Will epiphyte loss exacerbate climate change effects in tropical montane cloud forests?

Epiphytes in tropical montane cloud forests (TMCFs) are vulnerable to decline and mortality as cloud base heights gradually rise, diminishing a vital source of moisture. Despite epiphytes’ key roles in TMCF ecosystems, few studies have assessed the potential effects of their loss on host tree health and hydrological processes. To investigate potential cascading effects, we conducted a replicated whole-tree epiphyte removal experiment in a TMCF near Monteverde, Costa Rica. The study consisted of ten pairs of trees where we removed all epiphytes (vascular and non-vascular) and arboreal soil mats from experimental trees, while leaving canopy communities intact in nearby control trees. Five pairs were in contiguous forest; the remaining five pairs consisted of trees in isolated pasture areas. Microclimate monitoring in multiple crown locations of study trees revealed that epiphyte removal was linked to increases in key drying variables such as solar radiation and wind speed. Surprisingly, epiphyte removal also contributed to wetting by allowing increased cloud and rain penetration into crown interiors. Our data also indicate that epiphyte loss increased the ratio of sensible to latent heat flux, which in our study was associated with a 0.48 increase in stripped tree Bowen ratios. Some effects of epiphyte removal were smaller than expected, which could be explained by the highly humid and wet atmosphere of TMCFs. Under future climatic conditions that are expected to be drier and hotter, we suggest that it is possible that effects of epiphyte loss will be more severe. Landscape-scale processes could also be influenced, as increased Bowen ratios represent shifts in surface energy partitioning and boundary-layer development that can further elevate the cloud base.