CASEBOOK OF DR. SLATER – No. 22 Cavities are not necessarily a tragedy

When you find a hole in a tree, it is important to look into it!?

Decay cavities in trees often lead to such trees being downgraded in terms of their structure and longevity – however, many trees live long lives with cavities – and cavities can be important microhabitats for wildlife.?

Most cavities in the aerial parts of trees have one of three origins:?

i)???????????????????A branch has died, decayed, and a branch cavity occurs in the main stem or limb where the dead branch was attached.

ii)??????????????????The tree’s stem is wounded (or bad pruning occurs) such that a significant area of decay develops associated with that wounding.

iii)????????????????At the base of the tree, mechanical damage or the death of one or more of the tree’s roots leads to an open cavity in that location.?

Branch cavities?

Most branch cavities do not significantly reduce the factor of safety of a mature tree. This is because of two key factors: first, that the associated decay is most-often limited to the ‘insertion cone’ or ‘knot’ of the branch that has died, the decay not often spreading further into the associated stem or limb unless the tree is already in decline; second, that the tree often forms a ring of tough woundwood around such a cavity (Fig. 1) which causes the stem or limb to regain its bending strength in the area of the cavity. Kane and Ryan (2003) found that the production of woundwood in red maple (Acer rubrum) resulted in an increase in stem toughness, compared to normal, unwounded stems.?

Take a walk in a woodland or park, look up, and you will find that there are many branch cavities, of relatively small size, in many of the mature trees. Rather than concluding that all these trees are defective in a minor way, such branch cavities are a big boon, in terms of supplying micro-homes for a wide range of wildlife: bats, bees, beetles, birds… Many species need these little cavities for setting up home (Fig. 2)?

I have many pictures of tree failures, but not one that implicates a branch cavity as the main cause of failure. That is not to say that a large diameter branch cavity could not be a focus for tree failure, but it would appear to be a rather rare event. Figure 3 shows that some branch cavities are around only for a limited time, before the tree occludes them completely. The cavity shown was inhabited, as can be seen by the scratching around the entrance hole – hopefully, whatever creature it was, it has not become entombed within the tree!?

?Stem cavities?

A zone of decay at the edge of a tree stem can be much more problematic, as it frequently reduces the factor of safety of a tree by reducing its bending strength – and that a large amount of wood has been opened up to decay (as opposed to a small branch cavity).?

The typical response by a mature tree is to grow two ‘pillars’ of response wood either side of the cavity opening to reinforce itself, whilst attempting to occlude the wound, which will often be a slow process. Many hollows in tree stems are caused in this way – and many trees cope with this level of damage, living their expected lifespan (Fig. 4).?

Basal cavities?

Decay right at the base of a tree (its ‘root crown’ or ‘root flare’) is often related to substantial root decay, although it can be initiated by superficial mechanical damage (especially by mowing equipment). The wood of trees decays more rapidly just under the surface of the soil, because of the perpetual dampness and temperature buffering that occurs just under the soil’s surface, as well as the prevalence of many microbes in that biologically active topsoil.?

If the tree can respond to the presence of basal decay, it will ‘bottle’, which is to create a bulge at its base, akin to a buttress, but noticeably more swollen out, around the basal cavity, than any normal tree’s growth form (Fig. 5).?

Basal cavities are, generally, the most important cavities to assess in detail, using micro-drills or tomography to assess the extent of current decay – and to predict the progress of that decay. However, do not neglect the fact that roots are likely to be decayed below the level of the visible cavity – and the tree is far more likely to fail because of root decay, than it is due to being somewhat hollow at its base.?

Lesson Learnt?

Cavities are not all the same: their origins, position in the tree, the extent of associated decay and how much of a ‘flaw’ they represent in a tree varies greatly. In general, small diameter branch cavities surrounded by a ring of woundwood are of the least concern, and large basal cavities of greater concern, when it comes to inspecting trees. It is also important to gauge if the internal decay is still progressing rapidly, and the extent of response that the tree can achieve, which relates directly to the tree’s vitality. A declining tree with active decay at a large basal cavity is a classic cause of tree failure: a healthy tree with a minor stem or branch cavity is likely to make it through OK.

References?

Kane, BCP & Ryan III, HDP (2003) Examining formulas that assess strength loss due to decay in trees: woundwood toughness improvement in red maple (Acer rubrum). Journal of Arboriculture, 29(4), 209-217.

About the Author?

Dr. Duncan Slater is a senior lecturer in arboriculture at Myerscough College, Lancashire. He is also a co-ordinator of Arbor Day UK (#ArborDayUK ) – an action group that seeks to put new trees and woodlands back into the landscape. To support or sponsor these Arbor Day efforts, please contact Dr. Slater at any time.

*** THIS ARTICLE FIRST APPEARED IN PRO ARB MAGAZINE - Autumn 2022 Edition ***