Temperature and air pollution affected tree ring δ¹³C and water-use efficiency of pine and oak trees under rising CO₂ in a humid temperate forest

To better predict forest productivity under rising atmospheric CO₂ concentration ([CO₂]), it is critical to understand how intrinsic water-use efficiency (WUE_i) and its relationship with tree growth are affected by the concomitant changes in environmental conditions such as precipitation, temperature, and air pollution that either enhance or undermine any potential CO₂ fertilization effect. We investigated changes in δ¹³C and WUE_i in annual rings and basal area increment (BAI) of Pinus densiflora (from 1968 to 2007) and Quercus variabilis (from 1970 to 2007) trees in relation to precipitation, temperature, and air pollution in a humid temperate forest. The WUE_i of P. densiflora increased by 39.9%, whereas that of Q. variabilis did not change over time in the study period. The WUE_i was not affected by precipitation for both species but increased (P<0.001) with temperature for P. densiflora and with SO₂ emissions for both species. Multiple regression models suggested that the effect of [CO₂] on tree growth was much higher than temperature; however, for the period (1998 to 2007) when SO₂ emissions data were available, SO₂ emission was the driver of changes in BAI and WUE_i, and temperature effects became stronger than [CO₂]. Overall, BAI and WUE_i were positively (P<0.001) correlated for P. densiflora, but not for Q. variabilis. We conclude that temperature and air pollution rather than precipitation were key determinants of WUE_i at the study site and that the two species had contrasting responses to environmental changes.