Abstract
The magnitude and frequency of soil frost events might increase in northern temperate regions in response to climate warming due to reduced insulation caused by declining snow cover. In temperate deciduous forests, increased soil frost severity can hamper tree growth and increase the mortality of fine roots, soil fauna and microorganisms, thus altering carbon and nutrient cycling. From single-site studies, however, it is unclear how the sensitivities of these responses change along continental gradients from regions with low to high snowfall. We conducted a gradient design snow cover and soil temperature manipulation experiment across a range of lowland beech forest sites to assess the site-specific sensitivity of tree growth and biogeochemical cycling to soil cooling. Even mild and inconsistent soil frost affected tree increment, germination, litter decomposition and the retention of added 15N. However, the sensitivity of response (treatment effect size per degree of warming or cooling) was not related to prevailing winter climate and snow cover conditions. Our results support that it may be valid to scale these responses to simulated winter climate change up from local studies to regional scales. This upscaling, however, needs to account for the fact that cold regions with historically high snowfall may experience increasingly harsh soil frost conditions, whereas in warmer regions with historically low snowfall, soil frost may diminish. Thus, despite the uniform biotic sensitivity of response, there may be opposing directions of winter climate change effects on temperate forests along continental temperature gradients due to different trends of winter soil temperature.
Highlights
The magnitude of climate warming over the current century is projected to vary both regionally and seasonally
Even at the shallowest soil measurement depth, the effect of snow exclusion (S-) was not strong enough to induce soil cooling across all sites, and soil cooling due to snow exclusion was most pronounced at the coldest site, which was expected based on there being little to no snowfall toward the warmer end of the gradient
In northern temperate forests, strong effects of snow manipulation are typically associated with a strong soil freezing effect, which provided the basis of our first hypothesis that soil cooling would decrease plant growth and disrupt biogeochemical cycles
Summary
The magnitude of climate warming over the current century is projected to vary both regionally and seasonally. Warming is generally projected to increase with latitude, with mean rates of warming exceeding the global average by up to 50% in cool and cold temperate climates (IPCC 2013). For these regions, snowfall during winter is a typical feature and soils are protected from frost events if the snow cover remains sufficiently deep and continuous (Sturm and others 1997; Groffman and others 2001; Wipf and others 2009; Kreyling 2020). A more frequent absence of snow cover in combination with the occurrence of cold winter air temperatures is anticipated to result in the phenomenon of increased soil frost frequency and magnitude in some regions despite global warming (Groffman and others 2001; Kreyling 2020)
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