A Comparative Study of Tree Establishment in Abandoned Pasture and Mature Forest of Eastern Amazonia

In Amazonia, millions of hectares of forest have been converted to cattle pasture then abandoned. On sites with histories of heavy use, forest recovery is slow. We compared the process of tree establishment in an abandoned pasture with a history of heavy use and in a mature forest through a series of field studies in northeastern Amazonia. Tree seedling and sprout emergence was ≥20 times lower in the abandoned pasture than in forest understory and forest gaps. Tree emergence was restricted in the abandoned pasture by a lack of tree seeds in the soil (3 tree genera vs 15 in the forest), and a low rate of tree and liana seed deposition by birds and bats in the open vegetation of the abandoned pasture (2 m^-2 yr^-1). Tree and liana seed deposition in the abandoned pasture was higher beneath treelets (990 m^-2 yr^-1). Rates of seed removal and consumption by ants and rodents were also higher in the abandoned pasture (>80% removal within 20 d for 6 tree species) than in forest understorey and forest gaps. Cutter ants (Atta sexdens) hindered tree seedling survivorship and growth in the abandoned pasture by clipping leaves and stems, and preferred tree seedlings to grass and shrub seedlings. In the absence of herbivores, survivorship and height growth of seedling transplants in the abandoned pasture were generally lower than in experimental treefall gaps, and were correlated with harsh environmental conditions in the former. Air temperature, air vapor pressure deficit and soil moisture stress in the abandoned pasture exceeded conditions measured in both treefall gaps and intact forest during the 5-month dry season. Seedling growth in the abandoned pasture was also restricted during the wet season. These barriers to tree establishment help explain the low density and emergence rates of tree seedlings in this abandoned pasture relative to the adjacent mature forest.