Nitrogen cycling and tree nutrition in boreal forests: interactions with understory vegetation and soil water availability

点击翻译按钮获取中文摘要

We have a poor understanding of the interactive effects of nutrient availability, water supply, and understory competition on stand productivity in boreal forests where competitive understory species such as Canada bluejoint grass are abundant and grow vigorously and where N is often the most limiting nutrient. In this study, I will use both field- and greenhouse-based approaches to study soil N dynamics, competition between understory and tree species, and how they are affected by resource (N and water) additions. The objectives are: 1) to investigate how understory vegetation and water availability interact to affect soil nutrient dynamics and tree nutrition; 2) to explore the linkage between soil resource availability (or competition) and tree ecophysiological processes (stomatal conductance, photosynthesis, stable C isotope fractionation, and plant N uptake); 3) to study the effects of competition and resource availability on the function and diversity of soil microbial populations; and 4) to gain a better understanding of the role of organic N in Canada bulejoint grass and white spruce nutrition. I will study net and gross rates of N mineralization, net and gross rates of nitrification, rate of heterotrophic nitrification, rates of soluble organic N production, N immobilization by microbes, N uptake by both tree and understory species, and the net primary productivity of the forest ecosystem. The stable N and C isotope (15N and 13C) techniques, ecophysiological (e.g., gas exchange) measurements, and vector analysis will be used to investigate the mechanisms for N competition and resource limitations. A greenhouse experiment will be conducted to test the hypothesis that Canada bluejoint grass can take up simple organic forms of N. Results from this study will help us better understand the factors controlling forest productivity in sites infested with competitive understory vegetation, design forest management strategies to adopt to the changing precipitation patterns, and provide information on the upper limits of forest productivity in the boreal region. The proposed research will advance ecological theories in soil-plant relations, resource limitation, competition, and boreal vegetation dynamics.