Tree Species Selection For Afforestation In Rocky Desertification Areas Of Southwestern Guangxi

The study site of rocky desertification (RD) control was located in southwestern Guangxi, across19counties,4cities, where areas of rock exposure widespread, soils seriously degenerated, natural disasters frequently occurred, coupled with high eco-geological importance and poor economy, so the management of RD was very difficult. Selecting suitable tree species for afforestation and recover the vegetation of the region was an urgent and difficult task for forestry sector. This paper tested9candidate tree species in the areas, investigated their survival rate of afforestation, growth and litter water-holding capacities, and identified suitable tree species and silvicultural practices for the area, to provide technical support for the control of RD of the region. The study has come to the conclusions as follows:(1) Among the9species, the highest survival rate of afforestation (100%) presented in Leucaena leucocephala, and that of Tectona grandis, Zenia insignis, Bischqfia javanica, Acrocarpus fraxinifolius was over80%, meeting or exceeding the National Afforestation Acceptance Criteria, and we thought these5tree species were suitable for afforestation in the RD area of Southwest Guangxi.(2) The tree height, diameter at breast height (DBH), individual volume varied significantly, and Z. insignis, T. grandis, L. leucocephala, A. fraxinifolius grew fast in tree height and DBH, among7year olds, annual average tree height growth>1.1m, annual average DBH growth>0.9cm. Syzygium hainanense, B. javanica, Cinnamomum camphora grew moderately in tree height and DBH, with annual average tree height growth of0.5m and annual average DBH growth>0.5cm. Dracontomelon duperreanum and Caesalpinia sappan grew relatively slow in tree height, and C. sappan’s DBH growth is the slowest. Z. insignis had the most individual volume, and followed by T. grandis, which7year-old stand volume reached63.02m3/ha and47.26m3/ha respectively. L. leucocephala and A. fraxinifolius were moderate in growth of individual volume, while S. hainanense, C. camphora, D. duperreanum, B. javanica and C. sappan had slow individual volume growth. These indicated that Z. insignis, T. grandis, L. leucocephala and A. fraxinifolius can be used as timber species to recover degenerated land and increase forest stock of the RD area. (3) Litter amounts of the9species varied obviously, ranging from1.55t/ha to6.74t/ha; and the maximum total litter presented in T. grandis, which was over4times of that of C. sappan. Total litter volumes were in an order as follows:T. grandis> C. camphora> L. leucocephala> A. fraxinifolius> B. javanica> D. duperreanum> S. hainanense> Z. insignis> C. sappan forest. Litter water-holding capacities differed among the species, and the highest water-holding capacity of undecomposed layer litter displayed in C. sappan forest, which were2.26times of their masses; while the highest water-holding capacity of decomposed layer litter displayed in Z. insignis forest, which were2.35of their masses. Litter water holding volumes were logarithmically related to the soaking times, which increasing the most in the first1/6h of soaking, slacking after2-4h, and saturated after24h. Litter water absorption rates were exponentially related to the soaking times, and all litter layers absorbed water fast in the early stage of soaking, reaching maximum at1/6h, then the absorbing rates gradually reduced; the absorbing rate decreased fastest during the first2h, moderately decreased from4h to8h, slowed down after8h, and saturated after24h soaking. Litters’effective water-holding volumes varied significantly among the9species, ranging from2.08t/ha to8.89t/ha; among them, T. grandis forest had the highest litter effective water-holding volume,8.89t/ha, which was4times of that of C. sappan forest. Litter effective water-holding volumes were in an order as follows:T. grandis> C. camphora> A. fraxinifolius> Z. insignis> B. javanica> L. leucocephala> D. duperreanum> S. hainanense> C. sappan forest.(4) Considering the findings of our research and the characteristics of RD in southwestern Guangxi, the slightly RD lands can be used to plant fast-growing timber species such as Z. insignis, T. grandis and A. fraxinifolius. Moderate RD lands can be used to plant Z. insignis, T. grandis, A. fraxinifolius, L. leucocephala and B. javanica etc. On severe RD or extreme RD lands, direct seeding forestation of L. leucocephala and C. sappan were recommended, and policies of "closing hillsides to facilitate afforestation" should be implemented.