We synthesize findings to date from the world’s largest and longest-running experimental study of habitat fragmentation, located in central Amazonia. Over the past 32
years, Amazonian forest ...fragments ranging from 1 to 100
ha have experienced a wide array of ecological changes. Edge effects have been a dominant driver of fragment dynamics, strongly affecting forest microclimate, tree mortality, carbon storage, fauna, and other aspects of fragment ecology. However, edge-effect intensity varies markedly in space and time, and is influenced by factors such as edge age, the number of nearby edges, and the adjoining matrix of modified vegetation surrounding fragments. In our study area, the matrix has changed markedly over the course of the study (evolving from large cattle pastures to mosaics of abandoned pasture and regrowth forest) and this in turn has strongly influenced fragment dynamics and faunal persistence. Rare weather events, especially windstorms and droughts, have further altered fragment ecology. In general, populations and communities of species in fragments are hyperdynamic relative to nearby intact forest. Some edge and fragment-isolation effects have declined with a partial recovery of secondary forests around fragments, but other changes, such as altered patterns of tree recruitment, are ongoing. Fragments are highly sensitive to external vicissitudes, and even small changes in local land-management practices may drive fragmented ecosystems in markedly different directions. The effects of fragmentation are likely to interact synergistically with other anthropogenic threats such as logging, hunting, and especially fire, creating an even greater peril for the Amazonian biota.
ABSTRACT
We synthesize findings from one of the world's largest and longest‐running experimental investigations, the Biological Dynamics of Forest Fragments Project (BDFFP). Spanning an area of ∼1000 ...km2 in central Amazonia, the BDFFP was initially designed to evaluate the effects of fragment area on rainforest biodiversity and ecological processes. However, over its 38‐year history to date the project has far transcended its original mission, and now focuses more broadly on landscape dynamics, forest regeneration, regional‐ and global‐change phenomena, and their potential interactions and implications for Amazonian forest conservation. The project has yielded a wealth of insights into the ecological and environmental changes in fragmented forests. For instance, many rainforest species are naturally rare and hence are either missing entirely from many fragments or so sparsely represented as to have little chance of long‐term survival. Additionally, edge effects are a prominent driver of fragment dynamics, strongly affecting forest microclimate, tree mortality, carbon storage and a diversity of fauna.
Even within our controlled study area, the landscape has been highly dynamic: for example, the matrix of vegetation surrounding fragments has changed markedly over time, succeeding from large cattle pastures or forest clearcuts to secondary regrowth forest. This, in turn, has influenced the dynamics of plant and animal communities and their trajectories of change over time. In general, fauna and flora have responded differently to fragmentation: the most locally extinction‐prone animal species are those that have both large area requirements and low tolerance of the modified habitats surrounding fragments, whereas the most vulnerable plants are those that respond poorly to edge effects or chronic forest disturbances, and that rely on vulnerable animals for seed dispersal or pollination.
Relative to intact forests, most fragments are hyperdynamic, with unstable or fluctuating populations of species in response to a variety of external vicissitudes. Rare weather events such as droughts, windstorms and floods have had strong impacts on fragments and left lasting legacies of change. Both forest fragments and the intact forests in our study area appear to be influenced by larger‐scale environmental drivers operating at regional or global scales. These drivers are apparently increasing forest productivity and have led to concerted, widespread increases in forest dynamics and plant growth, shifts in tree‐community composition, and increases in liana (woody vine) abundance. Such large‐scale drivers are likely to interact synergistically with habitat fragmentation, exacerbating its effects for some species and ecological phenomena. Hence, the impacts of fragmentation on Amazonian biodiversity and ecosystem processes appear to be a consequence not only of local site features but also of broader changes occurring at landscape, regional and even global scales.
The specific gravity (SG) of wood is a measure of the amount of structural material a tree species allocates to support and strength. In recent years, wood specific gravity, traditionally a ...forester's variable, has become the domain of ecologists exploring the universality of plant functional traits and conservationists estimating global carbon stocks. While these developments have expanded our knowledge and sample of woods, the methodologies employed to measure wood SG have not received as much scrutiny as SG's ecological importance. Here, we reiterate some of the basic principles and methods for measuring the SG of wood to clarify past practices of foresters and ecologists and to identify some of the prominent errors in recent studies and their consequences. In particular, we identify errors in (1) extracting wood samples that are not representative of tree wood, (2) differentiating wood specific gravity from wood density, (3) drying wood samples at temperatures below 100°C and the resulting moisture content complications, and (4) improperly measuring wood volumes. In addition, we introduce a new experimental technique, using applied calculus, for estimating SG when the form of radial variation is known, a method that significantly reduces the effort required to sample a tree's wood.
Significance Although forest succession has been approached as a predictable process, successional trajectories vary widely, even among nearby stands with similar environmental conditions and ...disturbance histories. We quantified predictability and uncertainty during tropical forest succession using dynamical models describing the interactions among stem density, basal area, and species density over time. We showed that the trajectories of these forest attributes were poorly predicted by stand age and varied significantly within and among sites. Our models reproduced the general successional trends observed, but high levels of noise were needed to increase model predictability. These levels of uncertainty call into question the premise that successional processes are consistent over space and time, and challenge the way ecologists view tropical forest regeneration.
Although forest succession has traditionally been approached as a deterministic process, successional trajectories of vegetation change vary widely, even among nearby stands with similar environmental conditions and disturbance histories. Here, we provide the first attempt, to our knowledge, to quantify predictability and uncertainty during succession based on the most extensive long-term datasets ever assembled for Neotropical forests. We develop a novel approach that integrates deterministic and stochastic components into different candidate models describing the dynamical interactions among three widely used and interrelated forest attributesâstem density, basal area, and species density. Within each of the seven study sites, successional trajectories were highly idiosyncratic, even when controlling for prior land use, environment, and initial conditions in these attributes. Plot factors were far more important than stand age in explaining successional trajectories. For each site, the best-fit model was able to capture the complete set of time series in certain attributes only when both the deterministic and stochastic components were set to similar magnitudes. Surprisingly, predictability of stem density, basal area, and species density did not show consistent trends across attributes, study sites, or land use history, and was independent of plot size and time series length. The model developed here represents the best approach, to date, for characterizing autogenic successional dynamics and demonstrates the low predictability of successional trajectories. These high levels of uncertainty suggest that the impacts of allogenic factors on rates of change during tropical forest succession are far more pervasive than previously thought, challenging the way ecologists view and investigate forest regeneration.
The responses of plant–animal interactions to forest fragmentation can vary. We hypothesized that large-seeded plant species would be more susceptible to forest fragmentation than small-seeded ...species because large-seeded species rely on a few, extinction prone dispersers. We compared seed dispersal of the large-seeded, mammal dispersed
Duckeodendron cestroides and the small-seeded, avian dispersed
Bocageopsis multiflora. The number, percentage, distance, and distributions of dispersed seeds were all reduced in fragments for
Duckeodendron but not for
Bocageopsis. Other fragmentation research in tropical communities supports this hypothesis through three lines of evidence: (1) Large-seeded plant species are more prone to extinction, (2) Fragmentation restricts or alters the movement of large animal dispersers more than small dispersers, and (3) Large and small-seeded species seem to be differentially linked to primary and secondary forest habitats. Therefore, small-seeded plants may be more resilient to forest fragmentation while large-seeded species may be more susceptible and should be a priority for conservation.
Amazon Rain Forest Succession MESQUITA, RITA DE CÁSSIA GUIMARÃES; MASSOCA, PAULO EDUARDO DOS SANTOS; JAKOVAC, CATARINA CONTE ...
BioScience/Bioscience,
09/2015, Volume:
65, Issue:
9
Journal Article
Peer reviewed
Open access
Land-use practices can dramatically shift the trajectories of rain forest recovery. In a 25-year study, Amazon rain forest regenerated following deforestation as long as seed availability and ...seedling recruitment were not interrupted. In contrast, rain forest converted to cattle pastures via cutting and burning prior to abandonment diverted succession, leading to highly simplified stands dominated by a single genus. Annual fires eliminated seedlings, saplings, coppice, and seeds in the soil, except for several Vismia species. Once established, Vismia regenerated by continual resprouting and resisted the establishment of other rain forest species, especially the normal suite of pioneers. Through time, succession both in abandoned clearcuts and pastures increased in stem density and biomass; however, species accumulation and ecosystem services were limited in pastures when compared with those in abandoned clearcuts. Therefore, prescribed burning to maintain pastureland leaves a legacy that is not readily extinguished, whereas abandoning clearcuts engenders an accelerated rain forest regeneration.
Old-growth tropical forests harbor an immense diversity of tree species but are rapidly being cleared, while secondary forests that regrow on abandoned agricultural lands increase in extent. We ...assess how tree species richness and composition recover during secondary succession across gradients in environmental conditions and anthropogenic disturbance in an unprecedented multisite analysis for the Neotropics. Secondary forests recover remarkably fast in species richness but slowly in species composition. Secondary forests take a median time of five decades to recover the species richness of old-growth forest (80% recovery after 20 years) based on rarefaction analysis. Full recovery of species composition takes centuries (only 34% recovery after 20 years). A dual strategy that maintains both old-growth forests and species-rich secondary forests is therefore crucial for biodiversity conservation in human-modified tropical landscapes.
The Amazon basin is experiencing rapid forest loss and fragmentation. Fragmented forests are more prone than intact forests to periodic damage from El Niño-Southern Oscillation (ENSO) droughts, which ...cause elevated tree mortality, increased litterfall, shifts in plant phenology, and other ecological changes, especially near forest edges. Moreover, positive feedbacks among forest loss, fragmentation, fire, and regional climate change appear increasingly likely. Deforestation reduces plant evapotranspiration, which in turn constrains regional rainfall, increasing the vulnerability of forests to fire. Forest fragments are especially vulnerable because they have dry, fire-prone edges, are logged frequently, and often are adjoined by cattle pastures, which are burned regularly. The net result is that there may be a critical "deforestation threshold" above which Amazonian rainforests can no longer be sustained, particularly in relatively seasonal areas of the basin. Global warming could exacerbate this problem if it promotes drier climates or stronger ENSO droughts. Synergisms among many simultaneous environmental changes are posing unprecedented threats to Amazonian forests.
Measuring wood specific gravity…Correctly Williamson, G. Bruce; Wiemann, Michael C.
American journal of botany,
March 2010, 2010-03-00, Volume:
97, Issue:
3
Journal Article
Peer reviewed
The specific gravity (SG) of wood is a measure of the amount of structural material a tree species allocates to support and strength. In recent years, wood specific gravity, traditionally a ...forester's variable, has become the domain of ecologists exploring the universality of plant functional traits and conservationists estimating global carbon stocks. While these developments have expanded our knowledge and sample of woods, the methodologies employed to measure wood SG have not received as much scrutiny as SG's ecological importance. Here, we reiterate some of the basic principles and methods for measuring the SG of wood to clarify past practices of foresters and ecologists and to identify some of the prominent errors in recent studies and their consequences. In particular, we identify errors in (1) extracting wood samples that are not representative of tree wood, (2) differentiating wood specific gravity from wood density, (3) drying wood samples at temperatures below 100°C and the resulting moisture content complications, and (4) improperly measuring wood volumes. In addition, we introduce a new experimental technique, using applied calculus, for estimating SG when the form of radial variation is known, a method that significantly reduces the effort required to sample a tree's wood.
The recovery capacity and the successional pathways of tropical forests after anthropogenic disturbance vary considerably and may depend on prior land‐use type and intensity. It is still unclear if ...forests subjected to high intensity impact, such as periodically burned pastures, are capable of restoring their original functional properties.
This study analysed the functional trait dynamics of the dominant species in successional trajectories following two land uses, pasture or clear‐cut, north of Manaus. Fourteen years of demographic data from the Biological Dynamics of Forest Fragments Project were used to determine the dominant species of the two successional trajectories, for which leaf area, leaf dry mass content, specific leaf area and wood density were collected, whereas seed mass was obtained from literature. Community weighted mean of each trait was weighted by basal area determined annually along succession. Prinicpal components analysis was used to analyse the extension and direction of the functional trajectories of plots.
Forests regenerating from pastures increased in wood density through successional time, but other traits did not change significantly. Succession after clear‐cut exhibited increasing leaf dry mass content and seed mass, and decreasing leaf area over time, but no change in wood density. Functional trajectories of plots after clear‐cut were more extensive and directional than those of pasture‐derived plots.
Synthesis and applications. We demonstrate how central Amazonian secondary forests subjected to different land uses show differences in functional trait trajectories, in ways parallel to previously shown changes in biomass, floristic diversity and forest structure. These results indicate that natural recovery of forest functional traits is affected by prior land‐use history, with implications for management and restoration. Thus, natural recovery of forests on abandoned pastures is much slower than clear‐cuts, even though seed sources from mature forests are very close to these areas, and the former may need intervention to counteract the diverted succession.
Resumo
A capacidade de recuperação e os caminhos sucessionais das florestas tropicais após perturbações antrópicas variam consideravelmente e podem depender do tipo e da intensidade prévia do uso da terra. Ainda não está claro se as florestas sujeitas a impactos de alta intensidade, como pastagens periodicamente queimadas, são capazes de restaurar suas propriedades funcionais originais.
Este estudo analisou a dinâmica de atributos funcionais das espécies dominantes em trajetórias sucessionais após dois usos da terra, pastagem ou corte raso, ao norte de Manaus. Quatorze anos de dados demográficos do Projeto de Dinâmica Biológica de Fragmentos Florestais foram utilizados para determinar as espécies dominantes das duas trajetórias sucessionais, para as quais foram coletadas área foliar, conteúdo de massa seca foliar, área foliar específica e densidade de madeira enquanto que a massa das sementes foi obtida da literatura. A média ponderada da comunidade (CWM) de cada atributo foi ponderada pela área basal determinada anualmente ao longo da sucessão. Uma PCA foi utilizada para analisar a extensão e direção das trajetórias funcionais das parcelas.
Florestas regeneradas de pastagens aumentaram a densidade da madeira através do tempo de sucessão, mas outras características não mudaram significativamente. A sucessão após corte apresentou maior conteúdo de massa seca foliar e massa de sementes, além de diminuir a área foliar ao longo do tempo, mas sem alteração na densidade da madeira. Trajetórias funcionais de parcelas após corte raso foram mais extensas e direcionais do que as de parcelas derivadas de pastagens.
Síntese e aplicações. Este trabalho demonstra como as florestas secundárias da Amazônia central submetidas a diferentes usos da terra apresentam diferenças nas trajetórias de seus atributos funcionais, de forma paralela às mudanças anteriormente mostradas na biomassa, na diversidade florística e na estrutura florestal. Estes resultados indicam que a recuperação natural de atributos funcionais da floresta é afetada pela história anterior de uso da terra, com implicações para o manejo e restauração. Assim, a recuperação natural de florestas em pastos abandonados é muito mais lenta do que cortes rasos, embora as fontes de sementes de florestas maduras sejam muito próximas dessas áreas, e as primeiras precisarão de intervenção para modificar a sucessão florestal desviada.
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