Thursday, August 9, 2007

PS 67-154: Spatio-temporal patterns of forest structure, understory species composition, and soil characteristics in longleaf pine flatwoods along Florida's Gulf Coast

George L. McCaskill, Shibu Jose, and Andrew V. Ogram. University of Florida

Little information exists on spatio-temporal patterns of overstory, understory, and soil characteristics in longleaf pine flatwoods communities along the lower coastal plain of the Gulf coast. In order to assess the restoration effort of a longleaf pine ecosystem using herbicides and fire, we examined patterns of forest structure, plant species richness, a set of soil quality factors, and soil microbial biomass as environmental indicators along a chronosequence. Three representative sites along a spatial gradient from Pensacola to Tampa Bay (720 km), each containing stands of three distinct successional stages (age groups) were used in this study to represent a chronosequence of 120 years.  Preliminary results indicated that stand volume initially followed stand density up to canopy closure when stand density dramatically declined.  Coarse woody debris (CWD) increased with mean stand diameter up to 30 cm and then significantly declined as stands became dominated by large diameter trees. Understory plant species richness increased up to 35 years, then declined under forest canopies and leveled off at 90 years. Plant species assemblage rates increased dramatically up to 35 years then slowed down to a flat rate at 45 years. Net Nitrogen mineralization levels were high and variable after stand initiation, but decreased as the forest matured. As longleaf pine stands reached 85 years, net nitrogen mineralization levels became more stable in magnitude and variability. Nitrifying bacterial populations were larger in young longleaf pine forest soils than in soils under mature forests. Soil microbial diversity initially increased after a disturbance as soil organic matter accumulated, but decreased as the fungal-to-bacterial ratio increased with longleaf pine basal area.  It appears that longleaf pine flatwoods stands reach steady state equilibrium in terms of our indicators at a threshold age of ninety years after establishment.

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