Topic 9.1: Collateral Damage to Trees and Their Responses

Logging affects trees in the residual stand directly and immediately when they are mechanically damaged, and indirectly and more slowly by the subsequent depredations of pathogenic and wood rotting fungi, along with insects and other invertebrates.  Given that some damage is unavoidable, it is important to understand how trees resist and respond to having their bark stripped, their stem and branches broken, and their roots abraded.

Bark represents the first and often the most effective line of defense for tree stems and roots.  The outer bark (phelloderm), which is mostly composed of dead cells, and the inner bark (phloem) resist and respond to damage in somewhat different ways.  Often both are extremely tough, being strengthened by abundant bast fibers.  Out bark can be peeled off without exposing the vascular cambium; new cork cambia re-differentiate and the outer bark can be rapidly replaced.  Removal of the inner bark exposes the vascular cambium and results in a much more serious sort of wound.

Barks of some trees have extremely high concentrations of compounds that impede the growth of fungal hyphae and deter feeding by insects; for example, up to 40% of the dry weight of bark of mangrove trees in the genus Rhizophora is tannin.  In response to wounding by logging equipment or attack by animals, some barks release sticky resins, gums, or latex that impede both further feeding and fungal growth.  Living cells in bark provide for dynamic responses to insect and fungal attack. These cells can exude compounds that effectively contain the attack.  Compartmentalization of decay, exudation of resins and gums, and other mechanisms of defense are also found in wood, but are often much less effective than in bark.

Exposure and death of portions of the vascular cambium due to bark removal can threaten the life of a tree or at least reduce the market value of its stem.  As has already been mentioned, trees protect themselves from diseases and decay not by attacking the attackers, as in the immune responses and phagocytotic responses of animals, but by compartmentalizing the damaged or diseased tissue.  By containing the “disease courts” of fungi and bacteria by sealing off the affected tissue with resins, gums, and/or thick walled cells, the threat is at least kept from threatening the entire tree.  Small compartments of decay might not represent a large problem for trees or the wood products made from them. In contrast, where the decay is substantial, the tree may be susceptible to breakage and its wood much reduced in value. In many cases the compartmentalization process in wood is not successful and pathogens can spread up and down the bole.   Pathogenic fungi that enter through damaged tissues may eventually kill the tree whereas heartrot fungi might leave the tree standing but hollow.

In response to stem breakage, many trees can resprout.  Root damage can also stimulate production of new stems referred to as “root suckers.”  Although resprouting is an important mode of forest regeneration, the junctions between new sprouts and old tissues may not be well sealed against pathogen attach and may not be mechanically strong.  Resprouts from small stems may develop mechanical integrity and become sealed against pathogens more rapidly than the broken stumps of large trees.  Where sprouting from cut stems is desired, as in stands managed by coppicing, leaving cleanly cut stems that are somewhat inclined to promote drainage may reduce the incidence of fungal attach and promote strong stump-sprout junctions.

Damage to standing trees during selective logging creates a silvicultural dilemma.  Damaged trees can be beneficial insofar as they help to maintain pre-logging environmental conditions (e.g., shade) and may produce fruits and seeds that help to maintain animal communities and promote forest regeneration.  Unfortunately, many of these damaged trees are likely to die within a decade or two after logging, or develop heartrots or other defects that substantially reduce their commercial value.  The dilemma for the silviculturalist is whether to retain these damaged trees or to harvest or at least kill them to provide better growing conditions for trees that are more likely to survive the cutting cycle and provide marketable wood.  Given the extremely high proportion of heartrot-effected trees several decades after destructive logging (up to 25-30% of large trees in some forests), the question of what to do about trees damaged during logging is very serious.  The only obvious answer is to avoid damage in the first place through careful planning and implementation of logging operations.