Strategies for cooler cities? Ecophysiological responses of semi-arid street trees to storm water harvesting

As the southwestern U.S. moves into an uncertain future in terms of water supply and climate, communities are seeking creative ways to harvest urban runoff. One such solution is to implement water-sensitive urban design features such as rain basins, which are designed to capture and facilitate infiltration of precipitation and storm water as it runs off impermeable surfaces like streets and sidewalks. Rain basins essentially act as temporary cisterns, allowing a given rain event to have a much larger impact in recharging soil water profiles. In this sense, even a 'small' rain may yield a more saturated soil profile and stimulate plant physiological activity well beyond plants that lack this additional moisture input. However, the impacts of rain basins on plant function remain unquantified. Therefore, the purpose of our research is to characterize the performance of native mesquite trees in basins relative to non-basin native mesquites. To answer our question, we randomly sampled basin and non-basin native mesquites in two different neighborhoods in Tucson, AZ, and characterized their response to precipitation events. We measured stomatal conductance, a proxy for transpiration, on the first and third days following rain events in 2013. Numerous environmental factors, such as photosynthetically available radiation (PAR), temperature, relative humidity, and soil moisture, were also measured in order to explore relationships with conductance. These measurements were conducted before and during monsoon season in order to determine the significance of water in basin performance, enabling us to better characterize plant response to medium (6 to 12 mm) rain events. Findings from this study indicate that basin and non-basin mesquites have similar pre-monsoon conductance rates, with a mean basin value of 70 +/-10 mmol/(m2*s) and a mean non-basin value of 57 +/-6 mmol/(m2*s) at peak conductance. In contrast, during the monsoon, basin mesquites showed significantly higher peak conductance rates (179 +/-22 mmol/(m2*s)) than non-basin trees (126 +/-9 mmol/(m2*s)). Perhaps more importantly, basin mesquite conductance remained elevated for an extended period of time into the afternoon as compared to non-basin mesquites. While this difference was negligible before the monsoon, it was significant during the monsoon. The day immediately after a medium rainfall event, non-basin mesquites shut down around 13:00, while basin mesquites never shut down completely before the end of the measurement period around 17:30. Soil moisture levels were elevated in the rain basins relative to the non-basin soils, suggesting that basins impact plant functioning through enhanced soil water availability. These preliminary results demonstrate that basins are an effective means of capturing water and irrigating plants. Here we have demonstrated how an appreciation of wildland plant ecophysiology can be applied to an urban setting in support of a suite of ecosystem services. Notably, there is a potential for enhanced urban heat island mitigation in semi-arid cities through the application of water-sensitive urban design features such as rain basins, due to their supporting a longer duration of latent heat flux cooling (i.e., transpiration) into the afternoon.