**4.1.3 Nutrient losses through harvest or wildfire**

The differences in nutrient removal between the three disturbances are presented in Table 4. The nutrient removals associated with timber harvesting generally were within the range of estimated wildfire removals; the P removal by WTH was equal to the largest value estimated for wildfire losses. WTH removed more N and P nutrients (about 2-fold) than SOH harvesting did, confirming that SOH is the more nutrient-conserving of the two harvesting methods.

Wildfire removes nutrient-rich crown material and forest floor, but leaves most large woody material in the ecosystem. In contrast, timber harvesting removes most large woody material, but leaves the nutrient-rich forest floor and part of the crown materials, depending on harvesting technique (e.g., SOH vs. WTH). Our estimates on N loss by wildfire, based on consumption of woody and litter materials, are reasonable due to volatilization and particulate convection. However, P loss by wildfire may be overestimated because P in

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

The single-exponential decay coefficient (k) is an indicator of the rate of the decay process (Swift et al., 1979). The higher the k value, the faster CWD decays. Table 3 showed that, when CWD is in early or medium decay stages (data limited to about <30 years of decay since disturbance in this study), k values associated with above-ground CWD on the harvested sites were significantly higher (*p* < 0.001) than those on the fire-killed sites. These differences were attributed to the degree of contact between CWD and ground. CWD after harvesting generally has full contact with the ground, while CWD after wildfire disturbance experiences a long period of time before fully reaching the ground (from snag to partial suspended CWD and finally to fully ground-contact CWD). Decomposition is accelerated when logs are in contact with the ground, probably as a result of higher moisture content

Decay class Years of decay\* Treatment Sample size (n)

Table 3. Single-exponential decay coefficients (k) for 10-20 cm diameter above-ground coarse woody debris (CWD) of different decay classes on both harvested and fire-killed sites (Wei et al. 1997) (Note: Means with the same letter within a row are not significantly different (*p* > 0.05) from each other (t-tests); standard error of the mean is shown in parentheses; \*data on various years of wood decay was grouped into two time classes as shown in the table)

The differences in nutrient removal between the three disturbances are presented in Table 4. The nutrient removals associated with timber harvesting generally were within the range of estimated wildfire removals; the P removal by WTH was equal to the largest value estimated for wildfire losses. WTH removed more N and P nutrients (about 2-fold) than SOH harvesting did, confirming that SOH is the more nutrient-conserving of the two

Wildfire removes nutrient-rich crown material and forest floor, but leaves most large woody material in the ecosystem. In contrast, timber harvesting removes most large woody material, but leaves the nutrient-rich forest floor and part of the crown materials, depending on harvesting technique (e.g., SOH vs. WTH). Our estimates on N loss by wildfire, based on consumption of woody and litter materials, are reasonable due to volatilization and particulate convection. However, P loss by wildfire may be overestimated because P in

Early 5 - 10 0.021 (0.004)a 0.004 (0.001)b 16 Medium 20 - 30 0.018 (0.005)a 0.009 (0.002)b 12

Harvested Fire-killed

ecosystems.

**4.1.2 Above-ground CWD decay** 

and increased interaction with the soil fauna and microflora.

**4.1.3 Nutrient losses through harvest or wildfire** 

harvesting methods.

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.


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 fire severities: see text for explanation)
