Frost is a major stress that greatly influences plant species distribution. Many herbaceous plants avoid freezing stress by placing buds at or below ground level, where they are insulated by snow, litter, or soil. Although it can protect, increased bud depth may have a cost (e.g. delayed emergence and reduced growth) when frost stress is absent. Our objective was to explore the strength of this trade-off for a range of rhizomatous plant species. We conducted a suite of experiments to isolate the direct effects of soil depth and litter cover on plant growth and survival from the effects of variation in frost exposure for six species. In one experiment, plants were buried in the fall at one of two depths with one of three litter treatments, and these treatments were combined with either snow removed (to increase frost stress) or ambient snow. In another experiment, plants subjected to the same depth and litter treatments were buried in the spring (i.e. potential benefits of frost avoidance were removed). In a final experiment, plants experienced one of six freezing temperatures under controlled conditions. The following summer all plants were harvested and dried, and height and biomass were measured.
Results/Conclusions
While species responses varied, half of the species exhibited interactions between the cost of depth and frost avoidance. Anemone canadensis exhibited an interaction between snow removal and the soil/litter treatment for most response variables. Total biomass was greatest with snow present and low litter and at the greatest soil depth. Convallaria majalis exhibited an increase in rhizome storage investment under high litter over winter, compared to under low litter and deep soil. For Viola sororia, winter stress and depth had no effect, but in the spring depth experiment, the greatest aboveground biomass occurred with the deepest soil placement and no litter, and the second highest biomass occurred under high litter, possibly as a result of increased soil moisture. Overall, the native species studied were generally less affected by freezing in the field than the non-native species. The two species that did not experience any negative response to depth also had the largest structures devoted specifically to nutrient storage. Our results revealed an important and under-studied interaction in herbaceous plants that may become increasingly important as climate change promotes increased frost penetration into soil as a result of reduced snow cover and increased temperature variability over winter.