WWRC 85-35
Water and Nutrient Outflow From Contrasting Lodgepole Pine Forests in Wyoming
Abstract
Factors affecting water and nutrient outflow beyond the rooting zone were studied during
a 3-yr period, using data from eight contrasting stands of lodgepole pine (Pinus contorta ssp. latifolia)
forest in southeastern Wyoming and the output of a hydrologic simulation model (H20TRANS)
based on tree physiology. Nutrient outflow during a specific time period was estimated by multiplying
simulated water outflow times element concentrations in the soil solution, the latter determined from
samples collected periodically near the bottom of the rooting zone.
Estimates of actual evapotranspiration (ET) for the period from early spring to late fall ranged
from 21 to 53 cm, which was 33-95% (x= 73%) of total annual precipitation. For all stands and
years, transpiration accounted for 50-61% of ET, and 9-44% of the transpiration occurred during the
spring drainage period (vernal transpiration, VT). Estimated VT and outflow varied considerably
among the stands, with VT accounting for 4-20% of the snow water. Outflow occurred only during
the snow melt period and accounted for 0-80% of the snow water. Snow water equivalent varied
annually by 300% or more. Nutrient outflow from the different stands also varied greatly. Ratios
between simulated annual outflow and atmospheric inputs (bulk precipitation) were consistently > 1.0
for Ca, Na, and Mg; were consistently < 1.0 for N; and ranged from 0.3 to 2.0 for P and from 0.2 to
3.3 for K. Much of the variability in water and nutrient outflow can be attributed to the degree of
biotic control, with water outflow affected by a different combination of factors than nutrient outflow.
H20TRANS was used to simulate the effects on outflow of different snow water equivalents and
different total leaf areas. One result of the simulations was that nitrogen appears to be retained even
at the highest levels of water outflow. Another was that increases in water outflow following reduction
in leaf area were proportional to the leaf area removed.
The results indicate that stands differing in site or habitat type experience different rates of water
and element losses at different times during the snow melt season, and contribute differentially to
streamwater quality and hydrograph shape. Factors affecting outflow are discussed in the context of
successional trends, common perturbations including timber harvest, and hypotheses pertaining to
nutrient conservation in terrestrial ecosystems. Nutrient retention in the snow-dominated lodgepole
pine ecosystem appears to be primarily dependent on evergreen leaf area, duration of the VT period,
and high carbon/nutrient ratios of the forest floor. Net losses of limiting nutrients probably occur
primarily in pulses after abiotic perturbations such as fire.
Key words: computer simulation; evapotranspiration; forest biomass; forest hydrology; habitat
types; immobilization; leaf area; nitrogen; nutrient input; nutrient outflow; Pinus contorta; Rocky
Mountains; transpiration; Wyoming.
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