The effects of environmental variables on montane longleaf pine ecosystems
Once the dominant tree in fire maintained ecosystems throughout Southeast, the longleaf pine (LLP, Pinus palustris) ecosystem’s range has dwindled from 37 million hectares to less than 1 million hectares. An important and unique fraction of this ecosystem is found in mountainous environments, but little is known about the ecology of these montane systems. Longleaf pines are found in two distinct topographic regions (foothills and ridge) within the montane ecosystem at Oak Mountain State Park in Alabama. Due to over 60 years of fire suppression, the forests are undergoing succession from LLP woodlands to broadleaf forests. This study investigated the effect of environmental variables (soil depth, slope steepness and canopy openness) and measures of competition (tree species richness and non-LLP basal area) on establishment of juveniles and maintenance of adult LLP (adult LLP basal area). We hypothesized that variables that increased stress (decreased soil depth, increased slope, decreased canopy openness) and competition (increased tree species richness and increased non-LLP basal area) would have negative relationships with the presence of juvenile and adult longleaf. Parameters were quantified in ridge and foothill sites by using twenty 20x50 m plots, each subdivided into ten 10x10 m subplots within which the variables were measured.
The regions significantly differed in each variable except for canopy openness. Slope, soil depth and species richness had a negative relationship with juvenile LLP frequency in the foothills, while in the ridge, species richness had a positive relationship to the juvenile frequency via multiple linear regression tests. None of the tested variables (canopy openness, slope, soil depth, species richness and non-LLP basal area) were significant in a multiple linear regression against adult basal area in the foothills, but slope and non-LLP basal area were negatively related to adult basal area in the ridge. These results suggest that variables affecting LLP establishment and growth were different between ridge and foothills sites. We found that environmental stress may be slowing the demise of LLP communities in this era of fire suppression. Further research should examine how temperature, slope aspect, and the average distance from the parent tree affect growth in LLP.