The cost of depth: frost avoidance trade-offs in herbaceous plants

Abstract

The positioning of bulbs and stem tubers deep in soil can decrease frost exposure, yet it can come at the cost of increased investment to reach the surface, and delayed emergence. We explored the strength and generality of this trade-off for a range of herbaceous, temperate species. To isolate the direct effects of soil depth on plant growth and survival from the effects of variation in frost exposure, one set of plants buried over winter at three depths experienced either ambient or increased frost (via snow removal), another set sheltered from winter frost stress was buried at three depths in spring, and a third set was exposed to freezing under controlled conditions. Growth generally increased with organ soil depth under increased frost. However, for many species, spring-planted organs exhibited a trade-off of decreased aboveground leaf biomass with greater organ depth. Increased freezing exposure alone generally decreased survival, with sub-lethal effects (particularly decreased leaf biomass) observed for some species. Our results highlight that optimal bud depth is influenced by trade-offs between frost avoidance and plant growth. The balance of this trade-off may shift in coming decades with reduced snow cover and resulting increases in soil frost exposure.