The influence of intensive grazing on canopy microclimate, grass leaf water relations and community evapotranspiration was investigated by comparing a grassland heavily grazed by a colony of black-tailed prairie dogs (Cynomys ludovicianus) with an adjacent lightly-grazed, uncolonized area in a northern-mixed prairie in South Dakota. Total above-ground plant biomass was lower on the colony through the growing season. Daytime leaf and air temperatures were higher on the colony. Although daytime air vapor pressure in the canopy was higher on the colony, higher leaf temperatures resulted in higher leaf-to-air vapor-pressure differences on the colony. Along with higher vapor-pressure differences, higher wind speeds within the canopy probably also contributed to higher evaporative demand. In spite of this, the dominant grasses, Agropyron smithii and Bouteloua gracilis, had consistently higher leaf conductances to water vapor and water potentials (ψ) on the colony, especially later in the day and growing season. Predawn leaf ψ were higher on the colony in July and August, suggesting available soil moisture in the rooting zone was higher there. At the community level, afternoon evapotranspiration rates were lower on the colony in June, but higher on the colony in July and August as soil water became more limiting. While intensive grazing by prairie dogs lead to a warmer canopy microclimate with higher evaporative demand, any detrimental effects on plant water status were apparently offset by greater available soil moisture, especially later in the day and season. Greater soil water availability on the colony was probably the result of smaller transpiring leaf mass (and area), although soil factors such as improved infiltration due to prairie dog burrows can not be ruled out as causative factors.
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