Spatiotemporal measurement and modeling of stand-level boreal forest soil temperatures

The spatial and temporal dynamics of soil temperature (T-SOIL) strongly influence a wide range of biotic and abiotic processes in boreal forests. Relatively few spatial and temporal T-SOIL measurements have been made in these systems; in addition, not all ecosystem models take into account the effect of changing stand structure during stand development on T-SOIL dynamics. The goals of this study were to measure T-SOIL and its spatiotemporal variability in a boreal forest chronosequence, develop and test a computationally simple empirical model to predict T-SOIL, and quantify the effects of different approaches to deriving T-SOIL on simulated ecosystem processes. TSOIL was measured hourly at six depths (0-100 cm) for 3-4 years, and sampled in a spatial grid monthly during one growing season, in a black spruce (Picea mariana (Mill.) BSP)-dominated boreal forest chronosequence. We report annual and daily T-SOIL patterns, air-soil hystereses, and T-SOIL changes at the freeze-thaw transition. An empirical model predicting T-SOIL as of the weighted sum of past air temperatures generally accounted for 90-95% of temporal T-SOIL variability at shallow depths, and 77-83% at 50-100 cm. A variety of stand structural characteristics affected the model parameters, with leaf area index (LAI) usually the most significant. Spatial T-SOIL correlation, measured at depths of 2 and 10 cm, was generally constant between 5 and 30 m. The new empirical model that accounts for changes in canopy structure greatly improved the prediction of T-SOIL for stands with high LAI in the Biome-BGC process model; the broader implications of this change are discussed. This model has simple data requirements - only air temperature and LAI - although the parameters given here should only be used for boreal stands with similar soil types. (c) 2005 Elsevier B.V. All rights reserved.