© 1998 Heron Publishing—Victoria, Canada
Seasonal changes in leaf nitrogen pools in two Salix species
Olevi Kull (1), Andres Koppel (2) and Asko Noormets (3)
1. Institute of Ecology, Riia 181, EE2400 Tartu, Estonia (olevi@zbi.ee ) / 2. Institute of Zoology and Botany, Riia 181, EE2400 Tartu, Estonia / 3. Department of Botany and Ecology, Tartu University, Lai 40, EE2400 Tartu, Estonia / Received December 6, 1996
Summary
Leaf nitrogen distribution pattern was studied four times during the growing season in a 2-year-old Salix viminalis L. and Salix dasyclados Wimm. plantation in Estonia. We measured the vertical distributions of leaf nitrogen concentration, dry mass, leaf area and
light environment (as fractional transmission of diffuse irradiance, ad) in the canopy. The light-independent nitrogen pool was evaluated as the intercept of the leaf nitrogen concentration versus
ad relationship, and the nondegradable nitrogen pool was evaluated as the nitrogen remaining in abscised leaves. A strong vertical
gradient of mass-based leaf nitrogen concentration was detected at the beginning of the growing season, and decreased steadily
during canopy development. This decline had at least three causes: (1) the amount of nitrogen in the foliage was larger at
the beginning of the growing season than at the end of the growing season, probably because of pre-existing root systems;
(2) with increasing leaf area index (LAI) during the growing season, the proportion of leaf nitrogen in total canopy nitrogen
that could be redistributed (light-dependent nitrogen pool) decreased; and (3) the photosynthetic photon flux density gradient
inside the canopy changed during the season, most probably because of changes in leaf area and leaf angle distributions. Total
canopy nitrogen increased almost proportionally to LAI, whereas the light-dependent nitrogen pool had a maximum in August.
Also, the proportion of the light-dependent nitrogen pool in the total canopy nitrogen decreased steadily from 65.2% in June
to 17.2% in September in S. dasyclados and from 63.3 to 15.1% in S. viminalis. The degradable nitrogen pool was always bigger than the light-dependent nitrogen pool.
Keywords:
acclimation, leaf nitrogen concentration, light extinction, light interception, willow.
