Developing Ozone Risk Assessment for Larch Species

Ozone (O-3) risk assessment for the protection of forests requires species-specific critical levels (CLs), based on either O-3 concentrations (AOT40) or stomatal uptake (PODY) accumulation over the growing season. Larch (Larix sp.) is a genus with O-3-susceptible species, widely distributed in the northern hemisphere and with global economic importance. We analyzed published and unpublished data of Japanese larch (Larix kaempferi) and its hybrid F-1 (Larix gmelinii var. japonica x L. kaempferi) stomatal responses for developing a parameterization of stomatal conductance model and estimating PODY-based CLs with two Y thresholds, that is, 0 and 1 nmol m(-2) s(-1) projected leaf area (PLA). In parallel, we estimated AOT40-based CLs. The results show that the AOT40-based CLs for a 2% and 4% biomass loss in Japanese larch were 5.79 and 11.59 ppm h, that is, higher than those in hybrid larch F-1 (2.18 and 4.36 ppm h AOT40), suggesting a higher O-3 susceptibility of the hybrid. However, the use of PODY reconciled the species-specific differences, because the CLs were similar, that is, 9.40 and 12.00 mmol m(-2) POD0 and 2.21 and 4.31 mmol m(-2) POD1 in Japanese larch versus 10.44 and 12.38 mmol m(-2) POD0 and 2.45 and 4.19 mmol m(-2) POD1 in the hybrid, for 2% and 4% biomass loss, respectively. Overall, the CLs were lower than those in other forest species, which suggests a relatively high susceptibility of these larches. These results will inform environmental policy-makers and modelers about larch susceptibility to O-3