When is a bush a tree?

We got very excited over lunch the other day arguing about whether olives grew on bushes or trees, leading to reflection on what exactly it meant to be a tree.

First thought was that trees had trunks. A tree is a plant consisting of a substantial cylindrical trunk with roots below, branches and leaves above.

Second thought was that, in order to keep trees separate from bushes, the first section of the trunk, accounting for at least 17% of the overall height of the tree above ground level, should be clear of branches. But this criterion fails on many fir trees which sprout branches all the way up the trunk. OK, so they may die away when the tree is in a forest, but they are certainly there when the tree is grown as a specimen, in the open.

Then how about the trunk being a substantial, vertical cylinder? There may be branches emanating from the trunk but these will be substantially less substantial, with diameters not exceeding 23% of that of the trunk at the point of emanation. This criterion fails on cedars from Lebanon, the sort of the thing that you have in the garden of Osborne House on the Isle of Wight, where the branches are sometimes of more or less the same diameter as the trunks. And I recall trees in the botanical garden at Puerto de la Cruz in Tenerife having trunks of very bizarre form, certainly when compared to those of, say, an ash tree.

To avoid confusion with large weeds like teazles, we can say that the trunk must persist from year to year. We do not allow annual trees.

To avoid confusion with climbers like clemati, we can say that the trunk must be rigid and carry the weight of the branches and so forth above.

But what about tree ferns? I suspect that the general appearance of the trunk is not going to be enough to keep tree ferns out of trees; maybe we are going to have to get into modes of reproduction. A tree must have seeds while a fern must have spores. But then, is distinguishing a spore from a seed as fraught as distinguishing a bush from a tree? Or do we have to rest our case on trees and ferns being on different branches of the evolutionary tree? What happens if the same species evolves two or more times on two or more branches of the evolutionary tree? Suppose one population of lions evolved from spiders and another population of lions evolved from cat fish, with the two populations looking identical and being able to interbreed? Perhaps this last is a bit unlikely; the genome of the spider lion being likely to be far too different from that of the cat fish lion for fertilization to work, however alike the two sorts of lions might look. Which might result in lots of infertile unions. One lion could fall in love with another lion of the wrong sort.

What about unnatural trees? Trees which have been deformed by human intervention like coppices and which do not have trunks in the ordinary way. Or trees which have been deformed by storm and where the original tree has fallen over and a new tree has sprouted out of the side of the trunk of the old tree? The old tree still having enough root left to support the new.

All very puzzling. I wonder if there is a simple answer to be found in an NVQ on botany? That there really is some simple botanical principle which separates out trees from the rest of the plant world, preferably a principle for which long range eyeball inspection is enough, which does not depend on microscopes and preferably keeping bushes out too.

PS: taken a peek now at Lecointre & Le Guyader which tells me that while trees might be on a very different part of the evolutionary tree to ferns, a birch tree is a lot more closely related to a cucumber than it is to a plane tree. So it all remains very puzzling and God has a lot to answer for.