Role of climate, crown position, tree age and altitude in calculated ozone flux into needles of Picea abies and Pinus cembra: a synthesis.

Ozone (O(3)) flux into Norway spruce (Picea abies) and cembran pine (Pinus cembra) needles was estimated under ambient conditions at six rural sites between 580 and 1950 m a.s.l. We also assessed age-related differences in O(3) flux by examining changes in leaf conductance across the life span of Norway spruce. At the leaf level O(3) flux into the needles was effectively controlled by stomatal conductance and, hence by factors such as temperature, irradiance and humidity, which control stomatal conductance. Seasonal variations in O(3) flux were mainly attributed to the course of the prevailing temperature. During the growing season, however, data have emphasised leaf-air vapour pressure difference as the environmental factor most likely to control stomatal conductance and O(3) flux into the needles. In the sun crown stomatal conductance averaged over the growing season decreased with increasing tree age from 42.0+/-3.5 mmol O(3) m(-2) s(-1) in 17-year-old trees to 7.1+/-1.0 mmol O(3) m(-2) s(-1) in 216-year-old trees, indicating that O(3) concentration in the substomatal cavities is higher in young than in old trees. Independent from tree age stomatal conductance and O(3) flux were approximately 50% lower in shade needles as compared to sun-exposed needles. Stomatal conductance was also greater in the current flush (24+/-5.6 mmol O(3) m(-2) s(-1)) and in 1-year old needles (16+/-4 mmol O(3) m(-2) s(-1)) than in older needle age classes (12+/-1 mmol O(3) m(-2) s(-1), averaged across the four older needle age classes). In trees similar in age (60-65 years old) average O(3) flux into sun needles increased from 0.55+/-0.36 nmol m(-2) s(-1) at the valley floor to 0.9 nmol m(-2) s(-1) in 1950 m a.s.l. Cumulative O(3) uptake during the vegetation period increased from 11.4+/-1.7 mol m(-2) in the valley to 14 mol m(-2) at the alpine timberline. Although stomatal conductance provides the principal limiting factor for O(3) flux, additional field research is necessary in order to improve our understanding concerning the quantitative 'physiological threshold dose' which internally can be active and can have adverse effects of O(3) on forest trees.

Environmental control of ozone uptake in Larix decidua Mill.: a comparison between different altitudes.

We studied ozone (O(3)) uptake by European larch (Larix decidua Mill.) needles under ambient conditions at both a low and a high elevation site. At a given ambient O(3) concentration, the rate of O(3) uptake by needles was effectively controlled by stomatal conductance and, hence, by factors such as light, humidity and water status, which control stomatal conductance. At both study sites, atmospheric water vapor pressure deficit (VPD) was the climatic factor most closely correlated with ambient O(3) concentration. Thus, when ambient O(3) concentrations were highest, O(3) flux into the needles tended to be restricted by narrowing of the stomata. Mitigation of potential O(3) stress by stomatal closure was most marked at the low elevation site where both soil water stress and atmospheric VPD were greater than at the high elevation site.

Effects of atmospheric humidity and acclimation temperature on the temperature response of photosynthesis in young Larix decidua Mill.

Larch (Larix decidua Mill.) seedlings of a low altitude (600 m) Austrian provenance were raised outdoors and acclimated in chambers for 14 to 24 days during August and September at either 8 degrees C and an atmospheric saturation vapor pressure deficit (DeltaW) of 2.5 Pa kPa(-1), or 24 degrees C and a DeltaW of 6.2 Pa kPa(-1). Subsequently, their rates of photosynthesis, dark respiration and transpiration were measured at temperatures between 5 and 30 degrees C with DeltaW either maintained below 10 Pa kPa(-1) or allowed to increase with temperature up to 38 Pa kPa(-1). Below 15 degrees C the photosynthetic rate of cold-acclimated plants was higher, but above 15 degrees C it was lower, than that of warm-acclimated plants. Temperature acclimation caused a greater shift in the temperature optimum for photosynthesis when DeltaW was kept small than when it was allowed to increase with temperature. When DeltaW was kept small, leaf conductance of cold-acclimated plants, unlike that of warm-acclimated plants, did not increase with temperature above 15 degrees C. When DeltaW increased with temperature, leaf conductance of cold-acclimated plants decreased more rapidly with temperature than that of warm-acclimated plants. Low temperature acclimation increased the rate of photosynthesis below 15 degrees C without affecting leaf conductance, which indicates that there was an adaptation in leaf internal processes. Further evidence of a metabolic adaptation to acclimation temperature is that dark respiration of cold-acclimated plants was twice that of warm-acclimated plants at all temperatures.

Environmental control of CO2-assimilation and leaf conductance in Larix decidua Mill. : I. A comparison of contrasting natural environments.

CO2-assimilation and leaf conductance of Larix decidua Mill. were measured in the field at high (Patscherkofel, Austria) and low (Bayreuth, Germany) elevation in Europe, and outside its natural range along an altitudinal gradient in New Zealand.Phenology of leaf and stem growth showed New Zealand sites to have much longer growing seasons than in Europe, so that the timberline (1,330 m) season was almost twice as long as at the Austrian timberline (1,950 m).The maximum rates of photosynthesis, A max, were similar at all sites after completion of leaf growth, namely 3 to 3.5 µmol m-2 s-1. Only the sun needles of the Bayreuth tree reached 3.5 to 5 µmol m-2 s-1. Light response curves for CO2-assimilation changed during leaf ontogeny, the slope being less in young than in adult leaves. The temperature optimum for 90% of maximum photosynthesis was at all sites similar between ca. 12-28°C for much of the summer. Only at the cooler high altitude timberline sites were optima lower at ca. 10-16°C in developing needles during early summer.A linear correlation existed between A max and leaf conductance at A max, and this showed no difference between the sites except for sun needles at Bayreuth.Leaf conductance responded strongly to light intensity and this was concurrent with the light response of CO2-uptake. A short-term and a long-term effect were differentiated. With increasing age maximum rates of CO2-uptake and leaf conductance at A max increased, whereas short-term response during changes in light declined. The stomata became less responsive with increasing age and tended to remain open. The stomatal responses to light have a significant effect on the water use efficiency during diurnal courses. A higher water use efficiency was found for similar atmospheric conditions in spring than in autumn.Stomata responded with progressive closure to declining air humidity in a similar manner under dissimilar climates. Humidity response thus showed insensitivity to habitat differences.From the diurnal course of gas-exchange stomata were more closed at timberline (1,330 m) than at lower elevations but this did not lead to corresponding site differences in CO2-exchange suggesting Larix may not be operating at high water use efficiency when air is humid.The main difference between habitats studied was in the time necessary for completion of needle development. Similarity in photosynthesis and leaf conductance existed between sites when tree foliage was compared at the same stage of development. Length of growing season and time requirement for foliar development appear to be a principle factor in the carbon balance of deciduous species. The evergreen habit may be more effective in counterbalancing the effects of cool short summers.