Influence of species pool, fire history, and woody canopy on plant species density and composition in tallgrass prairie1

Abstract

Understanding the processes contributing to long-term change in remnant plant communities is relevant to community ecology and conservation biology. Using a unique set of historical field data, we undertook a natural field experiment to examine how novel landscape conditions, fire history, and increased woody canopy affect plant composition and structure of tallgrass-prairie remnants in Wisconsin, USA. In 1999 we used 1-m2 quadrats to resample 21 remnant tallgrass-prairie sites that had been similarly sampled between 1947 and 1954. Results reveal that community change was largely recruitment driven, with mean recruitment (41.4 spp./site ± 14.4 SD) significantly different from mean extirpation (28.4 spp./site ± 11.5 SD). Species density increased significantly from 10.6 ± 2.3 to 12.8 ± 3.9 spp./m2, indicating that regional processes contribute substantially to species diversity. We propose that increased species density is the inevitable consequence of prairie's historical condition of seed limitation combined with modern seed subsidy from an altered landscape now rich in habitat generalist species. We hypothesized that species responses to fire suppression and increased shading would vary with their competitive versus reproductive ability. The bulk of increaser species were habitat generalists. Declines among habitat specialists and once-dominant species were exacerbated by fire suppression. Canopy effects were generally weaker than fire suppression effects, consistent with the theory of strong floristic similarity of prairie and savanna vegetation. Extirpations of rare species (i.e., original quadrat frequency ≤ 5%) ranged from 48 to 84 percent, but extirpations were independent of fire regime or woody canopy. Our results suggest that the instability of remnant communities is the consequence of regional, local, and stochastic processes.