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 dm(3) water week(-1) (HN+) or 0.9 dm(3) water week(-1) (HN-), or low-N supply (14 mg tree(-1) week(-1)) with 2 dm(3) (LN+) or 0.9 dm(3) (LN-) water week(-1). Until 1994, the N supplied to trees was enriched with (15)N 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 x 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 (15)N 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.
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