**3.2.2 Defining the disturbance scenarios and sustainability indicators**

Characteristics of disturbance can be best described by the frequency (return intervals for wildfire and rotation lengths for harvesting) and intensity (severity for wildfire and utilization levels for harvesting) of disturbance. Based on input from local ecologists and soil scientists, we defined three severity categories (low, medium, high) and three fire

Sustainable Forest Management in a Disturbance

to total WD loading on the disturbed sites in the study area.

low-quality trees.

Context: A Case Study of Canadian Sub-Boreal Forests 125

ground WD on the sites. These above-ground WD retention percents on the harvested sites were higher than we expected. This is because a relatively high proportion of the living stems in the stands that were studied were smaller than the utilization criteria; more than 92% of above-ground WD on the harvested sites was contributed by woody material with a diameter less than 15 cm, which was the lower limit for utilization on these sites. The remaining WD was contributed by small quantities of logs in advanced decay stages that were a carryover from previous natural disturbances, and unharvested

There were no significant differences in below-ground WD mass between the three types of disturbance (Tables 2). The amount of below-ground WD was similar to the amount of above-ground WD on the harvested sites, and was about 30% of the above-ground mass on the fire-killed sites. This suggests that below-ground WD makes an important contribution

WTH SOH Wildfire

Diameter size class (cm) Mass of WD (Mg.ha-1)

0.0 -- 0.49 0.25(0.0) 0.63(0.0) 0.49(0.0) 0.5 -- 0.99 0.75(0.1) 1.78(0.2) 1.37(0.1) 1.0 --2.49 1.61(0.1) 3.60(0.5) 2.77(0.3) 2.5 -- 4.99 1.28(0.1) 2.67(0.2) 2.53(0.1) 5.0 -- 6.99 1.73(0.2) 3.83(0.6) 4.34(0.4) 7.0 11.71(1.3) 15.51(2.3) 91.09(7.3) advanced decay 1.88(0.3) 3.68(1.4) 1.29(0.3) stump 1.62(0.1) 1.96(0.4) 0.00

above-ground CWD 18.22(1.2)a 27.65(1.6)a 99.25(4.2)b above-ground FWD 2.61(0.1)a 6.01(0.4)b 4.63(0.2)b total above-ground WD 20.83(1.1)a 33.66(2.2)a 103.88(4.3)b

total below-ground WD 31.98(1.9) a 31.53(2.2) a 37.12(1.6) a

total WD 52.81(3.0)a 65.19(4.1)a 141.00(5.8)b

Timber harvesting removed most of the above-ground biomass, but like natural disturbances it left all below-ground biomass on site. The below-ground biomass (total roots) in lodgepole pine stands accounts for an important portion of total tree biomass,

Table 2. Comparison of woody debris (WD) mass (Mg.ha-1) in the year immediately following disturbance between stem-only harvested (SOH), whole-tree harvested (WTH) and wildfire-killed sites, based on modification of the field data to account for mass losses due to decomposition since disturbance (Note: FWD: fine woody debris; CWD: coarse woody debris; standard error of the mean is in parentheses; the sample size (n) is 25 for WTH, and 20 for others; means with the same letter within a row are not significantly

different (*p* > 0.05) from each other (the Tukey test))

return intervals (40, 80, 120 years) for wildfire simulations, and two utilization levels (SOH, WTH) and three rotation lengths (40, 80, 120 years) for harvesting simulations. A description of those severity and utilization categories is given in Table 1. Each scenario was then simulated commencing with the initial ECOSTATE file described in Wei et al. (2003). The unrealistically short 40-year rotation length was included in the study in order to compare harvesting at this frequency with fire, and because it helps to define a response curve (Powers et al., 1994).

Four output parameters (production, mass of decomposing litter, total available soil nitrogen and nitrogen removal) were used in the assessment of the sustainability of site productivity. Total production is a direct indicator of achieved productivity, while decomposing litter, total available soil nitrogen and nitrogen removal are indirect indicators of site productivity potential. Woody debris, as part of decomposing litter, is also a source of asymbiotic nitrogen fixation (Wei and Kimmins, 1998), and can protect soil from erosion and play an important role in maintaining some aspects of biodiversity (Harmon et al*.,* 1986; Hunter, 1990).


Table 1. Definition of disturbance severity for both wildfire and harvesting for simulations of lodgepole pine forests (\*Fire-L: low severity fire; Fire-M: medium severity fire; Fire-H: high severity fire; SOH: stem-only harvesting; WTH: whole-tree harvesting)
