Mature Tree Management

The better we understand the biological and mechanical designs of trees, the better the chances are that treatments will be done that heal rather than hurt.

Alex Shigo

From the moment a seed germinates, it responds to its environment. If left alone it will grow primarily to what its genetic code dictates and secondly to the external environment it experiences throughout its life. It can’t get up and move - it either adapts to its environment or it dies. There is no other choice. It doesn’t care what it looks like, what form it has or how rare it is. Its role is to grow and adapt to its environment. If it lives long enough to reach maturity, it will sexually reproduce, pass adaptations on to its progeny and then die to make way for the next generation – all to ensure the survival of the species.  We on the other hand have many more expectations of trees we plant in our parks and gardens. We plant for shelter, shade, erosion, rarity, but mostly for a pleasant landscape. Every time we work with or around a tree, we have the potential to employ practices which either help or harm it. Everything we do to a young tree has a bearing on what it will be like as a mature tree.

A park or garden tree begins its life in a nursery and is re-potted three to four times before being planted in its chosen site. Correct planting techniques cause little stress, however, poor planting techniques can cause problems throughout the tree’s lifetime. Planted too deeply, it will be stressed from the start, may produce girdling roots or succumb to stem decay. Plant it where it is too exposed or the soil PH is wrong and it may survive, but not have the vigour or vitality of a well-sited tree. Grown in the open, it encounters a greater array of forces than a forest tree would. Its branching will be much denser and it will have a shorter stature because light will be in abundance. It will be subjected to winds which it has to endure alone and its roots will spread further to compensate for the extra stresses exposure brings – however, its biggest problem will be us.

The life of a tree is all about energy income versus energy expenditure. If the tree is wounded or under stress it uses energy to compensate rather than to grow. Proper arboricultural care from day one should ensure the mature tree has energy to grow – it should not have to use energy to compensate for a lifetime of harm. Caring for trees is a specific skill. An understanding of the tree as a whole leads to informed choices. Every time a branch is pruned it is wounding the tree and creating an entry point for pests and pathogens. Trees respond to this wounding by a process Alex Shigo calls ‘Compartmentalisation Of Decay In Trees’ or CODIT. The tree stops the spread of decay by creating internal barriers. Trees don’t heal (healing implies the regeneration of cells), they generate new cells and close wounds over time with new tissue. This process and every other process costs the tree energy. If we prune incorrectly, CODIT barriers may need to be more extensive and the tree has to use more energy to respond.

A tree has an internal record of every season, every year, good growth, poor growth and a record of everything we have done to it, good or bad. Every stress the tree encounters throughout its life is recorded within its growth rings. Every year it grows a new tree over the old one and the new growth responds to stresses such as an internal crack, wind, gravity, pests, decay or mechanical damage. Claus Mattheck refers to ‘The axiom of uniform stress’ as a tree growing new wood to uniformly distribute stress over the whole of its surface. This process of responsive growth is called shape optimisation.

Properly trained arborists weigh all these factors and more when assessing mature trees. Visual Tree Assessment (VTA) is a method used to identify potential faults and hazards whether easily seen, such as a hanging branch, or not so easily seen, such as shape optimisation on a stem indicating internal damage, decay, or a poor branch union. Mature trees in parks and public gardens are often targeted as buildings, car parks or walkways are may be within falling distance or the drip line. Potential hazards should be identified by regular scheduled VTA on mature trees. This also assesses the overall health of the tree. VTA is designed as a quick comprehensive assessment to give an overall picture and identify where more in-depth investigation or remedial work is needed.

Understand and care for the young tree and you will have a healthier, safer mature tree.

Danny Frazer

CURATOR (2010-2012)