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 cat tle pasture then abandoned. On sites with histories of heavy use, fore st recovery is slow. We compared the process of tree establishment in an abandoned pasture with a history of heavy use and in a mature fores t through a series of field studies in northeastern Amazonia. Tree see dling and sprout emergence was greater than or equal to 20 times lower in the abandoned pasture than in forest understory and forest gaps. T ree emergence was restricted in the abandoned pasture by a lad: of tre e seeds in the soil (3 tree genera vs 15 in the forest), and a low rat e of tree and liana seed deposition by birds and bats in the open vege tation 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 rodent s 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 tre e seedlings to grass and shrub seedlings. In the absence of herbivores , survivorship and height growth of seedling transplants in the abando ned pasture were generally lower than in experimental treefali gaps, a nd were correlated with harsh environmental conditions in the former. Air temperature, air vapor pressure deficit and soil moisture stress i n 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. Th ese barriers to tree establishment help explain the low density and em ergence rates of tree seedlings in this abandoned pasture relative to the adjacent mature forest.