The proposed research will examine basic physiological processes controlling plant growth and water used in native plants under conditions of fluctuating sunlight. Most previous studies have been done under constant sunlight but both native and agricultural plants are more commonly exposed to variations in sunlight due to clouds or other plants shading each other. Basic knowledge of how plants respond to fluctuating levels of sunlight in terms of their growth and water use is crucial for our understanding of native plant and crop productivity. The studies will build upon patterns of responses already documented and provide a mechanistic interpretation of them. No-steady state plant responses to humidity, reduced water availability, and temperature will be completed. Such a mechanistic understanding will be useful to basic ecologists as well as use in crops plants. The use of three different ecosystems for study sites (subalpine, desert, tallgrass prairie) will broaden the applicability of the results.