Environment Characteristics Of Typical Karst Rocky Desertification And The Effect Of Soil Water On Afforesting Tree Species

Karst widely distributed in the world, and southwest of China was the one of the most typical area. Because the fragile nature environment and human unfit disturbances, rocky desertification was serious in these zone. The area of karst rock desertification in China has always arrived 129.6 thousands km², and such area is expanding 2 500 km² annually. For the ecology control, the main ecology factors and the effects of water stress on the recovery tree species were studied in Guangxi province, Lingyun city. The results show that:(1) The natural succession process of vegetation in Lingyun city of Guangxi province after rocky desertification appearing was divided into the herbosa stage, the stage of herbosa-shrub, the stage of vine-shrub, the stage of subaltern tree forest and the climax stage. Typical plots were set up using space instead of time. The species composition was analyzed in each stage and the species diversity in the layers of herb, shrub, tree and community at different succession stages was measured by the methods of species abundance, Simpson index, Shannon index and Pielou's evenness index. The results showed that, with the exception of the herb layer, the species abundance in all layers rose steadily when the succession developed from the herb stage, and reached the maximum in the stage of subaltern tree forest. In the herb layer, species abundance and Shannon index fluctuated but Simpson index and evenness index decreased gradually, and reached the minimum in the climax stage. Simpson index, Shannon index and Pielou's evenness index first increased, followed by a decline, but the stage at which the index reached its maximum was not consistent. Diversity indices of the herb layer were higher than those of the shrub layer in the stage of herbosa. When the succession entered into the stage of herbosa-shrub, Diversity indices of the shrub layer rose and exceeded those of the herb layer. Diversity indices of tree layer were the lowest from the time when arbor appeared.(2) To study the effect of different disturbance to environment microclimate diurnal dynamics in the Karst zone, five typical modes were chosen and light intensity, air temperature, air relative humidity, soil temperature, and soil water content of -5cm were measured. The results showed that different plant types observably changed the composition, coverage, and height of overground vegetation, so the environment microclimate was changed at different degree. The effect of forest was the best, then was shrub, grass, farmland, and Rocky Desertification land. In the rockland, light intensity was strongest, air temperature and soil temperature were highest, air relative humidity and soil water content were lowest, and the variations of the five factors were notable. The microclimatic conditions of rockland developed towards much more dry and hot. While, compared to rockland, in forestland, shrubland, grassland, and farmland, the light intensity 96.4%,52.0%,17.0%,44.2% respectively, air temperature decreased 30.1%,20.2%,12.7%,17.8%, soil temperature decreased 11.5%,8%,2.5%,5.5%, and air relative humidity increased 129.2%,57.2%,18.0%,41.2%, soil water content increased 42.6%,33.2%,15.7%,14.0%. The five factors fluctuations were less than rockland. Light intensity, air temperature, and soil temperature were positive correlation, and negative correlation to air relative humidity and soil water content. Air temperature and soil water content were positive correlation.(3) The soil physical and chemical properties of different plant types in the typical Karst rocky desertification zone were analyzed. The plots included 7 types, such as farmland(FL), retired terrace farmland(RTL), retired sloping land(RSL), grassland (GL), shrubland(SL), coniferous forest land(CFL), broadleaf forest land(BFL). The physical properties indexes were bulk weight(BW), water content(WC), total porosity(TP), capillary porosity(CP), non-capillary porosity(NCP). And the chemical properties consisted of pH, organic matter(OM), total N, total P, total K, dissoluble N(DN), dissoluble P(DP), dissoluble K(DK), Exchangeable Ca²⁺(EC).The results show that different plant types did significantly effect on the soil physical and chemical properties. The difference between BW, WC, TP, CP and NCP of 7 types plant types soil was distinct(p<0.01). Coefficients of variation between 7 types plant types soil respectively were 12.37%,29.72%,18.07%,12.66%,84.70%. The difference OM, total N, total P, total K, DN, DP, DK and EC of 7 types plant types soil was distinct(p<0.01), too. BW was negative correlation with other indexes, while OM was positive correlation with other indexes. Relative soil quality indexes method was used to evaluated the soil quality, compared to the BFL soil, the relative soil degenerate indexes of SL, CFL, RTL, GL, RSL, FL respectively were -25.3%,-56.3%,-56.6%,-56.7%,-62.2%,-68.4%. Fuzzy multidimensional comprehensive evaluation's result show that the soil quality of FL was the worst, CFL, GL, RSL and RTL were middle, then the SL and BFL were higher.(4) The rainfall distribution is seasonal change, mainly happened in June to September and minimum in November. There were 108 times rain in whole year, and among them there were 60 times light rain, 25 times moderate rain, 11 times heavy rain, 8 times rainstorm, 4 times heavy rainstorm. Rainstorm and heavy rainstorm mainly effected the whole year rainfall. In research area, there were 53 stages (total 211 days) no-rain-date. No-rain-date was more than rain-date.Runoff could occur in KSD zone, and the runoff coefficient of every time changed between 1.5～13.8%, and the mean runoff was 75.86mm. There was prominence difference between different vegetation types(p<0.01), the runoff of farmland (122.75mm) was the highest. The sediment concentrations of runoff in spring and autumn were higher than in summer. The soil erosion modulus between vegetation types was prominence(p<0.01), and the mean erosion modulus was 45.88t/km²Â·a, belong to slight erosion. The total N concentrations declined with time, while, total P and total K were same to the sediment concentrations change. The difference of gross total N, P, K between different vegetations was prominence(p<0.01), and the gross was 230.491mg/m²Â·a, 4.408 mg/m²Â·a and 929.050 mg/m²Â·a.(5) The effects of water stress on three trees seeds from rocky desertification area was tested by the treatments with different concentrations of polyethylene glycol(PEG). The results showed that PEG could retard the germination of stressed seeds. the germination rate, germination index vigor index decreased in total. The change tendency of different drought resistance among three trees was similar, but the changes range was different remarkably.(6) Water stress had significant effects on three seedling height growth and biomass. The main results showed that: activity of root system, shoot length increment, diameter growtgh increment and total biomass of three seedlings declinde with the declining soil moisture. But shoot-root ratio of three tree species enhance with the declining soil moisture.(7) As stress increasing and time delaying, the net photosynthesis rate, transpiration, stomata conduct and WUE were gradually decreased but CO₂ concentration in stomata, COi/CO2 and Tleaf-Tair was increased, the effects becomes severity with the declining soil moisture. The midday depression of photosynthesis under both light and heavier stress may be induced by a combination of stomatal limitation and non-stomatal factors, but under extreme drought conditions, induced mainly by the non-stomatal factors.(8) The soluble protein of all seedling content decreased with water stress strengthened. The two trees seedling proline, Zenia insignis, and Acacia dealbata, increased sharply, but Catalpa fargesii was contrary with them. It demonstrated that different trees had different available osmotic substances when they were under the force of water stress. The acivity of SOD and POD enzymes of three trees seedlings were increased with the water stress, but the time of enhancing differed each other. The content of MDA, however, showed obviously negative corrlelation with activity of protective enzyme (POD and SOD) when water stress happened.