The Relationship Between Competition And Diameter Distribution Of The Forest Mixed With Conifer And Broadleaves In Subtropical Forest Ecosystems

Competition is the main factor influencing the growth of trees, whilst the diameter distribution of forest is the primary characteristic of forest structures. The study on the relationship between forest competition and diameter distribution of mixed forest could help us know the rule of coordinate of different forest ecosystems. This would great improved the understanding of the internal evolution mechanism of the subtropics forest ecosystems, as well as provided theoretical basis for the sustainable development, the correct prediction and scientific management of forest.The objectives of this study were to investigate the diameter distributions characteristics and the intensity of competition in the Pinus massonian-Lthocarpus glaber mixed forest (PL) and the Lthocarpus glaber-Cyclobalanopsis glauca mixed forest (LC), the typical natural forests in the low mountain and hilly areas in subtropical China. The diameter distributions of dominant species were analyzed using probability density functions. According to the result of Chi-square test, the optimal model which has the best fitness compared with the actual diameter distribution was selected for each dominant species. In addition, the intensity of competition of the dominant species of the two forest stands was measured by the distance-dependent competition index. The competition index both in intraspecific and interspecific competition was measured for each diameter class and dominant tree species. Finally, the correlation between the diameters of objective trees and competition index was used to quantization the relationship between competition and diameter distribution. The results of this study indicated that:(1) Trees with diameter bigger than4cm were found in the studying forest community belonged to66woody species,50genera, and28families. All of the diversity index, including species diversity index (i.e., Shannon-Weiner index and Simpson index), species richness index (i.e., Margalef index) and species evenness index (i.e., Pielou index) of LC were greater than PL. In PL, the relative important value of Pinus massonian was the largest compared to the other species, which was38.82%. As the two most dominated species, the total relative importance value of Pinus massonian and Lthocarpus glaber was73.18%. However, the total important value of the other species such as Choerospondias axillaris, Liquidambar formosana and Cyclobalanopsis glauca was only26.82%. In LC, the two most dominated species were replaced by Lthocarpus glaber and Cyclobalanopsis glauca, the relative important value were28.42%and12.55%, respectively. Whiles, the relative important value of Pinus massoniana was10.16%, which is the third dominated species in LC.(2) The diameter distributions of the two stands were consistent with the shape of reverse J, which was in agreement with the regular pattern of diameter distributions in natural mixed forest. The Skewnesses of the two forest stands were both positive, which means that the diameter distributions of the two forest stands are obviously left-skewed. The kurtosis of PL was minus, the relatively flat variation of diameter distribution of PL indicated that the domination of trees with large diameter in the stand. However, the kurtosis of LC was greater than PL, the relatively sharp variation of diameter distribution of LC indicated that the overwhelming superiority of trees with small diameter. As the dominate species, the Weibull distribution was the best for the diameter distribution of Pinus massoniana compared with Exponential, Lognormal, Log-gamma, Gamma and Normal distribution. The Log-gamma distribution and Gamma distribution provided a satisfactory fit of diameter distribution of Lthocarpus glaber at PL and LC, respectively. We observed a good fit of the Exponential distribution to the diameter distribution of Cyclobalanopsis glauca.(3) According to the correlation between diameter and competition index of objective trees, the index which calculated by the square of diameter and distance between objective trees and competition trees was selected to quantitative distribute the competition in this study. In PL, the large competition stresses of Lthocarpus glaber were mainly come from the other species. Whilst the competition stresses of Pinus massoniana were relatively less, and most of them were caused by the trees of same species. However, the competition between trees in LC was significantly less than PL. There was no significant difference between intraspecific competition and interspecific competition in Cyclobalanopsis glauca. And the interspecific competition of Lthocarpus glaber in LC was greater than intraspecific competition.(4) The competitive intensities of objective trees of small and middle diameter were significantly greater than trees with bigger diameter. Generally, the competition index decreased with diameter of objective trees increased in this study. And, there were significant negative correlation between diameter of objective trees and the competition index. The negative relationship indicated that the bigger objective trees, the smaller the interspecific and intraspecific competition. On the contrary, there is a strong possibility that the smaller trees which under lager competition stress would be eliminated in community succession.