دانلود رایگان مقاله بررسی نقش محدودیت آب خاک در تعیین آستانه دوز ازون سیتوتوکسیک

Abstract

Phytotoxic Ozone Dose (PODY), defined as the accumulated stomatal ozone flux over a threshold of Y, is considered an optimal metric to evaluate O3 effects on vegetation. PODY is often computed through the DO3SE model, which includes species-specific parameterizations for the environmental response of stomatal conductance. However, the effect of soil water content (SWC) on stomatal aperture is difficult to model on a regional scale and thus often ignored. In this study, we used environmental input data obtained from the WRF-CHIMERE model for 14,546 grid-based forest sites in Southern Europe. SWC was obtained for the upper 10 cm of soil, which resulted in a worst-case risk scenario. PODY was calculated either with or without water limitation for different Y thresholds. Exclusion of the SWC effect on stomatal fluxes caused a serious overestimation of PODY. The difference increased with increasing Y (78%, 128%, 237% and 565% with Y = 0, 1, 2 and 3 nmol O3 m−2 s−1, respectively). This behaviour was confirmed by applying the same approach to field data measured in a Mediterranean Quercus ilex forest. WRF-CHIMERE overestimated SWC at this field site, so under real-world conditions the SWC effect may be larger than modelled. The differences were lower for temperate species (Pinus cembra 50–340%, P. sylvestris 57–363%, Abies alba 57–371%) than for Mediterranean species (P. pinaster 87–356%, P. halepensis 96–429%, P. pinea 107–532%, Q. suber 104–1602%), although a high difference was recorded also for the temperate species Fagus sylvatica with POD3 (524%). We conclude that SWC should be considered in PODY simulations and a low Y threshold should be used for robustness.

5. Conclusions

Flux-based risk assessment of O3 effects on vegetation is gradually superseding the exposure-based assessment. Ideally, O3 risk assessment maps should be generated from measurements of stomatal O3 flux (Tuovinen et al., 2009). However, data sets suitable for this are scarce (e.g. Tuovinen et al., 2004, 2007; Fares et al., 2013b) and thus the regional-scale assessment of O3 risk to vegetation is dependent on modelling. By using the DO3SE flux model and modelled input data for 14,546 grid-based Southern European forest sites distributed in temperate and Mediterranean climates, we demonstrated that the differences between the accumulated stomatal O3 flux (PODY) values calculated either with or without SWC limitation (i) were significant, (ii) strongly increased with increasing flux thresholds, and (iii) increased from temperate to Mediterranean vegetation/climate. The quantification of these differences depends on the accuracy of the stomatal conductance model and of the meteorological/hydrological model used for producing the SWC and other input for the stomatal model. As the soil moisture model employed here overestimated SWC relative to the measurements at an Italian forest site, the magnitude of differences can be higher under real-world conditions than what our model results suggest. We therefore recommend to (i) include the soil water content (fSWC) function in any PODY simulation, in order to avoid significant overestimation of O3 risk to vegetation, in particular in water-limited environments, as the differences were higher for Mediterranean vegetation than for temperate vegetation; and (ii) use the a low Y threshold in O3 risk assessment due to the increasing sensitivity of PODY to inputs with increasing Y.