Environmental control of CO2-assimilation and leaf conductance in Larix decidua Mill.

Summary 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 mol m^-2 s^-1. Only the sun needles of the Bayreuth tree reached 3.5 to 5 mol 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.Dedicated with the greatest appreciation to the 75th birthday of Prof. Dr. M. Evenari