Community Hydraulics in a Tropical Montane Cloud Forest in Southeastern Brazil

Abstract:

Tropical montane cloud forests (TMCF) play important hydrological services as they contribute to the maintenance of local hydrology and water reservoirs. TMCF are currently under threat because climate models forecast an increase in the frequency and intensity of droughts associated with an uplift of cloud zone formation. Such droughts can directly impact the hydrological functioning of these ecosystems as they can trigger tree mortality and alter forest composition. Thus, understanding the drought sensitivity of key TMCF species is crucial to predict how these ecosystems will respond to future changes in water availability. We investigated the drought sensitivity of TMCF species in southeastern Brazil by analyzing a suite of hydraulic traits, stomatal control capabilities and the hydraulic safety margins of 16 dominant species, which represent more than 80% of the forest biomass. We found different stomatal regulation strategies that varied from isohydric (nine species) to anisohydric (seven species). The water potential inducing 50% loss of hydraulic conductance (P₅₀) varied from -1.5 to -4.9 MPa and it was negatively related to the degree of anisohydry and to leaf water potential variation, i.e., more resistant species controlled less their stomata. Most species showed a narrow hydraulic safety margin (P₅₀ - minimum xylem water potential), with a community average of 1.24 MPa (varying between 0.3 and 2.88 MPa). Species with denser wood had higher hydraulic safety margins. Our results suggest that more than 60% of the trees in this TMCF are very vulnerable to predicted droughts and if climate change decreases fog frequency in this region, we expect large-scale tree mortality and changes in vegetation composition. Considering the variation in species-specific tree hydraulic conductance, we discuss how the expected mortality of drought-sensitive trees will affect forest hydrology. The studied TMCF provide important hydrological services for many large cities in southeastern Brazil and changes affecting these forests will probably have significant economical and social impacts. Key words: drought, hydraulic safety margin, climate change, hydrological services