Tree Age
ASSESSING THE AGE OF A TREE
Unless one's lucky enough to have a record of the planting date of a tree, trying to determine its age is always tricky unless one has the opportunity to count its annual rings. However, unless you happen to have an increment borer, that generally involves cutting down the tree! But even then it can be less than straightforward as it is not always easy to distinguish the individual rings; and, of course, many old trees have decayed or hollow interiors where there will be no rings to count!
However, the fact that trees do in general grow incrementally, laying down additional tissue around the trunk each year, the size of the trunk can provide some guidance as to its age. If you put a tape around the tree and measure its girth at about chest height (regarded as being at 1.5m about ground level*), you can reckon that each inch (or 2.5cm) represents one year's growth. It should be stressed that this is only a very crude rule of thumb, and thumbs come in a range of sizes! However it works well enough to give one some idea for most trees in middle age. It doesn't work for young trees, which are generally growing fast, nor for very old trees, whose growth rate has slowed; and neither does it work for trees known for their fast growth rates, such as the Giant redwood (Sequoiadendron gigantea)
* Guidance on how to measure trees can be found HERE
The rate of tree growth varies widely, not only between different species but also as a result of different growing conditions, while the growth rate of individual trees can vary quite widely depending on their stage of maturity: they tend to put on incremental growth faster when young with growth slowing as they gain maturity, often becoming very slow in old age.
John White of the Forestry Commission has developed a system for assessing the age of trees based on their varying rates of increasing girth at different stages of their lives. It cannot be regarded as providing incontrovertible evidence of age as so many variable factors are involved, but it gives a very useful guide to the age of many species of trees based on their girths. The system is described in detail HERE;
various stages are involved as one works through the procedure, which can be quite time-consuming. Fortunately, someone on the Bristol Tree Forum has cleverly adapted the process that involves simply entering a few details into an online form. With the Bristol Tree Forum's kind permission we have been able to place a link below giving access but their Tree Age Estimator. .
Assessing the 'Age-Class' of a tree
As already mentioned, there are a wide range of factors that affect the rate of growth of a tree and indeed its longevity. Some such as Yews and Oaks are relatively slow-growing but can live for many centuries; others, like Willow, Poplars and Birch, are much shorter-lived but grow much faster. While the rate of tree growth varies widely between different species, it is also highly dependent on environmental factors such as soil type, climate and exposure and all these variables all add to the difficulty of age assessment. However, it is not always necessary to assess a tree's chronological age; in many cases it may be sufficient to consider what stage of life it has reached by defining its age-class. This may indeed be more informative as it can tell one more about a tree's vigour and how may be expected to develop in the coming years than would be provided by knowing its age in years.
Allocating a tree to an age-class is inevitably somewhat subjective, being largely based on a tree's appearance rather than on any measurable property. It must also be recognised that the ‘maturity’ of a tree (see below) is a phase within the overall lifespan of a particular tree. This means that while a short-lived tree such as Birch may be fully mature at 30 years, an oak of the same age would be regarded as still being ‘young’. Furthermore, the age classes that an individual tree might pass through are not of equal duration: 'maturity' and subsequent stages may persist for many decades; indeed, the 'ancient' phase of particularly long-lived trees (e.g. oak, yew, sweet chestnut) may extend to hundreds of years.
AGE CLASSES may be defined as follows:
SAPLING: Beyond the stage of being a newly-sprouted seedling but still very young, with a slender trunk, say under 75mm diameter. Saplings are generally growing fast but are likely to be vulnerable to damage from herbivores (livestock, rabbits, squirrels etc.). They are also easily subject to mechanical damage (e.g. by mowing equipment and vandalism) as well as being sensitive to drought
YOUNG trees will still growing fast, usually primarily increasing in height more than spread; although fairly well established in their location they will as yet be making a fairly limited impact within the wider landscape.
EARLY-MATURE trees are well-stablished young trees, normally of good vigour and still increasing in height, but beginning to spread laterally and starting to make an impact upon the local landscape and providing increasing habitat value to wildlife.
MATURE trees are fully established; upward growth may be slowing as they approach their full height but they will tend to be filling out and increasing in spread and will generally be at a stage when they have their greatest visual impact in the landscape. The bark may be beginning to crack & fissure and some development of internal deadwood is to be to be expected, along with other features associated with increased habitat value.
LATE MATURITY is a stage when trees are nearing their full height; trunk diameter will generally be large but any increase in size will be slow. Dead wood and some decay (potential indicators of a 'veteran' tree - see below) are likely to be present and some crown retrenchment ('growing down') may be beginning. Such trees tend to be of high habitat value.
OLD trees are now in the latter phase of the expected lifespan for the species; vigour, as indicated by shoot extension growth will generally be declining and there is likely to be some crown retrenchment. Old trees are also likely to possess veteran characteristics such as old wounds, decay pockets, large, ponderous branches etc. While these features could be regarded as potential hazards, they do not necessarily indicate heightened levels of risk; on the other hand, they do represent increased habitat value.
ANCIENT trees are those that have survived well beyond the typical life expectancy for the species. They will exhibit a range of ancient characteristics such as hollowing and decay in the trunk, branches or roots, with water pockets and often supporting fungal fruiting bodies. The crown may be reduced through branch loss or pollarding but may also have becoming smaller as a result of the natural process of retrenchment, involving the dieback of the outer crown and the possible development of 'stag-headedness'. Such trees are generally regarded as being of particularly high value for their benefits they offer to a wide range of organisms as well as for their aesthetic, social and historic significance.
'Veteran' is not really and age category at all, as veteran trees are defined largely by their possession of quailities that provide the special ecological attributes for which they are particulalry valued. While most veteran trees may indeed old or even ancient, their status is not determined by their chronological age alone, but by the characteristics that have developed as a result of events they have lived through.
While all ancient trees are veteran trees, not all veterans are ancient: veteran trees are identified by their features such as hollowing, branch death, the presence if significant decay etc, regardless of their age, while ancient trees are defined by reaching the end of their natural lifespan for their species. Veteran characteristics can develop due to age, but also from natural damage or environmental factors like wind and management practices. Thus a younger tree can become a veteran without being ancient
A detailed description of what constitutes a veteran an ancient and other catagories of trees can be found by clicking the button below.