Inter-annual variability in the leaf area index of a boreal aspen-hazelnut forest in relation to net ecosystem production

Abstract

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 ( F NEP). 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 F NEP and LAI were tightly coupled and the correspondence between their respective inter-annual differences was remarkable, particularly during leaf emergence in spring. Annual F NEP was positively correlated with the canopy duration and the annual maximum LAI, with increases in annual F NEP 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.