Judging the Condition of A Shade Tree
Making a tree condition appraisal can be highly subjective. One individual may judge a tree as fair condition while another may label it as good condition. In some instances, there can be a disparity of two or more condition classes—at Tree Matrix, we think this variance calls for a standardization of appraisal methods. Below is a summary of Bruce L. Webster’s 1978 whitepaper in the Journal of Arboriculture, a Guide to Judging the Condition of A Shade Tree.
The Forestry Division of the South Dakota Game Fish & Parks was faced with this very problem in 1973 when the community forestry program was launched. They hired a summer forestry student to conduct the street tree surveys. Those surveys required the student to note species, size, and condition. Since most upper-level students already took dendrology, species identification posed little problems. However, condition class proved to be difficult to demonstrate due to the lack of standardization.
By 1974 they devised a formula for determining tree condition. This formula utilized 5 factors and assigned a rating for each. These factors utilized the visibly identifiable characteristics of a shade tree to avoid subjective errors. The formula helped to create consistency among surveyors. The method was revised several times and then included in the Guide to the Professional Evaluation of Landscape Trees, Specimen Shrubs and Evergreens produced by ISA, AAN, and other related organizations.
The Condition Guide
The guide to judging shade tree condition utilizes 6 factors: trunk, growth rate, structure, insect and disease problems, crown development, and life expectancy. Each factor is given a rating between either one and three or one and five. These ratings are all based on easily identifiable visual characteristics.
Trunk Rating (1-5): A tree trunk receiving a five (the best rating), is sound and solid throughout with no visible deterioration or damage to the bark and cambium. Minor damages would subsequently drop the score. A tree with a rating of three would show signs of decay by the presence of a conk or other means. A tree with excessive decay would receive a two and would receive a one when coupled with a hollow trunk.
Growth Rate (1-3): Growth rate is determined by measuring annual twig elongation. If growth rate exceeds 6 inches it receives a three, 2-6 inches receives a 2, less than 2 inches, receives a rating of one. However, these ratings apply generally for medium growth trees. Other species that grow quickly or slowly may fall under a different range of growth values.
Structure (1 -5): The structure of a tree addresses the development and placement of the major limbs and branches or as we call it in horticulture, the scaffold. The rating is determined by three characteristics, radial placement of limbs, dead, broken or missing limb, and narrow crotch angles. A top rating of five means no major dead limbs, no narrow crotch angles and strong radial distribution of branches. Detriments to these factors subsequently reduce the score.
Insects and diseases (1-3): No pests or insects present in the tree warrants a top rating of three. If there are one or two minor insect or disease problems present, such as leaf feeders or leaf diseases, it would receive a two. If the insect or disease problem is serious, such as a canker disease, wilt disease, bark beetles or wood borers, the tree would receive a one. Other environmental factors such as air pollution, herbicidal damage and flooding can also contribute to insect and disease problems.
Crown development (1-5): Crown development is based on balance and crown density. It indicates problems such as over- crowdedness, competition, dominance, etc. A rating of 5 indicates a dense leafy crown that is evenly balanced on all sides. Lower ratings are warranted by unbalanced or thinning crowns.
Life expectancy (1 -5): Life expectancy is rather subjective because it is based on prediction and probability. Life expectancy is a catch-all factor as it considers all preceding rating factors. Some characteristics that influence life expectancy can include historical data about the tree or its quality of care. A tree with a history of defoliation might have a lower rating in life expectancy, even though all other factors point to a highly rated tree.
The ratings for life expectancy are 5 for over 30 years, 4 for 25-30 years, 3 for 15-20 years, 2 for 5-10 years, and 1 for 5 years or less.
Rating for Condition Classes
At the end of a condition evaluation, a tree can receive a score from 6 to 26. The below examples illustrate how a real-life evaluation might transpire:
Example #1: Tree has a sound trunk with undamaged bark (5). Its growth rate is 4" (2), and the structure or scaffold has no narrow crotch angles or broken, dead or missing limbs (5). There is a leaf-chewing insect present (2), and the crown is well balanced and quite dense (5). Its life expectancy is 25 years (4).
The sum of the ratings for the six factors is 23, hence it falls into the 80-100% or excellent condition class.
Example #2: Tree has a sound and solid trunk (5), growth rate is less than 1" per year (1). The structure is good except for one major limb is broken (3). There is a leaf disease present (2), and the crown Is relatively balanced but thin (3). Life expectancy is 15 years (3). The total rating of these six factors for tree two is 17, hence it has a classification of 40-60% or fair condition class.
Example #3: Tree is split open and hollow (1). Its growth rate is 4 " (2). There are two major limbs missing and another with a narrow crotch angle (1). Wood borers and a canker disease are present (1). What little crown does exist is fairly dense, but is very lopsided (2). Life expectancy is 5 years (1).
The ratings for tree three add up to 8, putting this tree into the very poor or 0-20% condition class.
This method of using numerical ratings to determine tree condition does not completely eliminate subjectivity, but it helps to greatly reduce it and opinions still vary over the criteria used for tree condition ratings as well as how to weight each factor.
If you’re looking for a better way to quantify tree condition, explore Tree Matrix’s Predictive Analytics Engine and remove uncertainty from your tree's survival before breaking ground on a project!