SUMMARY Norway spruce and red oak trees were planted directly into the soil and exposed to 700 μmol mol‐1 CO2 in open‐top chambers. There were large inter‐specific differences in response to naturally occurring drought during the second year of exposure to elevated CO2. Both species had decreased assimilation rates. CO2‐treated red oak had no loss of photosynthetic enhancement when undroughted, whereas CO2‐treated Norway spruce showed a relative increase in assimilation rates only when draughted. The effect of CO2 on radial growth of both species was less marked in the second growing season, but this may have been a result of different biomass partitioning as Norway spruce shoot extension had a different pattern of growth in elevated CO2. Stomatal density and chlorophyll content were largely unaffected by the CO2 treatment. A precise method for measuring Norway spruce needle surface area was also developed.

Two clones of eight-year old trees of Norway spruce and beech, planted directly into the soii and in open-top chambers, were exposed to ele\'ated ozone concentrations and subjected to mild soil drought. When ambitnt ozone was increased by 50 pph, the shoot extension of both Norway spruce clones was signiticantly reduced. The mild drought stress was begun in July 1993 and following a soii-dr\ing associated with a spell of fine weather, the drought remained relatively constant for over a month. During this period, gas exchange parameters were regularly monitored between 7'00 and 900 GMT and on two occasions followed throughout the day. Both clones responded to the mild drought by reducing stomatal conductance and photosynthesis. There were no apparent interactions between drought and ozone. The reaction of beech was more complicated. Ozone caused a significant decrease in stomata! conductance and photosynthesis in well-watered treatments. All the ozone-drought stress treated trees had greater stomatal conductantc than the non-drought stressed equivalents in tht morning, but the situation was reversed as the day went on. As peak ozone concentrations occur in the afternoon, it is hypothesized that over a period of time the drought stressed trees are less affected by ozone as they recei\e a lower cumulati\e dose. Coupled with tht* low values of vapour pressure deficit occurring in the morning, the ozone-treated, droughtstressed trees can support relatively higher rates of photosynthesis than the well-watered equivalents. The chlorophyll contents of beech lent support to this hypothesis, as ozone associated reductions were less marked in drought stressed trees.

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Eight-year-old Norway spruce (Piceaabies (L.) Karst.) and 6-year-old red oak (Quercusrubra L.) trees planted directly into the soil were enclosed in open-top chambers and exposed to either 350 or 700 μmol•mol−1 of CO2 for three growing seasons. During the third year a natural drought was allowed to develop, reducing the predawn leaf water potential to between −0.80 and −1.15 MPa. Intensive gas-exchange measurements were performed before, during, and after the drought. CO2 response curves revealed mesophyll limitation to photosynthesis in drought-stressed trees grown in elevated levels of CO2. The water-use efficiency was greater for trees grown at elevated CO2, but less so during drought in red oak and the same between treatments for drought-stressed spruce. Diurnal measurements showed that enhancement of assimilation rates of trees grown at 700 μmol•mol−1 depended upon the time of day that measurements were made. There was an acclimation to increased CO2 in both species that could not be explained by leaf area differences, available soil for roots, nutrient limitation, or starch accumulation.