Hydrologic significance of stand density variations in Alberta lodgepole pine (Pinus contorta Dougl. (Moench) Voss. var. latifolia Engelm.) forests

Abstract

With twin objectives of (1) determining whether any hydrologic effect of thinning persisted 25 years after implementation, and (2) examining the hydrologic significance (if any) of variations in stand density, studies of snow accumulation, canopy interception of rainfall, and soil moisture, were undertaken in natural and previously thinned stands of lodgepole pine in west central Alberta. Secondary objectives dealt with precipitation redistribution within forest stands, the utility of trough gauges in throughfall estimation, and sample numbers required in soil moisture sampling. Snowpack was ephemeral, and no definitive results on snow accumulation were obtained.

There was no evidence that thinning, after 20 years, increased throughfall. Two natural stands with identical basal areas, but differing widely in stem numbers and mean DBH, had significantly different net precipitation. In explaining these differences, a number of crown variables were tested. Canopy density appeared most helpful. Stemflow was negligible.

None of three plots showed any greater or lesser variability in throughfall. Throughfall was greater at the crown perimeter than at other positions beneath crowns. In storms < 0.50 inches, throughfall beneath canopy gaps was greater than at the crown perimeter. In storms > 0. 50 inches, this was reversed. Throughfall, as a function of distance from tree stems, was approximately 85 percent randomly distributed.

Trough gauges tended to undercatch. The relationship of mean throughfall (standard gauge) and mean throughfall (trough gauge) was very precise, however.

Highly significant differences in plot soil moisture contents were found, but differences in soil moisture change were not significant. Soil moisture change was concentrated in the surface soil layers. Little change took place at depths greater than eight feet, in spite of water being available there. No significant differences in evapotranspiration were found, no effect of thinning remaining after 25 years. Mean daily evapotranspiration rate in July to September inclusive, 1966 was 0.09 inches water.

Number of samples required to yield a precision of estimate of one percent soil moisture by volume was calculated. For soil moisture content, 40 samples were required. For soil moisture change, at least eight samples appeared necessary. To estimate total soil moisture change in the whole instrumented solun, consistently with the same precision required more than eight access tubes. Soil moisture variance was greatest in early season.

Wide variations in stand density do not necessarily signify any difference in ecosystem hydrology. Better measures of stand description for hydrologic purposes are desirable.