Inter-annual variability in the leaf area index of a boreal aspen-hazelnut forest in relation to net ecosystem production Alan G. Barr a, * , T.A. Black b , E.H. Hogg c , N. Kljun b , K. Morgenstern b , Z. Nesic b a Climate Research Branch, Meteorological Service of Canada, 11 Innovation Blvd., Saskatoon, SK, Canada, S7N 3H5 b Department of Agroecology, University of British Columbia, Vancouver, Canada c Canadian Forest Service, Edmonton, Canada Received 20 April 2004; accepted 26 June 2004 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. Crown Copyright # 2004 Published by Elsevier B.V. All rights reserved. Keywords: Leaf area index; Net ecosystem production; Boreal deciduous forest; Inter-annual variability 1. Introduction The seasonal cycle of leaf area index (LAI) in deciduous forest ecosystems is dynamic and has a major influence on the global carbon, water and radiation balances. The timing of leaf emergence and senescence are major determinants of net ecosystem production (F NEP ) in boreal and temperate deciduous forests (Goulden et al., 1996; Black et al., 2000; Barr et al., 2002; Saigusa et al., 2003). Leaf emergence and senescence also affect climate at the regional scale www.elsevier.com/locate/agrformet Agricultural and Forest Meteorology 126 (2004) 237–255 * Corresponding author. E-mail address: alan.barr@ec.gc.ca (A.G. Barr). 0168-1923/$ – see front matter. Crown Copyright # 2004 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2004.06.011