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CONNECTING SOILS WITH
FORESTPRODUCTnnTY
Dan Binkley
Table 1-Demonstrated
ABSTRACT
nutrient limitations in the Rocky Mountain
region
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.
Species
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 Nand 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
Nutrient limitation
Lodgepole pine
N, P?, K? (Weetman and Fournier 1982)
N, P, S? (Yang 1985a)
N , S, P? (Yang 1985b)
N (Waring and Pitman 1985)
N (H unt and others 1988)
N, P (Weetman and others 1988)
N?, P?, S? (Cochran 1989)
N, P?, K? (Binkley and others 1990)
White pine
N, P?, K? (Loewenstein and Pitkin
Ponderosa pine
N (Heidmann and others
N, P? (Cochran 1979)
Douglas-fir
N, K? (IFTNC 1989)
N (Shafii and others 1989)
Grand fir
N, P?, K? (Loewenstein and Pitkin
N (Shafii and others 1989)
1963)
1979)
1971)
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 ofN are lost for every 1,000 kg of biomass burned
(Binkley and Christensen 1991). A typical slashfire might
consume about 50,000 kg/ha offuel, reducing the capital
of nitrogen by 250 kg/ha. How important is this amount
of nitrogen? Two comparisons are enlightening. A stemonly harvest of a typical Rocky Mountain forest would
remove about 60 to 150 kg/Ita 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 kglha 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
Paper presented at the Symposium on Management and Productivity
of Western-Montane Forest Soils, Boise, ID, April 10-12, 1990.
Dan Binkley is Professor, Department of Forest Sciences, Colorado
State University, Fort CoIlins, CO 80523.
66
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-kglha 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 offire 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.
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 mmlyr, 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 larchIDouglas-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 OF),
and the high on the same day was 56°C (133 OF). Under
an intact forest, the low and high were 2 °C (36 OF) and
35°C (95 OF). 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.
FOREST EFFECTS ON SOILS
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
IMPLICATIONS
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 postcutting losses
amount to only a few kg/ha ofN, 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.
67
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 offertilized 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 oflong-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. RM149. 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 offorests 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 ini tial predictions. Soil Science Society of America Journal. 52: 833-839.
Yang, R. C. 1985a. Ten-year growth response of 70-yearold lodgepole pine to fertilization in Alberta. Infor. Rep.
NOR-X-266. Edmonton, AB: Canadian Forestry Service.
17 p.
Yang, R. C. 1985b. Effects offertilization on growth of 30year-old lodgepole pine in west-central Alberta. Infor.
Rep. NOR-X-268. Edmonton, AB: Canadian Forestry
Service. 11 p.
research, which sounds predictably familiar from an academic such as myself. However, the most cri tical 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 ofleaf 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 R~search 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
68
Speakers answered questions from the audience following their presentations. Following are the questions and
answers on this topic:
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.
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
69