Beyond hectares: four principles to guide reforestation in the context of tropical forest and landscape restoration

New climate change agreements emerging from the 21st Conference of the Parties and ambitious international commitments to implement forest and landscape restoration (FLR) are generating unprecedented political awareness and financial mobilization to restore forests at large scales on deforested or degraded land. Restoration interventions aim to increase functionality and resilience of landscapes, conserve biodiversity, store carbon, and mitigate effects of global climate change. We propose four principles to guide tree planting schemes focused on carbon storage and commercial forestry in the tropics in the context of FLR. These principles support activities and land uses that increase tree cover in human‐modified landscapes, while also achieving positive socioecological outcomes at local scales, in an appropriate contextualization: (1) restoration interventions should enhance and diversify local livelihoods; (2) afforestation should not replace native tropical grasslands or savanna ecosystems; (3) reforestation approaches should promote landscape heterogeneity and biological diversity; and (4) residual carbon stocks should be quantitatively and qualitatively distinguished from newly established carbon stocks. The emerging global restoration movement and its growing international support provide strong momentum for increasing tree and forest cover in mosaic landscapes. The proposed principles help to establish a platform for FLR implementation and monitoring based on a broad set of socioenvironmental benefits including, but not solely restricted, to carbon mitigation and wood production.     

Recovery of floristic diversity and basal area in natural forest regeneration and planted plots in a Costa Rican wet forest

When compared to planted reforestation, natural unassisted regeneration is often reported to result in slow recovery of biomass and biodiversity, especially early in succession. In some cases, naturally regenerating forests are not comparable to the community structure of primary forests after many decades. However, direct comparison of the outcomes of tropical forest restoration and natural regeneration is hindered by differences in metrics of forest recovery, inconsistency in land use histories, and dissimilarities in experimental design. We present the results of a replicated reforestation experiment comparing natural regeneration and polyculture tree planting at multiple diversity levels (3, 6, 9, or 12 native tree species), with uniform land use history and initial edaphic conditions. We compare the recovery of basal area and floristic diversity in these treatments after 5 yr of succession. Total basal area was higher in planted plots than in naturally regenerating plots, but it but did not vary among the different planted diversity levels. The basal area of woody recruits did not differ among treatments. The diversity of woody recruits increased substantially over time but did not vary among planting treatments. Species composition trajectories showed directional turnover over time, with no consistent differences among treatments. The convergence of restoration trajectories and similarity of floristic community diversity and composition across all treatments, after only 5 yr, provides evidence of the viability of natural regeneration for rapid restoration of forest biodiversity.     

The role of land-use history in driving successional pathways and its implications for the restoration of tropical forests

Secondary forests are increasingly important components of human-modified landscapes in the tropics. Successional pathways, however, can vary enormously across and within landscapes, with divergent regrowth rates, vegetation structure and species composition. While climatic and edaphic conditions drive variations across regions, land-use history plays a central role in driving alternative successional pathways within human-modified landscapes. How land use affects succession depends on its intensity, spatial extent, frequency, duration and management practices, and is mediated by a complex combination of mechanisms acting on different ecosystem components and at different spatial and temporal scales. We review the literature aiming to provide a comprehensive understanding of the mechanisms underlying the long-lasting effects of land use on tropical forest succession and to discuss its implications for forest restoration. We organize it following a framework based on the hierarchical model of succession and ecological filtering theory. This review shows that our knowledge is mostly derived from studies in Neotropical forests regenerating after abandonment of shifting cultivation or pasture systems. Vegetation is the ecological component assessed most often. Little is known regarding how the recovery of belowground processes and microbiota communities is affected by previous land-use history. In published studies, land-use history has been mostly characterized by type, without discrimination of intensity, extent, duration or frequency. We compile and discuss the metrics used to describe land-use history, aiming to facilitate future studies. The literature shows that (i) species availability to succession is affected by transformations in the landscape that affect dispersal, and by management practices and seed predation, which affect the composition and diversity of propagules on site. Once a species successfully reaches an abandoned field, its establishment and performance are dependent on resistance to management practices, tolerance to (modified) soil conditions, herbivory, competition with weeds and invasive species, and facilitation by remnant trees. (ii) Structural and compositional divergences at early stages of succession remain for decades, suggesting that early communities play an important role in governing further ecosystem functioning and processes during succession. Management interventions at early stages could help enhance recovery rates and manipulate successional pathways. (iii) The combination of local and landscape conditions defines the limitations to succession and therefore the potential for natural regeneration to restore ecosystem properties effectively. The knowledge summarized here could enable the identification of conditions in which natural regeneration could efficiently promote forest restoration, and where specific management practices are required to foster succession. Finally, characterization of the landscape context and previous land-use history is essential to understand the limitations to succession and therefore to define cost-effective restoration strategies. Advancing knowledge on these two aspects is key for finding generalizable relations that will increase the predictability of succession and the efficiency of forest restoration under different landscape contexts.     

The potential for automating assisted natural regeneration of tropical forest ecosystems

Assisted (or accelerated) natural regeneration (ANR) will play an important role in meeting the UN target to restore forest to 350 million hectares of degraded land, by 2030. However, since most accessible land is already used for agriculture, most of the sites, available for ANR, are far from roads and/or on difficult terrain, where implementing ANR with human labour is not practical. Therefore, this paper explores the potential of emerging technologies, such as low‐cost UAVs (drones) and new imaging devices, to automate ANR tasks, including site monitoring (to assess site potential for natural regeneration, plan interventions and assess progress), maintenance of natural regeneration (particularly weeding) and species enrichment through aerial seeding. The usefulness of existing technologies is reviewed and future innovations needed, to provide practicable support for ANR, are discussed. Intensive collaboration, among technologists and forest ecologists, will be essential to ensure that technological innovations are based firmly on sound restoration science.     

Establishment of tree seedlings in the understory of restoration plantations: natural regeneration and enrichment plantings

Little is known about the potential of restoration plantations to provide appropriate understory conditions to support the establishment of seeds arriving from neighboring native forests. In this article, we investigated how seedling establishment is affected in the understory of restoration sites of different ages and assessed some of the potential environmental factors controlling this ecological process. We first compared the density and richness of native tree seedlings among 10‐, 22‐, and 55‐year‐old restoration plantations within the Atlantic Forest region of southeastern Brazil. Then, we undertook a seed addition experiment in each study site, during the wet season, and compared seedling emergence, survival, and biomass on local versus old‐growth forest soil (transferred from a reference ecosystem), in order to test whether local substrate could hamper seedling establishment. As expected, the oldest restoration site had higher density and richness of spontaneously regenerating seedlings. However, seedling establishment was less successful both in the oldest restoration planting and using substrate transferred from a reference ecosystem, where emergence and survival were lower, but surviving seedlings grew better. We attribute these results to lower light availability for seedlings in the understory of the oldest site and speculate that higher incidence of pathogens on old‐growth forest soil may have increased seedling mortality. We conclude that the understory of young restoration plantations provides suitable microsite conditions at the early establishment phases for the spontaneous regeneration or enrichment planting of native trees.     

Incorporating natural regeneration in forest landscape restoration in tropical regions: synthesis and key research gaps

Extensive tropical forest loss and degradation have stimulated increasing awareness at the international policy level of the need to undertake large‐scale forest landscape restoration (FLR). Natural regeneration offers a cost‐effective way to achieve large‐scale FLR, but is often overlooked in favor of tree plantations. The studies presented in this special issue show how natural regeneration can become an important part of FLR and highlight the ecological, environmental, and social factors that must be considered to effectively do so. They also identify major knowledge gaps and outline a research agenda to support the use of natural regeneration in FLR. Six central questions emerge from these studies: (1) What are the ecological, economic, and livelihood outcomes of active and passive restoration interventions?; (2) What are the tradeoffs and synergies among ecological, economic, and livelihood outcomes of natural regeneration, restoration and productive land uses, and how do they evolve in the face of market and climate shocks?; (3) What diagnostic tools are needed to identify and map target areas for natural regeneration?; (4) How should spatial prioritization frameworks incorporate natural regeneration into FLR?; (5) What legal frameworks and governance structures are best suited to encourage natural regeneration and how do they change across regions and landscapes?; (6) What financial mechanisms can foster low‐cost natural regeneration? Natural regeneration is not a panacea to solve tensions and conflicts over land use, but it can be advantageous under some circumstances. Identifying under what conditions this is the case is an important avenue for future research.     

Environment and landscape rather than planting design are the drivers of success in long‐term restoration of riparian Atlantic forest

Question

Identifying the factors that lead to the success of restoration projects has been a major challenge in ecological restoration. Here we ask which factors, aside from time since restoration began, drive the recovery of tree biomass, density and richness of the understorey in riparian forests undergoing restoration.

Location

Semideciduous Atlantic Forest with tropical climate and deep, fertile soils, southeast Brazil.

Methods

We sampled tree basal area (DBH ≥ 5 cm), density and richness of the understorey (DBH < 5 cm) in 26 riparian forests undergoing restoration (a chronosequence spanning 4–53 years). We assessed the following variables as possible factors, besides time, influencing community attributes: (1) planting design: density and richness of seedlings planted; (2) landscape features: proximity index measuring forest cover within a 1.5‐km radius, distance and size of the nearest forest remnant; and (3) environmental factors: invasive grasses, soil fertility, drought, average annual precipitation and proportion of fine particles in the soil. We performed correlation analyses including predictor and response variables, followed by stepwise backward regression (AIC), multiple and simple linear regressions, to investigate the relationships between those factors and the community attributes.

Results

Tree basal area was primarily influenced by the proportion of small particles in the soil (+) and secondarily by rainfall (?). Understorey richness was influenced by the combination of size (+) and distance (?) of the nearest patch, rainfall (?) and soil fertility (+). Understorey density was primarily influenced by the size of the nearest forest remnant (+) and secondarily by invasive grasses (?). No influence of density or richness of the seedlings planted was observed.

Conclusion

Environmental factors and landscape configuration drive the recovery of tree biomass, density and richness in communities undergoing restoration. The most relevant ecological filters influencing restoration success are availability of soil water and nutrients and the distance and size of the nearest remnant of native vegetation. The expected influence of richness and density of seedlings planted, considered for many years as important drivers of forest restoration success, was not confirmed in this study.     

A joint individual‐based model coupling growth and mortality reveals that tree vigor is a key component of tropical forest dynamics

Tree vigor is often used as a covariate when tree mortality is predicted from tree growth in tropical forest dynamic models, but it is rarely explicitly accounted for in a coherent modeling framework. We quantify tree vigor at the individual tree level, based on the difference between expected and observed growth. The available methods to join nonlinear tree growth and mortality processes are not commonly used by forest ecologists so that we develop an inference methodology based on an MCMC approach, allowing us to sample the parameters of the growth and mortality model according to their posterior distribution using the joint model likelihood. We apply our framework to a set of data on the 20‐year dynamics of a forest in Paracou, French Guiana, taking advantage of functional trait‐based growth and mortality models already developed independently. Our results showed that growth and mortality are intimately linked and that the vigor estimator is an essential predictor of mortality, highlighting that trees growing more than expected have a far lower probability of dying. Our joint model methodology is sufficiently generic to be used to join two longitudinal and punctual linked processes and thus may be applied to a wide range of growth and mortality models. In the context of global changes, such joint models are urgently needed in tropical forests to analyze, and then predict, the effects of the ongoing changes on the tree dynamics in hyperdiverse tropical forests.     

Demographic costs and benefits of herbicide‐based restoration to enhance habitat for an endangered butterfly and a threatened plant

Restoring habitat degraded by invasive species is often a primary focus of conservation strategies, yet few studies investigate the effects of invasive species control on multiple at‐risk taxa. Selective herbicides are increasingly used because they can selectively reduce aggressive invasive plant species with the aim of minimizing effects on other taxa within the habitat. We conducted a four‐year experiment to test how annual application of grass‐specific herbicide affected the demography on Fender's blue butterfly (Icaricia icarioides fenderi) and Kincaid's lupine (Lupinus oreganus), two federally protected species which persist in highly degraded prairie remnants in western Oregon, USA. Effects of herbicide application were transitory for the butterfly; reduction of invasive grasses increased fecundity and led to higher annual population growth (λ) at one of two conservation areas in the first season. There were no detectable differences in λ in subsequent seasons—suggesting that treatments caused neither extensive harm nor extensive benefit to the butterfly population. For the lupine, there were no detectable differences in leaf and flower abundance between control and herbicide treatments. However, greater seed production in herbicide plots in the first and third seasons suggests that lupines in herbicide‐treated plots have greater potential reproductive success. While treatments do not have a long‐term benefit to annual population growth for the butterfly, increasing reproductive success of the threatened plant may justify integrating this strategy into restoration plans. Considering the impact of restoration practices on the demography of multiple at‐risk taxa within a community is critical to effective recovery strategies.     

海南岛热带天然针叶林主要树种的空间格局及关联性

天然针叶林在热带地区虽较为少见, 但其对维持热带地区的生物多样性和生境异质性具有特殊意义。在我国热带天然针叶林集中分布面积最大的海南霸王岭林区, 作者选择伴生阔叶树种优势度不同的两种典型南亚松(Pinus latteri)天然林(简称纯林和混交林), 采用点格局法分析了其林冠层、亚林层和林下层主要树种的空间分布格局及其关联性。结果表明: (1)纯林中林冠层的南亚松主要为聚集分布, 混交林中在较小尺度上为聚集分布, 在较大尺度上为随机分布。(2)纯林中亚林层树种在较小尺度上为聚集分布, 在较大尺度上为随机分布, 在混交林中主要为聚集分布。(3)纯林中林下层树种主要呈现为随机分布, 而在混交林中主要为聚集分布。(4)随着尺度的增加, 林冠层与其他两个层次的树种, 在纯林中表现出从空间无关联到正关联的变化趋势, 而在混交林中则表现出从空间无关联到负关联的变化趋势。(5)亚林层与林下层树种在各个尺度上都表现为空间正关联。由此可见, 热带天然针叶林中优势种南亚松对伴生阔叶树种的分布格局具有重要影响。     

Natural regeneration in the context of large‐scale forest and landscape restoration in the tropics

Large‐scale and long‐term restoration efforts are urgently needed to reverse historical global trends of deforestation and forest degradation in the tropics. Restoration of forests within landscapes offers multiple social, economic, and environmental benefits that enhance lives of local people, mitigate effects of climate change, increase food security, and safeguard soil and water resources. Despite rapidly growing knowledge regarding the extent and feasibility of natural regeneration and the environmental and economic benefits of naturally regenerating forests in the tropics, tree planting remains the major focus of restoration programs. Natural regeneration is often ignored as a viable land‐use option. Here, we assemble a set of 16 original papers that provide an overview of the ecological, economic, and social dimensions of forest and landscape restoration (FLR), a relatively new approach to forest restoration that aims to regain ecological integrity and enhance human well‐being in deforested or degraded forest landscapes. The papers describe how spontaneous (passive) and assisted natural regeneration can contribute to achieving multiple social and ecological benefits. Forest and landscape restoration is centered on the people who live and work in the landscape and whose livelihoods will benefit and diversify through restoration activities inside and outside of farms. Given the scale of degraded forestland and the need to mitigate climate change and meet human development needs in the tropics, harnessing the potential of natural regeneration will play an essential role in achieving the ambitious goals that motivate global restoration initiatives.