PS 25-91
Nutrients, light, earthworm biomass and deer herbivory as predictors of seedling survival at the prairie-forest ecotone in western Minnesota
As part of larger study examining forest dynamics across a climate gradient at the prairie-forest ecotone, we transplanted over 4000 commonly-sourced seedlings representing four tree species into six forested study sites across a 400 km transect in the spring of 2012. Study species include three native trees—Acer saccharum (sugar maple), Quercus macrocarpa (bur oak), and Tilia americana (basswood)—and the invasive Rhamnus cathartica (European buckthorn). Here we report on seedling survival after two growing seasons as a function of deer density, light, soil nutrients and invasive earthworm biomass. At each site, seedlings are growing in 12 plots, arranged into four clusters, with high, medium, and low seedling density plots in each cluster. Each plot is divided into two subplots, one of which is protected from deer herbivory by a small exclosure (2.5 x 1.5 m). The other subplot serves as a control. We estimated light using hemispherical photos, deer density using fecal pellet counts, and earthworm biomass using hot mustard extraction. Soil properties analyzed include nitrogen, phosphorous, organic carbon, and pH.
Results/Conclusions For all four tree species, protection from deer herbivory dramatically decreased mortality (odds ratio for protected vs. control seedling mortality: 0.06 (0.04-0.09) for A. saccharum, 0.20 (0.15-0.28) for Q. macrocarpa, and 0.23 (0.17-0.32) for T. americana). Compared to the three native species, invasive R. cathartica appears relatively indifferent to herbivory (OR: 0.56 (0.40-0.77)). Deer densities vary considerably within our study region, but seedling survival was only impacted by density for R. cathartica. Soil carbon and nitrogen had a modest impact on seedling survival, but phosphorous had no impact. In the alkaline soils of western Minnesota, increasing pH led to substantially higher mortality risk for the native species (OR: 1.68 (1.01-2.60) for A. saccharum and 2.29 (1.53-3.43) for T. americana). Increased light positively impacted the survival of T. americana, but negatively affected R. cathartica, consistent with previous ecotonal work where we showed poor survival of R. cathartica saplings in high-light microsites. T. americana survival was poorer in sites with more invasive earthworm biomass, but the other species were unaffected. Overall, our results suggest some environmental niche partitioning among tree species at Minnesota’s prairie-forest ecotone, but these effects are subtle when compared with the strong impact of deer browsing.