I have had a phylostachys nigra and a sambucus nigra growing happily as neighbours in a corner of my garden for about 15y. In that time, a tree on the other side of the fence to the south has been growing taller and wider. I expect that's why the bamboo has started leaning more and more on the elder, and the elder has started growing away from the bamboo.
For several years both plants have been thriving. They don't seem stressed. This year the elder flowered gloriously as usual. But I've just noticed that the part of it which has grown away from the bamboo has lost all its leaves. It still has its berries. The part leant upon by the bamboo still has its leaves.
What is going on?