Understory vegetation is an important contributor to ecosystem productivity, biodiversity, and wildlife habitat that is strongly influenced by weather and fire. Along the forest-grassland ecotone of the southcentral USA, frequent fires maintain savanna ecosystems with productive understories while fire exclusion results in closed canopy forests with minimal understory. We used a study established in southeastern Oklahoma in 1984 consisting of treatments with various combinations of pine harvest, hardwood thinning, and fire return intervals (1, 2, 3, 4 years and none) that have resulted in ecosystems ranging from mature, closed canopy forest to open savanna. By comparing 30+ years of understory aboveground net primary productivity (ANPP) to annual variation in weather and time since fire, we ask the questions 1) how does understory productivity vary among ecosystems, 2) how does variation in temperature and precipitation affect understory productivity, and 3) how does time since fire affect understory productivity.
Results/Conclusions
Across all experimental units, mean ANPP of non-woody vegetation ranged from 9 g/m2/y (for harvest-thinning, no fire) to 342 g/m2/y (harvest-thinning, annual burning). Among the other burning treatments, ANPP decreased with fire frequency (234, 227, and 172 g/m2/y for 2, 3, and 4-year return intervals) and this trend was inversely related to both overstory density and litter accumulation. Annual precipitation and temperature trends had little effect on productivity. However, June precipitation (r = 0.63; p <0.0001) had a strong positive correlation with productivity while June maximum temperature (r = -0.37; p < 0.0001) had a strong negative correlation. Analyses regarding time since fire and the woody components are ongoing. These results indicate that early summer drought has the most severe effect on productivity of understory vegetation and that the interactive effect of fire and tree density is the most important factor controlling understory productivity.