It is that time of year. Warming weather accelerates vascular activity, which makes foliage heavier. If evapotranspiration is inhibited by humidity and a lack of wind, the foliage can get too heavy to be supported by the trees that produce it. All that increasing weight can bring down big limbs or entire trees at the most unexpected times. The process is spontaneous limb failure.
By ‘unexpected’, I mean that it happens when there is no wind. It is startling because broken limbs and fallen trees are typically associated with wind rather than a lack of it. Gentle wind actually accelerates evapotranspiration, which relives affected vegetation of some of its weight and susceptibility to spontaneous limb failure. Aridity helps too, by absorbing more moisture.
Of course, even a gentle breeze at the wrong time can have disastrous results for vegetation that is already about to succumb to spontaneous limb failure. I suspect that is what happened here, since the air was not completely still at the time. It was just a bit warmer than it had been, and slightly more humid than typical. It is too late and pointless to analyze the situation now.
At about noon on Thursday, someone who works nearby alerted me to the sound a a big tree falling. I was in the same neighborhood, but was driving by with the radio on. The tree is precisely where I was told it would be. No one was nearby when it fell. Damage was originally minimal, with a portion of trail displaced by roots, and a rail on a bridge crushed by the trunk.
By ‘originally’, I mean that this was not the extent of the damage. After barricading the trails and road leading to the site, and leaving, we heard another loud crash from the same location as a bay tree that had been leaning against the already fallen fir tree collapsed in pieces on top of the whole mess. Fortunately, the damage to the bridge, although worse, is not too terribly bad.