Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the diversity of hydraulic traits (including ...variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of individual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among individuals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (Amax), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.
Abstract
Tropical ecosystems adapted to high water availability may be highly impacted by climatic changes that increase soil and atmospheric moisture deficits. Many tropical regions are experiencing ...significant changes in climatic conditions, which may induce strong shifts in taxonomic, functional and phylogenetic diversity of forest communities. However, it remains unclear if and to what extent tropical forests are shifting in these facets of diversity along climatic gradients in response to climate change. Here, we show that changes in climate affected all three facets of diversity in West Africa in recent decades. Taxonomic and functional diversity increased in wetter forests but tended to decrease in forests with drier climate. Phylogenetic diversity showed a large decrease along a wet-dry climatic gradient. Notably, we find that all three facets of diversity tended to be higher in wetter forests. Drier forests showed functional, taxonomic and phylogenetic homogenization. Understanding how different facets of diversity respond to a changing environment across climatic gradients is essential for effective long-term conservation of tropical forest ecosystems.
The future of tropical forests under global environmental change is uncertain, with biodiversity and carbon stocks at risk if precipitation regimes alter. Here, we assess changes in plant functional ...composition and biomass in 19 plots from a variety of forest types during two decades of long‐term drought in Ghana. We find a consistent increase in dry forest, deciduous, canopy species with intermediate light demand and a concomitant decrease in wet forest, evergreen, sub‐canopy and shade‐tolerant species. These changes in composition are accompanied by an increase in above‐ground biomass. Our results indicate that by altering composition in favour of drought‐tolerant species, the biomass stocks of these forests may be more resilient to longer term drought than short‐term studies of severe individual droughts suggest.
In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns in the Atlantic forest, Brazil, and assessed whether and why patterns may vary ...among species. We found large leaf‐to‐air temperature differences that were influenced strongly by radiation and differences in leaf temperature between 2 species due to variation in leaf width and stomatal conductance. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.
Given anticipated climate changes, it is crucial to understand controls on leaf temperatures including variation between species in diverse ecosystems. In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns on 3 tree species in the Atlantic forest, Brazil, over a 10‐day period and assessed whether and why patterns may vary among species. We found large leaf‐to‐air temperature differences (maximum 18.3 °C) and high leaf temperatures (over 35 °C) despite much lower air temperatures (maximum 22 °C). Leaf‐to‐air temperature differences were influenced strongly by radiation, whereas leaf temperatures were also influenced by air temperature. Leaf energy balance modelling informed by our measurements showed that observed differences in leaf temperature between 2 species were due to variation in leaf width and stomatal conductance. The results suggest a trade‐off between water use and leaf thermoregulation; Miconia cabussu has more conservative water use compared with Alchornea triplinervia due to lower transpiration under high vapour pressure deficit, with the consequence of higher leaf temperatures under thermal stress conditions. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.
In recent years, LiDAR technology has provided accurate forest aboveground biomass (AGB) maps in several forest ecosystems, including tropical forests. However, its ability to accurately map forest ...AGB changes in high-biomass tropical forests has seldom been investigated. Here, we assess the ability of repeated LiDAR acquisitions to map AGB stocks and changes in an old-growth Neotropical forest of French Guiana. Using two similar aerial small-footprint LiDAR campaigns over a four year interval, spanning ca. 20km2, and concomitant ground sampling, we constructed a model relating median canopy height and AGB at a 0.25-ha and 1-ha resolution. This model had an error of 14% at a 1-ha resolution (RSE=54.7Mgha−1) and of 23% at a 0.25-ha resolution (RSE=86.5Mgha−1). This uncertainty is comparable with values previously reported in other tropical forests and confirms that aerial LiDAR is an efficient technology for AGB mapping in high-biomass tropical forests. Our map predicts a mean AGB of 340Mgha−1 within the landscape. We also created an AGB change map, and compared it with ground-based AGB change estimates. The correlation was weak but significant only at the 0.25-ha resolution. One interpretation is that large natural tree-fall gaps that drive AGB changes in a naturally regenerating forest can be picked up at fine spatial scale but are veiled at coarser spatial resolution. Overall, both field-based and LiDAR-based estimates did not reveal a detectable increase in AGB stock over the study period, a trend observed in almost all forest types of our study area. Small footprint LiDAR is a powerful tool to dissect the fine-scale variability of AGB and to detect the main ecological controls underpinning forest biomass variability both in space and time.
•Repeated LiDAR maps are compared with long-term tropical forest inventories.•LiDAR accurately predicts AGB stocks in space even in a high-biomass forest.•Forest dynamics is seen differently from a ground or a LiDAR perspective.
Climatic changes have profound effects on the distribution of biodiversity, but untangling the links between climatic change and ecosystem functioning is challenging, particularly in high diversity ...systems such as tropical forests. Tropical forests may also show different responses to a changing climate, with baseline climatic conditions potentially inducing differences in the strength and timing of responses to droughts. Trait‐based approaches provide an opportunity to link functional composition, ecosystem function and environmental changes. We demonstrate the power of such approaches by presenting a novel analysis of long‐term responses of different tropical forest to climatic changes along a rainfall gradient. We explore how key ecosystem's biogeochemical properties have shifted over time as a consequence of multi‐decadal drying. Notably, we find that drier tropical forests have increased their deciduous species abundance and generally changed more functionally than forests growing in wetter conditions, suggesting an enhanced ability to adapt ecologically to a drying environment.
•Leaf-air temperature difference decreases with increasing environmental temperature.•Hot-dry habitat increases transpirational cooling, decreases leaf warming.•Plants minimize transpirational ...cooling, facilitate leaf warming in cold forest.•Leaf area and water content play significant roles in physical thermal regulation.
The ecophysiological processes of leaves are more related to leaf temperature (Tl) than air temperature (Ta). Transpiration and leaf physical traits enable plants to maintain Tl within a thermal range. However, it is challenging to quantitatively study leaf thermal regulation strategies, due to the complex interaction between thermal effects of transpiration and leaf physical traits. We utilized a 3-T method that compares Tl, Ta, and Tn (the temperature of non-transpiring leaves) investigate thermal regulation strategies of dominant canopy species in four vegetation types, including a savanna woodland, a tropical rain forest, a subtropical evergreen broad-leaved forest, and a temperate mixed forest. Our results indicate that the difference between Tl and Ta decreased as the site mean temperature increased. Transpirational cooling was strongest in savanna woodland, and decreased from the hottest site to the coldest site. Without transpiration, sun-exposed leaves were consistently hotter under sunshine than air. This physical warming effect increased from the hottest site to the coldest site. We observed leaf area, water content and leaf angle played a significant role in physical thermal regulation. The present research quantitatively measured leaf thermal regulation strategies across a temperature and precipitation gradient, which advances our understanding of how plants adapt to their thermal environments.
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Abstract
Land regions are warming rapidly. While in a warming world at extra-tropical latitudes vegetation adapted to higher temperatures may move in from lower latitudes this is not possible in the ...tropics. Thus, the limits of plant functioning will determine the nature and composition of future vegetation. The most temperature sensitive component of photosynthesis is photosystem II. Here we report the thermal safety margin (difference between photosystem II thermotolerance (T
50
) and maximum leaf temperature) during the beginning of the dry season for four tree species co-occurring across the forest-savanna transition zone in Brazil, a region which has warmed particularly rapidly over the recent decades. The species selected are evergreen in forests but deciduous in savannas. We find that thermotolerance declines with growth temperature >40 °C for individuals in the savannas. Current maximum leaf temperatures exceed T
50
in some species and will exceed T
50
in a 2.5 °C warmer world in most species evaluated. Despite plasticity in leaf thermal traits to increase leaf cooling in hotter environments, the results show this is not sufficient to maintain a safe thermal safety margin in hotter savannas. Overall, the results suggest that tropical forests may become increasingly deciduous and savanna-like in the future.
Empirical evidence and modelling both suggest that global changes may lead to an increased dominance of lianas and thus to an increased prevalence of liana‐infested forest formations in tropical ...forests. The implications for tropical forest structure and the carbon cycle remain poorly understood. We studied the ecological processes underpinning the structure and dynamics of a liana‐infested forest in French Guiana, using a combination of long‐term surveys (tree, liana, seedling and litterfall), soil chemical analyses and remote‐sensing approaches (LiDAR and Landsat). At stand scale and for adult trees, the liana‐infested forest had higher growth, recruitment and mortality rates than the neighbouring high‐canopy forest. Both total seedling density and tree seedling recruitment were lower in the liana‐infested forest. Stand scale above‐ground biomass of the liana‐infested forest was 58% lower than in the high‐canopy forest. Above‐ground net primary productivity (ANPP) was comparable in the liana‐infested and high‐canopy forests. However, due to more abundant leaf production, the relative contribution of fast turnover carbon pools to ANPP was larger in the liana‐infested forest and the carbon residence time was half that of the high‐canopy forest. Although soils of the liana‐infested forest were richer in nutrients, soil elemental ratios suggest that liana‐infested forest and high‐canopy forest soils both derive from the same geological substrate. The higher nutrient concentration in the liana‐infested forest may therefore be the result of a release of nutrients from vegetation after a forest blowdown. Using small‐footprint LiDAR campaigns, we show that the overall extent of the liana‐infested forest has remained stable from 2007 to 2012 but about 10% of the forest area changed in forest cover type. Landsat optical imagery confirms the liana‐infested forest presence in the landscape for at least 25 years. Synthesis. Because persistently high rates of liana infestation are maintained by the fast dynamics of the liana‐infested forest, liana‐infested forests here appear to be the result of an arrested tropical forest succession. If the prevalence of such arrested succession forests were to increase in the future, this would have important implications for the carbon sink potential of Amazonian forests.
Over the last 30 yr, our understanding of the factors that control the structure and function of coastal wetlands has shifted from a narrative that focused on bottom‐up control to one that explicitly ...includes top‐down factors. However, the emphasis of this transition has been on how consumers influence wetland vegetation; comparatively, few studies evaluate how consumers influence broader ecosystem properties and functions such as nutrient cycling and decomposition while fewer still investigate how trophic interactions affect these relationships. Here, we review the literature on consumer impacts on coastal wetland ecosystems and highlight areas where additional studies are sorely needed. It is our hope that identifying the current gaps in our knowledge will provide additional impetus for the study of community and ecosystem ecology within these globally important ecosystems.
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