Soil-atmosphere fluxes of trace gases (especially nitrous oxide (N 2 O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N 2 O, methane (CH 4 ) and carbon dioxide (CO 2 ) in field chambers monthly (bi-weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO 2 and CH 4 showed a strong seasonal pattern, with low rates in winter, but N 2 O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N 2 O flux and decreased CH 4 uptake in both treatment years, especially during winter. Annual N 2 O fluxes in sugar maple treatment plots were 207 and 99 mg N m À2 yr À1 in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N 2 O or CO 2 fluxes, but CH 4 uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N 2 O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N 2 O fluxes from northern hardwood forests.

Abstract:Measurements were conducted in coniferous forests of differing density, insolation and latitude to test whether air temperatures are suitable surrogates for canopy temperature in estimating sub-canopy longwave irradiance to snow. Air temperature generally was a good representation of canopy radiative temperature under conditions of low insolation. However during high insolation, needle and branch temperatures were well estimated by air temperature only in relatively dense canopies and exceeded air temperatures elsewhere. Tree trunks exceeded air temperatures in all canopies during high insolation, with the relatively hottest trunks associated with direct interception of sunlight, sparse canopy cover and dead trees. The exitance of longwave radiation from these relatively warm canopies exceeded that calculated assuming canopy temperature was equal to air temperature. This enhancement was strongly related to the extinction of shortwave radiation by the canopy. Estimates of sub-canopy longwave irradiance using either two-energy source or two thermal regime approaches to evaluate the contribution of canopy longwave exitance performed better than did estimates that used only air temperature and sky view. However, there was little evidence that such corrections are necessary under cloudy or low solar insolation conditions. The longwave enhancement effect due to shortwave extinction was important to sub-canopy longwave irradiance to snow during clear, sunlit conditions. Longwave enhancement increased with increasing solar elevation angle and decreasing air temperature. Its relative importance to longwave irradiance to snow was insensitive to canopy density. As errors from ignoring enhanced longwave contributions from the canopy accumulate over the winter season, it is important for snow energy balance computations to include the enhancement in order to better calculate snow internal energy and therefore the timing and magnitude of snowmelt and sublimation.

Understanding how exogenous and endogenous factors control the distribution, production and mortality of fine roots is fundamental to assessing the implications of global change, yet our knowledge of control over fine root dynamics remains rudimentary. To improve understanding of these processes, the present study developed regression relationships between environmental variables and fine root dynamics within a northern hardwood forest in New Hampshire, USA, which was experimentally manipulated with a snow removal treatment. Fine roots (< 1 mm diameter) were observed using minirhizotrons for 2 years in sugar maple and yellow birch stands and analyzed in relation to temperature, water and nutrient availability. Fine root dynamics at this site fluctuated seasonally, with growth and mortality peaking during warmer months. Monthly fine root production was strongly associated with mean monthly air temperature and neither soil moisture nor nutrient availability added additional predictive power to this relationship. This relationship exhibited a seasonal temperature hysteresis, which was altered by snow removal treatment. These results suggest that both exogenous and endogenous cues may be important in controlling fine root growth in this system. Proportional fine root mortality was directly associated with mean monthly soil temperature, and proportional fine root mortality during the over‐winter interval was strongly related to whether the soil froze. The strong relationship between fine root production and air temperature reported herein contrasts with findings from some hardwood forest sites and indicates that controls on fine root dynamics vary geographically. Future research must more clearly distinguish between endogenous and exogenous control over fine root dynamics in various ecosystems.

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