Understanding how winter climate change will affect forest ecosystem C and N dynamics is challenging.  Many approaches can, and need to be taken to address this question including modeling, long-term studies, experimental manipulations and comparative studies.  Natural gradient studies provide information on the long-term effects of altered climate on whole ecosystems.  Here we exploited the natural climate gradient in the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF) to evaluate the effects of climate variation similar to what is predicted to occur with global warming over the next 50 – 100 years for northeastern North America. Our objectives were to, 1) characterize the elevation gradient in summer and winter climate for the HBEF and its effects on soil temperature, moisture and frost and 2) evaluate variation in total soil (TSR) and microbial respiration, mineralization, nitrification, denitrification, nitrous oxide flux and methane uptake along this gradient. 

Results/Conclusions

Low elevation sites were consistently warmer (1.5 – 2.5 ^o C) and drier than high elevation sites.  Despite higher temperatures, low elevation plots had less snow and more soil frost than high elevation plots.  Net nitrogen (N) mineralization and nitrification were lower in warmer, low elevation plots, in both summer and winter.  In summer, this pattern was driven by lower soil moisture in warmer soils and in winter the pattern was linked to less snow and more soil freezing in warmer soils.  These data suggest that N cycling and supply to plants in these northern hardwood ecosystems will be reduced in a warmer climate due to changes in both winter and summer conditions.  TSR was consistently higher in the warmer, low elevation plots, suggesting that climate change may reduce soil carbon (C) sequestration in the northern hardwood forests.  While variation in N cycling was controlled by moisture, C cycling was more strongly influenced by temperature.