Summary1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34 -36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

The aim of the present study was to determine how long-term nutrient optimisation of Norway spruce stands affects the chemical composition of stem wood. Material for the study was collected from Flakaliden (Sweden) where Norway spruce [Picea abies (L.) Karst.] stands have been grown either without fertilisation or under nutrient optimisation treatment, by supplying a complete nutrient mix in the irrigation water every 2nd day during the growing season. The experiment was established in 1987 and in the autumn of 1998, 12 trees were harvested both in control (no fertilisation) and irrigated-fertilised (IL) stands. The increased growth rate caused by the IL treatment affected the chemical composition of the stem wood. The most pronounced effect was a 7% increase in lignin concentration caused by the IL treatment. Increases in concentrations of acid-soluble lignin (1.1-fold), extractives (1.2-fold), soluble sugars (1.3-fold), sterols (1.3-fold) and dehydroabietic acid (1.6-fold) as well as a decrease in the proportional quantity of terpinolene were also found. These results demonstrate that nutrient optimisation affected the chemical composition of Norway spruce wood, which may influence the suitability of such wood for specific end-use purposes.

SUMMARYConcentrations of soluble and bound phenolic cotnpounds were measured in needles of 3-yr-old loblolly pme {Pinus taeda L.) trees exposed from May to November 1993 to a range of ozone (O3) concentrations in open-top fidd chambers. The treatments were charcoal-filtered air (CF). non-filtered air (NF), and NF air with O, added at 1-5 times (NF 1-5) and 2-0 times (NF 2-0) the ambient O, concentration for 12 h daily. Average daily (0800-2000 hours) O3 concentrations in the CF.NF, NF ISandNF 2 0 treatments were r. 29, 47, 76 and 98 nl 1"', respectively, for the 140 d treatment period. At tbe end of the treatment period, total phenolic and proanthoc\anidin concentrations in the previous year's needles were 25-29'\, higher in the NF 2'0 treatment compared v\ ith the lower O., treatments. Catechin concentration increased in the previous year's needles by as mucb as 81 ''" between the NF 2-0 treatment and tbe lower O^ treatments. Catechin is an effective antioxidant, and elevated levels migbt confer some protection against O^ injury. No significant differences in total phenolics and proanthocyanidins in the previous year's needles were detected among the remaining O^ treatments, or among any C>. , treatment for tbe current year's needles. Lignin content in needles of botb years was not significantly affected by O,j exposure. Changes in the phenolic content of older needles in response to elevated O^ could alter plant-patbogen interactions and slow down microbial decomposition, which could contribute to a decline in site soil quality.

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Three-year old Betula pendula Roth clones were grown at two nutrient levels in a field experiment to investigate the responses and recovery in growth and wood properties to a range of defoliation levels (0–100%). No general threshold value of defoliation level for negative effects in growth was found, since the sensitivity of saplings to defoliation varied according to plant traits studied. However, responses were related to defoliation intensity. Saplings compensated for 25% defoliation in terms of height growth and number of current branches and were able to tolerate 50% defoliation without effects on diameter growth 1 year after the defoliation. Nutrient availability was significant only in determining how total biomass responded to defoliation. Fertilized saplings were able to tolerate 25% defoliation without reduction in total biomass, but nonfertilized saplings were not. The interaction between defoliation and fertilization disappeared in the second growing season after the defoliation. Saplings were not able to compensate for 75% defoliation in terms of total biomass or for 100% defoliation in terms of growth and branching even in 2 years' recovery time. In stemwood, complete defoliation reduced growth ring width and vessel diameter simultaneously and also induced a narrow zone of secondary xylem with defects. Our results suggest that defoliation level and recovery time played a crucial role in compensatory growth of birch saplings, while nutrient availability had a minor role.