Forests embedded across developed landscapes simultaneously experience multiple global change factors (e.g., warm temperatures, increased CO2), and thus, are a natural experiment to measure forest carbon (C) and nitrogen (N) cycling in response to global change. Small forests embedded across urbanized areas may disproportionally impact global C and N cycles considering the proximity to human activities that elevate atmospheric CO2 concentrations and N deposition. The overall objective of our research is to determine mechanistic controls on C and N cycles in urban forests. Our research utilizes a long-term urban forest network and provides an ideal study system to investigate separate and combined effects of abiotic (altered climate and biogeochemical cycles) and biotic (invasive plant spread) factors controlling forest C and N cycling. Preliminary data on forest floor litter volume (mid-summer) indicated that urbanization impacts offset decreased productivity and increased decomposition due to plant invasion in these forests. We hypothesized that urban environmental conditions stimulate tree productivity (e.g., urban heat island) and slow decomposition rates (e.g., recalcitrant leaves). We studied foliar production and decomposition rates in a full-factorial design of urban/rural and invaded/un-invaded forests to test the prediction that urbanization impacts offset altered C and N cycling due to plant invasion.
Results/Conclusions
As expected, total foliar production was greater in urban (210 ± 7.2 g m2) than rural (190 ± 7.5 g m2) forests, yet this was not significant (p = 0.16). More interestingly, significantly greater foliar biomass was collected after November in urban (105 ± 5.1 g m2) than rural (82.0 ± 5.5 g m2) forests suggesting altered leaf drop phenology due to the urban heat island (p < 0.001). Furthermore, in support of our hypothesis, urbanization offset reductions in foliar production in urban invaded forests (180 ± 7.0 g m2) compared to rural invaded forests (165 ± 5.7 g m2; p < 0.004). Similarly, decomposition rates supported our expectation that urban forests (49 ± 0.8 % oven dry mass remaining [ODM]) would have significantly slower decay than rural forests (45 ± 1.0 %ODM, p < 0.05). As expected, rural invaded forests had significantly greater decay rates (42 ± 0.9 %ODM) than rural uninvaded forests (47 ± 0.4 %ODM, p < 0.05). Our research findings indicate altered abiotic conditions in urban forests may offset the negative consequences of plant invasion by increasing foliar inputs to soils while decreasing decay rates, which in-turn enhances C storage and N retention in these forests.