Differential Impacts of Passive versus Active Irrigation on Semiarid Urban Forests

Abstract

Trees provide benefits to the urban environment and irrigation is common to support these ecosystem services. In dryland communities where water resources are limited, collection and retention of stormwater runoff is used to passively irrigate the urban forest. However, the effects of passive irrigation versus regular, controlled moisture inputs, or active irrigation, is largely unquantified. We monitored the ecohydrology of urban mesquite trees (Prosopis spp) under these contrasting irrigation regimes in semiarid Tucson, AZ. Measurements included soil moisture, sap flow, canopy greenness, and leaf-area index. We expected both irrigation types to provide additional deep (>20 cm) soil moisture compared to natural conditions, and that trees would depend on this deep soil moisture for transpiration and phenological activity. Results show that active irrigation supported higher soil moisture, sap flow, and greenness during the dry conditions of spring. Following summer rain, greenness was higher under passive irrigation, despite sustained elevated soil moisture under active irrigation. Deep soil moisture had only slightly stronger controls over mesquite productivity than shallow moisture, and these relationships were stronger in the spring, rather than summer months. Finally, passive irrigation generally failed to provide additional deep soil moisture, though treatments in closer proximity to impervious surfaces did provide wetter soil conditions. This research aims to contribute empirical observations of green infrastructure performance and improved understanding of urban forest function for watershed management and planning.