Dynamics Of Restoration Of Disturbed Evergreen Broad-leaved Forests And Ecological Strategies Of Main Woody Speceis In Tiantong National Forests Park, Zhejiang

Evergreen broad-leaved forest (EBLF) is a zonal forest ecosystem of Eastern China. Under the long-term, frequent disturbance of human activities, most of EBLF have been deteriorated to many degradation types, such as secondary forests, secondary shrub communities, and shrub-grassland, even to bare lands. It is very important to learn the restoration process and mechanism of EBLF following different disturbance levels.The study site is located in Tiantong National Forest Park (TNFP, 29°53'N, 121°39'E), Zhejiang Province, Eastern China, several kilometers inland from the East China Sea. Five 20 m×20 m plots, at 260m above sea level, with a 25-30°slope, were established within TNFP, in October, 2003. The main composition of the community was evergreen broad-leaved species of Fagaceae, Camellia, and Symplocaceae and was dominated by Schima superba, Castanopsis carlesii and Lithocarpus glaber. Four plots were disturbed by diversity treatments according to the common natural or artificial disturbance types of this area, and the other was free of disturbances. The four treatments were: PlotⅠ, removal of the aboveground canopy trees over 8 m, to simulate canopy gaps caused by selective logging or a typhoon; PlotⅡ, removal of vegetation that simulated clear-cutting; PlotⅢ, removal of vegetation, litter and the topsoil (0-10 cm) layer to represented landslides or mining disturbances; PlotⅣ, removal of the understorey vegetation below 8 m, which is standard forestry management practice. PlotⅤwas left undisturbed.Long-term monitoring investigations of floristic composition, community structures, and habitat factors were carried out sequentially (2006-2008) to clarify the restoration pattern and dynamics of vegetation in different degradation plots. From the invading, growing and regenerating stage, combining leaf traits and crown architecture of main woody species, the restoration mechanism of destroyed EBLF was tempted to discuss.The main results as follows:(1) Species richness of disturbed plots increased sharply during the 5 years after disturbance and was higher than the richness of the pre-disturbance plot. The richness increased exponentially in the early stage, reached the summit in the 3^rd year after disturbance, and followed the inflexion. The richness was higher in the plot with light disturbance.(2) The life-form was changed significantly in disturbed plots. The annual and perennial herb dominated in the 1^st year after disturbance. Wood species was maily composed of deciduous broad-leaved trees, whose numbers increased along disturbance levels. As the process of restoration, the stratification was formed with the decreasing domination of deciduous broad-leaved species and increasing domination of evergreen broad-leaved species. In the 5^th year after disturbance, Choerospondias axiliaris and Sassafras tzumu in the plot removal of vegetation and even removal litter and the topsoil reached the height of 6.4 and 7 m respectively.(3) On the base of previous research, according to the appearance and disappearanceof species, we divided the species in each destroyed plot into 4 species groups, residual species, disappeared species, invading species and once-appeared species. In the early stage of restoration, the species in heavy disturbed plots were mainly dispersed by wind, and the ratio of resprouting individual of evergreen broad-leaved trees was the highest. Due to the strong ability of resprouting, the climax sepceis appeared in the early stage of restoration.(4) According to the photosynthetic photon flux density, air and soil temperature and moisture, soil chemistry characteristics, and the restoration pattern following different disturbance levels, the destroyed plots could be divided to 4 types. 1)Accelerated regeneration (plotⅠ), the evergreen broad-leaved species and shrub species understory grew rapidly after the remove of canopy layer. 2)Restrained regeneration (plot IV), the remove of understory leaded to the regeneration of fern, but its predominance restrained the regeneration and growth of woody plants. 3)Progressive succession (plotⅡ), perfect soil and light condition, abundant seed resources, resprouting and soil seed bank accelerated the succession. 4)Delayed succession (plotⅢ), poor soil nutrient and destroyed soil seed bank limited some pioneer species distribution and growth, the restoration rate of vegetation was slowly. The 4 restoration types proved the multi-approach succession in this region.(5) The number dynamic, spatial distribution, and height-growth trajectories of 7 main tree species of the main species group, invading species and residual species, were studied to discuss mechanisms of pattern and dynamics of the initial stage of restoration. The invading species included decidous broad-leaved Litsea cubeba, Sassafras tzumu and Choerospondias axiliaris. The residual species included evergreen broad-leaved Schima superba, Lithocarpus glaber, Castanopsis carlesii and Castanopsis fargesii. The results shown that, the distribution and number of invading species were higher than residual species, which were related with seed mass and dispersal type. For residual species, the population deponded on the pre-disturbed population and the resprouting ability. The height gowth of invading species was higher than residual species.(6) Fast height growth was an important aspect of the dominantion at the early stage of restoration. Therefore, leaf traits (including the the maximum of net photosynthesis based on area (Amax), specific leaf area (SLA), leaf nitrogen and phosphor content (N, P)) and crown architecture of 7 main species were analysed to explain the height growth pattern via the light transform and capture. The results shown that, the decidous invading species had high ability of light transformation, and simple crown architecture, flat-shaped crown with low investment, while high invested in the branch growth. The acclimation to strong light condition ensured the dominance of deciduous broad-leaved species at the early stage of restoration. The evergreen residual species had low ability of light transformation, and complex crow architecture, deep crown with high investment, while low invested in the branch growth. The acclimation to shade condition made the slow height growth of evergreen broad-leaved species at the early stage of restoration, but it was the powerful instrument to become the dominant species or the understory companion species at the late stage of restoration.(7) The intensity and type of disturbance caused the change of resource availability, which affected the leaf trait and crown architecture. Light and soil nutrition were the main environmental fators. In light disturbed plot with shade condition, plant owned low Amax, large and thin leaflet, low bifurcation ratio, small branch and leaf angle to horizon, short branch and flat crown to enlarge photosynthetic area and cut down consume of self-maintenance, then forming the short individual. Along the disturbance level, light increased, and plant had higher Amax, small and thick leaflet and large branch and leaf angle to horizon, which benefited the fast growth and self-protection under strong light. Individual density was another factor affecting the crown architecture. In the plot of vegetation removal, the high individual density caused the higher bifurcation ratio, the larger branch angle to horizon, longer branch length and narrow crown, which benefited growing fast and increasing competitiveness for light. The correspondence of leaf trait, crown architecture and restoration type explained effectively the mechanism of restoration.(8) The middle-long term prediction of the future trajectory of each destroyed community shown that, the plot removal of vegetation, litter and the topsoil would enter the climax stage early than the plot only removal of vegetation. It might because the poor soil caused the slow growth of deciduous broad-leaved species, while the powerful accumulation of nutrition of the resprouting individual of evergreen broad-leaved species ensured the fast growth. It would make the destroyed vegetation span the deciduous and evergreen broad-leaved forest.