Trees are complex
Mark Chester MICFor looks at some of the amazingly complex interactions within and between trees which are beginning to give us a new insight into tree health and how best to plant and manage our trees for the future.
Trees Are Complex
The ability of trees to absorb carbon dioxide is well known. During the 2019 General Election, the benefits of planting trees for this purpose was appreciated and pledges were made to plant tens of millions of trees in this decade. However, trees contribute far more than this, and indeed, can be truly complex organisms.
A young, vigorous and rapidly growing tree can absorb large quantities of carbon dioxide. As it grows, it can contribute so much more. Trees in urban settings can help to reduce air pollution, interrupt the development of wind tunnels caused by tall buildings and bring nature in to an often concrete landscape. Being around trees is good for both mental and physical health, reducing stress and aiding recovery from illness.
Shade is valued in the heat of summer and evapotranspiration aids cooling. As a tree matures, it becomes an attractive habitat for birds, insects and fungi, keen to make a home and to access the resources within. Indeed, as a tree ages, it often becomes more valuable as a habitat, an ecosystem, even as absorption of carbon dioxide reduces. Veteran trees can provide unique conditions and host rare species of fauna and flora, and the presence of dead wood is a highly sought resource in the natural world. Like a village elder, no longer bringing in a wage but making an invaluable contribution to life.
Response to pathogens can be complex
Trees such as birch and ash, are shorter lived and their focus is on growing quickly and setting abundant seeds. Defence against pathogens is limited. Longer lived trees such as London plane, oak and yew invest in defence. Chemicals such as toxins are stored within to enable resistance to pathogens. Indeed, there are fungi that can only attack when the host is stressed, and the arborist can use this to detect health issues.
We are beginning to appreciate how dynamic the internal mechanisms of a tree are. A beech tree can detect a caterpillar attacking a single leaf and release a toxin in all of the leaves in seconds to deter further attacks. As an arborist, most of my skills are based on observing the external condition of a tree. However, by measuring the response of a leaf to exposure to sunlight (chlorophyll fluorescence), stress from drought or herbicides can be detected sometimes several weeks before it is externally evident!
The interactions between generations can be truly fascinating. As seeds develop on the parent of some trees (my own research involved elms), they receive sunlight. If the sunlight passes through leaves en-route, it changes across the spectrum to ultra-violet and the seed is unable to germinate until it is in an unshaded place and the position on the spectrum is reversed. This helps to ensure that germination won’t happen on a crowded woodland floor where other plants will compete for light. Beech trees release a chemical from the roots to prevent vegetation growing around the base, so that there is no competition from seedlings. By contrast, trees such as pines release chemicals to encourage rooting of seedlings around them, providing a nursery as it were.
Ted Green, Conservation Advisor to the Windsor Estate and someone who has lived in the shadow of Windsor Great Park all his life, shared with me how, when younger, he watched forestry workings planting pine seedlings amongst mature pines, leaving them there for a couple of years. His research led him to conclude that myccorizae among the roots of the mature pines were stimulating growth of the seedlings. He then took several pine saplings from this ‘nursery’ and planted them in a spot at Windsor. For contrast, he then planted several pine saplings from a conventional source. Ten years later, the local Pines are thriving, and those from the conventional source are still adjusting.
We are beginning to appreciate the relationship between trees and soil, and how well aerated soil is important to tree health. I pondered this during the recent floods. In my adopted Herefordshire, fields of pasture with trees were sodden and puddling, the trees intercepting rain fall and slowing the drainage process. However, fields with arable crops and few trees were very different. The plants that had germinated were very isolated in a swamp of cold mud. The sodden surface could take no more water and it was pouring off fields and on to the roads.
Where trees are able to become established, the soil benefits and a dynamism becomes established that we are only just beginning to appreciate. Aerated soil can host fauna and flora which finds compacted ground inhospitable.
About ten years ago, a veteran tree in Wales, potentially more than 1000 years old, was blown over in a storm. The loss was avoidable. A quick-thinking local arborist took cuttings from the old giant and sent them to several people he knew could propagate them. One was Peter Wells, founder of Barcham Trees and a nurseryman with half a century of experience. He watched as his cuttings became established, and some cuttings produced more than a metre of new growth in the first year (ten times greater than expected). He is still pondering how. What is it in the genetics of this veteran that enabled such vigour? And can we introduce it to today’s trees? Current research is focusing on tolerance to drought and waterlogging, so that the trees being propagated today are equipped for the harsh urban conditions of tomorrow.
Trees are truly complex. Viewing them simply as a source of carbon storage is to miss so much, observing in monochrome when colour is available. There is so much to know and understand. The good news is that we appreciate this and we are gradually exploring more.
Mark Chester, MICFor, is Principal Consultant of Cedarwood Tree Consultants, based in the Midlands. He also runs the Consulting Arborist Society, an organisation which supports tree consultants in their professional development. He is the author and presenter of the Amazing Trees video series.