The seasonal phenology of the leaf area index (LAI) is a major determinant of net ecosystem production in deciduous forest ecosystems. This study describes seasonal and inter-annual differences in LAI in a boreal aspen-hazelnut forest in central Saskatchewan, Canada, between 1994 and 2003, and relates the differences in LAI to annual net ecosystem production (FNEP). A robust method is developed to fill gaps in the annual LAI cycle from systematic but sparse measurements using associated radiation and temperature indices. The ratio of the photosynthetically-active radiation reflectance to the shortwave reflectance is shown to have a particularly distinct LAI signature.Optical estimates of the fully-leafed LAI agreed well with measurements from autumn litterfall and showed moderate inter-annual variability for the trembling aspen overstory (mean ± S.D. of 2.44 ± 0.30) and the hazelnut understory (1.98 ± 0.44). Two features of the annual LAI cycle differed among years—the timing of leaf emergence in spring, which varied by up to four weeks, and the fully-leafed value for LAI, which varied between 3.66 and 5.22. The timing of leaf senescence in autumn was nearly constant among years. The seasonal cycles of FNEP and LAI were tightly coupled and the correspondence between their respective inter-annual differences was remarkable, particularly during leaf emergence in spring. Annual FNEP was positively correlated with the canopy duration and the annual maximum LAI, with increases in annual FNEP of 6.9 g C m−2 for each additional day in full leaf and 83 g C m−2 for each additional unit of LAI.