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)

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... 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.