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Impacts of forest management on northern forest soils
Forest Ecology and Management 133 (2000) 37±42
Impacts of forest management on northern forest soils T.M. Ballard* Faculties of Agricultural Sciences and Forestry, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z4, Canada Accepted 6 October 1999
Abstract Many effects of forest management on northern soil environments are characteristic of other latitudes, as well. Nutrient removals in harvested timber are substantial, and on some sites this may in¯uence not only the amount but also the balance of remaining plant-available nutrients in the long term. Canopy removal during harvesting in¯uences soil temperature and moisture regimes. Physical effects of ground-based skidding may include soil structural change, in¯uencing water retention and ¯ow, and reducing aeration and root penetration. Higher soil temperatures in the daytime and during the growing season tend to result from the forest ¯oor displacement and other disturbances which may result from harvesting and site preparation activities. Impairment of soil gas exchange, due to management activities, can result in increased leaching of nutrient cations where soil pH is not very low, as a consequence of carbonic acid formation. Impaired gas exchange also results in anoxic microenvironments and may result in denitri®cation and the reduction of manganese, iron and sulfate. Prescribed ®re results in substantial nutrient losses through volatilization (notably of N and S) and, in some cases, ¯y-ash losses. Slashburning yields base oxides in the ash. Hydrolysis of these oxides results in increased soil pH and both, the magnitude and the duration of the pH change are in¯uenced by soil-buffering capacity. Many of the remaining ash nutrients are soluble, plant-available, and highly susceptible to leaching. However, increased pH and sorption after burning may limit availability of micronutrient metals and phosphorus in the soil. Hydrologic behavior can be in¯uenced by ®re effects on soil hydrophobicity. Urea fertilizer use can increase soil pH in the short run and lead to increased leaching of metals and biocides associated with dispersible organic colloids. In the longer run, the soil acidi®cation resulting from nitri®cation of fertilizer N can result in leaching of some heavy metal cations. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Management impacts; Soils
1. Introduction Management impacts on forest soils have many similarities across different climatic zones. Consequently, this paper will mention several impacts which are not unique to the north. However, northern environments also offer variations on the general themes because of such factors as soil frost, snow cover, low soil temperatures, and in many situations, thick forest * Tel.: 1-604-822-2300; fax: 1-604-822-8639. E-mail address: [email protected] (T.M. Ballard)
¯oors. These are of particular interest because low soil temperature is often a growth-limiting factor for vegetation and soil organisms at high latitudes. This paper is intended as a brief overview of management impacts, rather than an exhaustive review. 2. Impacts of timber harvesting and mechanical site preparation Nutrient removals in harvested timber are substantial, and can be especially high where whole-tree
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 2 9 6 - 0
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T.M. Ballard / Forest Ecology and Management 133 (2000) 37±42
harvesting is practiced. In a 65-year-old upland black spruce stand (107 dry t/ha) studied by Weetman and Webber (1972), N, P, K, Ca, and Mg in bolewood and bark amounted to 43, 12, 25, 98 and 8 kg/ha, respectively. Whole-tree amounts of these nutrients in the stand were 167, 42, 84, 277 and 27 kg/ha, respectively. In a 65-year-old jack pine stand, Foster and Morrison (1976) found N, P, K, Ca and Mg amounting to 185, 14, 93, 132, and 20 kg/ha, on a whole-tree basis. Principally because of between-stand biomass variability, such magnitudes vary among different sites. However, these data are somewhat suggestive of magnitudes and proportions of macronutrient removals by conifer logging. Widespread nitrogen de®ciency in northern forests lends plausibility to the notion that nitrogen removals by cropping should be of greatest concern. However, when nutrient reserves and replacement mechanisms are considered, long-term implications for other nutrients often seem more important. For example, potassium losses by cropping can sometimes be about twice as high for whole-tree logging as for conventional (bolewood and bark) logging, and the exchangeable potassium reserves in low-potassium soils sometimes amount to less than the aboveground potassium in a mature stand. Consequently, as a new stand grows, available potassium reserves may become depleted unless mineral weathering is rapid (Goulding and Stevens, 1988). Particularly problematic are those soils in which potassium-bearing minerals are neither abundant nor susceptible to rapid weathering, and coarse-textured soils, which offer little speci®c surface at which weathering release of potassium can occur. Forest canopy removal by timber harvesting results in greater soil-temperature amplitudes, partly due to changes in the radiation balance at the soil surface, although reduced transpiration losses and other factors may also affect partitioning of energy. Differences in canopy development during early succession after logging may result in large differences in soil thermal behaviour, as demonstrated in the study of Hogg and Lieffers (1991) in northern Alberta, where Calamagrostis canadensis had a much greater soil-insulating effect than did Epilobium. Clearcut edges in¯uence wind and shade, affecting snow accumulation, melting, and early-season soil temperature and moisture regimes.
Ground-based skidding may result in soil compaction and other soil structural changes, in¯uencing soil water retention, and reducing soil aeration, drainage, and root penetration. Moist, ®ne-textured soils are particularly susceptible, whereas frozen soils tend to be quite resistant to structural degradation induced by traf®c. Reductions in tree height, diameter and volume growth are often observed where soils have been affected by skidding activity (Froehlich et al., 1986; Wert and Thomas, 1981). The greatest changes in soil bulk density are associated with the ®rst trips over the ground, and even quite low ground pressures (e.g. 35± 65 kPa) can result in substantial compaction (Froehlich, 1978). Even where additional trips do not result in signi®cant increases in bulk density, changes in pore size distribution may continue to occur, with large pores collapsing to form smaller ones (Lenhard, 1986). This loss of large pores profoundly affects water retention, aeration and drainage, with implications for the balance of aerobic and anaerobic populations and processes. Work by Hassink et al. (1993) in grassland soils found a strong correlation of bacterial biomass with soil pore volume in the 0.2±1.2 mm range, and of nematode biomass with pore volume in the 30±90 mm range, while fungal and protozoan biomass showed no discernible relationship with pore size classes. Such observations raise questions about possible effects of pore size changes on populations of forest soil biota. Reduction of porosity and particularly of macroporosity results in a substantial reduction of saturated hydraulic conductivity, which limits soil in®ltration capacity. Although many undisturbed forest soils have saturated hydraulic conductivity exceeding normal peak rainfall intensities, the compaction associated with harvesting traf®c often results in localized surface runoff, which may be channeled by wheel ruts and gouges, causing some loss of erodible surface materials. Haul roads and ditches also divert water. Although hard evidence is limited, ®eld observations sometimes suggest that signi®cant local changes in site water regime can be caused by such diversion. Mechanical site preparation, like agricultural cultivation, may have several bene®ts, but can also potentially cause problems, as has been recognized in the British Columbia Forest Practices Code. Soil structural degradation sometimes occurs where such site preparation is carried out in wet soils. Also, some
T.M. Ballard / Forest Ecology and Management 133 (2000) 37±42
methods result in scalping: localized forest displacement. Forest ¯oor displacement can also occur during harvesting, although cable yarding systems, snow cover, and other factors may greatly limit disturbance of forest ¯oors. Exposure of mineral soil to erosive rainfall impact is only one of several possible impacts of forest ¯oor displacement Localized nutrient removals can be signi®cant. In the case of small scalps, a signi®cant fraction of the displaced nutrients may remain within reach of a tree seedling's extending root system. In the southern interior of British Columbia, Hickling (1997) observed that this often occurred in skidder-logged areas that had been replanted. However, large scalps, which may result from blading, raking, and piling for burning, can deplete nutrients over a large contiguous area. In a study in northern B.C., the forest ¯oor displacement associated with piling for burning was estimated to have removed about 500 kg N/ha over the affected area. Alder invasion on part of the area restored that much nitrogen over a period of just eight years, resulting in faster growth of spruce seedlings near alders. Elsewhere in the treated area, nitrogen depletion remained serious as a tree growth-limiting factor (Ballard and Hawkes, 1989). Soil biota may be strongly in¯uenced by forest ¯oor displacement too, by virtue of food source removal and other habitat changes. Scalping of forest ¯oors during harvesting or site preparation can affect soil thermal behaviour. In comparison with organic layers, mineral soil tends to have higher thermal conductivity, heat capacity, thermal admittance and thermal diffusivity (Ballard et al., 1977). As a consequence, a bare mineral soil surface tends to undergo less extreme ¯uctuations of temperature than an organic surface. However, the bare mineral soil tends to undergo larger and more rapid temperature responses in response to surface temperature ¯uctuations. Because low temperature is often an important limitation of soil biological activity at high latitudes, soil scalping during harvesting or site preparation is often of great signi®cance for biological activity. Of course, as mentioned earlier, the removal of a canopy, including an understory canopy, during harvesting, also signi®cantly in¯uences soil temperatures by changing the radiation balance. Not only is more shortwave radiation received at the soil surface during daytime, but cooling at night tends to be greater
39
as well, due to reduced long-wave radiation downward in the absence of a canopy. Changes in temperature regime are likely to be signi®cant in contributing to increased biological activity, following timber harvest. Implications include increased rates of organic matter decomposition and, in some but not all cases, increased net mineralization of nitrogen (Bonan and Van Cleve, 1992; Pare and Van Cleve, 1993). Increased leaching loss of nutrients from soil may be observed after logging (Cole and Gessel, 1965); increased net mineralization by soil organisms as well as reduced uptake by plants is presumably an important factor in this. Effects of temperature change on decomposition activity vary widely, with the Q10 ranging from the vicinity of ca. 1.8 in some studies (Winkler et al., 1996) to as much as 6.0 in a harvested forest wetland (Trettin et al., 1996). Some of the soil physical impacts of these management activities can in¯uence soil chemical properties, through a chain of indirect effects. It has been noted that various management activities may affect soil aeration. For example, pore size reduction and other changes conducive to increased soil water retention impede gas exchange. This is because the diffusivity of gas in water is only ca. 0.0001 as large as in air. Reduction of soil porosity by compaction also impedes gas exchange. And local changes in snowpack thickness and density, associated with management activities, may impede gas exchange. Where gas transfer is limited, metabolism of roots and soil microbes results in an elevated carbon dioxide concentration. As the gas phase equilibrates with the liquid phase, this results in an increased dissolved CO2 concentration, which, due to reaction with H2O, elevates the carbonic acid concentration. If the pH is not too low, a large fraction of the carbonic acid dissociates, yielding Hions and bicarbonate ions. Because of mass action, the increased hydrogen ion activity results in desorption of exchangeable cations, e.g. Ca2, Mg2, and K, into the soil solution. As water percolates down through the soil, these nutrient cations may be carried along by mass ¯ow, with the mobile bicarbonate ions providing charge balance, so that leaching is not prevented by electrical forces. This mechanism was described by McColl and Cole (1968). While carbon dioxide is accumulating, oxygen is being depleted. The ®rst microsites to become anoxic
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T.M. Ballard / Forest Ecology and Management 133 (2000) 37±42
are likely to be the centres of large aggregates and deeper zones in the soil. When oxygen is depleted, the oxidation±reduction potential falls. In the course of anaerobic respiration by various facultatively and/or obligatory anaerobic heterotrophs, various mineral nutrient forms may serve as terminal electron acceptors, dramatically affecting soil nutrient status. Although denitri®cation, reduction of manganese and iron, and sulfate reduction might be expected to occur in sequence as the oxidation-reduction potential falls (Bohn et al., 1985), spatial variability of potentials may cause these processes to be concurrent within a small soil volume. 3. Impacts of prescribed burning Removal of vegetation and slash cover, forest ¯oor reduction, and albedo reduction by burning modify the temperature regime. Although plant nutrient status, inferred from foliar analysis, is sometimes poorer on burned than on unburned sites, tree seedling growth is often better on burned sites in northern environments, presumably because of higher growing-season temperatures (Ballard and Hawkes, 1989). Hydrophobicity sometimes results from prescribed burning. An example of this in coastal British Columbia was described by Henderson and Golding (1983). Pyrolysis can contribute to the soil's content of nonpolar organic substances. Volatilization of hydrophobic substances in the heating zone, their diffusion through soil along a concentration gradient, and condensation on soil-particle surfaces in a cooler, subsurface zone can induce hydrophobicity in a near-surface mineral soil layer. Coarse-textured soils are especially susceptible to development of extreme hydrophobicity, because of their low speci®c surface, i.e. very little condensation of lipid material is suf®cient to coat virtually all of the particle surfaces (DeBano and Letey, 1969). Hydrophobicity can dramatically alter soil-water regime on a small scale, and when widespread and severe, can induce overland ¯ow and erosion over large watershed areas. Other physical effects include a tendency for soil pore sizes to be reduced where repeated burning has occurred (Arend, 1941; Moehring et al., 1966; Wells et al., 1979). Slashburning results in some nutrient losses from the site. Estimates of nitrogen volatilization loss dur-
ing combustion of forest ¯oor and other fuels range from ca. 50 to nearly 100% of the N content (DeBell and Ralston, 1970; Feller, 1982, 1983; Little and Ohlmann, 1988). Estimates of S loss by volatilization range from ca. 20 to 90% of the S contained in the fuel, with higher temperatures and more prolonged burns tending to result in greater losses (Allen, 1964; Sanborn and Ballard, 1991; Tiedemann, 1987). Fly-ash losses of macronutrients, such as Ca, Mg, K and P, may be signi®cant (Feller, 1983). Soil pH tends to increase after a burn, due to hydrolysis of the base cation oxides which are abundant in ash. The ®rst wetting fronts after a ®re are of extremely high pH (Grier and Cole, 1971). This high pH is presumably a factor in the prevalence of Fe and Cu de®ciencies on some northern sites where burning has occurred (Ballard and Hawkes, 1989; Majid and Ballard, 1990). However, reduced production of siderophores by soil microbes may also be implicated in some cases of post-®re iron de®ciency (Perry et al., 1984). The magnitude and duration of pH rise may be quite large for poorly buffered soils, but the response curve tends to be short and broad for soils rich in clay and/or organic matter. Soluble ash constituents may be readily leached or subject to loss in surface runoff. Phosphate is often strongly sorbed by ash or charcoal (Beaton et al., 1960). Elevated total phosphate concentrations in stream water may be observed while dissolved ortho-phosphate concentrations are low (Stednick et al., 1982), suggesting loss of organic phosphates and/or erosion of soil particles to which phosphate is sorbed. In some cases, accumulation of mineral N in soil is greater in burned than unburned soils (Mroz et al., 1980). Changes in temperature regime, pH and other factors affecting microbial activity, and changes in competition for available N may be implicated. Jorgensen and Wells (1971) reported increased nonsymbiotic nitrogen ®xation after burning in an area on the Atlantic Coastal Plain. However, some other studies have failed to ®nd such an effect (Jorgensen, 1975; Vance et al., 1983). Possibilities which might be implicated where the effect was observed were reviewed by Vance et al. (1983). They include increased cation availability and pH, cyanobacterial response to more sunlight reaching the soil surface on burned sites, and favouring of Clostridiumspp. and microaerophilic nitrogen ®xers by increasingly anoxic
T.M. Ballard / Forest Ecology and Management 133 (2000) 37±42
conditions associated with reduced pore sizes after burning. As noted in the review by Feller (1983), ®re can affect populations of many microbes and invertebrates, particularly those reliant on slash and forest ¯oor in terms of habitat. 4. Impacts of fertilizer and biocides Fertilizers are sometimes used in the management of northern forest soils. Nitrogen is the fertilizer element most commonly applied. Although ammonium nitrate, ammonium sulfate and other salts have been used in some cases, urea is very commonly used. Urea application tends to cause a short-term rise in soil pH due to reactions involving the ammonium carbonate produced during urea hydrolysis. Where very large amounts of urea are used, the rise in pH and concentration of the monovalent ammonium ion results in dispersion of humic acids from the forest ¯oor and, in some cases, of clays from near-surface mineral soil, resulting in downward movement and, in some cases, pore clogging by the colloidal material. Along with the colloidal material, adsorbed substances, e.g. biocides (Ballard, 1971) and organically complexed metal cations such as copper, iron, and aluminium (Otchere-Boateng and Ballard, 1981), may be translocated. In many cases where urea or ammonium salts are applied, the longer-term effect is a drop in pH, associated with the release of hydrogen ions in the ®rst stage of nitri®cation. This pH drop may result in accelerated leaching of certain metal cations, such as manganese, zinc, and aluminium (Otchere-Boateng and Ballard, 1981). The change in nutrient status effected by fertilizers may result in accelerated soil microbial activity. A priming effect of nitrogen fertilizer on nitrogen mineralization has sometimes been observed. Enhanced nitrogen status may be bene®cial for many of the soil biota, but high nitrogen concentrations may be toxic for some taxa. Among the various forest management activities in¯uencing forest soil environments, the use of biocides also deserves mention. At one time, DDT was commonly used in some northern forest environments. Sometimes, arthropods, such as springtails and ¯y larvae, became more numerous after such treatments, presumably due to accumulation of the pesticide in their predators higher up the food chain. Many bio-
41
cides tend to be strongly sorbed at soil particle surfaces, and their consequent inaccessibility limits their effect on biota in the soil environment. Herbicides, through their effects on canopy reduction, sometimes have a pronounced effect on soil temperatures (Wood and von Althen, 1993). 5. Conclusions Many commonly employed forest management practices affect northern forest soil environments. Among these are timber harvesting, mechanical site preparation, prescribed burning, fertilizer use and biocide application. Some of their impacts on soil physical and chemical properties and processes have important implications for plant growth and soil biota. References Allen, S.E., 1964. Chemical aspects of heather burning. J. Appl. Ecol. 1, 347±367. Arend, J.L., 1941. Infiltration rates of forest soils in the Missouri Ozarks as affected by woods burning and litter removal. J. For. 39, 726±728. Ballard, T.M., 1971. Role of humic carrier substances in DDT movement through forest soil. Soil Sci. Soc. Am. Proc. 35, 145±147. Ballard, T.M., Black, T.A., McNaughton, K.G., 1977. Summer energy balance and temperatures in a forest clearcut in southwestern British Columbai. In: 6th B.C. Soil Science Workshop Report, B. C. Min. Agr., Victoria. pp. 74±85. Ballard, T.M., Hawkes, B.C., 1989. Effects of burning and mechanical site preparation on growth and nutrition of planted white spruce. Forestry Canada Inf. Report. BC-X-309. Beaton, J.D., Peterson, H.B., Bauer, N., 1960. Some aspects of phosphate adsorption by charcoal. Soil Sci. Soc. Am. Proc. 24, 340±345. Bohn, H.L., McNeal, B.L., O'Connor, G.A., 1985. Soil Chemistry, second ed. Wiley, New York. 341 pp. Bonan, G.B., Van Cleve, K., 1992. Soil temperature, nitrogen mineralization, and carbon source±sink relationships in boreal forests. Can. J. For. Res. 22, 629±639. Cole, D.W., Gessel, S.P., 1965. Movement of nutrients through a forest soil as influenced by tree removal and fertilizer additions. In: C.T. Youngberg (Ed.), Forest±Soil Relationships in North America. Proc. 2nd N. Am. For. Soils Conf., Oregon State University Press, Corvallis, OR. pp. 95±104. DeBano, L.F., Letey, J., 1969. Water repellent soils. Proc. Symp. Water Repellent Soils, University of California, Riverside, CA, 354 pp.
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DeBell, D.S., Ralston, C.W., 1970. Release of nitrogen by burning light forest fuels. Soil Sci. Soc. Am. Proc. 34, 936±938. Feller, M.C., 1982. The ecological effects of slashburning with particular reference to British Columbia: a review. B.C. Min. For. Land Manage. Report No. 13. 60 pp. Feller, M. C., 1983. Impacts of prescribed fire (slashburning) on forest productivity, soil erosion and water quality on the coast. In: Trowbridge, R.L., Macadam, A. (Eds.), Prescribed Fire± Forest Soils Symposium Proceedings. Brit. Col. Min. For. Land Manage. Report 16, pp. 57±91. Foster, N.W., Morrison, I.K., 1976. Distribution and cycling of nutrients in a natural Pinus banksiana ecosystem. Ecology 57, 110±120. Froehlich, H.A., 1978. Soil compaction from low ground-pressure, torsion-suspension logging vehicles on three forest soils. Oregon State Univ. For. Res. Lab. Res. Pap. 36. 13 pp. Froehlich, H.A., Miles, D.W.R., Robbins, R.W., 1986. Soil bulk density recovery on compacted skid trails in central Idaho. Soil Sci. Soc. Am. J. 49, 1015±1017. Goulding, K.W.T., Stevens, P.A., 1988. Potassium reserved in a forested, acid upland soils, acid upland soils and the effect on them of clear-felling versus whole-tree harvesting. Soil Use Manage. 4, 45±51. Grier, C.C., Cole, D.W., 1971. Influence of slashburning on ion transport in a forest soil. Northwest Sci. 45, 100±106. Hassink, J., Bouwman, L.A., Zwart, K.B., Brussaard, L., 1993. Relationship between habitable pore space, soil biota and mineralization rates in grassland soils. Soil Biol. Biochem. 25, 47±55. Henderson, G.S., Golding, D.L., 1983. The effect of slashburning on the water repellency of forest soils at Vancouver, British Columbia. Can. J. For. Res. 13, 353±355. Hickling, J., 1997. Some effects of forest floor displacement on soil properties and lodgepole pine productivity in the Boundary Forest District. Unpubl. M.Sc. thesis, Dept. of Soil Sci., University of British Columbia, Vancouver, 212 pp. Hogg, E.H., Lieffers, V.J., 1991. The impact of Calamagrostis canadensis on soil thermal regimes after logging in northern Alberta. Can. J. For. Res. 21, 387±394. Jorgensen, J.R., 1975. Nitrogen fixation in forested coastal plain soils. USDA For. Serv. Res. Pap. SE-130. Jorgensen, J.R., Wells, C.G., 1971. Apparent nitrogen fixation in soil influenced by prescribed burning. Soil Sci. Soc. Am. Proc. 35, 806±810. Lenhard, R.J., 1986. Changes in void distribution and volume during compaction of a forest soil. Soil Sci. Soc. Am. J. 50, 462±464. Little, S.N., Ohlmann, J.L., 1988. Estimating nitrogen lost from forest floor during prescribed fires in Douglas-fir/western hemlock clearcuts. For. Sci. 34, 152±164. Majid, N.M., Ballard, T.M., 1990. Effects of foliar application of copper sulphate and urea on the growth of lodgepole pine. Forest Ecol. Manage. 37, 151±165.
Moehring, D.M., Grano, C.X., Bassett, J.R., 1966. Properties of forested loess soils after repeated prescribed burns. USDA For. Serv. Res. Note. SO-40. McColl, J.G., Cole, D.W., 1968. A mechanism of cation transport in a forest soil. Northwest Sci. 42, 134±140. Mroz, G.D., Jurgensen, M.F., Harvey, A.E., Larsen, M.J., 1980. Effects of fire on nitrogen in forest floor horizons. Soil Sci. Soc. Am. J. 44, 395±405. Otchere-Boateng, J., Ballard, T.M., 1981. Effect of urea fertilizer on leaching of micronutrient metals and aluminum from forest soil columns. Can. J. For. Res. 11, 763±767. Pare, D., Van Cleve, K., 1993. Soil nutrient availability and relationships with aboveground biomass production on postharvested upland white spruce sites in interior Alaska. Can. J. For Res. 23, 1223±1232. Perry, D.A., Rose, S.L., Pilz, D., Schoenberger, M.M., 1984. Reduction of natural ferric iron chelators in disturbed forest soils. Soil Sci. Soc. Am. J. 48, 379±382. Sanborn, P.T., Ballard, T.M., 1991. Combustion losses of sulfur from conifer foliage: implications of chemical form and soil nitrogen status. Biogeochem. 12, 129±134. Stednick, J.D., Tripp, L.N., McDonald, R.J., 1982. Slashburning effects on soil and water chemistry in southeastern Alaska. J. Soil Water Cons. 37, 126±128. Tiedemann, A.R., 1987. Combustion losses of sulfur from forest foliage and litter. For. Sci. 33, 216±223. Trettin, C.C., Davidian, M., Jurgensen, M.F., Lea, R., 1996. Organic matter decomposition following harvesting and site preparation of a forested wetland. Soil Sci. Soc. Am. J. 60, 1994±2003. Vance, E.D., Henderson, G.S., Blevins, D.G., 1983. Nonsymbiotic nitrogen fixation in an oak-hickory forest following longterm prescribed burning. Soil Sci. Soc. Am. J. 47, 134± 137. Weetman, G.F., Webber, B.D., 1972. The influence of wood harvesting on the nutrient status of two spruce stands. Can. J. For. Res. 2, 351±369. Wells, C.G., Campbell, R.E., DeBano, L.F., Lewis, C.E., Fredriksen, R.L., Franklin, E.C., Froelich, R.C., Dunn, P.H., 1979. Effects of fire on soil A state-of-knowledge review. USDA For. Serv. Gen. Tech. Rep. WO-7. Wert, S., Thomas, B.R., 1981. Effects of skid roads on diameter, height, height and volume growth in Douglas-fir. Soil Sci. Soc. Am. J. 45, 629±632. Winkler, J.P., Cherry, R.S., Schlesinger, W.H., 1996. The Q10 relationship of microbial respiration in a temperate forest soil. Soil Biol. Biochem. 28, 1067±1072. Wood, J.E., von Althen, F.W., 1993. Establishment of white spruce and black spruce in boreal Ontario: effects of chemical site preparation and post-planting weed control. For. Chronol. 69, 554±660.
Impacts of forest management on northern forest soils T.M. Ballard* Faculties of Agricultural Sciences and Forestry, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z4, Canada Accepted 6 October 1999
Abstract Many effects of forest management on northern soil environments are characteristic of other latitudes, as well. Nutrient removals in harvested timber are substantial, and on some sites this may in¯uence not only the amount but also the balance of remaining plant-available nutrients in the long term. Canopy removal during harvesting in¯uences soil temperature and moisture regimes. Physical effects of ground-based skidding may include soil structural change, in¯uencing water retention and ¯ow, and reducing aeration and root penetration. Higher soil temperatures in the daytime and during the growing season tend to result from the forest ¯oor displacement and other disturbances which may result from harvesting and site preparation activities. Impairment of soil gas exchange, due to management activities, can result in increased leaching of nutrient cations where soil pH is not very low, as a consequence of carbonic acid formation. Impaired gas exchange also results in anoxic microenvironments and may result in denitri®cation and the reduction of manganese, iron and sulfate. Prescribed ®re results in substantial nutrient losses through volatilization (notably of N and S) and, in some cases, ¯y-ash losses. Slashburning yields base oxides in the ash. Hydrolysis of these oxides results in increased soil pH and both, the magnitude and the duration of the pH change are in¯uenced by soil-buffering capacity. Many of the remaining ash nutrients are soluble, plant-available, and highly susceptible to leaching. However, increased pH and sorption after burning may limit availability of micronutrient metals and phosphorus in the soil. Hydrologic behavior can be in¯uenced by ®re effects on soil hydrophobicity. Urea fertilizer use can increase soil pH in the short run and lead to increased leaching of metals and biocides associated with dispersible organic colloids. In the longer run, the soil acidi®cation resulting from nitri®cation of fertilizer N can result in leaching of some heavy metal cations. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Management impacts; Soils
1. Introduction Management impacts on forest soils have many similarities across different climatic zones. Consequently, this paper will mention several impacts which are not unique to the north. However, northern environments also offer variations on the general themes because of such factors as soil frost, snow cover, low soil temperatures, and in many situations, thick forest * Tel.: 1-604-822-2300; fax: 1-604-822-8639. E-mail address: [email protected] (T.M. Ballard)
¯oors. These are of particular interest because low soil temperature is often a growth-limiting factor for vegetation and soil organisms at high latitudes. This paper is intended as a brief overview of management impacts, rather than an exhaustive review. 2. Impacts of timber harvesting and mechanical site preparation Nutrient removals in harvested timber are substantial, and can be especially high where whole-tree
0378-1127/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 9 9 ) 0 0 2 9 6 - 0
38
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harvesting is practiced. In a 65-year-old upland black spruce stand (107 dry t/ha) studied by Weetman and Webber (1972), N, P, K, Ca, and Mg in bolewood and bark amounted to 43, 12, 25, 98 and 8 kg/ha, respectively. Whole-tree amounts of these nutrients in the stand were 167, 42, 84, 277 and 27 kg/ha, respectively. In a 65-year-old jack pine stand, Foster and Morrison (1976) found N, P, K, Ca and Mg amounting to 185, 14, 93, 132, and 20 kg/ha, on a whole-tree basis. Principally because of between-stand biomass variability, such magnitudes vary among different sites. However, these data are somewhat suggestive of magnitudes and proportions of macronutrient removals by conifer logging. Widespread nitrogen de®ciency in northern forests lends plausibility to the notion that nitrogen removals by cropping should be of greatest concern. However, when nutrient reserves and replacement mechanisms are considered, long-term implications for other nutrients often seem more important. For example, potassium losses by cropping can sometimes be about twice as high for whole-tree logging as for conventional (bolewood and bark) logging, and the exchangeable potassium reserves in low-potassium soils sometimes amount to less than the aboveground potassium in a mature stand. Consequently, as a new stand grows, available potassium reserves may become depleted unless mineral weathering is rapid (Goulding and Stevens, 1988). Particularly problematic are those soils in which potassium-bearing minerals are neither abundant nor susceptible to rapid weathering, and coarse-textured soils, which offer little speci®c surface at which weathering release of potassium can occur. Forest canopy removal by timber harvesting results in greater soil-temperature amplitudes, partly due to changes in the radiation balance at the soil surface, although reduced transpiration losses and other factors may also affect partitioning of energy. Differences in canopy development during early succession after logging may result in large differences in soil thermal behaviour, as demonstrated in the study of Hogg and Lieffers (1991) in northern Alberta, where Calamagrostis canadensis had a much greater soil-insulating effect than did Epilobium. Clearcut edges in¯uence wind and shade, affecting snow accumulation, melting, and early-season soil temperature and moisture regimes.
Ground-based skidding may result in soil compaction and other soil structural changes, in¯uencing soil water retention, and reducing soil aeration, drainage, and root penetration. Moist, ®ne-textured soils are particularly susceptible, whereas frozen soils tend to be quite resistant to structural degradation induced by traf®c. Reductions in tree height, diameter and volume growth are often observed where soils have been affected by skidding activity (Froehlich et al., 1986; Wert and Thomas, 1981). The greatest changes in soil bulk density are associated with the ®rst trips over the ground, and even quite low ground pressures (e.g. 35± 65 kPa) can result in substantial compaction (Froehlich, 1978). Even where additional trips do not result in signi®cant increases in bulk density, changes in pore size distribution may continue to occur, with large pores collapsing to form smaller ones (Lenhard, 1986). This loss of large pores profoundly affects water retention, aeration and drainage, with implications for the balance of aerobic and anaerobic populations and processes. Work by Hassink et al. (1993) in grassland soils found a strong correlation of bacterial biomass with soil pore volume in the 0.2±1.2 mm range, and of nematode biomass with pore volume in the 30±90 mm range, while fungal and protozoan biomass showed no discernible relationship with pore size classes. Such observations raise questions about possible effects of pore size changes on populations of forest soil biota. Reduction of porosity and particularly of macroporosity results in a substantial reduction of saturated hydraulic conductivity, which limits soil in®ltration capacity. Although many undisturbed forest soils have saturated hydraulic conductivity exceeding normal peak rainfall intensities, the compaction associated with harvesting traf®c often results in localized surface runoff, which may be channeled by wheel ruts and gouges, causing some loss of erodible surface materials. Haul roads and ditches also divert water. Although hard evidence is limited, ®eld observations sometimes suggest that signi®cant local changes in site water regime can be caused by such diversion. Mechanical site preparation, like agricultural cultivation, may have several bene®ts, but can also potentially cause problems, as has been recognized in the British Columbia Forest Practices Code. Soil structural degradation sometimes occurs where such site preparation is carried out in wet soils. Also, some
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methods result in scalping: localized forest displacement. Forest ¯oor displacement can also occur during harvesting, although cable yarding systems, snow cover, and other factors may greatly limit disturbance of forest ¯oors. Exposure of mineral soil to erosive rainfall impact is only one of several possible impacts of forest ¯oor displacement Localized nutrient removals can be signi®cant. In the case of small scalps, a signi®cant fraction of the displaced nutrients may remain within reach of a tree seedling's extending root system. In the southern interior of British Columbia, Hickling (1997) observed that this often occurred in skidder-logged areas that had been replanted. However, large scalps, which may result from blading, raking, and piling for burning, can deplete nutrients over a large contiguous area. In a study in northern B.C., the forest ¯oor displacement associated with piling for burning was estimated to have removed about 500 kg N/ha over the affected area. Alder invasion on part of the area restored that much nitrogen over a period of just eight years, resulting in faster growth of spruce seedlings near alders. Elsewhere in the treated area, nitrogen depletion remained serious as a tree growth-limiting factor (Ballard and Hawkes, 1989). Soil biota may be strongly in¯uenced by forest ¯oor displacement too, by virtue of food source removal and other habitat changes. Scalping of forest ¯oors during harvesting or site preparation can affect soil thermal behaviour. In comparison with organic layers, mineral soil tends to have higher thermal conductivity, heat capacity, thermal admittance and thermal diffusivity (Ballard et al., 1977). As a consequence, a bare mineral soil surface tends to undergo less extreme ¯uctuations of temperature than an organic surface. However, the bare mineral soil tends to undergo larger and more rapid temperature responses in response to surface temperature ¯uctuations. Because low temperature is often an important limitation of soil biological activity at high latitudes, soil scalping during harvesting or site preparation is often of great signi®cance for biological activity. Of course, as mentioned earlier, the removal of a canopy, including an understory canopy, during harvesting, also signi®cantly in¯uences soil temperatures by changing the radiation balance. Not only is more shortwave radiation received at the soil surface during daytime, but cooling at night tends to be greater
39
as well, due to reduced long-wave radiation downward in the absence of a canopy. Changes in temperature regime are likely to be signi®cant in contributing to increased biological activity, following timber harvest. Implications include increased rates of organic matter decomposition and, in some but not all cases, increased net mineralization of nitrogen (Bonan and Van Cleve, 1992; Pare and Van Cleve, 1993). Increased leaching loss of nutrients from soil may be observed after logging (Cole and Gessel, 1965); increased net mineralization by soil organisms as well as reduced uptake by plants is presumably an important factor in this. Effects of temperature change on decomposition activity vary widely, with the Q10 ranging from the vicinity of ca. 1.8 in some studies (Winkler et al., 1996) to as much as 6.0 in a harvested forest wetland (Trettin et al., 1996). Some of the soil physical impacts of these management activities can in¯uence soil chemical properties, through a chain of indirect effects. It has been noted that various management activities may affect soil aeration. For example, pore size reduction and other changes conducive to increased soil water retention impede gas exchange. This is because the diffusivity of gas in water is only ca. 0.0001 as large as in air. Reduction of soil porosity by compaction also impedes gas exchange. And local changes in snowpack thickness and density, associated with management activities, may impede gas exchange. Where gas transfer is limited, metabolism of roots and soil microbes results in an elevated carbon dioxide concentration. As the gas phase equilibrates with the liquid phase, this results in an increased dissolved CO2 concentration, which, due to reaction with H2O, elevates the carbonic acid concentration. If the pH is not too low, a large fraction of the carbonic acid dissociates, yielding Hions and bicarbonate ions. Because of mass action, the increased hydrogen ion activity results in desorption of exchangeable cations, e.g. Ca2, Mg2, and K, into the soil solution. As water percolates down through the soil, these nutrient cations may be carried along by mass ¯ow, with the mobile bicarbonate ions providing charge balance, so that leaching is not prevented by electrical forces. This mechanism was described by McColl and Cole (1968). While carbon dioxide is accumulating, oxygen is being depleted. The ®rst microsites to become anoxic
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are likely to be the centres of large aggregates and deeper zones in the soil. When oxygen is depleted, the oxidation±reduction potential falls. In the course of anaerobic respiration by various facultatively and/or obligatory anaerobic heterotrophs, various mineral nutrient forms may serve as terminal electron acceptors, dramatically affecting soil nutrient status. Although denitri®cation, reduction of manganese and iron, and sulfate reduction might be expected to occur in sequence as the oxidation-reduction potential falls (Bohn et al., 1985), spatial variability of potentials may cause these processes to be concurrent within a small soil volume. 3. Impacts of prescribed burning Removal of vegetation and slash cover, forest ¯oor reduction, and albedo reduction by burning modify the temperature regime. Although plant nutrient status, inferred from foliar analysis, is sometimes poorer on burned than on unburned sites, tree seedling growth is often better on burned sites in northern environments, presumably because of higher growing-season temperatures (Ballard and Hawkes, 1989). Hydrophobicity sometimes results from prescribed burning. An example of this in coastal British Columbia was described by Henderson and Golding (1983). Pyrolysis can contribute to the soil's content of nonpolar organic substances. Volatilization of hydrophobic substances in the heating zone, their diffusion through soil along a concentration gradient, and condensation on soil-particle surfaces in a cooler, subsurface zone can induce hydrophobicity in a near-surface mineral soil layer. Coarse-textured soils are especially susceptible to development of extreme hydrophobicity, because of their low speci®c surface, i.e. very little condensation of lipid material is suf®cient to coat virtually all of the particle surfaces (DeBano and Letey, 1969). Hydrophobicity can dramatically alter soil-water regime on a small scale, and when widespread and severe, can induce overland ¯ow and erosion over large watershed areas. Other physical effects include a tendency for soil pore sizes to be reduced where repeated burning has occurred (Arend, 1941; Moehring et al., 1966; Wells et al., 1979). Slashburning results in some nutrient losses from the site. Estimates of nitrogen volatilization loss dur-
ing combustion of forest ¯oor and other fuels range from ca. 50 to nearly 100% of the N content (DeBell and Ralston, 1970; Feller, 1982, 1983; Little and Ohlmann, 1988). Estimates of S loss by volatilization range from ca. 20 to 90% of the S contained in the fuel, with higher temperatures and more prolonged burns tending to result in greater losses (Allen, 1964; Sanborn and Ballard, 1991; Tiedemann, 1987). Fly-ash losses of macronutrients, such as Ca, Mg, K and P, may be signi®cant (Feller, 1983). Soil pH tends to increase after a burn, due to hydrolysis of the base cation oxides which are abundant in ash. The ®rst wetting fronts after a ®re are of extremely high pH (Grier and Cole, 1971). This high pH is presumably a factor in the prevalence of Fe and Cu de®ciencies on some northern sites where burning has occurred (Ballard and Hawkes, 1989; Majid and Ballard, 1990). However, reduced production of siderophores by soil microbes may also be implicated in some cases of post-®re iron de®ciency (Perry et al., 1984). The magnitude and duration of pH rise may be quite large for poorly buffered soils, but the response curve tends to be short and broad for soils rich in clay and/or organic matter. Soluble ash constituents may be readily leached or subject to loss in surface runoff. Phosphate is often strongly sorbed by ash or charcoal (Beaton et al., 1960). Elevated total phosphate concentrations in stream water may be observed while dissolved ortho-phosphate concentrations are low (Stednick et al., 1982), suggesting loss of organic phosphates and/or erosion of soil particles to which phosphate is sorbed. In some cases, accumulation of mineral N in soil is greater in burned than unburned soils (Mroz et al., 1980). Changes in temperature regime, pH and other factors affecting microbial activity, and changes in competition for available N may be implicated. Jorgensen and Wells (1971) reported increased nonsymbiotic nitrogen ®xation after burning in an area on the Atlantic Coastal Plain. However, some other studies have failed to ®nd such an effect (Jorgensen, 1975; Vance et al., 1983). Possibilities which might be implicated where the effect was observed were reviewed by Vance et al. (1983). They include increased cation availability and pH, cyanobacterial response to more sunlight reaching the soil surface on burned sites, and favouring of Clostridiumspp. and microaerophilic nitrogen ®xers by increasingly anoxic
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conditions associated with reduced pore sizes after burning. As noted in the review by Feller (1983), ®re can affect populations of many microbes and invertebrates, particularly those reliant on slash and forest ¯oor in terms of habitat. 4. Impacts of fertilizer and biocides Fertilizers are sometimes used in the management of northern forest soils. Nitrogen is the fertilizer element most commonly applied. Although ammonium nitrate, ammonium sulfate and other salts have been used in some cases, urea is very commonly used. Urea application tends to cause a short-term rise in soil pH due to reactions involving the ammonium carbonate produced during urea hydrolysis. Where very large amounts of urea are used, the rise in pH and concentration of the monovalent ammonium ion results in dispersion of humic acids from the forest ¯oor and, in some cases, of clays from near-surface mineral soil, resulting in downward movement and, in some cases, pore clogging by the colloidal material. Along with the colloidal material, adsorbed substances, e.g. biocides (Ballard, 1971) and organically complexed metal cations such as copper, iron, and aluminium (Otchere-Boateng and Ballard, 1981), may be translocated. In many cases where urea or ammonium salts are applied, the longer-term effect is a drop in pH, associated with the release of hydrogen ions in the ®rst stage of nitri®cation. This pH drop may result in accelerated leaching of certain metal cations, such as manganese, zinc, and aluminium (Otchere-Boateng and Ballard, 1981). The change in nutrient status effected by fertilizers may result in accelerated soil microbial activity. A priming effect of nitrogen fertilizer on nitrogen mineralization has sometimes been observed. Enhanced nitrogen status may be bene®cial for many of the soil biota, but high nitrogen concentrations may be toxic for some taxa. Among the various forest management activities in¯uencing forest soil environments, the use of biocides also deserves mention. At one time, DDT was commonly used in some northern forest environments. Sometimes, arthropods, such as springtails and ¯y larvae, became more numerous after such treatments, presumably due to accumulation of the pesticide in their predators higher up the food chain. Many bio-
41
cides tend to be strongly sorbed at soil particle surfaces, and their consequent inaccessibility limits their effect on biota in the soil environment. Herbicides, through their effects on canopy reduction, sometimes have a pronounced effect on soil temperatures (Wood and von Althen, 1993). 5. Conclusions Many commonly employed forest management practices affect northern forest soil environments. Among these are timber harvesting, mechanical site preparation, prescribed burning, fertilizer use and biocide application. Some of their impacts on soil physical and chemical properties and processes have important implications for plant growth and soil biota. References Allen, S.E., 1964. Chemical aspects of heather burning. J. Appl. Ecol. 1, 347±367. Arend, J.L., 1941. Infiltration rates of forest soils in the Missouri Ozarks as affected by woods burning and litter removal. J. For. 39, 726±728. Ballard, T.M., 1971. Role of humic carrier substances in DDT movement through forest soil. Soil Sci. Soc. Am. Proc. 35, 145±147. Ballard, T.M., Black, T.A., McNaughton, K.G., 1977. Summer energy balance and temperatures in a forest clearcut in southwestern British Columbai. In: 6th B.C. Soil Science Workshop Report, B. C. Min. Agr., Victoria. pp. 74±85. Ballard, T.M., Hawkes, B.C., 1989. Effects of burning and mechanical site preparation on growth and nutrition of planted white spruce. Forestry Canada Inf. Report. BC-X-309. Beaton, J.D., Peterson, H.B., Bauer, N., 1960. Some aspects of phosphate adsorption by charcoal. Soil Sci. Soc. Am. Proc. 24, 340±345. Bohn, H.L., McNeal, B.L., O'Connor, G.A., 1985. Soil Chemistry, second ed. Wiley, New York. 341 pp. Bonan, G.B., Van Cleve, K., 1992. Soil temperature, nitrogen mineralization, and carbon source±sink relationships in boreal forests. Can. J. For. Res. 22, 629±639. Cole, D.W., Gessel, S.P., 1965. Movement of nutrients through a forest soil as influenced by tree removal and fertilizer additions. In: C.T. Youngberg (Ed.), Forest±Soil Relationships in North America. Proc. 2nd N. Am. For. Soils Conf., Oregon State University Press, Corvallis, OR. pp. 95±104. DeBano, L.F., Letey, J., 1969. Water repellent soils. Proc. Symp. Water Repellent Soils, University of California, Riverside, CA, 354 pp.
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DeBell, D.S., Ralston, C.W., 1970. Release of nitrogen by burning light forest fuels. Soil Sci. Soc. Am. Proc. 34, 936±938. Feller, M.C., 1982. The ecological effects of slashburning with particular reference to British Columbia: a review. B.C. Min. For. Land Manage. Report No. 13. 60 pp. Feller, M. C., 1983. Impacts of prescribed fire (slashburning) on forest productivity, soil erosion and water quality on the coast. In: Trowbridge, R.L., Macadam, A. (Eds.), Prescribed Fire± Forest Soils Symposium Proceedings. Brit. Col. Min. For. Land Manage. Report 16, pp. 57±91. Foster, N.W., Morrison, I.K., 1976. Distribution and cycling of nutrients in a natural Pinus banksiana ecosystem. Ecology 57, 110±120. Froehlich, H.A., 1978. Soil compaction from low ground-pressure, torsion-suspension logging vehicles on three forest soils. Oregon State Univ. For. Res. Lab. Res. Pap. 36. 13 pp. Froehlich, H.A., Miles, D.W.R., Robbins, R.W., 1986. Soil bulk density recovery on compacted skid trails in central Idaho. Soil Sci. Soc. Am. J. 49, 1015±1017. Goulding, K.W.T., Stevens, P.A., 1988. Potassium reserved in a forested, acid upland soils, acid upland soils and the effect on them of clear-felling versus whole-tree harvesting. Soil Use Manage. 4, 45±51. Grier, C.C., Cole, D.W., 1971. Influence of slashburning on ion transport in a forest soil. Northwest Sci. 45, 100±106. Hassink, J., Bouwman, L.A., Zwart, K.B., Brussaard, L., 1993. Relationship between habitable pore space, soil biota and mineralization rates in grassland soils. Soil Biol. Biochem. 25, 47±55. Henderson, G.S., Golding, D.L., 1983. The effect of slashburning on the water repellency of forest soils at Vancouver, British Columbia. Can. J. For. Res. 13, 353±355. Hickling, J., 1997. Some effects of forest floor displacement on soil properties and lodgepole pine productivity in the Boundary Forest District. Unpubl. M.Sc. thesis, Dept. of Soil Sci., University of British Columbia, Vancouver, 212 pp. Hogg, E.H., Lieffers, V.J., 1991. The impact of Calamagrostis canadensis on soil thermal regimes after logging in northern Alberta. Can. J. For. Res. 21, 387±394. Jorgensen, J.R., 1975. Nitrogen fixation in forested coastal plain soils. USDA For. Serv. Res. Pap. SE-130. Jorgensen, J.R., Wells, C.G., 1971. Apparent nitrogen fixation in soil influenced by prescribed burning. Soil Sci. Soc. Am. Proc. 35, 806±810. Lenhard, R.J., 1986. Changes in void distribution and volume during compaction of a forest soil. Soil Sci. Soc. Am. J. 50, 462±464. Little, S.N., Ohlmann, J.L., 1988. Estimating nitrogen lost from forest floor during prescribed fires in Douglas-fir/western hemlock clearcuts. For. Sci. 34, 152±164. Majid, N.M., Ballard, T.M., 1990. Effects of foliar application of copper sulphate and urea on the growth of lodgepole pine. Forest Ecol. Manage. 37, 151±165.
Moehring, D.M., Grano, C.X., Bassett, J.R., 1966. Properties of forested loess soils after repeated prescribed burns. USDA For. Serv. Res. Note. SO-40. McColl, J.G., Cole, D.W., 1968. A mechanism of cation transport in a forest soil. Northwest Sci. 42, 134±140. Mroz, G.D., Jurgensen, M.F., Harvey, A.E., Larsen, M.J., 1980. Effects of fire on nitrogen in forest floor horizons. Soil Sci. Soc. Am. J. 44, 395±405. Otchere-Boateng, J., Ballard, T.M., 1981. Effect of urea fertilizer on leaching of micronutrient metals and aluminum from forest soil columns. Can. J. For. Res. 11, 763±767. Pare, D., Van Cleve, K., 1993. Soil nutrient availability and relationships with aboveground biomass production on postharvested upland white spruce sites in interior Alaska. Can. J. For Res. 23, 1223±1232. Perry, D.A., Rose, S.L., Pilz, D., Schoenberger, M.M., 1984. Reduction of natural ferric iron chelators in disturbed forest soils. Soil Sci. Soc. Am. J. 48, 379±382. Sanborn, P.T., Ballard, T.M., 1991. Combustion losses of sulfur from conifer foliage: implications of chemical form and soil nitrogen status. Biogeochem. 12, 129±134. Stednick, J.D., Tripp, L.N., McDonald, R.J., 1982. Slashburning effects on soil and water chemistry in southeastern Alaska. J. Soil Water Cons. 37, 126±128. Tiedemann, A.R., 1987. Combustion losses of sulfur from forest foliage and litter. For. Sci. 33, 216±223. Trettin, C.C., Davidian, M., Jurgensen, M.F., Lea, R., 1996. Organic matter decomposition following harvesting and site preparation of a forested wetland. Soil Sci. Soc. Am. J. 60, 1994±2003. Vance, E.D., Henderson, G.S., Blevins, D.G., 1983. Nonsymbiotic nitrogen fixation in an oak-hickory forest following longterm prescribed burning. Soil Sci. Soc. Am. J. 47, 134± 137. Weetman, G.F., Webber, B.D., 1972. The influence of wood harvesting on the nutrient status of two spruce stands. Can. J. For. Res. 2, 351±369. Wells, C.G., Campbell, R.E., DeBano, L.F., Lewis, C.E., Fredriksen, R.L., Franklin, E.C., Froelich, R.C., Dunn, P.H., 1979. Effects of fire on soil A state-of-knowledge review. USDA For. Serv. Gen. Tech. Rep. WO-7. Wert, S., Thomas, B.R., 1981. Effects of skid roads on diameter, height, height and volume growth in Douglas-fir. Soil Sci. Soc. Am. J. 45, 629±632. Winkler, J.P., Cherry, R.S., Schlesinger, W.H., 1996. The Q10 relationship of microbial respiration in a temperate forest soil. Soil Biol. Biochem. 28, 1067±1072. Wood, J.E., von Althen, F.W., 1993. Establishment of white spruce and black spruce in boreal Ontario: effects of chemical site preparation and post-planting weed control. For. Chronol. 69, 554±660.