Adult Tree Communities Research Articles

Chronic browsing by an introduced mammalian herbivore in a tropical island alters species composition and functional traits of forest understory plant communities

AbstractMammalian herbivores have large‐scale impacts on vegetation, altering structure, and species composition, especially in tropical grasslands and savannas. However, there is limited understanding of the potential impacts of mammalian herbivores in tropical wet forests, where they are typically less abundant. We investigated the effects of an introduced mammalian herbivore chital (Axis axis) on vegetation structure, composition and leaf functional traits of tropical evergreen forests of the Andaman Islands, India. Across seven islands, representing a gradient of herbivore densities, increasing chital presence was associated with decreased understory richness, understory density and adult tree richness, but was not related to adult tree density or size class distributions. We also found a significant decrease in community level leaf palatability traits (specific leaf area decreased and leaf thickness increased) with increasing chital habitat use. This community level shift in leaf trait values was better explained by intraspecific variation in leaf traits across islands rather than changes in species composition. In summary, we show persistent long‐term impacts of an introduced mammalian herbivore on understory tropical tree communities although there is little impact on adult tree communities. Our results also show that functional traits of species can be altered in response to novel herbivory, even at herbivore densities where there are no detectable impacts on adult forest structure or composition. Such altered functional traits may potentially alter ecosystem functioning in these forests even without changes in vegetation structure.Abstract in Hindi is available with online material.

Evidence for Alternate Stable States in an Ecuadorian Andean Cloud Forest

Tree diversity inventories were undertaken. The goal of this study was to understand changes in tree community dynamics that may result from common anthropogenic disturbances at the Reserva Los Cedros, a tropical montane cloud forest reserve in northern Andean Ecuador. The reserve shows extremely high alpha and beta tree diversity. We found that all primary forest sites, regardless of age of natural gaps, are quite ecologically resilient, appearing to return to a primary-forest-type community of trees following gap formation. In contrast, forests regenerating from anthropogenic disturbance appear to have multiple possible ecological states. Where anthropogenic disturbance was intense, novel tree communities appear to be assembling, with no indication of return to a primary forest state. Even in ancient primary forests, new forest types may be forming, as we found that seedling community composition did not resemble adult tree communities. We also suggest small watersheds as a useful basic spatial unit for understanding biodiversity patterns in the tropical Andes that confound more traditional Euclidean distance as a basic proxy of dissimilarity. Finally, we highlight the conservation value of Reserva Los Cedros, which has managed to reverse deforestation within its boundaries despite a general trend of extensive deforestation in the surrounding region, to protect a large, contiguous area of highly endangered Andean primary cloud forest.

Forest structural analysis of a seasonal semideciduous area in the northeast Brazil

As a prominent aspect of forest fragmentation, arboreal individuals’ diameter and height structure are impaired. The forest structure allows identifying each population’s behavior and habit and collects information on growth and survival that assists in establishing and managing conservation strategies. This work aimed to perform the diametric and height distribution of an adult tree community and the five highlighted species of the area located in the Northeast Brazil. We implanted 20 permanent plots with 250 m² and measured all arboreal individuals sampled with a circumference at breast height (CBH) ≥ 15 cm in addition to heights. The tree community presented a graph of the negative exponential type in the diametric distribution and a higher concentration in the 6-8 m in height median class, a typical pattern found in remnants of Atlantic Forest. Guapira nitida, Manilkara sp., Byrsonima crassifolia, and Sloanea obtusifolia presented the same pattern as the community due to their ecological attributes as succession group and dispersion syndrome. These data are compatible with the ecological succession process of Brazilian’s northeast Seasonal Semideciduous Forest. Moreover, the species Buchenavia tetraphylla diverged because of its ecological characteristics that do not allow new individuals to develop in the study area. The fragment was classified according to CONAMA Resolution n°. 31 as secondary vegetation in the medium regeneration stage. The analyzed remnant is still in the regeneration process, resulting from the disturbances caused. The most important species of the fragment still present young individuals, representing young communities with potential for establishment.

Seed fates of four rainforest tree species in the fragmented forests of Anamalais in the southern Western Ghats, India

Seed predation is one of the key ecosystem processes governing the plant population and community structure in forests. Forest fragmentation and habitat loss have been shown to affect seed predation, leading to altered tree recruitment. However, the effects of fragmentation and habitat loss on seed predation are highly variable and context-specific, with limited information from South Asia. For four rainforest tree species in a production landscape of tea and coffee, we examined the influence of forest type (benchmark, N = 3, and fragmented, N = 5, forests) and seed size on the proportion of seeds, 1) predated by vertebrate seed predators, 2) predated by invertebrate seed predators, and 3) removed by vertebrate seed predators. The seed fates were tracked by placing the seeds in four 1 × 1 m plots under the canopy of the tree (N ≥ 11 individuals per tree species and 48 trees overall), and camera traps were placed under a subset of trees (N = 29, trap-nights = 712). Overall, we recorded nine species of seed predators, and that the seed predator species composition differed among the four tree species. However, there was no significant difference in either seed predator species composition or their visitations across the forest types. The seeds were more likely to be predated (on average 6% higher predation) by vertebrate seed predators in the fragments than in the benchmark forests. Medium-seeded species were more likely to be predated by vertebrate seed predators than large-seeded species (on average 41% higher predation). Invertebrate seed predation and seed removal by vertebrate seed predators had a weak relationship with forest type, with higher invertebrate seed predation in the fragments and higher seed removal in the benchmark forests. With altered seed dispersal patterns resulting from habitat fragmentation and habitat loss, differential seed predation can alter regeneration patterns, thereby influencing adult tree communities in fragmented forests.

Instability of insular tree communities in an Amazonian mega‐dam is driven by impaired recruitment and altered species composition

Abstract Mega‐dams create highly fragmented archipelagos, affecting biodiversity and ecosystem functioning in remnant forest isolates. This study assessed the long‐term impact of dam‐induced fragmentation on insular tropical tree communities, with the aim of generating robust recommendations to mitigate some of the detrimental biodiversity impacts associated with future dam development. We inventoried adult and sapling trees across 89 permanent plots, located on 36 islands and in three mainland continuous forest sites in the Balbina Dam, Brazilian Amazon. We examined differences in recruitment, structure, and composition of sapling and adult tree communities, in relation to plot‐, patch‐ and landscape‐scale attributes including area, isolation, and fire severity. Islands harboured significantly lower sapling (mean ± 95% CI 48.6 ± 3.8) and adult (5 ± 0.2) tree densities per 0.01 ha, than nearby mainland continuous forest (saplings, 65.7 ± 7.5; adults, 5.6 ± 0.3). Insular sapling and adult tree communities were more dissimilar than in mainland sites, and species compositions showed a directional shift away from mainland forests, induced by fire severity, island area, and isolation. Insular sapling recruitment declined with increasing fire severity; tree communities with higher community‐weighted mean wood density showed the greatest recruitment declines. Our results suggest that insular tree communities are unstable, with rare species becoming extinction‐prone due to reduced tree recruitment and density on islands, potentially leading to future losses in biodiversity and ecosystem functioning across Balbina's >3,500 reservoir islands. Policy implications. In Balbina, fire and reduced habitat area and connectivity were drivers of tree community decay after only 28 years of insularization, despite strict protection provided by the ~940,000 ha Uatumã Biological Reserve. Given that many dams are planned for lowland, moderately undulating Amazonia, we recommend that dam development strategy explicitly considers (a) dam location, aiming to minimize creation of small (<10 ha) and isolated islands, (b) maintaining reservoir water levels during droughts to reduce fire risk, and (c) including aggregate island area in environmental impact and offset calculations. Ideally, we recommend that alternatives to hydropower be sought in lowland tropical regions, due to the far‐reaching biodiversity losses and ecosystem disruption caused by river impoundment.

Defaunation impacts on seed survival and its effect on the biomass of future tropical forests

Large animal species, which provide important ecological functions such as dispersal of seeds or top–down control of seed predators, are very vulnerable in fragmented forests, being unable to survive in small fragments, and facing increasing hunting pressure. The loss of large animals affects two main ecological processes crucial for the tree reproductive cycle: seed dispersal of large seeds (e.g. provided by tapirs) and control of seed predator population (e.g. provided by large cats). The changes in both processes are expected to increase seed mortality since seeds are not dispersed away from conspecifics (causing increased pre‐dispersal mortality due to negative density dependent effects) and/or face increased predation after a dispersal event (post‐dispersal mortality). Although an extensive body of empirical knowledge exists on seed predation, the link between seed loss and adult tree community composition and structure is not well established, as well as the temporal scale seed changes affect adults. Using an individual‐based forest model (FORMIND), we evaluate the long‐term consequences of increased pre and post‐dispersal seed mortality on the future forest biomass retention of a Brazilian northeastern Atlantic forest. Our results show that forest biomass is significantly affected after 80–93% pre‐dispersal loss of large seeds, or post‐dispersal predation densities of 20–25 predators per parent tree. Large‐seeded tree species are at increased risk of local extinction causing up to 26.2% loss of forest biomass when both pre and post‐dispersal processes are combined. However, these changes can last up to 100 years after the occurrence of defaunation. In summary we conclude that large animal loss has the potential to reduce future forest biomass and tree species‐richness by impacting seed survival, and should be considered in the planning of biodiversity friendly landscapes as well as in calculations of the global carbon budget.

Functional trait shifts after disturbance reveal broad‐scale variability in temperate forest regional recruitment processes

AbstractQuestionDecreased above‐ground biomass and subsequent changes in resource availability exert a strong influence on seedling recruitment in post‐disturbance forests. Ecological trade‐offs underlie recruitment processes, as species vary in resource allocation, dispersal and stress tolerance strategies. Functional traits that indicate resource acquisition–conservation trade‐offs include leaf N content, wood density, maximum height, shade tolerance and drought tolerance, while seed mass can indicate species’ dispersal strategies. This study looks for generality in post‐disturbance trait responses, and asks whether these responses are additionally constrained or amplified by disturbance characteristics, climate and ecological provinces.LocationEastern US temperate forests.MethodsI leveraged a longitudinal, continental‐scale database of post‐disturbance forests to examine recruitment dynamics. Using multivariate, hierarchical Bayesian models, I examined how disturbance affects the traits of seedling layer communities where recruitment dynamics should be most evident, and compared this to undisturbed communities. I also examined how the traits underlying trade‐off axes varied across disturbance properties, ecological provinces and climatic gradients.ResultsAll traits except leaf N showed significant, study‐wide shifts in disturbed communities consistent with expectations of post‐disturbance stands. In undisturbed plots, all traits had significant shifts in the opposite direction to those observed in disturbed plots, except leaf N and drought tolerance, where no change was observed. Disturbance severity increased the magnitude of response of several, but not all, trait responses in disturbed plots. The traits that had significant shifts were idiosyncratic across ecological provinces, indicating unique processes influencing disturbance responses across the system. Moreover, while climate strongly correlated with all traits prior to disturbance in the initial sampling periods, it was not correlated with disturbance responses.ConclusionsThis study demonstrates general, biome‐wide trait shifts towards resource acquisition and bet‐hedging dispersal strategies in post‐disturbance forest seedling communities. At finer spatial scales, the traits underlying these trade‐offs varied, indicating that disturbance‐dependent species differ in life‐history strategies across the system. This variation across ecological provinces appears independent of temperature and precipitation, suggesting that unmeasured abiotic or biotic variables influence recruitment. Seedling recruitment is critical in shaping future adult tree communities, and this study reveals large‐scale contingencies in trait patterns associated with this process.

Functional convergence and phylogenetic divergence during secondary succession of subtropical wet forests in Puerto Rico

AbstractQuestionUnderstanding how the relative importance of different community assembly processes changes during secondary succession of diverse systems remains elusive. Functional and phylogenetic approaches that place species along continuous axes of niche differentiation and evolutionary relatedness, however, are deepening our understanding of the mechanisms that drive successional dynamics. We ask whether successional shifts in the functional and phylogenetic composition of post‐agricultural tropical forests provide evidence for niche partitioning or competitive dominance hierarchies as drivers of successional change.LocationSubtropical wet forests, Puerto Rico.MethodsWe combined data on four functional traits [leaf dry mass per area (LMA), wood density (WD), maximum height (Hmax), seed dry mass] and a well‐resolved molecular phylogeny to characterize taxonomic, functional and phylogenetic composition of sapling and adult tree communities along a regionally replicated chronosequence. We used a null model approach to assess how functional and phylogenetic diversity change with forest age.ResultsCorresponding increases of community‐weighted mean LMA, Hmax and seed mass with forest age reflected a shift in dominance of species with acquisitive resource‐use strategies and small seeds towards species with more conservative resource use and larger seeds. A negative relationship between forest age and local diversity of Hmax and seed mass suggested increased importance of competitive hierarchies for light capture and shade‐tolerant regeneration in older forests. In contrast, the colonization of palms in older forest plots led to a positive relationship between forest age and local phylogenetic diversity, suggesting functional convergence of distantly related lineages on traits that confer competitive dominance under low resource conditions.ConclusionsWe linked both functional and phylogenetic dimensions of community diversity with successional trajectories of post‐agricultural tropical forests. Contrasting patterns of these dimensions of diversity shed light on the underlying community assembly processes. We argue that integrating traits and phylogeny with specific hypotheses about physiological and historical mechanisms is essential for advancing our understanding of the drivers of community change during succession.

Species Turnover across Different Life Stages from Seedlings to Canopy Trees in Swamp Forests of Central Brazil

Processes driving the assembly of swamp forest communities have been poorly explored. We analyzed natural regeneration and adult tree communities data of a swamp gallery forest in Central Brazil to discuss the role of ecological filters in shaping plant species turnover in a successional gradient. Species data of 120 plots were used to assess species turnover between natural regeneration and adult tree communities. Our analyses were based on 4995 individuals belonging to 72 species. Community patterns were discerned using ordination analyses. A clear floristic turnover among plant life stages was distinguished. Regeneration community of swamp forests was richer in species composition than the adult community. Tree species commonly found in nonflooded gallery forests were present in the regeneration plots but not in the adult community. Differences in the floristic composition of these two strata suggest that not all species in the seedling stage can stand permanent flooding conditions and only a few tolerant species survive to become adult trees. We propose that natural disturbances play an important role by altering limiting resources, allowing seeds of nonflooded forest species to germinate. This paper elucidates the turnover between plant life stages in swamp forests and suggests mechanisms that may shape these communities.

Effects of high-frequency understorey fires on woody plant regeneration in southeastern Amazonian forests

Anthropogenic understorey fires affect large areas of tropical forest, yet their effects on woody plant regeneration post-fire remain poorly understood. We examined the effects of repeated experimental fires on woody stem (less than 1 cm at base) mortality, recruitment, species diversity, community similarity and regeneration mode (seed versus sprout) in Mato Grosso, Brazil. From 2004 to 2010, forest plots (50 ha) were burned twice (B2) or five times (B5), and compared with an unburned control (B0). Stem density recovered within a year after the first burn (initial density: 12.4-13.2 stems m(-2)), but after 6 years, increased mortality and decreased regeneration--primarily of seedlings--led to a 63 per cent and 85 per cent reduction in stem density in B2 and B5, respectively. Seedlings and sprouts across plots in 2010 displayed remarkable community similarity owing to shared abundant species. Although the dominant surviving species were similar across plots, a major increase in sprouting occurred--almost three- and fourfold greater in B2 and B5 than in B0. In B5, 29 species disappeared and were replaced by 11 new species often present along fragmented forest edges. By 2010, the annual burn regime created substantial divergence between the seedling community and the initial adult tree community (greater than or equal to 20 cm dbh). Increased droughts and continued anthropogenic ignitions associated with frontier land uses may promote high-frequency fire regimes that may substantially alter regeneration and therefore successional processes.

Secondary forests of central Panama increase in similarity to old‐growth forest over time in shade tolerance but not species composition

AbstractQuestionDoes species and functional composition of secondary tropical forests more closely follow a deterministic or a stochastic model of succession?LocationThe Barro Colorado Nature Monument (BCNM), central Panama. The BCNM comprises Barro Colorado Island (BCI) and five adjacent mainland peninsulas and is a mosaic of tropical old‐growth (OG) and secondary forests (SF).MethodsWe studied the species and functional composition of seedling, sapling and adult tree communities in a chronosequence of SF stands ranging in age from 20 to 100 yrs since abandonment, as well as two OG stands for comparison. Functional composition was defined as the representation of species with different degrees of shade tolerance. We also examined patterns of species diversity and dominance to better understand processes driving compositional changes.ResultsSpecies diversity recovered rapidly, and, by 20 yrs, species richness for all size classes was equivalent if not higher in SF compared to OG. Species diversity tended to be lower and species dominance higher for seedlings than saplings or adults. Seedling, sapling and adult tree species composition did not clearly increase in similarity to OG with forest age. Instead, differences in species composition among stands reflected variation in dominance by particular species. Light‐demanding species became less common and shade‐tolerant species more common over stand age, with the functional composition of older secondary forests converging on that of OG. The seedling and sapling communities across the chronosequence were composed of a shade‐tolerant subset of those species found in the stands as adults.ConclusionOur results suggest that recruitment into the seedling community of SF is determined by both predictable (e.g. selection for shade‐tolerant species) and unpredictable factors (e.g. species‐specific reproduction events and dispersal limitation). Recruitment into the sapling and tree communities reflects stronger, and potentially compounded, selection for shade‐tolerant species. Therefore, changes in the species composition of regenerating forests in this area appear to be unpredictable, even though the functional composition follows a more deterministic and predictable trajectory with convergence on OG over time.

Patterns of tree community composition along a coastal dune chronosequence in lowland temperate rain forest in New Zealand

Soil chronosequences provide an opportunity to examine the influence of long-term pedogenesis on the biomass and composition of associated tree communities. We assessed variation in the species composition of trees, saplings, and seedlings, and the basal area of adult trees, in lowland temperate rain forest along the Haast chronosequence on the west coast of the South Island of New Zealand. The sequence consists of Holocene dune ridges formed following periodic earthquake disturbance and is characterized by rapid podzol development, including a marked decline in phosphorus concentrations, accumulation of a thick organic horizon, and formation of a cemented iron pan. Tree basal area increased for the first few hundred years and then declined in parallel with the decline in total soil phosphorus, consistent with the concept of forest retrogression. There were also marked changes in the composition of the tree community, from dominance by conifers on young soils to a mixed conifer–angiosperm forest on old soils. Although a variety of factors could account for these changes, partial Mantel tests revealed strong correlations between tree community composition and soil nutrients. The relationships differed among life history stages, however, because the adult tree community composition was correlated strongly with nutrients in the mineral soil, whereas the seedling community composition was correlated with nutrients in the organic horizon, presumably reflecting differences in rooting depth. The changes in the tree community at Haast are consistent with disturbance-related succession in conifer–angiosperm forests in the region, but the opposite of patterns along the nearby Franz Josef post-glacial chronosequence, where conifers are most abundant on old soils. The Haast chronosequence is therefore an important additional example of forest retrogression linked to long-term soil phosphorus depletion, and provides evidence for the role of soil nutrients in determining the distribution of tree species during long-term succession in lowland temperate rain forests in New Zealand.

Differential diameter-size effects of forest management on tree species richness and community structure: implications for conservation

In this paper we tested the hypothesis that logging effects in the adult tree community reverberate upon the regeneration contingent. We examined the differences on the tree community between forest reserves and 10 year-old logged areas in the Yucatan Peninsula. We used a paired design in three independent sites to estimate the effects of logging on tree species richness, diversity, composition and structure. Analyses were conducted differentiating individuals of four diameter-size classes: 1-5, 5-10, 10-25, and (25 cm DBH. We found out that there were differential effects by size. Species richness in the smaller and larger diameter-size classes was significantly lower in logged areas. Floristic composition was also different between logged and unlogged areas, with a trend towards more secondary forest associated species and less primary forest associated spe- cies in logged areas, and a higher density of species represented by a single individual in unlogged reserves. In terms of structure, trees DBH \10 cm, lianas, and re-sprouting stumps were more abundant in logged areas. Our findings suggest that 10 years after logging, harvested areas show alterations in structure, and potentially a reduction in species richness. We suggest that to make timber extraction and forest conservation compatible at this site, it is necessary to gain a better understanding of the ecology and regeneration requirements of the less abundant species, and to assess whether current logging practices might hinder their permanence in the study area.

Tree spacing and area of competitive influence do not scale with tree size in an African rain forest

AbstractQuestions:Is the area of influence of individual trees determined by tree size? Does competition, inferred from spatial pattern between neighbouring trees, affect adult tree spacing patterns in an tropical forest? At what size‐class or stage is competition between neighbours most likely to affect adult tree spacing patterns?Location:Kibale National Park, western Uganda.Methods:Relationships between focal tree size and nearest neighbour distance, size, density, and species in a 4‐ha permanent plot, using point pattern analyses.Results:We found non‐random patterns of distribution of nearest tree neighbours (stems > 10 cm DBH). Independent of identity, tree density was highest and neighbours were regularly spaced within 3–5m of an individual. Tree densities were lower and relatively constant at distances >5m and neighbours were typically randomly spaced. In general, conspecific patterns conformed to the latter trends. Thus, individual area of influence was small (within a radius of 3–5 m). Rarer species were more clumped than common species. Weak competitive thinning occurred among more densely packed small trees (<20 cm DBH), and rapidly disappeared with increasing tree size and distance from an individual. The clumping and density of individuals was not significantly affected by tree size.Conclusions:Negative effects of competition among trees are weak, occur within the crown radius of most individuals, and are independent of adult tree size and identity. The density of neighbouring trees (aggregation) did not decline with increasing focal tree size at either the conspecific or the community level and tree diameter (tree size) was not a good estimator of the implied competitive influence of a tree. Mechanisms operating at the recruitment stage may be important determinants of adult tree community diversity and spacing patterns.

Fragmentation and Alteration of Seed Dispersal Processes: An Initial Evaluation of Dung Beetles, Seed Fate, and Seedling Diversity1

ABSTRACTGiven current accelerated trends of tropical land conversion, forest fragments are being incorporated into many conservation programs. For investing in fragments to be a viable conservation strategy, forest fragments must maintain their ecological integrity over the long term. Based on fieldwork in 22 forest fragments in the crater lakes region of western Uganda and in the continuous forest of Kibale National Park, we examined (1) seed predation on experimentally dispersed seeds, (2) abundance and composition of the dung beetle community that may play a major role in removing seeds from sites of high seed predation, and (3) compared the fragments’ seedling community composition to adult tree community composition and the seedling community in continuous forest. First, the rate of seed removal at experimental stations was lower in forest fragments (85% remaining after 1 day) than at stations in the continuous forest (79% remaining) and the probability of stations being discovered by seed predators was lower in fragments (23%) than in the intact forest (41%). Second, there was a 62 percent decline in fragment dung beetle abundance. The magnitude of this decline varied among dung beetle guilds that process dung and seeds in different fashions. The abundance of large rollers that move large seeds away from sites of defecation did not differ, but medium and smaller rollers and burying beetles that process small and medium‐sized seeds were less common in the fragments than in the intact forest. Finally, we compared the seedling community composition relative to adult tree community composition by identifying all adult trees in each fragment and by sampling the composition of the seedling community. We found some evidence to suggest that there was movement of seeds among forest fragments by large‐bodied dispersers, particularly chimpanzees (Pan troglodytes) and hornbills (Ceratogymna subcylindricus).

Fragmentation and Alteration of Seed Dispersal Processes: An Initial Evaluation of Dung Beetles, Seed Fate, and Seedling Diversity1

Given current accelerated trends of tropical land conversion, forest fragments are being incorporated into many conservation programs. For investing in fragments to be a viable conservation strategy, forest fragments must maintain their ecological integrity over the long term. Based on fieldwork in 22 forest fragments in the crater lakes region of western Uganda and in the continuous forest of Kibale National Park, we examined (1) seed predation on experimentally dispersed seeds, (2) abundance and composition of the dung beetle community that may play a major role in removing seeds from sites of high seed predation, and (3) compared the fragments' seedling community composition to adult tree community composition and the seedling community in continuous forest. First, the rate of seed removal at experimental stations was lower in forest fragments (85% remaining after 1 day) than at stations in the continuous forest (79% remaining) and the probability of stations being discovered by seed predators was lower in fragments (23%) than in the intact forest (41%). Second, there was a 62 percent decline in fragment dung beetle abundance. The magnitude of this decline varied among dung beetle guilds that process dung and seeds in different fashions. The abundance of large rollers that move large seeds away from sites of defecation did not differ, but medium and smaller rollers and burying beetles that process small and medium-sized seeds were less common in the fragments than in the intact forest. Finally, we compared the seedling community composition relative to adult tree community composition by identifying all adult trees in each fragment and by sampling the composition of the seedling community. We found some evidence to suggest that there was movement of seeds among forest fragments by large-bodied dispersers, particularly chimpanzees (Pan troglodytes) and hornbills (Ceratogymna subcylindricus).