Branch growth and gas exchange in 13 - year - old loblolly pine ( Pinus taeda ) trees in response to elevated carbon dioxide concentration and fertilization CHRIS

We used whole-tree, open-top chambers to expose 13-year-old loblolly pine (Pinus taeda L.) trees, growing in soil with high or low nutrient availability, to either ambient or elevated (ambient + 200 µmol mol –1) carbon dioxide concentration ([CO 2 ]) for 28 months. Branch growth and morphology , foliar chemistry and gas exchange characteristics were measured periodically in the upper, middle and lower crown during the 2 years of exposure. Fertilization and elevated [CO 2 ] increased branch leaf area by 38 and 13%, respectively, and the combined effects were additive. Fertilization and elevated [CO 2 ] differentially altered needle lengths, number of fascicles and flush length such that flush density (leaf area/flush length) increased with improved nutrition but decreased in response to elevated [CO 2 ]. These results suggest that changes in nitrogen availability and atmospheric [CO 2 ] may alter canopy structure, resulting in greater foliage retention and deeper crowns in lob-lolly pine forests. Fertilization increased foliar nitrogen concentration (N M), but had no consistent effect on foliar leaf mass (W A) or light-saturated net photosynthesis (A sat). However, the correlation between A sat and leaf nitrogen per unit area (N A = W A N M) ranged from strong to weak depending on the time of year, possibly reflecting seasonal shifts in the form and pools of leaf nitrogen. Elevated [CO 2 ] had no effect on W A , N M or N A , but increased A sat on average by 82%. Elevated [CO 2 ] also increased photosynthetic quantum efficiency and lowered the light compensation point, but had no effect on the photosyn-thetic response to intercellular [CO 2 ], hence there was no accli-mation to elevated [CO 2 ]. Daily photosynthetic photon flux density at the upper, middle and lower canopy position was 60, 54 and 33%, respectively, of full sun incident to the top of the canopy. Despite the relatively high light penetration, W A , N A , A sat and R d decreased with crown depth. Although growth enhancement in response to elevated [CO 2 ] was dependent on fertilization , [CO 2 ] by fertilization interactions and treatment by canopy position interactions generally had little effect on the physiological parameters measured.