Deep-water access is arguably the most effective, but under-studied mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep-water access may delay plant dehydration. Here, we tested the role of deep-water access in enabling survival within a diverse tropical forest community in Panama using a novel data-model approach. •We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics to vapor pressure deficit, water potentials in the whole-soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water-access depths.• Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years , especially in evergreen species. Species exposure to water-stress declined with deeper ERD indicating that trees compensate for waterstress related mortality risk through deep-water access.• The role of deep-water access in mitigating mortality of hydraulically vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

Questions persist about interpreting isotope ratios of bound and mobile soil water pools, particularly relative to clay content and extraction conditions. Interactions between pools and resulting extracted water isotope composition are presumably related to soil texture, yet few studies have manipulated the bound pool to understand its influence on soil water processes. Using a series of drying and spiking experiments, we effectively labelled bound and mobile water pools in soils with varying clay content. Soils were first vacuum dried to remove residual water, which was then replaced with heavy isotope-enriched water prior to oven drying and spiking with heavy isotope-depleted water. Water was extracted via centrifugation or cryogenic vacuum distillation (at four temperatures) and analysed for oxygen and hydrogen isotope ratios via isotope ratio mass spectrometry. Water from centrifuged samples fell along a mixing line between the two added waters but was more enriched in heavy isotopes than the depleted label, demonstrating that despite oven drying, a residual pool remains and mixes with the mobile water. Soils with higher clay + silt content appeared to have a larger bound pool. Water from vacuum distillation samples have a significant temperature effect, with high temperature extractions yielding progressively more heavy isotope-enriched values, suggesting that Rayleigh fractionation occurred at low temperatures in the vacuum line. By distinctly labelling bound and mobile soil water pools, we detected interactions between the two that were dependent on soil texture. Although neither extraction method appeared to completely extract the combined bound and mobile (total water) pool, centrifugation and high temperature cryogenic vacuum distillations were comparable for both δ 2 H and δ 18 O of soil water isotope ratios. K E Y W O R D Sclay mineralogy, extraction, methods, soil texture, soil water, stable isotopes, two water world hypothesis

Tropical ecosystems offer a unique setting for understanding ecohydrological processes, but to date, such investigations have been limited. The purpose of this paper is to highlight the importance of studying these processes—specifically, how they are being affected by the transformative changes taking place in the tropics—and to offer an agenda for future research. At present, the ongoing loss of native ecosystems is largely due to agricultural expansion, but parallel processes of afforestation are also taking place, leading to shifts in ecohydrological fluxes. Similarly, shifts in water availability due to climate change will affect both water and carbon fluxes in tropical ecosystems. A number of methods exist that can help us better understand how changes in land use and climate affect ecohydrological processes; these include stable isotopes, remote sensing, and process‐based models. Still, our knowledge of the underlying physical mechanisms, especially those that determine the effects of scale on ecosystem processes, remains incomplete. We assert that development of a knowledge base concerning the effects of transformative change on ecological, hydrological, and biogeochemical processes at different spatio‐temporal scales is an urgent need for tropical regions and should serve as a compass for emerging ecohydrologists. To reach this goal, we advocate a research agenda that expands the number and diversity of ecosystems targeted for ecohydrological investigations and connects researchers across the tropics. We believe that the use of big data and open source software—already an important integrative tool/skill for the young ecohydrologist—will be key in expanding research capabilities.

We have characterized for the first time the complete primary structure of the main protamine components of the sperm from four reptiles: Chrysemys picta (turtle), Elaphe obsoleta (snake), Anolis carolinensis (lizard), and Alligator mississipiensis (crocodilian). These species were chosen to represent one of each of the main phylogenetic branches of this taxonomic group. Comparison of these protamine sequences with those already available from other vertebrate groups allows us to define properly the chemical consensus composition of protamines and provides a unique insight into their molecular evolution and classification.

Vegetation processes are fundamentally limited by nutrient and water availability, the uptake of which is mediated by plant roots in terrestrial ecosystems. While tropical forests play a central role in global water, carbon, and nutrient cycling, we know very little about tradeoffs and synergies in root traits that respond to resource scarcity. Tropical trees face a unique set of resource limitations, with rock-derived nutrients and moisture seasonality governing many ecosystem functions, and nutrient versus water availability often separated spatially and temporally. Root traits that characterize biomass, depth distributions, production and phenology, morphology, physiology, chemistry, and symbiotic relationships can be predictive of plants’ capacities to access and acquire nutrients and water, with links to aboveground processes like transpiration, wood productivity, and leaf phenology. In this review, we identify an emerging trend in the literature that tropical fine root biomass and production in surface soils are greatest in infertile or sufficiently moist soils. We also identify interesting paradoxes in tropical forest root responses to changing resources that merit further exploration. For example, specific root length, which typically increases under resource scarcity to expand the volume of soil explored, instead can increase with greater base cation availability, both across natural tropical forest gradients and in fertilization experiments. Also, nutrient additions, rather than reducing mycorrhizal colonization of fine roots as might be expected, increased colonization rates under scenarios of water scarcity in some forests. Efforts to include fine root traits and functions in vegetation models have grown more sophisticated over time, yet there is a disconnect between the emphasis in models characterizing nutrient and water uptake rates and carbon costs versus the emphasis in field experiments on measuring root biomass, production, and morphology in response to changes in resource availability. Closer integration of field and modeling efforts could connect mechanistic investigation of fine-root dynamics to ecosystem-scale understanding of nutrient and water cycling, allowing us to better predict tropical forest-climate feedbacks.

Stocking density, both current and past, is a major determinant of the nature and condition of rangelands. Despite this fact, there have been few detailed examinations of historical trends in stocking density. We used data from the U.S. Census of Agriculture to track the density of domestic livestock from the early 1900s to the present, for six rangeland regions in the State of Texas: (1) the Edwards Plateau; (2) the Trans-Pecos; (3) the Lampasas Cut Plain; (4) the South Texas Plains; (5) the Rolling Plains; and (6) the High Plains. We find that stocking densities have declined across the state-ranging from a decline of about 40% in some regions to as much as 75% in the Trans-Pecos and Edwards Plateau regions. The period of sharpest decline, which began in the middle of the last century, reflects important, but not fully understood, socioeconomic changes. These most likely include changing land ownership, fragmentation of land holdings, and increasing emphasis on wildlife conservation. Other factors potentially contributing to the destocking of Texas rangelands include woody plant encroachment and a rise in predation. We argue that the dramatic reduction in stocking densities documented here has profound socioeconomic, ecological, and hydrological implications that need to be better understood. Resumen La densidad de carga en el paso y actual, es el principal determinante de la naturaleza y condición del agostadero. Después de éste hecho, ha habido pocas revisiones de la tendencia histórica y la densidad de carga en detalle. Usamos datos del Censo de Agricultura de los Estados Unidos para rastrear la densidad de animales domesticos desde inicios de los 1900s al presente, en seis regiones de pastizales en el estado de Texas. (1) La Altiplanicie Edwards, (2) La Región de Trans-Pecos, (3) La Planicie de Lampasas, (4) Las Planicies del Sur de Texas, (5) Las Planicies Onduladas y (6) Las Planicies Altas de Texas. Encontramos que las densidades de carga se han reducido a lo largo del estado-variando de una disminución del 40% en algunas regiones hasta un 75% en las regiones Trans-Pecos y La Planicie Edwards. El periodo de más reducción que inicio a mediados del siglo pasado refleja cambios socioeconómicos importantes que no están bien comprendidos. Los más probables son el cambio de propiedad, fragmentación de la tierra y el aumento en el interés de conservación de la fauna. Otros factores que potencialmente contribuyen en la descarga de los agostaderos de Texas incluyen, la invasión de plantas leñ osas y un aumento en los predadores. Discutimos que esta dramática reducción de la densidad de carga aquí documentada tiene profundas implicaciones socioeconómicas, ecológicas e hidrológicas que requieren ser entendidas.

In seasonally dry tropical forests, plant functional type can be classified as deciduous low wood density, deciduous high wood density, or evergreen high wood density species. While deciduousness is often associated with drought‐avoidance and low wood density is often associated with tissue water storage, the degree to which these functional types may correspond to diverging and unique water use strategies has not been extensively tested. We examined (a) tolerance to water stress, measured by predawn and mid‐day leaf water potential; (b) water use efficiency, measured via foliar δ13C; and (c) access to soil water, measured via stem water δ18O. We found that deciduous low wood density species maintain high leaf water potential and low water use efficiency. Deciduous high wood density species have lower leaf water potential and variable water use efficiency. Both groups rely on shallow soil water. Evergreen high wood density species have low leaf water potential, higher water use efficiency, and access alternative water sources. These findings indicate that deciduous low wood density species are drought avoiders, with a specialized strategy for storing root and stem water. Deciduous high wood density species are moderately drought tolerant, and evergreen high wood density species are the most drought tolerant group. Synthesis. Our results broadly support the plant functional type framework as a way to understand water use strategies, but also highlight species‐level differences.

With current observations and future projections of more intense and frequent droughts in the tropics, understanding the impact that extensive dry periods may have on tree and ecosystem-level transpiration and concurrent carbon uptake has become increasingly important. Here, we investigate paired soil and tree water extraction dynamics in an old-growth upland forest in central Amazonia during the 2018 dry season. Tree water use was assessed via radial patterns of sap flow in eight dominant canopy trees, each a different species with a range in diameter, height, and wood density. Paired multi-sensor soil moisture probes used to quantify volumetric water content dynamics and soil water extraction within the upper 100 cm were installed adjacent to six of those trees. To link depth-specific water extraction patterns to root distribution, fine root biomass was assessed through the soil profile to 235 cm. To scale tree water use to the plot level (stand transpiration), basal area was measured for all trees within a 5 m radius around each soil moisture probe. The sensitivity of tree transpiration to reduced precipitation varied by tree, with some increasing and some decreasing in water use during the dry period. Tree-level water use scaled with sapwood area, from 11 to 190 L per day. Stand level water use, based on multiple plots encompassing sap flow and adjacent trees, varied from ∼1.7 to 3.3 mm per day, increasing linearly with plot basal area. Soil water extraction was dependent on root biomass, which was dense at the surface (i.e., 45% in the upper 5 cm) and declined dramatically with depth. As the dry season progressed and the upper soil dried, soil water extraction shifted to deeper levels and model projections suggest that much of the water used during the month-long dry-down could be extracted from the upper 2–3 m. Results indicate variation in rates of soil water extraction across the research area and, temporally, through the soil profile. These results provide key information on whole-tree contributions to transpiration by canopy trees as water availability changes. In addition, information on simultaneous stand level dynamics of soil water extraction that can inform mechanistic models that project tropical forest response to drought.