Processes responsible for changes in plant species abundance following disturbance

Change in the dominant plant species following the creation of a gap has been well documented. The most broadly accepted idea to explain these changes is competitive reduction: disturbance removes biomass, reducing overall competitive intensity and allowing species that are poor competitors but good colonizers to persist. Because this idea is broadly accepted, its assumptions and predictions are rarely tested. I test three of the main predictions of the competitive reduction hypothesis: that there is a tradeoff between colonization and competition ability; that matrix species (those characteristic of undisturbed vegetation) are always competitively superior to disturbance species; and that the major mechanism driving the changes in abundance is the removal of biomass by disturbance.;I test the first prediction, whether there is a tradeoff between competition and colonization abilities, by quantifying tradeoffs within and among several levels of response (traits, processes, field demography, abundance). I did not find the expected tradeoff to be important. Instead, seed traits, tolerance ability, and relative growth rate best explained species abundance response.;In tests of the second prediction, I found that species formed a competitive hierarchy under matrix conditions but have similar competitive abilities following mound disturbance. Following a fire disturbance, the competitive hierarchy completely reversed as the community recovered from a fire disturbance. Both these experiments indicate that disturbance has the potential to change the competitive rankings of species and support an alternative hypothesis, competitive change.;A meta-analysis review of published research also confirms that gap creation shifts the competitive rankings of species: matrix species were competitive superior in matrix conditions, but were competitively equivalent to disturbance species under gap conditions.;To test the third prediction, I experimentally separated the influence of several components of disturbance (biomass, size-structure, neighbor identity, abiotic environment). While neighborhood characteristics influenced overall competitive intensity, abiotic environment alteration (primarily soil moisture and compaction in this case) was the only component that shifted competitive rankings.;This work indicates the broadly accepted assumption of competitive release is not justified: disturbances affect more processes than competition as well as change the competitive rankings of species.