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  1. Abstract

    In tropical forests, both vegetation characteristics and soil properties are important not only for controlling energy, water, and gas exchanges directly but also determining the competition among species, successional dynamics, forest structure and composition. However, the joint effects of the two factors have received limited attention in Earth system model development. Here we use a vegetation demographic model, the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) implemented in the Energy Exascale Earth System Model (E3SM) Land Model (ELM), ELM‐FATES, to explore how plant traits and soil properties affect tropical forest growth and composition concurrently. A large ensemble of simulations with perturbed vegetation and soil hydrological parameters is conducted at the Barro Colorado Island, Panama. The simulations are compared against observed carbon, energy, and water fluxes. We find that soil hydrological parameters, particularly the scaling exponent of the soil retention curve (Bsw), play crucial roles in controlling forest diversity, with higherBswvalues (>7) favoring late successional species in competition, and lowerBswvalues (1 ∼ 7) promoting the coexistence of early and late successional plants. Considering the additional impact of soil properties resolves a systematic bias of FATES in simulating sensible/latent heat partitioning with repercussion on water budget and plant coexistence. A greater fractionmore »of deeper tree roots can help maintain the dry‐season soil moisture and plant gas exchange. As soil properties are as important as vegetation parameters in predicting tropical forest dynamics, more efforts are needed to improve parameterizations of soil functions and belowground processes and their interactions with aboveground vegetation dynamics.

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  2. Summary

    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 (1990–2015) 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 (1981–2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress‐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.

  3. Many tropical regions are experiencing an intensification of drought, with increasing severity and frequency of the events. However, the forest ecosystem response to these changes is still highly uncertain. It has been hypothesized that on short time scales (from diurnal to seasonal), tropical forests respond to water stress by physiological controls, such as stomata regulation and phenological adjustment, to control increasing atmospheric water demand and cope with reduced water supply. However, the interactions among biological processes and co-varying environmental factors that determine the ecosystem-level fluxes are still unclear. Furthermore, climate variability at longer time scales, such as that generated by ENSO, produces less predictable effects, which might vary among forests and ecoregions within the tropics. This study will present some emerging patterns of response to water stress from five years of observations of water, carbon, and energy fluxes on the seasonal tropical forest in Barro Colorado Island (Panama), including an increase in productivity during the 2015 El Niño. We will show how these responses will depend critically on the combination of environmental factors experienced by the forest along the seasonal cycle. These results suggest a critical role of plant hydraulics in mediating the response to water stress on a broadmore »range of temporal scales, including during the wet seasons when water availability is not a limiting factor. The study also found that the response to large-scale drought events is contingent and might produce a different outcome in different tropical forest areas.« less
  4. Although early theoretical work suggests that competition for light erodes successional diversity in forests, verbal models and recent numerical work with complex mechanistic forest simulators suggest that disturbance in such systems can maintain successional diversity. Nonetheless, if and how allocation tradeoffs between competitors interact with disturbance to maintain high diversity in successional systems remains poorly understood. Here, using mechanistic and analytically tractable models, we show that a theoretically unlimited number of coexisting species can be maintained by allocational tradeoffs such as investing in light-harvesting organs vs. height growth, investing in reproduction vs. growth or survival vs. growth. The models describe the successional dynamics of a forest composed of many patches subjected to random or periodic disturbance, and are consistent with physiologically mechanistic terrestrial ecosystem models, including the terrestrial components of recent Earth System Models. We show that coexistence arises in our models because species specialize in the successional time they best exploit the light environment and convert resources into seeds or contribute to advance regeneration. We also show that our results are relevant to non-forested ecosystems by demonstrating the emergence of similar dynamics in a mechanistic model of competition for light among annual plant species. Finally, we show that coexistencemore »in our models is relatively robust to the introduction of intraspecific variability that weakens the competitive hierarchy caused by asymmetric competition for light.« less