Ecophysiology of primary production in black alder (Alnus glutinosa (L.) Gaertn.)

Black alder (Alnus glutinosa) is known as an early successional tree species and as typical for wet sites. Investigations of microclimatic conditions, gas exchange, leaf area index (LAI) and elucidated the physiological base of niche. The investigated adult trees of black alder (Alnus glutinosa) were growing on the banks of Lake Belau in Northern Germany. Their maximum LAI was 4.8, which is relatively low compared to other broad-leaved tree species. Black alder exhibits an unusual litterfall pattern: while in the periphery still new leaves are growing, litterfall starts already in July in the inner, shaded parts of the crown. With regard to chlorophyll content and gas exchange characteristics the leaves in the various parts of the alder crown are not differentiated, rather they are uniformly helio-phil. They exhibit a high maximum net photosynthesis rate of 16 mu mol CO2 m(-2) s(-1). All of these properties characterize black alder as a less competitive pionier tree species. Because of always relatively high transpiration rates of all leaves, leaves from the shaded inner crown achieve only a poor WUE (shown by balances of carbon and wafer exchange). Under drought stress conditions this causes increased litterfall and consequently a decreased primary production. Thus, black alder prefers sites with all-year water supply.