(1) Alnus nepalensis D. Don, a species with symbiotic Frankia in its root nodules, is common in the Eastern Himalayas. (2) Five different ages of plantations from the Pankhasari range of the Kalimpong forest division in the Eastern Himalayas have been studied. Rates of nitrogen fixation (nitrogenase activity) by three different age-classes of root nodules was measured in July, October and December 1981 and April 1982. These months represent the rainy, transition, winter and early summer seasons. (3) Nitrogenase activity (acetylene reduction) was highest in July (high moisture and 25 ⚬C average temperature) and lowest in December (dry and cold (9 ⚬C), with litterfall). Activity was highest in younger nodules irrespective of tree age. (4) It is concluded that A. nepalensis plants in the Himalayas fix significant amounts of nitrogen.
