Long-term NxP fertilization effects on herbaceous layer communities in ten northern hardwood forest stands

Background/Question/Methods

Herbaceous plants in northern hardwood ecosystems play key roles in nutrient cycling and forest diversity. The importance of nitrogen (N) availability on herbaceous communities has been better studied than that of phosphorus (P), although both N and P have been found to be limiting to forest growth in this forest type. To determine the sensitivity of herbaceous layer community composition to N and P availability, I surveyed herb transects in ten stands distributed across three sites of a long term NxP factorial fertilization study in which pre-treatment transects were surveyed in 2010.

Results/Conclusions

Species richness was lower for plots that received N, indicated by ANCOVA (p = 0.01) with pre-treatment data as a covariate, while P addition had no detectable effect on species richness (p = 0.38). Herb abundance was not sensitive to either N (p = 0.50) or P (p = 0.28) addition. The decrease in species richness from N addition could be a result of a removal of niches for N-limited conditions. Herbaceous communities differed by site and age as indicated by non-metric multidimensional scaling (NMDS) and permutational multivariate ANOVA (PERMANOVA). Differences across the three sites are likely due to soil fertility, while differences with stand age may be due to factors such as light and moisture availability.