Abrupt loss and uncertain recovery from fires of Amazon forests under low climate mitigation scenarios.

Tropical forests contribute a major sink for anthropogenic carbon emissions essential to slowing down the buildup of atmospheric CO2 and buffering climate change impacts. However, the response of tropical forests to more frequent weather extremes and long-recovery disturbances like fires remains uncertain. Analyses of field data and ecological theory raise concerns about the possibility of the Amazon crossing a tipping point leading to catastrophic tropical forest loss. In contrast, climate models consistently project an enhanced tropical sink. Here, we show a heterogeneous response of Amazonian carbon stocks in GFDL-ESM4.1, an Earth System Model (ESM) featuring dynamic disturbances and height-structured tree-grass competition. Enhanced productivity due to CO2 fertilization promotes increases in forest biomass that, under low emission scenarios, last until the end of the century. Under high emissions, positive trends reverse after 2060, when simulated fires prompt forest loss that results in a 40% decline in tropical forest biomass by 2100. Projected fires occur under dry conditions associated with El Niño Southern Oscillation and the Atlantic Multidecadal Oscillation, a response observed under current climate conditions, but exacerbated by an overall decline in precipitation. Following the initial disturbance, grassland dominance promotes recurrent fires and tree competitive exclusion, which prevents forest recovery. EC-Earth3-Veg, an ESM with a dynamic vegetation model of similar complexity, projected comparable wildfire forest loss under high emissions but faster postfire recovery rates. Our results reveal the importance of complex nonlinear responses to assessing climate change impacts and the urgent need to research postfire recovery and its representation in ESMs.

Patterns and inferred causes of liana mortality in a tropical forest.

Lianas are major contributors to tropical forest dynamics, yet we know little about their mortality. Using overlapping censuses of the lianas and trees across a 50 ha stand of moist tropical forest, we contrasted community-wide patterns of liana mortality with relatively well-studied patterns of tree mortality to quantify patterns of liana death and identify contributing factors. Liana mortality rates were 172% higher than tree mortality rates, but species-level mortality rates of lianas were similar to trees with 'fast' life-history strategies and both growth forms exhibited similar spatial and size-dependent patterns. The mortality rates of liana saplings (<2.1 cm in diameter), which represent about 50% of liana individuals, decreased with increasing disturbance severity and remained consistently low during post-disturbance stand thinning. In contrast, larger liana individuals and trees of all sizes had elevated mortality rates in response to disturbance and their mortality rates decreased over time since disturbance. Within undisturbed forest patches, liana mortality rates increased with increasing soil fertility in a manner similar to trees. The distinct responses of liana saplings to disturbance appeared to distinguish liana mortality from that of trees, whereas similarities in their patterns of death suggest that there are common drivers of woody plant mortality.

Patterns of early post-disturbance reorganization in Central European forests.

Disturbances catalyse change in forest ecosystems, and a climate-driven increase in disturbance activity could accelerate forest reorganization. Here, we studied post-disturbance forests after the biggest pulse of tree mortality in Central Europe in at least 170 years, caused by drought and bark beetle (Scolytinae) outbreaks in 2018-2020. Our objectives were to characterize the early state of tree regeneration after mortality, quantify patterns of reorganization relative to undisturbed reference conditions and assess how management and patch size affect forest reorganization after disturbance. We surveyed 1244 plots in 120 patches under managed (salvage-logged, often planted) and unmanaged (deadwood remaining on site, no planting) conditions in Germany. We found that regeneration density on disturbed sites was high (median 11 897 stems ha-1), resulting from a cohort of advance regeneration. Disturbances were strong drivers of change, with indications for resilience on only 36.3% of patches. Reassembly (i.e. a change in species composition) was the dominant pattern of reorganization (61.5%), and Picea abies forests changed most strongly. Post-disturbance management facilitated forest change, particularly promoting a change in species composition. The strength of reorganization increased with patch size. We conclude that the recent wave of tree mortality will likely accelerate forest change in Central Europe.

Aerodynamic effects cause higher forest evapotranspiration and water yield reductions after wildfires in tall forests.

Wildfires are increasing in frequency, intensity, and extent globally due to climate change and they can alter forest composition, structure, and function. The destruction and subsequent regrowth of young vegetation can modify the ecosystem evapotranspiration and downstream water availability. However, the response of forest recovery on hydrology is not well known with even the sign of evapotranspiration and water yield changes following forest fires being uncertain across the globe. Here, we quantify the effects of forest regrowth after catastrophic wildfires on evapotranspiration and runoff in the world's tallest angiosperm forest (Eucalyptus regnans) in Australia. We combine eddy covariance measurements including pre- and post-fire periods, mechanistic ecohydrological modeling and then extend the analysis spatially to multiple fires in eucalypt-dominated forests in south-eastern Australia by utilizing remote sensing. We find a fast recovery of evapotranspiration which reaches and exceeds pre-fire values within 2 years after the bushfire, a result confirmed by eddy covariance data, remote sensing, and modeling. Such a fast evapotranspiration recovery is likely generalizable to tall eucalypt forests in south-eastern Australia as shown by remote sensing. Once climate variability is discounted, ecohydrological modeling shows evapotranspiration rates from the recovering forest which reach peak values of +20% evapotranspiration 3 years post-fire. As a result, modeled runoff decreases substantially. Contrary to previous research, we find that the increase in modeled evapotranspiration is largely caused by the aerodynamic effects of a much shorter forest height leading to higher surface temperature, higher humidity gradients and therefore increased transpiration. However, increases in evapotranspiration as well as decreases in runoff caused by the young forest are constrained by energy and water limitations. Our result of an increase in evapotranspiration due to aerodynamic warming in a shorter forest after wildfires could occur in many parts of the world experiencing forest disturbances.

Recolonization of secondary forests by a locally extinct Caribbean anole through the lens of range expansion theory.

Disturbance and recovery dynamics are characteristic features of many ecosystems. Disturbance dynamics are widely studied in ecology and conservation biology. Still, we know less about the ecological processes that drive ecosystem recovery. The ecological processes that mediate ecosystem recovery stand at the intersection of many theoretical frameworks. Range expansion theory is one of these complementary frameworks that can provide unique insights into the population-level processes that mediate ecosystem recovery, particularly fauna recolonization. Although the biodiversity patterns that follow the fauna recolonization of recovering forests have been well described in the literature, the ecological processes at the population level that drive these patterns remain conspicuously unknown. In this study, we tested three fundamental predictions of range expansion theory during the recolonization of recovering forests in Puerto Rico by a shade specialist anole, Anolis gundlachi. Range expansion theory predicts that individuals at the early stages of recolonization (i.e., younger forests) would have a high prevalence of dispersive traits, experience less density dependence, and suffer less parasitism. To test these predictions, we conducted a chronosequence study applying space-for-time substitution where we compared phenotypic traits (i.e., body size, body condition, and relative limb size), population density, population growth rates, and Plasmodium parasitism rates among lizard populations living in young (<30 years), mid (~40-70 years), and old-growth forests (>75 years). Lizard populations in younger forests had lower densities, higher population growth rates, and lower rates of Plasmodium parasitism compared with old-growth forests. Still, while we found that individuals had larger body sizes, and longer forelimbs in young forests in one site, this result was not consistent among sites. This suggests a potential trade-off between the traits that provide a dispersal advantage during the initial stages of recolonization and those that are advantageous to establish in novel environmental conditions. Overall, our study emphasizes the suitability of range expansion theory to describe fauna recolonization but also highlights that the ecological processes that drive recolonization are time-dependent, complex, and nuanced.

Understorey Seedling Bank in Forest Areas with a Differing Period of Recovery in Mabira Central Forest Reserve, South Central Uganda.

Seedling banks are very important in forest regeneration following forest disturbances such as crop cultivation. In 2011 and 2013 the Uganda National Forestry Authority (NFA) evicted encroachers from parts of Mabira Central Forest Reserve that had been under crop cultivation for over 40 years. This gave an opportunity for the vegetation to recover. In this study, we assessed the recovery process based on seedling bank diversity, richness and density in three blocks differing in recovery time. Two disturbed blocks, the Western Block (WB) (abandoned by encroachers 1-3 years), and the Eastern Block (EB) (abandoned 4-5 years) before this study and a nearby undisturbed area (intact) were included in the study. We recorded 48 seedlings species; 37 in WB, 30 in EB and 27 in intact. Differences in species richness were not statistically significant among blocks (F2, 88 = 1.2420, p = 0.294). All seedling species found in the intact were found in the EB and WB. There were statistically significant differences in species diversity (Shannon-Wiener: F2, 88 = 5.354, p = 0.006), density (P < 0.001) and composition (ANOSIM; R = 0.55, p = 0.001) among blocks. Apart from Broussonetia papyrifera, other species contributing to the dissimilarity (Acalypha neptunica, Antiaris toxicaria, Blighia unijugata, Funtumia elastica were late succession species. Animal dispersed species dominated intact. Seed-regenerating species were found in both WB and EB, but re-sprouts were more common in EB. These results show that proximity to intact forest aids forest recovery, even for areas with long-term cultivation history.

Native multispecies and fast-growing forest root biomass increase C and N stocks in a reclaimed bauxite mining area.

This study is aimed at evaluating C and N stocks in fractions of soil organic matter (SOM) in an area of bauxite mining under recovery with tree species. We have analyzed the long-term recovery of C and N stocks of organic matter fractions from five types of forest cover (Eucalyptus, Anadenanthera peregrina, mixed plantation of 16 native species, a mined area without vegetation cover as a control site, and a natural forest cover as a reference site). The total organic C (TOC) and N (TN) stocks and also organic matter fractions, particulate organic matter (POM), mineral-associated organic matter (MAOM), microbial biomass (MB), and labile C (LC), were determined, as well as the C/N ratio and the carbon management index (CMI). Although the stocks of TOC and LC, CMI, and MB did not differ between the types of forest cover in the 0-60 cm layer, they were lower than the values in the native forest. Forest cover increases the stocks of TOC, LC, MB, and CMI in the area of bauxite mining compared to the control site. In addition, we found that the TOC C and TN stocks and also SOM fractions (LC, C-MAOM, C-POM, N-MAOM, and N-POM) are positively correlated (r ≥ 0.71 for all cases) with volume of roots larger than 2 mm. Therefore, Eucalyptus, A. peregrina, and a mixed of 16 native trees contribute for restoring stocks of soil C and N following bauxite mining in the Brazilian Atlantic Forest.

Effects of thinning on the demography and functional community structure of a secondary tropical lowland rain forest.

Tropical secondary forests play a prominent role in conserving biodiversity and providing ecosystem services, but their recovery can be slow and their succession trajectory is distinct from old-growth forests. Thinning is an essential silvicultural approach to enhance the recovery rate and timber production of forests. However, the selection of trees to thin has been mainly based on size class rather than on species identity. There is little empirical or experimental evidence of species-focused thinning with the goal of altering species composition. We examined the effects of thinning on community structure, demographic rates, species richness and functional diversity of woody plants in a detailed investigation of 60-year-old secondary tropical lowland rain forest on Hainan Island, South China. The density and basal area of trees ≥5 cm DBH (diameter at breast height) increased significantly after five years' recovery with no significant change for saplings (DBH < 5 cm). Species composition after thinning changed significantly and mid-to late-successional species of both saplings and trees were more abundant after five years' recovery. The relative growth rates (RGR) and recruitment rates were significantly higher in thinning plots for both saplings and trees, and RGRs increased by 127% and 48%, respectively. The mortality rate decreased by 13% for trees and increased by 47% for saplings in thinning plots compared to the control. The community weighted mean (CWM) of the specific leaf area (SLA) of saplings showed a significantly decreasing trend while CWMs of wood density (WD) and mean maximum height (Hmax) of saplings increased after thinning. By contrast, CWMs of SLA and Hmax of trees were significantly higher, but WD was significantly lower- in thinning plots than the control. RGR and recruitment rate of saplings and trees increased significantly as thinning intensity increased. However, the thinning intensity had a weak or nil effect on the mortality rate. Our results support the selective removal and girdling of pioneer and mid-successional species in a way that could accelerate recovery and improve the growth and recruitment of late-successional species in tropical secondary forests. Thinning at a relatively low intensity can maintain species diversity and alter species functional composition. This outcome shows promise for improved future management of tropical forests in human-modified tropical forest landscapes.

Largely underestimated carbon emission from land use and land cover change in the conterminous United States.

Carbon (C) emission and uptake due to land use and land cover change (LULCC) are the most uncertain term in the global carbon budget primarily due to limited LULCC data and inadequate model capability (e.g., underrepresented agricultural managements). We take the commonly used FAOSTAT-based global Land Use Harmonization data (LUH2) and a new high-resolution multisource harmonized national LULCC database (YLmap) to drive a land ecosystem model (DLEM) in the conterminous United States. We found that recent cropland abandonment and forest recovery may have been overestimated in the LUH2 data derived from national statistics, causing previously reported C emissions from land use have been underestimated due to the definition of cropland and aggregated LULCC signals at coarse resolution. This overestimation leads to a strong C sink (30.3 ± 2.5 Tg C/year) in model simulations driven by LUH2 in the United States during the 1980-2016 period, while we find a moderate C source (13.6 ± 3.5 Tg C/year) when using YLmap. This divergence implies that previous C budget analyses based on the global LUH2 dataset have underestimated C emission in the United States owing to the delineation of suitable cropland and aggregated land conversion signals at coarse resolution which YLmap overcomes. Thus, to obtain more accurate quantification of LULCC-induced C emission and better serve global C budget accounting, it is urgently needed to develop fine-scale country-specific LULCC data to characterize the details of land conversion.

Stream Conditions after 18 Years of Passive Riparian Restoration in Small Fish-bearing Watersheds.

Many of the ecological processes in the riparian forests and streams across the Pacific Northwest have become impaired through production forestry practices common prior to the 1990s. Some of these practices included forest harvest without stream buffers, removal of instream wood, road construction and use, and harvesting large proportions of watersheds. Passive ecological restoration (the use of natural processes of succession and disturbance to alleviate anthropogenic impacts over time) is a common practice used in the management of riparian forests previously subjected to production forestry. Eighteen years after the implementation of passive restoration of riparian forests, we used four common stream indicators (stream temperature, canopy closure, instream wood, and salmonid densities) to assess the effects of restoration in small fish-bearing streams. Summer stream temperatures have decreased below unmanaged reference levels, whereas riparian forest canopy closure has increased beyond that in reference watersheds. Instream wood and age-1 or older salmonids appear to be either stable at reduced levels or declining, compared with production forestry and unmanaged reference watersheds. Overall, second-growth riparian forests need more time to develop allowing more light into streams (increasing primary productivity), while also allowing for the continuous recruitment of larger pieces of instream wood (improving habitat for salmonids). Using only passive restoration, stream conditions in second-growth forests are unlikely to increase salmonid production in the near future.

Evidence for declining forest resilience to wildfires under climate change.

Forest resilience to climate change is a global concern given the potential effects of increased disturbance activity, warming temperatures and increased moisture stress on plants. We used a multi-regional dataset of 1485 sites across 52 wildfires from the US Rocky Mountains to ask if and how changing climate over the last several decades impacted post-fire tree regeneration, a key indicator of forest resilience. Results highlight significant decreases in tree regeneration in the 21st century. Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985-1999, suggesting increasingly unfavourable post-fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non-forests after wildfires. Major climate-induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future.

Taxonomic and functional divergence of tree assemblages in a fragmented tropical forest.

Tropical forests are being exposed to increasing levels of habitat loss and fragmentation, threatening the maintenance of global biodiversity. However, the effect that land-use change may have on the spatial dissimilarities in taxonomic and functional composition of remaining assemblages (i.e., taxonomic/functional ß-diversity) remains poorly understood. We examined a large vegetation database from an old and severely fragmented Atlantic forest landscape to test two alternative hypotheses: (1) tree assemblages experience a taxonomic and functional homogenization (reduced ß-diversity) between forest fragments and along forest edges, or alternatively, (2) these edge-affected forests show increased taxonomic and functional differentiation (increased ß-diversity) when compared to forest interior (reference) stands. Taxonomic and functional ß-diversity were examined via novel abundance-based metrics and considering functional traits related to plant dispersion, recruitment, and growth. Overall, taxonomic ß-diversity among fragments was significantly higher than among edge and reference plots. Edge plots also showed higher ß-diversity than reference plots, but only when considering dominant species. In functional terms, ß-diversity among reference plots was also lower than among forest fragments and among edge plots. These patterns support the landscape-divergence hypothesis, which postulates that variable human disturbances among forest fragments and along forest edges can lead to contrasting trajectories of vegetation changes, thus increasing the compositional and functional differentiation of tree communities in these emerging environments. Our results also show that such differentiation can preserve landscape-wide biodiversity, thus overriding negative effects of habitat fragmentation on local (α) diversity. Therefore, our findings demonstrate that forest fragments and forest edges can be more valuable for maintaining species diversity and ecosystem function in fragmented tropical landscapes than previously thought.

Rural aquaculture as a sustainable alternative for forest conservation in the Monarch Butterfly Biosphere Reserve, Mexico.

Forest conservation plays a significant role in environmental sustainability. In Mexico only 8.48 million ha of forest are used for conservation of biodiversity. Payment for Environmental Services in the Monarch Butterfly Biosphere Reserve, one of the most important national protected areas, contributes to the conservation of these forests. In the Reserve, production of rainbow trout has been important for the rural communities who need to conserve the forest cover in order to maintain the hibernation cycle of the butterfly. Aquaculture is a highly productive activity for these protected areas, since it harnesses the existing water resources. In this study, changes from 1999 to 2012 in vegetation and land-use cover in the El Lindero basin within the Reserve were evaluated in order to determine the conservation status and to consider the feasibility of aquaculture as a means of sustainable development at community level. Evaluation involved stereoscopic interpretation of digital aerial photographs from 1999 to 2012 at 1:10,000 scale, comparative analysis by orthocorrected mosaics and restitution on the mosaics. Between 1999 and 2012, forested land recovered by 28.57 ha (2.70%) at the expense of non-forested areas, although forest degradation was 3.59%. Forest density increased by 16.87%. In the 46 ha outside the Reserve, deforestation spread by 0.26%, and land use change was 0.11%. The trend towards change in forest cover is closely related to conservation programmes, particularly payment for not extracting timber, reforestation campaigns and surveillance, whose effects have been exploited for the development of rural aquaculture; this is a new way to improve the socio-economic status of the population, to avoid logging and to achieve environmental sustainability in the Reserve.

Maintenance of different life stages of old-growth forest trees in deforested tropical landscapes.

Tropical tree species are increasingly being pushed to inhabit deforested landscapes. The habitat amount hypothesis posits that, in remaining forest patches, species diversity in equal-sized samples decreases with decreasing forest cover in the surrounding landscape. We tested this prediction by taking into account three important factors that can affect species responses to forest loss. First, forest loss effects can be linear (proportional) or nonlinear, as there can be threshold values of forest loss beyond which species extirpation may be accelerated. Second, such effects are usually scale dependent and may go unnoticed if assessed at suboptimal scales. Finally, species extirpation may take decades to become evident, so the effects of forest loss can be undetected when assessing long-lived organisms, like adult old-growth forest trees. Here, we evaluated the linear and nonlinear effects of landscape forest loss across different spatial scales on site-scale abundance and diversity of old-growth forest trees, separately for four plant-life stages (seeds, saplings, juveniles, and adults) in two rainforest regions with different levels of deforestation. We expected stronger (and negative) forest loss effects on early plant-life stages, especially in the region with the highest deforestation. Surprisingly, in 13 of 16 study cases (2 responses × 4 life stages × 2 regions), null models showed higher empirical support than linear and nonlinear models at any scale. Therefore, the species richness and abundance of local tree assemblages seem to be weakly affected by landscape-scale forest loss independently of the spatial scale, life stage, and region. Yet, as expected, the predictive power of forest cover was relatively lower in the least deforested region. Our findings suggest that landscape-scale forest loss is poorly related to site-scale processes, such as seed dispersal and seedling recruitment, or, at least, such effects are too small to shape the abundance and diversity of tree assemblages within forest patches. Therefore, our findings do not support the most important prediction of the habitat amount hypothesis but imply that, on a per-area basis, a unit of habitat (forest) in a highly deforested landscape has a conservation value similar to that of a more forested one, particularly in moderately deforested rainforests.

Impacts of Water Stress on Forest Recovery and Its Interaction with Canopy Height.

Global climate change is leading to an increase in the frequency, intensity, and duration of drought events, which can affect the functioning of forest ecosystems. Because human activities such as afforestation and forest attributes such as canopy height may exhibit considerable spatial differences, such differences may alter the recovery paths of drought-impacted forests. To accurately assess how climate affects forest recovery, a quantitative evaluation on the effects of forest attributes and their possible interaction with the intensity of water stress is required. Here, forest recovery following extreme drought events was analyzed for Yunnan Province, southwest China. The variation in the recovery of forests with different water availability and canopy heights was quantitatively assessed at the regional scale by using canopy height data based on light detection and ranging (LiDAR) measurements, enhanced vegetation index data, and standardized precipitation evapotranspiration index (SPEI) data. Our results indicated that forest recovery was affected by water availability and canopy height. Based on the enhanced vegetation index measures, shorter trees were more likely to recover than taller ones after drought. Further analyses demonstrated that the effect of canopy height on recovery rates after drought also depends on water availability—the effect of canopy height on recovery diminished as water availability increased after drought. Additional analyses revealed that when the water availability exceeded a threshold (SPEI > 0.85), no significant difference in the recovery was found between short and tall trees (p > 0.05). In the context of global climate change, future climate scenarios of RCP2.6 and RCP8.5 showed more frequent water stress in Yunnan by the end of the 21st century. In summary, our results indicated that canopy height casts an important influence on forest recovery and tall trees have greater vulnerability and risk to dieback and mortality from drought. These results may have broad implications for policies and practices of forest management.

Does Environment Filtering or Seed Limitation Determine Post-fire Forest Recovery Patterns in Boreal Larch Forests?

Wildfire is a primary natural disturbance in boreal forests, and post-fire vegetation recovery rate influences carbon, water, and energy exchange between the land and atmosphere in the region. Seed availability and environmental filtering are two important determinants in regulating post-fire vegetation recovery in boreal forests. Quantifying how these determinants change over time is helpful for understanding post-fire forest successional trajectory. Time series of remote sensing data offer considerable potential in monitoring the trajectory of post-fire vegetation recovery dynamics beyond current field surveys about structural attributes, which generally lack a temporal perspective across large burned areas. We used a time series of the normalized difference vegetation index (NDVI) and normalized difference shortwave infrared reflectance index (NDSWIR) derived from Landsat images to investigate post-fire dynamics in a Chinese boreal larch forest. An adjacent, unburned patch of a similar forest type and environmental conditions was selected as a control to separate interannual fluctuation in NDVI and NDSWIR caused by climate from changes due to wildfire. Temporal anomalies in NDVI and NDSWIR showed that more than 10 years were needed for ecosystems to recover to a pre-fire state. The boosted regression tree analysis showed that fire severity exerted a persistent, dominant influence on vegetation recovery during the early post-fire successional stage and explained more than 60% of variation in vegetation recovery, whereas distance to the nearest unburned area and environmental conditions exhibited a relatively small influence. This result indicated that the legacy effects of fire disturbance, which control seed availability for tree recruitment, would persist for decades. The influence of environmental filtering could increase with succession and could mitigate the initial heterogeneity in recovery caused by wildfire.

Multiple successional pathways in human-modified tropical landscapes: new insights from forest succession, forest fragmentation and landscape ecology research.

Old-growth tropical forests are being extensively deforested and fragmented worldwide. Yet forest recovery through succession has led to an expansion of secondary forests in human-modified tropical landscapes (HMTLs). Secondary forests thus emerge as a potential repository for tropical biodiversity, and also as a source of essential ecosystem functions and services in HMTLs. Such critical roles are controversial, however, as they depend on successional, landscape and socio-economic dynamics, which can vary widely within and across landscapes and regions. Understanding the main drivers of successional pathways of disturbed tropical forests is critically needed for improving management, conservation, and restoration strategies. Here, we combine emerging knowledge from tropical forest succession, forest fragmentation and landscape ecology research to identify the main driving forces shaping successional pathways at different spatial scales. We also explore causal connections between land-use dynamics and the level of predictability of successional pathways, and examine potential implications of such connections to determine the importance of secondary forests for biodiversity conservation in HMTLs. We show that secondary succession (SS) in tropical landscapes is a multifactorial phenomenon affected by a myriad of forces operating at multiple spatio-temporal scales. SS is relatively fast and more predictable in recently modified landscapes and where well-preserved biodiversity-rich native forests are still present in the landscape. Yet the increasing variation in landscape spatial configuration and matrix heterogeneity in landscapes with intermediate levels of disturbance increases the uncertainty of successional pathways. In landscapes that have suffered extensive and intensive human disturbances, however, succession can be slow or arrested, with impoverished assemblages and reduced potential to deliver ecosystem functions and services. We conclude that: (i) succession must be examined using more comprehensive explanatory models, providing information about the forces affecting not only the presence but also the persistence of species and ecological groups, particularly of those taxa expected to be extirpated from HMTLs; (ii) SS research should integrate new aspects from forest fragmentation and landscape ecology research to address accurately the potential of secondary forests to serve as biodiversity repositories; and (iii) secondary forest stands, as a dynamic component of HMTLs, must be incorporated as key elements of conservation planning; i.e. secondary forest stands must be actively managed (e.g. using assisted forest restoration) according to conservation goals at broad spatial scales.

Can we set a global threshold age to define mature forests?

Globally, mature forests appear to be increasing in biomass density (BD). There is disagreement whether these increases are the result of increases in atmospheric CO2 concentrations or a legacy effect of previous land-use. Recently, it was suggested that a threshold of 450 years should be used to define mature forests and that many forests increasing in BD may be younger than this. However, the study making these suggestions failed to account for the interactions between forest age and climate. Here we revisit the issue to identify: (1) how climate and forest age control global forest BD and (2) whether we can set a threshold age for mature forests. Using data from previously published studies we modelled the impacts of forest age and climate on BD using linear mixed effects models. We examined the potential biases in the dataset by comparing how representative it was of global mature forests in terms of its distribution, the climate space it occupied, and the ages of the forests used. BD increased with forest age, mean annual temperature and annual precipitation. Importantly, the effect of forest age increased with increasing temperature, but the effect of precipitation decreased with increasing temperatures. The dataset was biased towards northern hemisphere forests in relatively dry, cold climates. The dataset was also clearly biased towards forests <250 years of age. Our analysis suggests that there is not a single threshold age for forest maturity. Since climate interacts with forest age to determine BD, a threshold age at which they reach equilibrium can only be determined locally. We caution against using BD as the only determinant of forest maturity since this ignores forest biodiversity and tree size structure which may take longer to recover. Future research should address the utility and cost-effectiveness of different methods for determining whether forests should be classified as mature.

Reduced aboveground tree growth associated with higher arbuscular mycorrhizal fungal diversity in tropical forest restoration.

Establishing diverse mycorrhizal fungal communities is considered important for forest recovery, yet mycorrhizae may have complex effects on tree growth depending on the composition of fungal species present. In an effort to understand the role of mycorrhizal fungi community in forest restoration in southern Costa Rica, we sampled the arbuscular mycorrhizal fungal (AMF) community across eight sites that were planted with the same species (Inga edulis, Erythrina poeppigiana, Terminalia amazonia, and Vochysia guatemalensis) but varied twofold to fourfold in overall tree growth rates. The AMF community was measured in multiple ways: as percent colonization of host tree roots, by DNA isolation of the fungal species associated with the roots, and through spore density, volume, and identity in both the wet and dry seasons. Consistent with prior tropical restoration research, the majority of fungal species belonged to the genus Glomus and genus Acaulospora, accounting for more than half of the species and relative abundance found on trees roots and over 95% of spore density across all sites. Greater AMF diversity correlated with lower soil organic matter, carbon, and nitrogen concentrations and longer durations of prior pasture use across sites. Contrary to previous literature findings, AMF species diversity and spore densities were inversely related to tree growth, which may have arisen from trees facultatively increasing their associations with AMF in lower soil fertility sites. Changes to AMF community composition also may have led to variation in disturbance susceptibility, host tree nutrient acquisition, and tree growth. These results highlight the potential importance of fungal-tree-soil interactions in forest recovery and suggest that fungal community dynamics could have important implications for tree growth in disturbed soils.