Edge effects in a three-dimensional world: height in the canopy modulates edge effects on the epiphyte Sarmienta repens (Gesneriaceae)

In fragmented landscapes, the sharp transitions between remnant habitats and those created by human disturbances result in “edge effects”. Changes in microhabitat characteristics (for example, the height at which epiphytic plants grow on their host trees) may, however, modulate these effects. We evaluated how distance to the edge and height on the host tree affect seed germination and adult occurrence of the austral-forest epiphyte Sarmienta repens. The negative effect of edges on germination and adult occurrence was modulated by a vertical gradient (height) with contrasting effects on seed germination and survival to adulthood. Seed germination was affected by both height and distance to the forest edge, increasing away from forest edges for plants growing on trees in mid-story (3–6 m) and canopy zones (6–12 m) but being unaffected by the distance to the edge for plants located in understory zones (<3 m above the ground). Adult occurrence, in contrast, increased with both height and distance to the edge. As a result, seeds deposited near edges germinated best at a microhabitat (understory zone) inadequate for subsequent survival. Our results stress the importance of maintaining intact forest areas, free from edge influences and hosting large trees, to preserve the rich epiphytic flora characteristic of humid forests.