Seed predators are also important roadblocks to the establishment of forest trees in degraded pastures. To study the fate of seeds that are artificially dispersed to degraded pastures, we placed seeds in groups of 8 that are 10 centimeters apart at 30 widely spaced stations. By repeatedly revisiting these stations, we are able to determine disappearance rates and, if we are lucky, the animal species that are removing the seeds. In studies on the tree Inga sp., leaf-cutter ants (Atta sexdens) were carrying off the seeds to their subterranean burrows within minutes of placement. Large seeds of other species were slowly consumed in situ, presumably by rodents, judging from teeth marks on partially consumed seeds.
In recently completed studies, we placed seeds of forest tree species simultaneously in pastures, in closed forest, and in canopy openings caused by treefalls. For all six species tested, seed removal was much more rapid in the degraded pasture than in the forest environments. All these findings suggest that seed predation is an important impediment to forest regeneration in degraded pastures.
Harsh environmental conditions present a final impediment to the establishment of forest trees in degraded pastures. The physical conditions of the pasture and forest contrast sharply and may significantly impede seedling survival and growth in the pasture. During the 6- to 7-month wet season usually ending in June or July, daily rains maintain pasture and forest soils near saturation. In the ensuing dry season, rain falls episodically, usually totalling less than 100 millimeters per month, and soil moisture levels decline between rain events. In the pastures, tensions of 2.0 megapascals are reached in soil 15 centimeters below the surface, whereas in forests, moisture is rarely held at tensions greater than 1.5 megapascals. This means that pasture plants rooted in the 15 centimeters of soil are subjected to intense drought (i.e., permanent wilting point conditions), whereas water availability in the surface soil of forests is adequate.
In short, the seasonal changes in moisture and energy conditions near the ground surface of degraded pastures are dramatic compared to conditions in the forest. During the wet season, the availability of soil moisture and radiation load in degraded pastures is very similar to conditions beneath closed-canopy forest. During drought episodes, soil and air moisture deficits and air temperatures in pastures greatly exceed those of the forest.
A summary of the factors that limit the establishment of forest trees in degraded pastures is provided in Figure 37–1.
We have begun a series of field experiments to try to overcome these impediments to forest recovery on degraded lands. The seed dispersal limitation is straightforward and can be overcome, since humans can act as the seed dispersal agent. Alternatively, instead of individual seeds, humans could disperse onto degraded lands packets of forest surface soil with its component seed bank. This might allow whole