PARENT SESSION
Contributed Oral Session 163: Forest Ecology: Vegetation Change and Succession
Friday, August 12, 8:00 AM - 11:30 AM, Meeting Room 520 B, Level 5, Palais des congrès de Montréal

Woody colonists in Costa Rican premontane successional pastures are more common on microscale slopes than level ground.

Peterson, Chris^*,1, Haines, Bruce¹, ¹ University of Georgia, Athens, GA, U.S.A.

ABSTRACT- Secondary succession in neotropical post-agricultural lands is strongly influenced by land use history, the availability of nurse plants or remnant trees, and various types of microsites such as logs or fern patches. For three successional pastures, we report what we believe is the first demonstration that microscale steepness of slope, for 4 m x 4 m grid cells, is positively correlated with density of tree and shrub colonists. In 1996, we fenced five sites at interfaces of mature forest and active cattle pastures in the premontane zone of southern Costa Rica, to study natural forest regeneration from the initial abandonment. Beginning in 1998, we marked, mapped, and measured all woody colonists > 1.5 m tall, and have resurveyed them roughly annually since December 2000. Elevations were interpolated by kriging at corners of 4 m x 4 m grid cells and overlaid on stem maps, to determine woody colonist density per grid cell. The 1998 colonist density was positively correlated with slope in three of the sites: Site 1, r = 0.31, p<.001, n=240 cells; Site 2, r=.387, p<.001, n=189 cells; and Site 4, r=.298, p<.001, n=191 cells. In two sites, there was a marginally significant positive correlation between cell slope and density of colonists establishing between 1998 and December 2000. We are unaware of any similar report for neotropical secondary successional sites. We hypothesize that, prior to fencing, the steeper areas of each pasture had repeated trampling of the same spots by cattle, creating microscale gaps in the graminoid cover, which, after cattle exclusion, were favorable microsites for woody plant establishment. If so, then a substantial portion of the spatial pattern of woody colonists may be a consequence of repeatedly-used cattle paths through the grass of these steeply-sloping pastures.

Key words: succession, tropical, oldfield, colonization