EFFECTS OF LONG-TERM CO2 AND TEMPERATURE ELEVATION ON CROWN NITROGEN DISTRIBUTION AND DAILY PHOTOSYNTHETIC PERFORMANCE OF SCOTS PINE

Single Scots pines (Pinus sylvestris L.), aged 20-25 years, were grown in open-top chambers and exposed to elevated temperature (Elev. T), e levated CO2 (Elev. C) and a combination of elevated CO2 and temperatur e (Elev. C + T) for 3 years. The vertical distribution of needle nitro gen concentration was measured simultaneously with gas exchange of att ached shoots. Based on the measurements, the dependencies on needle ni trogen concentrations of four photosynthetic parameters, i.e., RuP2 (r ibulose 1,5-bisphosphate)-saturated rate of carboxylation (V-cmax), ma ximum potential electron transport (J(max)), the rate of respiration i n the light (R-d) and light-use-efficiency factor (delta), were determ ined. Using a crown multilayer model, the performance of daily crown p hotosynthesis in Scots pine was predicted. Compared to the control tre atment, the mean concentration of nitrogen in the foliage decreased by 20% and by 17% for trees grown under Elev. C and under Elev. C + T, r espectively, but increased by 4% for trees grown under Elev. T. Howeve r, the total content of foliage nitrogen per unit ground area increase d by 25% for trees grown under Elev. C, by 19% for trees grown under E lev. C + T and by 6% for trees grown under Elev. T; these were due to the increase in the total needle area index. Regressions showed that t he foliage grown under Elev. C and Elev. C + T had steeper slopes repr esenting the responses of V-cmax, and R-d and delta to leaf nitrogen c oncentrations, while Elev. C + T and Elev. T had steeper slopes repres enting the response of J(max) to needle nitrogen concentrations. Predi ctions showed that, on a typical sunny day, the daily total of crown p hotosynthesis increased 22% and 27%, separately for Elev. C and Elev. C + T, and by only 9% for Elev. T alone. Furthermore, the increased da ily crown photosynthesis, resulting from treatments involving elevated CO2, can be attributed mainly to an increase in the ambient CO2 conce ntration and the needle area index, while modification of the intrinsi c photosynthetic capacity had only a marginal effect. Based on the cur rent pattern of crown nitrogen allocation, the prediction showed also that the relationship between daily crown photosynthesis and crown nit rogen content was strongly dependent on the daily incident PAR and air temperature. The CO2-elevated treatments led to an increase in the se nsitivity of daily crown photosynthesis to changes in crown nitrogen c ontent, daily incident PAR and temperature, while the temperature-elev ated treatment had the opposite effect on the sensitivity. (C) 1997 El sevier Science B.V.