Root-zone soil temperature variation associated with microsite characteristics in high-elevation forest openings in the interior of British Columbia

Abstract

Low soil temperatures have been suggested as a factor contributing to poor rooting of planted conifer seedlings and hence a cause for failure of some high-elevation reforestation efforts. Few studies have attempted to document soil temperature variation and its causal factors in post-logged, high-elevation openings during the growing season. A field survey of afternoon soil temperature (at a depth of −10 cm from the mineral soil surface) was conducted in six high-elevation clear-cuts located in the Engelmann Spruce-Subalpine Fir (ESSF) biogeoclimatic zone in central British Columbia, Canada. On a clear summer day, a difference of 8°C in afternoon soil temperatures was measured between the warmest microsites (bare mineral soil, 17.6 ± 2.5°C) and the coolest microsites (covered by forest floor and vegetation, 9.3 ± 0.7°C). Heavily covered microsites showed little diurnal or spot-to-spot (microsite) variation in soil temperature. Conversely, microsites with little surface cover exhibited the greatest spatial and temporal variability in soil temperature. Variation in afternoon root-zone soil temperature ( T) was most strongly associated with the combination of thickness of duff (forest floor) ( D), cumulative % vegetation cover ( C), and % soil moisture ( M). Comprised of these three microsite attributes, a multiple regression model, T = 24.06 − [1.05(ln( C) + 1.64(ln( D)) + 1.38(ln( M))] ( R 2 + 0.73) was created which predicts high-elevation afternoon soil temperatures to within ±2.0°C. Multiple regression models including relative surface irradiance (as a percentage of above canopy light), cumulative % vegetation height, micro-slope and micro-aspect were less predictive. The relationship between afternoon soil temperature and each microsite attribute generally showed a strongly negative curvilinear pattern, described by double exponential decay functions. Microsites associated with suboptimally low afternoon soil temperatures (< 12°C) during the growing season were widespread (70 to 80% of the sampled areas) in the surveyed high-elevation openings.