Microclimate and facilitation trade-offs along a Los Angeles urban gradient

Abstract Background and aims Urbanization is a stressor that can exacerbate climate change impacts. Urban ecosystems are hotter and drier than their rural counterparts, due to a lack of vegetation and an abundance of impermeable surfaces that characterize cities. Urban greening projects can reverse these trends through microclimate cooling and humidification. Vegetation and aridity gradients have the capacity to modify how neighboring plants interact with one another: plant neighbors may benefit one another through facilitation in hot and dry environments but experience more competition in cooler and more humid areas. We examined how herbaceous plants can ameliorate heat and water stress across an established urban aridity gradient in the greater Los Angeles area. Methods We measured 162 plants at 81 plots in 27 sites across a well-established urban-to-rural gradient. In each plot, we identified two pre-existing focal individuals: we removed the neighboring plants for one of these, and left the community intact around the other. We then measured growth of the two focal plants and placed temperature and humidity sensors near each. We also measured light above and below the canopy and total precipitation. Results We found that across the urban aridity gradient, the hottest sites experienced the most vegetative microclimate amelioration. We also found that neighboring plants had positive effects on growth (facilitation), but only when neighbors also cooled microclimate temperature and decreased microclimate vapor pressure deficit (VPD). Facilitation between neighbors was also greatest at the sites with the least precipitation. Conclusion Previous studies have shown that plants may compete for limited resources but we show that interactions can shift to be more facilitative along an urban aridity gradient. Future work should use this information to augment restoration plans in urban environments.