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

Abstract

The effect of O3 exposure or uptake on carbon acquisition (net assimilation (A) or gross photosynthesis (Pg)), 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 Pg generally declined with cumulative O3 exposure or uptake at all sites. As a response variable, Pg was slightly more sensitive than A to cumulative O3 exposure. As a metric, O3 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 O3 exposure or uptake to reduce Pg. The general additive model allows the user to define a deleterious level of cumulative O3 exposure or uptake, and to quantitatively assess biological response. © New Phytologist (2002).