Foliar δ13C and δ18O reveal differential physiological responses of canopy foliage to pre-planting weed control in a young spotted gum (Corymbia citriodora subsp. Variegata) plantation
Zhiqun Huang (1, 2, 3), Zhihong Xu (2, 4), Timothy J. Blumfield (2, 4) and Ken Bubb (5)
1. Griffith School of Environment, Griffith University, Nathan, QLD 4111, Australia / 2. Centre for Forestry and Horticultural Research, Griffith University, Nathan, QLD 4111, Australia / 3. Corresponding author () / 4. School of Biomolecular and Physical Sciences, Griffith University, Nathan, QLD 4111, Australia / 5. Forestry Plantations Queensland, Gympie, QLD 4570, Australia / Received February 23, 2008; accepted May 14, 2008; published online August 1, 2008
Summary
Weed control may improve the growth of forest plantations by influencing soil water and nutrient availability, but our knowledge
of leaf-level physiological responses to weed control at different within-canopy positions is limited for tropical and subtropical
plantations. Foliar carbon (δ13C) and oxygen (δ18O) isotope compositions, gas exchange, and nitrogen (Nmass) and phosphorus (Pmass) concentrations at four canopy positions were assessed in a young spotted gum (Corymbia citriodora subsp. Variegata (F. Muell.) A.R. Bean & M.W. McDonald) plantation subjected to either weed control or no weed control treatment, to test
if leaves at different positions within the tree canopy had the same physiological responses to the weed control treatment.
Weed control increased foliar δ13C but lowered δ18O in the upper-outer and upper-inner canopy, indicating that weed control resulted in a higher foliar photosynthetic capacity
at upper-canopy positions, a conclusion confirmed by gas exchange measurements. The increased photosynthetic capacity resulting
from weed control can be explained by an increase in foliar Nmass. In the lower-outer canopy, weed control reduced foliar δ13C while lowering δ18O even more than in the upper-canopy, suggesting strong enhancement of the partial pressure of CO2 in the leaf intercellular spaces and of foliar stomatal conductance in lower-canopy foliage. This conclusion was supported
by gas exchange measurements. Foliar photosynthesis in the lower-inner canopy showed no significant response to weed control.
The finding that leaves at different canopy positions differ in their physiological responses to weed control highlights the
need to consider the canopy position effect when examining competition for soil nutrient and water resources between weeds
and trees.
Keywords:
foliar nitrogen concentration, light-saturated net photosynthesis, stomatal conductance.
