Can Interspecific Variation in Whole-Plant Resource Economies and Underlying Physiological and Morphological Traits Provide a Mechanistic Basis for Forest Composition?

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Human activities are substantially influencing the availability of nitrogen, calciumand water in north temperate forests. These changes may strongly affect forest composition and dynamics because water, nitrogen and calcium limit - in a species-specific manner -the growth and survival of trees in northern temperate forests. The ability to predict forest responses will be aided by characterizing the functional mechanisms that underlie tree species differences in growth and survival across soil resource gradients and how these species traits interact with light availability in determining species competitive abilities. We hypothesize that seedlings of tree species with greater growth and survival in low soil resource environments have collections of traits that enhance whole-plant use efficiency of and access to limiting soil resources, that these traits underlie adaptation to low soil resources, and that these traits occur at a tradeoff with shade tolerance and/or high light growth rates in high soil resource environments.A seedling transplant experiment to field plots and a closely linked potted seedling experiment are proposed to address these hypotheses. Both experiments will use the same nine broad-leaved deciduous species that are common in northern temperate forests, but that vary in abundance across light and soil resource environments. For the field experiment, seedlings have been transplanted into fertilized (Ca, nitrogen, Ca + nitrogen) and unfertilized plots distributed across variation in light and independent landscape-level gradients of water and nutrients in glaciated northwestern lower Michigan. Over the three-year duration of the experiment, we will monitor seedling growth and survival and measure whole-plant morphological and physiological traits that we hypothesize underlie species differences in performance. The outdoor plastic pot experiment will allow us to more thoroughly examine fine root physiology and dynamics than could be accomplished in the field experiment.Collectively these data will allow us to determine: 1) species specific growth and survival responses to natural gradients of nitrogen, calcium, water and light; 2) with fertilized plots, the degree to which N and Ca limit growth and survival and how it varies over the landscape gradient; 3) interspecific variation in access to, and use efficiency of nitrogen, calcium and water; 4) the physiological and morphological components of whole-plant resource economies that are associated with species differences in access to, and use efficiency of soil resources; 5) whether interspecific variation in whole-plant resource economies and related traits underlie variation in growth and survival at low soil resource availability; 6) interspecific trade-offs in resource-based growth and survival and the particular morphological / physiological traits underlying species differences.