Why don’t trees rot in the ground, while wooden structures do if they aren’t built properly?
Chris Daniel
Colwyn Bay, Conwy, UK
Trees are living organisms that can respond actively to infection, whereas timber products must be treated chemically to make them resistant to rotting.
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Not having immune systems, trees have evolved ways to beat infections by means of chemical and mechanical barriers
Living trees can become infected by bacteria, or viruses due to physical injury or damage, but, not having immune systems to neutralise infections, they have evolved ways of combatting pathogens by means of chemical and mechanical barriers. This is done in three distinct stages.
The first is strengthening of the walls between cells to isolate the living tissues from the source of harm and minimise the spread of damage.
The second is the setting up of four kinds of defence around the damaged area, in a process called , which provides barriers to the spread of infection. The first of these is the formation of a gum that plugs the vascular channels in the sapwood, or xylem, to limit the spread of fungal filaments upwards and downwards to the rest of the tree. The second is the production in the xylem of chemical compounds that resist the spread of decay into the centre of the branch or trunk. The third prevents the spread of decay circumferentially round the active growth ring, or cambium, and is promoted by chemicals in a type of xylem cell with radial walls. The fourth is the generation by the cambium of a callus, just under the bark. This is called the barrier zone, and it consists of chemicals that spread outwards to prevent fungus from infecting new xylem. Woody tissue called woundwood then grows over time to close the damaged area.
The third stage of infection control is the growth of new tissues around the infected compartment, which is now full of compounds that kill the infective organisms and deprive them of their food supply. As long as the growth of new tissue in the tree outpaces the infection, the will survive.
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