Tree transpiration in a multi-species Mediterranean garden

Urban trees provide benefits and services, like improving environmental quality and mitigating impacts of climate change on human health, e.g. through the reduction of greenhouse gases effects, the removal of pollutants from the atmosphere and the improvement of water quality through interception of pollution. Tree transpiration in a multi-species urban garden in southern Italy was investigated for one year, on hourly, daily, seasonal and annual time scale. Water status of trees was determined by means of the Crop Water Stress Index (CWSI). The difference between canopy and air temperature was calculated using transpiration measurements and the canopy energy balance. Transpiration was measured by heat dissipation method (HDM) in more than 14 tree species, grouped in four classes (Olea, Citrus and Eriobotrya, Conifers and Broadleaves) according to number of specimens in the garden and similarity in plant functional category. At single tree scale, the radial trend of sap flux density was modeled considering the sapwood area, while the upscaling to the garden level was performed using the quantile method integrated by trunk diameter classification. Complete daytime time series of sap flux density were obtained by gap filling algorithms, based on multivariate models based on environmental drivers of transpiration. Tree transpiration showed that Olea and Citrus and Eriobotrya had a good adaptation to Mediterranean climates. CWSI values indicated that Conifers suffered a moderate water shortage, while the other species were subjected to higher water shortage. Conifers showed the maximum efficacy in lowering air temperature, followed by Broadleaves, Olea and Citrus and Eriobotrya. The uncertainties were evaluated by error analysis.