“From acorn to ancient” – contrasting open-grown and forest trees

In spite of Ted Green remarking that he is “stating the obvious” in this chapter of his, it’s nonetheless good to be reminded of the benefits of open-grown trees and how they compare to closed-canopy trees of a similar age. Despite certain similarities, there really are many differences in today’s age, and we’ll explore some of them here. Before that however, it is interesting to note that the utilisation of open-grown trees during ‘ancient’ times was not uncommon. Orchard trees, for example, demonstrate man’s understanding of open-grown trees producing a much greater abundance of fruit than closed canopy trees.

Open-grown individuals

Morphologically, a tree that has grown without any marked competition will be vastly different to a tree that has grown amongst its competitive peers. Due to a reduced competition for light, and also because of the greater effect of wind upon its structure, an open-grown tree will be shorter, have a fatter trunk, a wider crown (perhaps with vastly-spreading and near-horizontal limbs), and a more significant anchorage root system. This ensures it is well adjuested to its setting (principally in allowing for optimal photosynthesis whilst retaining structural stability). Amazingly, investigations into the rooting systems of open-grown veteran trees by the Ancient Tree Forum revealed roots with a 2.5cm diameter at a distance of 50m from the base of the tree.

A mature open-grown oak pollard, complete with an absurdly broad crown with many near-horizontal limbs. Certainly an effective means of balancing photosynthetic needs with stability!

On a biodiversity level, we must also recognise that roots are host to a massive array of mycorrhizal associations. If an open-grown tree has a huge rooting system, associations with the tree (directly) will be potentially vast, though perhaps isolated somewhat (assuming no other trees are about). Shed roots may also support organisms of the decmposition subsystem (fungi, in particular).

Looking at the above-ground structure of an open-grown tree, we can also observe associations with a great number of species. For instance, the large limbs and vast canopy area of an open-grown tree will provide habitat for many insect species, and entire trophic systems surrounding their presence (from parasitic wasps to mites, and from birds to fungi and bacteria). Similarly, the very large and broad main stem will have a vast quantity of substrate (wood, cavities, hollows, and so on) available for saproxylic species and nesting birds, which will typically revel in the warmer conditions. The herbaceous plant species surrounding the tree will also provide support for insects, including saproxylic beetles.

Closed-canopy individuals

Because trees growing within a woodland or forest have to compete for all types of resource (light, water, nutrients, rooting space, branching space), their form will be vastly divorced from the open-grown tree. Investing most of its resources into growing towards the light, stems will be thin, lack taper, and rooting systems will be largely insufficient to support the tree structurally (which is evident when woodland edge trees fall following localised tree felling). However, root grafts (between a single species) and mycorrhizal relationships between individuals (across different species) may ensure that edge trees, which have larger canopies, support trees (via grafted roots and mycorrhizae) in more sheltered but still very local settings. By that same token, older trees may support younger individuals (particularly of the same species).

In time, deadwood within woodland stands may build up to significant quantities. As a stand ages, there is a progressive ‘self-thinning’ of the stand, eventually leading to only some (usually mature and quite large) trees remaining. This accumulation of deadwood ensures that the decomposition subsystem can mineralise nutrients, allowing these remaining trees to utilise these resources once again. The same can also be said for leaf litter, which is of far greater quantity within a closed stand (and retained, courtesy of a more sheltered micro-climate). For these reasons, and compiled both with the fact trees will ‘trade’ resources in a sort of peer-to-peer economy and their sheltered setting, rooting systems may be less expansive (compared to open-grown trees). Buttressing may also be lacking, unlike in open-grown individuals.

A stand of beech (Fagus sylvatica) last pollarded before 1830. We can see how the crowns of each individual are much more upright and narrow, and the stems have less taper than they would in an open setting. There is however a great abundance of leaf litter on the floor, though an unfortunate lack of coarse woody debris (excluding standing deadwood).

The decomposition associated with woodland stands will also be far quicker, given the cooler and moister conditions. As fungi tend to operate most effectively in such settings, the mineralisation of locked-away nutrients will be a much swifter process, when compared to the decomposition of deadwood on and around an open-grown tree. For instance, a large stem of a fallen tree within a woodland may decay in a few deacdes, whilst a fallen and significantly-sized limb of an open grown tree may take up to a century to fully decay.

Clearly it’s not possible to compare and contrast trees in different setting in any short space of time, though I felt that this brief entry by Ted Green in the below book was a rewarding read, if not to simply cement existing knowledge.

Source: Green, T. (2007) Stating the obvious: the biodiversity of an open-grown tree – from acorn to ancient. In Rotherham, I. (ed.) The History, Ecology, and Archaeology of Medieval Parks and Parklands. UK: Wildtrack Publishing.

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