Can disturbances increase plant competition?

Background/Question/Methods

Classical disturbance theory predicts that most disturbances reduce the strength of biotic interactions (competition/facilitation). Most support for this prediction is based primarily on the results of removal experiments that show reduced responses of targets to neighbor removal following disturbances that are relatively intense and non-selective in their immediate effects on vegetation. In nature, however, most physical disturbances cause little mortality of entire genets and are selective in their impact on vegetation. Furthermore, most removal experiments measure responses to competition or facilitation and do not account for interspecific differences in disturbance resistance or competitive/facilitative effects of species recovering from disturbance. A better understanding of how disturbances affect interaction strength might be achieved by examining disturbances with selective impacts on vegetation using experimental designs in which the interaction-reduction treatment is not effectively equivalent to the disturbance treatment (i.e., density/biomass reduction).  This field study presents preliminary results from a neighborhood analysis of the effect of fire on competitive interactions between groups of sapling species known to differ in fire-resistance and growth rate.

Results/Conclusions

Preliminary results suggest that, when light was not limiting to sapling growth (e.g., within canopy gaps), a prescribed fire in spring 2010 increased the competitive effect of a fire-resistant sapling group (oaks) on a group of more fire-sensitive (but potentially faster-growing) saplings (mesophytes, e.g., gums, maples, elms).  In a burned plot, mesophytes experienced a post-fire height growth disadvantage to oaks when mixed with oaks, but had a slight advantage in monotypic neighborhoods. In an unburned plot, mesophytes experienced an equivalent growth disadvantage to oaks in mixtures and monotypic neighborhoods. The fact that mesophytes did not grow as fast as or faster than oaks in the unburned plots was unexpected and requires additional study. To complement these suggestive but inconclusive results, we have initiated an experiment that examines the interaction between fire disturbance and selective removal of saplings within neighborhoods. We predict that growth responses to experimental removal will depend on the identity of the neighbor removed and the disparity in resistance to fire.