Arboriculture & Urban Forestry 36(4): July 2010 Canopy diameter as a predictor of root spread Canopy diameter is convenient for purposes of visually estimat- ing tree root spread, but the relationship between canopy and roots is highly species dependent (Tubbs 1977; Gilman 1988). In one study, the largest roots of young nursery trees were exca- vated to their full length and the relationship between canopy and root spread determined (Gilman 1988). Root system diameter averaged 2.9 times the diameter of the canopy, but ranged from 1.68 times the canopy for Fraxinus pennsylvanica (green ash) to 3.77 for Magnolia grandiflora (Southern magnolia), and in Quer- cus virginiana (live oak), no relationship could be established. In a study of young forest trees on a clearcut, Acer saccharum (sugar maple) average root spread (not the maximum) was found to equal canopy spread, while in Betula allegheniensis (yellow birch) the average root spread was greater than canopy spread (Tubbs 1977). In addition to species variation, root spread may not be symmetrically situated beneath the canopy, even in unre- stricted soil, especially if a tree is leaning. For example, Tubbs (1977) documented root systems that were approximately the same diameter as the canopy spread, but the distribution was not directly beneath the canopy—one side of the root system extend- ed far beyond the dripline while the other side extended much less than half the canopy diameter. Tubbs (1977) also observed the root systems were generally distributed away from the lean of the tree. A similar phenomenon has been observed for trees on slopes: a majority of the root system may be located on the uphill side of the tree (Di Iorio et al. 2005). All reviewed studies relating tree canopy and root system spread used trees less than 18 cm in trunk diameter, meaning it is possible these relationships would change for mature trees. In summary, canopy spread is not likely to be a successful predictor of root spread unless a relationship is established for a particular species and it is clearly recognized that root distribution may not correspond to canopy distribu- tion. Even then, these relationships may not hold for older trees. Trunk diameter as a predictor of root spread Trunk diameter is often used to estimate tree root spread; mu- nicipal ordinances frequently specify this method for determin- ing tree protection zones (TPZs) and ensuring adequate soil resources for preserved trees. For the current study, the authors employed nonlinear regression to investigate the relationship between trunk diameter and maximum root spread using avail- able published data (Figure 2). A much stronger relationship (R2 = 0.89) was found when relating root spread to trunk diameter rather than tree height. The relationship reaches an asymptote at approximately 25–30 cm of trunk diameter. Analysis of the linear portion of the regression (0–20 cm trunk diameter range) deter- mined the average ratio of predicted root system radius to trunk diameter is 38:1. Thus on young trees, root system radius may increase by 38 cm for every cm of trunk diameter. However, on older trees, this relationship changes, and root extent increases very slowly relative to trunk diameter. Most, but not all, of the species assessed were medium or large-stature trees, and all were trees that experience secondary growth (i.e., not palm trees). It would be expected that smaller stature species would exhibit diminished root system expansion at a smaller trunk diameter. The strong relationship between trunk diameter and root spread shown above supports the practice of designating TPZs based on trunk diameter. Harris et al. (2004) suggest suitable Figure 2. The relationship between trunk diameter and maximum root radius from summarized literature. Each data point repre- sents a study average, see Appendix for data sources and N values. CONCLUSIONS AND FUTURE RESEARCH Root depth and extent can be severely limited and highly irreg- ular in urban settings. When root restrictions are minimal, root spread shows a strong relationship with trunk diameter, which is a more reliable predictor than canopy diameter or tree height. During the first part of a tree’s lifespan, the ratio of root sys- tem radius to trunk diameter is about 38:1. However, consider- able variation can be expected due to species and site conditions. Expansion of the root system relative to trunk growth appears to slow down as a tree matures. Root depths greater than 2 m have been documented for several urban species, and genetic control over rooting depth is evident within species. Nonethe- less, urban sites frequently restrict rooting depth, and vegetative propagation of deep-rooted selections has not been successful. Deep roots may confer a number of advantages including the ac- quisition of additional water and mineral resources, potential for ©2010 International Society of Arboriculture 153 TPZs have a ratio anywhere from 6:1 (radius of TPZ:trunk di- ameter) for young or tolerant trees, to 18:1 for old trees of sen- sitive species (note these ratios are unitless). According to the authors’ predictive model, prescribing a TPZ on the low end of this scale protects a relative small portion of the root system. Thus, at young ages, root systems would only be partially pro- tected. As trees age and become more vulnerable to injury from disturbance, more of the root system would be included in the TPZ. On very large and old trees, it is likely that the entire root system should be protected, even allowing for some irregulari- ties in root distribution, which would be wise given their rela- tive intolerance of root disturbance. In summary, trunk diameter can provide a reasonable estimate of tree root spread as long as one recognizes: 1) individual trees will vary from the estimate, perhaps considerably; 2) root spread may be irregular and not uniformly distributed around the trunk, especially when trees are leaning or located on a slope; and 3) physical constraints, such as confined urban planting pits (Gerhold and Johnson 2003), or other structures may limit root growth in certain dimensions.
July 2010
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