Climate warming effects in the winter are significant and are likely to become stronger. In high elevation and high latitudes environments, such as the Arctic, biological activity continues in frozen soil. However, this activity is extremely temperature-sensitive, and small increases in soil temperature can have large effects on biological processes. Conversely and surprisingly, warmer air temperatures in the temperate zone, such as Northeastern North America, may result in colder soils. In these areas, warmer winters result in a loss of the insulating snowpack (Hayhoe et al. 2006). With the snowpack as a good insulator, soils may remain thawed, but a loss in snowpack depth and duration can increase soil frost. Seasonally frozen ground is widespread, occupying approximately 55% of the exposed land surface in the Northern Hemisphere (Zhang et al. 2004). Therefore, changes in the depth and duration of frozen ground have the potential to affect the hydrology, biology and chemistry of ecosystems across broad regions. Variation in wintertime conditions can have a substantial influence on ecosystem processes by affecting the physical properties of soils, as well as plant and microbial activity. Recent research focused on the impacts of soil freezing and thawing on plant and microbial processes have produced a wealth of data on the impacts of changes in snowpack on alpine tundra, arctic tundra and northeastern forest ecosystems. Although studies have examined the impact of soil freezing on plant and microbial activity in these terrestrial ecosystems, there have been few attempts at cross-site syntheses. Here we propose to lead an organized oral session at the 2009 Ecological Society of America meeting to bring together ecosystem-scale experiments across a wide geographic range of ecosystem types. Our session will include two types of talks. The first set will showcase key ecosystem types and novel techniques and the second set will focus on synthesis across studies. Through these topics, we will address questions about the effects of winter climate change on the hydrology, biology and chemistry of tundra and forest ecosystems.