In many parts of the world, climate change projections include increases in drought which may impact plant communities and alter ecosystem function. The Tropical Montane Cloud Forest (TMCF), which is characteristically moist, exists in compressed ecotones and may be particularly vulnerable to climate change. Epiphytes are an important component of the TMCF and are likely vulnerable to projected changes in precipitation since they are physically isolated from terrestrial soil resources. For this work, we took advantage of a record El Niño drought in 2016 to conduct a field-based sap flow study in the TMCF region of Costa Rica. We also conducted a common garden greenhouse experiment to test the resilience of these epiphytes to drought. In the field study, we instrumented plants from three sites bracketing the current cloud base height with sap flow sensors and measured a number of water relations parameters. In the greenhouse experiment, plants from a cloud forest and premontane rainforest site underwent a month-long drought under native temperature conditions, while a second set of cloud forest plants were subjected to an additional warming treatment. In this experiment we measured gas exchange, growth, phenology, and leaf thickness throughout the drought and during a recovery period.
Results/Conclusions
Despite the unprecedented nature of the 2016 dry season and the severe drought conditions imposed during the greenhouse experiment, most of the plants in the study survived, indicating drought resilience in epiphytes. While this outcome was similar across sites and plant groups, there were distinct strategies that were employed across sites and taxa that related to variation in microclimate. In the field, plants from the highest elevation site exhibited the lowest volumetric sap flow throughout the study, and these plants were most affected by the drought. In the middle and lower elevation sites, initial sap flow was higher, there was less of an effect of drought, and recovery of the initial sap flow rates was greater. Plants in lower elevations exhibited greater osmotic adjustment, which may have aided in the maintenance of water balance at these sites. In the greenhouse experiment, plants responded to the drought by decreasing gas exchange, removing water from leaf water stores, or by reducing leaf area, and these responses differed across functional groups. While the effects of the drought were substantial, nearly all treatment plants experienced a significant recovery of gas exchange, leaf thickness, and overall health during the recovery period after receiving ample water.