Extreme Droughts Push Heterotrophic Functions Above Baseline Levels in a Neotropical Ecosystem.

Abstract

Droughts are intensifying in the humid Neotropics, raising concerns about the impacts on ecosystem processes related to C cycling, such as decomposition and CO₂ respiration. In particular, the resilience of multiple functions to extreme droughts in Neotropical aquatic systems remains poorly understood, limiting our ability to predict drought-driven feedbacks on C cycling. Here, we used rain shelters placed above tank bromeliads, plants that hold small freshwater ecosystems within their leaf axils, to emulate drought events ranging from the current norm to different IPCC scenarios. We then quantified the resilience of three key ecosystem functions (microbial respiration, litter decomposition, and photosynthetic efficiency) during a post-drought rewetting phase of 60 days. To assess the role of biotic recolonization during rewetting, we used mosquito nets over half of the bromeliads to prevent macroinvertebrates from recolonizing bromeliads from adjacent source patches. We found that extreme droughts (94 days) pushed heterotrophic functions above baseline levels during the rewetting phase. Microbial respiration and litter decomposition increased during this rewetting period, relative to undisturbed bromeliads. This boost was even faster when macroinvertebrate recolonization was allowed. Structural equation models suggested that nutrient release from dead organic matter during the rewetting phase, along with changes in bacterial density and shredder biomass, drove the positive shifts in heterotrophic functions and ecosystem multifunctionality. Extreme droughts accelerated C processing in tank bromeliads, particularly when external recolonization occurred, releasing a noticeable amount of carbon to the atmosphere. Our study shed light on the mechanisms underlying post-drought ecosystem multifunctionality trajectory and its link with C cycling, encouraging future works considering these small but abundant water bodies as sources of C in the Neotropics in the face of drought intensification.