© 2007 Heron Publishing—Victoria, Canada
Defoliation and nitrogen effects on photosynthesis and growth of Eucalyptus globulus
Elizabeth A. Pinkard (1–3), Michael Battaglia (2, 4) and Caroline L. Mohammed (1, 2, 4)
1. School of Agriculture, University of Tasmania, Private Bag 12, Hobart, TAS 7001, Australia / 2. CRC for Forestry, Private Bag 12, Hobart, TAS 7001, Australia / 3. Corresponding author (libby.pinkard@ensisjv.com) / 4. Ensis, Private Bag 12, Hobart, TAS 7001, Australia / Received July 18, 2006; accepted September 11, 2006; published online April 2, 2007
Summary
Plant responses to defoliation are complex. We established a field experiment in a nine-month-old Eucalyptus globulus Labill. plantation to examine the effects of pattern (upper crown versus lower crown removal), frequency (single, double
or triple defoliation within a 12-month period) and severity (25 versus 38% of leaf area removed) of defoliation and the effect
of soil nitrogen (N) on photosynthetic processes and stem growth. The photosynthetic responses observed following defoliation
could be attributed to changes in source:sink ratios. Light-saturated CO2 uptake (Amax) increased with increasing severity and frequency of defoliation irrespective of defoliation pattern. Seedlings defoliated
in autumn did not exhibit increases in Amax until the following spring, whereas there was no such delay in photosynthetic responses associated with spring defoliation.
Application of N before defoliation allowed trees to compensate for the effect of defoliation on stem diameter growth, which
could not be explained simply in terms of increases in Amax. The observed increases in stem diameter increment following N fertilization of defoliated trees suggested increases in leaf
area development, and there were changes in the leaf area:leaf dry mass ratio that may have increased light absorption by
the crown. Nitrogen fertilization also increased partitioning of dry mass to branches at the expense of main stems, suggesting
that N supply was important in rebuilding crowns following a defoliation event.
Keywords:
artificial defoliation, biomass, CO2 assimilation, light response of CO2 uptake, source:sink ratio.
