Spatially and temporally discrete episodes of plant recruitment occur when relaxation of the resources that limit establishment and growth coincides with propagules.  There are many ecological factors that affect below- and aboveground resource availability in a community and this research integrates important spatially and temporally varying components.  In a managed grassland at the Pymatuning Laboratory of Ecology, we used a balanced split-plot factorial design to measure the interactive effects of grazing intensity (GI), soil resource availability (SRA) and soil disturbance patch size (DPS) on the invasion success of the annual smooth pigweed Amaranthus hybridus (L.) and velvetleaf Abutilon theophrasti (L.).  Importantly, we considered two life history stages of each focal invader (i.e., emerged and established seedlings) because their invasion success probability (ISP; measured as biomass accumulation) may depend on when resource shifts coincide with particular life history stages. 

Results/Conclusions

Based on models developed by Renne and Tracy (unpublished), we tested the following hypotheses: 1) thresholds in ISP exist across DPS, the magnitude of which increases with higher SRA levels, 2) as DPS declines, the interactive effects of invader life history stage, SRA and GI on ISP increase and 3) in the absence of grazing, SRA within a given DPS can have opposing effects on ISP.  Final dry weight biomass for emerged and established seedlings of each focal invader species differed by more than two orders of magnitude across treatment combinations.  Based on quantile regression analysis, our empirical data generally corroborate models of grassland invasibility, where the interactive effects of timing and/or intensity of GI, SRA and DPS on invader life history stage largely dictate whether establishment and subsequent growth occurs.  Integrating disturbance type and intensity across other ecological dimensions is important in elucidating the scale-dependent mechanisms governing community diversity and invasibility as well as offering sound recommendations to practitioners of managed grassland communities.