The Pressure Is On - Epiphyte Water-Relations Altered Under Elevated CO₂

Vascular epiphytes are a major biomass component of forests across the globe and they contribute to 9% of global vascular plant diversity. To improve our understanding of the whole-plant response of epiphytes to future climate change, we investigated for the first time both individual and combined effects of elevated CO₂ (560 ppm) and light on the physiology and growth of two epiphyte species [Tillandsia brachycaulos (CAM) and Phlebodium aureum (C3)] grown for 272 days under controlled conditions. We found that under elevated CO₂ the difference in water loss between the light (650 μmol m^-2s^-1) and shade (130 μmol m^-2s^-1) treatment was strongly reduced. Stomatal conductance (g _s) decreased under elevated CO₂, resulting in an approximate 40-45% reduction in water loss over a 24 h day/night period under high light and high CO₂ conditions. Under lower light conditions water loss was reduced by approximately 20% for the CAM bromeliad under elevated CO₂ and increased by approximately 126% for the C3 fern. Diurnal changes in leaf turgor and water loss rates correlated strong positively under ambient CO₂ (400 ppm) and high light conditions. Future predicted increases in atmospheric CO₂ are likely to alter plant water-relations in epiphytes, thus reducing the canopy cooling potential of epiphytes to future increases in temperature.