Improving plant water use efficiency (WUE) has the potential to lower plant susceptibility to drought. Amending soils with biochar has been suggested as a way to improve WUE, as it has been shown to increase the water holding capacity of soils. Here, we investigated the influence of two different biochar soil amendments on WUE measured by gas exchange and carbon isotopes of pine-oak ecosystem species. We measured WUE of individuals grown in soils where either lignocellulose or hemicellulose biochar was applied (10% v/v). WUE increased under lignocellulose, but not hemicellulose, biochar amendment during both late spring and early fall measurement periods in a single year study. However, net photosynthesis and stomatal conductance decreased in plants grown with biochar amendments. Physiochemical and sorption data provide a partial explanation of how biochar mechanisms impact soil-water-plant relations. Our results demonstrated that lignocellulosic biochar may be added to forest soils to reduce drought stress in pine-oak systems, but amendments may not lead to increases in carbon uptake rates.