Topic Introduction
The pattern of individual tree growth is periodic over days and weeks depending on favorable temperature and moisture conditions. Tree growth pattern over years follows a typical sigmoid shaped curve. The pattern of tree growth in a stand varies between individuals because trees growing together compete for light, water and nutrients. Trees regenerating from seed start off at about the same size but some trees grow more rapidly and others more slowly. Over time the vigorous or fast growing trees suppress the slow growing trees that may become stagnant or die. Through these interactions between trees – or competition – the forest stand changes through time, it is dynamic.
Between tree competition develops as they begin to compete for aboveground resources (light) and below ground resources (moisture and nutrients). In moist environments the onset of competition usually occurs around the time of canopy closure or in drier environments, characterized by more wide-spaced trees, competition is belowground between roots for scarce moisture resources.
In woodlands the trees do not close canopy and the competition is mainly between roots for soil moisture. In a forest the trees usually achieve canopy closure at which time competition for light and tree height becomes important.
Trees in a stand differentiate in to crown classes because the growth of some trees slows more than others so that as the stand ages, a range of tree sizes develops that can be classified in to different crown classes – according to the trees relative position in the canopy.
Disturbance is a determinant of forest structure and competition between trees because forests are continually impacted by external forces to modify the structure of the stand – often through killing and felling of trees. Sources of disturbance include wind, fire, drought, water, ice and biota. There is a whole area of ecology devoted to describing and defining disturbance and our intention here is to briefly identify the interaction between trees and disturbance in creating and modifying stand structure.
Terminology and concepts definition of a stand, even-aged, mixed-age, competition, canopy, stand structure, gap formation, shifting-mosaic steady state, old growth, landscape scale.