As we hiked in Olympic Park and through Deception Pass, we kept seeing branches fallen to the ground. A surprising percentage of the time, the branches had fruticose lichen on them. In fact, the fallen ones seemed always to be the most lichen-covered of branches.
Were the branches dying from the lichen? Or were the trees killing off infected branches deliberately?
The fruticose lichen had to interfere with the leaves photosynthesizing on branches below, at least. On some trees, they visibly grabbed a significant share of sunlight. But they might have done worse. They might have been sapping energy directly from the trees, probably in the form of sucking out glucose from the lining under the bark. That would go against the generally accepted rule for lichens now but I think it's good to be suspicious of what seems like overly-quick, overly-general conclusions. We seem to be in the infancy of understanding biochemical relationships between different plants. It wouldn't seem like any surprise to learn that some lichen behave chemically different from others. Their fungal components might be partially parasitic or at least opportunistic if they happen upon cracks in bark.
The deadfalls seemed to lead into the central question: were these branches breaking accidentally? Were they killed by lichen robbing them of their leaves? Or were the trees cutting off resources to those branches deliberately so as to let them snap off in the next strong wind? At this point, we know trees have intentionality. But do they have it in response to these lichen?
For a while, I contemplated experiments that might reveal the intentions of the trees, if any. None of them would be easy. It is not enough that the branches hit the ground, not enough that the falls correlate to the heaviest growths of lichen (if they do), not even enough to show the branch losses are far greater than chance. After all, the lichen could be killing those branches even in a non-parasitic way.
No, to really demonstrate that the deadfalls were intentional, you would have to know the basic chemistry of trees, perhaps of each individual tree, and you would need a way to track individual changes in their living chemistry. That's how it was done when spotting how acacia trees turn their leaves bitter when they need to discourage herbivores. In that case, biologists took leaf samples. That's probably not applicable here. You might not pick up distinctive signals in leaf chemistry. And you wouldn't want to conduct an experiment by cutting trees or branches down. (That has too often seemed to be the current state of forestry practice.)
My suspicion as a casual observer is that the trees do respond to lichen invasions. But it's only a suspicion.