Mechanisms of Negative Impacts of Three Forest Treatments on Nutrient Availability

Many forest management treatments are directly aimed at maintaining or enhancing forest productivity. There may also be secondary effects that detract from this goal. We discuss three case studies in Washington state in which several mechanisms may have led to adverse secondary impacts. In the first study, pulp and paper (PII) sludges were mixed into soil and growth of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], noble fir (Abies procera Render) and western white pine (Pinus monticola Douglas ex D. Don) was monitored. There was a significant negative correlation of height and diameter growth and C/N ratio for Douglas-fir and western white pine (0.05 level). In a second study, effects of 50 yr of red alder (Alnus rubra Bong.) and Douglas-fir growth on soil chemistry and stand productivity were compared. When the 50-yr-old stands were cut and red alder was established by planting into the soil of the former Douglas-fir and red alder forests, lowered available P in the soil of the previous red alder stand was observed. In a third study, high rates of low C/N ratio organic matter (300 Mg ha") were added in municipal biosolids (N amount about 8000 kg ha~') to Douglas-fir and grand fir [Abies grandis (Dougl.) Forbes] plantations. Excess organic N in the biosolids apparently mineralized, nitrified, and contributed to soil acidification and accelerated cation leaching. Severe Mg deficiency (0.25 g kg" in biosolids-treated vs. 0.93 g kg" in untreated area) might be the cause of observed foliar chlorosis and poor growth rates. H COTTA noted as early as 1817 that the study of forest science and efforts to increase forest productivity in Germany were driven by a dwindling supply of wood (Anonymous, 1992). Today, maintaining high and diverse productivity in forests not managed for traditional wood products is also increasingly important. Considering the variety of "products" that forests can provide, defining forest productivity itself is not simple (Grier et al., 1989). For instance, Smith et al. (1994) found that N fertilization increased total aboveground biomass in a 5-yr-old radiata pine (Pinus radiata D. Don) stand by 48 %, but stem biomass (the most merchantable portion) increased by only 28%. Much of the biomass was branches, which increased by 94% with fertilization. Thus, what is defined as a suitable "response" to forest treatment can depend on the viewpoint of the forest manager. Practices to increase forest productivity often include additions of limiting nutrients. Others advocate avoiding potential negative effects, such as not causing soil compaction or minimizing the removal of organic matter. Few treatments impact only one aspect of forest productivity. One mechanism for adverse effects from forest fertilization is nutrient imbalance (Davey, 1968). For instance, Teng and Timmer (1995) found P deficiency R.B. Harrison, S.P. Gessel, D. Zabowski, C.L. Henry, D. Xue, D.W. Cole, College of Forest Resources, AR-10, University of Washington, Seattle, WA 98195; and I.E. Compton, Harvard Forest, P.O. Box 68, Petersham, MA 01366. Received 1 Feb. 1995. *Corresponding author (RobH @ u. Washington. edu). Published in Soil Sci. Soc. Am. J. 60:1622-1628 (1996). in white spruce [Picea glauca (Moench) Voss] was initiated by N fertilization only. Jokela et al. (1991) found subacute Mn deficiency in slash pine (Pinus elliottii Engl. ssp. elliottii) in Florida and hypothesized that intensive culture and macronutrient fertilization might increase Mn deficiency where Mn is not applied. Fertilizer nutrients could also interact with native soil elements, potentially either increasing or decreasing availability. Burton et al. (1990) found that heavy applications of sewage biosolids induced nitrification and NOf and cation leaching. Van Miegroet and Cole (1985) found acidification and leaching of nutrient cations associated with N2 fixation in a red alder stand. Cole (1979) reported significant losses of K, Ca, and Mg after fertilization with urea without any observation of NOf leaching, apparently due to NH/ replacement of exchangeable cations on the soil CEC. Nutrient additions can also affect competing biota. Fertilizing with N alone can increase the susceptibility of a forest stand to insect or disease attack (Ballard, 1979). Treatments designed to reduce competing biota and prepare a suitable bed for seed germination or planting can compact soil and destroy soil structure, and remove or mix soil horizons (Swanston, 1984; Clay ton, 1981). In New Zealand, Dyck et al. (1989) found 4.5to 17-yr-old radiata pine volumes were lower between windrows where logging slash had been scraped than on windrows and heaps; they also observed overall per-area volume reductions for windrowed compared with unwindrowed areas. In the state of Washington, a colder climate than that of the study of Dyck et al. (1989), Lopushinsky et al. (1992) observed that retention of logging slash resulted in reduced soil temperatures compared with broadcast burning, piling and burning, and slash removal. They attributed lower growth rates and higher mortality in planted Douglas-fir and lodgepole pine (Pinus contorta Douglas ex Louden) to these lower soil temperatures and to increased predation on seedlings by small mammals because of added cover. Fire can be a useful management tool for preparing a suitable planting site and also for vegetation control, and can possibly stimulate N mineralization and availability for planted seedlings (Ellis and Graley, 1983). However, fire can also directly volatilize nutrients and possibly result in additional losses through leaching (Grier, 1969; Boyer, 1987; Raison et al., 1990). Categorizing various management practices according to their effect on productivity is virtually impossible (Burger and Powers, 1991; Grier et al., 1989), and is outside the scope of this study. Instead, we took three case studies from the state of Washington where treatments designed to increase forest productivity by addition of nutrients instead resulted in reduced growth rates. Our primary objective was to elucidate potential mechanisms, Abbreviations: P&P, paper and pulp; CEC, cation-exchange capacity.