Adaptive significance of C partitioning and regulation of specific leaf area in Betula pendula
Carbon allocation and regulation of specific leaf area (σ) define key processes underlying the adaptation of plants to varying habitats. In this study, the general principles governing adaptation and a dynamic optimality model of plant adaptation are reviewed. The central new elements of this model are: (i) differential root carbon costs for maintaining a defined nutrient status; (ii) a simple formula for optimal σ at steady-state as a function of nitrogen (N) status and irradiance; and (iii)
... neric rules for the time propagation of adapting traits. The model was applied to a large data set compiled by Ingestad et al. (1995) and McDonald et al. (1986a McDonald et al. ( , 1986b for birch seedlings (Betula pendula Roth) during stationary logarithmic growth and during transient changes in response to a range of irradiances and nutrient supply rates. In the stationary case, large variations in the fraction of leaf dry mass to total dry mass ( f L ), σ and N concentration were simulated with high accuracy. The independently calibrated model described the temporal response of seedlings following a sharp decrease in N supply, which includes phenomena such as the temporary C accumulation in leaves and damped oscillations in N concentration. Dynamics in σ were more sensitive to variation in light than in N supply. Nevertheless, adaptive adjustments in f L , σ and N concentration were strongly coupled, underlining the relevance of a whole-plant perspective when modeling plant growth and regulation. The high coincidence between model calculations and measured values supports the notion that plant acclimation can be both understood and predicted as a growth-optimizing mechanism.