Patterns of herbaceous plant diversity in southeastern Louisiana pine savannas

Question: How diverse are Louisiana pine savanna plant communities and how is diversity affected by time since burn and removal of a competitively dominant species? Location: Lake Ramsay, southeastern Louisiana, USA. Methods: Species-area curves were constructed from nine nested quadrats in open savanna differing in time since burn (6, 18 and 30 months). Species frequency was determined for 1001-m(2) quadrats. The dominant grass, Andropogon virginicus, was removed with herbicide from moist and dry sites to test for possible effects of competition. Results: Slopes of log-log species-area relationships were steep (0.195 to 0.379). Time since burn did not affect the richness of herbaceous plants, only woody species. More than half of all species recorded (43/79, 54 %) were infrequent (in < 10 % of quadrats). After two years, there were no differences in species richness and composition for plots with and without A. virginicus. Conclusions: The high species diversity is typical of other savannas across the coastal plain. The large number of infrequent species indicates that the core-satellite pattern of species occurrence found in temperate grasslands does not apply to southern pine savannas. The absence of effects due to removal of a dominant may be due to insufficient observation time, or low competition. Most species have traits, such as diminutive life forms, that suggest they are weak competitors for light in the presence of robust matrix grasses and in the absence of fire. Many species in Pinus palustris savannas are likely either fugitive or peripheral species.