Adaptation of Willows in River Lowlands to Flooding under Arctic Amplification: Evidence from Nitrogen Content and Stable Isotope Dynamics

With the global warming, the frequent and severe river floods in Arctic regions have affected the distribution of willows in river lowlands. To investigate the effect of floods, especially waterlogging, on nitrogen conditions for willows, we measured their foliar nitrogen content and isotopic composition (δ15N), under various hydrological conditions in Northeastern Siberia, which was classified into four conditions according to the previous study: dry, wet, waterlogging [WL], long-period waterlogging [LWL] respects to willows’ physiological response. The δ15N values of willow leaves, which correlate to those of the soil, were significantly lower under dry (−3.6 ± 1.3‰) and wet (−3.5 ± 1.7‰) than WL (0.3 ± 2.6‰) and LWL (2.7 ± 2.4‰), indicates acceleration of soil nitrogen dynamics for waterlogging. There is no higher foliar nitrogen contents found under LWL (2.3 ± 0.3%) than WL (2.4 ± 0.4%). This might be caused by little decreased nitrogen uptake related to the low activity of carbon fixation in willows under LWL. Interestingly, LWL occurred over a wide area following extreme flooding in 2017; yet, willows survived, whereas potential competing species (larch) died. We further predict that the distribution of willows in Arctic riparian areas will expand, particularly after extreme floods under Arctic amplification.