Big pruning cuts need to be approached and considered with care. The TCAA’s Jim McArdle looks at the pros and cons of heavy pruning.
It’s important to ask the reason for the scope with regard to big segments of tree stems being pruned or shaped.
There are three examples of Big Pruning Cuts (BPC) where lopping is engineered with specific outcomes. The first indicates a sheltered environment; the second to evoke a regrowth; and the third to choose species able to respond well to BPC while pruning in season.
Four points considered where BPC are required in development
1. Within asset-protection zones and/ or Inner Protection Zones for the reduction of fire hazard and pruning clearances of touching canopies. In bushfire reduction, BPC of existing trees with poor structure reduces the weight and allows for a ‘half’ tree to remain – which is probably a psychological issue more than a tree older trees to reduce branch drop and tears that have large surface areas to compartmentalise with response wood is indeed an issue for the tree;
2. In access areas for driveways;
3. In visual aspects(view); and
4. In balancing damaged trees, including fungal or lightning damage, where the trees are required to be reduced prior to being further colonised by fungus or to diminish struck wood.
Managing remediation of big cuts
After big pruning cuts it’s a regular practice to keep up moisture and to mulch with certified, clean, composted eucalyptus mulch. In my experience pine mulch tends to bring fungal colonisation.
It’s also important to allow soil areas to be friable and aerated, not compacted, and to ensure the tree is, as best circumstances will allow, shielded from unusual exposure to factors like vibration, compactions, further pruning, excessive winds and so forth.
On the plus side
Correctly executed BPCs are preferable to removal of a tree. The tree’s amenity is preserved and the benefits of the tree are still utilised, even though they’re reduced.
Where I recommend a big cut is if there are competing leaders with an occluding or compression fault.
Rather than remove the whole tree we pursue the removal of the less-dominant side and retain the larger side, or the side which will function as a tree.
But when this practice is inexpertly carried out – precuts, flush cuts and non-splayed cuts – it can allow secondary issues like faster decay. With arboricultural practice, branchunion- type cuts may also be too close to the union, and a cut further along may allow the typical dieback to the branch union. This has not been industry practice on all trees, but is useful with compartmentalisation.
Wood Density is Strongly Related to a Tree’s Physiological Performance (Markesteijn et al, 2011) and Mechanical Structure, for example, Risk of Trunk Failure Under Wind Loads (Telewski, 2012) and Density is a Key Trait for Determining Wood Quality, (Zhang and Morgenstern, 1995; Rozenberg et al, 2000) all speak to this.
It is rare to have failure in a branch collar.
Higher density trees are more decay and disease resistant (according to Chave et al 2009) , and where trees are loaded for ice or snow they also tend to have higher densities than there same species along the coast.
The response of trees with dense timbers in the branch’s collar outside of the union is less dense than the union. Recommendations of the AS4373 2007 Pruning of Amenity Trees allows for cutting at the branch collar, not for disease resistance, but for mobility of the new growth. Pruning a tree can cause it to grow in the future.
This can be an issue for trees when sprouting epicormics that race towards the sun and present for heading back branches, especially the leader, results in vigorous growth from buds just below the cut. The further back the branch is cut, the more numerous the number of shoots will usually be. If you just cut back some of this year’s growth, you usually won’t get a lot of regrowth. If you cut way back into previous year’s growth, you are more likely to have problems with suckers, water sprouts, and huge growing shoots. Examples of this can easily occur in fruit trees.
To read more of Jim McArdle, and to stay in touch with the ongoing work of Tree Contractors & Arborists Association Australia, log on to tcaa.com.au.