Thaw circles around tree trunks provide spring ephemeral plants with a big head start on the growing season

Where winter lasts half the year, naturalists rejoice at the sight of early spring plants emerging from the cold and damp forest floor: the first sign of the imminent and long-awaited eruption of life from above and below. In temperate deciduous forests across the globe, a set of plants known as “spring ephemerals” emerges upon the melting of snow, and immediately enters a race against time. Spring ephemerals in northeastern North America, such as spring beauties (Claytonia caroliniana Michx.) and trout lilies (Erythronium americanum Ker-Gawl.), have only a month or so of high light during which to complete all or nearly all of their yearly photosynthesis, before the leaf-out of canopy trees cuts off their light supply. Extending or contracting this brief window of opportunity by even just a week would, therefore, represent a substantial change to the length of their growing season (Lapointe 2001). On 19 April 2017, we walked with snowshoes on top of about 50–70 cm of snow to the prospective location of a new field experiment in Parc national du Mont M egantic, a protected area in southern Quebec, Canada, about 15 km north of the border with New Hampshire and Maine, USA. The low-elevation forest here is dominated by sugar maple (Acer saccharum Marsh.), with minor components of American beech (Fagus grandifolia Ehrh.) and yellow birch (Betula alleghaniensis Britt.). Upon entering the forest, we were immediately struck by circles of bare ground extending about one trunk’s width out from the edges of most medium-sized to large trees. Interestingly, the depth of snow did not increase gradually with distance from the trunk; rather, snow-free ground abutted up against near-vertical walls of snow upward of 50 cm high. More importantly for this story, plants of several species, including spring beauty, trout lily, and red trillium (Trillium erectum L.), the latter of which emerges in early spring but with leaves that remain green through midsummer, were emerging in these “thaw circles” (a term we borrow from Veblen et al. [1977]). We returned to the site five days later for some high-quality photos (Fig. 1). During the transition from winter to spring, tree trunks receive direct sunlight, become considerably warmer than the surrounding air, and contribute to relatively early snowmelt in the vicinity of the trunk (Fig. 1; Geiger 1965). Thaw circles have thus been described previously, and the potential impact on the microsite-scale growing season noted (e.g., Brooke et al. 1970). To our knowledge, however, the potentially major impact of thaw circles on the phenology and distribution of spring ephemeral plants has not been studied. Having returned to our field site every few days after the initial visit, we estimate that the onset of the growing season for spring ephemerals (i.e., the time of snowmelt) occurred about a week earlier in thaw circles than in microsites just 1 or 2