Question: Herbaceous plant communities in hardwood forests are important for maintaining biodiversity and associated ecosystem services, such as nutrient storage. Are there differences in herbaceous layer nutrient storage for urban park and state preserve forests, and is there seasonal variation?

Location: United States,Midwestern region (Iowa).

Methods: We examined the herbaceous layer in three 20-m2 plots at six forest sites, in urban parks (city parks with high human visitation) and state preserves (under permanent protection as state preserves). We harvested herbaceous plants from quadrats in each plot in spring, summer and autumn to analyse above- and below-ground tissues for total C, N and P concentrations. Biomass and tissue nutrient concentration data were used to estimate nutrient storage per plot and per hectare. We also collected soil samples at each plot in each season to determine soil C, N and P content.

Results: State preserve and urban park forests did not differ in herbaceous species richness, although state preserve forests were qualitatively distinct. State preserve forests had relatively higher biomass and nutrients than urban park forests. In both forest types, above-ground concentration for N and P were higher in spring than in autumn, whereas below-ground concentration of these nutrients was consistently higher in autumn. In urban parks, total soil N was higher in spring compared to summer and autumn. However, soil nutrient content did not appear to drive differences in plant tissue nutrient content in urban park forests.

Conclusions: Subtle qualitative differences in herbaceous layer composition affected seasonal biomass quantities and nutrient concentrations in urban park and state preserve forests. These differences influenced C, N and P storage and led to consistent trends for relatively higher biomass and nutrient storage in state preserve forests. Above-ground plant tissue provides important storage of N and P in spring, and below-ground plant tissue provides important storage of N and P in autumn. Since spring and autumn are seasons of limited vegetative cover in the regional landscape,with subsequently higher potential for nutrient loss from terrestrial systems, this finding is crucial for provision of ecosystemservices.