| This doctoral thesis focuses on identifying environmental drivers of the variability in leaf area index (LAI) and, more in particular, focuses on the determinants of maximal, temporal, spatial and age-related LAI. Therefore, homogeneous even-aged stands of three tree species (i.e. pedunculate oak, European beech, and Scots pine) were analysed in 2008;The first three chapters deal with the environmental determinants of LAI in mature forest stands. First, the relationships between LAI and environmental determinants were assessed, tested, and validated on a European dataset (CarboEurope-IP) at the time of the highest measured LAI value. Species-specific models of maximum LAI for beech and oak were primarily driven by management practices affecting stand characteristics and tree growth. Scots pine models were more affected by site quality and meteorological variables. The beech meteorological model showed very good agreement with LAI at several European sites. Scots pine's stand model predicted well LAI across Europe. Second, temporal LAI development was studied in mature beech and oak stands. Three distinct phases of LAI development, highly similar throughout all stands and for both species, were identified: (i) budbreak and leaf unfolding (early spring), (ii) constant LAI (for 3 weeks), and (iii) LAI increase until the timing of attainment of maximum LAI (mid-July). The main drivers of the intra-seasonal LAI development were the meteorological variables. They explained the stagnating LAI during the second phase. In addition, they also explained the significant LAI development of beech (radiation) and oak (air temperature). Third, spatial LAI was affected by the high crown variability within small-scale plots. Therefore, a large number of samples were necessary to estimate the average stand LAI. We proposed a new sampling strategy: instead of a random approach, we proposed a scattered sampling protocol as a function of the local canopy pattern.;In the final chapter, we studied the age-related changes in LAI for young beech and oak plantations, from initial canopy closure until first thinning. LAI development was related to stand characteristics and site quality variables. Oak plots showed an increase in LAI with stand development, while beech plots expressed a negative relation between LAI and stand development. |
