Resprouting of the woody plant Pyrus calleryana influences soil ecology during invasion of grasslands in the American midwest.
Biological invasion of woody plants into grasslands is a widespread phenomenon that threatens the cultural value, biodiversity, and ecosystem function of these unique systems. In the American Midwest, grasslands are increasingly threatened by invasion of the tree Pyrus calleryana (Callery pear) which is particularly challenging to manage and has strong potential to alter ecosystem function. Mowing is a standard practice for maintaining Midwestern grasslands; however, P. calleryana exhibits an aggressive sprout response to cutting and the ecological implications of this behavior are not well understood. We measured the response of soil moisture and pH, and soil enzyme activities representing labile carbon cycling (β-glucosidase), recalcitrant carbon cycling (peroxidase and phenol oxidase), nitrogen cycling (leucine aminopeptidase) and phosphorus cycling (phosphatase) to determine how P. calleryana trees that are untreated and single stemmed alter nutrient cycling compared to their cut and resprouting counterparts. We found lower β-glucosidase activity and higher peroxidase activity underneath single stemmed trees than underneath those which had resprouted, indicating that there may be lower nutrient availability underneath untreated trees accounting for differences in enzyme activity . Generally, invasive species leaf litter has faster decay rates than native species, which results in higher activities of enzymes that degrade labile materials in soil underneath the plants. Because soils underneath P. calleryana do not follow this pattern, it is possible that its leaf material is not as labile as other common invaders. We also found that increasing P. calleryana basal diameter was associated with reductions in soil pH, which indirectly increased peroxidase and phenol oxidase activities, enzymes which are indicative of recalcitrant C sources such as lignin. This demonstrates that P. calleryana may alter carbon cycling by altering the C inputs to the soil system from its leaf litter. Taken together, P. calleryana may reduce plant richness and promote further invasion by lowering the availability of labile carbon and lowering soil pH, indicating that this invasion may be soil-mediated and self-reinforcing. Further, we recommend continued mowing as a treatment for P. calleryana invasion to ameliorate the impacts of its invasion even though there will likely be a sprout response to treatment.