I was pruning some forsythia, and I noticed that there was a hole through the center of many of the major woody parts. It looks as if (only) the pith had been bored out. I'm wondering what could cause this, or if it's common.
I saw only a few mentions of this online: one question with no answer and another that seems to suggest that this is normal in older wood.
I hope the description is adequate; I could take a picture of the trimmings if it's not clear (not my plants and I didn't think to snap a picture). The plants are otherwise reasonably healthy-looking, and flowered well this past season. Is this normal/common in healthy forsythia? Wood-boring insects?