Plant-soil feedbacks and long-term coexistence dynamics in the field
Plant-soil feedbacks (PSF) have high potential as a stabilizing mechanism of coexistence in plant communities. Past research has shown that the effects of soil microbes on individual plant fitness are correlated with patterns of relative species abundance in some communities. Here, we first explicitly tested the role of density-dependence in microbe-mediated plant competition in the greenhouse, then implemented a field study which manipulated PSF along a historical gradient of species turnover. Finally, we relate the results from these studies to long-term species dominance and turnover patterns to determine the importance of microbe-driven PSF as a coexistence mechanism in nature.
In the greenhouse experiment, we isolated the microbial contribution to PSF by comparing sterile and live inocula from conspecific and heterospecific rhizospheres. Plants were grown in a response-surface design to determine frequency-dependent and independent effects of soil inocula on competition coefficients. In the field experiment, transplants were installed along a transect in regions with differing long-term turnover dynamics. Different sized mesh cylinders allowed rhizosphere fungi from established plants to colonize or be excluded from transplants. This on-going experiment will allow us to correlate strengths of fungal-driven feedbacks with known patterns of turnover and coexistence between two dominant plant species.
In the greenhouse experiment, the competitive dominant, Bouteloua gracilis, was more strongly limited by negative plant-soil feedbacks than was the competitive subordinate, Bouteloua eriopoda. We saw both frequency-dependent and frequency-independent effects of PSF on B. gracilis performance, whereas B. eriopoda did not show differences among feedback treatments. The presence of live PSFs decreased the per capita biomass of B. gracilis by 25% (frequency-independent), and increased the steepness of the slope of its intraspecific competitive response by 39% (frequency-dependent). Root colonization microscopy suggests that the negative feedbacks may be driven by dark septate endophytes in particular. These results indicate the presence of frequency-dependent negative plant-soil feedbacks that may increase coexistence in a semi-arid grassland system. In the field, 25-year transect data show that these two species coexist, but with spatial variation in rates of turnover. Patch-based analysis of long term data indicated the presence of interspecific competition (negative correlation between change in patch cover of the two species across years) and density dependence in the field (negative correlation between patch size and rate of cover increase). These results set the stage for novel methods in linking manipulative feedback experiments in the field to long-term coexistence dynamics, which we will introduce here.