Seedling Response of Four Birch Species to Simulated Nitrogen Deposition: Ammonium vs. Nitrate

Chronic nitrogen deposition has the potential to alter seedling shade tolerance and growth in the temperate forests of northeastern United States, by affecting both the form and the quantity of available nitrogen. Simulated deposition treatments were applied to seedlings of four birch species that co-occur at Harvard Forest (Betula lenta, B. alleghaniensis, B. populifolia, and B. papyrifera). Seedlings were individually potted in forest soil, and grown under light treatments representative of forest understory and treefall gap light levels. In a split-plot design, N was applied at 25 and 50 kg°ha^-1 °yr^-1 , as either nitrate, ammonium nitrate, or ammonium, within each light environment. While B. populifolia and B. papyrifera, and B. lenta all showed increased biomass allocation to leaves with increased N, only B. lenta showed a significant growth response to the type of N added, and this response was conditional on rate of N application and light environment. At low light, nitrate-fed B. lenta grew best, and also at low rate of supply, nitrate treatments out-performed ammonium treatments. Greater growth under these conditions is probably the result of higher biomass allocation to leaves, and greater specific leaf area, which increased the leaf area ratio, and improved the capacity for carbon gain. Under N deposition regimes that increase soil nitrate availability, the differences in response of B. lenta and B. alleghaniensis to nitrate at low light may potentially lead to changes in the species composition of the seedling communities in the understory. When a treefall occurs, a different species mix of seedlings will be released, with potential consequences for sapling and tree species mix.