CONTENTS: I. II. III. IV. V. VI. VII. References SUMMARY: The Brazilian Atlantic Forest hosts one of the world's most diverse and threatened tropical forest biota. In many ways, its history of ...degradation describes the fate experienced by tropical forests around the world. After five centuries of human expansion, most Atlantic Forest landscapes are archipelagos of small forest fragments surrounded by open‐habitat matrices. This ‘natural laboratory’ has contributed to a better understanding of the evolutionary history and ecology of tropical forests and to determining the extent to which this irreplaceable biota is susceptible to major human disturbances. We share some of the major findings with respect to the responses of tropical forests to human disturbances across multiple biological levels and spatial scales and discuss some of the conservation initiatives adopted in the past decade. First, we provide a short description of the Atlantic Forest biota and its historical degradation. Secondly, we offer conceptual models describing major shifts experienced by tree assemblages at local scales and discuss landscape ecological processes that can help to maintain this biota at larger scales. We also examine potential plant responses to climate change. Finally, we propose a research agenda to improve the conservation value of human‐modified landscapes and safeguard the biological heritage of tropical forests.
► Biotic homogenization (BH) results from biological invasion and loss of biodiversity. ► BH may also emerge from proliferation of native disturbance-adapted species. ► Human-caused disturbances ...promote BH driven only by native species. ► More attention should be given to BH driven by native disturbance-adapted species.
Despite its relevance to contemporary biodiversity conservation, the ecological mechanisms controlling nonrandom species replacements and biotic homogenization of native biotas remain poorly understood. Here, we advocate for the widespread occurrence of a pattern of winner–loser species turnover exhibited in tropical forest floras: the replacement of old-growth tree species by a small set of pioneer or successional tree species across edge-dominated habitats in hyper-fragmented landscapes. A growing body of evidence on biotic reassembly in human-degraded forest sites suggests that winner–loser replacements are often advanced by native rather than exotic plant species, leading to taxonomic homogenization at multiple spatial scales (i.e. from local to regional assemblages). This process does not rely on biological invasions and associated shifts in geographic ranges of nonindigenous species. Moreover, this form of biodiversity erosion is not limited to restricted-range and endemic species but can include any species of many ecological groups that are apparently intolerant to highly modified habitat conditions, such as desiccated/illuminated forest edges and fire-degraded forest fragments persisting in open-habitat matrices. This unidirectional pattern of species turnover is expected to set in motion devastating cascading effects onto higher trophic levels that will eventually disrupt the structure of the entire forest ecosystem. Our framework not only reaffirms the winner–loser replacement paradigm, but also contends that the proliferation of disturbance-adapted native organisms has a pivotal role in sealing the fate of tropical biodiversity in particular contexts.
ABSTRACT
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.
Summary
Competitor, stress‐tolerator, ruderal (CSR) theory is a prominent plant functional strategy scheme previously applied to local floras. Globally, the wide geographic and phylogenetic coverage ...of available values of leaf area (LA), leaf dry matter content (LDMC) and specific leaf area (SLA) (representing, respectively, interspecific variation in plant size and conservative vs. acquisitive resource economics) promises the general application of CSR strategies across biomes, including the tropical forests hosting a large proportion of Earth's diversity.
We used trait variation for 3068 tracheophytes (representing 198 families, six continents and 14 biomes) to create a globally calibrated CSR strategy calculator tool and investigate strategy–environment relationships across biomes world‐wide.
Due to disparity in trait availability globally, co‐inertia analysis was used to check correspondence between a ‘wide geographic coverage, few traits’ data set and a ‘restricted coverage, many traits’ subset of 371 species for which 14 whole‐plant, flowering, seed and leaf traits (including leaf nitrogen content) were available. CSR strategy/environment relationships within biomes were investigated using fourth‐corner and RLQ analyses to determine strategy/climate specializations.
Strong, significant concordance (RV = 0·597; P < 0·0001) was evident between the 14 trait multivariate space and when only LA, LDMC and SLA were used.
Biomes such as tropical moist broadleaf forests exhibited strategy convergence (i.e. clustered around a CS/CSR median; C:S:R = 43:42:15%), with CS‐selection associated with warm, stable situations (lesser temperature seasonality), with greater annual precipitation and potential evapotranspiration. Other biomes were characterized by strategy divergence: for example, deserts varied between xeromorphic perennials such as Larrea divaricata, classified as S‐selected (C:S:R = 1:99:0%) and broadly R‐selected annual herbs (e.g. Claytonia perfoliata; R/CR‐selected; C:S:R = 21:0:79%). Strategy convergence was evident for several growth habits (e.g. trees) but not others (forbs).
The CSR strategies of vascular plants can now be compared quantitatively within and between biomes at the global scale. Through known linkages between underlying leaf traits and growth rates, herbivory and decomposition rates, this method and the strategy–environment relationships it elucidates will help to predict which kinds of species may assemble in response to changes in biogeochemical cycles, climate and land use.
Lay Summary
Global climate change alters the dynamic of natural ecosystems and directly affects species distributions, persistence and diversity. The impacts of climate change may lead to dramatic changes in ...biotic interactions, such as pollination and seed dispersal. Life history traits are extremely important to consider the vulnerability of a species to climate change, producing more robust models than those based primarily on species distributions. Here, we hypothesized that rising temperatures and aridity will reduce suitable habitats for the endemic flora of the Caatinga, the most diverse dry tropical forest on Earth. Specifically, species with specialized reproductive traits (e.g. vertebrate pollination, biotic dispersal, obligatory cross-pollination) should be more affected by climate change than those with generalist traits. We performed two ecological niche models (current and future) to simulate the effects of climate change on the distribution area of endemic species in relation to life-history traits. We used the MIROC-ESM and CCSM4 models for both intermediate (RCP4.5) and highest predicted (RCP8.5) GHG emission scenarios, with a resolution of 30' (~1 km2). Habitat with high occurrence probability (>80%) of endemic species will be reduced (up to ~10% for trees, ~13% for non-arboreous, 10-28% for species with any pollination/reproductive system), with the greatest reductions for species with specialized reproductive traits. In addition, the likely concentration of endemic plants in the extreme northeastern portion of the Caatinga, in more mesic areas, coincides with the currently most human-modified areas of the ecosystem, which combined with climate change will further contract suitable habitats of endemic species. In conclusion, plant species endemic to the Caatinga are highly vulnerable to even conservative scenarios of future climate change and may lose much of their climatic envelopes. New protected areas should be located in the northeastern portion of the Caatinga, which hosts a more favorable climate, but is currently exposed to escalating agricultural intensification.
Habitat fragmentation and forest management have been considered to drastically alter the nature of forest ecosystems globally. However, much uncertainty remains regarding the causative mechanisms ...mediating temperate forest responses, such as forest physical environment and the structure of woody plant assemblages, regardless of the role these forests play for global sustainability. In this paper, we examine how both habitat fragmentation and timber exploitation via silvicultural operations affect these two factors at local and habitat spatial scales in a hyper-fragmented landscape of mixed beech forests spanning more than 1500 km
2
in SW Germany. Variables were recorded across 57 1000 m
2
plots covering four habitats: small forest fragments, forest edges within large control forests, as well as managed and unmanaged forest interior sites. As expected, forest habitats differed in disturbance level, physical conditions and community structure at plot and habitat scale. Briefly, diversity of plant assemblages differed across all forest habitats (highest in edge forests) and correlated with integrative indices of edge, fragmentation and management effects. Surprisingly, managed and unmanaged forests did not differ in terms of species richness at local spatial scale, but managed forests exhibited a clear signal of physical/floristic homogenization as species promoted by silviculture proliferated; i.e. impoverished communities at landscape scale. Moreover, functional composition of plant communities responded to the microclimatic regime within forest fragments, resulting in a higher prevalence of species adapted to these microclimatic conditions. Our results underscore the notion that forest fragmentation and silvicultural management (1) promote changes in microclimatic regimes, (2) alter the balance between light-demanding and shade-adapted species, (3) support diverse floras across forest edges, and (4) alter patterns of beta diversity. Hence, in human-modified landscapes edge-affected habitats can be recognized as biodiversity reservoirs in contrast to impoverished managed interior forests. Furthermore, our results ratify the role of unmanaged forests as a source of environmental variability, species turnover, and distinct woody plant communities.
1. Secondary forests are increasingly dominant in human-modified tropical landscapes, but the drivers of forest recovery remain poorly understood. Soil conditions influence plant community ...composition, and are expected to change over a gradient of succession. However, the role of soil conditions as an environmental filter driving community assembly during forest succession has rarely been explicitly assessed. 2. We evaluated the role of stand basal area and soil conditions on community assembly and its consequences for community functional properties along a chronosequence of Atlantic forest regeneration following sugar cane cultivation. Specifically, we tested whether community functional properties are related to stand basal area, soil fertility and soil moisture. Our expectations were that edaphic environmental filters play an increasingly important role along secondary succession by increasing functional trait convergence towards more conservative attributes. 3. We sampled soil and woody vegetation features across 15 second-growth (3-30 years) and 11 old-growth forest plots (300 m² each). We recorded tree functional traits related to resource-use strategies (specific leaf area, SLA; leaf dry matter content, LDMC; leaf area, LA; leaf thickness, LT; and leaf succulence, LS) and calculated community functional properties using the community-weighted mean (CWM) of each trait and the functional dispersion (FDis) of each trait separately and all traits together. 4. With exception of LA, all leaf traits were strongly associated with stand basal area; LDMC and SLA increased, while LT and LS decreased with forest development. Such changes in LDMC, LT and LS were also related to the decrease in soil nutrient availability and pH along succession, while soil moisture was weakly related to community functional properties. Considering all traits, as well as leaf thickness and succulence separately, FDis strongly decreased with increasing basal area and decreasing soil fertility along forest succession, presenting the lowest values in oldgrowth forests. 5. Synthesis. Our findings suggest that tropical forest regeneration may be a deterministic process shaped by soil conditions. Soil fertility operates as a key filter causing functional convergence towards more conservative resource-use strategies, such as leaves with higher leaf dry matter content.
Aim To examine whether the tree flora of the Atlantic forest of northeastern Brazil has experienced detectable taxonomic homogenization via the proliferation of native pioneer species in response to ...habitat loss and fragmentation. Location Biotic homogenization (BH) was examined across the Atlantic forest of northeast Brazil, i.e. a 56,000 km² piece of tropical forest and a distinct centre of species endemism in South America. Methods We assessed a dataset consisting of 5122 tree records and compared the similarity of tree floras from 12 semi-natural sub-regions of the Atlantic forest between two time periods: pre-1980 (plant records between 1902 and 1980), and post-1980 (between 1981 and 2006). To understand the mechanisms leading to BH (1) tree floras were ordered (via non-metric multidimensional scaling - NMDS) by date (pre/post 1980) based on species occurrence and frequency, (2) NMDS axes were regressed against the proportion of those species that increased their occurrence post-1980 (i.e. the winner species), and (3) patterns of geographic distribution and frequency of particular life-history traits were examined across winner species and a control group. Results Tree floras across the Atlantic forest became c. 20-40% more similar to each other post-1980, but patterns of species similarity were also influenced by between-plot geographical distance. NMDS ordination clearly segregated pre- and post-1980 floras with a clear signal of floristic convergence. Furthermore, winner tree species were largely composed of short-lived and small-seeded pioneer species that exhibit wide geographic distributions. Main conclusions Our results suggest that tropical forest biotas are susceptible to taxonomic homogenization (i.e. increasing levels of similarity) in the context of severe human-disturbance via the proliferation of particular groups of native species comprised mainly by ecologically-plastic, generalist species. We are thus extending the concept of homogenization to address and highlight a pervasive biological shift in the structure of tropical forest communities currently taking place across hyper-fragmented landscapes.
Drastic changes in vegetation structure caused by exceeding ecological thresholds have fueled the interest in tropical forest responses to climate and land-use changes. Here, we examine the potential ...successional trajectories experienced by the largest dry tropical forest region in South America, driven by climate conditions and human disturbance. We built potential distribution models for vertebrate taxa associated with forest or shrub habitats to estimate natural vegetation cover. Distribution patterns were compared to current vegetation across the entire region to identify distinct forest degradation levels. Our results indicate the region has climatic and soil conditions suitable for more forest cover than is currently found, even in some areas with limited precipitation. However, 11.04% of natural cover persists across such an immense region, with only 4.34% consisting of forest cover. Forest degradation is characterized by the dramatic expansion of shrubland (390%), farming, and non-vegetation cover due to changes in land-use, rather than climatic conditions. Although different climate conditions have been the principal drivers for natural forest distribution in the region, the forest seems unable to resist the consequences of land-use changes, particularly in lower precipitation areas. Therefore, land-use change has exceeded the ecological thresholds for the persistence of forests, while climate change may exacerbate vegetation-type transitions.
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