Functional Restoration of Desertified, Water-Limited Ecosystems: The Israel Desert Experience

Abstract

Ecological restoration has recognized explicitly the recovery of ecosystem functions. The emphasis on ecosystem functions in restoration efforts can be defined as functional restoration. Functional restoration, i.e., rehabilitation of ecosystem functions, is particularly applicable to highly degraded landscapes such as desertified areas, where restoration to the original state is impossible. This review paper produced a general conceptual model of the fundamental processes that regulate ecosystem functions in water-limited ecosystems. The Israeli Negev Desert was used as a case study to test the model results in the field. We developed general guiding principles for functional restoration of degraded landscapes that integrate functional restoration methods and include four successive steps: (1) identifying the fundamental processes that regulate ecosystem functions in alternative states; (2) detecting drivers leading to degraded states; (3) functional restoration: changing the state of the degraded landscape; and (4) monitoring the impact of the restoration effort and assessing its success in terms of ecosystem services. Our case study, the Negev, provided us with insights on how to reverse desertification in water-limited systems by restoring source–sink networks as a subset of functional restoration. The four suggested steps of functional restoration are essential for ecosystem recovery in the Anthropocene.

1. Introduction

Recently, ecological restoration has explicitly recognized that the recovery of ecosystem functions, such as soil formation, nitrification, carbon sequestration, and decomposition, can be objectives of restoration projects in their own right [1,2,3,4]. Restoration ecology started to take an ecosystem-functioning perspective [5,6,7,8] because land degradation and global change do not only affect the biotic composition of ecosystems but also their functions, sometimes having unpredictable trajectories [9]. This change, which emphasizes ecosystem functions in restoration, can be defined as functional restoration. Functional restoration, i.e., restoring ecosystem functions, is particularly applicable to highly degraded landscapes such as desertified areas where restoration to the original state is impossible [10]. We define functional restoration as rehabilitation of the main functions of the ecosystem, i.e., increasing the three main ecosystem processes: primary and secondary productivity; decomposition; nutrient cycling, and its ecosystem services (provisioning, regulating, cultural and supporting).

The semi-arid region of the Negev desert represents a desertified area where long-term land uses, including agriculture, water storage by building dams and canals, uncontrolled grazing, and removal of trees and shrubs, have led to changes in the landscape’s structure and its biodiversity. These changes influence the rate of ecological processes associated with primary and secondary productivity, the water cycle, and soil processes [11,12,13,14]. The changes in landscapes and their diversity are interwoven with changes to ecological processes such as water loss, soil erosion, and reduction in biological productivity [15,16,17,18].

In the Negev, land use and landscape changes have caused an increase in the cover of soil biocrusts (crusts formed by cyanobacteria, lichens, and mosses that adhere soil particles). These biocrusts spread as the area covered by woody vegetation declines. As they spread, soil water infiltration decreases, thus increasing surface runoff and causing an overall water loss to the ecosystem. These processes reduce biological productivity on hillslopes, causing changes in hillslopes function in relation to water leakage that significantly impact channel processes by higher runoff flow, which causes soil erosion [19,20].

Although past and present human over-grazing and clear cutting of woody vegetation have caused changes to the landscapes of the Negev Desert, Israel, the basic structure and function of the landscape can still be decoded by focusing on the main processes that control dryland functions, such as hydrological connectivity and source–sink relationships of the landscape’s resources [21,22]. Hydrological connectivity defines the length of pathway of runoff that connect one or more patches with enriched water content. These processes link between water and soil resource distribution, landscape mosaic and ecosystem function, and determine the state of the ecosystem [23]. The connectivity and source–sink relationship are natural processes that are modified by human land use.

Research at long-term ecological research sites (LTER IL; https://lter-israel.org.il/, 1 February 2023) in the Negev, Israel, has identified the links between resource distribution in the landscape mosaic, ecosystem functions, and ecosystem state [24]. The results enabled the development of a general conceptual model of the fundamental processes that regulate ecosystem functions in water-limited ecosystems. The model serves as a roadmap for functional restoration and provides a means to identify human impacts on the Negev ecosystems that result in desertification and in widespread loss of ecosystem functions and services. To integrate the model with the practice of functional restoration, we developed general guiding principles for functional restoration of degraded landscapes. The general guiding principles include four successive steps: (1) identifying the fundamental processes that regulate ecosystem functions in alternative states such as shrub land, crust land, or grassland; (2) detecting drivers leading to degraded states; (3) functional restoration: changing the state of the degraded landscape; and (4) monitoring the impact of the restoration and assessing its success in terms of ecosystem services.

The functional restoration practice follows the guiding theoretical concept that ecosystems are likely to exist in alternative states [25] depending on land-use history, but in all states, the basic dryland processes that govern ecosystem functions prevail, such as source–sink dynamics and connectivity.

The aim of the paper is to report on the Israeli experience in applying the general guiding principles of functional restoration to the desertified ecosystems of the Negev.

2. Functional Restoration of Degraded Landscapes—Guiding Principles

2.1. Identifying the Fundamental Processes That Regulate Ecosystem Functions

A general model of the Negev region that integrates and synthesizes the fundamental processes that regulate ecosystem functions was used as the theoretical guide for the functional restoration of degraded landscapes. The model is based on long-term eco-hydrological studies undertaken at the Negev LTER sites [26,27,28,29,30]. This model serves as the foundation for assessing the state of the ecosystem and its functioning today and in the past, and as a roadmap for the functional rehabilitation of desertified ecosystems. The model indicates that the central process shaping the structure and functioning of the Negev ecosystems is the redistribution of rainwater by surface runoff, creating spatially heterogeneous soil moisture [18]. The heterogeneous moisture distribution is expressed as a spatial mosaic of moisture-rich and moisture-poor patches, existing side by side, at different spatial scales [31]. In moisture-rich patches, soil moisture is high and water penetrates deeply into the soil. Without these moisture-rich patches, most of the rainwater would percolate to a shallow depth and quickly evaporate, resulting in low levels of primary production and biodiversity. The high-water content in the water-enriched soil patches supports increased primary production and species richness in these patches. The central functional basis of the system is therefore the concentration of surface runoff and the creation of water-enriched soil patches [32].

In contrast to these soil patches, soil biocrusts reduce rainwater infiltration over time, resulting in surface runoff generation [33]. Patches differing in soil moisture levels form in response to the functional interactions between biotic soil crusts and the soil patches found under the canopy of woody plants [34,35]. The physical-chemical-biological properties of soil beneath woody canopies differ from those of the biocrust patches between the shrubs. The soil beneath the woody plants is friable, rich in organic matter, and the root system within it creates soil porosity [36]. These characteristics increase rainwater and surface runoff permeability, thus creating moisture-rich soil patches.

In the Negev, soil biocrust patches and woody patches are found adjacent to each other in a double-patched landscape mosaic, maintaining functional connections between them. The soil biocrust patch acts as a source of water, soil, nutrients, and organic matter, which are transported by surface runoff to the woody patch. The woody patch is capable of “absorbing” runoff water and the materials within it. The functional relationship between these two types of patches is referred to as “source–sink relationship” [37]. This relationship regulates biological production, diversity, and distribution in the Negev [35]. In the source (biocrust) patches, primary production, species richness, and the number of individuals of each species are low. In the sink (woody) patches, primary production is high and species richness and the number of individuals of each species are high as well (Figure 1).

The main factor determining the functional level of the entire landscape system is the number of water-enriched soil patches and their degree of water enrichment and their pattern formation. Under any given rainfall regime, an ecosystem with higher-moisture patches will have higher levels of biological production and diversity. Increased production and biodiversity are usually accompanied by increased levels of ecosystem services. Human disturbances, such as agriculture, deforestation, and grazing, reduce the number of water-rich patches, and as a result, the ecosystem deteriorates, production and biodiversity diminish gradually, and ecosystem services decrease [38,39]. Functional ecosystems in the Negev produce a relatively high amount of plant biomass, which supports food webs. The relationship that contributes to ecosystem functioning is illustrated as a feedback model between source–sink feedback loops (Figure 2). The feedback model allows us to analyse the system’s response to disturbances or rehabilitation processes in terms of patchiness, soil moisture, and biological production. This model can warn against system degradation and assist us in examining the degree to which the integration of rehabilitation activities obtains the desired system functions.

Two positive feedbacks were identified in the Negev system: source feedbacks (Figure 2, processes 1, 2, 3) and sink feedbacks (processes 4, 5, 6). The source feedback loop starts with the formation of soil biocrusts by cyanobacteria. The crust produces runoff that disperses the cyanobacteria, causing the biocrust patch to expand. Biocrust expansion produces additional runoff that further increases the range of soil biocrusts. Without interference, crusts will eventually cover the entire area. The sink feedback loop begins with the germination of woody vegetation. The vegetation is nourished by runoff from the soil biocrusts. The feedback loop increases with increasing soil permeability under the plants’ canopy, due to the developing root system and accumulated leaf litter [36,40]. The two joint feedback loops contribute to the growth of woody soil patches and soil biocrust patches, creating a two-phase mosaic patch that maintains a sustainable source–sink relationship. The relationship between the feedback loops stabilizes the system (Figure 2, processes 7,8). These processes and feedback loops ultimately create a stable, two-phase mosaic that optimally utilizes water, soil, and nutrient resources. In addition, the feedback loops determine the patterns of moisture-rich soil patches reflected in the landscape. In sum, these patches affect biological production and biodiversity (Figure 2, processes 9, 10, 11) as well as the structure and function of the entire system. An ecosystem feedback model facilitates the prediction of system behaviour under changing conditions. For example, if woody vegetation is destroyed by grazing or trees and shrubs are cut down, one can expect the sink feedback loop to be reduced, soil biocrusts patches to expand, and the source feedback loop to reactivate. In this case, a landscape system with extensive source areas and limited sinks will be created. Resource conservation in the new source–sink relationship matrix will be reduced, and the system will lose water, soil, organic matter, and minerals (Figure 2, process 12). The result is low biological production and biodiversity, and ultimately ecosystem desertification.

Similarly, it is possible to predict the effects of creating artificial, human-made sinks on hillslopes and in stream channels. Such sinks directly increase the conservation of soil and water resources, and subsequently support primary production and biodiversity; they rehabilitate ecosystem functioning, soil quality, the water cycle, biomass production, and biodiversity. The feedback model shows that the Negev ecosystem can exist under three states: (A) a functioning state, (B) a desertified state, and (C) a rehabilitated state.

The system consists of two main source–sink feedback loops. The source feedback loop is controlled by soil crust, which is formed by cyanobacteria (1), and the biological soil crust produces runoff water (2) that helps the cyanobacteria to spread (3). The sink feedback loop begins when woody vegetation engineers a soil mound (4) which functions as a sink and increases soil moisture in patches (5). The two feedbacks are connected by a source–sink relationship: runoff generated by biological soil crusts increases soil moisture (8) and this affects the cyanobacteria (7). The growth of woody patches (sink) comes at the expanse of biocrust patch area (source) (7), but as a result, runoff to woody patches is reduced as is their growth (8). The overall impact is an increase in herbaceous vegetation under the woody vegetation (10) which uses the soil moisture in the sink patch (9) and provides food for animals (11). In the case of desertification, the source feedback loop generates runoff water that leaks from the system (12), whereas under functional restoration, the sink feedback loop conserves the water and prevents leakage of resources from the system.

2.2. Detecting Drivers Leading to Degraded States

Studies of the drivers leading to desertification processes focus on the natural and anthropogenic contributing forces, including grazing, tree removal and drought. They assess their influence on landscape patchiness (the spatial relationship between woody patches and soil biocrust patches), resource leakage, and the effects on ecosystem functioning [41,42,43,44].

A controlled experiment was conducted to assess the effect of grazing on shrub-cover density. Reduced shrub-cover density is the main cause of desertification [35], as it leads to the expansion of runoff-generating soil biocrust areas that cause water to leak from the system and increase soil erosion. Shrub mortality was found to be responsible for the decline in ecosystem productivity. Studies in the Negev found that seasonal grazing of sheep and goats during the green season, for three years only, significantly reduced the number of shrubs compared to control (non-grazed) plots [45,46]. The study clearly indicates the rapid rate of desertification processes under the influence of grazing. Considering that these processes have been going on for hundreds of years, it can be concluded that large areas of the Negev have been subjected to desertification processes.

To examine the effect of grazing on the spread of soil biocrusts over large areas, satellite imagery taken between 2003 and 2010 was analysed [47]. Studies and analyses of satellite imagery prove that the landscape structure changed: shrub-dominant landscape degraded into crust-dominant landscape following anthropogenic (grazing) and climatic (drought) influences [47]. In conclusion, landscape-desertification processes in the Negev, which previously resulted from uncontrolled land use, continue in the present and appear to be growing due to increases in grazing pressure and recurring drought events [48].

To determine the connection between landscape structure, surface runoff, and soil erosion, the following manipulative experiment was conducted at the LTER Park Shaked site: shrubs were removed from a plot and the effect on runoff and erosion was examined [48]. Likewise, the relationship between floods and soil erosion was also researched [35]. The effects of changes in shrub-cover density and the expansion of soil biocrusts on soil erosion processes at the watershed level were examined in a study of headward erosion of stream channels (Figure 3). This experiment aimed to examine the hypothesis that a decrease in shrub-cover density accelerates desertification processes due to a loss of water and soil resources. The results showed that during rain events, areas with soil biocrusts and without shrubs generated twice as much surface runoff as the amount generated from areas covered with shrubs. In addition, the experiment proved that areas with a shrub-cover density of only 25% reduced surface runoff by 50%. Additional findings confirm the main assumption that shrubs play a central role in conserving ecosystem resources [49]. Disturbances, such as grazing and trampling, reduce the functioning of shrubs as water sinks and of ecosystem engineers, thus eliminating the sink-enriched patches and leading to ecosystem degradation [31,49].

The Negev study’s assumption is that the loss of shrub patches and increased runoff volume from the watershed’s slopes, resulting from slope degradation processes, result in high-intensity floods downstream. The consequential floods cause soil erosion, and lead to the head-cutting of stream channels. Analysis of aerial photographs and field measurements indicate the rate of stream channel head-cut (Figure 3) [19,20]. Past studies have found that as long as a system retains the components that increase plant production and nutrient cycling, its carrying capacity increases, as does its ability to support a variety of organisms [47,50]. A decline in biomass production and species diversity indicates desertification processes [41]. It was shown that shrub removal caused damage to the site’s productive capacity, as reflected in a 50% reduction in herbaceous cover density. The degree of shrub-cover density has implications for the production of biomass and herbaceous species diversity [51].

In conclusion, research in the Negev attests that desertification processes result from the loss of shrub cover due to drought events and grazing. Reduced shrub cover and the expansion of soil biocrusts increase runoff intensity and soil erosion in the stream channel. These processes lead to ecosystem degradation, which is expressed in low productivity, reduced rates of nutrient cycling and a decreased level of biological diversity. The model shows that degradation changes the relationship between the source–sink feedback by increasing the source and decreasing the sink (Figure 2).

2.3. Functional Restoration: State Change in Degraded Landscape

Integrating fundamental processes that regulate ecosystem functions and detecting the drivers leading to degraded states are the basics of functional restoration programs of the Negev [18,51,52]. Surface runoff management is the foundation of the functional rehabilitation of ecosystems in the Negev. This is because runoff is the main regulator of water redistribution and the formation of water-enriched patches. Runoff management can regulate the intensity and volume of runoff and prevent erosional processes, thereby reducing damage to the soil resources and subsequent ecosystem degradation. Runoff rainwater harvesting is a major means of concentrating resources in the soil while increasing ecosystem productivity and biodiversity. Intelligent management of surface runoff makes it possible to transform runoff from a factor causing ecosystem degradation into an important resource for functional rehabilitation and state change. A multi-year analysis, over a nineteen-year monitoring period in Northern Negev, shows that the average annual rate of runoff is about 12% of the average annual precipitation [48].

In functional restoration by runoff manipulation, the surface runoff is collected into contour furrows. The furrows are created on hillslopes and dams that are erected on the floors of valleys (or gullies). Runoff from the source area is drained [53] from the slopes and the riverbeds and the water is allowed to penetrate the soil at a rate of at least three times the average annual rainfall (Figure 4). The large quantities of runoff enable deep infiltration of water into the soil profile, thus reducing water loss from evaporation, and providing water to vegetation for extended periods of time.

Studies show that water retention is particularly critical during drought years, when there is a risk of extensive desiccation and plant mortality [47,51]. During the drought of 2007–2010 in the Shaked Park LTER, the average annual precipitation was about 100 mm per year (compared to about 200 mm average annual), yet runoff harvested during these dry years facilitated the survival of woody and herbaceous vegetation in the human-made sinks throughout the watersheds. On the other hand, outside of the functional restoration area, where runoff was not collected, there was massive dieback of shrubs (Figure 5) [54].

These findings attest to the importance of runoff harvesting in drought years as a tool for preserving system functionality and protecting the ecosystem from catastrophic state shift. This is even more significant considering climate change scenarios that predict an increase in the frequency and intensity of droughts in the Negev [55,56]. In addition to collecting runoff water, the human-made sink patches constructed along the slopes and the valley of the watersheds alter the flow of resources between the natural and constructed patches and modulate the spatial heterogeneity of resources. Resource distribution and retention in drylands are key to productivity, and are dictated by runoff connectivity patterns and source–sink dynamics [18,33,51].

Paz-Kagan et al. [57] found that the construction of human-made sink patches in dry riverbeds significantly increases soil organic matter, available water content, and total N, P, and K in comparison to natural sinks. They attribute soil resource concentration to the combined effect of the connectivity pattern along the riverbed, and size of the human-made sink. Studies in the Negev have shown that in natural watersheds, runoff connectivity between the slopes and the riverbed is related to the surface texture of the slopes and rainfall properties [58,59].

Human-made sinks, constructed over the slopes and the riverbed as a component of functional restoration, decrease the length of the connectivity pathway and increase resource concentration. These sinks store abundant water and associated nutrients that support a high biomass of trees and understory herbaceous vegetation. Importantly, at the overall watershed scale, human-made sinks increase biomass production [60]. A positive relationship between biomass and diversity was found in the human-made sinks in the Negev [61]. The positive effect of functional restoration on biomass production relates to the negative correlation between biomass and abiotic stress under low water availability [62]. Inevitably, every human-made sink along the connectivity pathway creates two adjacent patches—one enriched with water and nutrients and the other deprived of water and nutrients. By creating landscape patchiness, most important functional features of the desert ecosystems can be restored [22].

On the watershed scale, runoff management on the slopes and riverbeds, as the first stage in functional restoration, has cascading effects on soil quality that in turn restore bio-productivity and biodiversity.

Comprehensive scientific work was carried out to identify the effects of functional rehabilitation activities on soil quality in Israel, Northern Negev. Soil quality is a key issue because soil is the basis for the functioning of natural terrestrial ecosystems and human-managed systems. Soil quality reflects the carrying capacity of the ecosystem and its ability to support soil-plant feedback and flora-fauna diversity. In a previous study, soil quality was determined by a combination of several physical, chemical, and biological properties [50]. The central research question focused on how the soil quality of sinks that capture runoff changes over time (at least 20 years). A soil quality index was calculated using indicators that represent the physical, chemical, and biological properties of the soil. The long-term storage of runoff in human-made sinks was found to significantly increase soil quality [50]. The increase in soil quality is attributed to increased soil moisture levels, and the accumulation of organic matter and nutrients that increase soil resources through soil biodiversity functioning. The combination of stored runoff and organic matter and increased biological activity triggers a positive feedback loop that improves soil quality.

Concentrating water and soil resources to increase soil quality in the human-made patches supports human-induced ecosystem state change from a degraded to a productive state. In the specific case of the Negev, the state change is from degraded shrubland to a novel ecosystem: a human-made savanna [18] (Figure 6).

The Negev research aimed to study the effect of resource redistribution on annual plant species diversity in runoff-harvesting systems along the runoff pathway of a watershed. The research tested the effect of resource redistribution (water and nutrients) on biomass, and the abundance of grass and forb as mediators of species diversity. The research was conducted in five small watersheds in the central Negev of Israel. The research quantified the addition of soil nitrogen, phosphorus, potassium, water, and organic matter in the human-made sinks, and two natural sites (upstream and downstream) along the watersheds. Biomass, grass and forb abundance, and species richness were quantified during two successive years [57]. Structural equation modelling was used to study the effect of grass and forb abundance and biomass on the species richness of herbaceous plant communities. All tested soil properties significantly improved in the human-made sinks compared to the natural sites. Annual biomass increased significantly in the human-made sinks compared to the downstream site. The results indicate a decline in forb abundance in the human-made sinks and the downstream site, and increase in grass abundance in the human-made sinks. The conclusion is that diversity responses to biomass production are both resource- and functional-group-specific.

Numerous studies show that the level of system function increases as functional diversity increases [63]. Differences in the functional diversity of grasses and forbs growing on soil biocrust landscape units and runoff-enriched landscape units have been identified [64]. On the other hand, it was found that when grasses dominate the landscape, there is a significant decrease in primary production, in plant species diversity, and in their influence on the food web [32,36].

Primary production is the basis for ecosystem functioning, as it determines the energy reserves available to plants and animals in the system. The level of primary production is an important indicator for identifying the level of activity of each ecosystem type [65]. Functional rehabilitation activities aim to increase the level of system activity by increasing primary production as an indicator of the success of rehabilitation works. Many studies have shown a positive correlation between increased primary production and the elevated biodiversity of herbaceous plants. In general, increased primary production and diversity of herbaceous species increases the variety of herbivores and decomposers [66,67,68,69].

A study at Shaked Park LTER station examined primary production during a drought year (2010) and a rainy year (2011) in an ecosystem rehabilitated by runoff harvesting that was established twenty years ago. About twenty years after the system was established, it is clear that the runoff-enriched landscape unit supports higher primary production compared to the soil biocrust landscape unit that loses runoff [47].

These findings indicate that the construction of human-made sinks can compensate for the loss of sinks during degradation processes and can transform desertified landscapes to novel ecosystems that function with high sink density; such novel ecosystems were also developed in ancient terraces in the Negev and usually occur when agricultural landscape is abandoned. The observations agree with our model, which shows that a system dominated by sources can be transformed to a system dominated by sinks (Figure 2).

2.4. Monitoring the Restoration Impact and Assessing Its Success in Terms of Ecosystem Services

The restoration effort and state change in the desertified ecosystem contribute additional benefits to society, which are not part of the multi-functional landscape. The novel multi-functional cultural landscape is designed to serve human needs; it is thoroughly integrated with the concept of ecosystem services, a concept that has developed over recent decades [70,71]. The Millennium Ecosystem Assessment (MA) defined four types of ecosystem services [72]: provisioning, regulating, and cultural and supporting services. Provisioning services are “products obtained from ecosystems” and can include food, fuel, and fresh water, among many others (MA, 2003, 8–11). Regulating services include air quality maintenance, climate regulation, water regulation, pollination and more. Cultural services are “nonmaterial benefits people obtain from ecosystems through spiritual enrichment, cognitive development, relaxation, recreation and aesthetic experiences”. Supporting services “are necessary for the production of all other ecosystem services” (MA 2003, 11); in practice they represent ecosystem function [73,74].

For monitoring the functional restoration project, we adopted the term “ecosystem services” as defined by Burkhard et al. [75]: “Ecosystem services are the contributions of ecosystem structure and function—in combination with other inputs—to human well-being”, using the four-groups defined by the MA.

Provisioning services: This category includes the material and energy that the ecosystem provides to humans, i.e., food, fuel and energy sources, genetic resources used for breeding animals and plants, and bio-technological-chemical development [76]. In the Negev, functional restoration of desertified areas provides pasture and shade resources to herds of cattle, sheep, and goats. In addition, these projects provide a variety of pasture plants, including grasses, herbs, and shrubs, and fruits from forest trees [46].

Regulating services: Regulating services, provided from ecosystems, maintain the quality of air and soil, support pollination, prevent floods, and control the spread of disease. Functional restoration in the Negev facilitates the main regulating services concerning the water cycle and soil conservation and enhancement by constructing contours and human-made sinks. Functional rehabilitation practices enable water conservation through runoff harvesting and by employing specific tree-planting techniques; consequently, water flow is regulated over the entire watershed basin. Flow regulation distributes water throughout the basin according to the needs of humans and nature, prevents floods and loss of resources, facilitates the management of water resources under extreme climatic conditions, and regulates the movement of soil, energy, and recycled nutrients. Runoff harvesting systems regulate the flow of water originating from the basin’s upper sections. The two main objectives of water flow regulation are: (1) preventing flood damage, and (2) reducing soil erosion. The spatial framework for functional rehabilitation is the watershed basin. The basic assumption behind flood-control activity is that the creation of runoff “sinks” throughout the watershed basin reduce the intensity of floods downstream. This reduction is important for mitigating extreme rain events characterized by large quantities of precipitation and high intensities of runoff. The ability of runoff harvesting systems established in forested areas to absorb runoff and regulate flood intensity has been proven by numerous events at various sites during the long-term monitoring period [77].

Cultural services: Creating a human-made savanna through the functional restoration program provides cultural services such as cultural diversity, environment-linked spiritual and religious values, environment-based information systems, educational values derived from ecological systems, inspiration from an environment that arouses stimuli, aesthetic values, social relations interwoven with human–environment connections, heritage of place, cultural values resulting from human–environment connections, creativity and inspiration, and ecology-inspired tourism.

The development of leisure culture, followed by the growth of urban areas and the need for open areas, increases the number of local and regional visitors that use the human-made savanna and open spaces for hiking and bicycling, thus advancing the development of tourist-support services [73].

Supporting services: Research and long-term monitoring programs have proven that restoration activities that collect and store runoff reduce losses of water and organic matter within the system, as well as evaporation losses. These actions greatly increase ecosystem productivity and biodiversity compared to areas that do not absorb runoff [64]. Since primary production is the main energy resource for all ecosystem activities, increased levels of primary production in a habitat provide sufficient energy to sustain a wide variety of organisms. Studies show that rehabilitation activities, such as human-made stream channels, contribute to the conservation of habitats of cliff-nesting birds [78] and increase microbial functional diversity due to the collection of runoff water [50]. In addition, several species critical to system function are found in the human-made savanna in the Negev, including termites [44], ants [79], snails [80], and spiders [81].

In conclusion, in addition to the state change by functional restoration, the novel human-made savanna provides of a set of ecological services that contribute to the wellbeing of humans living in the desert.

3. Discussion

The Negev studies suggest four successive steps for reconstruction of the soil resources and the source–sink network in a desertified, water-limited system:

3.1. Identifying the Fundamental Processes That Regulate Ecosystem Functions in Alternative States

In water-limited systems, the fundamental processes that regulate ecosystem state are water flow processes. They are initiated by rainfall pulses that are redistributed by runoff pulses and transferred into soil moisture reserves that regulate the pulses of ecosystem functions, such as primary and secondary productivity. The specific ecosystem state is determined by the function of the water flow regulators. If the regulators increase soil moisture pulses and reserves, the ecosystem processes operate on a high level and indicate a healthy state. However, if the regulator malfunctions, the runoff water leaks from the landscape and the ecosystem desertifies. For functional restoration, it is essential to identify the state of the regulators. This implies verifying whether the regulators operate to conserve resources or are unable to prevent the loss of water and associated resources. The regulators of water flow are linked with source–sink dynamics. The main water flow regulators in water-limited systems are landscape surface properties that interact with rainfall pulses to generate runoff, which redistributes water. Physical and biological surface properties regulate water flow. The physical regulator is associated with geodiversity, i.e., the network of rock and soil patches within the landscape. Rocks function as a source of runoff while soil patches act as sinks for the water that supports ecosystem activities. The biological surface properties are related to the activity of two ecosystem engineers: the biological soil crust that functions as a source of runoff and the shrubs that create mounds under their canopy that function as sinks. In both cases, the source–sink network controls the state of the system. If the source-to-sink ratio of a self-organized ecosystem conserves resources, then the system represents a state of high ecological functioning. If self-organization is disturbed and the source-to-sink network does not efficiently conserve resources, the system moves towards a desertified state. In summary, by identifying the state of the source–sink network before implementing the functional restoration activities, we can plan the means to restore the system to a desired state that is characterized by high productivity and diversity.

3.2. Detecting Drivers Leading to Degraded States

In water-limited systems, anthropogenic stressors and climate change can shift the source–sink network towards the desertified state. Many factors may modify the source–sink network. Uncontrolled grazing is a main factor that disturbs both the physical and biological regulators of the water flow [82]. Grazing may induce shrub mortality, and as a consequence, destruct the sink formed by shrub engineering. In this case, the biological soil crust cover will increase, increasing source patches and runoff generation, and resulting in a landscape characterized by the loss of resources. In addition, the trampling of grazing animals disturbs the physical network of source–sink networks by causing soil erosion. Soil erosion decreases the sink patches in the landscape and increases resource leakage. The second factor that induces a state change and requires functional restoration is climate change, specifically periods of intensive droughts. Under drought conditions, the source–sink network deteriorates due to shrub mortality and the destructions of the associated sink patches. In terms of ecosystem function, the consequences of drought are similar to the effects of grazing, i.e., a system dominated by resources and an ecosystem that cannot conserve resources.

3.3. Functional Restoration Actions Aimed at Changing the State of the Degraded Landscape

After identifying the main function of the system and its state, and collecting knowledge on factors that change the water flow regulation, the functional restoration measures can be applied. In water-limited systems, as was found in the Negev research, the main factor that causes state change toward desertification is the destruction of the source–sink network. Therefore, the main objective of the functional restoration is to rebuild a healthy source–sink network. The Negev studies propose that there are two ways to increase the sink functions of a water-limited watersheds: by concentrating surface runoff into contour collection furrows created on hillslopes, or constructing dams or gullies along the valley floor. The ideal design of a human-made sink depends on the degree of network deterioration, the pattern and quantity of predicted rainfall pulses, and the natural geodiversity. The specific design of the human-made sink should consider the theory of vegetation pattern formation in relation to water input to the system [83].

3.4. Monitoring the Restoration Impact and Assessing Its Success in Terms of Ecosystem Services

The functional restoration procedure must consider its contribution to human wellbeing, i.e., what ecosystem services are expected from the restored source–sink network. Studies in the Negev have shown that rebuilding source–sink networks can benefit ecosystem service provision, such as air quality maintenance, climate regulation, water regulation, and pollination.

4. Conclusions

Human activities have impacted most ecosystems on a global scale [84,85,86]. Ecosystem restoration, defined as the process of assisting the recovery of ecosystems that have been degraded [87] has become a tool to overcome human pressures on the structure and function of ecosystems. Under ecosystem degradation, the ability of ecosystems to maintain ecological functions and provide benefits to society declines [86,88]. Therefore, the functional restoration of a degraded ecosystem could be an essential means to offset the loss caused by anthropogenic activities [89].

The Negev functional restoration studies, which focus on state change by the restoration of the basic landscape structure and function, propose an approach focusing on restoring essential ecosystem attributes. The essential ecosystem attributes are the creation of water-enriched patches in a two-phase mosaic that operates as a source–sink feedback of soil resources and regulates ecological productivity and diversity (Figure 2). The Negev functional restoration program agrees with recent studies that have shown that ecosystem functioning is largely dependent on key ecological processes with major regulatory and feedback roles [90].

In water-limited ecosystems, the essential attributes that regulate ecological productivity and diversity are the redistribution of rainfall by runoff, thus creating water-enriched patches that increase ecosystem functions, such as energy flow and nutrient cycling [59]. The redistribution is regulated by the interactions between rainfall properties and the network of resources, and source and sink patches over the landscape [64]. Functional restoration in water-limited systems is basically the reconstruction of the soil resources, and the source–sink network that has been destroyed by anthropocentric activities.

The Negev studies provided us with insights into a potential method to reverse desertification in water-limited systems by restoring the source–sink network as a subset of functional restoration. The four steps of functional restoration that we suggest in this paper are a first attempt to establish the theory and practice of functional restoration that is essential for ecosystem recovery in the Anthropocene.