Ecophysiological Investigations on Lichens of the Negev Desert: VI. Annual Course of the Photosynthetic Production of Ramalina macifarmis (Del.) Bory

Abstract

Diurnal courses oflight, temperatures, water content, and CO2 exchange of the thalli of Ramalina maciformis were measured in its natural habitat in the Negev desert from March to September 1971. The measurements of water content of the lichen thalli by means of a Hiltner dew balance were extended over a period of about two years. Water content was the most prominent factor controlling the CO2-gas exchange of the lichen. The different types of water uptake by rain-, dew-, or water-vapour moistening and the corresponding daily courses of net photosynthesis are presented. The experimental data, formed the basis of a correlation model for net photosynthesis of R. maciformis by means of which the daily photosynthetic production due to dew and water-vapour uptake was calculated for a, whole year according to the registrations of the thallus water contents, temperatures of a neighbouring weather station, and photoperiod. The net photosynthesis due to rain-water imbibition and the respiration during the night were separately calculated. The photosynthetic CO2 gain which was yielded by dew on c. 200 days was as high as that calculated for the 29 rain days of the extremely rainy annual period of 1971/72. In years with precipitation near the long term average, as was the subsequent period 1972/73, rain induced production of R. maciformis was only 2/3 of that yielded by dewfall and may be only a small fraction in dry years, whereas dewfall maintains almost always its regular occurrence. Rainfall in the Negev, although being very effective for high photosynthetic productivity, is too scarce to provide life of R. maciformis. The existence of many lichens in the Negev is only possible because of the high frequency and regularity of dew falls. It is discussed whether the calculated amount of the annual photosynthetic gain is representative for the natural production and growth.