Timing of Disturbance and Coexistence in a Species-Rich Ruderal Plant Community

Theoretical models show how temporal variability can promote plant species richness by slowing or preventing competitive exclusion. This can come about as a result of variation in the frequency, timing, or intensity of events such as frosts, rainfall, soil disturbance, herbivore outbreak, or drought, as long as species exhibit niche differences in their responses to one or more of these factors. Here, I describe an experiment on a community of >50 predominantly annual plant species, in which the timing of soil disturbance was varied experimentally over a 10-year period (1992–2001). Replicate plots were plowed once per year in October, March, or May, and disturbance timing had a marked impact on botanical composition from the outset. Following cultivation of all plots in fall 2002 or spring 2003, the effects of long-term cultivation timing on biomass in July 2003 were significant for most species, and the effects were large (10-fold to 100-fold) for many species. Three guilds of species were apparent: fall germinators, spring germinators (seeds with a chilling requirement to break dormancy), and indifferent species (species germinating after disturbance, irrespective of timing). Some species were driven close to local extinction by annual soil disturbance at the “wrong” time of year; presumably their seed banks were small or rapidly depleted, and recruitment to their populations may have been more reliant on the annual seed rain. Proof that temporal variability of disturbance is an important contributor to long-term species coexistence requires a demographic approach showing both a storage effect and relative nonlinearity of competition. This experiment represents a first step, by showing that different timings of disturbance can have large effects on the biomass, frequency, and relative abundance of coexisting species.