Ecological consequences of C4 grass invasion of a C4 grassland:: A dilemma for management

Many successful exotic invasive species are functionally distinct from the dominant native species they displace. Occasionally invasion occurs where the exotic species possesses functional traits relatively similar to those of the dominant native. We examined the ecological consequences of such an invasion within a mesic, temperate grassland at the Konza Prairie Long-Term Ecological Research site. We assessed potential changes in carbon (C) and nitrogen (N) cycling and plant diversity following the invasion of a C-4 bunch grass species, Andropogon bladhii, into a tallgrass prairie dominated by the native C-4 grass species, A. gerardii. In these prairies burning is an important management tool used to maintain native-species dominance. We determined how frequent spring fires affected the impacts of A. bladhii in this system. Over a two-year study our results show that burning regulated the effects that the invasive species has on the native prairie. Compared to the native species, A. bladhii exhibited significantly greater plant biomass, significantly lower pools of soil N, significantly lower rates of decay and C cycling, and higher foliar and root tissue C:N ratio in response to burning. Notable spatial heterogeneity in C and N cycling was evident in areas dominated by the invasive bunch grass. In addition to altered ecosystem processes, areas dominated by the invasive, A. bladhii, had significantly lower plant species diversity. In a grassland ecosystem where burning is an important management tool for controlling exotic-species establishment, maintaining native-species dominance, and increasing productivity, A. bladhii may be able to successfully out-compete the native C-4 grass species by using traits typically used to explain the dominance of the native species. With frequent fire, the invasive species has the potential to decrease long-term fertility by lowering N inputs in litter and increasing erosion in non-vegetated soil between bunches, while also having a negative effect on plant diversity. By using fire to promote native C-4 grasses and maintain these tallgrass prairies, the threat of invasion by normative C-4 species may raise a dilemma for future management of these C-4 grasslands.