In many management applications, the degree of precision from a best fit equation may be less useful than a more easily applied equation with only a good fit. Because height class is the easiest and quickest measure, it is the tree characteristic used below to estimate understory tree biomass, even though for most species studied it gives a good but not best fit to the biomass data.
The methodological scheme outlined below is but one of several ways that biomass estimation may be approached. It is one of the broadest and least precise methods but is also fairly simple to apply. It can be integrated with measures of dead and down fuel (Brown, 1974) and litter or duff depth/mass regressions (Agee, 1973) to provide estimates of available fuel and created fuel from prescribed fires.
In this example, 20 plots in mixed conifer forest comprise the sample. It will usually be better to sample a larger number due to variability in understory tree density. Each square fixed plot is 3 m on a side. Basal area of the stand is estimated with a prism from the center of the fixed 9 m2 plot; however, the prism sampling is not constrained by the fixed-plot boundaries. Understory trees are tallied by a grouped species (pines or firs) and by 1 m height classes within each plot. The data are shown in Table 11.
The biomass is estimated by the height-biomass regressions using the combined pine and combined fir equations by overstory density class (Table 8). The procedure involves: 1) separating plots into “open” or “dense” overstories; 2) summing the number of pines and firs by height class in each overstory class; 3) calculating an average tree biomass within each category identified under (2); and 4) computing average biomass per unit area.
1. Separation of plots. The criteria for open and dense overstories is basal area of the stand. In this study areas classed as “open” tended to have basal areas below 45 m2/ha, and “dense” stands had basal areas above 45 m2/ha. Therefore, the sample data here was segregated using this criterion; 11 plots were classed as “open” and 9 were “dense.”
2. Sumning the data. The data grouped by species and height class are shown in Table 12.
3. Computing an average tree biomass by category. These are 12 categories shown in Table 12, and it is necessary to calculate an average tree biomass for each category. This can then be multiplied by the number of trees in each category to compute total biomass. The equations to be used are at the bottom of Table 8: both pines dense, both pines open, both firs dense, and both firs open. A sample calculation for the 0-1 m height class, dense overstory, both firs equation is shown below:
Equation: lnY = -0.0628 + 2.3576 ln(X2)
where Y = biomass (g) + 1
X2 = tree height times 10
In this example the mean of the height class, 0.5 m, is used to represent the average height of the trees tallied in this category.
h = 0.5 m
(X2) = 5
ln(X2) = 1.6094
ln(Y) – -0.0628 + 2.3576 (1.6094)
ln(Y) – 3.7315
Y = 41.74
so biomass is equal to 40.74 g for the average 0-1 m height class tree. Similar calculations are done for each category, resulting in the average per tree biomass shown in Table 13.
4. Computing average biomass per unit area. The total biomass on “open and “dense” plots is obtained by multiplying the figure in Table 12 by the equivalent category figures in Table 13.
For this example, total biomass is shown in Table 14. This must be converted to an area basis by dividing by the area of all plots within that overstory class. In this example, there were 11 “open” plots and 9 “dense” plots. Because each plot is 9 m2, total area in “open” plots is 99 m2 and total area in “dense” plots is 81 m2 . When the totals shown in Table 14 are divided by these figures the average biomass per unit area is 872 g/m2 for “open” areas and 417 g/m’ for “dense” areas. A weighted average for the entire area can be calculated if the chosen plots represent the actual proportions of the area in “open” and “dense” overstories: 872(.55) + 417(.45) – 667 g/m2.
This figure can then be used as an estimate of the total biomass. On a first burn, very little understory tree biomass will be consumed, but can be considered a biomass transfer from live to dead fuel, depending on scorch height and damage to roots.