Standing volume yield, canopy structure and allometric relationships of dominant urban trees in Melbourne, Australia

Most allometric equations currently used to quantify above-ground biomass of urban trees are derived from natural forest stands and can produce unreliable estimates of biomass for individual trees grown in open conditions. In addition, distribution of standing volume between stem, branch and foliage in solitary grown urban trees is poorly understood. In this study, a total of 45 trees of three species (Ulmus procera, n = 15, Corymbia maculta, n = 15 and, Platanus x acerofolia, n = 15) of different size (small, medium and large) were measured. For six trees of each species (18 in total), the stem and all major branches were fully mapped so that an accurate wood volume was established, and various random branch samples (RBS) and composites could be used to estimate standing volume. For the remaining nine trees of each tree species standing volume was estimated by only measuring five RBS pathways. Allometric relationships between diameter and volume were explored using linear, quadratic, cubic, log and exponential models. Slenderness ratio (SR) and bifurcation ratio (BR) were derived to explore canopy structure among species. U. procera trees showed the greatest BR, followed by P. x acerifolia and lowest in C. maculata. In U. procera, SR displayed a decreasing trend from scaffolds to third order branches while the opposite was true for C. maculata and P. x acerifolia. Number and dimensions of scaffolds, first and second order branches of small and medium size trees were similar but differed from that of large trees. Overall, standing volume distribution among species was 40% to the main stem and 60% to the crown. Across species and tree size class, RBS systematically under- or over-estimated standing volume in the range of -30% to +42% as compared to full tree measurements. Regardless of tree size, allometric relationships were similar within species but varied among crown structural parts. Linear models better explained diameter-volume relationships of whole tree and main stem across species. Log-Log and polynomial models better described diameter-volume relationships of branches of all orders. These findings suggest that a set of allometric equations could be developed to more accurately predict standing volume yield of urban trees.