Effects of light availability on crown structure, biomass production, light absorption and light-use efficiency of Hopea odorata planted within gaps in Acacia hybrid plantations

<p><strong>Background:</strong> Young saplings of <i>Hopea odorata</i>, a native dipterocarp in Vietnam, require shading to prevent photoinhibition but they must avoid competition that stagnates their growth.</p> <p><strong>Aims:</strong> To develop a silvicultural regime by examining how the biomass production of <i>H. odorata</i> changes along a light gradient in gaps within <i>Acacia</i> hybrid plantations.</p> <p><strong>Methods:</strong> <i>Hopea odorata</i> saplings were planted in 22-m-diameter circular gaps within a 3-year-old <i>Acacia</i> hybrid plantation and in 5-m and 7.5-m-wide strip gaps within a 2.5-year-old <i>Acacia</i> hybrid plantation.</p> <p><strong>Results:</strong> In the circular gap, biomass growth increased nearly tenfold from the gap perimeter (GP) to about 9 m from the perimeter, and then decreased. This was associated with a fivefold increase in the absorption of photosynthetically active radiation (APAR) and a nearly twofold increase in light-use efficiency (LUE). The increasing APAR was primarily related to increasing <i>H. odorata</i> crown sizes and a reduction in shading from the <i>Acacia</i> hybrid. In the strip gaps, the APAR was similar to that at the GP in the circular gap, however LUE was about threefold higher.</p> <p><strong>Conclusions:</strong> <i>Hopea odorata</i> expressed plasticity in crown structure in response to incident light. While it grows under a wide range of availability of photosynthetically active radiation, growth increases strongly to a certain level of PAR. The best conditions for <i>H. odorata</i> growth were near the centre of the circular gaps, where PAR was 60% of full sunlight, while the strip gaps, where PAR was 20%, were too narrow owing to shading from the <i>Acacia</i> hybrid.</p>