A New Approach for Studying Carbon Content of Urban Trees Using Non-Destructive Measurements of Tree Structure and Wood Traits.

Abstract

Accurate quantification of urban tree carbon is important for climate change mitigation. However, many estimates rely on allometric equations developed in natural forests, published wood density values, and a standard assumption that dry biomass is 50% carbon. These assumptions may not represent urban trees, which grow under very different and heterogeneous conditions. For example, street trees often face greater site constraints and stress than park trees, which can affect physiological processes and carbon accumulation. In this study, we combined terrestrial laser scanning (TLS) for aboveground woody volume estimation with direct measurements of wood density, elemental analysis of wood carbon concentration (C%), and microscopic analysis of xylem vessel anatomy from increment cores. We sampled 90 trees of three species: Quercus falcata, Quercus lyrata, and Taxodium distichum, equally from street and park environments in Auburn, Alabama, USA. The first chapter provides the conceptual and methodological foundation for the thesis. In the second chapter, we examined how species and growing environment affect wood C%, total aboveground carbon (AGC) biomass, and xylem vessel anatomy. Wood C% differed among species, with T. distichum having the highest and Q. falcata the lowest. In the two oak species, park trees had higher wood C% than street trees, whereas T. distichum showed no significant difference between environments. AGC scaled similarly with diameter across environments, but park trees accumulated more carbon at a given age than street trees. Microscopic analysis showed that park-grown oaks developed fewer but larger vessels, while street-grown oaks had more but smaller vessels. Vessel size was positively related to main-stem carbon biomass, but it explained only a small portion of the overall variation.

In the third chapter, we used TLS to quantify carbon allocation within sampled trees and compared TLS-based estimates with i-Tree-based estimates. Using measured wood density and carbon content instead of published defaults significantly affected AGC estimates in a species-dependent manner. TLS enabled component-level analysis, which showed that carbon allocation between the main stem and branches was species dependent, with Q. lyrata showing higher branch carbon relative to stem carbon than Q. falcata and T. distichum. Branch carbon decreased sharply with increasing branch order and was concentrated in large, low-order branches in the lower crown. TLS-based and i-Tree estimates were strongly correlated, but absolute agreement varied by species and was sensitive to the Crown Light Exposure (CLE) adjustment (open-grown biomass adjustment factor in i-Tree), which improved estimates for some species but worsened them for others. Collectively, this study demonstrates that species and growing environment significantly influence both the biology of carbon storage and the precision of carbon estimation methods. The findings support the use of species-specific wood properties and TLS-based structural measurements for improving urban forest carbon accounting, developing urban-specific allometric equations, and guiding species selection and management strategies that enhance urban forest carbon sequestration.