Recovery and resilience of tropical forests after disturbance

The time taken for forested tropical ecosystems to re-establish post-disturbance is of widespread interest. Yet to date there has been no comparative study across tropical biomes to determine rates of forest re-growth, and how they vary through space and time. Here we present results from a meta-analysis of palaeoecological records that use fossil pollen as a proxy for vegetation change over the past 20,000 years. A total of 283 forest disturbance and recovery events, reported in 71 studies, are identified across four tropical regions. Results indicate that forests in Central America and Africa generally recover faster from past disturbances than those in South America and Asia, as do forests exposed to natural large infrequent disturbances compared with post-climatic and human impacts. Results also demonstrate that increasing frequency of disturbance events at a site through time elevates recovery rates, indicating a degree of resilience in forests exposed to recurrent past disturbance

Mixed-forest species establishment in a monodominant forest in Central Africa: Implications for tropical forest invasibility

Background: Traits of non-dominant mixed-forest tree species and their synergies for successful co-occurrence in monodominant Gilbertiodendron dewevrei forest have not yet been investigated. Here we compared the tree species diversity of the monodominant forest with its adjacent mixed forest and then determined which fitness proxies and life history traits of the mixed-forest tree species were most associated with successful co-existence in the monodominant forest. Methodology/Principal Findings: We sampled all trees (diameter in breast height [dbh]≥10 cm) within 6x1 ha topographically homogenous areas of intact central African forest in SE Cameroon, three independent patches of G. dewevrei-dominated forest and three adjacent areas (450-800 m apart). Monodominant G. dewevrei forest had lower sample-controlled species richness, species density and population density than its adjacent mixed forest in terms of stems with dbh≥10 cm. Analysis of a suite of population-level characteristics, such as relative abundance and geographical distribution, and traits such as wood density, height, diameter at breast height, fruit/seed dispersal mechanism and light requirement-revealed after controlling for phylogeny, species that co-occur with G. dewevrei tend to have higher abundance in adjacent mixed forest, higher wood density and a lower light requirement. Conclusions/Significance: Our results suggest that certain traits (wood density and light requirement) and population-level characteristics (relative abundance) may increase the invasibility of a tree species into a tropical closed-canopy system. Such knowledge may assist in the pre-emptive identification of invasive tree species. © 2014 Peh et al

Historical Human Footprint on Modern Tree Species Composition in the Purus-Madeira Interfluve, Central Amazonia

Background: Native Amazonian populations managed forest resources in numerous ways, often creating oligarchic forests dominated by useful trees. The scale and spatial distribution of forest modification beyond pre-Columbian settlements is still unknown, although recent studies propose that human impact away from rivers was minimal. We tested the hypothesis that past human management of the useful tree community decreases with distance from rivers. Methodology/Principal Findings: In six sites, we inventoried trees and palms with DBH≥10 cm and collected soil for charcoal analysis; we also mapped archaeological evidence around the sites. To quantify forest manipulation, we measured the relative abundance, richness and basal area of useful trees and palms. We found a strong negative exponential relationship between forest manipulation and distance to large rivers. Plots located from 10 to 20 km from a main river had 20-40% useful arboreal species, plots between 20 and 40 km had 12-23%, plots more than 40 km had less than 15%. Soil charcoal abundance was high in the two sites closest to secondary rivers, suggesting past agricultural practices. The shortest distance between archaeological evidence and plots was found in sites near rivers. Conclusions/Significance: These results strongly suggest that past forest manipulation was not limited to the pre-Columbian settlements along major rivers, but extended over interfluvial areas considered to be primary forest today. The sustainable use of Amazonian forests will be most effective if it considers the degree of past landscape domestication, as human-modified landscapes concentrate useful plants for human sustainable use and management today. © 2012 Levis et al

Long-Term Vegetation Change in Central Africa: The Need for an Integrated Management Framework for Forests and Savannas

peer reviewedTropical forests and savannas are the main biomes in sub-Saharan Africa, covering most of the continent. Collectively they offer important habitat for biodiversity and provide multiple ecosystem services. Considering their global importance and the multiple sustainability challenges they face in the era of the Anthropocene, this chapter undertakes a comprehensive analysis of the past, present, and future vegetation patterns in central African forests and savannas. Past changes in climate, vegetation, land use, and human activity have affected the distribution of forests and savannas across central Africa. Currently, forests form a continuous block across the wet and moist areas of central Africa, and are characterized by high tree cover (>90% tree cover). Savannas and woodlands have lower tree cover (<40% tree cover), are found in drier sites in the north and south of the region, and are maintained by frequent fires. Recent tree cover loss (2000–2015) has been more important for forests than for savannas, which, however, reportedly experienced woody encroachment. Future cropland expansion is expected to have a strong impact on savannas, while the extent of climatic impacts depends on the actual scenario. We finally identify some of the policy implications for restoring ecosystems, expanding protected areas, and designing sustainable ecosystem management approaches in the region

Reconciling methodologically different biodiversity assessments

Ongoing large-scale habitat disturbance requires quick identification of conservation priorities such as targeting sites rich in species and/or endemics. Biodiversity assessments are time consuming and expensive, so surveys often rely on partial sampling. Optimal use should be made of all currently available sources of information, but methodological differences between surveys hamper direct comparison. Because diversity depends on spatial scale, diversity characteristics of different sites are best compared on the basis of species-area relationships. As a result of the incompleteness of sampling, the observed species-area relationship deviates from the "true" species-area relationship. In this paper, we identify five key factors affecting the shape of the species-area relationship due to incomplete sampling: (1) the total spatial extent of the observations, (2) the spatial distribution of the observations, (3) the proportion of the total extent sampled, (4) the proportion of the individuals in the sampled area included in the survey, and (5) the proportion of the included individuals successfully identified. We outline how methodologically different surveys can be combined to optimize the use of existing data in the evaluation of conservation needs, particularly for tropical forests. As an illustration, we analyzed four methodologically different botanical surveys in the same area of old growth lowland forest in South Cameroon with the aim of reconciling these surveys. The four surveys were (1) reconnaissance scale vegetation mapping, (2) detailed botanical assessment (all individuals), (3) incomplete botanical assessment (10% individuals), and (4) herbarium collections. By correcting for the five key factors we were able to match the results of the four different biodiversity surveys. The five key factors affected the recorded number of species and endemics differently; partial sampling of extent (3) and individuals (4) and partial identification of individuals (5) were the three most important factors. We conclude that reconciliation of biodiversity assessments is possible if the differences between methods can be accounted for. We advocate reliable documentation of survey methods, especially the five key factors, because it greatly enhances the potential of combining methodologically different surveys for comparative biodiversity analyses

Reconciling Methodologically Different Biodiversity Assessments

Ongoing large-scale habitat disturbance requires quick identification of conservation priorities such as targeting sites rich in species and/or endemics. Biodiversity assessments are time consuming and expensive, so surveys often rely on partial sampling. Optimal use should be made of all currently available sources of information, but methodological differences between surveys hamper direct comparison. Because diversity depends on spatial scale, diversity characteristics of different sites are best compared on the basis of species–area relationships. As a result of the incompleteness of sampling, the observed species–area relationship deviates from the ‘‘true’’ species–area relationship. In this paper, we identify five key factors affecting the shape of the species–area relationship due to incomplete sampling: (1) the total spatial extent of the observations, (2) the spatial distribution of the observations, (3) the proportion of the total extent sampled, (4) the proportion of the individuals in the sampled area included in the survey, and (5) the proportion of the included individuals successfully identified. We outline how methodologically different surveys can be combined to optimize the use of existing data in the evaluation of conservation needs, particularly for tropical forests. As an illustration, we analyzed four methodologically different botanical surveys in the same area of old growth lowland forest in South Cameroon with the aim of reconciling these surveys. The four surveys were (1) reconnaissance scale vegetation mapping, (2) detailed botanical assessment (all individuals), (3) incomplete botanical assessment (10% individuals), and (4) herbarium collections. By correcting for the five key factors we were able to match the results of the four different biodiversity surveys. The five key factors affected the recorded number of species and endemics differently; partial sampling of extent (3) and individuals (4) and partial identification of individuals (5) were the three most important factors. We conclude that reconciliation of biodiversity assessments is possible if the differences between methods can be accounted for. We advocate reliable documentation of survey methods, especially the five key factors, because it greatly enhances the potential of combining methodologically different surveys for comparative biodiversity analyses.

Plants and People in the African Past: Themes and Objectives of Archaeobotany

Editorial del volum

Soil charcoal to assess the impacts of past human disturbances on tropical forests

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and-burn agricultural activities that created large openings, while a decline of these activities since the colonial period could explain their deficit of regeneration. To verify this hypothesis, we compared soil charcoal abundance, used as a proxy for past slash-and-burn agriculture, and tree species composition assessed on 208 rainforest 0.2 ha plots located in three areas from Southern Cameroon. Species were classified in regeneration guilds (pioneer, non-pioneer light-demanding, shade-bearer) and characterized by their woodspecific gravity, assumed to reflect light requirement. We tested the correlation between soil charcoal abundance and: (i) the relative abundance of each guild, (ii) each species and family abundance and (iii) mean wood-specific gravity. Charcoal was found in 83% of the plots, indicating frequent past forest fires. Radiocarbon dating revealed two periods of fires: ‘‘recent’’ charcoal were on average 300 years old (up to 860 BP, n = 16) and occurred in the uppermost 20 cm soil layer, while ‘‘ancient’’ charcoal were on average 1900 years old (range: 1500 to 2800 BP, n = 43, excluding one sample dated 9400 BP), and found in all soil layers. While we expected a positive correlation between the relative abundance of light demanding species and charcoal abundance in the upper soil layer, overall there was no evidence that the current heterogeneity in tree species composition can be explained by charcoal abundance in any soil layer. The absence of signal supporting our hypothesis might result from (i) a relatively uniform impact of past slash-and-burn activities, (ii) pedoturbation processes bringing ancient charcoal to the upper soil layer, blurring the signal of centuries-old Human disturbances, or (iii) the prevalence of other environmental factors on species composition