Seasonal variation in soil and plant water potentials in a Bolivian tropical moist and dry forest

We determined seasonal variation in soil matric potentials (¿soil) along a topographical gradient and with soil depth in a Bolivian tropical dry (1160 mm y-1 rain) and moist forest (1580 mm y-1). In each forest we analysed the effect of drought on predawn leaf water potentials (¿pd) and drought response (midday leaf water potential at a standardized ¿pd of -0.98 MPa; ¿md) of saplings of three tree species, varying in shade-tolerance and leaf phenology. ¿soil changed during the dry season and most extreme in the dry forest. Crests were drier than slopes and valleys. Dry-forest top soil was drier than deep soil in the dry season, the inverse was found in the wet season. In the moist forest the drought-deciduous species, Sweetia fruticosa, occupied dry sites. In the dry forest the short-lived pioneer, Solanum riparium, occupied wet sites and the shade-tolerant species, Acosmium cardenasii drier sites. Moist-forest species had similar drought response. The dry-forest pioneer showed a larger drought response than the other two species. Heterogeneity in soil water availability and interspecific differences in moisture requirements and drought response suggest great potential for niche differentiation. Species may coexist at different topographical locations, by extracting water from different soil layers and/or by doing so at different moments in tim

Drought tolerance of tropical tree species : functional traits, trade-offs and species distribution

KEY-WORDS: Bolivia, drought tolerance, shade tolerance, functional traits, trade-offs, ecophysiology, species distribution Tropical forests occur under rainfall regimes that vary greatly in the rainfall pattern and frequency and intensity of drought. Consequently water availability is one of the most important environmental factors influencing community structure, species composition, and plant functioning across large-scale rainfall gradients and small-scale topographic gradients within forests. The relative success of tree species to establish along these gradients of water availability and their success in dealing with future changes in water availability will depend on how they are adapted to tolerate drought. In this dissertation I applied a multi-species, multi-trait approach in field studies and a controlled experiment to give detailed information on the mechanisms of drought-tolerance of a large set of tropical dry and moist forest tree species. The following research questions were addressed; 1) How do dry and moist forests differ in soil water availability? 2) How are dry and moist forest species adapted to drought and what different drought-strategies can be distinguished? 3) Is there a trade-off between drought- and shade-tolerance? and 4) How do drought- and shade-tolerance determine local and regional tree species distribution? Dry season soil water availability is clearly lower in the dry forest than in the moist forest. Especially in the dry forest there is a lot of temporal and spatial variation in soil water availability. Temporal variation depends on the annual cycle of precipitation. Spatial heterogeneity is two-dimensional; 1) water availability varies with topography of the landscape; elevated crests are dry in comparison to slopes and low valleys, and 2) soil water is vertically redistributed with soil depth; in the dry season more water is available in deep soil layers while in the wet season most water is found in the top soil. When combining temporal and spatial dimensions, a complex mosaic of soil water availability emerges that shows great potential for niche partitioning among species at various levels, if species are adapted to exploit this variation. Seedlings of dry forest species have evolved mechanisms that enhance their access to water in deep soil layers, increase drought-induced cavitation resistance and increase water conservation. Seedlings of moist forest species show adaptations that improve their light foraging capacity and increase nutrient and water acquisition. Associations among functional traits show that there are three major drought strategies among tropical tree species, 1) physiological drought-tolerance, 2) drought-intolerance and 3) drought-avoidance. No conclusive evidence for a direct trade-off between species drought- and shade-tolerance was found, and the association between drought- and shade-tolerance is mainly subject to the scale of observation. On small scales, within the dry forest, drought- and shade-tolerance are positively related, as species hydraulic properties are integrally linked with niche differentiation for both light and water. This implies that in their distribution, light-demanding species will be restricted to habitats that combine high light and high moisture availability, while shade-tolerant species will be the better competitors in drier and shadier habitats. On larger scales a strong trade-off between above and belowground biomass allocation was found, which should in theory have resulted in a trade-off between drought- and shade-tolerance, but in practice it did not. Plants can compensate for a low root mass fraction by producing relatively cheap roots with a large specific root length and compensate for a low leaf mass fraction by making cheap leaves with a large specific leaf area. Drought- and shade-tolerance thus depend largely on different suites of morphological traits and can be uncoupled. Species distribution along the rainfall gradient was not directly explained by species drought survival, mainly because deciduousness was the most important factor contributing to survival and deciduous species are well represented in both dry and moist forests. The occurrence of evergreen species at the dry end of the rainfall gradient largely depends on drought related traits as a high wood density and a large biomass allocation to deep roots. Species occurrence at the moist end of the rainfall gradient was mainly determined by traits related to light-demand, as a high leaf mass fraction and long, branched root systems. In conclusion, I propose that at small scales, within forests, species distribution along water gradients depends on the interaction between species drought- and shade-tolerance while at larger scales distribution of (evergreen) species is mainly determined by their drought-tolerance. <br/

Non-linear effects of drought under shade: reconciling physiological and ecological models in plant communities

The combined effects of shade and drought on plant performance and the implications for species interactions are highly debated in plant ecology. Empirical evidence for positive and negative effects of shade on the performance of plants under dry conditions supports two contrasting theoretical models about the role of shade under dry conditions: the trade-off and the facilitation hypotheses. We performed a meta-analysis of field and greenhouse studies evaluating the effects of drought at two or more irradiance levels on nine response variables describing plant physiological condition, growth, and survival. We explored differences in plant response across plant functional types, ecosystem types and methodological approaches. The data were best fit using quadratic models indicating a humped-back shape response to drought along an irradiance gradient for survival, whole plant biomass, maximum photosynthetic capacity, stomatal conductance and maximal photochemical efficiency. Drought effects were ameliorated at intermediate irradiance, becoming more severe at higher or lower light levels. This general pattern was maintained when controlling for potential variations in the strength of the drought treatment among light levels. Our quantitative meta-analysis indicates that dense shade ameliorates drought especially among drought-intolerant and shade-tolerant species. Wet tropical species showed larger negative effects of drought with increasing irradiance than semiarid and cold temperate species. Non-linear responses to irradiance were stronger under field conditions than under controlled greenhouse conditions. Non-linear responses to drought along the irradiance gradient reconciliate opposing views in plant ecology, indicating that facilitation is more likely within certain range of environmental conditions, fading under deep shade, especially for drought-tolerant species

TRY plant trait database - enhanced coverage and open access

Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives

Development of a compounded propofol nanoemulsion using multiple non-invasive process analytical technologies

Propofol is the preferred anaesthetic for induction and maintenance of sedation in critically ill mechanically ventilated COVID-19 patients. However, during the outbreak of the COVID-19 pandemic, regular supply chains could not keep up with the sudden increase in global demand, causing drug shortages. Propofol is formulated as an oil-in-water emulsion which is administered intravenously. This study explores the extemporaneous preparation of a propofol emulsion without specialized manufacturing equipment to temporally alleviate such shortages. A commercially available lipid emulsion (IVLE, SMOFlipid 20 %), intended for parenteral nutrition, was used to create a propofol loaded nanoemulsion via addition of liquid propofol drug substance and subsequent mixing. Critical quality attributes such as mean droplet size and the volume-weighted percentage of large-diameter (>5µm) droplets were studied. The evolution of droplet size and propofol distribution was monitored in situ and non-destructively, maintaining sterility, using Spatially Resolved Dynamic Light Scattering and Near Infrared Spectroscopy, respectively. Using response surface methodology, an optimum was found for a 4 % w/v propofol formulation with a ∼15 min mixing time in a flask shaker at a 40° shaking angle. This study shows that extemporaneous compounding is a viable option for emergency supply of propofol drug product during global drug shortages

Do differences in understory light contribute to species distributions along a tropical rainfall gradient?

In tropical forests, regional differences in annual rainfall correlate with differences in plant species composition. Although water availability is clearly one factor determining species distribution, other environmental variables that covary with rainfall may contribute to distributions. One such variable is light availability in the understory, which decreases towards wetter forests due to differences in canopy density and phenology. We established common garden experiments in three sites along a rainfall gradient across the Isthmus of Panama in order to measure the differences in understory light availability, and to evaluate their influence on the performance of 24 shade-tolerant species with contrasting distributions. Within sites, the effect of understory light availability on species performance depended strongly on water availability. When water was not limiting, either naturally in the wetter site or through water supplementation in drier sites, seedling performance improved at higher light. In contrast, when water was limiting at the drier sites, seedling performance was reduced at higher light, presumably due to an increase in water stress that affected mostly wet-distribution species. Although wetter forest understories were on average darker, wet-distribution species were not more shade-tolerant than dry-distribution species. Instead, wet-distribution species had higher absolute growth rates and, when water was not limiting, were better able to take advantage of small increases in light than dry-distribution species. Our results suggest that in wet forests the ability to grow fast during temporary increases in light may be a key trait for successful recruitment. The slower growth rates of the dry-distribution species, possibly due to trade-offs associated with greater drought tolerance, may exclude these species from wetter forests