Climate sensitivity of thinleaf alder growth on an interior Alaskan floodplain1

This study examined the climate sensitivity of the growth of riparian Alnus incana ssp. tenuifolia (thinleaf alder), a keystone nitrogen-fixer, on the Tanana River floodplain of interior Alaska. We investigated correlations between alder radial growth and inter-annual variation in monthly meteorology and hydrology, spatial patterns of alder climate sensitivity in relationship to depth of subsurface water, and long-term climatic trends. Annual radial growth of alder was positively correlated with June precipitation, river discharge, and Palmer Drought Severity Index values and was negatively correlated with June temperature, suggesting a susceptibility of growth to early-season moisture limitation, due to the co-occurrence of dry meteorological conditions and low levels of hyporheic flow. Alder radial growth was positively correlated with August discharge and July temperature, suggesting that moisture may also be limiting in August, but not in July. The sensitivity of alder growth to variation in temperature and precipitation was greater on higher terraces where depth to water table was greater, while the sensitivity to variation in river level was greater on lower terraces, suggesting that subsurface water more strongly influences moisture availability at these sites. Long-term climatic trends in this region suggest increasing drought conditions; however, the overall influence on alder growth and associated rates of nitrogen fixation are uncertain due to the contrasting relationship of growth with climate in June/August versus July.