Coarse Woody Debris: #2 Recruitment
The recruitment, transformation and loss of coarse woody debris (CWD) within the forest ecosystem varies by forest type, can be spatially explicit, and can be influenced by temporal patterns. While there are few long-term studies (greater than a decade) to provide guidance, forests of southeastern North America appear to produce less CWD than coniferous forests in northwestern North America.
Because of the episodic nature of CWD recruitment (insect outbreaks, storms, etc.), mortality that provides large inputs is often aggregated and exhibits an orientation that reflects prevailing storm patterns and winds. Seasonal variation in hurricanes and ice and windstorms will influence major recruitment. Amounts and types of input will also vary according to the successional stage of the forest (photo: Bill Byrne, 2011 tornado impact).
Tree mortality agents include wind, fire, insects, disease, suppression and competition. Wind causes uprooting, snapping of the boles, breaking of branches. Trees may also be broken by flying debris during large wind events. Fire can kill trees directly by girdling the stem, scorching the crown, or burning the root system (USDA Forest Service 1981). The heat from the fire may simply weaken trees and make them more susceptible to insects.
Insect attack can lead to outright mortality or to a continued weakening of the tree stem and a greater susceptibility to future wind damage. Diseases, usually caused by fungi, generate small amounts of CWD directly, while synergistically contributing to tree mortally associated with biological and environmental stressors.
In closed canopy forests, including unthinned pine plantations, trees that are not part of the dominant canopy layer are often suppressed through shading. This form of competition leads to a loss of vigor and consequently a higher susceptibility to fatal insect and pathogen infestation. Suppression mortality occurs in stands of all ages, but is most important as a source of CWD in mature forests with closed canopies, because of the size of the dead woody materials.
The next blog in this series will look at the process of coarse woody debris decomposition.