Dynamic tree aggregation patterns in a species-poor temperate woodland disturbed by fire

Question: How does fire affect the aggregation patterns of trees in a species-poor oak woodland? Location: East-central Minnesota, USA. Methods: More than 10000 trees with DBH > 2 cm (comprising more than 11000 stems) were monitored in a 16-ha grid on an annual basis from 1995- 2001 in a species-poor temperate woodland. Different portions of the grid experienced different frequencies of controlled burns. Aggregation indices were calculated for individual species and individual size classes within species. A community-wide aggregation index was also calculated for different burn units. Spatial data were managed. and many of the aggregation indices calculated using a GIS ArcInfo (TM) (ESRI). Results: Fire initially increased clumping, although repeated fires reduced it, a finding that suggests a corollary to the intermediate-disturbance hypothesis, the corollary stating that intermediate levels of disturbance are expected to maximize community-wide patterns of aggregation. Analyses also showed that all species are aggregated at small scales, that the degree of aggregation of a stem type (species or size) declines with distance from individual stems, that the degree of aggregation of large stems is usually less than that of small stems, and that rare species are more aggregated than common species. Findings from this study are consistent with those from similar studies in other temperate and tropical forests, woodlands, and savannas. Conclusion: The spatial patterns of trees in this woodland are dynamic, continually changing in response to the relative strengths of the often opposing forces of competition, which tends to reduce clumping, and disturbance, which, at low and intermediate frequencies, tends to increase it.