Despite its importance for urban planning, landscape management, and water management, there are very few in situ estimates of urban-forest transpiration. Because urban forests contain an unusual and diverse mix of species from many regions worldwide, we hypothesized that species composition would be a more important driver of spatial variability in urban-forest transpiration than meteorological variables in the Los Angeles (California, USA) region. We used constant-heat sap-flow sensors to monitor urban tree water use for 15 species at six locations throughout the Los Angeles metropolitan area. For many of these species no previous data on sap flux, water use, or water relations were available in the literature. To scale sap-flux measurements to whole trees we conducted a literature survey of radial trends in sap flux across multiple species and found consistent relationships for angiosperms vs. gymnosperms. We applied this relationship to our measurements and estimated whole-tree and plot-level transpiration at our sites. The results supported very large species differences in transpiration, with estimates ranging from 3.2 +/- 2.3 kg x tree(-1) x d(-1) in unirrigated Pinus canariensis (Canary Island pine) to 176.9 +/- 75.2 kg x tree(-1) x d(-1) in Platanus hybrida (London planetree) in the month of August. Other species with high daily transpiration rates included Ficus microcarpa (laurel fig), Gleditsia triacanthos (honeylocust), and Platanus racemosa (California sycamore). Despite irrigation and relatively large tree size, Brachychiton populneas (kurrajong), B. discolor (lacebark), Sequoia sempervirens (redwood), and Eucalyptus grandis (grand Eucalyptus) showed relatively low rates of transpiration, with values < 45 kg x tree(-1) x d(-1). When scaled to the plot level, transpiration rates were as high as 2 mm/d for sites that contained both species with high transpiration rates and high densities of planted trees. Because plot-level transpiration is highly dependent on tree density, we modeled transpiration as a function of both species and density to evaluate a likely range of values in irrigated urban forests. The results show that urban forests in irrigated, semi-arid regions can constitute a significant use of water, but water use can be mitigated by appropriate selection of site, management method, and species.

Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m(T); slope of the relationship between tree transpiration and ln D) and transpiration at D=1 kPa (E(Tref)) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m(T) was significantly related to vulnerability to cavitation of branches (P(50)). While m(T) did not appear to differ between ring- and diffuse-porous species, the relationship between m(T) and P(50) was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring- versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.

Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban water budgets in semi‐arid regions. A detailed understanding of plot‐scale ET and its sensitivity to plant species composition is necessary to improve estimates of urban water vapour fluxes and water balance. We used portable enclosed chambers and empirical equations to quantify ET from (1) unshaded urban lawns covered exclusively by turfgrass and (2) urban lawns comprised of open‐grown trees and turfgrass groundcover in the Los Angeles Metropolitan area. Turfgrass at all locations had a non‐limiting supply of soil water because of regular sprinkler irrigation. ET of irrigated turfgrass reached a maximum of 10. 4 ± 1·3 mm d−1 and was always higher than plot‐scale tree transpiration, which did not exceed 1 mm d−1. In summer, total plot ET of the lawns with trees was lower than lawns without trees by 0·9–3·9 mm d−1. Turfgrass ET was highly sensitive to solar radiation, and the ratio of ET of lawns with trees to ET of lawns without trees decreased with tree canopy cover. Hence, reductions in turfgrass ET caused by shading effects of open‐grown trees were more important in influencing total landscape ET than the addition of tree transpiration. This suggests that low‐density planting of trees that partially shade irrigated urban lawns may be a water‐saving measure in semi‐arid irrigated environments. Copyright © 2013 John Wiley & Sons, Ltd.

1 environment: an in situ evaluation of microclimatic 2 conditions and watering recommendations 3 Abstract 14As many regions worldwide are increasingly affected by water scarcity caused by unprecedented 15 climate change and population growth, current practices of landscape irrigation need to be 16 evaluated to develop scientifically based water-saving recommendations. We compared widely 17 used evapotranspiration (ET) based guidelines of turfgrass watering requirements with ET from 18 8 irrigated turfgrass lawns (4 unshaded and 4 shaded by various degree) measured in situ by 19 portable chambers in the Los Angeles Metropolitan area. ET from unshaded lawns was 40% 20 higher than recommended irrigation inputs and ET from shaded lawns was mostly within 21 recommended ranges. We evaluated the microclimate coefficient (k mc ), a factor used to adjust 22ET from reference surfaces to local microclimatic conditions. While in situ k mc was mostly within 23 the ranges recommended for California, (1) k mc ranges for particular landscape types deviated 24 from observations, (2) minimum and maximum k mc were beyond recommended ranges, and (3) 25 k mc had clear seasonal changes overlooked by current methodology. We propose a method to 26 quantitatively estimate the effect of shade on k mc and improved categories and coefficients for 27 applying the landscape coefficient method under realistic urban conditions. 28 . The central concept of this method is reference ET (ET 0 ), or ET from 54 vegetated surfaces that completely cover the ground and have access to non-limiting soil water. 55 ET 0 is an essentially climatic parameter that characterizes the local atmospheric potential of 56 driving ET (Allen et al. 1998). To standardize methodology, ET 0 is defined as ET from a vast 57 surface of healthy turfgrass that has uniform height and completely covers the soil. ET 0 is 58 estimated using the Penman-Monteith equation (Penman 1948, Monteith 1965, Eichinger et al. 59 1996, Pereira and Perrier 1999 and intended to represent the maximum rate of ET of flat and 60 homogenous vegetated surfaces under current meteorological conditions. To estimate ET from 61 specific agricultural crops (ET c ), ET 0 is multiplied by experimentally determined coefficients (k c ): 62 .

Evapotranspiration (ET), an essential process in biosphere‐atmosphere interactions, is highly uncertain in cities that maintain cultivated and irrigated landscapes. We estimated ET of irrigated landscapes in Los Angeles by combining empirical models of turfgrass ET and tree transpiration derived from in situ measurements with previously developed remotely sensed estimates of vegetation cover and ground‐based vegetation surveys. We modeled irrigated landscapes as a two‐component system comprised of trees and turfgrass to assess annual and spatial patterns of ET. Annual ET from vegetated landscapes (ETveg) was 1110 ± 53 mm/yr and ET from the whole city (vegetated and nonvegetated areas, ETland) was three times smaller, reflecting the fractional vegetation cover. With the exception of May and June, monthly ETland was significantly higher than predicted by the North American Land Data Assimilation System. ETveg was close to potential ET, indicating abundant irrigation inputs. Monthly averaged ETveg varied from 1.5 ± 0.1 mm/d (December) to 4.3 ± 0.2 mm/d (June). Turfgrass was responsible for ∼70% of ETveg. For trees, angiosperm species (71% of all trees) contributed over 90% to total tree transpiration, while coniferous and palm species made very small contributions. ETland was linearly correlated with median household income across the city, confirming the importance of social factors in determining spatial distribution of urban vegetation. These estimates have important implications for constraining the municipal water budget of Los Angeles and improving regional‐scale hydrologic models, as well as for developing water‐saving practices. The methodology used in this study is also transferable to other semiarid regions for quantification of urban landscape ET.