Carbon dioxide and ozone affect needle nitrogen and abscission in Pinus ponderosa

Summary

To determine whether CO₂ (a key contributor to climate change), and O₃ (an important air pollutant), interact to affect needle N dynamics; we grew Pinus ponderosa Dougl. ex Laws. seedlings for three seasons (1998–2000) in outdoor chambers with ambient or elevated CO₂ combined with low or elevated O₃. Nitrogen concentration (area basis) was measured for attached and abscised needles. Nitrogen retranslocation was determined by comparing N for attached needles in July vs. October, and N resorption by comparing N for needles which abscised from July to October vs. attached needles in October. Ozone, and to a lesser extent CO₂, reduced needle N depending on age class and sampling date, but there were no CO₂ × O₃ interactions. There was a significant CO₂ × O₃ interaction for N retranslocation for 1998 needles in 1999 (O₃ reduced higher retranslocation with elevated CO₂), but CO₂ and O₃ had only marginal effects on N retranslocation or resorption for other needles. Abscission of year-old needles increased with elevated CO₂, but not O₃. Long-term studies are needed to determine if the responses to elevated CO₂ and O₃ in our seedlings persist as trees mature and have multiple age-classes of needles.