With the new method, surveyors tally well-spaced trees in a
3.99 m radius plot and record the basal area of layer 1 (dbh >= 12.5
cm) trees around the plot centre. These two measurements are
combined to estimate understorey stocking at the sample point.
The new method is suitable for many blocks with dispersed retention.
In assessing regeneration stocking in this small opening, the
new method accounts for the edge effect around the perimeter
of the opening and the influence of the retained trees in the
middle of the opening.
The new method has three main uses. First, it can provide information
useful for treatment decision making. Second, it can be used
to specify “non-traditional” stocking standards (subject
to approval by the Ministry of Forests). Third, it provides insight
into the yield implications of a given understorey density.
DFP by well-spaced trees per plot and overstorey basal area
The well-spaced tree count and the basal area around the plot
determine the DFP at the plot. DFP (deviation from potential)
is a measure of understorey stocking. Three stocking classes are
shown – stocked (green), partially stocked (yellow), and open
(red) – based on the second approximation DFP table. (Click for high resolution PDF version of table)
The focus of the new method is on the growing space available
to seedlings. For this reason, when determining overstorey basal
area with the new method, all live overstorey trees are counted – regardless
of their economic value, form, species, or health. If standards
are required for the retained trees, they must be established
Overstorey trees reduce the growth and future harvest yield of
understorey trees by several mechanisms, including a) shading them
and b) providing a physical barrier to understorey tree crown
expansion. Other mechanisms include competition for soil resources
and, in some locations, reducing soil temperature.
Skid trails through areas of heavy retention within the NAR
(net area to be reforested) pose problems for existing stocking
assessment systems. However, the new method detects the heavy
overstorey in the vicinity of plots that fall on the trail and
indicates that the trail is “stocked” – in
the sense that the trail provides no growth opportunity for seedlings.
Under heavy retention, seedling height growth is reduced. This
must be accommodated by either delaying surveys or specifying
a minimum height standard that is achievable with the prescribed
amount of overwood.
In stands with retention, it is important to establish seedlings with shade tolerance that is appropriate for the amount of overstorey around them.
Understorey trees must meet standards for health, species, height, freedom from brush, etc., in the new method.