Enhancing the performance of native tree plantings in a seasonally dry mountain forest: the effects of elevation, irrigation and herbaceous cover

Abstract

Mountain forests have been extensively degraded by human activities, making their conservation and restoration urgent. In subtropical dry mountain forests, successful planting is conditioned by seasonal temperature and precipitation patterns, their variation with elevation, and interactions with co-occurring vegetation. We studied the effects of elevation (700 and 1,200 m asl), irrigation (with and without), and co-occurring herbaceous plant cover on the performance of the native Lithraea molleoides saplings. After 2 years, survival was significantly higher at 1,200 m (32%) than at 700 m (16%), whereas irrigation had no significant effect. Increased herbaceous plant cover significantly enhanced survival at low elevation (from 5 to 20%), but significantly reduced survival at high elevation (from 80 to 20%). Stem growth declined over 2 years due to aerial die-back in all treatments. Increased herbaceous cover significantly reduced stem growth across both elevation and irrigation treatments, with a stronger and significant effect at 1,200 (2.91 cm) than at 700 m asl (1.83 cm). Unexpectedly, at low elevations, freezing temperatures were lower, and the number of freezing days was greater than at high elevations, significantly hindering sapling performance. Our results suggest that freezing stress, rather than water deficit, appears to be a more limiting factor for L. molleoides saplings at low elevation. Microsite selection should consider herbaceous cover, which differentially affects survival and growth at each elevation.