OOS 20-2 - Can vascular epiphytes acclimate to a drier microclimate?: Results from a cloud exclusion experiment in a Peruvian Tropical Montane Cloud Forest

Tropical Montane Cloud Forests (TMCF) are characterized by frequent cloud immersion and host a rich community of canopy epiphytes. Epiphytes rely on atmospheric inputs of water, making them susceptible to reductions in cloud immersion. We used an experimental cloud reduction experiment in a Peruvian TMCF to examine the potential of five functional groups of vascular epiphytes (orchids, ferns, shrubs, bromeliads, and succulents) to acclimate to reduced cloud immersion via plasticity in morphological and physiological traits related to water use, water storage, and productivity. Complementary data for vascular and nonvascular epiphyte growth and dieback collected over a 3-year period provided additional context for epiphyte performance under reduced cloud immersion. We found that epiphytic functional groups varied in their ability to acclimate to cloud water reduction by trait plasticity. Stomatal length was significantly reduced in the cloud reduction treatment plot in the greatest number of functional groups. Succulents and shrubs reduced foliar water uptake capacity and minimal conductance (gmin) in the treatment plot. Orchids had no detectable change in physiology, while ferns increased gmin and reduced maximum efficiency of PSII photochemistry (Qy). Dieback of vascular epiphytes was greater in the treatment than control plot in 2017, but not in 2018 or 2019. Trait combinations and plasticity of functional groups reflect three different strategies to atmospheric drought stress: avoidance, tolerance, and intolerance. These strategies come with inherent trade-offs to either prioritize water storage or the foliar uptake of water. The cloud reduction experiment provided a novel opportunity to examine epiphyte vulnerability to projected changes in climate. Epiphytes’ dependence on atmospheric water and sensitivity to changes in microclimate make them important indicators of the alterations in TMCF ecosystem function under future climate scenarios. Understanding these impacts on TMCF ecosystems can inform management efforts to promote more climate-adapted and resilient ecosystems.