From Your Forest Newsletter Vol. 2
Scheduling a harvest – Part 1
As I write this, we’re in the midst of a relatively long stretch of classic winter weather, with consistently cold temperatures, periodic snowfalls and most notably, no thaws. These conditions are what foresters and loggers hope for between November and April because it allows us to carry out timber harvests on a certain subset of forest stands where an operation is not ecologically responsible during the rest of the year. For example, stands that have wet ground from seeps and springs or stands that support rare herbaceous plants are best harvested in the winter because heavy machinery used in a harvest mostly ride on top of the snow and ice, protecting the ground from damage.
There are many specific factors that determine when a harvest can and should be completed but before looking at the specific timing of a timber harvest, let’s first consider the fundamental timing – when is the forest ready to be harvested? One key part of a forest management plan is the proposed schedule of management activities that outlines when treatments, such as timber harvests, should occur. The dates proposed in the plan are not arbitrary. Not every acre of forest can be harvested at any one time, just like your entire vegetable garden can’t be harvested at any one time. In general, the number of trees and their size dictates the relative growth of a forest (obviously overlooking soils, weather, climate, species, etc) and as the density (the number of trees per given area) and size increase, the growth rate decreases. If a forest is too crowded, tree growth decreases and if the crowded condition lasts long enough, tree health and timber quality can suffer. Conversely, having too few trees can reduce timber quality because it allows for the growth of branches on the lower trunk of the tree, compromising the log quality.
During the forest inventory process (one of the steps of developing a management plan), data are collected for each forest stand (or area of forest with similar composition and structure) that allow foresters to measure the parameters to determine how crowded the forest is. The three main metrics are stem density, typically measured in trees per acre, basal area (see definition below), measured in square feet per acre and average stem diameter in inches. But these are just numbers and they need to be interpreted. Based on decades of research, guides have been created for major forest types outlining what conditions allow forests to grow best for timber production. These are known as stocking guides and allow foresters to interpret the inventory data. A stocking guide for upland central hardwood forest type is presented to the right as a sample. Although this forest type doesn’t exist in our region, this particular chart does a good job showing how to interpret the guide. As is standard with stocking guides, three lines have been delineated, A, B and C that break the chart into growth conditions, understocked, fully stocked and overstocked. Between the A and B lines, a forest is considered fully stocked and conditions are suitable for good forest growth. Below the B line and above the A line, conditions are not optimal for the growth of timber for reasons discussed above.
Using the forest inventory data and the appropriate stocking guide for the forest type, foresters can determine about how crowded the forest is and can begin to determine when the best time to harvest would be. The forester also integrates qualitative data collected during the inventory, such as crown size, tree height, tree health and apparent tree growth rates. If the stocking is nearing or above the A line or if the tree growth appears to have stagnated, a harvest is scheduled sooner, say in the next five to 10 years. If the stocking is closer to or below the B-line, a harvest is not warranted at the time. Depending how understocked a forest is, a harvest could be scheduled for the future, say 10 or 20 years down the road, but it would be advisable to recheck the conditions as that date gets closer to determine the current stocking.
Although harvests are typically only scheduled when the stocking is around the A-line, there are cases when a harvest is acceptable when stocking is low. When the quality of the stand, as determined by the proportion of trees that could be sold to a sawmill, is poor and there is not much potential for the stand to increase in quality, a stand could be harvested when the stocking is low in order to “reset the clock” by establishing another generation of trees. There are also cases when a harvest would not be scheduled immediately for a stand that is highly stocked. For example, delaying harvests and allowing the forest to become more highly stocked allows for the forest to sequester more carbon in the form of biomass, thus acting as a better carbon sink for the reduction in atmospheric carbon dioxide.
This was a quick synopsis of the main factor that foresters use for proposing dates for timber harvests, whether it’s in one year, or 20. And as it turns out, that is usually the easy step of scheduling a harvest. When the harvest actually occurs is determined by another set of factors (including weather, as discussed in the opening paragraph) and will be discussed in a future article.
Definition: Basal area - A measurement of the density of trees in an area, determined by estimating the cross sectional area of all trees at breast height (4.5 feet above ground) in a given area. Basal area is typically expressed in square feet per acre.