© 1996 Heron Publishing—Victoria, Canada
Impacts of water and nitrogen supplies on the physiology, leaf demography and nitrogen dynamics of Betula pendula
Renate Wendler (1) and Peter Millard (1, 2)
1. Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB9 2QJ, U.K. / 2. Author to whom correspondence should be addressed / Received March 2, 1995
Summary
We determined the response of Betula pendula Roth. trees to a restricted water supply, and quantified the interactions between tree N and water status on leaf demography
and internal N cycling. In April 1993, 3-year-old trees were planted in sand culture and four treatments applied: high-N supply
(56 mg tree–1 week–1) with either 2 dm3 water week–1 (HN+) or 0.9 dm3 water week–1 (HN–), or low-N supply (14 mg tree–1 week–1) with 2 dm3 (LN+) or 0.9 dm3 (LN–) water week–1. Until 1994, the N supplied to trees was enriched with 15N to 5.4 atom %.
During 1993, there were few differences in the growth or leaf demography of trees in the LN+ and LN– treatments, but the high-N
treatment increased tree growth. Leaf mass and area were initially similar in trees in the HN+ and HN– treatments, but the
trees in the HN– treatment had a smaller root system. Net assimilation rate under saturating light was higher in trees in
the HN+ treatment than in trees in the LN+ treatment. There was an N × water supply interaction as a result of trees in the
HN– treatment closing their stomata by the beginning of August. However, there was no difference in gas exchange characteristics
of leaves in the LN+ and LN– treatments. Although leaf senescence and abscission started in the HN– treatment by mid-August
and continued for about 90 days, whereas leaf abscission in the other treatments did not start until the beginning of October
and only lasted 25–30 days, the trees in the HN+ and HN– treatments remobilized similar amounts of 15N for leaf growth in the spring of 1994. There were no differences in predawn water potential among treatments and no evidence
of osmotic adjustment. We conclude that B. pendula trees avoid rather than tolerate drought. The interaction between the effects of nitrogen and water supplies on leaf demography
and internal cycling of N are discussed.
Keywords:
birch, internal cycling, leaf senescence, remobilization, water relations.
