Minisymposium 20: Global Change

Abs # 34004: Growth and photosynthesis of two silver birch clones exposed to elevated CO2 and O3: three- year field experiment

Presenter:	Riikonen, Johanna , johanna.riikonen@uku.fi
Authors	Riikonen, Johanna  (A) (B)  Holopainen, Toini  (A)   Oksanen, Elina  (A)   Vapaavuori, Elina  (B)
Affiliations:	(A): University of Kuopio, Department of Ecology and Environmental Science (B): Finnish Forest Research Institute, Suonenjoki Research Station

Two clones of 8-year-old silver birch (Betula pendula Roth) were exposed to elevated CO2 and O3 concentrations during the growing seasons 1999-2001, using open top chambers. In this field experiment the treatments were: outside control, chamber control, 2*ambient CO2, 2* ambient O3, and 2*ambient CO2 + 2* ambient O3, replicated 4 times. Growth, gas exchange and photosynthesis related biochemical properties of clones 4 and 80 were studied. The aims of the study were to examine: whether silver birch benefits from increasing CO2 in the atmosphere as anticipated, whether elevated O3 causes damage and growth losses and whether CO2 can compensate for the negative effects of O3 if such losses exist. The clones responded differently to the treatments. In clone 80, total biomass increased by 40% under elevated CO2 but was not affected by elevated O3. In clone 4, total biomass reduced by 26% under elevated O3, but no response to elevated CO2 was found. In clone 4, the negative effect of O3 was mediated by reduced root growth and by a tendency to reduced leaf area and accelerated leaf abscission. Elevated CO2 protected the trees of clone 4 from negative effects of O3, possibly by increasing the amount of carbohydrates available for detoxification and repair. Biomass allocation was not affected by the treatments, except under elevated O3 which decreased allocation to roots in clone 4. Net photosynthesis was higher in clone 80 than in clone 4, due to higher stomatal conductance and amount and activity of Rubisco. When measured at growth CO2 concentration, net photosynthesis increased under elevated CO2 treatments on an average by 23 % and 43% in clones 80 and 4, respectively. The discrepancy between growth and net photosynthesis in response to elevated CO2 may be due to differences in leaf area between the clones that increased in clone 80 but was unaffected in clone 4. Downregulation of photosynthesis was evident in both clones under elevated CO2 treatments. This was related to decreased amount and activity of Rubisco, reduced N concentration and to increased C:N ratio and accumulation of starch in the leaves. In clone 4 elevated O3 reduced the amount of Rubisco and chlorophyll whereas net photosynthesis was not affected in either of the clones.