**4.2 Long-term implications of these differences in sustainability of productivity 4.2.1 Impacts of disturbance frequencies (rotation length or intervals)**

As expected, the total productivity over a 240-year simulation increased with the length of the interval for all disturbance types (Figure 1a). This is clearly related to lower nitrogen losses over the 240-year simulation period (Figure 1b) and consequently more nitrogen and

et al., 1989; Sollins, 1982; Roskoski, 1980). Cushon and Feller (1989) found much lower values; aerobic conditions during incubation in their assay might be responsible for this

Aug. 22, 94 Sept. 25, 94 May 15, 95 June 18, 95 July 20, 95 mean

early 2.60 (0.10) 1.27 (0.32) 0.00 1.86 (0.91) 0.92 (0.30) 1.33 (0.36) c medium 3.91 (0.92) 3.85 (0.71) 0.14 (0.10) 10.94 (2.66) 9.00 (3.32) 5.58 (1.14) b advanced 6.32 (1.72) 8.69 (1.49) 0.33 (0.20) 12.05 (6.40) 2.81 (0.71) 5.64 (1.61) b dead roots 11.40 (2.82) 14.87 (4.56) 1.23 (0.46) 37.15 (16.1) 10.94 (3.11) 15.1 (4.75) a branches 4.70 (2.20) 4.96 (1.98) 0.00 0.29 (0.24) 0.00 1.99 (0.75) c floor litter 6.92 (3.11) 7.10 (2.02) 0.00 8.75 (4.11) 4.23 (1.52) 5.40 (1.43) b humus 1.33 (0.32) 0.10 (0.03) 0.21 (0.20) 7.05 (3.47) 1.68 (1.30) 2.07 (0.87) c mineral soil 0.00 0.05 (0.03) 0.00 0.50 (0.10) 0.00 0.11 (0.04) d

early 3.22 (0.30) 3.62 (1.14) 0.00 3.49 (0.88) 0.72 (0.24) 2.21 (0.74) c medium 3.90 (1.01) 3.60 (1.20) 0.00 10.70 (2.56) 7.37 (2.10) 5.20 (1.10) b advanced 10.02 (3.07) 1.76 (0.52) 0.60 (0.40) 10.11 (2.72) 9.11 (3.82) 6.32 (1.24) b dead roots 11.90 (3.01) 10.38 (1.34) 6.20 (0.31) 11.34 (1.29) 13.52 (5.94) 10.7 (2.01) a branches 3.01 (0.80) 5.57 (1.67) 1.41 (2.01) 1.06 (0.14) 2.57 (1.01) 2.72 (0.98) c floor litter 12.1 (2.05) 8.30 (2.89) 2.30(2.21) 1.20 (0.22) 4.30 (1.12) 5.64 (1.47) b humus 1.13 (0.24) 0.34 (0.15) 0.49 (0.28) 2.28 (0.89) 0.41 (0.31) 1.16 (0.37) c mineral soil 0.10 (0.05) 0.15 (0.07) 0.00 0.00 0.10 (0.10) 0.07 (0.03) d

Table 5. Estimated nitrogenase activity for each substrate and sampling time and the mean nitrogenase activities for all sampling dates in 1994-1995 (from Wei et al. 1998) (Note: Each value is the mean and (the standard error) of 4 samples; means with the same letter within a column are not significantly different (*p* > 0.05) from each other (the

**4.2 Long-term implications of these differences in sustainability of productivity** 

As expected, the total productivity over a 240-year simulation increased with the length of the interval for all disturbance types (Figure 1a). This is clearly related to lower nitrogen losses over the 240-year simulation period (Figure 1b) and consequently more nitrogen and

**4.2.1 Impacts of disturbance frequencies (rotation length or intervals)** 

**Substrate Nitrogen fixation rates (nm C2H4g-1day-1)** 

deviation.

Fire-killed sites decaying wood

Harvested sites decaying wood

Tukey test))

forest floor accumulation (Figures 1c,d, respectively). This indicates, as expected, that the sites we studied would be more productive under less frequent disturbance by the regimes defined in this study.

The rate of increase in productivity between disturbance scenarios varies, with a sharp increase from intervals of 40 years to intervals of 80 years, but only a modest increase from 80 years to 120 years. This reflects not only the difference in percentage change in interval length between these two scenarios, but a decrease in stem mass accumulation at stands ages greater than 80 years. The combined effects of genetically-determined, age-related decline in growth rates and the altered geochemical balance at longer disturbance intervals results in a declining sensitivity of total productivity to disturbance frequency at intervals longer than 80 years.

Figure 1a also shows that the difference in total productivity between the five disturbance types becomes progressively smaller as the disturbance interval increases, suggesting that these lodgepole pine ecosystems are fairly resilient in the face of a disturbance interval of 120 years. This is particularly evident for timber harvesting (SOH and WTH) and lowseverity wildfire disturbance. However, rotation lengths of longer than 120 years may not be suitable from a timber management perspective because (1): they lead to little gain of productivity within a rotation, and a decline of total productivity over the 240-year simulation period; and (2) they increase problems with mistletoe. Therefore, we conclude that 120 years would be the upper limit for rotation length in terms of maximization of site productivity for the medium quality site.

The trend of site productivity over multiple consecutive rotations is a useful indication of sustainability. Figure 2 shows that with a harvest (WTH, SOH) or low-severity wildfire interval of 80 years or longer, site productivity is sustainable over a 240-year simulation. In contrast, site productivity at 40-year frequency is only sustainable with SOH; the other four scenarios were not shown to be sustainable (Figure 1a and Table 6). Therefore, 80 years appears to be the lower limit of sustainable rotation lengths of the three examined for the management system that we simulated, and 80 to 120 years would probably be the range of suitable rotation lengths for medium quality sites in the study area. Simulations at intermediate rotation lengths would be needed to define sustainable rotation length more accurately.

Lodgepole pine forests in the study area are thought to have been recycled for thousands of years under natural wildfire return intervals of about 100-125 years. This is similar to the disturbance interval that was estimated by this simulation to be sustainable, and suggests that the study of natural disturbance regimes can be helpful in designing sustainable management strategies. However, although the average wildfire return interval is 100 –125 years in the study sites, its variability is very high, ranging from 40 to 200 years (Pojar, 1985). This is much different from human-caused disturbance such as timber harvesting which tends to apply roughly equal harvest frequencies in a specific type of forest. The variability in frequency of natural disturbance may be important for the maintenance of certain ecosystem values because it affects the dynamics of WD loading and stand structures. Our study has demonstrated that both above-ground and below-ground WD plays an important role in the nitrogen economy in these lodgepole pine forests. The implications of this natural disturbance variability for other ecological attributes, such as wildlife habitat, remain unknown and are beyond the scope of this study, as are the implications of imposing a more uniform disturbance frequency.

Sustainable Forest Management in a Disturbance

**c**

Mg/ha

20

80

140

200

260

320

380

100

500

900

1300

Kg/ha

Context: A Case Study of Canadian Sub-Boreal Forests 131

**d: total site N at the end (year 240) of simulations** 

**d: total floor mass at the end (year 240) of simulations** 

Disturbance interval (years)

0 40 80 120

Disturbance interval (years)

0 40 80 120

Fig. 1. (a-d). Four simulation output indicators (total productivity, total nitrogen removal, available soil nitrogen and forest floor mass) under five disturbance scenarios on a site of

medium quality over a period of 240-year simulation (from Wei et al. 2003)

Disturbance interval (years)

Fire-L Fire-M Fire-H SOH WTH

Disturbance interval (years)

**b: total nutrient removed or burned over 240 years** 

Disturbance interval (years)

0 40 80 120

0 40 80 120

Mg/ha

Kg/ha

200

600

1000

1400

500

1000

1500

2000

2500

**a: total productivity over 240 years** 

Fig. 1. (a-d). Four simulation output indicators (total productivity, total nitrogen removal, available soil nitrogen and forest floor mass) under five disturbance scenarios on a site of medium quality over a period of 240-year simulation (from Wei et al. 2003)

Sustainable Forest Management in a Disturbance

**4.2.2 Impacts of disturbance severities** 

severity wildfire simulations (Figure 1b).

Disturbance Interval (years)

Context: A Case Study of Canadian Sub-Boreal Forests 133

40 1157 847 709 1457 1275 80 1745 1525 1349 1887 1801 120 1942 1843 1734 2034 2002

Figures 1a-d show that medium and high-severity wildfire had the largest impact on most of the simulated indicators. Total productivity (Figure 1a) and total soil nitrogen (Figure 1c) are much less for the moderate and severe fire simulations than for the harvested or lowseverity wildfire simulations. This reflects the greater loss of N in the medium and high-

The simulations suggest that timber harvesting (SOH, WTH) is a relatively nutrient conservative disturbance compared to wildfire. All simulated indicators for the harvesting treatments are within the range of the wildfire treatment (Figures 1a-d), and close to those for the low-severity wildfire treatment. These simulation results support one of the conclusions from our field investigation of the differences between harvested and fire-killed stands in the study area: the nutrient removals caused by harvesting were within the

The difference in all indicators between SOH and WTH treatments declined as the rotation lengths increased (Figures 1a-d). There is only a minor difference in total productivity over the 240-year simulation at a rotation of 120 years. This suggests that both WTH and SOH are acceptable harvesting methods for the maintenance of long-term site productivity in these lodgepole pine forests if a rotation of 120 years is used. However, the simulations suggest that WTH could reduce productivity by up to 20% compared with SOH if the rotation length was as short as 40 years. SOH is a more nutrient conservative harvest method because it leaves more of the relatively nutrient-rich crown materials on the ground, and should be used instead of WTH for rotations less than 80 years. Our results from both simulation (Figure 1d) and field studies demonstrate that the total mass of decomposing organic matter on wildfire-killed sites, particularly for long-interval, lower-severity wildfires, would be much higher than on harvested sites. This reflects the larger accumulation of aboveground CWD on the wildfire-killed sites, and slower decomposition of this material on fire sites than on harvested sites because much of it is suspended above the ground on branch-stubs on the burned sites. The WD left on the harvested sites is smaller in diameter and in closer contact with the ground, resulting in faster decomposition and, therefore, lower persistence. The lower level of decomposing litter on high-severity wildfire sites (Figure 1d) is attributed to much larger loss of forest floor and crown materials compared with lower severity fire. Because of these differences, harvesting conserves nutrients more than wildfire does at time of disturbance. However, because decomposing

Table 6. Difference in total tree productivity (Mg/ha) between disturbance severity scenarios and between disturbance interval scenarios over a period of 240-year simulation

(from Wei et al. 2003) (Note: Abbreviations are given in Table 1)

estimated range of nutrient removals caused by wildfire.

Disturbance Severity

Fire-L Fire-M Fire-H SOH WTH

Fig. 2. Dynamics of site productivity under fiver disturbance scenarios on a site of medium quality over a simulation period of 240 years (from Wei et al. 2003)

In some forest types, and depending on how it is done, clear-cut harvesting reverts the ecosystem to an earlier stage of the sere (defined as the sequence of plant and animal communities which successively occupy a site over a period of time). However, in some forests, or with some techniques, clear-cutting may simply recycle the existing seral stage, promptly replacing the mature trees with young trees of the same species with little or no change in the understory. In other forests, clear-cutting in the absence of fire and soil disturbance may accelerate succession by facilitating earlier development of the subsequent seral stage. This generally involves the release of shade-tolerant seedling of the next seral stage. Clear-cutting in pure lodgepole pine forests in the study area generally tend to recycle the existing seral stage.

Fig. 2. Dynamics of site productivity under fiver disturbance scenarios on a site of medium

In some forest types, and depending on how it is done, clear-cut harvesting reverts the ecosystem to an earlier stage of the sere (defined as the sequence of plant and animal communities which successively occupy a site over a period of time). However, in some forests, or with some techniques, clear-cutting may simply recycle the existing seral stage, promptly replacing the mature trees with young trees of the same species with little or no change in the understory. In other forests, clear-cutting in the absence of fire and soil disturbance may accelerate succession by facilitating earlier development of the subsequent seral stage. This generally involves the release of shade-tolerant seedling of the next seral stage. Clear-cutting in pure lodgepole pine forests in the study area generally tend to recycle

quality over a simulation period of 240 years (from Wei et al. 2003)

the existing seral stage.


Table 6. Difference in total tree productivity (Mg/ha) between disturbance severity scenarios and between disturbance interval scenarios over a period of 240-year simulation (from Wei et al. 2003) (Note: Abbreviations are given in Table 1)
