Wind protection rather than soil water availability contributes to the restriction of high-mountain forest to ravines

Abstract

Tropical and subtropical high-mountain forests are often restricted to ravines, with forest presence being rare in other topographies such as ridges. We tested the hypothesis that this distribution is partly due to increased soil water content and/or protection from winds in ravines as compared to ridges. We planted 396 saplings of the dominant tree species in a three-factor experiment that included topography (ridge/ravine), wind protection (protected/unprotected) and water addition (watered/not watered). We monitored wind speed and soil water content for one complete annual cycle and sapling survival and growth for 27 months. Sapling survival presented marginally significant differences for topography, being slightly greater in ridges than in ravines (0.88 and 0.78, respectively), with no significant differences for wind protection or water addition treatments. By contrast, sapling growth was considerably reduced in ridges as compared to ravines (16.1 and 26.0 cm, respectively); the wind protection treatment showed a significant positive effect on growth (saplings grew 25.5 and 16.5 cm with and without wind protection, respectively), whereas the water addition treatment had no effect. Importantly, saplings under wind protection treatment in ridges had similar growth to that of unprotected saplings in ravines, and growth difference due to the wind protection treatment was positively correlated with wind intensity, suggesting that reduced wind impact in ravines is an important abiotic factor that promotes forest occurrence in ravines.