The effect of urban temperature gradients on grassland microclimate amelioration in Los Angeles, USA

Aim: Microclimate amelioration between neighboring plants may be more common in environments with greater abiotic stress. This pattern has been shown in deserts, alpine systems, and forests, but has not been explored along urban severity gradients. In this study we hypothesized that strong temperature gradients in the greater Los Angeles area might be driving changes in microclimate amelioration in annual grasslands. Location: Twenty-seven sites along a 100-km latitudinal, 72-km longitudinal urban gradient across the greater Los Angeles area in California, USA. Methods: We measured macro- and microclimate variables during the 2019 growing season. We took measurements of temperature, humidity, and vapor pressure deficit (VPD) at the site level as well as under grass canopies. Results: We found strong cooling effects of the vegetation during the day and warming effects from vegetation at night. We found that these effects were strongest on the hottest/driest days and at the hottest (and often most urban) sites. Conclusions: Our microclimate amelioration data suggest that positive interactions might become stronger along urban temperature gradients and may be determining plant interactions in these areas in a way that was not previously considered.