| Anthropogenic nitrogen deposition to the forests of Northeastern America may change both forest species composition and productivity, by altering soil N availability and the ratio of ammonium to nitrate available. N deposition was simulated in forest and shadehouse studies to investigate changes in seedling growth due to different N availability, and ratio of ammonium to nitrate, and possible interactions between N deposition and seedling light environment for four species of birch, Betula populifolia, B. papyrifera, B. lenta and B. alleghaniensis. The species responded differentially, as B. papyrifera seedlings grew larger given either ammonium or nitrate, B. papyrifera and B. lenta responded to nitrate, and B. alleghaniensis responded to neither. In high light, different forms of N deposition may lead to changes in the species composition of the seedling community. Under shaded conditions, nitrate increased the growth of B. lenta but not B. alleghaniensis, which could also change community species composition.; The thesis also investigates whether B. lenta seedlings can maximize growth by foraging selectively for nitrate or ammonium, and the effect of the ratio of ammonium to nitrate. Root architecture was examined, to ask whether root architecture is adapted to maximize efficiency of uptake of nutrients locally, and the effect of integration of patches of different N type. Spatial distribution of ammonium and nitrate in soils affected seedling growth; those offered ammonium and nitrate homogeneously grew larger than those offered a heterogeneous patch of nitrate and a patch of ammonium, although all seedlings took up a similar ratio of ammonium and nitrate. Lower growth in the heterogeneous treatment may reflect the additional cost of searching for nitrate separately from ammonium, compared with the homogeneous choice plants that encountered both together. As nitrate-rich deposition continues to fall, B. lenta seedlings will encounter nitrate more readily, and grow faster, like the seedlings offered a homogeneous choice of ammonium nitrate. Ultimately, they may have to search for scarce ammonium in a patchy environment. The exploitation of appropriate patches may be facilitated by changes in root architecture that increase the soil volume searched, and increase the likelihood of encountering new ammonium rich patches. |
