Influence of tree cover on summertime surface energy balance fluxes, San Gabriel Valley, Los Angeles

Trees are an important but little studied component of the urban canopy which have distinct climatic effects. This study investigates the influence of trees on local-scale surface energy balance fluxes. Simultaneous energy balance observations were conducted using eddy correlation methods for 2 suburban neighborhoods with higher (30%) and lower (10%) tree and shrub cover, in the San Gabriel Valley of the Los Angeles Metropolitan Area, California, USA. Data were collected on the materials and morphology of the urban surface through a combination of aerial photo analysis and field surveys and analyzed using a geographic information system. Information on external water use was obtained from questionnaires and the analysis of water use data from bi-monthly bills. In terms of the relative partitioning of energy, the effects of the trees are as expected: at the higher tree coverage neighborhood (HTN) the latent heat flux is increased as a fraction of net all-wave radiation, so too is the storage heat flux, whereas the sensible heat flux is decreased. However, in absolute terms, all fluxes, including the sensible heat flux, are enhanced at the HTN. A combination of lower albedos and lower surface temperatures in the HTN result in reduced loss of solar and longwave radiation respectively. Thus at the HTN there is greater net all-wave radiation, hence a greater amount of energy to be dissipated. Above the canopy, temperatures are slightly greater in the neighborhood with higher tree cover.