Using Acacia as a nurse crop for re-establishing native-tree species plantation on degraded lands in Vietnam

摘要: Acacia mangium, A. auriculiformis and their hybrid, the leguminous fast-growing tree species has been widely adopted as a nurse crop for re-establishing native-tree plantations on degraded lands in Vietnam, but with little success. This may be attributed to not fully understanding site requirements of target native potential negative well positive between-species interactions. The most planted is Hopea odorata Roxb., dipterocarp that thought shade-tolerant. To resolve how best apply manage such system mixed-species silviculture, this study first investigated resource H. its natural habitat soils change under consecutive short-rotations hybrid plantations. Plantings within circular strip gaps 3- 2.5-yr-old plantations, respectively, were used assess growth physiological responses competition light water. gradients created circular-gap experiment different condition strip-gap changes rate associated efficiency use by H.odorata. Site regeneration three representative 50  m inventory plots secondary evergreen forest southern Vietnam. upper canopy was dominated four species; odorata, Shorea roxburghii G. Don., Anisoptera costata Korth., Dipterocarpus alatus Roxb. ex Don. prevailing stand structure supported vigorous germination, development seedlings due low levels near floor. Seedling germination when daily transmitted photosynthetically active radiation (PAR) between 2.2% 6.6%, seedling only observed PAR 11.4%. slightly acidic sandy nutrient concentration apparently constraint given adequate conditions. results suggest re-establishment sites using crops should possible provided high shading are avoided. The improve soil conditions assessed gravelly Central from second- or third-rotation five age classes (0.5- 5-yr old) adjacent abandoned controls. Compared land, stock total carbon, nitrogen, exchangeable calcium, magnesium sodium significantly higher some years rotation. However, extractable phosphorus potassium affected. Electrical conductivity bulk density lower all ages. Soil pHCaCl2 at ages 0.5 5 yrs, pHH2O yrs. Within rotation, properties did plantation age, although they appeared decrease during years; carbon then recovered initial levels, nitrogen cations remained lower. Some strongly related gravel content elevation, rate. Thus plantings short-rotation land can lead properties. Growth environmental nurse-crop examined field where 22 m-diameter opened 3-yr-old At 2 stem diameter, height crown diameter increased gap perimeter (GP) centre (GC). response correlated significant increases incident 24% 61% PAR. Net photosynthetic 1500 mol m-2 s-1 (A1500) stomatal conductance (gs) trees GP than those GC. Light-saturated photosynthesis (Asat) GC 1 yr, Apparent quantum yield (Φ), dark respiration (Rdark), biochemical parameters (Vcmax Jmax) similar treatments. Chlorophyll content, chlorophyll fluorescence (Fv/Fm), leaf N P concentrations also unaffected treatment. Nevertheless, specific area other Despite substantial difference treatments, received irradiation >500 μmol 12% day v. 38% Significant reductions water (Ψleaf) end dry season treatments close compared suggested acacia trees, pre-dawn Ψleaf >-0.5 MPa. Thus, despite being regenerates naturally shade, quickly acclimates much environments. Understanding alters architectural traits changing environments essential designing manipulating containing species. Seedlings into referred above, 7.5 plantation. Crown structure, absorption (APAR) whole-plant light-use (LUE; above-ground biomass wood per unit APAR) over gradient across two years. Biomass production exponentially largely an exponential increase APAR minor LUE. large reduction trees. Conversely, decreased towards centre, probably steeper branch angles order avoid radiation. In planting, gaps; however pattern sun flecks direct sunlight gap. While LUE more shaded higher, enough make up hence production. shows able grow wide range availability strong influence growth. too narrow, indicated effective silvicultural design establishing resources managed. concluded recovery key chemical physical properties. It lands. Although shade-adapted species, it great plasticity acclimate However systems these interspecific needs addressed management