Vegetative microclimate effects on temperature and humidity in urban grasslands

Background/Question/Methods

Cities impose strong temperature gradients over very short land areas, and thus provide a perfect opportunity to test theories about the role of vegetation in moderating temperature effects. This type of microclimate amelioration has been shown in Mediterranean ecosystems in the past, but never before within the context of urban heat islands. Importantly, temperature gradients may drive changes in mechanisms of coexistence and maintenance of biodiversity in urban plant communities. In particular, facilitation due to microclimate amelioration is strongest when environmental conditions are severe, thus facilitation may be more important in hot urban parks and less important in neighboring rural areas. Here we used 27 sites in the greater Los Angeles area to assess how urbanization affects biodiversity and exotic species invasion in these urban grasslands and how vegetation ameliorates temperature and humidity along this urban temperature gradient.

Results/Conclusions

We found that grassland vegetation cools the microclimate of hotter urban sites by as much as 4°C during the day, and warms the microclimate of cooler rural sites by as much as 6°C at night. We found that vegetation can increase the humidity of hotter urban sites by as much as 20-30%. We also found that alpha diversity of urban grasslands was lower than nearby rural grasslands, but beta diversity was higher at our urban sites (due mostly to performance of invasive species). We use this framework to propose future experimental work to assess how competition and facilitation shift along urban temperature gradients and drive changes in plant community dynamics.