Ecophysiology of Populus fremontii: Effects of inundation, and interactions among nitrogen form, nutrient level, and water availability

As the climate changes and land continues to be developed for agricultural and urban use, ecosystems around the globe will experience shifts in hydrologic cycles, CO2 concentrations, water availability, and nutrient availability. Semi-arid riparian forests in the North American West will be particularly affected by these changes since establishment of riparian vegetation depends on the river hydrograph, urban and agricultural runoff, water and air temperature, CO2 concentration, and soil nutrient availability. Fremont cottonwood (Populus fremontii) is an early successional foundation species in these ecosystems that establishes along barren point bars and contributes many important ecosystem functions. The three chapters of this dissertation investigate responses of P. fremontii seedlings to 1) variation in inundation depth, duration, and temperature, 2) variation in atmospheric CO2 concentration and N form (NO3- and NH4+), and 3) variation in nutrient level, soil water availability, and N form.;Investigations of seedling tolerance of inundation showed that mortality increased linearly with days of complete shoot submergence (mortality % = 4.6 + (2.5 days)), resulting in greater than 60% mortality after four weeks of complete submergence. Cooler water temperature (18/11°C day/night) during complete submergence positively affected seedling dry weight and survival resulting in 25% greater seedling survival than at warmer water temperatures (25/18°C day/night). Results indicated that establishment of new P. fremontii populations in the riparian corridor will be more successful when flows do not completely cover the shoots of seedlings for more than two weeks and if water temperatures during inundation are cool.;Investigations of seedling response to N form and CO2 level found that seedlings grown with NH4+ had 42% greater average dry weight than those grown with NO3- regardless of CO2 level. The lack of an interaction countered our hypotheses that inhibition of leaf NO3- reduction at elevated CO2 would lead to lower dry weights in NO3- fed plants at elevated CO2. Instead we found that NO3 - reduction occurred in roots and was not coupled to carbon metabolism in seedling leaves. The observed differences in dry weight between NO 3- and NH4+ plants were likely due to the increased costs of NO3- reduction and maintenance of charge balance in P. fremontii seedling roots. Regardless of form, there was no significant difference in average dry weights between plants grown at ambient and elevated CO2, and there was evidence of CO2 acclimation.;The final chapter quantified relative effects of water stress (stressed vs. well watered), nutrient level (low near field conditions, medium, and high non-limiting), and N form (NO3- vs. NH 4+) on growth. Low nutrient level decreased average plant dry weight to a large extent. Decreases caused by low nutrients were at least six-fold greater than any decrease in biomass resulting from water stress. N form had little effect on plant dry weights. Both water stress and N form interacted with nutrient level such that effects of either factor on growth and allocation were only observed at high and medium nutrient levels. Thus, effects of water stress and N form are likely to affect plant growth and dry weight allocation only when high, non-limiting nutrients are present, whereas nutrient level consistently affects seedling growth and allocation and dampens effects of the other factors. Together these chapters provide an assessment of the effects of inundation, nutrient level, CO2 level, N form, and intermittent, moderate water stress on the growth and survival of P. fremontii seedlings during their first season of growth. (Abstract shortened by UMI.).