ok i think we have established that it may be good on dead trees and where wildlife values are high. Kathy H talked about coronet cutting on a declining backyard tree here once, and I lit into it, rather unmercifully, in retrospect. As as safety and amenity are values in urban trees, intentional wounding does not fit imo. Strongly recommend this site-http://www.treeworks.co.uk/-way ahead of US arb study imo. I asked the author about it last week--here's his reply:
GPM Is it ever proper to begin retrenchment pruning before the tree starts to decline? Aside from individual branch reduction for specified reasons.
**By ‘decline’, if you mean before the tree begins to retrench, then no; unless carried out for scientific reasons, I can’t see why one would want to induce or mimic retrenching for its own sake.** However we typically find ourselves professionally drawn to intervene with trees when this is not in their interests; for economic, safety and cultural reasons, prompted by some anthropocentric requirement (management for risk control or visual amenity). Professionally we are often drawn to intervene to mitigate decline, perhaps indicated by stress or disease symptoms (though I suspect we may differ in our definitions).
When is intervention appropriate? This would depend on the objectives of the client, also hopefully informed by the CA professionals’ expertise and advice. Intervention needs to consider time and priority, (if it is necessary at all, if so, how soon and how long can it reasonably be put off?), whether any intervention with the above ground tree / parts can be avoided (can intervention relating to health or stress remediation be confined to the root-soil system, mitigating rhyzosphere condition, again when and how long and how little).
We are drawn to intervene with trees for a variety of reasons. Typically though, safety and amenity have been the main drivers in our western culture (amenity usually driven by shifting notions of visual landscape aesthetics – balance, light and form). From the point of view of conservation arboriculture the driver is the tree (as host to its colonisers); its longevity and continued existence is its focus for a number of reasons, though some, I suspect, you are not convinced about. Nonetheless, tree management form a conservation arboriculture standpoint is based on the precautionary principle, i.e. a reasonable calculation of the risk of causing harm is sufficient before proportionate measures are taken to control such risks, so full scientific certainty is not necessary that we may be causing harm (Rio Summit). On this basis and in light of research about ecological continuity (1,700 different species of invertebrates whose lifestyles depend on British saproxylic habitat; their continued existence depends on veteran trees and their habitat in all their contexts). A key conservation arboriculture objective is therefore that there should be ‘no avoidable loss of veteran and ancient trees’. Given that mature trees are the veteran and ancients of tomorrow, this objective extends to the management of the younger, mature cohort (important also for its contribution to ecosystem services, including urban climate control).