Fire and vegetation interact in complex ways over space and time in southeastern U.S.A. pine savannas. Fires filter for plant species that are able to persist after fire, and those persisting plants fuel the subsequent fires. Although we know variation in fire intensity alters local plant species richness, it is unclear how this varies across soil moisture gradients and fire history. We tested the effects of fire intensity on the species richness of understory plant species in dry and mesic pine savannas that differed in fire history: long-unburned (>20 years), infrequently- (3-5-year fire intervals) or frequently-burned (1-2 year fire intervals). Within the two savanna types and three fire frequencies, we randomly selected trees and arranged plots to test whether fire intensity and plant species richness differed between driplines (i.e., where pine needles increase fuel loads) and inside driplines where duff often accumulates, especially in long unburned sites. We found that species richness is greater in mesic than dry pine savannas. Initially, there was an interaction between fire frequency and intensity, which decreased over time. Overall, frequently burned sites had significantly more species than long unburned sites. Spatial variation in fire intensity, especially in fire-frequented pine savannas likely promotes high biodiversity.