| Ranching and logging operations in eastern Amazonia are transforming mature forests into mosaics of frequently burned abandoned pastures and regrowth forests. Tree seedling establishment and, hence, forest regrowth proceed slowly in some abandoned pastures with histories of very heavy use. These "semi-permanent" ecosystems may persist for decades, fueling the fires that periodically set back forest recovery processes.;The limitations to tree seedling survival and growth were studied in a semi-permanent, abandoned pasture near Paragominas, Brazil. I compared seedling microenvironments in habitats of abandoned pasture and mature forest ecosystems and experimentally identified factors that limited seedling survival and growth in the abandoned pasture. Although seedling microenvironments during the wet season were similar in these ecosystems, surface soil water availability during the 5-month dry season was lower in the abandoned pasture than in treefall gaps or beneath the intact canopy of the mature forest. The relatively low survivorship and height growth of woody seedling transplants in the abandoned pasture vs. forest treefall gaps corresponded with relatively low xylem pressure potentials during rainless periods. Seedling height growth was also limited by root competition with pasture vegetation.;Seed supply, seed predation and seedling predation also restricted tree seedling establishment in the abandoned pasture. Although mostly small-seeded tree species were carried into the abandoned pasture from nearby forests, large-seeded tree species with drought-tolerant, deep-rooting seedlings that could sprout following shoot removal were the most likely to survive in this ecosystem. Islands of trees that emerged above abandoned pasture vegetation were important foci of tree seedling establishment because they attracted seed-carrying animals and ameliorated surface soil water deficits. Forest regrowth in semi-permanent, abandoned pastures may proceed through tree island formation and expansion, but only if the incidence of fire is reduced. |
