Green deserts, but not always: A global synthesis of native woody species regeneration under tropical tree monocultures.

Tree monocultures constitute an increasing fraction of the global tree cover and are the dominant tree-growing strategy of forest landscape restoration commitments. Their advantages to produce timber are well known, but their value for biodiversity is highly controversial and context dependent. Therefore, understanding whether, and in which conditions, they can harbor native species regeneration is crucial. Here, we conducted meta-analyses based on a global survey of the literature and on a database created with local, unpublished studies throughout Brazil to evaluate the regeneration potential of native species under tree monocultures and the way management influences this regeneration. Native woody species regeneration under tree monocultures harbors a substantial fraction of the diversity (on average 40% and 68% in the global and Brazilian surveys, respectively) and abundance (on average 25% and 60% in the global and Brazilian surveys, respectively) of regeneration observed in natural forests. Plantations with longer rotation lengths, composed of native tree species, and located adjacent to forest remnants harbor more species. Pine plantations harbor more native individuals than eucalypt plantations, and the abundance of regenerating trees is higher in sites with higher mean temperatures. Species-area curves revealed that the number of woody species under pine and eucalypt plantations in Brazil is 606 and 598 species, respectively, over an aggregated sampled area of ca. 12 ha. We highlight that the understory of tree monocultures can harbor a considerable diversity of regenerating native species at the landscape and regional scales, but this diversity strongly depends on management. Long-rotation length and favorable location are key factors for woody regeneration success under tropical tree monocultures. Therefore, tree monocultures can play a role in forest landscape restoration and conservation, but only if they are planned and managed for achieving this purpose.

Conflicts and opportunities for commercial tree plantation expansion and biodiversity restoration across Brazil.

Substantial global restoration commitments are occurring alongside a rapid expansion in land-hungry tropical commodities, including to supply increasing demand for wood products. Future commercial tree plantations may deliver high timber yields, shrinking the footprint of production forestry, but there is an as-yet unquantified risk that plantations may expand into priority restoration areas, with marked environmental costs. Focusing on Brazil-a country of exceptional restoration importance and one of the largest tropical timber producers-we use random forest models and information on the economic, social, and spatial drivers of historic commercial tree plantation expansion to estimate and map the probability of future monoculture tree plantation expansion between 2020 and 2030. We then evaluate potential plantation-restoration conflicts and opportunities at national and biome-scales and under different future production and restoration pathways. Our simulations show that of 2.8 Mha of future plantation expansion (equivalent to plantation expansion 2010-2020), ~78,000 ha (3%) is forecast to occur in the top 1% of restoration priority areas for terrestrial vertebrates, with ~547,500 ha (20%) and ~1,300,000 ha (46%) in the top 10% and 30% of priority areas, respectively. Just ~459,000 ha (16%) of expansion is forecast within low-restoration areas (bottom 30% restoration priorities), and the first 1 Mha of plantation expansion is likely to have disproportionate impacts, with potential restoration-plantation overlap starkest in the Atlantic Forest but prominent in the Pampas and Cerrado as well. Our findings suggest that robust, coherent land-use policies must be deployed to ensure that significant trade-offs between restoration and production objectives are navigated, and that commodity expansion does not undermine the most tractable conservation gains under emerging global restoration agendas. They also highlight the potentially significant role an engaged forestry sector could play in improving biodiversity outcomes in restoration projects in Brazil, and presumably elsewhere.

Convergence and divergence in science and practice of urban and rural forest restoration.

Forest restoration has never been higher on policymakers' agendas. Complex and multi-dimensional arrangements across the urban-rural continuum challenge restorationists and require integrative approaches to strengthen environmental protection and increase restoration outcomes. It remains unclear if urban and rural forest restoration are moving towards or away from each other in practice and research, and whether comparing research outcomes can help stakeholders to gain a clearer understanding of the interconnectedness between the two fields. This study aims to identify the challenges and opportunities for enhancing forest restoration in both urban and rural systems by reviewing the scientific evidence, engaging with key stakeholders and using an urban-rural forest restoration framework. Using the Society for Ecological Restoration's International Principles as discussion topics, we highlight aspects of convergence and divergence between the two fields to broaden our understanding of forest restoration and promote integrative management approaches to address future forest conditions. Our findings reveal that urban and rural forest restoration have convergent and divergent aspects. We emphasise the importance of tailoring goals and objectives to specific contexts and the need to design different institutions and incentives based on the social and ecological needs and goals of stakeholders in different regions. Additionally, we discuss the challenges of achieving high levels of ecological restoration and the need to go beyond traditional ecology to plan, implement, monitor, and adaptively manage restored forests. We suggest that rivers and watersheds could serve as a common ground linking rural and urban landscapes and that forest restoration could interact with other environmental protection measures. We note the potential for expanding the creative vision associated with increasing tree-containing environments in cities to generate more diverse and resilient forest restoration outcomes in rural settings. This study underscores the value of integrative management approaches in addressing future forest conditions across the urban-rural continuum. Our framework provides valuable insights for policymakers, researchers, and decision-makers to advance the field of forest restoration and address the challenges of restoration across the urban-rural continuum. The rural-urban interface serves as a convergence point for forest restoration, and both urban and rural fields can benefit from each other's expertise.

Integrated global assessment of the natural forest carbon potential.

Forests are a substantial terrestrial carbon sink, but anthropogenic changes in land use and climate have considerably reduced the scale of this system1. Remote-sensing estimates to quantify carbon losses from global forests2-5 are characterized by considerable uncertainty and we lack a comprehensive ground-sourced evaluation to benchmark these estimates. Here we combine several ground-sourced6 and satellite-derived approaches2,7,8 to evaluate the scale of the global forest carbon potential outside agricultural and urban lands. Despite regional variation, the predictions demonstrated remarkable consistency at a global scale, with only a 12% difference between the ground-sourced and satellite-derived estimates. At present, global forest carbon storage is markedly under the natural potential, with a total deficit of 226 Gt (model range = 151-363 Gt) in areas with low human footprint. Most (61%, 139 Gt C) of this potential is in areas with existing forests, in which ecosystem protection can allow forests to recover to maturity. The remaining 39% (87 Gt C) of potential lies in regions in which forests have been removed or fragmented. Although forests cannot be a substitute for emissions reductions, our results support the idea2,3,9 that the conservation, restoration and sustainable management of diverse forests offer valuable contributions to meeting global climate and biodiversity targets.

The global biogeography of tree leaf form and habit.

Understanding what controls global leaf type variation in trees is crucial for comprehending their role in terrestrial ecosystems, including carbon, water and nutrient dynamics. Yet our understanding of the factors influencing forest leaf types remains incomplete, leaving us uncertain about the global proportions of needle-leaved, broadleaved, evergreen and deciduous trees. To address these gaps, we conducted a global, ground-sourced assessment of forest leaf-type variation by integrating forest inventory data with comprehensive leaf form (broadleaf vs needle-leaf) and habit (evergreen vs deciduous) records. We found that global variation in leaf habit is primarily driven by isothermality and soil characteristics, while leaf form is predominantly driven by temperature. Given these relationships, we estimate that 38% of global tree individuals are needle-leaved evergreen, 29% are broadleaved evergreen, 27% are broadleaved deciduous and 5% are needle-leaved deciduous. The aboveground biomass distribution among these tree types is approximately 21% (126.4 Gt), 54% (335.7 Gt), 22% (136.2 Gt) and 3% (18.7 Gt), respectively. We further project that, depending on future emissions pathways, 17-34% of forested areas will experience climate conditions by the end of the century that currently support a different forest type, highlighting the intensification of climatic stress on existing forests. By quantifying the distribution of tree leaf types and their corresponding biomass, and identifying regions where climate change will exert greatest pressure on current leaf types, our results can help improve predictions of future terrestrial ecosystem functioning and carbon cycling.

Native diversity buffers against severity of non-native tree invasions.

Determining the drivers of non-native plant invasions is critical for managing native ecosystems and limiting the spread of invasive species1,2. Tree invasions in particular have been relatively overlooked, even though they have the potential to transform ecosystems and economies3,4. Here, leveraging global tree databases5-7, we explore how the phylogenetic and functional diversity of native tree communities, human pressure and the environment influence the establishment of non-native tree species and the subsequent invasion severity. We find that anthropogenic factors are key to predicting whether a location is invaded, but that invasion severity is underpinned by native diversity, with higher diversity predicting lower invasion severity. Temperature and precipitation emerge as strong predictors of invasion strategy, with non-native species invading successfully when they are similar to the native community in cold or dry extremes. Yet, despite the influence of these ecological forces in determining invasion strategy, we find evidence that these patterns can be obscured by human activity, with lower ecological signal in areas with higher proximity to shipping ports. Our global perspective of non-native tree invasion highlights that human drivers influence non-native tree presence, and that native phylogenetic and functional diversity have a critical role in the establishment and spread of subsequent invasions.

Phylogenetic diversity and the structure of host-epiphyte interactions across the Neotropics.

Understanding the mechanisms driving community assembly has been a major focus of ecological research for nearly a century, yet little is known about these mechanisms in commensal communities, particularly with respect to their historical/evolutionary components. Here, we use a large-scale dataset of 4,440 vascular plant species to explore the relationship between the evolutionary distinctiveness (ED) (as measured by the 'species evolutionary history' (SEH)) of host species and the phylogenetic diversity (PD) of their associated epiphyte species. Although there was considerable variation across hosts and their associated epiphyte species, they were largely unrelated to host SEH. Our results mostly support the idea that the determinants of epiphyte colonization success might involve host characteristics that are unrelated to host SEH (e.g., architectural differences between hosts). While determinants of PD of epiphyte assemblages are poorly known, they do not appear to be related to the evolutionary history of host species. Instead, they might be better explained by neutral processes of colonization and extinction. However, the high level of phylogenetic signal in epiphyte PD (independent of SEH) suggests it might still be influenced by yet unrecognized evolutionary determinants. This study highlights how little is still known about the phylogenetic determinants of epiphyte communities.

Potential aboveground biomass increase in Brazilian Atlantic Forest fragments with climate change.

Fragmented tropical forest landscapes preserve much of the remaining biodiversity and carbon stocks. Climate change is expected to intensify droughts and increase fire hazard and fire intensities, thereby causing habitat deterioration, and losses of biodiversity and carbon stock losses. Understanding the trajectories that these landscapes may follow under increased climate pressure is imperative for establishing strategies for conservation of biodiversity and ecosystem services. Here, we used a quantitative predictive modelling approach to project the spatial distribution of the aboveground biomass density (AGB) by the end of the 21st century across the Brazilian Atlantic Forest (AF) domain. To develop the models, we used the maximum entropy method with projected climate data to 2100, based on the Intergovernmental Panel on Climate Change Representative Concentration Pathway (RCP) 4.5 from the fifth Assessment Report. Our AGB models had a satisfactory performance (area under the curve > 0.75 and p value < .05). The models projected a significant increase of 8.5% in the total carbon stock. Overall, the projections indicated that 76.9% of the AF domain would have suitable climatic conditions for increasing biomass by 2100 considering the RCP 4.5 scenario, in the absence of deforestation. Of the existing forest fragments, 34.7% are projected to increase their AGB, while 2.6% are projected to have their AGB reduced by 2100. The regions likely to lose most AGB-up to 40% compared to the baseline-are found between latitudes 13° and 20° south. Overall, although climate change effects on AGB vary latitudinally for the 2071-2100 period under the RCP 4.5 scenario, our model indicates that AGB stocks can potentially increase across a large fraction of the AF. The patterns found here are recommended to be taken into consideration during the planning of restoration efforts, as part of climate change mitigation strategies in the AF and elsewhere in Brazil.

Beyond ecology: ecosystem restoration as a process for social-ecological transformation.

Ecosystem restoration conventionally focuses on ecological targets. However, while ecological targets are crucial to mobilizing political, social, and financial capital, they do not encapsulate the need to: integrate social, economic, and ecological dimensions and systems approaches; reconcile global targets and local objectives; and measure the rate of progress toward multiple and synergistic goals. Restoration is better conceived as an inclusive social-ecological process that integrates diverse values, practices, knowledge, and restoration objectives across temporal and spatial scales and stakeholder groups. Taking a more process-based approach will ultimately enable greater social-ecological transformation, greater restoration effectiveness, and more long-lasting benefits to people and nature across time and place.

Ecological integrity of tropical secondary forests: concepts and indicators.

Naturally regenerating forests or secondary forests (SFs) are a promising strategy for restoring large expanses of tropical forests at low cost and with high environmental benefits. This expectation is supported by the high resilience of tropical forests after natural disturbances, yet this resilience can be severely reduced by human impacts. Assessing the characteristics of SFs and their ecological integrity (EI) is essential to evaluating their role for conservation, restoration, and provisioning of ecosystem services. In this study, we aim to propose a concept and indicators that allow the assessment and classification of the EI of SFs. To this end, we review the literature to assess how EI has been addressed in different ecosystems and which indicators of EI are most commonly used for tropical forests. Building upon this knowledge we propose a modification of the concept of EI to embrace SFs and suggest indicators of EI that can be applied to different successional stages or stand ages. Additionally, we relate these indicators to ecosystem service provision in order to support the practical application of the theory. EI is generally defined as the ability of ecosystems to support and maintain composition, structure and function similar to the reference conditions of an undisturbed ecosystem. This definition does not consider the temporal dynamics of recovering ecosystems, such as SFs. Therefore, we suggest incorporation of an optimal successional trajectory as a reference in addition to the old-growth forest reference. The optimal successional trajectory represents the maximum EI that can be attained at each successional stage in a given region and enables the evaluation of EI at any given age class. We further suggest a list of indicators, the main ones being: compositional indicators (species diversity/richness and indicator species); structural indicators (basal area, heterogeneity of basal area and canopy cover); function indicators (tree growth and mortality); and landscape proxies (landscape heterogeneity, landscape connectivity). Finally, we discuss how this approach can assist in defining the value of SF patches to provide ecosystem services, restore forests and contribute to ecosystem conservation.

The role of incentive mechanisms in promoting forest restoration.

Forest restoration has been proposed as a scalable nature-based solution to achieve global environmental and socio-economic outcomes and is central to many policy initiatives, such as the Bonn Challenge. Restored forests contain appreciable biodiversity, improve habitat connectivity and sequester carbon. Incentive mechanisms (e.g. payments for ecosystem services and allocation of management rights) have been a focus of forest restoration efforts for decades. Yet, there is still little understanding of their role in promoting restoration success. We conducted a systematic literature review to investigate how incentive mechanisms are used to promote forest restoration, outcomes, and the biophysical and socio-economic factors that influence implementation and program success. We found that socio-economic factors, such as governance, monitoring systems and the experience and beliefs of participants, dominate whether or not an incentive mechanism is successful. We found that approximately half of the studies report both positive ecological and socio-economic outcomes. However, reported adverse outcomes were more commonly socio-economic than ecological. Our results reveal that achieving forest restoration at a sufficient scale to meet international commitments will require stronger assessment and management of socio-economic factors that enable or constrain the success of incentive mechanisms. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Invasive alien acacias rapidly stock carbon, but threaten biodiversity recovery in young second-growth forests.

Under the UN-Decade of Ecosystem Restoration and Bonn Challenge, second-growth forest is promoted as a global solution to climate change, degradation and associated losses of biodiversity and ecosystem services. Second growth is often invaded by alien tree species and understanding how this impacts carbon stock and biodiversity recovery is key for restoration planning. We assessed carbon stock and tree diversity recovery in second growth invaded by two Acacia species and non-invaded second growth, with associated edge effects, in the Brazilian Atlantic Forest. Carbon stock recovery in non-invaded forests was threefold lower than in invaded forests. Increasingly isolated, fragmented and deforested areas had low carbon stocks when non-invaded, whereas the opposite was true when invaded. Non-invaded forests recovered threefold to sixfold higher taxonomic, phylogenetic and functional diversity than invaded forest. Higher species turnover and lower nestedness in non-invaded than invaded forests underpinned higher abundance of threatened and endemic species in non-invaded forest. Non-invaded forests presented positive relationships between carbon and biodiversity, whereas in the invaded forests we did not detect any relationship, indicating that more carbon does not equal more biodiversity in landscapes with high vulnerability to invasive acacias. To deliver on combined climate change and biodiversity goals, restoration planning and management must consider biological invasion risk. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.

Overcoming biotic homogenization in ecological restoration.

Extensive evidence shows that regional (gamma) diversity is often lower across restored landscapes than in reference landscapes, in part due to common restoration practices that favor widespread species through selection of easily-grown species with high survival and propagation practices that reduce genetic diversity. We discuss approaches to counteract biotic homogenization, such as reintroducing species that are adapted to localized habitat conditions and are unlikely to colonize naturally; periodically reintroducing propagules from remnant populations to increase genetic diversity; and reintroducing higher trophic level fauna to restore interaction networks and processes that promote habitat heterogeneity. Several policy changes would also increase regional diversity; these include regional coordination amongst restoration groups, financial incentives to organizations producing conservation-valued species, and experimental designations for rare species introductions.

The biodiversity and ecosystem service contributions and trade-offs of forest restoration approaches.

Forest restoration is being scaled up globally to deliver critical ecosystem services and biodiversity benefits; however, there is a lack of rigorous comparison of cobenefit delivery across different restoration approaches. Through global synthesis, we used 25,950 matched data pairs from 264 studies in 53 countries to assess how delivery of climate, soil, water, and wood production services, in addition to biodiversity, compares across a range of tree plantations and native forests. Benefits of aboveground carbon storage, water provisioning, and especially soil erosion control and biodiversity are better delivered by native forests, with compositionally simpler, younger plantations in drier regions performing particularly poorly. However, plantations exhibit an advantage in wood production. These results underscore important trade-offs among environmental and production goals that policy-makers must navigate in meeting forest restoration commitments.