Guest posting from http://www.sydneytreecare.com.au/ - interesting natural phenomena I have seen recently. Two weeks ago (10th March 2012) a very large Gum tree branch failed and hit a house in Vaucluse in the Eastern Suburbs of Sydney. I was one of the first to attend the site and while I was there the team leader of the State Emergency Service arrived and we had a brief conversation of what he was able to do as a first response. In this conversation he mentioned that he had two other branch failures in the same or nearby suburbs to attend to that same morning. I thought that was strange. I was chatting to a friend about this and he relayed that that same night he was aware of another branch failure in his suburb of Engadine (30kms to the south). This seemed stranger. Particularly when considering that this was a very still and quiet night after some very wet and windy weather.
I am aware of the syndrome 'Summer Branch Drop', where, the syndrome goes, that while leaf size is at its maximum and fruit size at its maximum, this is most likely when failure will occur in a branch, particularly so after heavy dew-fall or wet and windy weather, which adds load to the branch. But this circumstance in Sydney two weeks ago does not meet those criteria. Dew falls after sun-down, so a mid-night break would seem strange. Also, the weather had been pretty awful up to a day before, but not that day and night when it was quite still. The branch failure was very counter-intuitive. Especially so endemically.
I think it maybe it could be a post-stress syndrome, maybe a bit like work-hardening in metals, when after a big viable load, it is the release of that load and inactivity that triggers the failure, perhaps brought on by a large difference in diurnal temperature. Seems a bit weird. I would welcome comment on this, as branch failure is a big deal when the tree is next to a house. (I am a consultant arboriculturalist working in Sydney.)