Tropical Dry Forest Restoration in an Era of Global Change: Ecological and Social Dimensions

1. Introduction

In 2019, the General Assembly of the United Nations proclaimed 2021 to 2030 to be the United Nations Decade on Ecosystem Restoration [1], with Resolution 73-284 identifying six core investment areas, including several directly related to tropical dry forest restoration: mobilizing resources, capacity building, scientific research and cooperation for ecosystem restoration at multiple scales; mainstreaming ecosystem restoration into policies and plans to address current national development; building on and reinforcing existing restoration initiatives; facilitating synergies to achieve international commitments and national priorities for ecosystem restoration; and promoting experience sharing and good practices in ecosystem conservation and restoration. In response to this UNO Resolution, this Special Issue on “Tropical Dry Forest Restoration in an Era of Global Change” draws much needed attention to the restoration of a geographically, ecologically, and socioculturally significant biome that is one of the Earth’s most threatened. We focus on the ecological and social dimensions of TDF restoration with the goal of identifying the local to global barriers to restoration and how teams of TDF researchers and practitioners are engaging with Resolution 73-284. These papers share the results of recent research and highlight promising future avenues for work that must be undertaken if TDFs are to remain a forested biome.

The restoration of tropical forests remains one of the most effective strategies for climate change mitigation and biodiversity conservation [2,3], but tropical restoration science and practice has historically focused on tropical rainforests. In contrast, the highly seasonal tropical dry forest (TDF) has received less attention from the research community—and society at large—than their more humid tropical counterparts [4]. Indeed, even landmark compilations of TDF ecology and conservation, e.g., [5,6], minimally address the topic of restoration, which is critical as this is one of the most threatened ecosystems on Earth [7,8]. The ten papers in this Sustainability Special Issue represent an effort to close that knowledge gap and advance our understanding of the science and practice of restoring these understudied ecosystems. In addition, the papers in this issue show that despite the dearth of information and their threat condition, there are many exciting and important opportunities to restore these important landscapes and highlight the potential of TDF restoration through the lens of socioecological research.

Seasonally dry tropical forests are distinguished from tropical moist and tropical rainforests by their lower annual rainfall and strong seasonality [9], with dry seasons that consist of rainless periods, which in some regions may exhibit a three-fold variation [10]. In addition, TDFs can occur from sea level to >2000 m above sea level [10], and on geological substrates ranging from limestone to basaltic parent material. The considerable variation in total annual precipitation, length of the dry season, topographic features, and edaphic conditions in which these forests occur contribute to their high species richness, diversity of life forms, endemism, physiognomies, and the diversity of functional groups [6,11], as well as the range of ecosystem functions [12] (see Figure 1). Encompassing 80 ecoregions, TDFs occur across the Americas, Africa, and Asia [12], typically on low soil fertility [13], where they are threatened by multiple anthropogenic stressors including land use change [14,15], fires [16,17], increases in atmospheric nitrogen deposition [18], climate change [19], and combinations thereof. Addressing these threats with TDF restoration programs will require science-based practices that engage the human dimensions of TDF stewardship. The topics addressed by the papers in this Special Issue aim to advance the state of knowledge on the functional diversity, environmental controls, and restoration strategies of these forests, addressing the foundations of TDF restoration, associated approaches and practices, new tools and advances, and the human dimensions of TDF restoration. Land use, usually involving fire, has been the dominant driver of change across this biome, and the pressures of land use change are also a focus of many papers.

2. Foundations for TDF Restoration

Precipitation does not explain the variation in plant species richness, but for Mexican TDFs, potential evapotranspiration (PET) appears to explain some of this diversity [10]. This suggests that some physical environmental factors determine TDF gradients of diversity. For example, although it is often cited that TDFs have higher soil fertility than their humid counterparts [9,14,20], the limited available soil data examining how ecosystem function varies along environmental gradients suggests that TDFs are a nutrient-limited ecosystem [21,22]. The pantropical analysis in Rivero-Villar’s paper [13] included in this Special Issue addresses an important soil fertility knowledge gap for TDFs. The results from the study do not corroborate the idea that TDFs grow on high fertility soils. Different soil metrics across 80 ecoregions show the prevalence of TDFs on soils with low fertility status and highlight the large soil biogeochemical heterogeneity occurring both across and within biogeographic regions, with notable, complex variation dependent on climate and parent material. The understanding of these interactions has important implications for TDF restoration planning in the context of the ongoing climate change.

Considering the large diversity of plant functional traits in TDF ecosystems [23,24,25,26], two studies appearing in this Special Issue explored the use of a functional trait approach to guide TDF restoration. Rosell et al. [27] suggest the existence of a higher diversity of functional groups within TDF communities in comparison to those in the tropical moist and rain forests and other forested biomes. The striking diversity of functional groups among plants in the TDF biome (dramatically evident in Figure 1) could be driven by environmental filtering resulting from stress imposed by seasonal droughts [12,28,29,30]. In another paper, Ware et al. [31] propose that the frequent ecophysiological adaptations of multi-stemmed trees across TDF species could provide climate resilience in restored landscapes. In summary, restoration practice must include information that is representative of the range of environmental parameters and strive for a better understanding of the functional diversity of original TDF plant communities to improve the efficacy of restoration efforts.

3. Approaches and Tools for Restoration of TDF Landscapes

The strong dry season that characterizes TDFs [12] imposes constraints to both active and passive restoration practices [32,33]. Three papers in this Special Issue address the sensitivity of TDF flora and fauna to climate across timescales. These papers employ a variety of tools including field experiments [34], collaborative research and transfer of techniques [35], and simulation modelling [36]. Questad et al. [34] used a field experiment to explore the restoration of three dryland plant communities in a gradient of rainfall and soil fertility in Hawaii. The authors found that plant survival improves in higher quality sites characterized by higher soil moisture and soil fertility. The seemingly low abiotic stress of the habitats examined by these researchers contrasts with the invasive species-driven biotic stress that is manifested in moderate- to low-quality habitats. They conclude that weed control activities are necessary to improve restoration outcomes in lower quality habitats. In a zoocentric example, Stephenson et al. [36] used modelling to assess arthropod and lizard sensitivities to the impacts of climate change (rainfall and temperature) on TDF fauna in a protected area of Jamaica. In their simulation analyses, they hypothesized that arthropod and lizard abundances are more sensitive to rainfall than temperature variations. Their results forecast a decrease in abundance of both faunal groups under drier but also warmer conditions. Climate modeling forecasts increases in the frequency and severity of extreme events [37], with many tropical locations already showing changes in drought seasonality [38]. These results underscore the nuanced sensitivity of fauna to expected climate change. Undoubtedly, climate change will interact with other drivers of global change, including land use change, leading to the dramatic declines in animal (vertebrates and invertebrates) populations we are witnessing in the Anthropocene age [39]. Finally, in the context of collaboration and academic synergies, Longman et al. [35] introduce the Pacific Drought Knowledge Exchange, a collaborative information-transfer and co-production consortium designed to support better integration of managers, emphasizing the importance of historical and projected future climate change into regional management planning. Improving our understanding of species’ sensitivity to climatic regime changes in TDF ecosystems and fostering collaborative efforts can guide the implementation of restoration models of these critical forests in the future [40].

4. Addressing Ecological and Social Complexity for TDF Restoration

Socioeconomic indicators are rarely used to evaluate restoration outcomes [41], and there are few studies including both ecological and socioeconomic indicators in TDF restoration practice [42]. Several papers in this Special Issue explore a broader range of social and environmental limitations for the effective restoration of TDFs in Mexico [33,43] and Hawaii [44], and review the opportunities for the integration of social science dimensions in TDF restoration [45]. These three papers of the present Special Issue stress the urgent need to integrate local people in order to not only develop better TDF restoration programs, but also to guide TDF restoration at regional and global scales.

5. Concluding Remarks

The papers in this Special Issue advance knowledge on the environmental and social dimensions of and constraints to TDF restoration and provide key information on the variety of plant functional traits and the large ecological heterogeneity that need to be included for this ecosystem’s restoration efforts. Future restoration science and practice should build upon these findings in a number of ways. First, the variability in climates within the TDF biome [12] and the enormous range in soil conditions and large functional diversity documented by the studies of this Special Issue [13,27,31] suggest that future TDF restoration practices should be founded on the links between biodiversity, soil heterogeneity, and climate, as well as the needs, motivations, and overall wellbeing of local people. Second, in order for restoration efforts to effectively match plant performance with restoration sites under changing climate conditions, a careful evaluation of a selection of native tropical dry forest species with respect to their functional traits and distribution along environmental gradients may prove a fruitful approach.

It is worth noting that the papers of this Special Issues are largely phytocentric, however TDF restoration needs to also consider the recovery of the animal component; clearly, this is an aspect that warrants further research. Finally, establishing social and biological indicators of success (ranging from plant survivorship and growth, to species richness and abundance, ecosystem functioning and service provisioning, community-led participation) and effectively monitoring these indices following the restoration of TDF sites will be necessary to evaluate the effectiveness of our restoration actions. Where success does not meet expectations, results from monitoring as well as more targeted research can inform adaptive management responses including the development of new strategies. Results from a global network of restoration sites ultimately could inform the development of policies that ensure the continued, successful restoration of the TDF biome into the Anthropocene.