Variations in relative stomatal and biochemical limitations to photosynthesis in a young blackbutt (Eucalyptus pilularis) plantation subjected to different weed control regimes
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 November 1, 2007; accepted February 14, 2008; published online May 1, 2008
Summary
Foliar gas exchange and carbon (δ13C) and oxygen (δ18O) isotope ratios were measured in a young blackbutt (Eucalyptus pilularis Sm.) plantation subjected to four weed control treatments defined by the width of the weed-free strip maintained for the
first 12 months after planting. Treatments were: 2-m-wide weed-free strip (50% of plot area, 2.0MWC), 1.5-m-wide weed-free
strip (37.5% of plot area, 1.5MWC), 1-m-wide weed-free strip (25% of plot area, 1.0MWC) and no weed control (NWC). Our objectives
were to determine (1) if decreasing the width of the weed control strip (decreasing herbicide use) affected growth and leaf
photosynthesis of the plantation, and (2) the effects of the weed control regimes on variations in relative stomatal and biochemical
limitations to photosynthesis. Trees in the 1.0MWC treatment had lower foliar light-saturated photosynthetic rate (Asat) than trees in the 2.0MWC treatment. An increase in metabolic limitation was responsible for the decrease in Asat in the 1.0MWC trees, which was also partly confirmed by the isotopic data. Compared with trees in the 1.0MWC, 1.5MWC and
2.0MWC treatments, Asat of NWC trees was significantly lower, a difference that was attributable mainly to stomatal limitation and to a lesser extent
to biochemical limitation. The results support the conclusion that different weed control regimes cause differences in relative
stomatal and biochemical limitations to plantation photosynthesis. This report contributes to a growing body of literature
on competition for soil resources between trees and weeds. Our results highlight the usefulness of the stable isotopic method
in supporting analysis of the response of net photosynthesis to varying intercellular CO2 concentration for determining the relative stomatal and non-stomatal limitations to photosynthesis under experimental conditions
in the field.
