We investigated the intra-annual variability and environmental controls over transpiration (E) in a planted, even-aged (58 years; 537 trees ha−1), experimental forest of invasive native Juniperus virginiana in the Nebraska Sandhills, with three canopy classes (dominant, co-dominant, and suppressed) by using sap flux techniques, in a year where drought was absent (2008, 34% above average precipitation). Daily E was closely linked to growing-season length and variability in the environment. Minimum and average daily air temperatures, photosynthetically active radiation, and precipitation explained the majority of the intra-annual daily variability in E. Vapour pressure deficit was a significant factor in spring and summer, shallow volumetric soil water content (VSWC 0·2 m) was important during summer particularly June, and deep VSWC (0·6 m) was a significant factor in January and August. E was highest in the dominant trees and contributed to the majority (~77%) of stand transpiration (Ec) on site because of their larger canopy size, greater tree density, more leaf area, and accessibility to water resources compared with the co-dominant and suppressed tree canopies, which contributed to 16% and 7%, respectively. Ec averaged ~413 mm year−1, corresponding to ~24% of potential evapotranspiration. Soils were significantly drier in the J. virginiana stand than in adjacent C4-dominated grasslands, which could be due to the longer growing season over which physiological activity extends in J. virginiana compared with C4-dominated grasslands in the region and precipitation interception by the canopy and forest floor, which evaporates before reaching the soil. Copyright © 2012 John Wiley & Sons, Ltd.