Study On The Threshold Of Selective Cutting Intensity And Management Strategies Of Natural Secondary Forests In Northern Fujian

As the climax forest ecosystem in northern Fujian Province, evergreen broad-leaved forests, which have rich flora composition and biological gene, play an important role in ecological function, and are of great benefit to local economy and society. However, under the unsuitable human intervention, most natural forests in Fujian Province tend to be degraded, and the situation is getting severer. Presently, except several nature conservations established by government, most evergreen broad-leaved forests have been degraded to natural secondary forests. In the levels of landscape and the specific stands, the traditional management has been proved to be unsustainable for these forests. To meet the need of practical management, it is necessary to do more research on management tactics for these natural secondary forests. As a major harvesting method, some previous researches showed that selective cutting following natural regenerating may be sound compared with clear cutting and shelterwood cutting, because it may make every tree in the stand necessary space to grow potentially, and increase species diversity and ability of resistance against diseases, and make the ecosystems more stable. Therefore, from the point of harvesting and regeneration, selective cutting may be a good selection for harvesting method of natural secondary forests.The sample plots for this study on selective cutting were set up in Dayuan Forest Logging and Regeneration Station located in Jian'ou County, Fujian Province. The experimental stands were harvested by selective cutting of different intensities (low intensity of 13.0% in volume, medium intensity of 29.1%, high intensity of 45.8%, over-high intensity of 67.1%) in November, 1996. The un-cutting and clear cutting plots were set as controls. All the cutting areas were recovered by means of natural regeneration following cutting operation. The regeneration investigation was conducted again in July, 2006. Vegetations in these plots were investigated adopting the regular methods of community ecology. Forest litters within plots were collected for further analysis, and soil sections were examined in every plot as well. The effects of different intensity selective cutting on the forest ecological factors, including stand structure, species diversity, regeneration, litter nutrient content, physical and chemical properties of soil, were investigated to determine the sound threshold of selective cutting intensity of these natural secondary forests. Environmental costs of logging operations were evaluated quantitatively also. Based on the results of investigation of these fixed plots, as well as the realistic structural characteristics of other natural secondary forests as comparison, the strategies of selective cutting operation for natural secondary forests were put forward to provide guidance for the management of natural secondary forests in northern Fujian. The main conclusions of this study are as follows:1) Selective cutting may preserve original stand structure basically, while the clear cutting results in great destruction in it. The more intensive of cutting, the more changes of tree species diversity and importance value of species in stands. Low, medium and high intensity selective cuttings caused a light variation to the tree layer flora, while the over-high intensity selective cutting changed it greatly.2) Species richness of tree layer peaked after 10 years of low intensity selective cutting, and the higher intensity of selective cutting, the less species richness. Diversity indexes of all the selective cutting plots after 10 years of cutting are more than the original ones. Selective cutting operation is beneficial to the development of tree layer species diversity, but the clear cutting results in the greatest decline of species diversity.3) All the skewnesses of diameters distribution of selective cutting plots after 10 years are negative, which suggests that the small-size trees dominate in the stands. The diameters of medium and high intensities cutting stands are distributed in negative exponential distribution. And Weibull distribution models are suitable for all the diameter distributions in the experimental natural forests after selective cutting.4) Diameter-class and stock structure of all plots are away from the ideal stand structure, particularly the over-high intensities selective cutting which led to a defective forest. The value of q on non-cutting plots is 1.404, while in other selective plots it is between 1.2 and 1.4. This suggests that if the selective cutting intensity is no more than that of high intensity cutting, i.e., 45.8%, the q value will still be reasonable.5) With selective cutting intensity increases, the volume growth rate is increasing, and the average annual growth and the total harvest volume inclines to change as a convex parabolic trend. For the high intensity selective cutting plots the average annual growth is the highest, i.e., 6.08m~3/(hm~2·a).6) The low and medium intensity selective cuttings may enhance the stability of forest community spatial structure effectively. The forest mingling intensity is the biggest in low selective cutting plots, that of medium intensity cutting ranks the secondary. Selective cutting intensity exerted little influence on diameter differentiation, but the positions of dominant species changed with different extents. Selective cutting did not change the spatial distribution of species. Aggregation distribution was the spatial pattern of either the forest as a whole or the dominant species, regardless of what selective cutting intensity is.7) The natural regeneration has been well on the way after selective cutting operation. The regeneration density goes up with the increasing intensity of selective cutting. All the distribution patterns in the five dominant populations follow the clumped pattern regardless of selective intensity, but the assemble intensity and mean crowding index varied with different trends.8) Biomass of forest litter and total nutrient contents were generally reduced with the cutting intensity increasing. Biomass of forest litter and total nutrient contents in high and over-high intensity selective cutting sites have not been recovered after 10 years, while that in low and medium intensity selective cutting sites were close to that of uncutting areas. It showed that vegetation in the low and medium intensity selective cutting plots have been recovered after 10 years.9) The soil aggregate stability, moisture content, porosity and nutrient of low and medium selective cutting plots have been recovered basically; furthermore some indexes have been partly improved after 10 years of cutting. But those in high intensity, over-high intensity and clear cutting plots, the physic-chemical properties of soil haven't recovered yet. And with the felling intensity increasing, the properties of soil deteriorated much more. By means of principle component analysis (PCA), the main indexes including variations of total K, available K, available P, organic matter, total N, non-capillary porosity, bulk density, maximum moisture capacity, minimum moisture capacity, total porosity, which responds most of the impacts of soil by cutting, were analyzed quantitatively. The orders of the disturbance degree are as followings: clear cutting>over-high intensity selective cutting>high intensity selective cutting>medium intensity selective cutting>low intensity selective cutting.10) Comprehending the research results, we concluded that for harvesting and management, the natural secondary forests in northern Fujian could be taken the selective cutting with low or medium intensities, and the intensity threshold should be no more than 40%.11) From the conversed values of water and soil conservation, carbon sequestration and oxygen releasing, the environment cost of selective cutting operating were evaluated as well. The results show that environmental cost of low intensity selective cutting is the least, only 1.7 Yuan/m~3, while that of medium and high intensity cutting were 5.1 Yuan/ m~3, 12.2 times of that of low intensity. Over-high intensity selective ranks the highest, which is 27.6 Yuan/m~3, and is 16.2 times of that of low intensity selective cutting. Due to its more timber volume harvesting, the cost of clear-cutting is lower than that of over-high selective which is 14.9 times of that of low intensity.12) According to fixed-plots survey data of natural secondary forests in northern Fujian, the realistic structural characteristics were analyzed, which can be divided into 4 different levels. The diameter and stock structure of each level were studied also.13) Based on the integration of research results, techniques model of selective cutting for natural secondary forests in northern Fujian is put forward, which contains the management objectives, determination of selective cutting methods, selective cutting intensity, selective cutting cycles, sound skidding methods and clean-up techniques for harvesting operations.