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Summary• The long-term response of leaf photosynthesis to rising CO 2 concentrations [CO 2 ] depends on biochemical and morphological feedbacks. Additionally, responses to elevated [CO 2 ] might depend on the nutrient availability and the light environment, affecting the net carbon uptake of a forest stand.• After 6 yr of exposure to free-air CO 2 enrichment (EUROFACE) during two rotation cycles (with fertilization during the second cycle), profiles of light, leaf characteristics and photosynthetic parameters were measured in the closed canopy of a poplar ( Populus ) short-rotation coppice.• Net photosynthetic rate ( A growth ) was 49% higher in poplars grown in elevated [CO 2 ], independently of the canopy position. J max significantly increased (15%), whereas leaf carboxylation capacity ( V cmax ), leaf nitrogen (N a ) and chlorophyll (Chl a ) were unaffected in elevated [CO 2 ]. Leaf mass per unit area (LMA) increased in the upper canopy. Fertilization created more leaves in the top of the crown.• These results suggest that the photosynthetic stimulation by elevated [CO 2 ] in a closed-canopy poplar coppice might be sustained in the long term. The absence of any down-regulation, given a sufficient sink capacity and nutrient availability, provides more carbon for growth and storage in this bioenergy plantation.

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Needle nitrogen partitioning and photosynthesis of Norway spruce were studied in a forest chronosequence in Järvselja Experimental Forest, Estonia. Current-and previous-year shoots were sampled from upper and lower canopy positions in four stands, ranging in age from 13 to 82 years. A/c i curves were determined to obtain maximum carboxylation rate (V cmax ) and maximum rate of electron transport (J max ), whereas needle nitrogen partitioning into carboxylation (P R ), bioenergetics associated with electron transport (P B ) and thylakoid light harvesting components (P L ) was calculated from the values of V cmax , J max and leaf chlorophyll concentration. The greatest changes in studied needle characteristics took place between tree ages of 13 and 26 years, and this pattern was independent of needle age and canopy position. Needle mass per projected area (LMA) was lowest in the 13-year-old stand and mass-based nitrogen concentration (N M ) was generally highest in that stand. The values of LMA were significantly higher and those of N M lower in the 26-year-old stand. Mass-based V cmax and J max were highest in the 13-year-old stand. Area-based photosynthetic capacity was independent of tree age. The proportion of photosynthetic nitrogen (P R , P B and P L ) was highest and that of non-photosynthetic nitrogen lowest in the 13-year-old stand. Current-year needles had lower LMA and P L , but higher photosynthetic capacity compared to 1-year-old foliage. Needles from lower canopy positions exhibited lower LMA, area-based nitrogen concentration and photosynthetic capacity than needles from upper canopy. The period of substantial reductions in needle photosynthetic capacity and changes in nitrogen partitioning coincides with the onset of reproductive phase during tree ontogeny.

Needle morphological, chemical and physiological characteristics of Norway spruce were studied in a forest chronosequence in Järvselja Experimental Forest, Estonia. Current-year shoots were sampled from upper canopy positions in five stands, ranging in height from 1.8 to 33.0 m (corresponding age range was 10-85 years). A/C(i) curves were determined to obtain maximum carboxylation rates (V(cmax)) and maximum rates of electron transport (J(max)). Needle nitrogen (N) partitioning into photosynthetic functions was calculated from the values of V(cmax), J(max) and leaf chlorophyll concentration. All needle size parameters (length, width, thickness, volume and cross-sectional areas of mesophyll and xylem) increased significantly with tree height. The needles of taller trees had lower mass-based N and chlorophyll concentrations (21% and 43% difference between shortest and tallest stands, respectively), but higher dry mass per area (35%), dry mass per volume (18%), number of cells per mesophyll cross-section area (40%) and partitioning of N into non-photosynthetic functions (12%). Light saturated net assimilation rate, V(cmax), J(max) and stomatal conductance decreased with tree age (35%, 16%, 12% and 29% difference, respectively). A path analysis model describing tree age-related reduction of photosynthetic capacity as a result of sink limitation provided the best fit to our data. However, since the path model corresponding to source limitation, where photosynthetic reduction derives from changes in needle structure and chemistry was not rejected, we conclude that the decline in photosynthesis with tree age results from several mechanisms (limited sink strength, stomatal and N limitation) operating simultaneously and sequentially.