Tree longevity drives conservation value of shade coffee farms for vascular epiphytes

Abstract

Forest loss and fragmentation threaten the high diversity of tropical forests. Tropical epiphytes are a key component of plant diversity and significant for ecosystem functioning, and they are vulnerable to these forces. We explored the potential of shade trees in agroforestry systems to sustain and restore epiphyte communities where forest cover has been lost. We investigated epiphyte colonization patterns using surveys of 748 trees within 87 variously-aged plots in coffee agroecosystems in northern Nicaragua. Species richness increased with stand age, reflecting effects of area, time, and the niche differentiation possible within larger trees. Young trees near mature trees were almost five times more likely to support epiphytes than those surrounded by other young trees, suggesting that propagule sources severely limit colonization in young, even-aged stands. We identified three stages of epiphyte colonization: an initial stage where trees lack epiphytes; a colonization stage where epiphyte abundance increases slowly; and a mature stage where accumulation accelerates as trees become large enough to support distinct microhabitats. Atmospheric bromeliads and several fern species with traits that facilitate dispersal, colonization, and persistence under open, sunny environmental conditions colonized young trees first. Thus, both environmental filtering and dispersal limitation drive community assembly in these agroforests. Because epiphytes colonize gradually, consistent land tenure and long-term management allow time for newly planted trees in shade coffee farms to develop to the size and structural complexity needed to support diverse epiphyte communities. Because ideal conditions for epiphytes may be at odds with managing for maximum coffee yields and some epiphyte species may never colonize coffee farms, even mature shade trees in coffee plantations cannot replace protecting native forests.