Why does needle photosynthesis decline with tree height in Norway spruce?

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.

Räim O ，Kaurilind E ，Hallik L ，Merilo E ... -  《-》

Stomatal conductance alone does not explain the decline in foliar photosynthetic rates with increasing tree age and size in Picea abies and Pinus sylvestris.

Niinemets U 《TREE PHYSIOLOGY》

Height-related decreases in mesophyll conductance, leaf photosynthesis and compensating adjustments associated with leaf nitrogen concentrations in Pinus densiflora.

Han Q 《-》

Fine-scale geographic variation in photosynthetic-related traits of Picea glauca seedlings indicates local adaptation to climate.

Climate-related variations in functional traits of boreal tree species can result both from physiological acclimation and genetic adaptation of local populations to their biophysical environment. To improve our understanding and prediction of the physiological and growth responses of populations to climate change, we studied the role of climate of seed origin in determining variations in functional traits and its implications for tree improvement programs for a commonly reforested boreal conifer, white spruce (Picea glauca (Moench) Voss). We evaluated growth, root-to-shoot ratio (R/S), specific leaf area (SLA), needle nitrogen (N(mass)), total non-structural carbohydrates (NSC) and photosynthetic traits of 3-year-old seedlings in a greenhouse experiment using seed from six seed orchards (SO) representing the different regions where white spruce is reforested in Québec. Height and total dry mass (TDM) were positively correlated with photosynthetic capacity (A(max)), stomatal conductance (g(s)) and mesophyll conductance (g(m)). Total dry mass, but not height growth, was strongly correlated with latitude of seed origin (SO) and associated climate variables. A(max), g(s), g(m) and more marginally, photosynthetic nitrogen-use efficiency (PNUE) were positively associated with the mean July temperature of the SO, while water use efficiency (WUE) was negatively associated. Maximum rates of carboxylation (V(cmax)), maximum rates of electron transport (J(max)), SLA, N(mass), NSC and R/S showed no pattern. Our results did not demonstrate a higher Amax for northern seed orchards, although this has been previously hypothesized as an adaptation mechanism for maintaining carbon uptake in northern regions. We suggest that gs, gm, WUE and PNUE are the functional traits most associated with fine-scale geographic clines and with the degree of local adaptation of white spruce populations to their biophysical environments. These geographic patterns may reflect in situ adaptive genetic differences in photosynthetic efficiency along the cline.

Benomar L ，Lamhamedi MS ，Villeneuve I ，Rainville A ，Beaulieu J ，Bousquet J ，Margolis HA ... -  《-》

Age-related effects on leaf area/sapwood area relationships, canopy transpiration and carbon gain of Norway spruce stands (Picea abies) in the Fichtelgebirge, Germany.

Köstner B ，Falge E ，Tenhunen JD 《TREE PHYSIOLOGY》