ENVIRONMENTAL DRIVERS OF VASCULAR EPIPHYTE ABUNDANCE, DENSITY, AND RICHNESS ALONG A HUMIDITY GRADIENT OF OLD-GROWTH ANDEAN MONTANE FOREST

Sammanfattning

This study investigates how host-tree size, vertical microhabitats, and local humidity levels affect vascular epiphytes. We measured epiphyte abundance, density, and species richness in 14 forest plots across three different humidity levels (high, medium, and low) to test how these community metrics vary with tree diameter (DBH), vertical stratification (Johansson Zones), and available humidity. Our results show that larger host trees hold a higher absolute abundance of epiphytes. However, we also found a clear "dilution effect" where epiphyte density decreases with increasing host-tree size. This could be explained by different mechanisms: tree bark surface area expands much faster than new plants can establish, causing a negative correlation between tree size and epiphyte density. Additionally, other factors like lower light penetration in larger tree canopies, as well as mechanical shedding, might be at play. Tree size alone did not predict species richness, and while plot humidity had a direct main effect on mean abundance, it also failed to affect richness. Vertically, however, epiphytes showed a highly stratified pattern. Abundance and richness consistently peaked in the mid-to-upper canopy and remained very low at tree-trunk base. This vertical distribution was statistically identical in both high- and low-humidity plots. These findings show that while humidity levels play a key role, internal forest structure and vertical microhabitats are more important in structuring canopy communities. Because large, mature trees hold the highest absolute number of epiphytes and the mid-canopy acts as a major diversity hotspot, conservation frameworks must prioritize protecting old-growth stands with large-diameter trees rather than relying on younger secondary forests.