Greater early season primary production in lowland vs. upland sites has been documented in a C4 dominated grassland in eastern Kansas (USA). Since lower production in uplands is probably not due to differences in water stress between sites at this time of year, an alternative hypothesis tested was that low air temperatures limit early season production in grassland and reduced windspeeds at lowland sites may allow leaf temperatures to be nearer their optimum for growth compared to leaves at exposed upland sites. Numerous microclimatic and physiological measurements were made early in the growing season at a lowland and at two upland sites (one shielded form the wind) in an eastern Kansas tallgrass prairie to test this hypothesis. During May, leaf temperatures of Andropogon gerardii, the dominant C4 grass in this grassland, were as much as 6 degrees Celcius greater (and windspeeds approximately 50% less) in the lowland and shielded plots compared to the upland plot. These higher leaf temperatures were attributed to reduced convective cooling of the grasses in the low wind plots. No substantial differences in water stress were detected between sites but greater stomatal conductance to water vapor diffusion was measured in lowland plants. The photosynthetic temperature response of A. gerardii indicated strong temperature limitation to carbon uptake at typically early season air temperatures. The net result of shielding an upland plot from wind was a significant increase in total aboveground primary production and shoot mass of A. gerardii compared to the unshielded upland plot. Furthermore, production in the shielded plot was similar to that in the lowland for the early season. Thus, greater early season production in lowlands in this grassland which cannot be attributed to increased moisture availability (important later in the season) may be due to a more favorable early season microclimate in lowlands for growth of these warm season grasses. Key words: Leaf temperature, Microclimate, Net photosynthesis, Primary production, Tallgrass prairie, Water relations, Wind