PS 52-76 - Herbaceous species losses associated with long-term woody encroachment of a species-rich savanna do not result from deterministic losses of functional groups

Thursday, August 10, 2017
Exhibit Hall, Oregon Convention Center
J. Stephen Brewer, Department of Biology, University of Mississippi, University, MS

Trait differences among plant species have been shown to be important in promoting species coexistence in assemblages of short-lived species and low-diversity assemblages of long-lived perennials. The role that such differences play in the assembly or dis-assembly of highly diverse plant communities dominated by long-lived perennials remains unresolved. Such community types, some of which may contain more than 40 species per 0.25-m2, remain among the greatest challenge for niche theory. In this study, I examined a 65-year chronosequence of herbaceous species losses associated with off-site pine encroachment in a species-rich wet savanna community during a period of fire exclusion. I tested the hypothesis that losses of herbaceous species resulted in greater loss of functional groups than expected from random species losses. Functional groups were identified by first describing 31 functional traits in 72 herbaceous species. Traits included maximum height, flowering phenology, fire-related traits, nutrient acquisition adaptations, dispersal syndrome, photosynthetic pathway, and depth of the deepest root (to name a few). Similarity profile permutation tests and K-R clustering identified 18 functional groups. I then used rarefaction to determine if the loss of functional groups with pine encroachment was greater (or lesser) than expected by chance.

Results/Conclusions The species were most distinguishable from one another with respect to height, photosynthetic pathway, root depth, leaf greenness before versus after fire, flowering phenology, fire-stimulated flowering, and fire-stimulated emergence. The C4 pathway was associated with greater height and root depth, fire-stimulated fall flowering, and distinctly greener leaves following fire than before fire. Several small forbs, exemplified by Polygala species, exhibited the C3 pathway, had shallow roots, flowered in the spring or summer, and showed fire-stimulated emergence. Encroachment of slash pine, Pinus elliottii, during a prolonged period of fire exclusion resulted on average in a 49% local reduction in herbaceous species. Rarefaction revealed that the observed reduction in functional groups resulting from woody encroachment was not significantly different from expected losses in functional group richness resulting from random species losses. Results suggest that frequent fires maintain diverse assemblages of herb species with a wide variety of adaptations to frequent fires in the wet savannas studied here. Although such fire-related traits likely permit tolerance of frequent fires (and of diffuse competition in years without fire), there is no evidence that functional differences in these or any other traits measured in this study can predict niche-based assembly or dis-assembly in this system.