Lessons from the old school: phenological responses of the horsetail Equisetum arvense to experimental air and soil warming in Interior Alaskan boreal forest

Abstract

Although there are many studies on plant phenological shifts due to climate change, few of them focus on the phenological responses of seedless vascular plant species to warming climate. The boreal forest biome contains ~ 30% of the carbon stored by forests globally and horsetails with their circumboreal distribution are abundant in the boreal forest. Understanding the phenological response of horsetails to warming air and the timing of ground thaw is an important component for understanding how much carbon will be fixed by plants in the boreal biome in the future warmer climate. To examine the effects of air and soil warming on the phenology of Equisetum arvense L. in interior Alaskan boreal forest near Fairbanks, Alaska, we carried out a two-by-two full factorial warming experiment using open topped warming chambers (air warming) and snow removals to advance ground thaw (soil warming). Warming soil by 1 °C caused E. arvense to emerge sooner, and warming air by 0.7 °C caused E. arvense to grow faster and advance to photosynthetic activity sooner. Warmed E. arvense stems also entered senescence earlier than stems in un-warmed control plots, but the advance was greater in the spring, leading to an overall maximum extension of their growing season by 6%, or 6.7 days, for plants that were exposed to both air and soil warming. This is double the average growing season extension of 3% (1.5 days) documented in similar warming experiments of arctic seed plants, and more than the mean growing season extension per decade for seed plants in Europe (4.8 days) and China (6.2 days). Such season expansion at relatively low temperature increases suggests that E. arvense has potential to fix more carbon in future boreal forests.