Extension > Garden > Yard and Garden > Trees and shrubs > A practioner's guide to stem girdling roots of trees > Physiological and structural effects of SGRs
Physiological and structural effects of SGRs
SGRs affect trees through stem compression, resulting in (1) physiological effects, (2) structural defects, and (3) adaptive growth. These responses, like xylem anatomy defects and changes in tree water relations, might go unnoticed, then become suddenly apparent when trees fail and expose a severely discolored and decayed stem, or by the tree's attempt to repair itself through adaptive growth, expressed as a bulge of stem tissue on the top of the SGR.
The presumed physiological cause of tree decline with SGRs is that the compression of stem tissues causes anatomical changes in wood and bark tissues, which in turn affect normal physiological processes. Anatomical changes have been documented, but the relationship with physiological processes has not been empirically measured.
When a root comes in contact with a tree stem, woody and bark tissues are compressed as the root and stem enlarge. Hudler and Beale (1981) found that in girdled woody stem tissue in a Norway maple, the numbers of vessel elements declined, vessel cross-sectional area was reduced by a factor in excess of 10, rays were skewed, and pits in rays were few. Bark tissue was compressed by a factor of 25. Cursory observation of phloem tissue suggested there was as much, or more, damage than there was to the xylem.
The transport of sap in trees is likely affected by compression from SGRs. Hudler and Beale (1981) suggested that stem compression causes tree decline by reducing stem conductivity and radial communication between tissues. Vessels and tracheids are cells that transport water in woody tissues of roots and shoots. Water transport occurs along a pathway from the soil to the atmosphere in response to a water potential gradient. Among the resistances to water transport found along the pathway is the diameter of the water transport cells. The smaller the diameter of the cell, the greater the resistance to water transport. In other words, greater pressure is required to pull water through a small-diameter water-conducting cell than through a larger-diameter one.
The Hagen-Poiseuille Law models liquid transport through circular tubes with rigid walls and laminar flow. The model can be used to estimate the impact of smaller diameter vessel elements and tracheids as a result of stem compression.
Using the above equation, the pressure gradient needed for similar water flow through vessel elements or tracheids under normal and compressed conditions can be approximated. Using data from Hudler and Beale (1981), a 10-fold difference in pressure required for water transport is estimated-0.007 MPa/meter (0.07 bars/meter) for normal vessel elements and 0.08 MPa/meter for altered vessel elements. Hence, a 10-meter-tall Norway maple would require 0.8 MPa of pressure to overcome resistance associated with the smaller-diameter vessel elements. As soil moisture is depleted, this resistance would plausibly influence tree water deficits earlier in trees with SGRs than in those lacking them. The percentage of total xylem-conducting area altered by girdling roots is an integral component influencing increased water deficits.
Damage to phloem tissue is another area in which SGRs can impact tree vitality. Phloem tissue is involved in the translocation of sugars, growth regulators and hormones, minerals, nitrogen compounds, and other substances within the tree. Damage to phloem tissue impedes translocation between branches and roots. A reduction of transport of photosynthesis products to the root system can result in root system decline and death and hasten overall tree decline and death.
No direct physiological measurements have been collected that quantify the effects of SGRs on tree health. Indirect indicators suggest negative physiological effects to varying degrees that influence tree survival, diameter growth, leaf size, leaf color, tissue structure, crown density, foliage dieback, crown height, tree vitality, and tree condition (structural integrity). These might or might not affect a tree's longevity or overall appearance.
Fig. 22 - Note the bulge in the stem over the girdling roots, which is stem wood embedding the roots.
Structural defects arise when stem compression results in tissue death. Compartmentalization of decay limits the vertical, radial, and tangential spread to healthy tissue. Decay organisms work to invade the dead area and, if successful, weaken the wood. Tree failure will occur when the strength of the wood holding the tree upright is less than the force acting upon the stem to topple it.
Trees respond to wounds through addition of new wood (Mattheck and Kubler 1995) in order to optimize themselves against external loading factors. A bulge is common evidence of such adaptive growth. Examples include addition of stem tissue near a cavity and growth around a foreign object (e.g., rock, fence post, or rope). Trees respond to SGRs by attempting to embed the root within the stem (Figure 22). In an SGR, however, both the foreign object (the root) and the stem tissue trying to embed the root are growing. The success of trees embedding roots within their stem tissue is unknown. However, the failure rate of trees successfully embedding SGRs must be great due to the abundance of SGR cases in which trees topple during a loading event.