Tipping the balance between ecosystem service and disservice: Considering light availability and soil carbon effects on carbon cycling in urban lawns

Background/Question/Methods

Evaluating the ecosystem service (ES) of urban vegetation requires integrating spatial and temporal heterogeneity. Much research has focused on discrete classifications of ES providers, such as tree or lawn. Environmental heterogeneity, however, can change ES outcomes. Here, we tested two environmental factors which exhibit fine-scale heterogeneity in urban ecosystems – soil carbon (C) concentrations and light availability – for their effects on C cycling in lawns. Both are expected to significantly affect ecosystem function, especially as management supplies otherwise limiting resources. Large shifts in ecosystem function indicate the potential for changes in ES outcomes. The study system consists of 15 lawns, commonly managed, and spanning a gradient of 1.1 to 5.8% total surface soil C. Plots (n=51) within each lawn covered a light gradient ranging from 15 to 90% of light energy reaching the ground. We quantified aboveground net primary productivity and specific leaf area (SLA; leaf area/dry mass) as measures of C inputs and litter quality, respectively; SLA positively correlates with the decomposition rate of litter. 

Results/Conclusions

Productivity ranged from 0.1 to 4.9 g dry biomass·m^-2·d^-1 and was positively correlated with light availability and soil C level. Light availability and soil C level together better explained productivity than either factor alone. In fact, the positive effect of light availability on productivity was greater at high than low soil C levels. Leaves also became denser and thicker under increasing light availability ranging from 67 to 23 mm² leaf area·mg^-1 dry mass. Representing approximately 40% of the global species level range, this range in SLA likely affects litter decomposability. Overall, C cycling in lawns had a large response to light availability and soil C level. Shifts in productivity and decomposition as well as elemental cycles coupled with C cycling can create different ES outcomes. Urban ES work focused on the structure providing the service (i.e., trees, lawns) needs to incorporate ecosystem heterogeneity in order to specify the conditions under which an ecosystem service or disservice is provided.