Seasonal timing and preceding moisture regime mediate impacts of heavy rainfall events on High Arctic plant growth

Abstract Arctic ecosystems are facing increases in heavy summer rainfall events and increased year‐to‐year hydrological variability. However, the evidence base of impacts of heavy rainfall on Arctic vegetation is limited. The role of seasonal timing in determining heavy rainfall impacts on plant growth and the legacy impacts of such effects have not been quantified. We set up an irrigation experiment in several sites on the High Arctic archipelago of Svalbard, in which we simulated a doubling of average summer rainfall (50 mm) in individual additions of 10 mm at different timings throughout the season (early or late summer). Plant growth and phenology indicators (normalized difference vegetation index, vegetation height, specific leaf area, and senescence) of key plant species were monitored under irrigation and in the years after to capture legacy effects. Late‐summer irrigation delayed end‐of‐summer declines in NDVI compared with control and early‐summer irrigation treatments. We found subtle positive legacy effects of early as well as late‐summer irrigation on NDVI in the following growing season. Irrigation only delayed senescence in Salix polaris at sites and moments where rainfall treatment compensated for low soil moisture levels prior to senescence. Other vegetation parameters did not show significant responses. Positive associations of plant growth variables with local soil moisture were strongest in late summer. Synthesis. Our findings indicate that the impact of heavy rainfall events on plant growth in the High Arctic is mediated by seasonal timing and local moisture regimes. Late‐summer rainfall can extend the growing season for plants experiencing end‐of‐season water‐limitation by delaying onset of senescence. Local soil moisture retention capacity appears to regulate this potential for growing season extension, implying that outcomes may be scalable across (micro)topographical and pedological gradients. The existence of subtle legacy effects suggests that previous years' hydrological conditions may marginally affect following years' plant growth.