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Using Acacia as a nurse crop for re-establishing native-tree species plantation on degraded lands in Vietnam
thesisposted on 2023-05-27, 00:53 authored by Tran, DL
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 the site requirements of target native species and the potential for negative as well as positive between-species interactions. The most planted native species is Hopea odorata Roxb., a dipterocarp that is thought to be shade-tolerant. To resolve how best to apply and manage such a system of mixed-species silviculture, this study first investigated the resource requirements of H. odorata in its natural habitat and how degraded soils change under consecutive short-rotations of A. hybrid plantations. Plantings of H. odorata within circular and strip gaps in 3- and 2.5-yr-old A. hybrid plantations, respectively, were used to assess the growth and physiological responses of H. odorata to competition for light and water. The light gradients created in the circular-gap experiment and the different light condition in the strip-gap experiment were used to assess how changes in growth rate were associated with the efficiency of use of light by the H.odorata. Site requirements for regeneration of H. odorata were investigated in its natural habitat in three representative 50 vîvá¬• 50 m inventory plots in a secondary evergreen natural forest in southern Vietnam. The upper canopy was dominated by four dipterocarp species; H. odorata, Shorea roxburghii G. Don., Anisoptera costata Korth., and Dipterocarpus alatus Roxb. ex G. Don. The prevailing stand structure supported the vigorous germination, but not development of H. odorata seedlings due to low levels of light near the forest floor. Seedling germination was supported when daily transmitted photosynthetically active radiation (PAR) was between 2.2% and 6.6%, but seedling development was only observed when PAR was 11.4%. The slightly acidic sandy soils with low nutrient concentration were apparently not a constraint on growth of H. odorata seedlings given adequate light conditions. The results suggest that the re-establishment of H. odorata on degraded sites using nurse crops should be possible provided that high levels of shading are avoided. The potential to improve soil conditions with Acacia hybrid was assessed on degraded gravelly and sandy soils in Central Vietnam, from second- or third-rotation plantations representative of five age classes (0.5- to 5-yr old) and adjacent abandoned lands as controls. Compared to abandoned land, stock of total soil carbon, total nitrogen, and exchangeable calcium, magnesium and sodium were significantly higher in some years of the 5-yr rotation. However, extractable phosphorus and exchangeable potassium were not affected. Electrical conductivity was significantly higher and bulk density was significantly lower in all ages. Soil pHCaCl2 was lower at ages 0.5 and 5 yrs, and pHH2O at age 5 yrs. Within a rotation, most soil properties did not change significantly with plantation age, although they appeared to decrease during the first three years; total carbon then recovered to initial levels, but total nitrogen and exchangeable cations remained lower. Some soil properties were strongly related to gravel content and elevation, but not with growth rate. Thus consecutive plantings of short-rotation Acacia hybrid on degraded and abandoned land can lead to changes in some soil properties. Growth and physiological responses of H. odorata to different environmental conditions created in a nurse-crop plantation were examined in a field experiment where H. odorata seedlings were planted within three 22 m-diameter gaps opened in a 3-yr-old Acacia hybrid plantation in Central Vietnam. At age 2 yrs, stem diameter, total height and crown diameter of the H. odorata increased significantly from gap perimeter (GP) to gap centre (GC). This positive response correlated with significant increases in daily incident photosynthetically active radiation (PAR) from 24% to 61% of total incident PAR. Net photosynthetic rate at 1500 vîvÖ‚â†mol m-2 s-1 (A1500) and stomatal conductance (gs) were significantly lower for trees near the GP than those near the GC. Light-saturated photosynthesis (Asat) was significantly lower for trees near the GP than GC at age 1 yr, but not at age 2 yrs. Apparent quantum yield (˜í¬¿), dark respiration (Rdark), and photosynthetic biochemical parameters (Vcmax and Jmax) were similar between treatments. Chlorophyll content, chlorophyll fluorescence (Fv/Fm), and leaf N and P concentrations were also unaffected by treatment. Nevertheless, specific leaf area was higher in the GP than other treatments. Despite the substantial difference in PAR between treatments, trees near the GP received levels of irradiation >500 ˜í¬¿mol m-2 s-1 for 12% of the day v. 38% at the GC. Significant reductions of leaf water potential (˜í¬Æleaf) at the end of the dry season in treatments close to the GP compared to those near the GC suggested competition for water between H. odorata and the acacia nurse trees, although pre-dawn ˜í¬Æleaf remained >-0.5 MPa. Thus, despite being a species that regenerates naturally in shade, H. odorata quickly acclimates to much higher light environments. Understanding how H. odorata alters its architectural traits and growth rate in response to changing light environments is essential when designing and manipulating mixed-species plantations containing this species. Seedlings of H. odorata were planted into the circular gaps referred to above, and in 5 and 7.5 m strip gaps within a 2.5-yr-old A. hybrid plantation. Crown structure, absorption of photosynthetically active radiation (APAR) and whole-plant light-use efficiency (LUE; above-ground biomass growth or wood growth per unit APAR) of the seedlings over a gradient of light across the circular gap were examined for two years. Biomass production increased exponentially from the GP to the GC. This was largely due to an exponential increase in APAR and a minor increase in LUE. The large increase in APAR was associated with an increase in leaf area and a reduction in shading from the nurse trees. Conversely, APAR per unit leaf area decreased towards the gap centre, probably due to steeper branch and leaf angles in order to avoid high radiation. In the strip-gap planting, the PAR was similar to that at the perimeter of the circular gaps; however the light pattern was dominated by sun flecks in the strip gap and direct sunlight in the circular gap. While the LUE of the more shaded H. odorata trees in the strip gaps was much higher, this was not enough to make up for the much lower APAR and hence biomass production. This study shows that H. odorata is able to grow under a wide range of PAR and that the availability of PAR has a strong influence on its growth. While the strip gaps used in this study appeared to be too narrow, the circular gap indicated that nurse plantings are an effective silvicultural design for establishing H. odorata provided that competition for other resources is managed. The study concluded that Acacia hybrid is a potential species for recovery of some key soil chemical and physical properties. It is a potential nurse crop for re-establishment of native-tree species on degraded lands. Although H. odorata is shade-adapted species, it has great plasticity to acclimate to a range of light environments. However for mixed-species systems using these species, interspecific competition for light and soil water between A. hybrid and H. odorata needs to be addressed during the design and then management of the plantations.
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