COMBINED EFFECTS OF PARTIAL DEFOLIATION AND NUTRIENT AVAILABILITY ON CLONED BETULA-PENDULA SAPLINGS .1. CHANGES IN GROWTH, PARTITIONING AND NITROGEN UPTAKE

The combined effects of partial defoliation and nutrient availability on dry matter accumulation and partitioning, and on nitrogen uptake and partitioning, were studied in cloned Betula pendula Roth saplings. The saplings were randomly assigned to different nutrient levels (5, 1.5 and 0.5 mol N m-3) in aerated nutrient culture and to the following defoliation treatments: (1) control (no damage), (2) damage of the developing main stem leaves (half of the leaf lamina removed), and (3) removal of the developing main stem leaves (entire leaf lamina removed). Measured in terms of cumulative whole-plant dry weight (including removed leaf tissue), the birch saplings were unable to compensate for the loss of the developing leaves (treatment 3) during the 14 d study period. In response to leaf removal (treatment 3) the mean final percentage reduction in whole-plant dry weight was actually greater than the initial mean percentage reduction caused by the removal itself; the magnitude of the final reduction was independent of nutrient availability. After removal of the developing leaves, branch growth was favoured at the expense of the growth of the rest of the shoot; the relative branch growth was most pronounced at the highest nutrient level. At the two highest nutrient levels the nitrogen uptake of the saplings with the developing leaves removed was less than that of undamaged saplings. We suggest, however, that the incapacity of the saplings for compensatory growth after removal of the developing leaves was primarily due to the decreased total carbon gain of the saplings rather than to the decreased nitrogen uptake rate.