[1] Heat pulse probes (HPP) have been widely utilized to determine soil thermal properties and water content in unfrozen soils; however, their applications in frozen soils are largely restricted by phase change and the presence of unfrozen water. This study explores the possibility of using HPP to determine total water content of frozen soils by (1) establishing the optimum heat applications to limit melting, (2) improving the mathematical representations for frozen conditions, and (3) evaluating the applicability of HPP methods under various temperature and moisture conditions. A custom-built HPP was tested at total moisture levels that varied from full saturation to oven dry and initial soil temperatures from 20°C to −11°C. The applied heat pulse durations varied from 8 to 60 s, with total heat strength varying from 100 to 2000 J m −1 . Comparison of mathematical methods involved two analytical solutions and a one-dimensional finite difference numerical model. While both analytical methods assumed no phase change, the numerical model considered ice melting and unfrozen water. Conclusions include the following: (1) the numerical model with phase change is the only appropriate method to represent the temperature change curve once melting occurs; (2) below −4°C, ice melting could be limited, and measurement errors of total moisture content were within ±0.05 m 3 m −3 ; (3) application of HPP between −2°C and 0°C is difficult because of the retarded response of probe temperature to changing moisture contents and heat applications; and (4) probe spacing is a sensitive parameter requiring calibration once reinstallation of the probe or the thawing and freezing process occurs.Citation: Zhang, Y., M. Treberg, and S. K. Carey (2011), Evaluation of the heat pulse probe method for determining frozen soil moisture content, Water Resour. Res., 47, W05544,

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Density-dependent regulation in plants may occur at the level of the entire community and may differ in magnitude and direction at different life history stages such as germination, survival and growth, and under different abiotic conditions. We constructed semi-natural communities of boreal forest understory species planting 10 of the most abundant species. The experimental communities were established from seed or from cuttings and grown in sandboxes at six densities that extended far above and below average natural field density, while maintaining constant relative species proportions (a community density series (CDS)). We used two watering and fertilization levels in a factorial design. At the community level, both emergence and final per-plant shoot mass were negatively density dependent, while survival to the end of the season was facilitative. The effect of water was positive at seed emergence, whereas fertilizer negatively affected survival. Species-specific responses were also dependent on life stage. We demonstrated that density dependence is important in structuring this unproductive boreal understory habitat. The CDS approach allows us to quantify the effects of plant competition at the community and species levels, and to determine whether the importance of these biotic interactions depend on abiotic factors.

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