Spatial and temporal dynamics in savanna ground-layers are regulated by interactions between plant functional groups, fire regimes and canopy cover gradients. Frequent fire selects for C4 grasses and prairie forbs in canopy openings, C3 graminoid species and shade-adapted forbs and shrubs in canopy shade, and maximum heterogeneity and richness in partial canopy cover. Edaphic factors are rarely integrated with a canopy cover gradient model. We examine temporal vegetation dynamics along an edaphic gradient under changing canopy cover during 25 years of repeated burning at Tefft Savanna, a 200-ha sand savanna in northwest Indiana. Its 210-230 m elevation gradient supports black oak at high-, white oak at mid-, and pin oak at low elevations. Following 3 decades of fire protection, Tefft has received 3 dormant season fires/decade beginning in 1982. We ask whether vegetation shifts toward an expected canopy gradient model, how this pattern is moderated by an edaphic gradient, and how it might further respond to climate change.
Results/Conclusions
After 25 years, elevation accounted for 62 % of the variation in an NMS ordination, and species richness was positively correlated with soil pH and negatively correlated with organic content, CEC, and increasing soil moisture at lower elevations. Canopy cover ranged from 24-88 % and had a significant secondary effect. Ground-layer composition varied with elevation and canopy cover, while functional group diversity corresponded to the canopy cover gradient. Species richness was predominantly forbs. The C3 sedge Carex pensylvanica was the dominant graminoid species throughout the savanna, while the C4 grasses Schizachyrium scoparius and Sorghastrum nutans were restricted to < 50 % canopy cover. Over time, repeated burning decreased canopy cover 20-50% at mid to high elevations, accompanied by a 20-100 % increase in species richness to 11.4 species/m-sq. Woody species dynamics were driven by time-since-fire due to post-fire re-sprouting. Temporal change in other functional groups co-varied with both canopy cover and elevation. Increasing cover and richness of C4 grasses and summer-flowering prairie and woodland forbs correlated with decreasing canopy cover, nitrogen-fixing species occurred in both shade and canopy openings, and heterogeneity was maximized at mid-canopy cover. These results indicate that repeated burning increases species richness and abundance of C4 and prairie forb species by decreasing canopy cover, but that edaphic factors underlie many patterns. We project vegetation dynamics under climate change effects on climatic extremes as well as on soil processes that regulate vegetation.
