The opposing effects of shortened days and cooling temperatures on tundra plant senescence

Plant responses to global change are well-studied in spring and early summer but less so in late summer, particularly in tundra ecosystems. It remains unclear whether senescence is driven more by temperature or photoperiod, and whether species respond similarly under late-season warming. We used growth chambers to test how photoperiod and late-season warming affect leaf senescence over two growing seasons in three tundra species collected from the same latitude in southwestern Yukon, Canada—Artemisia norvegica, Poa alpina, and Trisetum spicatum, representing one forb and two grasses, respectively. Senescence was quantified by digitally measuring leaf colour change in the final weeks of the experimental season. Both grasses were sensitive to temperature and daylength, but A. norvegica was only marginally affected by daylength. Interestingly, while cooling temperatures increase senescence over stable temperatures, autumn-like shortening daylengths decrease senescence over constant daylengths. In Poa alpina, reproductive individuals showed faster leaf senescence, and panicle senescence was temperature sensitive. The phenology of foliar and reproductive tissues may respond independently, showing that well-documented early-season responses to northern warming do not always predict late-season phenology.