Stand structure is the distribution of trees by species and size within a stand. The structure is the result of several factors:
Stands may be grouped into difference structural classes for different reasons. However, three common stand structure classes are described as:
- Growth habit of the tree species, especially the degree of shade tolerance.
- Ecological conditions.
- History of disturbance and management.
Even-aged stands tend to have normal or skewed-normal distributions for their frequency curve of diameter at breast height (dbh). Thus, an estimate of the mean and standard deviation of dbh is a useful summary of the stand table. Young and slow sorting stands tend to be positively skewed with more trees occurring in the smaller classes (i.e. median and mode less than mean). Conversely, an over-mature stand tends to have relatively few small trees and may be negatively skewed (median and mode greater than mean). Normality can be tested by visual inspection of a normal quantile plot where a reasonably straight line should be seen. Alternatively, a statistical test like a Shapiro-Wilk test can be carried out. Many statistical packages are available to present these plots or calculate the test statistics.
- Even-aged. Trees are of an identical age having been planted at the same time or germinated within a short period of each other (e.g. in response to a fire). Small differences in age may not be of any practical significance and are ignored.
- Uneven-aged. Stands are intermixtures of species and ages with no obvious boundaries separating them.
- Multi-aged. These stands are intermediate between even- and uneven-aged stands. Two or more age or species groups can be distinguished within the stand, although the boundaries may not be clearly defined. Examples of multi-aged stands include stands where regeneration is occurring in patches opened in the stand (due to timber harvesting or death of over-mature trees), and two-tiered or two-storeyed forests.
Normal distributions can however be truncated or distorted by management actions. For example, thinning operations in a plantation may disproportionately remove smaller (or larger) trees.
Multi-aged stands tend to have bi- or multi-nodal distributions - two or more peaks in the frequency curve. In the easiest cases, two independent normal distributions can be isolated.
The frequency distribution for dbh of uneven-aged stands is typically reverse-J shaped. However, this apparent shape may be an artefact of the scale. Small areas or patches within the stand may be normally or bimodally distributed, but when these patches are added together, the overall effect appears to be a reverse-J distribution. Small samples in the uneven-aged forests in Eden found normal, bimodal and reverse-J distributions within one 66 ha compartment. When added together, these distributions resulted in the typical reverse-J pattern.
Research is currently underway to determine an optimal distribution pattern for uneven-aged forests. There is no reason to believe that the simple geometric pattern of De Liocourt or even the continuous distribution modelled by a negative exponential function is optimal for site occupancy or biodiversity.