Is heterogeneity in plant communities informative of heterogeneity in soil properties? Comparisons of a native and invaded grassland

Mary Ellyn DuPre and Ylva Lekberg, MPG Ranch, Missoula, MT, Cara R. Nelson, Department of Ecosystem and Conservation Sciences, W.A. Franke College of Forestry and Conservation, University of Montana, Missoula, MT, Ylva Lekberg, Department of Ecosystem and Conservation Sciences, University of Montana and MPG Ranch, Missoula, MT

Background/Question/Methods
Shifts in plant communities associated with invasions are easy to appreciate and measure, but whether this corresponds to altered soil properties, especially at small spatial scales where plant soil feedback (PSF) operates is less known. Essentially, does the homogenization of plant communities associated with invasion translate to less variation in below ground properties? The answer is important not only to understand the impacts of invasion, but also to develop better sampling strategies to capture variation in plant communities that differ in heterogeneity. In this study, we recorded plants present in 49, 30 cm diameter plots located within two, 15 x 15 m areas dominated by either leafy spurge (Euphorbia esula L.) or mixed native grasses and forbs. These plots were located on a grid with pairwise sample distances ranging from 0.1 to 15 m. We collected soil cores (5 x 10 cm) from the center of each plot and quantified NO3-, NH4+, pH, soil moisture, nematode abundance, and arbuscular mycorrhizal fungal inoculum potential (MIP). We used dissimilarity-based multivariate analysis, one-way regression analysis, and Mantel tests to compare differences in measured variables between the two areas and to assess spatial autocorrelation between plant communities and soil properties.
Results/Conclusions
The dissimilarity among plant communities was greater among native plots than leafy spurge plots, which supports the homogenization of plant communities often associated with invasions. The trend above ground did not correspond to soil properties, as soil properties differed more among soil cores taken from the leafy spurge area than the native area. Leafy spurge soil cores had higher pH, soil moisture, and NO3- levels than native soil cores, but did not differ from cores in the native community in NH4+, MIP, or nematode abundance. Mantel tests revealed correlation between soil properties and plant communities in the native area but not in leafy spurge, possibly due to differences in PSF among native species. We also detected spatial autocorrelation in native plant community plots, but not in leafy spurge plots, most likely due their greater evenness. Soil properties showed the opposite pattern and were spatially autocorrelated in the leafy spurge but not native area. Overall, our results indicate that different plant communities can be associated with different soil properties. However, variation above ground does not appear to be informative of variation below ground, and we should not assume that more homogeneous plant communities require fewer samples to characterize adequately below ground attributes.