| The dramatic, yet predictable, changes in light availability beneath the canopy of temperate deciduous forests have given rise to communities of sympatric herbaceous plants that differ in their life histories. I have investigated the physiological and growth characteristics of three co-occurring herbs of a northern hardwood forest, each representative of a different phenological guild: the spring ephemeral Allium tricoccum, the summergreen Viola pubescens, and the semi-evergreen Tiarella cordifolia . In addition, I examined the influence of the spring ephemeral guild on ecosystem-level nitrogen (N) cycling.; Leaf CO2 exchange, structure, and biochemistry differed, both among species and within species between seasons, to match the changing light environment below the forest canopy. Viola exhibited remarkable plasticity of photosynthesis across the spring-summer transition. Tiarella acclimated both from spring to summer, and from summer to fall. Modeling of seasonal assimilation, and an analysis of whole-plant growth, demonstrated that acclimation allowed Viola and Tiarella to exploit the high light intensities of spring and fall (Tiarella only), without going into negative carbon balance in the deep shade of summer. These results demonstrate the importance of brief, seasonal periods of high light availability to the growth of deciduous forest herbs, even to shade-tolerant species that are primarily associated with the shaded conditions of summer.; There were dramatic disjunctions between the physiology of C assimilation and that of N capture in Tiarella and Allium. The most striking example of this was in Allium, which took up the bulk of its yearly N increment in the summer months when it was leafless. These data suggest that Allium should only be considered a spring ephemeral in terms of photosynthetic C assimilation. Finally, contrary to the "vernal dam" hypothesis, I found no evidence that spring ephemerals in this northern hardwood forest have a significant influence on ecosystem-level patterns of N-cycling and loss. Instead, it appears that immobilization of N by soil microrganisms is the primary mechanism of springtime N retention in this ecosystem. |
