**4.1.1 Difference in WD mass between SOH, WTH and wildfire disturbances**

There were significant differences in above-ground CWD (*p* < 0.001) and total WD mass (*p* < 0.001) between the fire-killed and the harvested sites that we sampled. The greatest CWD and WD mass was created by wildfire while the lowest was left on the WTH sites; wildfire left about 3 to 5 fold more above-ground WD on the sites than did clearcutting. No significant differences in above-ground CWD and total WD mass were detected between SOH and WTH (Table 2), except the difference in above-ground FWD (*p* < 0.001) (Table 2). However, the means of CWD and WD mass were greater on the SOH sites than on the WTH sites.

Timber harvesting removed far more above-ground WD than did the wildfire disturbance that was investigated. In the case of a severe wildfire, the difference would be much less. However, harvesting did leave between 18% (WTH) and 24% (SOH) of total above-

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

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;

(Fire-L) 0 10 10 20 0 0 10 10

(Fire-M) 15 60 50 60 20 20 20 60

(Fire-H) 50 95 95 100 30 30 60 95

WTH 90 90 90 90 0 0 0 90

Harvesting SOH 90 90 0 0 0 0 0 0

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:

There were significant differences in above-ground CWD (*p* < 0.001) and total WD mass (*p* < 0.001) between the fire-killed and the harvested sites that we sampled. The greatest CWD and WD mass was created by wildfire while the lowest was left on the WTH sites; wildfire left about 3 to 5 fold more above-ground WD on the sites than did clearcutting. No significant differences in above-ground CWD and total WD mass were detected between SOH and WTH (Table 2), except the difference in above-ground FWD (*p* < 0.001) (Table 2). However, the means of CWD and WD mass were greater on the SOH sites than on the

Timber harvesting removed far more above-ground WD than did the wildfire disturbance that was investigated. In the case of a severe wildfire, the difference would be much less. However, harvesting did leave between 18% (WTH) and 24% (SOH) of total above-

high severity fire; SOH: stem-only harvesting; WTH: whole-tree harvesting)

**4.1.1 Difference in WD mass between SOH, WTH and wildfire disturbances** 

**4.1 Differences immediately following disturbances** 

root

Small root Cones

Disturbance Severity\* Biomass burned or removed for each component (%) Stemwood Stembark Branch Foliage Large root Medium

response curve (Powers et al., 1994).

Low

High

**4. Results and discussion** 

WTH sites.

Hunter, 1990).

Wildfire Medium

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 low-quality trees.

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


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))

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,

Sustainable Forest Management in a Disturbance

fire severities: see text for explanation)

N

P

dates.

Context: A Case Study of Canadian Sub-Boreal Forests 127

burned material may be lost as fly-ash, or simply added to the ground as ash. No attempt was made in this study to compare the nutrient losses through soil leaching process following the SOH, WTH and fire disturbances. However, nitrogen leaching losses after harvesting or wildfire disturbances are believed to be low in these dry interior ecosystems.

total removed1 97.8 (3.9) 57.0 (4.6) 50.0 (1.5) 166.7 (4.8)<sup>3</sup> removal ratio<sup>2</sup> 0.31 (0.01) 0.18 (0.01) 0.15(0.01) 0.49(0.02)

total removed 8.8 (0.4) 5.2 (0.4) 2.8 (0.1) 9.5 (0.5) removal ratio 0.44 (0.01) 0.26 (0.01) 0.13(0.01) 0.44(0.02)

Table 4. Estimated nutrient losses (kg.ha-1) caused by stem-only harvesting (SOH), whole tree harvesting (WTH) and wildfire (from Wei et al. 1997)(Note: Standard error of the mean is shown in parentheses; 1. total removed is the amount of nutrients removed during disturbance; 2. the removal ratio is the amount of nutrients removed during disturbance divided by total amount of nutrients in biomass (above and below-ground biomass and forest floor) prior to disturbance; 3. the range given here is based on an assumed range of

**4.1.4 Asymbiotic nitrogenase activity in decaying wood, forest floor and soil** 

in stumps and litter was higher than that in soil and early decaying stems.

The nitrogen fixation rate in dead root was significantly higher than in the other substrates examined (Table 5), and the rate in mineral soil was the lowest. Table 5 also showed that nitrogen fixation rates in more advanced decay wood (medium or advanced decay classes) were significantly higher than in early decay wood. The difference in nitrogen fixation rates between medium and advanced decay wood was not significant. The nitrogen fixation rate in May 15, 1995 was the lowest, due to low temperature, among all other sampling

The differences between early and medium decay classes of WD for both wildfire-killed and harvested sites indicate that nitrogen fixation activity increases as wood decay progresses (Table 5). However, there was little change in activity between medium and advanced decay stages on the wildfire-killed sites, suggesting that the increase in activity may only occur during the early stages of decay and then reach a steady level, depending on moisture content. The nitrogen fixation activity associated with dead roots was the highest among all substrates we studied (Table 5) probably due to the high moisture content of this material. The lowest activity occurred in mineral soil probably because of insufficient carbon substrates and low moisture content and also because soil weights more per volume. The nitrogen fixation activity

Rates of nitrogen fixation in our study are generally consistent with other measures in northern forest ecosystems (Jurgensen et al., 1987 and 1989; Hendrickson, 1988; Harvey

WTH SOH Wildfire

Nutrient variables Disturbance types

ranging from 20% on mesic sites to 28% on xeric sites (Comeau and Kimmins, 1989). This below-ground biomass may play an important role in long-term site productivity on these disturbed sites because our data show that asymbiotic nitrogen fixation rates in belowground WD are significantly higher than in above-ground WD (see the section on asymbiotic nitrogenase fixation). Most WD studies have ignored below-ground WD, which may result in an incomplete understanding of the post-disturbance role of WD in forest ecosystems.
