No abstract

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 °yr , 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.

Rates of leaf extension have been studied with electronic auxanometers at mid-altitude in the Austrian Alps, where both low and high altitude species co-occur. The results demonstrate a clear differentiation in the temperature responses of extension between these two groups of species. For the low or mid-altitude species of Achillea millefolium, Agrostis stolonifera, Poa alpina and Rumex arifolius, the average rate of leaf extension increases from 0.1 to 0.4 mm h between 10 and 20° C. For the high-alpine species of Achillea erba-rotta ssp moschata, Poa alpina ssp vivipara and Polygonum viviparum the average rate of leaf extension was considerably lower from 0.016 to 0.064 mm h, between 10 and 20° C.Leaf extension in the lowland species was not observed below an average temperature of about 5° C, whilst no limit was observed for the upland species, down to a temperature of about 0° C.In the cases of the dicotyledons that were studied, leaf plus petiole shrinkage was observed to occur, for as much as 2 to 4 h, during periods of high water vapour pressure deficits. This response was not observed for the monocotyledons.The observations of leaf extension show that daily totals of extension in species from high altitudies will be much less sensitive to day, to day variations in local climate than will the species from low altitudes. The lowland species will have higher rates of extension during clear and warm weather conditions but lower rates in cold, cloudy weather.

Inorganic soil nitrogen is often heterogeneously distributed, both spatially and in form (ammonium versus nitrate). Here we present information on the architecture of black birch (Betula lenta L.) root systems exposed to homogeneous and heterogeneous nitrogen environments. The major effects on root architecture were at the whole root system level in response to heterogeneity of nitrogen form rather than the effect of local of local nitrate or ammonium supply on local root growth. In the heterogeneous treatment, plant root systems had greater link lengths and more simple branching patterns. Root architectural responses to heterogeneous nitrogen, independent of localized responses to patches, suggest that in a seedling of B. lenta whole plant integration of its environment may override local control of root growth.

Soil nitrogen environments are bigbly beterogeneous, containing microsites tbat differ in relative and absolute ammonium and nitrate contents. We investigated wbetber black bircb {Betula lenta h.) seedlings can maximize growth by foraging preferentially for a particular form of nitrogen, and wbetber supply rate of N and ammonium: nitrate in a patcb affects preferences. Seedlings were transplanted into 5 1 split pots witb roots partitioned evenly, and assigned to 1 of 4 nitrogen form treatments: nitrate (NO3") botb sides, ammonium (NH^+) botb sides, NH^NOg, 'homogeneous cboice', and NO3" one side/NH,* on tbe otber, 'patcby cboice', crossed witb two rates of nitrogen supply. Use of '°N-labelled nitrate revealed tbe proportion of seedling N from nitrate. Seedlings offered a patcby cboice did not significantly out-perform seedlings in bomogeneous ammonium and nitrate treatments, w-bereas seedlings offered a bomogeneous cboice did out-perform bomogeneous nitrate and patcby cboice treatment plants. In cboice treatments, approx, 35 "" of seedling N came from a nitrate source, Tbis selectivity was unaffected by rate of nitrogen application and ammonium: nitrate ratio. Tbat patcby cboice seedlings integrated patches to acbieve tbis ammonium: nitrate uptake suggests tbat black bircb bas a preferred balance, Tbe seedlings offered a bomogeneous cboice were significantly heavier tban tbe seedlings offered a patcby cboice, perbaps reflecting an additional cost of foraging for ammonium and nitrate separately.