City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!
Issue link: http://read.dmtmag.com/i/232632
CT: What sort of observations need to be made? Do they take a lot of time? Because of the differences among species and sizeclasses in the display of health traits, the method in Urban Tree Health begins with the supposition that no single trait suffices to estimate the health of every tree. Instead, five different traits, or "parameters," are evaluated to the extent permitted by season, individual tree, local conditions, etc.: ratio: proportion of tree height occupied by live crown opacity: proportion of darkness in upper crown (inverse of transparency) vitality: proportion of crown free of young twig death (inverse of dieback) growth: average shoot extension quality: proportion of foliage in the upper crown not chlorotic, necrotic, or undersized To make field work faster, these parameters are placed into 20% (for growth: 2 in/5 cm) categories. This simplification does not compromise the results, however, because small observational differences—say, between 20% and 30% vitality or between 40% and 45% opacity—cannot be interpreted reliably as a difference in health due to the large amount of variability in the natural world. CT: Why do you change the names of two parameters from the ones already well-established by the USDA Forest Service protocols? No question that it would be easier to keep "transparency" and "dieback" instead of switching to "opacity" and "vitality," respectively. But it is important that the scales be consistent: the highest category should indicate the same value judgment for each parameter. A consistent order is not only easier for field work, but also facilitates the derivation of a health statement from the observations that is the final result needed in urban forestry work. CT: How do you get a health statement from observations like that? (left) Observation of this London plane tree (Platanus x acerifolia) would record an opacity rating of 40-60%. That raw score needs to be compared to the norm for the species and size-class to produce the number of deviations (here: 1) used to produce the Health Score. www.urban-forestry.com To begin with, even the most objective measurement must be interpreted against the expected norm for species and size-class. For instance, a reading of 20-40% opacity may not have the same maintenance implications for many young trees as it does for older ones of the same species. Similarly, the same opacity reading might be 3 categories below the norm (80-100%) for a mature basswood (Tilia americana), 2 categories below for a London plane (Platanus x acerifolia), 1 below for a honeylocust (Gleditsia triacanthos), but 0 below for a paloverde (Parkinsonia florida). The solution to this problem is to normalize the five parameters with respect to species and size-class, then determine the deviations in category from expected norms. The record of the deviations from the norms for each parameter makes up the tree's Health Score, which might look something like this for a particular tree (Health Score is in the blue ellipse): Parameter Ratio Opacity Observed 5 2 5 3 5 Deviations 0 3 0 0 0 Vitality Growth Quality Such a Health Score is much easier to understand than the raw observations, and unlike them it can be used to draw conclusions about the tree's health. I put it directly into my reports and inventories, as I find it a very persuasive and professional method of discussing tree health when trying to answer questions about life expectancy, resistance to decay, etc. CT: How has the professional community reacted to the book and its method? Overall, very well indeed. I have received—and greatly appreciate—many emails from people telling me they now use the method for everyday work, which is very gratifying. Moreover, I know of two national organizations that are incorporating the method into their work. Finally, it is significant for me that formal reviews by consultants and academics (excerpts on the ISA Webstore page and full texts downloadable from my website, see below) have been extremely positive. For further information about Urban Tree Health: ISA Webstore: http://www.isa-arbor.com/store/product.aspx?ProductID=363 Urban Forest Analytics store: http://www.urbanforestanalytics.com/store 17