Effects of nitrogen, pH, and mycorrhizal fungi on the growth, water relations, and physiology of trembling aspen (Populus tremuloides) and balsam poplar (Populus balsamifera)

This dissertation consists of four studies aimed at examining: (1) trembling aspen's response to inorganic nitrogen; (2) how ectomycorrhizal associations may influence trembling aspen's growth and response to inorganic nitrogen; (3) the ectomycorrhizal influence on aspen's response to short-term pH changes, and; (4) how mycorrhizal associations affect growth and water relations of balsam poplar. In the first study, effects of nitrate and ammonium on solution culture-grown trembling aspen were examined. High ammonium concentrations were toxic to aspen, which was not due to pH effects alone. Effects of high ammonium exposure were partly reversible. Although aspen exhibited increased growth with high concentrations of nitrate vs. ammonium, aspen had slightly more growth at low and moderate ammonium concentrations vs. nitrate, suggesting that aspen is capable of moderate ammonium tolerance. In the second study, effects of nitrate and ammonium were examined in sand-grown aspen associated with Hebeloma crustuliniforme (Hc). Aspen showed minimal preference for nitrate vs. ammonium, although aspen growth was reduced when nitrate and ammonium were supplied together. N form preference was not related to pH effects. Mycorrhizal aspen had increased nitrogen uptake and ammonium assimilation at higher nitrogen concentrations, but had higher mortality at low nitrogen concentrations. Hc may therefore be important for aspen's increased tolerance of ammonium. In the third study, short-term effects of pH (4-9) on Hc-associated aspen seedlings were examined. Mycorrhizal aspen showed an increased alkaline pH tolerance with increased Lpr at pH 7, which was not likely due to changes in AQP activity. By comparison, L pr and AQP of non-mycorrhizal seedlings were relatively unaffected by pH. In the fourth study, the effects of Hc and Wilcoxina mikolae var. mikolae (Wm) on balsam poplar growth and water relations were examined. Wm increased balsam poplar's stomatal conductance, shoot water potential, and root volume, but Hc increased balsam poplar's Lpr. The different effects of the two fungi on balsam poplar may have been due to the different characteristics of the two fungal species or different types of associations that poplar roots formed with Wm (ectendomycorrhizal) and Hc (ectomycorrhizal).