Der CO2- und H2O- Gasaustausch von Pflanzen in der Krautschicht eines Kalkbuchenwaldes in Abhängigkeit von Standortsfaktoren.: III. CO2-Bilanzen und Netto-Primärproduktion

Abstract

In the understorey layer of a beech forest on limestone the CO 2 gas exchange of different herbs was studied. This was possible continuously in 1982 for Mercurialis perennis . In 1983 and spring 1984 six different species were investigated alternatingly: two spring geophytes ( Allium ursinum and Arum maculatum ), a wintergreen herb ( Asarum europaeum ), the summergreen herb M. perennis and two grasses ( Hordelymus europaeus and Melica uniflora ). In addition to the gas exchange measurements the seasonal course of climatic factors (photosynthetic active radiation (PAR), air temperature (Ta), water vapour saturation deficit of the air (SD)) and of endogeneous factors (Chlorophyll content (Chl a + b) and water potential of the leaves (θ L )) were measured. Applying multiple regression analysis an attempt was made to explain the variability of the daily CO 2 balance of the six plant species during their vegetation period with aid of these exogeneous and endogeneous variables. Additionally, time was taken into account as an independent variable for this analysis. The course of the daily CO 2 balance during the vegetation period of the plants can be attributed to the course of the PAR, air temperature and saturation deficit of the air with a coefficient of determination up to 99%. The other variables normally are of little influence. By using the independent variables in the regression equations a complete seasonal course of the daily CO 2 gain of all six plant species can be calculated for the year 1983. Because the PAR was the most important independent variable and because the PAR in the herb layer was quite different in both the summers 1982 and 1983, the calculations for the six plant species were done twice: at first with the daily PAR values of 1983, subsequently with the daily PAR values of 1982. The CO 2 gain of the two spring geophytes, which are photosynthetically active only as long as overstorey canopy closing has not been achieved, was little influenced by the different light climates in 1982 and 1983. The species A. ursinum and A. maculatum are characterized by high CO 2 gain, calculated on leaf area basis, because of their high photosynthetic capacity. The CO 2 gain of the summergreen herbs M. perennis and A. europaeum in spring only ran up to half of the values of the spring geophytes. In the summer 1982 the PAR values as well as the daily CO 2 gain of M. perennis were very small. In 1983 the PAR values and the CO 2 balances increased. Nevertheless, in comparison to the spring conditions the values stay very low. In the year 1982 M. perennis and A. europaeum fixed 75% of their total CO 2 balance in the first six weeks of the vegetation period before canopy closure. In spring and summer 1983 the CO 2 balance of these species was nearly equal due to the increase of light in the ground layer during this summer. The leaf development of the two grasses started after canopy closure. The CO 2 gain of the two grasses in 1982 was low due to their late leaf development and the insufficient light conditions. In the bright summer 1983 the grasses — especially M. uniflora — assimilated high amounts of CO 2 . The calculation of the CO 2 gain per ground area is possible by multiplying the leaf related CO 2 gain of the six species with their leaf area index (LAI) in the research plot. On this basis M. perennis or A. ursinum achieved the highest CO 2 balances because of their high LAI. The corresponding values of the grasses as well as the value of A. maculatum are very low. These species are not able to build a high LAI in this plant community. That is why they cannot use their high photosynthetic capacity for high CO 2 gains.