RESORPTION OF NITROGEN AND DRY-MATTER PRIOR TO LEAF ABSCISSION - VARIATION AMONG INDIVIDUALS, SITES AND YEARS IN THE MOUNTAIN BIRCH

1. Resorption of nitrogen and dry matter prior to leaf fall was monitored in 40 mountain birch trees [Betula Ehrh. pubescens ssp tortuosa (Ledeb.) Nyman] at four sites during 4 consecutive years. The variation in resorption of dry matter and nitrogen was related to the tree individual, summer temperature sum, leaf weight:area ratio, leaf nitrogen content and site. 2. Because of the sampling design the contribution of variation between individuals could only be assessed for 2 of the 4 years. For these 2 years 'individual within site' was the most important factor resolving the variation in the resorption of dry matter and nitrogen and in the respective resorption efficiencies. 3. On average, 26% of the dry matter was resorbed before leaf abscission. Dry-matter-resorption efficiency for individual trees varied between 0 and 45%. The leaf weight:area ratio was the factor explaining most of the variation in mass-resorption and mass-resorption efficiency. A significant negative relationship was also found between each of these two variables and the temperature sum. 4. Leaf nitrogen resorption for individual trees varied between 49 and 86%, with an overall mean of 72%. The nitrogen content of green leaves and the summer temperature were the most and next most important factors determining N resorption and resorption efficiency. A weak relationship was found between nitrogen resorption and mass resorption.