A cumulative ozone uptake-response relationship for the growth of Norway spruce saplings.

Norway spruce saplings [Picea abies (L.) Karst.] were exposed during four growing seasons to different ozone treatments in open-top chambers: charcoal filtered air (CF), non-filtered air (NF) and non-filtered air with extra ozone (NF+, 1.4xambient concentrations). The CF and NF+ ozone treatments were combined with phosphorous deficiency and drought stress treatments. The total biomass of the trees was harvested at different intervals during the experimental period. The ozone uptake to current-year needles of the Norway spruce saplings was estimated using a multiplicative stomatal conductance simulation model. There was a highly significant correlation between the reduction of total biomass and the estimated cumulative ozone uptake, which did not vary when different thresholds were applied for the rate of ozone uptake. The reduction of the total biomass was estimated to 1% per 10 mmol m(-2) cumulated ozone uptake, on a projected needle area basis.

Impact of ozone and reduced water supply on the biomass accumulation of Norway spruce saplings.

Norway spruce saplings [Picea abies (L.) Karst.] were exposed during four growing seasons to two different ozone treatments in open-top chambers: charcoal filtered air (CF), and non-filtered air with extra ozone (NF+, 1.4xambient concentrations). Within each ozone treatment the saplings were either kept well watered or treated with a 7-8 week period with reduced water supply each growing season. The total biomass of the trees was measured in April and September during each of the last three growing seasons. NF+ significantly reduced the total biomass accumulation of Norway spruce saplings during the fourth growing season. No interaction between ozone and reduced water supply could be detected. The magnitude of the ozone impact after 4 years of exposure was an 8% reduction of the total plant biomass and a 1.5% reduction of the RGR. The reduced water supply reduced the total biomass 29% and the RGR 12%.

Simulations of stomatal conductance and ozone uptake to Norway spruce saplings in open-top chambers.

A simulation model was developed to estimate the stomatal conductance and ozone flux to Norway spruce saplings in open-top chambers. The model was parameterized against needle conductance measurements that were made on 4-6-year-old spruce saplings, grown in open-top chambers, in July-September during three different seasons. The spruce saplings were either maintained well watered or subject to a 7-8 week drought period in July-September each year. The simulated conductance showed a good agreement with the measured conductance for the well-watered as well as the drought stress-treated saplings. The simulations were significantly improved when different vapour pressure deficit (VPD) functions were applied for well-watered and drought-stressed spruce saplings. The cumulated ozone uptake which was calculated from the conductance simulations showed less variation between years, compared to the cumulative ozone exposure index AOT40 (accumulated exposure over a threshold of 40 ppb or nl l(-1)) for the corresponding time periods. Measurements in May 1995 demonstrated the occurrence of long-term 'memory-effects' from the drought stress treatments on the conductance. Memory-effects need to be considered when simulation models for stomatal conductance are to be applied to long-lived forest trees under a multiple stress situation.