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

• The effect of O exposure or uptake on carbon acquisition (net assimilation (A) or gross photosynthesis (P )), 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 generally declined with cumulative O exposure or uptake at all sites. As a response variable, P was slightly more sensitive than A to cumulative O exposure. As a metric, O 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 exposure or uptake to reduce P . The general additive model allows the user to define a deleterious level of cumulative O exposure or uptake, and to quantitatively assess biological response.