Seed production in Polylepis australis (Rosaceae) as influenced by tree size, livestock and interannual climate variations in the mountains of central Argentina

Abstract

The expansion of tree species within a grassland-woodland matrix relies heavily on an abundant seed crop, which may be affected by many factors; among these, browsing by livestock has gone largely ignored. Here we explore how seed production is affected by tree size, grazing by large domestic herbivores and interannual climate fluc-tuations in a small-sized tree (Polylepis australis, Rosaceae), which dominates the remnant forests in the high mountains of central Argentina. Seed number and seed mass per tree were quantified in 100 different-sized trees located in areas with and without livestock. Using seed traps we also determined seed rain variation in 15 plots between 2005 and 2008. The estimated number of seeds per tree ranged from 0 to 224,000 and increased with tree height (power relation); for trees smaller than 354 cm in height, seed production was lower in areas with livestock and higher in areas without livestock. Mass per seed increased with tree height, with no significant livestock effect. The largest seed rain was recorded in 2008, probably due to growth of seed trees and not in response to climate. To promote seed production and forest expansion, livestock should be excluded from areas with small trees, such as forest edges.