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

Abstract

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.