OOS 20-4 - Drought stress decreases water interception capacity of two arboreal epiphytes with differing ecohydrological traits

Arboreal epiphytes influence canopy hydrology by increasing rainfall interception and storage, reducing throughfall, and increasing evaporation rates. However, since precipitation is expected to decrease in many places due to climate change, the hydrological role of epiphytes may depend on their response to drought conditions. Plant functional traits that influence rainfall interception, including leaf hydrophobicity may shift with drought. While the effects of drought on plants is well documented, the potential feedbacks of drought response on canopy hydrology have not been well investigated. We tested the effects of drought on the interception capacity of two epiphytes, resurrection fern (Pleopeltis polypodioides) and Spanish moss (Tillandsia usneoides), from maritime live oak (Quercus virginiana) forests of Georgia, U.S.A. We simulated drought-induced dehydration and measured interception capacity and drought response of epiphyte leaves at 100% hydration and dehydration to 75%, 50%, and ~25% of fresh weight. We measured interception capacity as mass changes of leaves, using a misting system in fog boxes. We examined the effect of dehydration on minimum leaf conductance (gmin) and leaf hydrophobicity, which affect canopy water exchange. Drought significantly reduced interception capacity of both species on a per trial basis. Hydrophobicity was greater in dehydrated leaves of both species, indicating that drought conditions may reduce canopy moisture through shedding of water droplets from leaves. The two species differed in their drought response. Tillandsia limits water loss under dehydration, as indicated by the lower minimum leaf conductance (gmin), a measure of the water loss under dehydration. Pleopeltis showed the opposite pattern with increasing gmin when dehydrated, perhaps because as “resurrection” plant it has an extraordinary ability to withstand water loss, rather than prevent it. Despite their differing drought tolerance strategies, the overall reduction in interception with dehydration did not differ between the two species. Our results suggest that increased drought would likely reduce the interception capacity of arboreal epiphytes in our Southeastern forests, with potentially large consequences for canopy hydrological cycling. Ultimately, the resilience of vital ecosystem services such as forest water cycling will depend on the ability of plants to maintain hydrological functions through the many effects of climate change, including altered precipitation patterns. Our results emphasize the importance of connecting foliar level drought response to hydrological processes such as rainfall interception. Connecting organismal processes with broader ecosystem functioning will create a more predictive understanding of potential feedbacks between climate change and ecosystem functioning.