To improve our predictive understanding of daily total evapotranspiration (ET), we quantified the differential impact of environmental drivers, radiation (Q), and vapor pressure deficit (D) in a wetland and upland forest. Latent heat fluxes were measured using eddy covariance techniques, and data from four growing seasons were used to test for (1) environmental drivers of ET between the sites, (2) interannual differences in ET responses to environmental drivers, and (3) changes in ET responses to environmental drivers between the leaf expansion period and midsummer. Two simple ET models derived from coupling theory, one radiation-based model, and another using mass transfer were used to examine the mechanisms underlying the drivers of ET. During summer months, ET from the wetland was driven primarily by Q, whereas it was driven by D in the upland. During the leaf expansion period in the upland forest the dominant driver was Q. ET from the wetland was linearly related to net radiation using coupling coefficients ranging from a low of 0.3–0.6 to a high of 1.0 between early May and midsummer. Interannually, ET from the upland forest exhibited near linear responses to D, with an effective reference canopy stomatal conductance varying from 1 to 5 mm s−1. The results show that ET predictions in northern Wisconsin and other mixed wetland-upland forests need to consider both wetland and upland forest processes. Furthermore, leaf phenology effects on ET represent a knowledge gap in our understanding of seasonal environmental drivers.