Defining Tree Health

As part of their role surveying trees Observatree volunteers are asked to judge whether a tree is healthy or not. But what exactly do we mean by a healthy or unhealthy tree within the context of Observatree?

Trees support and interact with a range of other life-forms throughout their life cycle, from fungi, bacteria, mosses, and lichens, to invertebrates, birds and mammals. Humans view some of these interactions as having a negative impact on the tree and where they are biological in origin we class them as pests and diseases. Environmental factors can also be viewed as having a negative impact on trees; drought, flooding, high winds and soil disturbance for example, and these are classed as disorders. In its very simplest form, a healthy tree can be defined as one which is free from any of the negative impacts of these biological (biotic) and environmental (abiotic) interactions.

However, there is no clear distinction between healthy and unhealthy trees and any definition is only ‘correct’ in the context of how and when it is applied. What we can do though is make some general statements which are useful in understanding what we must consider when assessing a tree as healthy or unhealthy. There are three key points that must be taken into consideration with any tree being assessed; tree species and growing conditions, tree age and lifecycle, and visible signs and symptoms.

So why is it important to define what we mean by a healthy or an unhealthy tree? A key purpose of Observatree is to build up a long-term dataset which can be used by researchers to look at questions such as ‘how has disease ‘X’ or pest ‘Y’ spread across the UK?’. To enable Observatree volunteers to build the most useful dataset we need to standardise how we assess the trees we’re looking at and minimise any variation between observers.

Tree species and growing conditions 

Each tree species has a unique growth habit, which will encompass a range rather than being tightly bound. Growing conditions play an important role too, trees growing in a forestry setting for example will tend to be more upright and narrower in form when compared to an open grown tree of the same species (fig. 1).

Parks and gardens frequently contain examples of horticultural selections of trees that can display a very different form or colour from their wild ancestors. Weeping branches, contorted stems/leaves, cut-leaved forms, variegated or red foliage, and dwarf trees are all examples of this (fig. 2). In addition, if the tree is in a park or garden, it could have been trained or pruned to adopt a particular form which can also be quite different from the form seen in the wild; bonsai, coppicing or pollarding for example.

Figure 1: different growth forms of Pedunculate Oak (Quercus robur):

 Figure 2: variation in foliage of Common beech (Fagus sylvatica):

Tree age and lifecycle 

Trees go through a sequence of changes as they age; a young sapling oak will look very different from a mature oak tree which could again appear very different from a 500-year-old ancient oak. Bark texture will go from smooth to fissured, branches may fall off leaving stumps or wounds (in ancient and veteran trees there may be substantial accumulation of deadwood), the amount of leaf cover might diminish, etc. This is all part of the normal lifecycle of the tree and should not be judged as unhealthy.

Visible signs and symptoms 

Our assessment of the health status of a tree is based solely on visible signs and symptoms. Whilst the cause of poor health in trees may be cryptic or hidden from view (e.g. root infections caused by a Phytophthora) changes in the outward appearance of parts that are above ground provide our first clue that the tree might be unhealthy. 

 Summary

• Knowing the tree species (and sometimes horticultural variety!), how it has been grown, and what is ‘typical’ for that particular tree gives a standard against which poor health characteristics can be assessed.
• Knowing the approximate age of the tree is important. When assessing a tree as healthy or unhealthy we need to do so against the typical state for a tree of that species at that stage in its life cycle.
• If there are no visible signs nor symptoms of pest infestation, disease, or abiotic stress (including physical damage) we must conclude that at the time of assessment the tree is believed to be healthy.
• Tree health is a continuum, from healthy to unhealthy. No definition is absolute and there will always be exceptions; there is no right or wrong answer.

If in doubt use the precautionary principle, if you see a tree that you judge to currently be unhealthy report it on TreeAlert.