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A Guide to Soil Quality Monitoring for Long Term Ecosystem Sustainability on Northern Region National Forests
CONNECTING SOILS WITH FOREST PRODUCTIVITY
ABSTRACT
The productivity of Rocky Mountain forests is lower than forests in most other regions due to shorter growing seasons and low precipitation. Nutrient availability also appears to limit most forests in the region. Although operational fertilization remains uncommon in the region, standard forest management practices have large impacts on soils that can increase or decrease nutrient availability. Sustained productivity cannot be assured without improving our understanding of the effects of management on soils.
INTRODUCTION
We all know that soils are a crucial factor in the productivity of forests, and that forest management practices alter soil properties that in turn alter forest productivity. Extreme cases are perfectly clear; fast-growing plantations of Eucalyptus in Brazil cannot be sustained without frequent fertilization. What about the Rocky Mountain region, where short growing seasons, cool temperatures, and dry soils conspire to limit growth? Surprisingly little work has focused on the connections between soils and forest productivity in the Rockies; available evidence demonstrates strong, reciprocal relationships between forests and soils.
Connections between soil nutrient availability and forest growth have been well established in the Pacific Northwest and Southeastern United States, where each year thousands of hectares are fertilized with N and P to compensate for limiting soil supplies (Allen 1987; Binkley 1986). To what extent does the availability of soil nutrients limit growth in the Rocky Mountain region? Only a handful of studies have examined nutrient limitations in this region, but all have found that nitrogen availability limits growth on at least some of the sites examined (table 1). Conclusive evidence for limitation by other elements is less common, but indications of responses to fertilization with P and K have been reported.
Low rates of forest productivity limit the investments that are warranted in forest management; profitable opportunities for fertilization may be limited in this region to late-rotation periods. Given limited interest in intensive management and fertilization, how much interest is warranted in soil nutrient availability? Sustained-yield forestry probably cannot be achieved in the Rockies without careful management of stand nutrition. Operations such as harvesting and burning remove nutrients and alter the rates at which nutrients recycle. If current rates of forest production are nutrient limited, such changes are likely to change future productivity.
IMPLICATIONS OF SIMPLE CALCULATIONS
Some implications become clear from simple calculations. For example, a wide range of studies have shown that the amount of nitrogen lost in fires is directly proportional to the amount of organic matter consumed; about 5 kg of N are lost for every 1,000 kg of biomass burned (Binkley and Christensen 1991). A typical slash fire might consume about 50,000 kg/ha of fuel, reducing the capital of nitrogen by 250 kg/ha. How important is this amount of nitrogen? Two comparisons are enlightening. A stem-only harvest of a typical Rocky Mountain forest would remove about 60 to 150 kg/ha of nitrogen (Clayton and Kennedy 1985; Stark 1982), so a typical slash fire could remove as much nitrogen as the biomass harvested in two or more rotations. The N content of precipitation in the region is about 2 to 4 kg N/ha annually, and free living N fixation (see Jurgensen, this proceedings) probably adds another kilogram or so. At a rate of 3 to 5 kg/ha of inputs annually, a loss of 250 kg-N/ha equals 50 to 80 years of inputs. This might lead a forester to figure that given a 80- to 100-year rotation, the effects of slash burning on N loss are quite acceptable. The forester might be right if sustained production were the only goal. However, the forester might take less comfort in that conclusion if she realized the site was N limited, and that the fire prescription prevented a natural increase in productivity that might have accompanied a 250-kg/ha increase in soil N over that period. The fertility of soils is malleable, and the productive capacity of a site at any particular time is likely increasing due to nutrient accumulation, or decreasing due to recent effects of fire or harvest.
Not all forest management operations reduce nutrient availability. In many cases, forest harvest (and even burning) increases the availability of nutrients at a time when the demand for nutrients to expand the canopies of regenerating trees is great. Again, little quantitative information is available for the Rockies, but what little we know is consistent with results from other regions (see Page-Dumroese, this proceedings).
Nutrient availability may be even more fundamental to forest management in the Rockies than these examples would suggest. Yield tables for our region typically show maximum periodic annual increment near age 40 to 80, followed by substantial declines in later decades. This observation of declining growth in relatively young (<100-yr) stands is so common that we typically don't bother to ask why it occurs. Recent work in central Colorado (M. G. Ryan, personal communication) and in southeastern Wyoming (F. W. Smith, personal communication) shows that leaf area of lodgepole pine forests declines substantially after about age 50 to 70, coincident with declines in stem growth. Fertilization trials have shown no response of leaf area in stands younger than 50 years, but substantial increases in leaf area in older stands (Binkley and others 1990). The implications are far reaching: declining nutrient availability may lower stand leaf area, which in turn lowers stand productivity. If such patterns of soil nutrient availability with stand age applied across the region, all our site index estimates and stand yield tables would have built into them the effects of declining nutrient availability (which might result from accumulation of readily available nutrients in noncycling biomass, but we don't know for sure). Any management activity that altered nutrient availability would alter the trajectory of stand yield, and alter such calculations as the culmination of mean annual increment and optimal rotation age.
FOREST EFFECTS ON SOILS
In addition to the role of soils in determining potential rates of forest production, the state of a forest also has reciprocal effects on soils. Forest harvesting can greatly increase the concentrations of nitrate leaching in soil solutions by an order of magnitude or more (Knight and others 1991; Stottlemyer 1987). These post-cutting losses amount to only a few kg/ha of N, and are much lower than the responses of N-rich northern hardwood forests (such as Hubbard Brook; Likens and others 1978); nonetheless, they demonstrate dramatic interactions between the soil system and trees.
The state of a forest also has a strong influence on soil moisture in our region. Harvesting a mixed conifer forest in the Fraser Experimental Forest in central Colorado resulted in a reduction in Evapotranspiration of about 50 mm/yr, plus a greater snowpack accumulation (due to reduced sublimation of snow held in the canopy) of 150 mm/yr (Troendle and Kaufmann 1987). These hydrologic changes after forest removal result in large increases in soil moisture. For example, Newman and Schmidt (1980) found that soils in a larch/Douglas-fir forest in Montana contained less than half the water (on a weight basis) contained in soils in clearcut sites throughout the growing season. Such changes in soil moisture likely produce large changes in microbial activity and nutrient availability, but such responses remain virtually unexamined in the Rocky Mountain region.
Water losses due to interception and Evapotranspiration also differ among species under the same site conditions. For example, Kaufmann (1985) estimated that Evapotranspiration in aspen stands removed only about half as much soil water as in lodgepole pine stands, and only one-third as much as in Englemann spruce stands.
Forests also exert considerable control over the temperature regimes at the surface of soils. A classic paper by Hungerford (1980) illustrates the dramatic differences in temperature extremes at the soil surface for different degrees of canopy removal. The low temperature at the soil surface in a clearcut in Montana's Lubrecht Experimental Forest on August 29 of 1978 was −5°C (23°F), and the high on the same day was 56°C (133°F). Under an intact forest, the low and high were 2°C (36°F) and 35°C (95°F). High temperatures in the clearcut may be lethal to regenerating seedlings, and the temperature effects on microbial processes are likely substantial (especially coupled with higher soil moisture).
It is difficult to remove logs from a forested landscape without compacting soils, and it is amazing how little we know in the Rocky Mountains about the degree and pervasiveness of soil compaction, the time course of recovery, and the implications for forest productivity (see Froelich, this proceedings). This is perhaps the most critical gap in our understanding of the connection between forest management and the maintenance of forest productivity in our region.
IMPLICATIONS
Soil fertility constrains forest productivity in the Rockies, and forest management activities certainly alter soils. We currently have very little information on the pluses and minuses of common forest practices for this large region of the country. We can continue getting by with the status quo, but that would not be responsible land management.
Two critical needs are obvious. The first is to educate foresters about the fundamental connections between soils and forest productivity (highlighting the particular importance of soil organic matter), which was a primary goal of this workshop. The second is a need for more research, which sounds predictably familiar from an academic such as myself. However, the most critical sort of research is not esoteric investigations that only a scientist could love. What we need to understand (= conduct research on) is simply what our current management practices are doing to the productivity of our soils and forests (see Powers, this proceedings). How can we be confident that our current practices lead to sustainable (or preferably, increasing) productivity if we have almost no information on the effects on soils?
REFERENCES
Allen, H. L. 1987. Forest fertilizers. Journal of Forestry. 85: 37-46.
Binkley, D. 1986. Forest nutrition management. New York: Wiley. 290 p.
Binkley, D.; Christensen, N. L. 1991. The effects of canopy fire on nutrient cycles and plant productivity. In: Laven, R.; Omi, P., eds. Pattern and process in crown fire ecosystems. Princeton University Press. [In press].
Binkley, D.; Smith, F. W.; Long, J. N. 1990. Nutrient limitation of leaf area with stand age in lodgepole pine forests. Bulletin of the Ecological Society of America. 71: 92.
Clayton, J. L.; Kennedy, D. A. 1985. Nutrient losses from timber harvest in the Idaho batholith. Soil Science Society of America Journal. 49: 1041-1049.
Cochran, P. H. 1979. Response of thinned ponderosa pine to fertilization. Res. Note PNW-339. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 8 p.
Cochran, P. H. 1989. Growth rates after fertilizing lodgepole pine. Western Journal of Applied Forestry. 4: 18-20.
Heidmann, L. J.; Rietveld, W. J.; Trujillo, D. P. 1979. Fertilization increases cone production and diameter growth of a 55-year-old ponderosa pine stand in Arizona. In: Bonner, F., ed. Proceedings: a symposium on flowering and seed development in trees; May 15-18, 1978; Mississippi State University: 197-205.
Hungerford, R. D. 1980. Microenvironmental response to harvesting and residue management. In: Environmental consequences of timber harvesting in Rocky Mountain coniferous forests. Gen. Tech. Rep. INT-90. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 37-74.
Hunt, H. W.; Ingham, E. R.; Coleman, D. C.; Elliott, E. T.; Reid, C. P. P. 1988. Nitrogen limitation of production and decomposition in prairie, mountain meadow, and pine forest. Ecology. 69: 1009-1016.
IFTNC (Intermountain Forest Tree Nutrition Cooperative). 1989. Ninth annual report. Moscow, ID: College of Forestry, Wildlife and Range Sciences, University of Idaho. 44 p.
Kaufmann, M. R. 1985. Annual transpiration in subalpine forests: large differences among four tree species. Forest Ecology and Management. 13: 235-246.
Knight, D. H.; Yavitt, J. B.; Joyce, G. D. 1991. Water and nitrogen outflow from lodgepole pine forests after two levels of tree mortality. Forest Ecology and Management. [In press].
Likens, G. E.; Bormann, F. H.; Pierce, R. S.; Reiners, W. A. 1978. Recovery of a deforested ecosystem. Science. 199: 492-496.
Loewenstein, H.; Pitkin, F. H. 1963. Response of grand fir and western white pine to fertilizer applications. Northwest Science. 37: 23-30.
Loewenstein, H.; Pitkin, F. H. 1971. Growth responses and nutrient relations of fertilized and unfertilized grand fir. Sta. Pap. 9. Moscow, ID: University of Idaho. Forest, Wildlife and Range Experiment Station. 13 p.
Newman, H. C.; Schmidt, W. C. 1980. Silviculture and residue treatments affect water use by a larch/fir forests. In: Environmental consequences of timber harvesting in Rocky Mountain coniferous forests. Gen. Tech. Rep. INT-90. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 75-110.
Shafii, B.; Moore, J. A.; Olson, J. R. 1989. Effects of nitrogen fertilization on growth of grand fir and Douglas-fir stands in northern Idaho. Western Journal of Applied Forestry. 4: 54-57.
Stark, N. 1982. Soil fertility after logging in the northern Rocky Mountains. Canadian Journal of Forest Research. 12: 679-686.
Stottlemyer, R. 1987. Natural and anthropic factors as determinants of long-term streamwater chemistry. In: Troendle, C. A.; Kaufmann, M. R.; Hamre, R. H.; Winokur, R. P., eds. Management of subalpine forests: building on 50 years of research. Gen. Tech. Rep. RM-149. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 86-94.
Troendle, C. A.; Kaufmann, M. R. 1987. Influence of forests on hydrology of the subalpine forest. In: Troendle, C. A.; Kaufmann, M. R.; Hamre, R. H.; Winokur, R. P., eds. Management of subalpine forests: building on 50 years of research. Gen. Tech. Rep. RM-149. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 68-78.
Waring, R.; Pitman, G. 1985. Modifying lodgepole pine susceptibility to mountain pine beetle attack. Ecology. 66: 889-897.
Weetman, G. F.; Fournier, R. M. 1982. Graphical diagnoses of lodgepole pine response to fertilization. Soil Science Society of America Journal. 46: 1280-1289.
Weetman, G. F.; Fournier, R. M.; Schnorbus, E. 1988. Lodgepole pine fertilization screening trials: four-year growth response following initial predictions. Soil Science Society of America Journal. 52: 833-839.
Yang, R. C. 1985a. Ten-year growth response of 70-year-old lodgepole pine to fertilization in Alberta. Infor. Rep. NOR-X-266. Edmonton, AB: Canadian Forestry Service. 17 p.
Yang, R. C. 1985b. Effects of fertilization on growth of 30-year-old lodgepole pine in west-central Alberta. Infor. Rep. NOR-X-268. Edmonton, AB: Canadian Forestry Service. 11 p.
Speakers answered questions from the audience following their presentations. Following are the questions and answers on this topic:
Q. (from Hersh McNeil)Do you think something could be done to increase N availability later in stand development?
A. Only a few percent of the total quantity of nutrients bound in soil organic matter become available each year for plant uptake; any treatment that could increase this percentage could (1) reduce nutrient limitation of current growth rates, and (2) increase the rate at which nutrients accumulate in harvestable biomass. The most proven approach to increasing turnover of soil nutrients is harvesting; warmer and wetter conditions following harvest often increase nitrogen availability by two-fold (we need to know more about the size and variability of this response). The response of nutrient availability to thinning has not been examined in this region; in many cases, thinned stands respond well to fertilization, which suggests indirectly that any increase in nutrient cycling is slight (and may in fact be negative, if thinning slash allows microbes to compete with plants for N). Prescribed fire is another possibility; W. W. Covington and colleagues demonstrated that prescribed burning in ponderosa pine in northern Arizona resulted in both an immediate increase in soil ammonium, and an increase in the rate of decomposition of the remaining forest floor. Fertilization with one element (such as P) may increase the release of other elements (such as N) through accelerated decomposition, but this hasn't been examined in our region. Finally, the choice of species may be the greatest opportunity for influencing nutrient cycling rates. The inclusion of N-fixing species typically accelerates the cycling of all elements. Less is known about the differences among non-N-fixing species, but studies from the Eastern United States and elsewhere suggest that rates of decomposition and nutrient cycling may differ substantially among species. Unfortunately, no one has examined nutrient availability in replicated plantations of different species in our region. I suspect some of the greatest differences would be found between stands of aspen and conifers; the nutrient-rich litter of the aspen combined with the frequent presence of understory N-fixers probably promotes much faster nutrient cycling under aspen.
Paper presented at the Symposium on Management and Productivity of Western-Montana Forest Soils, Boise, ID, April 10-12, 1990.
Dan Binkley is Professor, Department of Forest Sciences, Colorado State University, Fort Collins, CO 80523.