Life in the treetops: Ecophysiological strategies in canopy epiphyte communities along a microclimate gradient in the tropical montane cloud forest
Tropical montane cloud forests (TMCFs) are rich in biodiversity, and they play important roles in local and regional water cycles. Canopy plants such as epiphytes and hemiepiphytes are an important component of the biodiversity in the TMCF and play a significant role in the carbon, nutrient, and water cycles. With only partial or no access to resources on the ground, canopy plants may be vulnerable to changes in climate that increase canopy temperatures and decrease atmospheric humidity or precipitation inputs. Despite their importance in the TMCF, there is little information regarding drought tolerance and water balance in this community. We quantified variation in microclimate, functional traits, and water relations in common species of epiphytes and hemiepiphytes across a precipitation gradient in a Costa Rican TMCF. We also determined the capacity for foliar water uptake (FWU) in the laboratory and analyzed leaf cross-sections to quantify variation in cuticle thickness, leaf thickness and hydrenchymal thickness across the sites. In three sites, we also measured whole-plant transpiration.
Functional traits, water relations, and microclimate varied significantly across sites. We predicted that allocation to hydrenchymal thickness would be greatest at the lowest elevation (i.e., the driest site) but instead found that water storage was greatest in mid-elevation sites. This may be in response to variability in microclimate at middle elevations. As expected, specific leaf area was highest in wetter sites and lowest in dry sites. Other functional traits varied across the gradient, but there was no clear pattern across the sites. We found stronger and more significant trait correlations in the drier sites than the wetter sites, indicating that there are fewer adaptive ecophysiological strategies in drier sites. All species studied had the capacity for foliar uptake. Variation in this trait was negatively correlated with hydrenchymal thickness and cuticle thickness. In general, we found a trade-off between traits that confer relative drought resistance and foliar water uptake capacity versus traits that confer leaf capacitance and relative drought avoidance. This trade-off may represent an additional axis of the leaf economics spectrum that is unique to epiphytes. Our results indicate that canopy plants that have a greater ability for FWU may be vulnerable to projected changes in climate including changes in the overall precipitation patterns or increase in cloud base heights.