By Brandon Gallagher Watson
Simply put, the point of tree health care is to promote the health of trees. Easy, right? Except one minor point of contention — what do we mean by “health” and how do we know if we achieved it?
These questions appear simple, but start to peal it back and this question becomes more difficult. The definition or, for that matter, even the existence of health has been a hotly debated topic by doctors, scientists and philosophers for thousands of years. To avoid getting into an existential debate on ‘The Meaning of Health,” for our purposes here we will define health somewhere between “a state of being free from illness or injury” and “a tree’s ability to fulfill on its genetic potential.” Now, what data can we collect that would show if our actions as tree health professionals is making a difference?
Before we get into that, first let’s define another term. When we talk about human fitness, we often use the terms “health” and “condition” interchangeably, but with trees these are two different things. Good health means the tree has all the air, water, light and nutrients it needs to perform photosynthesis, grow and reproduce. Good condition means the tree is structurally sound and is a low risk to fail either partially or completely. A tree can be in great health but in poor condition, and, conversely, a tree can be in good physical condition but if, say, the tree was suffering from chlorosis, we would say the tree has a health issue to address. Tree owners tend to collapse these two terms together, and can be surprised when an arborist recommends a removal for a tree seen by the professional as in poor condition while the homeowner just sees green leaves and thinks the tree is fine.
Assessing a tree’s condition is most often done subjectively through various condition rating systems. Commonly these systems use a scale with “flawless specimen” on one end and “dead” on the other. The number of increments on the scale will vary, and can be adjusted depending on the purpose. The International Society of Arboriculture’s (ISA) version uses a 1-10 rating that ranks trees Excellent, Very Good, Good, Fair, Poor, Critical, and Dead. The ISA provides definitions for each category, making determination fairly simple. Other systems may use fewer categories, such as 1-4 rating, with 4 being “excellent condition” and 1 being “remove.” No mater how many condition categories are used, the most important factor for successfully using these systems is consistency, ensuring every evaluator is rating the trees the same.
Certain condition criteria are readily apparent when simply looking at a tree. Red flags such as splits, leaning, and large dead limbs are obvious, but other factors may not be easily seen. Using simple tools like tapping the trunk with a rubber mallet to detect decay cavities can be useful. If a more empirical method is needed, more advanced tools, such as a Resistograph, can give you some real data regarding the thickness of the walls or the extent of a decay column. Other tools, such as a clinometer, can be used to measure not just tree height, but also the angle or severity of tree’s lean.
There are ways we can attempt to quantify a tree’s health as well. Some measurements require specialized equipment or even a laboratory, but others can be simple. Observational rating systems can be useful in quantifying health just as they were when addressing condition. A common leaf color-rating chart, first developed in the 1970s, is a very useful system for visually assessing a leaf’s chlorophyll content. The scale goes from 1-10, but uses the number 7 as “normal” for that species. For example, say we are doing research on optimal rates for an injectable nutrient supplements used to correct an issue such as chlorosis. We would take baseline data on the color of each tree, assigning a number based on how yellow or how green the tree is. Treatments of various rates would be applied to the trees, and follow-up observation ratings would be made. Comparing to the baseline data, we can determine which rates improved the trees to their “normal” color, which were not strong enough, and which rates caused the tree to be abnormally green.
This exact trial could utilize more empirical data by taking leaf samples into a lab and determining the precise quantity of chlorophyll molecules by chemical analysis. Lab work can also yield data regarding nutrient content, secondary metabolites, or even the presence of toxins. Root samples can be analyzed for stored starch content. All of this data can be used as a proxy measurement for a tree’s health. While collecting samples and having them analyzed may yield more accurate results, it also adds to the labor, time, and cost of the trial. Researchers are constantly evaluating which data collection methods give the most accurate and pertinent information while balancing that with the time and budget allotted for the trial.
Evaluating the fullness, or density, of a tree’s canopy is another common way researchers and arborists evaluate tree health. Measurements such as percent fine twig dieback can give an indication of how stressed a tree’s canopy is. Subjective ratings, such as percent-canopy, are frequently used in tree health care. For example, it is commonly said that an infested ash tree can be saved from emerald ash borer if the tree has less than 30 percent canopy decline. There have been visual guides developed to help establish guideline for what 10-, 20- or 30-percent decline looks like that can help create some consistency to this subjective method.
If a more objective measurement is needed, canopy density can be assessed through more empirical methods. Imaging tools such as hemispheric photos taken from below the tree can give data on the fullness of a canopy. Leaf area index (LAI) is a canopy density metric that is frequently used in forestry as a way to predict forest productivity, but it can be used in arboriculture as well. LAI is quantified as the average leaf area per unit of ground surface area, and can be measured by taking a statistically significant number of leaves from four sides of the tree. Area is then either measured manually, by using an electronic area meter, or by using specialized scanning software. The leaf area is then divided by the area under the tree, measured from drip line to drip line, to give you the LAI. This index can be useful for measuring health, as trees that are performing well tend to have normal sized leaves for that species while trees that are struggling tend to produce smaller leaves. Using LAI, researchers can evaluate the responses to different treatments, but, as most trees only produce one set of leaves per year, this can be a difficult metric for more immediate responses.
Measuring how the health of a tree is varying day to day can be more challenging, but, with the right equipment, can yield valuable data. Just as your doctor measures your vital signs — blood pressure, pulse rate, breathing — plants have metabolic processes that can be measured too. Chlorophyll florescence is one of the more common measurements taken to assess overall tree vigor. One of the most pronounced physiological responses to stress is a reduction in net photosynthesis; and using a chlorophyll florescence meter can be a quick way to determine the photosynthetic potential of a leaf. Chlorophyll can also be measured in the field using a handheld SPAD meter that quickly reads the chlorophyll content of leaves.
Arborists of the future will, of course, simply scan the tree with their ArborTron 3000 Healthometer and get instant info on the current health and remedies for every malady conceivable. But until then, we will continue to measure health in all these different ways. There are many other ways not mentioned here that scientists can use to attempt and quantify health; but no matter how or what data is collected, turning that data into actions that help improve the life and vitality of trees is the ultimate goal. Tree health care is still an emerging science and new analysis tools are being created, tested and adopted all the time. Keeping up with the current ways we can quantify tree health can help make you a better tree health professional — at least until the ArborTron 3000 comes out.
Brandon Gallagher Watson is creative director at Rainbow Treecare Scientific Advancements, and is an ISA Certified Arborist (#MN-4086A).