Variation in leaf area density drives the rainfall storage capacity of individual urban tree species

Abstract

A rapid rise of urban population is making cities denser. Consequently, the proportion of impervious surface cover has enlarged, increasing the amount and speed of run‐off reaching urban catchment areas, which may cause flash flooding. Trees play a key role to reduce run‐off in the city, as they intercept rainfall and store part of it on their leaves and branches, reducing the amount and speed of water running onto impervious surfaces. Storage capacity will depend on the rainfall event, the climate conditions and tree characteristics and canopy density. These canopy characteristics vary greatly among different species and their phenology. Furthermore, these canopy characteristics can vary greatly among individual trees of the same age, size, and species. This study tested how canopy density and leaf characteristics of three different tree species affect storage capacity under simulated rainfall conditions. Three species were selected (Ulmus procera, Platanus × acerifolia, and Corymbia maculata), each being of the same height and similar canopy dimensions. Storage capacity was measured using a mass balance approach during a 15‐min indoor, simulated rainfall event (2.5 mm/hr). Canopy metrics were estimated using a terrestrial laser scanner. Canopy surface area was measured through destructive harvest and leaf/twig/branch scanning. To investigate variations in the canopy leaf density, leaves were systematically removed to create four treatments: full, half, quarter, and woody. Canopy storage capacity was well correlated to plant surface area (m2), plant area index (m2/m2), and plant area density (m2/m3). All analyses indicated U. procera as the most efficient species for storing rainfall water within a canopy of equal volume or area index. Results reveal the complexity of evaluating interception of rainfall by tree canopies. This study contributes to the discipline and practice by distinguishing how variation in the leaf density is important to consider when selecting urban tree species to be planted.

Terrestrial Laser Scanning to Predict Canopy Area Metrics, Water Storage Capacity, and Throughfall Redistribution in Small Trees

“…These leaves were oven-dried and their surface area estimated from the ratio of leaf area to dry leaf mass. The canopy water storage measurements and TLS scanning were then performed on these quarter canopies, and then finally, all remaining leaves were removed and oven-dried, and then their leaf area was calculated for each canopy density treatment [43]. The final leaf removal process resulted in a canopy without leaves (or 'Woody').…”

“…After water storage capacity measurements and TLS scanning, the woody material for each tree was collected and divided into two groups depending on their diameter class: ≥1 cm or <1 cm. Each branch group had its length, area, and volume estimated [43].…”

“…Full details for this method and for keeping constant precipitation rates, uniformity, and raindrop size are available in Refs. [43,44]. their leaf area was calculated for each canopy density treatment [43].…”

“…[43,44]. their leaf area was calculated for each canopy density treatment [43]. The final leaf removal process resulted in a canopy without leaves (or 'Woody').…”

“…Full details on how the rainfall simulator was constructed, trialled, and operated with trees beneath are presented in Ref. [43].…”