Surface water storage capacity of mature street trees in Davis, California

Rainfall interception by the urban forest benefits stormwater runoff management because it reduces the amount of runoff and delays the time of peak runoff. Canopy surface water storage capacity (S) and maximum water storage capacity (Smax) are critical parameters influencing the rainfall interception process. The purpose of this study was to directly measure S and Smax for mature street trees in Davis, California. To determine values for these parameters leaves and stems were weighed before immersion in distilled water, immediately after immersion (Smax), and after excess water dripped off the samples (S). Samples were taken randomly from three crown layers: top, middle, and bottom. All samples were at least 15,625 cm3 (25cm X 25cm X 25cm). Leaf and stem surface areas were directly measured after the samples were wetted and weighed. Initial results from this study suggest that S and Smax vary by species and by age of the segment of stem sampled. S for leaf surfaces of the Holly oak was 52% and 62% greater than the Callery pear and Canary island pine tree. However, S was 72% greater for stem surfaces of the Canary island pine than for the Holly oak. Bark surface roughness controls the S, while absorbtivity also influences Smax. Chinese hackberry, which has rough bark, held 58% more water than the sycamore, which has a relative smooth bark surface. The Smax varied by 50% for these two species. S and Smax varied less than 30% for tree species with bark that retained approximately the same roughness regardless of age, such as pear, sycamore, and zelkova. S was influenced more by stem age than crown layer. S varied by as much as 59% for young and old branches of gingko and by 25% for different aged branches of Chinese hackberry.