The regulation on water and heat factors during the restoration process of an Acacia mangium plantation

Previously, Heshan County, Guangdong province was covered with degraded grassland, bare land and other degraded plantation ecosystems due to the removal of natural vegetation. In recent years, vegetation rehabilitation has succeeded in this region by integration with intensive scientific studies and practices, which also resulted in a great progress in water and heat environments there. This experiment was carried out in Heshan hillyland interdisciplinary station (22°41′ N, 112°54′ E), a very good place for studies on the rehabilitation and reconstruction of degraded ecosystems in the northern subtropical zone. Based on long term field measurement data, this paper reported the development of water and heat factors during the restoration progress, started in degraded grassland and using Acacia mangium forest stands as the pioneer community. Interactions between hydro_thermal and forest and their temporal and spatial dynamics were analyzed. The monsoon climate brings abundant rainfall to the experimental area, but more than 85% of annual rainfall falls in the wet season (April to September). Owing to the uneven distribution of the monthly rainfall in a year but evenly distributed monthly rich solar radiation, the potential of soil erosion during the wet season and water deficit in the dry season is high. The high productivity potential can be anticipated in an ecosystem if it can reduce the water demand in the dry season and increase water storage in the wet season, such as is seen in Acacia mangium plantation. On the contrary, degraded ecosystems in this region do not have the ability to adjust, tend to be further degraded and are hard to restore. Reflectivity in restoration process for Acacia mangium plantation was much less than that of grassland or bare land, which decreased with tree growth before 11 years old and fluctuated between 6.7%_7. 0% after 11 years old. The fluctuations of air temperature indexes in Acacia mangium plantation stand were less than those in bare land, for example, the fluctuation of the highest extreme monthly mean air temperature in bare land is 9.2 ℃, but only 5.0 ℃ in Acacia mangium plantation stand. The same pattern was seen with soil temperature. It benefited the rehabilitation and reconstruction of function and structure of the ecosystem. The relative humidity of Acacia mangium plantation was much higher than that of bare land in the dry season, but the potential evapotranspiration was opposite. This pattern greatly reduced the water demand and evapotranspiration in this plantation, and lessened its tension of water supply. Therefore, the growth of Acacia mangium plantation can sustain the dry season, which made the ecosystem rehabilitate smoothly. With the succession of Acacia mangium plantation, its evapotranspiration and groundwater increased but the total runoff was decreasing. At the same time, the soil moisture of Acacia mangium plantation became higher than that of grassland. Based on the above, it could be concluded that there was evidence of vegetation controlling the heat balance and the water distribution in an ecosystem. As a whole, the seasonal water consumption in Acacia mangium plantation became more reasonable and water and heat conditions were improved. The hydrological and thermal interactions of the ecosystem resulted in the maintenance of a healthy forest in a region where soil degradation, erosion and loss of fertility have often followed the removal of natural vegetation.