When stomatal flux is predictable from AOT40: results of a 13 years stomatal flux calculation exercise at an Alpine spruce forest with the DO3SE model

The evaluation of the ozone risk for mountain forests in remote areas often starts with the measurements of in situ ozone concentrations by means of passive samplers. Thirteen years (1996-2009) of summer ozone measurements taken with passive samplers in an Alpine spruce forest at Passo Lavazé (Italy, 1750 m a.s.l.) have been used for a stomatal ozone dose estimation exercise by means of a DO3SE type model. The hourly ozone concentrations required by the model have been derived by disaggregation of the weekly concentration means assuming an average daily ozone concentration course typical of the mountain areas located at the same relative elevation of the testing site (Loibl et al., 1994; Gerosa et al. 2007). Meteorological data have been taken from an in situ monitoring station running from 2002. For the years preceding 2002 a mean meteorological season has been estimated by averaging hour by hour all the meteo data from 2002 and 2009. Model simulations have been made for a 28 m tall forest canopy with a constant LAI of 2.94. A maximum stomatal conductance value of 125 mmol m-2 s-1 has been employed as indicated by (UN/ECE 2009). Soil moisture has been estimated by a daily water balance taking into account the soil texture and a rooting depth set to 1 m. The results show a high exposure of forest to ozone (AOT40 of the daylight hours between a minimum of 30,000 ppb•h and a maximum of 87,000 ppb•h) and an important stomatal flux (AFst0) between 36.3 and 49.4 mmol m-2. Even taking into account the detoxifying capacity of the plants, simulated by the introduction of the 1.6 nmol m-2 s-1 instantaneous flux threshold (AFst1.6), the phytotoxycal ozone dose resulted between a minimum of 22.1 and a maximum of 35.7 mmol m-2, well above the critical level of 8 mmol m-2 proposed by the UN/ECE for Norway spruce. The ozone exposure and dose are well linearly correlated (R2 = 0.85), revealing the almost negligible influence of water availability on the stomatal conductance for this site. Conversely, stomatal conductance, and the absorbed ozone dose, seem to be more strongly limited by the air temperature and the VPD values.