In montane ecosystems of the western US, winter snowpack is predicted to decline with rising air temperatures. Because winter snowpack both provides water for tree growth and influences nitrogen (N) mineralization through thermal insulation of soils, the decline in snowpack is expected to affect forest productivity. However, despite the importance of N availability to temperate forest productivity, the effects of declining snowpack on current N availability across complex mountain terrain is not clear. We examined the impacts of topography (slopes vs. hollows) and spatial differences in snowpack on N availability across the Lubrecht Experimental Forest in western Montana. We hypothesized that N availability generally declines with declining snowpack: higher N availability on deeper snowpack sites (high elevation, north-facing site) than on shallower snowpack sites (low elevation, south-facing site), and at hollows relative to slope positions. We measured ammonium (NH4+) and nitrate (NO3-) in soil KCl-extracts and in soil water collected from 24 lysimeters for two growing seasons, immediately following the spring snowmelt of 2015 and 2016. Soil temperature and moisture were recorded continuously during this time using hydrometeorology stations.
Results/Conclusions
Our results show that soil N availability was generally greater at higher elevation sites and in the hollow locations, where the winter snowpack is deepest, supporting our hypothesis that winter thermal insulation and greater water availability under deeper snowpack contribute to higher N availability. In contrast, N availability on slope locations was consistently lower than hollow locations, regardless of elevation and aspects. Our results further suggest that warmer winters with reduces snowpack influences N availability by narrowing windows for N mineralization. Finally, large differences in seasonal availability of NH4+ and NO3- between 2015 and 2016 that correspond to inter-annual variations in the snowpack depths further support our hypothesis that N availability is closely associated with snowpack durations and magnitude in western montane forests. Our findings imply that declining snowpack under warmer climate may greatly affect the productivity of western montane forests through the reduction of N availability. Our results also suggest that the effects of warmer climate differ across landscape positions. This may lead to spatial differences in N availability with changing snowpack dynamics.