THE EFFECTS OF SLIGHTLY ELEVATED OZONE CONCENTRATIONS AND MILD DROUGHT STRESS ON THE PHYSIOLOGY AND GROWTH OF NORWAY SPRUCE, PICEA-ABIES (L) KARST AND BEECH, FAGUS-SYLVATICA L, IN OPEN-TOP CHAMBERS

Two clones of eight-year old trees of Norway spruce and beech, planted directly into the soil and in open-top chambers, were exposed to elev ated ozone concentrations and subjected to mild soil drought. When amb ient ozone was increased by 50 ppb, the shoot extension of both Norway spruce clones was significantly reduced. The mild drought stress was begun in July 1993 and following a soil-drying associated with a spell of fine weather, the drought remained relatively constant for over a month. During this period, gas exchange parameters were regularly moni tored between 7.00 and 9.00 GMT and on two occasions followed througho ut the day. Both clones responded to the mild drought by reducing stom atal conductance and photosynthesis. There were no apparent interactio ns between drought and ozone. The reaction of beech was more complicat ed. Ozone caused a significant decrease in stomatal conductance and ph otosynthesis in well-watered treatments. All the ozone-drought stress treated trees had greater stomatal conductance than the non-drought st ressed equivalents in the morning, but the situation was reversed as t he day went on. As peak ozone concentrations occur in the afternoon, i t is hypothesized that over a period of time the drought stressed tree s are less affected by ozone as they receive a lower cumulative dose. Coupled with the low values of vapour pressure deficit occurring in th e morning, the ozone-treated, drought-stressed trees can support relat ively higher rates of photosynthesis than the well-watered equivalents . The chlorophyll contents of beech lent support to this hypothesis, a s ozone associated reductions were less marked in drought stressed tre es.