Running bamboo is notoriously fast spreading and difficult to remove. What keeps its population balanced in the wild, and prevents it from crowding out the competition? I tried googling, but was inundated with gardening advice, horror stories, and assault / offensive gardening (some of the latter two presumably covering the same incident from both sides). My google-fu failed, I couldn’t really find any info about natural population controls of running bamboo in the thicket of tall tales and gardening advice.
Trees are not a taxonomic group. It’s rather a description of characteristics the most important of which is having a woody trunk. For example there are tree legumes and non-tree legumes. A species of tree can therefore be more closely related to a non-tree than to other trees. However it’s totally true bamboo is not a tree. A grass could in theory however hit all the characteristics that are required to be tree and would then be considered as such, however no such grass happens to exist.