What's going on in your trunks?

I spend a lot of time looking at trees.

Yes, I really am that dull.

But I like trees. They fascinate me. And I often find myself wondering about things like ... why do some trees have a single main trunk while others seem to have several? Are they all the same tree? Or are they several trees that have become conjoined? Do the different trunks have different DNA? Are the roots shared?

My favourite local Oak tree is this beauty (photographed here across four seasons).

From a distance it looks like three or maybe four separate trees. But they all merge at the base - so are we looking at one, two, tree or four trees?

And looking at other Oaks I find lots that are single and lots that have two or more trunks.

And it's not just Oaks of course. As you can see, multiple trunks are common on other trees too like these Hawthorn, Ash and Crab Apple trees (apologies for the nosy pug).

We know how trees grow (see here) so why this disparity? 

I endeavoured to find out.

And it turns out that scientists are wrestling with the same problem. There doesn't appear to be any one single classification system for forms of trees (in respect of multiple trunks). Also, because some trees have multiple trunks, scientists have had to find new ways to measure their size and width. 

The best thing I've found so far is a discussion paper by a scientist called Ed Frank from the Native Tree Society where he suggests a 10 category system. It goes like this:

Category 1: Single Trunk Tree. This is the standard growth form of most trees consisting of a single large trunk growing from a simple root mass below ground. A single trunk tree is defined as one that has a single pith at ground level. It should be measured around its girth at around 4.5 feet from the ground. Or, if the tree branches below 4.5 feet, then the girth is measured at the narrowest point below the lower branching. Sometimes, especially in open areas, small sucker shoots may grow around the base of larger trees. These are not considered as multiple stems as they tend to be very small and short lived. 

Category 2: Multitrunk Trees. This is the second largest category. These are trees and shrubs that have more than one stem growing from a single root mass. these trees have a fused base area that consist of multiple piths at ground level, These generally form when the original stem of the tree was damaged, broken, or browsed by animals. This results in new stem(s) sprouting from the root mass (Aha! So that's what happens!). In general these stems are all of similar age and size, but will often reduce to developing one, or a few dominant stems. The fused base should be measured at a height of 4.5 feet, if it extends that high, or measured at the narrowest point below 4.5 feet when it does not extend that high.  

Category 3: Clonal Coppice. Some tree species form coppices in which the initial tree sends out lateral root branches, or rhizomes, and new tree stems sprout from these lateral roots (Hazel is a good example). This process continues and the result is a clonal cluster of trees derived from a single original tree. This spreading by roots is not the only method of propagation for these colonies, as they also produce seeds. It may be difficult or impossible to distinguish whether something is a clonal coppice, or simply a cluster of individual trees growing close to each other based upon simple observation alone. However there are some clues that can be used. In a clonal coppice the trees typically are oldest in the centre and younger toward the edges. In a clonal coppice, the largest individual tree should be measured for comparison with single trunk trees. 

Category 4: Clonal Colonies. This in many ways is the same process as described for clonal coppices, but on a far larger scale. The primary type example of a clonal colony is the Aspen. All Aspens (including White Poplar) typically grow in large colonies derived from a single seedling, and spreading by means of root suckers; new stems in the colony may appear at up to 30-40m from the parent tree. Each tree only lives for 40-150 years above ground, but the root system of the colony is long-lived, in some cases for many thousands of years, sending up new trunks as the older trunks die off above ground. For this reason it is considered to be an indicator of ancient woodlands. For measurement purposes, the largest individual specimens should be measured for comparison with single trunk trees, the area of the colony should be mapped, and the number of individual stems in the colony estimated. 

Category 5: Fused and Hugging Trees. Sometimes two trees may grow to large size adjacent to each other and grow together. These may be of the same species or even trees of two different genera or families. These consist of two basic forms: (a) Fused trees are two trees that have become grafted together. Generally this grafting is between two trees of the same or closely related species or genera; and b) Hugging - two trees that are not grafted together, but are physically touching. rubbing, intertwined, or entangled. These need to be considered on a case by case basis. In general the standard height, girth, and crown spread measurements can be made for each individual of the conjoined or hugging pair. 

Category 6: Fallen Trees. There are cases where an individual tree has fallen, from wind events or other causes. On these fallen trees the upright branches form new vertical trunks, and roots may sprout from the surface of the tree where it touches the ground. These are unusual and should be considered on a case by case basis as there is no standard protocol for documenting them. 

Category 7: Tree Complexes. Situations occasionally occur where re-sprouting, grafting, and other processes mean that there is no other category that can be applied other than a 'complex'. For example, a tree falls down but remains alive and puts out new upright branches. These become larger and eventually blow down themselves and their branches sprout new uprights. Sometimes it isn't even clear if they (genetically) are one tree; if another tree falls across a downed tree, it might graft to the other. They need to be documented on a case by case basis.

Category 8: Banyan-Like Trees.  Banyans and Figs send down shoots from their branches that take root and become new trunks. A single tree thus may spread over a large area and look like a small forest. I actually saw one such fig tree in Sri Lanka that had 350 large trunks and over 3,000 small ones and yet, genetically, was still a single tree.

Category 9: Mangroves and other trees with large aerial roots. No protocol has yet been defined. Options might include documenting the spread of the aerial roots and their height. 

Category 10: Epiphytic Trees. In general these are not unique in any way aside from their placement in the branches or canopy of other trees. Woody Vines are generally considered to be a plant that contains woody material but does not stand upright on its own (e.g. Ivy, Grape vines etc.). They tend to climb on the branches and trunks of other plants. Where possible try to find the rooted end and measure the girth 4.5 feet from the end of the vine.

Who knew that looking at trees could be so complicated?