I walked past a couple of people today looking at a large Horse Chestnut tree near my house. I missed the start of their conversation but as I walked past I heard them commenting on its health and appearance; the lady said she thought the tree looked sturdy enough.
It reminded me of many a discussion I have had with people, distraught at a particular tree being felled when they believed the tree to be perfectly healthy, and the hours I have spent trying to reassure someone that a tree near their house is no cause for concern when they are convinced that it is dangerous.
Trees can tolerate very poor conditions before they are so effected that they fall over, or fail, however for the most there are signs and symptoms that the arboricultural consultant can detect to aid appropriate management.
The term signs is used when there is an external indication of defects on a tree such as the fruiting body of a parasitic fungus, (parasitic - lives off live woody tissue) or a particular saprophytic fungus, (saprophytic - lives of dead woody tissue) associated with trees in decline, or significant deadwood and secondary pathogens. I will explain all of this in greater detail later.
The term symptoms refers to indicators of defects that the tree develops itself such as the swelling of reaction wood that may indicate internal defects such as decay and cavities, or deadwood indicating issues with the trees root system.
Both symptoms and signs may be subtle however significant internal defects may be present.
It is the job of the arboricultural consultant to inspect trees for such signs and symptoms and calculate their severity and significance for any given tree in an individual environment.
To return to parasitic and saprophytic fungus, there are many parasitic fungus that degrade the live, woody tissue of trees, each with their own decay strategy.
There are also many saprophytic fungus that live off deadwood and hundreds of species can be found within a woodland however some associated with growth on deadwood within trees can be an indicator or trees in severe decline.
Some even imply the presence of a specific parasitic primary fungus within in a tree that may not visible at that time. Primary fungus result in the initial infection of a tree but may pave the way for associated secondary diseases, including fungus. Primary diseases are usually more significant, secondary diseases may indicate the severity of the situation.
The fruiting body of a fungus is just that, the fruit. The remainder of the fungus exists predominantly in what is termed its mycelial form within the tree. This is the vegetative part of the fungus as seen here on a rotting log.
Some fruiting bodies of fungus are annual and therefore only visible on an infected tree for a limited period every year, however some are perennial and increase in incremental growth each year.
There are other species specific symptoms that can indicate tree defects. Severe stem taper can indicate stem base decay or root decay.
Also extensive epicormic growth on a tree, (prolific shoot generation on a main stem or within the crown of a tree) can be an indicator of stress however some species of tree, such as Lime trees, generate dense epicormic growth by design as seen around the base of the Lime tree in the photo opposite.
Recognising defects and understanding their significance therefore requires knowledge and experience of individual tree species, their natural features and their typical form.
So to return to the Horse Chestnut tree near my house, the tree is in fact suffering some stress as a result of root disturbance and ground compaction. It is also mature to over mature in age and has some epicormic growth on the main stem however for the most I would have to agree with their current conclusion, it does (at present) look sturdy enough!
Image Credit for Mycelium: Dr. George Knaphus Iowa State University
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