Resource‐based habitat associations in a neotropical liana community

Summary 1. Lianas are a conspicuous element of many tropical forests, accounting for up to 40% of woody stem density and 20% of species richness in seasonal forests. However, lianas have seldom been surveyed at sufficiently large spatial scales to allow an assessment of the importance of habitat variables in structuring liana communities. 2. We compare the association patterns of 82 liana species and an equivalent sample of tree species on the 50 ha Forest Dynamics Project plot on Barro Colorado Island, Panama, with topographic habitat variables (high and low plateau, slope, swamp and streamside), and thirteen mapped soil chemical variables. In addition, we test for liana species associations with canopy disturbance using a canopy height map of the plot generated using light detection and ranging. 3. For all liana species combined, densities differed among topographic habitat types in the plot, with significantly higher densities on the seasonally drier lower plateau habitat (1044 individuals ha−1) than the moister slope habitat (729 individuals ha−1). Lianas were also significantly more abundant than expected in areas with low canopy height. 4. The proportion of liana species associated with one or more topographic habitat variables (44%) was significantly lower than that for trees (66%). Similarly, liana species were significantly less frequently associated with PC axes derived from soil chemical variables (21%) than trees (52%). The majority of liana species (63%) were significantly associated with areas of the plot with low canopy height reflecting an affinity for treefall gaps. 5. Synthesis. The habitat associations detected here suggest that liana density is associated primarily with canopy disturbance, and to a lesser extent with topography and soil chemistry. Relative to trees, few liana species were associated with local variation in topography and soil chemistry, suggesting that nutrient availability exerts only weak effects on liana community composition compared to trees. Results from this study support the contention that increases in forest disturbance rates are a driver of recently observed increases in liana abundance and biomass in neotropical forests

Edaphic Factors and Initial Conditions Influence Successional Trajectories of Early Regenerating Tropical Dry Forests

Edaphic factors and initial conditions can regulate the speed of forest succession. Edaphic factors, which include soil chemistry and topography, determine soil resource availability and can filter species as forests mature. Initial plant cover early in succession can determine the rates at which secondary forests change in structure, richness, biomass and composition over time. While some of the effects of edaphic factors and initial conditions on forest succession have been studied, how they simultaneously modify young regenerating tropical forest has rarely been examined. We surveyed 22 young forests plots in Panama for 7 years (11, 6 and 3‐year‐old stands when censuses began). We study how tree and liana species composition change early in succession, as well as how edaphic factors (soil nutrients and topography) and initial conditions (initial basal area and forest canopy cover) influence changes in tree and liana abundance, species richness, biomass and composition throughout succession. We found that edaphic factors and initial conditions explained up to 45% of the variation in the successional trajectories for trees and lianas. Soil nutrients had a significant positive effect on the changes in tree biomass accretion, while topography significantly contributed to community similarity of large lianas over time. Initial basal area had a significant negative effect on the changes in sapling abundance and tree richness over time and a positive marginal effect on tree biomass accretion. Forest canopy cover only had a positive marginal effect on changes in sapling abundance. Tree abundance, biomass and richness increased over time, while sapling abundance, biomass and richness remained stable or decreased, probably due to community thinning. However, changes over time of small and large lianas diverged, probably due to differential resource availability that affected lianas but not trees. Synthesis. Soil fertility, topography and initial basal area influence early forest regeneration. Higher soil fertility can allow trees to fix carbon faster, and lianas might show habitat association to ridges and slopes. Basal area can determine how fast saplings and trees change in abundance, richness and biomass over time by possibly affecting space availability for recruitment and light availability for growth

The Liana assemblage of a Congolian rainforest : diversity, structure and dynamics

Key words: Liana assemblage, species composition, community, dynamics, canopy openness, Manniophyton fulvum, functional traits, population density, pervasive change. This study analyzes the diversity, composition, and dynamics of the liana assemblage of the Ituri rain forest in northeastern DR Congo. I used data from two 10-ha plots of the Ituri Forest Dynamics Plots, in which all liana stems ≥2 cm diameter at breast height (dbh) were marked, mapped, measured and identified in 1994, 2001 and 2007. In addition, the plot topography and canopy structure were measured. Chapter 2 analyzes the liana assemblage (in terms of species richness, abundance and diversity), characterizes liana functional traits and determines effects of forest structure, topography and edaphic variation on liana species composition. In 20 ha, 15008 liana individuals were found, representing 195 species, 83 genera and 34 plant families. Per hectare species number averaged 64, basal area was 0.71 m2 and Fisher alpha, Shannon and Simpson diversity indices were 17.9, 3.1 and 11.4, respectively. There was oligarchic dominance of 10 plant families that represented 69% of total species richness, 92% of liana abundance and 92% of basal area, while ten dominant species accounted for 63% of abundance and 59% of basal area. Forty-one species (21%) were represented by one individual only. Most lianas were light-demanding, climbed their hosts by twining, and had conspicuous flowers, medium-sized leaves and animal-dispersed propagules. Liana abundance increased with abundance of medium-sized and large trees but was, surprisingly, independent of small-tree abundance. Canopy openness, soil moisture, and tree size were the most important environmental factors influencing abundance and distribution of lianas. In Chapter 3 I investigate changes in structural characteristics, diversity, recruitment, mortality and growth of the liana community over the thirteen years (1994 ¬- 2007). Liana density decreased from 750 (1994) through 547 (2001) to 499 (2007) stems ha-1, with concomitant declines in basal area and above-ground biomass. Despite lower stem densities the species richness remained constant over time. Total liana recruitment rates decreased slightly from 8.6% per year in the first period to 6.6% in the second, but this decrease was not significant. Liana mortality rates decreased significantly from 7.2% to 4.4% per year over the two census intervals. Diameter growth rates and survival increased with liana stem diameter. Surprisingly, liana abundance in Ituri showed recent declines, rather than recent increases, as has been reported for tropical and temperate forests in the Americas. Interestingly, changes in overall liana community structure and composition were mostly driven by one species only: the dramatic collapse of superabundant Manniophyton fulvum between the first and the second census. In chapter 4 I investigated species-specific dynamics of the 79 most abundant liana species, representing 13,156 of the stems (97% of total) in two 10-ha plots. I evaluated their demographic performance and the relation if the vital rates (growth, mortality, recruitment) to the species abundance and four functional traits (climbing strategy, dispersal syndrome, leaf size and light requirements) to determine across species variations and major strategies characterizing species. Vital rates shared a wide interspecific variation; species-specific recruitment rates varied from 0.0-10.9%, mortality rates from 0.43-7.89% over 13-year, and growth rates from -0.03-3.51 mm y-1. Most species had low to moderate rates. Species that grew fast tended also to recruit and die fast, but recruitment and mortality rates were not directly related, suggesting that species shift in absolute abundance over the 13 year period. However, with the exception of the collapsing Manniophyton fulvum population, species maintained their rank-dominance over time. Species growth declined with abundance, but recruitment and mortality rates were not related to abundance. The demographic performance of liana species varied weakly with their climbing strategy and dispersal mode but was, surprisingly, not related to their lifetime light requirements. A principle components analysis of liana strategies in terms of functional traits and vital rates showed that light demand, and dispersal syndrome were the most determining traits. Based on the PCA three functional guilds were distinguished. I conclude that old-growth forest liana species show a large variation in abundance and vital rates, and that density-dependent mechanisms are insufficient to explain the species abundance patterns over time. Lianas are thought to globally increase in density, but we have limited knowledge about the taxonomic patterns of change in liana abundance, and the underlying vital rates that explain changes in liana density. In chapter 5 the changes in abundance of 79 relatively abundant liana species are evaluated. The Ituri forest showed a pervasive change in liana population density in the last decade. 37 species changed significantly in their abundance over time: 12 (15% of total) species increased, and 25 (32%) species decreased. 42 (53%) species did not change. Of the 48 genera, 40% decreased and 52% stayed the same. Five of the 12 increasing species belonged to the Celastraceae, which also was the only significantly increasing family. Surprisingly, none of the four functional traits (lifetime light requirements, climbing mechanism, dispersal mechanism, and leaf size) was significantly associated with species change in population density. Many decreasing species, however, are associated with disturbed habitats and are short-lived. Many increasing species are late successional and longer-lived. Increasing species have a slightly higher recruitment, decreasing species a higher mortality. This study suggests that changes in the liana community result from forest recovery from past disturbances. Rising atmospheric CO2 level was not a likely explanation for liana change: more species declined than increased, and increasing species did not have higher growth rates. In the Ituri Forest local stand dynamics override more global drivers of liana change. <br/

Ecological Succession in Tropical Forests: The Role of Edaphic Factors, Initial Conditions and Competition

Succession is a fundamental process in ecology in which ecosystems recover after disturbances. The goal of the study of ecological succession is to understand the mechanisms responsible for changes in species’ density, diversity, and ecosystem processes. Understanding the mechanisms that determine how young tropical forests change during succession is crucial because approximately half of the world’s tropical forests are regenerating after farmland abandonment, and successional forests are now expected to supply the vast majority of ecosystems services that were provided by old growth forests (e.g. carbon sequestration). Edaphic factors, initial conditions, and competition have been proposed to be key drivers that influence tropical forest succession; however, how these drivers alter succession remains poorly understood. For my doctoral dissertation research, I used census data from a young tropical dry forest, and a large-scale field experiment in a tropical moist forest to examine the combined effects of edaphic factors and initial conditions on forest succession, as well as the effect of lianas on trees, an intense form of plant competition, on forest succession.In the dry forest, edaphic factors and initial conditions were strong determinants of succession. Soil fertility accelerated tree biomass accretion. Topography made liana composition more similar over time. Initial conditions decreased sapling recruitment and biomass accretion, probably due to lower light levels when there is more basal area early in succession. The accumulation of tree species was slowed while tree composition similarity increased with more basal area early in succession. Competition for space may have delayed tree recruitment in the canopy and homogenized composition. In the moist forest, liana competition significantly influenced succession. Lianas contributed 20% of the foliage to the forest canopy, and thus significantly reduced light level and tree biomass accumulation. Lianas reduced tree biomass accumulation even when trees received full sunlight and their canopies were intact. Finally, using a comprehensive literature review on liana removal experiments, I report that lianas decrease tree establishment, growth, biomass accumulation and reproduction across the world’s tropical forests. In summary, liana competition, edaphic factors, and initial conditions all influence the rate and direction of succession in young tropical forests

Tree Function and Habitat Niche Partitioning in Tropical Forests: Implications for Responses to Environmental Change

Tropical forests possess exceptional levels of tree species richness but explaining this diversity has presented a long existing challenge. Habitat niche partitioning provides a hypothesis for species co-existence, whereby species avoid competitive exclusion by partitioning demands on multiple resources within an environment. However, limited understanding concerning how tree function is influenced by multiple environmental variables has limited the support for this hypothesis. This knowledge gap also limits our ability to predict how tropical forest tree communities will respond to environmental change, given multiple dimensions of a species’ niche are likely to be affected. In this thesis, I investigate the role of niche partitioning in supporting co-existence of species and the turnover of species across edaphic gradients, as well as how long-term changes to the environment from selective logging and drought affect niche space of tropical tree species. I use species distribution models and measurements of leaf physiological traits to determine the key dimensions of tree species’ niches in primary forests. In chapter 2 I demonstrate niche partitioning is strong within tropical forests with at least 60-86% of abundant species occupying their own unique niche. Species partition a wide range of abiotic environments, including soil nutrient, topographic and light environments, with greater environmental heterogeneity enhancing the scope for niche partitioning. Building on this, in chapter 3 I find that variation in nutrient availability explains more variation in leaf physiology and habitat preferences than light availability of species from the Dipterocarpaceae family that dominates South-East Asian forests. This highlights the importance of edaphic environments in structuring tropical forest communities. I also find different leaf nutrients are related to photosynthetic capacity in different forest types, revealing that multiple different nutrients may limit productivity and affect species distributions in tropical forests. Many tropical forest tree species are highly specialised with limited ability to adjust their traits between environments, underlining their potential vulnerability to environmental change. In chapter 4 I show seedlings from selectively logged Bornean forests have different community weighted mean trait values, with greater belowground investment in logged forests. These adaptations are sufficient to overcome soil stress and to maintain foliar nutrient concentrations. However, I show seedlings of species that dominate old-growth forests are less able to adapt their traits and experience elevated mortality rates in logged forests. I attribute this to greater soil nutrient limitation as they are unable to maintain leaf nutrient concentrations. Selective logging will therefore likely drive shifts in species composition towards greater dominance of earlier-successional species that have traits capable of surviving in disturbed environments. This could result in local-scale reductions in species diversity and functional diversity, which could reduce long-term resilience to environmental change. In contrast, in Chapter 5 I demonstrate small trees in Amazonian forests are able to respond to changes in their environment following long-term drought conditions. Following mortality of large canopy trees, small trees can respond to increased light availability even under reduced water availability by adjusting resource allocation and by increasing nutrient use efficiency. Despite evidence of resilience to long-term drought conditions, hyper-dominant species show a greater capacity to respond, which may further enhance the dominance of these species under future climates. In conclusion my results highlight the importance of habitat niche partitioning in structuring tropical forest tree communities and identify key environmental variables that determine species distribution and tree function. My results have important implications for the conservation and restoration of tropical forests under environmental change. Avoidance of environmental homogenisation and changes to as few environmental conditions as possible is likely to be important in maintaining high species diversity in tropical forests and to avoid increased dominance by few generalist species. Many current conservation and restoration projects focus on recovering vegetation, but my research highlights the additional need to maintain and restore soil environments, especially for the long-term persistence of highly specialist species

Determinants of change in subtropical tree diameter growth with ontogenetic stage

We evaluated the degree to which relative growth rate (RGR) of saplings and large trees is related to seven functional traits that describe physiological behavior and soil environmental factors related to topography and fertility for 57 subtropical tree species in Dinghushan, China. The mean values of functional traits and soil environmental factors for each species that were related to RGR varied with ontogenetic stage. Sapling RGR showed greater relationships with functional traits than large-tree RGR, whereas large-tree RGR was more associated with soil environment than was sapling RGR. The strongest single predictors of RGR were wood density for saplings and slope aspect for large trees. The stepwise regression model for large trees accounted for a larger proportion of variability (R 2 = 0.95) in RGR than the model for saplings (R 2 = 0.55). Functional diversity analysis revealed that the process of habitat filtering likely contributes to the substantial changes in regulation of RGR as communities transition from saplings to large trees. © 2014 Springer-Verlag Berlin Heidelberg

Landscape Ecology of Birds on Mount Leconte, Great Smoky Mountains National Park

Landbirds form a significant component of wildlife resources in the Great Smoky Mountains National Park. The present study explored how forest structure and composition of deciduous-coniferous ecotones influenced diversity, richness, and relative abundance of bird species and how bird species responded to the spruce-fir community. Using a form of the variable-circular plot method, I conducted audio-visual censuses of diurnal birds on Mount LeConte. I established 212 geo-referenced census points on six trails, which were used as gradient-oriented transects (gradsects). I measured habitat characteristics at the same census points. I used forest community types for each point on gradsects to delineate boundaries. I applied The Tasseled-Cap (T-CAP) , a graphic description of the spectral-temporal development of locations, to analyze bird-habitat relationships in order to investigate the utility of Landsat T-CAP indices in predicting forest patterns and bird species\u27 richness and abundance. I derived elevation, slope, and aspect from differentially corrected GPS coordinates using ArcView Spatial Analyst and T-Cap indices from Landsat TM remotely sensed data for forest community types and each vegetation sampling station using Earth Resources Data Analysis System. My results showed correlations among the abundances of many bird species and elevational, floristic, and physiogonomic features of their habitat, both for univariate and multivariate characters. Both cover type and size class (dbh) were important to the breeding avifauna; various groups of breeding birds were associated with either one or both variables. Ecotones along the gradsects among forest types were perceived by many bird species as significant discontinuities. Zones of both rapid and gradual change in bird abundance were observed. For certain bird species, patterns of bird distribution and forest types coincided. Individual species responded to patchiness, vegetation structure, and elevation, sometimes in a predictable manner. I observed clusters of species and communities along my derived zones that appeared to be different. Species expanded or contracted their distributions in localities where the homogenous ecotone was displaced upward or downward in elevation relative to the location of the ecotone on Mount LeConte. Spatial fluctuations were, in general, related to zonal transitions in forest types. T-CAP indices were related to species\u27 responses to changes in landscape structure and composition. Distinct patterns in vegetation that corresponded with different forest types and zones of rapid and gradual change in bird abundance were observed. Greenness and wetness values differentiated closed canopy fir stands from all other classes

Distribution and ecological effects of Azteca Chartifex/Trigona in the Barro Colorado Nature Monument.

This dissertation explores the behavior and ecology of a conspicuous and behaviorally dominant ant species in the tropical rainforest of Panama. Competition with aggressive species is an important factor that shapes local community structure. Eavesdropping on the chemical communication systems of such behaviorally dominant species can help heterospecific species to avoid negative interactions. I review the chemical communication system and known ecological effects of the Neotropical canopy ant, Azteca chartifex/trigona (Chapter 1). There are over 40 known compounds produced by Azteca workers, yet how these compounds impact heterospecific species is unclear in many circumstances. I assessed if heterospecific species respond to A. trigona pheromones by exposing workers of 29 canopy ant species to A. trigona alarm pheromones (Chapter 2). Seven species showed distinct responses to A. trigona pheromones and responses were not associated with phylogeny. The pheromones produced near ant nests may additionally be a reliable source for eavesdropping species and I used open-air sampling techniques to determine whether the air space surrounding A. trigona carton nests has a distinct chemical composition (Chapter 3). The air around disturbed A. trigona nests had higher concentrations of compounds associated with worker alarm pheromones, whereas undisturbed nests were chemically indistinguishable from the surrounding forest air. Azteca trigona workers aggressively outcompete for some resources and I experimentally assessed the effects of A. trigona on the composition of resident and colonizing ants by installing artificial nests in 28 tree crowns (Chapter 4). The presence of A. trigona did not affect the colonization frequency of artificial nests nor species composition of the resident ants in a tree; however, species composition of nest occupants differed between trees and nests located within the foraging territories of A. trigona were colonized less frequently. Finally, I determined if A. trigona was affected by landscape scale factors, including liana presence and canopy height (Chapter 5). A. trigona nests were more frequent in taller trees within older forests and had smaller colony sizes in liana removal plots. Collectively, the results summarized in this dissertation improves our understanding of canopy ant interactions and distributions in a tropical forest

Effects of experimental lightgaps and topography on enrichment plantings in a central Amazonian secondary forest

Enrichment plantings into secondary forest are an important option in restoring species diversity and ecosystem services. However, little attention has been given to environmental requirements for species performance. This study evaluated the effects of lightgaps and topographic position on the growth and survival of four native tree species (Pouteria caimito, Garcinia macrophylla, Dipteryx odorata and Cynometra bauhiniaefolia) planted into a 26-year old secondary forest originating from abandoned pastures in the central Amazon Basin. Artificial lightgaps and control plots under closed canopy were uniformly distributed on plateaus and bottomlands near water bodies. Seedlings were planted randomly into the plots and monitored for 28 months. Seedling survival rate was high (93%) and did not differ among species. Overall, lightgaps produced a 38% increase in seedling height relative to the controls. Although the four species naturally occur in mature forest, two of the four grew significantly more in lightgaps than in dosed canopy secondary forest. Overall, bottomlands facilitated greater seedling growth in height (38%) relative to plateaus, but only one species exhibited a significant increase. This study shows the importance of the environmental variability generated with canopy openings along the topographic gradient, suggesting that both the selection of species and microsite conditions of planting sites have to be considered important criteria in the recovery of degraded areas