Functional Relationships Between Branch and Stem Wood Density for Temperate Tree Species in North America

David W Macfarlane

doi:10.3389/ffgc.2020.00063

Abstract

Wood density is strongly related to key aspects of tree physiological performance. While many studies have examined wood density in different parts of trees for a variety of reasons, there are very few studies that have compared within-tree density variation across many trees, of many species, drawn from a large geographic area. Here, a large data set representing trees of 78 species/genera in the Eastern United States was compiled and analyzed to explore branch to main stem wood basic density relationships. The expectation was that differences in stem versus branch wood density among trees would be due to both genetic constraints and plastic responses in wood properties, due to tree growth responses to external environments. The results show a wide tree-to-tree variation in average branch density, relative to main stem density. However, there was a general pattern for overstory tree species to have high tree branch density relative to stem density, at lower stem densities, and a declining branch to stem wood density ratio as stem density increased. Evergreen gymnosperms showed the strongest change in branch to stem wood density ratios over the range of stem wood densities and deciduous angiosperms the least; deciduous gymnosperms showed an intermediate pattern, but with generally higher branch- than stem- wood densities. More cold-hearty, shade-tolerant / drought-intolerant, evergreen gymnosperms, growing at higher latitudes, showed higher branch to stem density ratios than more shade-intolerant / drought tolerant evergreen gymnosperms growing at lower latitudes. Across all trees, canopy position had a significant influence on branch to stem density relationships, with higher branch to stem density ratios for canopy dominant trees and successively lower branch to stem density ratios for trees in successively inferior canopy positions (in terms of light availability). Understory tree species, which remain in the forest understory at maximum height, showed generally lower branch than stem densities over a wide range of stem densities. The results suggested that tradeoffs between mechanical safety and whole-tree hydraulic conductance are driving within-tree differences in wood density and highlighted the need for more detailed examinations of within-tree density variation at the whole-tree level.

Highlights

Wood density is strongly related to key aspects of a tree’s physiological performance and mechanical structure and is a key trait for determining wood quality, as it is strongly correlated with the properties of many forest products (Zhang and Morgenstern, 1995; Rozenberg et al, 2001)
There were clear differences in the relationships, depending on the functional type (Figure 3 and Table 3), with deciduous angiosperms being the only type for which both small understory trees and canopy trees were present in the data base
The results show a high degree of variation in average branch- vs. stem- wood density for North American tree species, and a strong linear relationship that generally supports

Summary

Introduction

Wood density is strongly related to key aspects of a tree’s physiological performance (e.g., hydraulic conductance, e.g., Markesteijn et al, 2011) and mechanical structure (e.g., risk of trunk failure under wind loads, Telewski, 2012) and is a key trait for determining wood quality, as it is strongly correlated with the properties of many forest products (Zhang and Morgenstern, 1995; Rozenberg et al, 2001). Databases of species-specific values of wood density are often derived from wood samples drawn from wood in the lower stem or trunk (e.g., Chave et al, 2009; Niklas and Spatz, 2010), often measured at breast height on standing trees (1.3 m above the ground) (e.g., Gao et al, 2017). This widespread sampling bias is partly a product of the convenience of measuring trees close to the ground, but likely because the most valuable wood is found in the lower trunk of tree. Recent studies have shown that trunk-based estimates of wood density can lead to bias in tree or stand -level estimates of forest biomass or carbon (e.g., Momo et al, 2018)

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