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Joint effects of competition, recruitment limitation, and fire suppression in an invaded oak savanna ecosystem MacDougall, Andrew Stewart

Abstract

Competition is often assumed to determine relative abundance in plant communities, especially in the absence of disturbance. At the community level, however, the relationship between competitive ability and abundance is rarely tested. Emerging evidence supports an alternative model where species abundance is determined as much or more by differences in dispersal ability. If true, this suggests that factors that restrict dispersal in contemporary landscapes, such as habitat fragmentation, may be more limiting for native species than competition by invasive flora. I tested the relative importance of competition and dispersal in an invaded and fire-suppressed oak savanna in southwestern British Columbia. Dominance by two exotic perennial grasses strongly suggests that competition is restricting the occurrence of native flora, many of which are presently rare. Extreme fragmentation of this savanna (habitat loss > 95%), however, suggests that dispersal may also limit their occurrence. I explored these alternatives using a combination of experimental (field and glasshouse) and biogeographical approaches. My results confirmed the importance of competition for structuring this plant community. Removal of the dominant grasses greatly increased the cover of previously subordinate native forbs. A pair-wise competition experiment revealed that the most abundant species, the invasive perennial grass Poa pratensis, dominates by limiting light availability and recruiting by tillering in the light-limited understory. Seed addition experiments, however, also confirmed the importance of recruitment limitation for native species. Despite low light, most native species established and survived in the grass sward. B y reducing the dominant grasses and eliminating the litter layer, experimental burning increased the survival of added species but was not a necessary pre-condition for recruitment. For community structure generally, burning transformed the savanna from a grass-dominated system to one dominated by native forbs. However, because forbs are substantially less productive than grasses, litter levels dropped so that fire could not occur continuously over time, which in turn favours recruitment by grasses. Based on these results, I surmise that this system formerly oscillated between domination by grasses and forbs with fire. Long-term fire suppression explains the domination by grasses in many remnant areas presently. In combination, my work revealed that competition and dispersal interact to structure this oak savanna. Disturbance, soil depth, and annual variations in climate also have an impact on the interaction of these two factors.

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