioned partially by inadequate sampling. Such data can also be obtained from other species occurrence databases such as Global Biodiversity Information Facility (GBIF) and herbarium records, and it is recommended that data from all available sources should be used together. Genetic data are also important in restoration seed sourcing, as it provides important information on the genetic structure of target species and populations [49]. The seed collection databases that are held by seed banks are therefore a valuable resource for enabling the identification of ecologically and genetically suitable germplasm. The eco-geographical data on seed bank collections may help in matching germplasm collection sites with seed production and restoration sites. This is essential in enhancing the ecological adaptation and persistence of the genetic material.

3.3. Seed Sourcing Capacity

Seed collecting constitutes the principal germplasm acquisition strategy for a majority of seed banks. The genetic quality of a seed bank collection, particularly its genetic composition, determines to a great extent its value in ecosystem restoration. Staff working in well-established seed banks with a focus on seed collecting are likely to possess great technical knowledge on how to collect seed samples with maximum intraspecific diversity that are representative of the variability in a natural population. In addition, expertise necessary for ensuring that the collected seeds are of good pathological and physiological quality is available in seed banks. Seeds should be collected at the point of natural dispersal as this will ensure optimum seed quality, although the exact stage at which seed quality is at its maximum depends on genotype and the environment [50,51]. Simple visual seed quality assessment protocols, such as cut tests, are often carrued out during seed collecting missions to determine whether seeds have attained optimum quality.

Because they regularly undertake germplasm collections, seed bank personnel usually have a good understanding of regulatory and administrative requirements, particularly in terms of licensing permits and conditions during seed collecting. National seed banks often have some advantage in seed collecting as they find it relatively easy to access government protected areas compared to independent seed collectors. Protected areas are valuable sources of planting material as they usually harbor wide inter- and intraspecific diversity of native species, thus enhancing species richness during restoration. Moreover, where international germplasm collecting efforts have been restricted as a result of unfavorable policies [52], national seed banks have proven to be vital links in facilitating such collecting initiatives as they are, in some cases, subjected to less stringent policies. For example, seed collecting under the Millenium Seed bank Partnership was relatively easy in some of countries, such as Kenya, as it was spearheaded by national agencies, and national seed banks among them.

Seed banks often establish close working relationships with various experts, herbarium staff and local contacts making it relatively easy to find information on species phenology, distribution patterns, optimum collecting time and sites with appropriate restoration seed. This information is important in ensuring the timely collection of high-quality seeds. In addition, the plant taxonomy expertise that is available in seed banks enables correct identification of native restoration species, which is a challenge when sourcing seeds from many commercial nurseries [47]. A lack of important information on seed source and taxonomic identity lowers the restoration value of seeds.

In addition to seed collecting, restoration seed can also be assembled through field-based seed production (Figure 2). Seed banks usually hold very limited seed quantities that are not sufficient to support large-scale restoration and will therefore need seed increase programs. Due to the need for routine undertaking of germplasm regeneration, characterization and seed multiplication, agricultural seed banks usually have access to a network of field sites. In order to meet the climatic requirements of diverse species conserved in seed banks, these sites are usually located in different agro-ecological and climatic zones. These field sites act as a vital physical resource that can be used for research and seed bulking activities for restoration. Field-based seed bulking, if well managed to prevent selection and genetic drift, provides a good seed sourcing option that reduces the pressure on natural populations, thus reducing overharvesting [7,53]. However, seed bulking has the potential to negatively affect genetic integrity, particularly in cases where several cycles of seed increase are required. There are several documented cases in which seed banks have participated in seed sourcing and supply for large-scale restoration projects. These include the restoration of the Sahel under the Action Against Desertification program (AAD), which has been supported by various national seed banks through seed mobilization (Box 1) [3]. The Seeds of Success (SOS) program, led by the Bureau of Land Management (BLM) in partnership with other government and non-government agencies, has over the years spearheaded the collecting of wild plant diversity for conservation and use in supporting habitat restoration efforts in the United States [54]. This serves as a classic example of how a national seed collecting program can support research and biodiversity conservation through restoration and establishment of a national seed bank collection.

Box 1. Role of the Genetic Resources Research Institute (GeRRI) of the Kenya Agricultural and Livestock Research Organization (KALRO) and other regional seed banks in supporting habitat restoration in the Sahel under the Great Green Wall Initiative.

The Great Green Wall Initiative is a flagship project of the African Union meant to promote the resilience and prosperity of communities through habitat restoration and arresting further degradation of fragile ecosystems. Under the GGW, a total of 166 million hectares of land are in need of restoration [55]. Some of the programs that have been implementing the GGW have partnered with seed banks in seed mobilization, demonstrating the great role that seed banks can play in restoration. The national seed banks in Burkina Faso, Mali and Niger have contributed large quantities of quality seed [3]. In addition, the Genetic Resources Research Institute of the Kenya Agricultural and Livestock Research Organization (KALRO) has provided close to one 1 tonne of seed of various forage species. These include Clitoria ternatea, Macroptilium atropurpureum, Neonotonia wightii and Stylosanthes guianensis. Out of about 60 species that have been planted in the Sahel, more than a third can be found at the Genetic Resources Research Institute (GeRRI). While some of the species are represented by multiple accessions collected from different provenances, others are represented by only a few ecotypes, which may not cover the full species range (Table 3).

Table 3. Species used in the restoration of degraded habitats in the Sahel under the Great Green Wall Initiative and can be found at the KALRO—Genetic Resources Research Institute (GeRRI), Kenya.

Species	Number of Accessions	Number of Provenances
Acacia nilotica	9	8
Acacia senegal	8	7
Acacia seyal	4	4
Adansonia digitari	2	2
Balanites aegyptica	6	4
Tamarindus indica	3	2
Ziziphus mauritiana	1	1
Andropogon gayanus	28	2
Cymbopogon giganteus	3	1
Cenchrus biflorus	1	1
Chamaecrista mimosoides	3	3
Cymbopogon giganteus	3	1
Diospyros mespiliformis	2	2
Grewia bicolor	1	1
Sclerocarya birrea	2	2
Strychnos spinosa	3	3
Stylosanthes hamata	34	3
Stylosanthes mucronata	5	3
Clitroia ternatea	393	20
Stylosanthes guianensis	750	11
Macroptilium atropurpureum	163	11
Neonotonia wightii	436	25

Table 3. Species used in the restoration of degraded habitats in the Sahel under the Great Green Wall Initiative and can be found at the KALRO—Genetic Resources Research Institute (GeRRI), Kenya.

Species	Number of Accessions	Number of Provenances
Acacia nilotica	9	8
Acacia senegal	8	7
Acacia seyal	4	4
Adansonia digitari	2	2
Balanites aegyptica	6	4
Tamarindus indica	3	2
Ziziphus mauritiana	1	1
Andropogon gayanus	28	2
Cymbopogon giganteus	3	1
Cenchrus biflorus	1	1
Chamaecrista mimosoides	3	3
Cymbopogon giganteus	3	1
Diospyros mespiliformis	2	2
Grewia bicolor	1	1
Sclerocarya birrea	2	2
Strychnos spinosa	3	3
Stylosanthes hamata	34	3
Stylosanthes mucronata	5	3
Clitroia ternatea	393	20
Stylosanthes guianensis	750	11
Macroptilium atropurpureum	163	11
Neonotonia wightii	436	25

3.4. Seed Processing

In order to ensure optimal physical, pathological and physiological seed quality, it is important that seeds are properly processed and cleaned. Seed banks mostly handle relatively small quantities of seed, usually less than 1 kg per accession. Advanced and sophisticated seed sorting equipment and technologies such as those based on seed imagery using a flat-bed scanner or a CCD camera analysis exist [56], but these may not be available in the majority of seed banks. These advanced pieces of equipment may not even be necessary for relatively less well-resourced and poorly developed seed banks, as basic seed processing equipment is generally sufficient for a wide variety of plant species. However, due to the large morphological diversity of seed and seed bearing structures, seed processing of native restoration species is usually more complex and challenging than that of many agricultural, horticultural and commercially available forestry species [57]. While some seed banks do not have the specialized equipment for handling these native species, they have wide experience in processing diverse wild species. In cases where there is no suitable seed processing equipment, there may be a need to adapt existing equipment so as to meet the processing requirements of a particular species [58]. Proper seed processing is critical for international restoration seed exchange, as phytosanitary regulatory authorities usually conduct stringent examinations of the physical seed purity before issuing the necessary permits to allow international germplasm transfer.

3.5. Seed Drying

Seed moisture content is one of the most important determinants of seed longevity, with total seed life span being extended significantly at low moisture content during storage. In the case of orthodox seeded species, which can survive drying to low moisture content and storage at sub-zero temperatures, it is imperative that restoration seed is properly dried. Non-orthodox seeded plant species cannot be conserved in conventional seed banks because their seeds do not withstand desiccation and/or freezing. For every 1% reduction in seed moisture content within the range of 5–14% moisture content, seed longevity is doubled for orthodox seeded species [59]. Seed banks have facilities for drying where seeds are sometimes dried to 3–7% moisture content to allow for long-term storage (Figure 3). However, apart from cases where stockpiling of restoration seed is required [60], drying to such low moisture contents may not be necessary as the restoration seeds are usually stored for a relatively short period of time. The duration of drying and the environment under which it is conducted, among other postharvest seed handling practices, determine seed quality and longevity. Seed banks conduct seed drying under controlled temperature and relative humidity conditions, thus maximizing seed viability and longevity.

3.6. Seed Storage

In addition to seed handling, storage conditions play a critical role in determining seed quality and longevity. Even though restoration seeds may not require to be put in long-term storage conditions, it is important to ensure they are stored in conditions that maintain viability [15]. According to Harrington’s rule of thumb, seed storage life doubles with every 5 °C reduction in storage temperature up to a certain limit [59]. Generally, restoration seed material is stored for about 1–3 years, although unexpected challenges could emerge which could lead to delays before the seeds are planted, thus necessitating longer storage. Moreover, due to challenges in obtaining enough seed from natural populations, it may be necessary to collect and store seeds over a period of time before initiating ecological restoration. Some seed banks usually have elaborate temperature-controlled storage facilities in the form of either cold stores or freezers. Although most species will maintain their viability even after a few years in ambient storage, there are some problematic species which will rapidly lose viability due to their short-lived nature [42]. Low-temperature storage may help extend the lifespan of such taxa, and this makes seed bank storage facilities an important asset for restoration seed sourcing. Storage of seeds in uncontrolled conditions where they are subjected to fluctuating temperature and relative humidity is a major cause of declining seed viability, with the rate of deterioration being species-dependent [61]. It may therefore be necessary for seed centers to hold seeds in short- or medium-term temperature-controlled storage awaiting planting at restoration sites. However, seed banks will not require large-scale climate-controlled restoration seed storage facilities such as those established by seed suppliers and restoration agencies such as the Bureau of Land Management in the USA [62]. In addition to demand-driven seed production, there may be a need to stockpile seed stocks in order to meet future restoration seed demands [60]. However, it may not be appropriate for seed banks to engage in stockpiling, as this may tie up physical resources required for other crucial seed banking activities.

3.7. Seed Quality Assurance and Control

Seed quality is one of the foundational pillars of a successful restoration program, and it is therefore critically important that high seed quality is assured. Information on seed viability is important to restoration practitioners as it helps in determining potential seeding densities. High seed quality results in better plant establishment and thus greater diversity of the resultant population. Despite its importance, most restoration projects give little consideration to seed quality [11,63]. There is also evidence indicating that the quality of commercially available native seed is sometimes not optimal [64]. Most suppliers of restoration seed material have inadequate technical and infrastructural capacity to undertake credible seed testing [65] making it difficult to monitor seed quality. Most of the native restoration species are currently not available in the existing commercial seed supply systems [66] and are therefore traded informally with little or no control on seed quality. Seed certification systems for most native restoration species are non-existent or poorly developed and no seed quality assurance mechanisms are available [58,67]. Certification schemes developed in some jurisdictions have been found to be unreliable as they largely address the requirements of the agricultural sector and not native seed industry [68]. In other cases, the seed quality assurance regulatory requirements are too stringent and costly to the extent that they stifle native seed supply [65]. This situation has potential for compromising the quality of seed available for restoration. This raises the need for alternative simple, cheap and effective seed quality control protocols for native restoration species.

Seed viability testing is a core gene banking activity that is routinely conducted in order to ensure that only viable seeds are conserved and to assist in seed conservation management decision making. Seed viability is tested at storage and later monitored periodically during storage usually after every 10 years resource allowing [69]. This routine seed testing is, however, usually carried out on species whose seed testing protocols are known and this may not be the case for some native restoration species. Due to limited availability of resources for seed viability testing, less priority is usually given to native species. Due to this capacity and experience, seed banks can potentially support restoration initiatives by developing seed quality control systems and offering independent seed testing services, particularly in cases where these do not exist in the formal seed sector. Unlike in the agricultural seed industry and some selected forestry species where there exists independent accredited seed testing laboratories and formal guidelines under the framework of the International Seed Testing Association (ISTA), such laboratories generally do not exist for native species [70]. Where such protocols and guidelines exist, they are limited to only a few native taxa and jurisdictions [57]. The need for internationally recognized accreditation schemes cannot be overemphasized. The potential of accrediting well equipped seed testing laboratories found in seed banks needs to be explored.

3.8. Experience and Capacity in Facilitating Germplasm Exchange

Large-scale restoration projects typically involve some level of local and international germplasm exchange. If seed bank collections are to be of value to human kind, they need to be easily accessible for use in breeding, research, education, restoration and other uses [71]. Germplasm exchange is subject to regulation by both national and international policies, although these policies are increasingly constraining trans-boundary germplasm transfers by imposing various types of restrictions [52]. These policies regulate both access and benefit sharing, as well as phytosanitary management. The nature of existing phytosanitary regimes is an important factor, especially in relation to international germplasm exchange, but is rarely given proper attention during restoration planning. In addition to the physiological seed quality, there is a need to ensure that the phytosanitary quality of the seeds meets the set standards. In case of transnational seed exchange, it is important that the seed supplier and recipient are able to meet the phytosanitary requirements of both countries.

As a result of undertaking regular international germplasm transfers, some seed banks have developed closer working relationships with phytosanitary regulatory authorities. Their experience in sharing germplasm in countries with diverse phytosanitary regimes is also an added advantage. Although phytosanitary management is a mandate of the phytosanitary regulatory agencies, seed banks have a responsibility to conserve and share healthy germplasm. A few seed banks have therefore built both technical and infrastructural capacity in this area by establishing phytosanitary units [72]. These factors give seed banks capacity to comply with phytosanitary requirements of different countries with relative ease, thus enabling international germplasm flows.

4. Role of Seed Banks in Assisted Migration and Enrichment Planting

Climate change and ecological degradation have, in some cases, resulted in the emergence of novel ecosystems where distinct changes in ecological functioning have been noted. Some local plant populations are unable to persist in these ecosystems due to reduced adaptive capacity and require management interventions. On the other hand, increased species richness has been recorded indicating the emergence of more suitable climates [73]. Shifts in distributional ranges have been noted for many species [74,75]. Due to these changes occasioned by climate change, assisted migration and enrichment planting have emerged as important management actions aimed at enabling populations to establish in new locations. Enrichment planting also enhances the capacity of populations to thrive in their old habitats. Analysis of the eco-physiological response of Picea glauca seeds obtained from different sources along a climate gradient revealed an adaptive response to climate change in terms of key functional traits [76]. This demonstrates that assisted migration presents a viable management option that helps species and populations occupy locations that are likely to experience climate change in the future. The success of assisted migration depends on the ability of conservation and natural resource managers as well as other conservation practitioners to select the appropriate seed source. Through climate modelling, it is becoming increasingly possible to predict the future climatic conditions of a given area, and seed banks can potentially help identify and access genetic materials that will adapt in these “future climates” [77]. It has been suggested that the chances of capturing adaptive variation can be enhanced by combining genetic materials collected from sites possessing “future climates” with locally sourced germplasm, thus facilitating assisted gene flow [31]. The availability of germplasm collections with ecotypes obtained from genetic sources spread along diverse climate gradients makes seed banks an important source of genetic material and information for assisted migration and enrichment planting.

5. Source of Genetic Materials for Characterizing Ecological and Climate Change Adaptive Capacity

Planning and implementing sustainable ecosystem restoration and in situ conservation programs would greatly benefit from insights on temporal and spatial patterns of intraspecific diversity and adaptive capacity responses of target restoration species. However, obtaining such information is often constrained by the lack of biological materials for studying plant adaptive processes in time and space [78,79]. In the absence of long-term experiments involving natural populations, the historical geographically diverse seed bank collections form important resources for studying the spatial and temporal responses of plants to environmental shifts, particularly those caused by climate change. This analysis helps to identify the major climate variables responsible for shaping climate change adaptive capacity [80]. Similarly, analysis of germplasm collections is enabling the identification of genetic markers associated with certain environments. A dedicated seed bank collection [78] that holds plant samples collected over broad geographical ranges has been established. The samples will be compared with modern samples, thereby providing insights on spatial and temporal patterns of plant evolutionary changes.

A comparative analysis of samples obtained from the same population at different times revealed diverse genetic and phenotypic changes [81,82,83]. In some cases, this analysis has revealed the emergence of novel alleles through adaptation and evolution. There could also be an increase in allelic frequency of rare alleles associated with certain adaptive traits [84]. These novel and rare alleles need to be prioritized for conservation. Adaptive changes after multi-decadal sampling have been witnessed in plant functional traits such as flowering time [79,85]. Changes in a plant phenotypic response are usually accompanied by changes in the genomic loci controlling the trait [85], thus distinguishing adaptive capacity from phenotypic plasticity. In addition to temporal sampling, analysis of samples collected along an environmental gradient will provide insights on loci responsible for climate change adaptation [86,87]. Such analysis enables the identification of genomic regions possessing signatures of selection. This is assessed based on the level of differentiation in the genomes from contrasting populations, with the focus being on genomic regions with unusually high genetic differentiation. These insights on factors responsible for successful climate change adaptation are important for guiding biodiversity restoration and in situ conservation programs.

6. Selection of Genetically Appropriate Material for Restoration, Reintroduction or Enrichment Planting

Identifying genetically appropriate germplasm for biodiversity restoration, genetic rescue and enrichment planting is one of the greatest challenges facing conservation managers and practitioners. As much as possible, efforts should be made to ensure the selected genetic resources possess the necessary adaptive traits to fit in the target restoration sites, or genetic characteristics that will allow compatibility with the target population. Owing to the large size of most seed bank collections, selecting such germplasm remains a daunting task for seed bank managers. This is compounded by a lack of information on the intraspecific diversity and adaptive potential of conserved germplasm, as germplasm evaluation has not been carried out in most cases. This calls for care and caution to be exercised when selecting ex situ seed bank samples to be used in restoring fragile ecosystems. In cases of reintroduction or enrichment planting, it has been argued that the source population should have high intraspecific diversity to enhance resilience to diverse stressors in the target environment [88]. However, using germplasm from the source and target population that are highly genetically diverged also carries the risk of reduced fitness as a result of outbreeding depression [89]. Improper selection of seed bank material, such as using germplasm subjected to human-mediated selection, may, for example, be counterproductive to restoration efforts, as it has the potential to cause maladaptation [90]. Knowledge of trait variation is important in selecting species and genotypes that possess the necessary adaptive capacity for a particular habitat. Trait-based approaches have found great application in a wide range of fields, among them plant improvement, conservation, agricultural production and ecological restoration. Various approaches that make use of morphological, molecular and ecological data have been deployed to assist in trait identification from seed bank collections [91]. One of these trait-based approaches that has found much application in seed banks is the Focused Identification of Germplasm Strategy (FIGS), which is an effective tool that has been used to identify materials possessing useful trait variation, mainly resistance to biotic and abiotic stresses [92,93,94].

FIGS can help biodiversity conservation and restoration practitioners to match germplasm collection areas with target restoration sites. It operates on the premise that the genetic variation found in a population is shaped by the prevailing environmental conditions and biotic and abiotic stresses as a result of natural selection. However, a major weakness of this strategy is that it focuses only on natural selection and ignores other evolutionary forces that have a potential impact on trait variation and its spatial distribution [95]. Genome environment associations (GEA) have been identified in various wild and cultivated species and are vital in guiding the selection of ecologically adapted germplasm [96,97]. Expert knowledge in the use and interpretation of the vast amounts of phenotypic, eco-geographical and molecular data that are available in seed banks is important in supporting the selection of suitable planting material for restoring natural capital. Although seed bank collections remain poorly studied, and their potential genetic and restoration value is largely unknown, there are tools that seed banks can use to gain understanding of within- and between-species functional trait diversity. This includes genomic tools that can help support seed sourcing and analyze adaptive capacity [98]. This knowledge is vital in guiding the selection of ecologically adapted germplasm from seed bank collections.

7. Role of Community Seed Banks

Community-based seed supply forms a major seed sourcing strategy for restoration projects [65]. Local communities have also demonstrated their capacity to supply sufficient quantities of restoration planting material and provide alternative livelihood support for poor households in the process [58]. Regulatory regimes need to adopt less stringent seed certification requirements for community-based seed production of native species [65]. The capacity of communities to supply quality and genetically diverse planting materials can be strengthened by the establishment of community seed banks. Community seed banks are an important platform in supporting community-based management of biodiversity and informal seed systems. These community seed banks can be organized into a network which can be linked to national seed banks, thus enabling the exchange of germplasm between them.

8. Challenges

8.1. Inadequate Knowledge on Native Species Ecology, Phenology and Seed Biology

Compared to major agricultural species, there is inadequate knowledge on diverse aspects of native species, thus affecting not only their effective conservation but also their use in restoration. Inadequate knowledge on species germination biology for a vast majority of native restoration species limits its use in informing seeding practices. This situation often leads to poor seed establishment in the field, thus hindering the recreation of resilient and diverse ecosystems possessing optimal ecological functionality [99]. This can principally be attributed to the presence of seed dormancy among other factors. Different species have varying types and levels of dormancy which may not have been properly characterized [100,101]. Some species have deep seated dormancy which, in addition to presenting challenges in field establishment, also make seed testing difficult.

Information on necessary seed germination requirements and appropriate dormancy-breaking treatments is lacking for many species [60]. Unlike in the case of most agricultural, horticultural and forestry species, seed testing protocols and methods for many native species are lacking [102,103]. This can be attributed to the huge number of native species available and the diverse morphological and physiological traits that they possess, which have varied impacts on seed dormancy and germination [104,105]. Even where such protocols exist, they are not always effective, thus raising the need for development of more appropriate methods and procedures. Some seed banks, particularly those dealing with wild species, have developed the necessary technical and scientific capacity in the development of appropriate seed testing and dormancy breaking protocols for those species where such information may be lacking. Such protocols are continually being documented in seed bank databases and publications. For example, the Seed Information Database (SID), which is now being hosted by the Society for Ecological Restoration (SER) in collaboration with the Royal Botanic Garden, Kew, provides information on germination requirements of various native species [24]. The development of appropriate seed testing and germination protocols is the basis of a robust and reliable seed quality control scheme.

There is also poor understanding of the phenology of some native species, particularly those that are not widely distributed, which has led to challenges in accurate prediction of timing of seed maturity during seed collecting, thus compromising seed quality and the ability to capture maximum within-species diversity [25,71]. Coupled with this is the seed-shattering nature of wild species, a factor that usually makes it difficult to collect enough high-quality seeds from the wild. Obtaining adequate seed quantities for seed increase to enable large-scale restoration is therefore often difficult for some ecotypes. In some cases, more than one generation of seed increase may be required, a situation that risks compromising genetic integrity. In order to reduce the number of seed increase cycles, restoration practitioners can supplement these seed-limited ecotypes with other taxa to create species-rich mixtures [16]. In such cases, SPAs can play an important role in enabling genetic diversification when selecting target restoration taxa and populations [53]. Similarly, the seed storage behavior of some species is not known and this could pose challenges in determining their longevity and appropriate storage conditions [43]. Prior to initiating restoration, it is imperative that efforts are made to gain a sound understanding of the ecology, phenology and biology of the target species.

8.2. Poor Seed Viability

Even though seed banks usually offer near-optimal seed storage conditions that reduce seed deterioration, natural seed ageing is inevitable and leads to viability loss. This viability decline may go undetected, particularly where routine seed viability monitoring systems are poor. Despite having great technical and infrastructural capacity in diverse seed bank operations, the majority of seed banks are poorly resourced and have huge backlogs in seed testing and germplasm regeneration [14]. This means a good proportion of their collections may be at risk, as they may be maintained below recommended conservation standards in terms of seed viability [69]. In such cases, the chances of losing an accession during propagation in the field are high. In some rare cases, seeds stored for many years—as happens in seed banks—may germinate but fail to develop further, most likely due to ageing [106], thus hampering restoration efforts. Where seed viability is evidently poor, it is advisable to take precautions, such as germinating the seeds in a controlled environment before taking them to the field. Laboratory-based pre-germination is one such option after which the materials can then be transferred to further controlled conditions in a screen or green house (Figure 4). This may however not be practical when restoring large areas. In addition, the success rate when transferring seedlings from the laboratory to the greenhouse or field may be low.

8.3. Collection Gaps and Incomplete Documentation

Despite the significant progress made in conserving plant diversity in seed banks, there are major biogeographic and taxonomic gaps in the existing collections, particularly for wild species [107]. The intraspecific diversity available in seed bank collections is often not fully representative of the diversity found in in situ populations, and this has principally been attributed to inadequate sampling during seed collecting [108]. Low intraspecific diversity also remains a major gap as most native restoration species are represented by very few ecotypes whose diversity is not representative of genetic variation inherent in a species or population. This presents a challenge in deploying genetically diverse plant material that has the capacity to adapt to present and future climates [31]. Seed bank collections also suffer from the problem of incomplete documentation where vital data on seed collections such as geo-referenced data of collection sites is in some cases lacking. This might make it difficult to select germplasm of the right provenance for restoration planting. As observed by Jalonen, Valette, Boshier, Duminil and Thomas [11], this challenge of incomplete seed collection records does however not appear to be unique to seed banks, as it has been reported by other collectors in the commercial native seed supply sector. In the commercial native seed supply sector, such cases of a lack of information on provenance have amplified the calls to establish seed accreditation programs in order to give certainty to seed buyers on seed origin and important characteristics [62,70,103]. In order to maintain trust and transparency between suppliers and buyers of native seed, as well as to assure seed quality, it is imperative for the restoration seed sector to develop robust seed traceability systems. Closely related to this is a clear information gap on seed sourcing strategies employed by some restoration projects. This hinders current restoration initiatives from learning from the experiences of past projects as far as accessing restoration germplasm is concerned. Reporting of habitat rehabilitation initiatives usually focus almost exclusively on the success of their restoration activities, with less attention being given on the source and acquisition strategies of their planting material.

8.4. Inadequate Knowledge on Potential Genetic Value of Seed Bank Collections

As already highlighted, the success and sustainability of a restoration program depends on the ability of restoration practitioners to select germplasm with good adaptive capacity. However, germplasm conserved in most seed banks remains poorly characterized and evaluated, and its functional variation is largely unknown [109]. This constrains the effective deployment of such germplasm in restoration activities. Through molecular and phenotypic analysis, great insights are being obtained on functional trait variation within and between populations and species. In the absence of phenotypic data, genomic predictive models have been developed and are assisting in selecting seed bank genetic materials that are likely to perform better than accessions whose trait performance is known [110]. Advances in genomics, and particularly in DNA sequencing, are providing unprecedented opportunities that are helping identify adaptive variation [111]. Seed banks are increasingly building their capacity, both technical and infrastructural, to leverage advances in genomics, thus enabling them to unravel the potential genetic value of their collections [91]. Population genomics is assisting in sourcing seed with suitable functional diversity based on its provenance [98]. This helps in not only selecting genetically appropriate and ecologically viable restoration plant material but also identifying the right seed sources.

8.5. Altered Genetic Make-Up of Seed Bank Collections

Both genetic and phenotypic changes have been observed between different generations during restoration seed production [112]. As already stated, seed banks conserve small seed samples which need to be multiplied in order to be of value to restoration activities. Moreover, agricultural seed banks undertake routine regeneration of seed samples to maintain seed viability. These seed bank samples may have been collected from a larger population which may have been poorly sampled. Undertaking seed increase using such samples with small founder population size may lead to undesirable genetic changes such as genetic drift, thus affecting plant characteristics [29]. Genetic drift lowers genetic variation and may lead to reduced fitness and fixation of deleterious alleles, thus increasing chances of extinction, particularly for small and threatened populations [113]. It is recommended that seed banks adhere to existing technical guidelines, e.g., [114,115] on seed collecting in order to minimize these risks. The possibility of producing inbred seed during seed increase is high, and with it comes the associated risk of inbreeding depression, which has an impact on various traits and could cause a reduction in plant fitness, or even lead to extinction [116,117]. Although hybridization and introgression are usually of great conservation concern, it is important to note that they are increasingly being recognized as a source of novel variation that is helping populations to adapt to the rapidly changing climate [88]. In cases where genetic diversity may have been lowered, restoration practitioners may need to use strategies such as post-restoration enrichment planting to enhance it. Comparative molecular analysis of ex situ conserved genetic material and their source populations after a period of ex situ seed bank storage have revealed differences in allelic diversity [118]. Some of these alleles led to changes in important phenotypic traits such as early flowering [85]. Ex situ conservation in seed banks halts adaptive and evolutionary processes, a situation that has the potential to present challenges during restoration as it is likely to result in a decline in fitness and maladaptation, particularly in the case of climate-smart provenancing. Due to this potential lack of adaptive fitness, it might be necessary for restoration managers to test the performance of seed bank-sourced seeds before embarking on large-scale restoration. However, this is only viable for species with short growth cycles.

8.6. Reluctance to Exchange Germplasm

Despite the importance of seed banks in biodiversity conservation, germplasm exchange between these facilities has faced various challenges over the years which have negatively impacted the international flow of germplasm [91]. Of great concern is the emergence of protectionist tendencies, where some national seed banks are often not willing to share their germplasm outside their national boundaries [119]. This is particularly the case for developing countries, where there is a feeling that their genetic resources are being accessed by other countries without proper benefit sharing arrangements in place. Compared to cultivated taxa, there appears to be more reluctance among seed banks to share wild species. This could mainly be attributed to the fact that they are more difficult and costly to collect from the wild and to maintain in ex situ conservation programs than cultivated taxa. In addition, the majority of wild species are not covered by international agreements, specifically the International treaty on Plant Genetic Resources for Food and Agriculture (ITPGRFA), and hence, sharing genetic resources of these taxa will require bilateral negotiations, which can be slow and costly. It is important for seed banks to recognize the fact that countries are highly interdependent in terms of plant genetic resources, as no country is self-sufficient in these important resources. While many international policies, treaties and agreements have been put in place with the aim of facilitating sharing of germplasm internationally, these have often failed to achieve the intended purposes [120]. In some cases, there is a feeling among germplasm conservation practitioners and stakeholders that these international polices do not satisfactorily address germplasm conservation needs at the national level. Unless favorable access and benefit sharing policies are in place nationally, coupled with strong political will on germplasm exchange, seed bank collections are unlikely to be of value to in situ conservation and restoration activities internationally.

9. Conclusions

Ex situ seed bank collections form an immensely valuable resource with broad genetic diversity that potentially confers the wide adaptive capacity that is necessary to withstand diverse climate related stresses during habitat restoration. Coupled with this is the broad capacity available in seed banks that not only ensures efficient conservation of these resources, but also enables their effective and sustainable deployment in biodiversity restoration, as well as in situ conservation. However, despite this great potential, seed banks remain underutilized in supporting in situ conservation and biodiversity restoration efforts. There are many taxa and populations in ex situ collections for which no efforts have been made to undertake their in situ conservation. Although ex situ and in situ conservation approaches are supposed to complement each other, the link between them has often been poor.

It is possible for ex situ conservation to be carried out at the expense of in situ conservation, particularly when unsustainable conservation practices such as overharvesting during seed collecting are used. With the continued global efforts for the restoration of degraded areas, this situation is likely to change, as in situ conservation will be greatly enhanced. This will require critical rethinking of funding strategies and resource allocation. There is a need to explore how the resources amassed through the largely successful ex situ conservation initiatives and efforts can be used to support in situ conservation and restoration activities. Seed banks are particularly important for the reintroduction of rare, threatened and extinct taxa which are conserved in ex situ seed banks, but their natural populations continue to decline due to habitat destruction, among other factors.