The European MOTTLES project set-up a new-generation network for ozone (O₃) monitoring in 17 plots in France, Italy and Romania. These monitoring stations allowed: (1) estimating the accumulated exposure AOT40 and stomatal O₃ fluxes (PODY) with an hourly threshold of uptake (Y) to represent the detoxification capacity of trees (POD1, with Y = 1 nmol O₃ m⁻² s⁻¹ per leaf area); and (2) collecting data of forest-response indicators, i.e. crown defoliation and visible foliar O₃-like injury over the time period 2017–2019. The soil water content was the most important parameter affecting crown defoliation and was a key factor affecting the severity of visible foliar O₃-like injury on the dominant tree species in a plot. The soil water content is thus an essential parameter in the PODY estimation, particularly for water-limited environments. An assessment based on stomatal flux-based standard and on real plant symptoms is more appropriated than the exposure-based method for protecting vegetation. From flux-effect relationships, we derived flux-based critical levels (CLef) for forest protection against visible foliar O₃-like injury. We recommend CLef of 5 and 12 mmol m⁻² POD1 for broadleaved species and conifers, respectively. Before using PODY as legislative standard in Europe, we recommend using the CLec for ≥ 25% of crown defoliation in a plot: 17,000 and 19,000 nmol mol⁻¹ h AOT40 for conifers and broadleaved species, respectively.

As determined through 11 subcultures of Taxus cuspidata callus, the growth in FW and level of Taxol production were stable by the ninth generation.