Poplar trees used for biomass production need to be both productive and efficient in using water. Researchers studied different poplar genotypes to see how well they balance growth and water use. They found that genotypes from cooler areas were more efficient in using water, even with a lot of leaves. Some genotypes lose less water at night, affecting their overall water use efficiency. When faced with drought, the trees improved their water use efficiency at the leaf level, but not overall. This suggests that other factors like respiration may offset the benefits of improved water use efficiency during drought.