Photosynthetic and stomatal conductance responses of Norway spruce and beech to ozone, acid mist and frost--a conceptual model

Ozone visible symptoms and reduced root biomass in the subalpine species Pinus uncinata after two years of free-air ozone fumigation

Concentrations of ozone often exceed the thresholds of forest protection in the Pyrenees, but the effect of ozone on Pinus uncinata, the dominant species in subalpine forests in this mountainous range, has not yet been studied. We conducted an experiment of free-air ozone fumigation with saplings of P. uncinata fumigated with ambient O(3) (AOT40 May-Oct: 9.2 ppm h), 1.5 × O(3amb) (AOT40 May-Oct: 19.2 ppm h), and 1.8 × O(3amb) (AOT40 May-Oct: 32.5 ppm h) during two growing seasons. We measured chlorophyll content and fluorescence, visible injury, gas exchange, and above- and below-ground biomass. Increased exposures to ozone led to a higher occurrence and intensity of visible injury from O(3) and a 24-29% reduction of root biomass, which may render trees more susceptible to other stresses such as drought. P. uncinata is thus a species sensitive to O(3), concentrations of which in the Pyrenees are already likely affecting this species.

Assessing the risk caused by ground level ozone to European forest trees: a case study in pine, beech and oak across different climate regions

Two different indices have been proposed for estimation of the risk caused to forest trees across Europe by ground-level ozone, (i) the concentration based AOT40 index (Accumulated Over a Threshold of 40 ppb) and (ii) the recently developed flux based AFstY index (Accumulated stomatal Flux above a flux threshold Y). This paper compares the AOT40 and AFstY indices for three forest trees species at different locations in Europe. The AFstY index is estimated using the DO(3)SE (Deposition of Ozone and Stomatal Exchange) model parameterized for Scots pine (Pinus sylvestris), beech (Fagus sylvatica) and holm oak (Quercus ilex). The results show a large difference in the perceived O(3) risk when using AOT40 and AFstY indices both between species and regions. The AOT40 index shows a strong north-south gradient across Europe, whereas there is little difference between regions in the modelled values of AFstY. There are significant differences in modelled AFstY between species, which are predominantly determined by differences in the timing and length of the growing season, the periods during which soil moisture deficit limits stomatal conductance, and adaptation to soil moisture stress. This emphasizes the importance of defining species-specific flux response variables to obtain a more accurate quantification of O(3) risk.

Modelling stomatal ozone flux across Europe

A model has been developed to estimate stomatal ozone flux across Europe for a number of important species. An initial application of this model is illustrated for two species, wheat and beech. The model calculates ozone flux using European Monitoring and Evaluation Programme (EMEP) model ozone concentrations in combination with estimates of the atmospheric, boundary layer and stomatal resistances to ozone transfer. The model simulates the effect of phenology, irradiance, temperature, vapour pressure deficit and soil moisture deficit on stomatal conductance. These species-specific microclimatic parameters are derived from meteorological data provided by the Norwegian Meteorological Institute (DNMI), together with detailed land-use and soil type maps assembled at the Stockholm Environment Institute (SEI). Modelled fluxes are presented as mean monthly flux maps and compared with maps describing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb or nl l(-1)), highlighting the spatial differences between these two indices. In many cases high ozone fluxes were modelled in association with only moderate AOT40 values. The factors most important in limiting ozone uptake under the model assumptions were vapour pressure deficit (VPD), soil moisture deficit (for Mediterranean regions in particular) and phenology. The limiting effect of VPD on ozone uptake was especially apparent, since high VPDs resulting in stomatal closure tended to co-occur with high ozone concentrations. Although further work is needed to link the ozone uptake and deposition model components, and to validate the model with field measurements, the present results give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation.

Some foliar characteristics of ash trees (Fraxinus excelsior) exposed to ozone episodes

Young ash trees (Fraxinus excelsior L.) growing in the field were exposed to episodes of 150 nl 1^-1 ozone, or to clean air, in open-top chambers at the University of Nottingham, Sutton Bonington Campus, UK, May-October, 1992-1994. The episodes were for 8 h daily and for 1-4 d in succession, with seasonal totals of 24-27 d. A nondestructive method for estimating whole-tree leaf area was developed in 1992 and used to monitor leaf area, leaf number and leaf size in 1993 and 1994. Stomatal density and leaf abscission were also studied. No effects of exposure to ozone on leaf area and leaf number were detected. In 1994 there were consistent trends towards greater mean leaf size and higher stomatal density in trees exposed to ozone, but these effects were not significant at the 95% confidence level. Leaf abscission was slower in trees exposed to ozone in 1992 and 1994, but not in 1993 when there were fewer exposure days late in the season.

Seasonal changes in water use of ash trees exposed to ozone episodes

Young ash trees (Fraxinus excelsior L.) growing in the field were exposed to episodes of 150 nl 1^-1 ozone, or to clean air, in open-top chambers at the University of Nottingham, Sucton Bonington Campus, UK, in the summer of 1992. The episodes were for 8 h daily and for 1-4 in succession, with a seasonal total of 27 d. From late-June until mid-September 1942, flow of water in the stems of individual trees was measured using a heat balance method. Short-term effects of ozone exposure on stem flow were not detected. However, in the longer-term, daily integrated stem flow values for the ozone treatment decreased throughout the measurement period relative to the clear air treatment. Further data analysis showed that integrated stem flow values for morning, evening and for the more stable central part of the day changed in a similar way, but this was statistically significant only for the evening period. No treatment differences, however, were detected in night-time water use values. This response was probably mediated by changes in stomatal resistance and, if applicable to other species, has important implications for the long-term growth of trees in regions where photochemical ozone is common.

The impact of constituent ions of acid mist on assimilation and stomatal conductance of Norway spruce prior and post mid-winter freezing

Norway spruce seedlings were sprayed twice weekly with one of a range of artificial mists at either pH 2.5, 3.0 or 5.6, for three months. The mists consisted of either (NH4)2SO4 (pH 5.6), NH4NO3 (pH 5.6), water (pH 5.6), HNO3 (pH 2.5), H2SO4 (pH 2.5). In late December 1988 and early January 1989 the light response of assimilation and stomatal conductance were assessed in the laboratory following a 4-day equilibration period at 12 degrees C. The intact trees were then subjected to a mild (-10 degrees C), brief (3 h) frost in the dark and the recovery of light saturated assimilation (Amax) was followed during the subsequent light period. The same trees were then subjected to a second 3 h (-18 degrees C) frost. The recovery of Amax during the next day was followed. All ion-containing mists stimulated Amax and apparent quantum yield relative to control trees, irrespective of pH. The mists containing SO4 made stomatal conductance unresponsive to light flux density and caused the stomata to lock open. Frosts of -10 degrees C and -18 degrees C did not inhibit the Amax of control trees for longer than 200 min into the light period. In contrast, the ion-containing mists exerted a significant inhibitory effect upon the recovery of Amax. Nitric acid inhibited Amax to 35% of the pre-frost value, whilst the remaining treatments inhibited Amax between 15% and 40% of the pre-frost value. It is concluded that SO4 causes increased mid-winter frost sensitivity and NO3 ameliortes this effect. The results are discussed in relation to forest decline.