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

Liana Abundance, Diversity, and Distribution on Barro Colorado Island, Panama

Lianas are a key component of tropical forests; however, most surveys are too small to accurately quantify liana community composition, diversity, abundance, and spatial distribution – critical components for measuring the contribution of lianas to forest processes. In 2007, we tagged, mapped, measured the diameter, and identified all lianas ≥1 cm rooted in a 50-ha plot on Barro Colorado Island, Panama (BCI). We calculated liana density, basal area, and species richness for both independently rooted lianas and all rooted liana stems (genets plus clones). We compared spatial aggregation patterns of liana and tree species, and among liana species that varied in the amount of clonal reproduction. We also tested whether liana and tree densities have increased on BCI compared to surveys conducted 30-years earlier. This study represents the most comprehensive spatially contiguous sampling of lianas ever conducted and, over the 50 ha area, we found 67,447 rooted liana stems comprising 162 species. Rooted lianas composed nearly 25% of the woody stems (trees and lianas), 35% of woody species richness, and 3% of woody basal area. Lianas were spatially aggregated within the 50-ha plot and the liana species with the highest proportion of clonal stems more spatially aggregated than the least clonal species, possibly indicating clonal stem recruitment following canopy disturbance. Over the past 30 years, liana density increased by 75% for stems ≥1 cm diameter and nearly 140% for stems ≥5 cm diameter, while tree density on BCI decreased 11.5%; a finding consistent with other neotropical forests. Our data confirm that lianas contribute substantially to tropical forest stem density and diversity, they have highly clumped distributions that appear to be driven by clonal stem recruitment into treefall gaps, and they are increasing relative to trees, thus indicating that lianas will play a greater role in the future dynamics of BCI and other neotropical forests

Soil resources and topography shape local tree community structure in tropical forests

Both habitat filtering and dispersal limitation influence the compositional structure of forest communities, but previous studies examining the relative contributions of these processes with variation partitioning have primarily used topography to represent the influence of the environment. Here, we bring together data on both topography and soil resource variation within eight large (24-50 ha) tropical forest plots, and use variation partitioning to decompose community compositional variation into fractions explained by spatial, soil resource and topographic variables. Both soil resources and topography account for significant and approximately equal variation in tree community composition (9-34% and 5-29%, respectively), and all environmental variables together explain 13-39% of compositional variation within a plot. A large fraction of variation (19-37%) was spatially structured, yet unexplained by the environment, suggesting an important role for dispersal processes and unmeasured environmental variables. For the majority of sites, adding soil resource variables to topography nearly doubled the inferred role of habitat filtering, accounting for variation in compositional structure that would previously have been attributable to dispersal. Our results, illustrated using a new graphical depiction of community structure within these plots, demonstrate the importance of small-scale environmental variation in shaping local community structure in diverse tropical forests around the globe. © 2012 The Author(s) Published by the Royal Society. All rights reserved

The effects of temperature and moisture on the efflux of CO2 from leaf litter.

Soil respiration represents an enormous output of carbon dioxide to the atmosphere. The possibility of a positive feedback effect between global climate change and soil respiration makes it important to understand how future climatic changes will affect soil respiration in forest ecosystems. One important component of soil respiration is the highly labile leaf litter. In a forest ecosystem the leaf litter carbon pool is closely tied to the net primary productivity, and it is subjected to greater variation over short periods of time in the main drivers of soil respiration, temperature and moisture, than the rest of the soil. Leaf litter samples were collected from four plots and on two dates in a temperate forest in northern lower Michigan. The litter samples were subjected to laboratory experiments to determine the relationship between temperature, moisture, and the rate of CO2 efflux from leaf litter. This study presents a predictive model of the efflux of CO2 from leaf litter based on the experimentally generated data. The model was used to estimate the contribution of the previous year's leaf litter to the total soil respiration in the same region. Both temperature and moisture are significantly correlated with leaf litter respiration, and the previous year's leaf litter was found to be a significant contributor (0-8% over a two week period, depending on conditions) to total soil respiration, with the proportional contribution of leaf litter varying under different environmental conditions. The results are consistent with other studies that have named primary production as an important factor in variation in soil respiration between ecosystems.http://deepblue.lib.umich.edu/bitstream/2027.42/55041/1/3484.pdfDescription of 3484.pdf : Access restricted to on-site users at the U-M Biological Station

The effects of local-scale resource heterogeneity on tropical tree communities

The role of niche assembly processes in shaping ecological communities is a subject of great interest to ecologists, especially in species rich communities such as tropical forests, as niche processes may play an important part in biodiversity maintenance. An important part of the environmental niche for tropical tree species is their specialization for particular soil resource conditions. My dissertation research examines the soil resource and topographic niches of tropical forest tree species and how they affect local ( 1 cm in diameter have been mapped. Additionally, topographic variation has been mapped within these plots and I use data from a recent, extensive soil sampling effort that mapped the small-scale heterogeneity in many soil variables (including P, Ca, Mg, K, Mn, Al, and pH) within these plots. Previous research using these data has demonstrated that many tree species are non-randomly distributed with respect to soil resource and topographic variation, indicating that local-scale soil resource specialization is common and widespread for tropical trees. I use a variety of multivariate techniques to investigate whether the soil resource and topographic niches of individual tree species have important emergent effects at the community level. I demonstrate that environmental variation is often a strong driver of variation in community composition within these forest plots. I also relate the soil resource and topographic niches of species to their evolutionary relationships and show that closely related species often have more similar habitat niches than distant relatives. The combined effects of habitat heterogeneity on community structure and phylogenetic signal in habitat niches create communities where soil resource and topographic variation affects the overall phylogenetic structure of the community. Furthermore, I examine a possible mechanism for the controls of soil resources on spatial variation in leaf chemistry, an important component of ecosystem biogeochemical cycling. I find that leaf nutrient profiles are highly conserved within a species and thus I predict that soil resources are likely to influence community-level variation in leaf chemistry through their effects on species composition, rather than by intraspecific responses to soil nutrients. Overall, my dissertation has helped to reveal and explain local-scale environmental controls on community structure and to clarify likely controls of soil resources on spatial variation in ecosystem biogeochemistry

Improving Remote Species Identification through Efficient Training Data Collection

Plant species identification and mapping based on remotely-sensed spectral signatures is a challenging task with the potential to contribute enormously to ecological studies. Success in this task rests upon the appropriate collection and use of costly field-based training data, and researchers are in need of ways to improve collection efficiency based on quantitative evidence. Using imaging spectrometer data collected by the Carnegie Airborne Observatory for hundreds of field-identified tree crowns in Kruger National Park, South Africa, we developed woody plant species classification models and evaluated how classification accuracy increases with increasing numbers of training crowns. First, we show that classification accuracy must be estimated while respecting the crown as the basic unit of data; otherwise, accuracy will be overestimated and the amount of training data needed to perform successful classification will be underestimated. We found that classification accuracy and the number of training crowns needed to perform successful classification varied depending on the number and spectral separability of species in the model. We also used a modified Michaelis-Menten function to describe the empirical relationship between training crowns and model accuracy, and show how this function may be useful for predicting accuracy. This framework can assist researchers in designing field campaigns to maximize the efficiency of field data collection, and thus the amount of biodiversity information gained from remote species identification models

Estimating Vegetation Beta Diversity from Airborne Imaging Spectroscopy and Unsupervised Clustering

Airborne remote sensing has an important role to play in mapping and monitoring biodiversity over large spatial scales. Techniques for applying this technology to biodiversity mapping have focused on remote species identification of individual crowns; however, this requires collection of a large number of crowns to train a classifier, which may limit the usefulness of this approach in many study regions. Based on the premise that the spectral variation among sites is related to their ecological dissimilarity, we asked whether it is possible to estimate the beta diversity, or turnover in species composition, among sites without the use of training data. We evaluated alternative methods using simulated communities constructed from the spectra of field-identified tree and shrub crowns from an African savanna. A method based on the k-means clustering of crown spectra produced beta diversity estimates (measured as Bray-Curtis dissimilarity) among sites with an average pairwise correlation of ~0.5 with the true beta diversity, compared to an average correlation of ~0.8 obtained by a supervised species classification approach. When applied to savanna landscapes, the unsupervised clustering method produced beta diversity estimates similar to those obtained from supervised classification. The unsupervised method proposed here can be used to estimate the spatial structure of species turnover in a landscape when training data (e.g., tree crowns) are unavailable, providing top-down information for science, conservation and ecosystem management applications

Savanna tree height class distribution under different fire management histories

Tree height class and species data collected with airborne LiDAR and imaging spectroscopy across two savanna hill slopes with different fire histories in Kruger National Park, South Africa. Tree heights are in meters. Tree IDs are a random selection of 1000 individuals per species per fire history treatment. Airborne data was collected in April 2008

Data from: Demographic legacies of fire history in an African savanna

Fire is a key determinant of woody vegetation structure in savanna ecosystems, acting both independently and synergistically through interactions with herbivores. Fire influences biodiversity and ecological functioning, but quantifying its effects on woody structure is challenging at both species and community scales. Deeper insight into fire effects, and fire-herbivore interactions, can be gained through the examination of species-specific demographic and dynamic changes occurring across areas with different fire regimes in the presence of large herbivores. We used the Carnegie Airborne Observatory (an integrated LiDAR and imaging spectroscopy system) to map woody tree structure, species and dynamics over a four-year interval across two adjacent savanna landscapes with contrasting fire histories in Kruger National Park, South Africa. A history of higher fire frequency was associated with reduced woody canopy cover (17% vs. 23%) and an increased overall rate of treefall (27% vs. 18%). The landscape with a history of higher fire frequency displayed a shift in woody canopy height distribution from a unimodal curve to a bimodal pattern at the community scale, with large reductions in height classes < 7 m. Differences in tree height distributions and treefall rates across sites were underpinned by species-specific responses to fire frequency. Acacia nigrescens displayed the highest rates of treefall, most likely related to elephant activity, with losses exceeding 40% in the 6-9 m height classes. Synthesis. Our findings indicate that fire history imparts demographic legacies not only on vegetation structure, but also on current vegetation dynamics. Current treefall rates of certain tree species are exacerbated by a history of higher fire frequency. Species-specific and context-conscious investigations are critical for elucidating the driving mechanisms underlying broader community patterns