COS 275-2 - CANCELLED - Is winter really the “dormant season” in northern temperate forests? Investigating overwinter nitrogen uptake in six contrasting tree species.

Abstract: Nutrient uptake is often assumed to cease in winter alongside photosynthesis and transpiration, leading to significant overestimation of nitrogen losses by global earth system models. However, evidence is building that some plants can take up meaningful amounts of nitrogen over winter. Data are still sparse, particularly for temperate forests, with additional questions on how winter N is allocated among plant tissues and how the seasonality of uptake is controlled by major functional traits. We hypothesized that winter uptake would be of negligible magnitude, but that the potential for winter N uptake might depend on major traits that affect trees’ C reserves (leaf habit), N foraging (mycorrhizal association), and overall C and N demand (leaf shape and N concentration). We expected leaf habit to be a particularly important control: in deciduous species, photosynthesis and transpiration stop after leaf-fall, while evergreen species can continue these processes when conditions are mild. We conducted an isotopic (¹⁵N-NH₄Cl) tracer study to measure summer and winter nitrogen uptake by six tree species of the northern Adirondack Mountains representing these functional trait axes. Samples of above- and belowground tissues were taken two months after tracer addition: in August 2021 for the summer tracer addition and January 2022 for the winter addition.

We found that surprisingly, all six species from across all functional groups took up substantial amounts of ¹⁵N in winter. Total N uptake in winter was of comparable magnitude as in summer, for both deciduous and evergreen species. ¹⁵N concentrations in fine roots in winter roughly doubled those in summer. Aboveground tissues showed only negligible enrichment in winter, but did show substantial enrichment in summer. It appears that while N is transported aboveground during summer, a considerable amount of nitrogen accumulates within the roots over winter. We will next analyze samples from additional time-points including snowmelt and the end of the subsequent growing season, allowing us to track the fate of winter-acquired N into new growth and examine longer-term N accumulation and allocation. Overwinter N uptake appears to be widespread and biogeochemically important across a variety of northeastern trees, with implications for our understanding of intra-annual N cycling in temperate forests.