Phenological responses of Equisetum arvense to experimental air and soil warming in boreal forest of interior Alaska

Across high-latitude regions, warmer spring and fall conditions are associated with expanded growing seasons. However, almost all work on plant phenological shifts in response to climate change has focused on seed plants. Equisetum species (horsetails) are seedless vascular plants abundant in circumboreal forests. We evaluated phenological responses of Equisetum arvense in boreal forest of interior Alaska to earlier ground thaw and warmer air temperatures. We performed a 2 x 2 factorial experiment using modified greenhouses (air warming) and snow removals to advance ground thaw (soil warming), and developed monitoring protocols for the phenology of seedless vascular plants. Warming soil caused vegetative and reproductive stems to emerge sooner, and warming air caused faster vegetative growth and earlier advancement to a photosynthetically active phenophase, but treatments had minimal impacts on time of senescence. Plants that were exposed to both air and soil warming extended the growing season by 6.7 days compared to plants in control conditions, despite the short duration of the treatments and unusually cold spring conditions. We conclude that E. arvense has the potential to substantially increase its growing season as spring conditions continue to advance.