O3 uptake and drought stress effects on carbon acquisition of ponderosa pine in natural stands

The effect of O-3 exposure or uptake on carbon acquisition (net assimilation (A) or gross photosynthesis (P-g)), with and without drought stress, is reported here in 40-yr-old-ponderosa pine (Pinus ponderosa) trees. Maximum daily gas exchange was measured monthly for 12 trees at four sites differing in pollutant exposure over two growing seasons with above- and below-average annual precipitation. Gas exchange measures were estimated between sampling periods using a generalized additive regression model. Both A and P-g generally declined with cumulative O-3 exposure or uptake at all sites. As a response variable, P-g was slightly more sensitive than A to cumulative O-3 exposure. As a metric, O-3 uptake vs exposure permitted slightly better statistical resolution of seasonal response between sites. The effect of late summer drought stress was statistically significant only at the moderate pollution site, and combined synergistically with O-3 exposure or uptake to reduce P-g. The general additive model allows the user to define a deleterious level of cumulative O-3 exposure or uptake, and to quantitatively assess biological response. (C) New Phytologist (2002) 154: 621-631.