65 research outputs found
Leaf traits of dipterocarp species with contrasting distributions across a gradient of nutrient and light availability
No full textBackground: Tree species composition at the landscape scale is often tightly associated with underlying soil type in tropical forests. Changes in soil type may have effects on forest structure that drive changes in both light and soil resource availability, since light availability in the understorey tends to be lower in more fertile sites. Plant functional traits may determine species distributions across gradients of light and soil resource availability. Aims: To test whether tree species with contrasting distributions exhibit leaf traits that reflect adaptation to the resources most limiting in their native environment. Methods: We measured foliar nutrient concentrations, stomatal density, leaf δ13C values, leaf mass per area, and leaf lifespan for saplings of nine common dipterocarp species at Sepilok Forest Reserve, Malaysian Borneo, possessing varying associations to soil resource habitats. Results: Species specialised in their adult distribution to nutrient-poor sandstone soils had traits indicative of a nutrient conservation strategy. Species specialised to more fertile alluvial soils had a wider spectrum of leaf N and P concentrations and LL, reflecting greater variance in strategies for resource acquisition and use among species in this habitat. Conclusions: Understorey light regimes co-vary with soil type, and both light and soil resource availability influence leaf trait adaptations that may contribute to species–habitat associations
Factors explaining alien plant invasion success in a tropical ecosystem differ at each stage of invasion.
No full text1 Understanding why some alien plant species become invasive when others fail is a fundamental goal in invasion ecology. We used detailed historical planting records of alien plant species introduced to Amani Botanical Garden, Tanzania and contemporary surveys of their invasion status to assess the relative ability of phylogeny, propagule pressure, residence time, plant traits and other factors to explain the success of alien plant species at different stages of the invasion process. 2 Species with native ranges centred in the tropics and with larger seeds were more likely to regenerate, whereas naturalization success was explained by longer residence time, faster growth rate, fewer seeds per fruit, smaller seed mass and shade tolerance. 3 Naturalized species spreading greater distances from original plantings tended to have more seeds per fruit, whereas species dispersed by canopy-feeding animals and with native ranges centred on the tropics tended to have spread more widely in the botanical garden. Species dispersed by canopy-feeding animals and with greater seed mass were more likely to be established in closed forest. 4 Phylogeny alone made a relatively minor contribution to the explanatory power of statistical models, but a greater proportion of variation in spread within the botanical garden and in forest establishment was explained by phylogeny alone than for other models. Phylogeny jointly with variables also explained a greater proportion of variation in forest establishment than in other models. Phylogenetic correction weakened the importance of dispersal syndrome in explaining compartmental spread, seed mass in the forest establishment model, and all factors except for growth rate and residence time in the naturalization model. 5 Synthesis. This study demonstrates that it matters considerably how invasive species are defined when trying to understand the relative ability of multiple variables to explain invasion success. By disentangling different invasion stages and using relatively objective criteria to assess species status, this study highlights that relatively simple models can help to explain why some alien plants are able to naturalize, spread and even establish in closed tropical forests
Using INLA to fit a complex point process model with temporally varying effects – a case study
No full textIntegrated nested Laplace approximation (INLA) provides a fast and yet quite exact approach to fitting complex latent Gaussian models which comprise many statistical models in a Bayesian context, including log Gaussian Cox processes. This paper discusses how a joint log Gaussian Cox process model may be fitted to independent replicated point patterns. We illustrate the approach by fitting a model to data on the locations of muskoxen (Ovibos moschatus) herds in Zackenberg valley, Northeast Greenland and by detailing how this model is specified within the R-interface R-INLA. The paper strongly focuses on practical problems involved in the modelling process, including issues of spatial scale, edge effects and prior choices, and finishes with a discussion on models with varying boundary conditions.Peer reviewe
The Re-Establishment of Desiccation Tolerance in Germinated Arabidopsis thaliana Seeds and Its Associated Transcriptome
No full textThe combination of robust physiological models with “omics” studies holds promise for the discovery of genes and pathways linked to how organisms deal with drying. Here we used a transcriptomics approach in combination with an in vivo physiological model of re-establishment of desiccation tolerance (DT) in Arabidopsis thaliana seeds. We show that the incubation of desiccation sensitive (DS) germinated Arabidopsis seeds in a polyethylene glycol (PEG) solution re-induces the mechanisms necessary for expression of DT. Based on a SNP-tile array gene expression profile, our data indicates that the re-establishment of DT, in this system, is related to a programmed reversion from a metabolic active to a quiescent state similar to prior to germination. Our findings show that transcripts of germinated seeds after the PEG-treatment are dominated by those encoding LEA, seed storage and dormancy related proteins. On the other hand, a massive repression of genes belonging to many other classes such as photosynthesis, cell wall modification and energy metabolism occurs in parallel. Furthermore, comparison with a similar system for Medicago truncatula reveals a significant overlap between the two transcriptomes. Such overlap may highlight core mechanisms and key regulators of the trait DT. Taking into account the availability of the many genetic and molecular resources for Arabidopsis, the described system may prove useful for unraveling DT in higher plant
Aboveground biomass estimation in tropical forests at single tree level with ALS data
No full textIn this paper we present a study on the estimation of the aboveground biomass in tropical forests at single tree level using airborne laser scanning (ALS) data. Individual tree crowns (ITCs) are firstly detected using a method based on an adaptive window that change its size according to tree height. The diameter at breast height (DBH) and the aboveground biomass (AGB) of each ITC then are predicted using standard allometric models. Lastly, the AGB values are aggregated at plot level, and compared with field measured values. The results show that it is possible to accurately predict the aboveground biomass of tropical forests at single tree level using ALS dat
Pairwise interaction point processes for modelling bivariate spatial point patterns in the presence of interaction uncertainty
No full textCurrent ecological research seeks to understand the mechanisms that sustain biodiversity and allow a large number of species to coexist. Coexistence concerns inter-individual interactions. Consequently, there is an interest in identifying and quantifying interactions within and between species as reflected in the spatial pattern formed by the individuals. This study analyses the spatial pattern formed by the locations of plants in a community with high biodiversity from Western Australia. We fit a pairwise interaction Gibbs marked point process to the data using a Bayesian approach and quantify the inhibitory interactions within and between the two species. We quantitatively discriminate between competing models corresponding to different inter-specific and intraspecific interactions via posterior model probabilities. The analysis provides evidence that the intraspecific interactions for the two species of the genus Banksia are generally similar to those between the two species providing some evidence for mechanisms that sustain biodiversity.Peer reviewe
Variation in tropical forest growth rates: combined effects of functional group composition and resource availability
No full textRates of tree growth in tropical forests reflect variation in life history strategies, contribute to the determination of species' distributional limits, set limits to timber harvesting and control the carbon balance of the stands. Here, we review the resources that limit tree growth at different temporal and spatial scales, and the different growth rates and responses of functional groups defined on the basis of regeneration strategy, maximum size, and species' associations with particular edaphic and climatic conditions.Variation in soil water availability determines intra- and inter-annual patterns of growth within seasonal forests, whereas irradiance may have a more important role in aseasonal forests. Nutrient supply limits growth rates in montane forests and may determine spatial variation in growth of individual species in lowland forests. However, its role in determining spatial variation in stand-level growth rates is unclear. In terms of growth rate, we propose a functional classification of tropical tree species which contrasts inherently fast-growing, responsive species (pioneer, large-statured species), from slow-growing species that are less responsive to increasing resource availability (shade-bearers, small-statured species). In a semi-deciduous forest in Ghana, pioneers associated with high-rainfall forests with less fertile soils, had significantly lower growth rates than pioneers that are more abundant in low-rainfall forests with more fertile soils. These results match patterns found in seedling trials and suggest for pioneers that species' associations with particular environmental conditions are useful indicators of maximum growth rate.The effects of variation in resource availability and of inherent differences between species on stand-level patterns of growth will not be independent if the functional group composition of tropical forests varies along resource gradients. We find that there is increasing evidence of such spatial shifts at both small and large scales in tropical forests. Quantifying these gradients is important for understanding spatial patterns in forest growth rates
Consistent effects of disturbance and forest edges on the invasion of a continental rainforest by alien plants
No full textContinental tropical forests are thought to be resistant to alien plant invasion due to a lack of disturbance, or low propagule pressure from introduced species. We assessed the importance of disturbance and edge effects by surveying areas of submontane and lowland forest of Amani Nature Reserve in the East Usambara mountains, Tanzania. These areas are in the vicinity of Amani Botanic Garden (ABG)—a propagule source for many alien plant species. We surveyed three edges in the vicinity of the ABG plantations, using plots interspersed along multiple 250 m transects. Survey plots were either in secondary or seminatural forest, representing a difference in past disturbance). Alien plant species richness and abundance declined with increasing distance from forest edges, indicating that edge effects were important. In addition, the effect of distance on richness and abundance of alien species as adults was much smaller in seminatural than secondary forest, emphasizing that invasion of seminatural forest is less likely to occur. Abundance and occurrence of individual species showed broadly similar declines with increasing distance from the forest edge, and lower abundance in seminatural compared to secondary forest. Alien species were dominant in 15 percent of plots surveyed. As 28 percent of the Amani nature reserve forest is within 250 m of an edge, the importance of disturbance and edges could make a potentially large proportion of the forest vulnerable to alien species invasion
Topography shapes the structure, composition and function of tropical forest landscapes
No full textTopography is a key driver of tropical forest structure and composition, as it constrains local
nutrient and hydraulic conditions within which trees grow. Yet, we do not fully understand how
changes in forest physiognomy driven by topography impact other emergent properties of forests,
such as their aboveground carbon density (ACD). Working in Borneo – at a site where 70-m-tall
forests in alluvial valleys rapidly transition to stunted heath forests on nutrient-depleted dip slopes
– we combined field data with airborne laser scanning and hyperspectral imaging to characterise
how topography shapes the vertical structure, wood density, diversity and ACD of nearly 15 km2
of old-growth forest. We found that subtle differences in elevation – which control soil chemistry
and hydrology – profoundly influenced the structure, composition and diversity of the canopy.
Capturing these processes was critical to explaining landscape-scale heterogeneity in ACD, highlighting
how emerging remote sensing technologies can provide new insights into long-standing
ecological question
Phenological differences in tree water use and the timing of tropical forest inventories: conclusions from patterns of dry season diameter change
No full textInterspecific variation in water-induced fluctuations in stem girth demonstrates the mechanisms promoting coexistence in seasonally dry tropical forest. In addition, these fluctuations are a potential, but unevaluated, source of bias in measurements of annual tree growth rates. To examine diurnal and seasonal patterns of stem diameter change, tree girth was measured over 2 years (1997-1999), using dendrometer bands, for three species (Celtis mildbraedii, C. zenkeri and Strombosia glaucescens) in semi-deciduous forest in Ghana. Soil matric potential was measured concurrently at 15 cm depth. In addition, measurements of all trees >20 cm dbh on three, 1 ha plots were made at the beginning and middle of the 1998/1999 dry season. During the severe 1997/1998 dry season, soil matric potential declined below -1.5 MPa and two species showed significant stem shrinkage. For the evergreen species, C mildbraedii, there was a significant positive effect of tree diameter on stem shrinkage, and shrinkage was greater in the second, compared to the first, half of the dry season. For the deciduous species, C zenkeri, shrinkage was reduced during the second half of the dry season, following leaf fall. During 1998/1999, soil matric potential, did not decline below - 1.5 MPa, and rates of girth change remained positive for all species. There were no significant effects of size or phenology on the rate of girth change in the plot-based study. Deviations in annual increment calculated over successive monthly intervals indicate that a 10-fold difference in soil water availability between measurement occasions can lead to a 4% bias in estimates of annual growth. Measurements of forest plots should be made when inter-annual variation in soil water availability is low. In this forest, measurements should, therefore, be made during the wet season, contrary to published recommendations. (C) 2002 Elsevier Science B.V. All rights reserved
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