Forest gaps are openings or holes created in forest canopy. Gaps may range in area from the openings created by the death of a single branch to larger scale blowdowmns by catastrophic or noncatastrophic disturbance. Usually the size of gaps was defined within the boundaries of the 4 100m². Forest gaps are divided into two kinds of gap, canopy gap and expanded gap. Based on the fact that forest gaps play an important role in the maintenance of tree species richness, thus it is useful to research the forest gaps for forest regeneration In this paper, the authors mainly analyzed the relationship between the diameter height ratio (D_EG/H) and the number of young trees in gaps. The concept of diameter height ratio (D_EG/H) is defined as the ratio of the gap diameter to gap height. It is a major feature factor of forest gaps, and is an important indicator for studying forest dynamics and evaluating forest harvesting.
Based on three kinds of the natural secondary coniferous forest gap (Larix principis-rupprechtii, Picea spp, P.tabulaeformis) in Guandi Mountain in Shanxi Province, firstly the paper analyzed the structure of the forest gap using D_EG/H. Second, the scatter distribution trends of gap size and D_EG/H to the number of saplings were compared. Finally, linear model, logarithm model and second-order multinomial distribution model were used to analyze the saplings density and D_EG/H, and to compare the models through the correlation coefficient and the P-value of parameters. The results are as follows:
In the three kinds of the forest gap, the average size of diameter height ratio (D_EG/H) of Picea spp is the biggest(average value=1.3327), followed by Larix principis-rupprechtii(average value=1.1315) and P.tabulaeformis is minimum(average=0.7121). D_EG/H values of forest Gap for picea spp mainly are between 0.6 and 1.6, which reach 81.82%, P.tabulaeformis are between 0.8 and 1.6, which reach 70.72%, Larix principis-rupprechtii are between 0.4 and 1, which reach 97.06%.
The scatter distributions of gap size and the number of saplings are not regular, it is difficult to reflect the specific relationship between the gap size and the number of saplings. However the scatter distribution of gap diameter height ratio and the density of saplings are very regular, it is easy to reflect the relationship between gap size and the number of saplings. From the scatter distribution of gap diameter height ratio and the density of samplings, we can include that there is a positive correlation between the number of young trees and D_EG/H. According to analysis of the linear relations between the diameter height ratio of expanded gap and the seeding density, the results demonstrated that Larix principis-rupprechtii has the most obvious positive correlation, followed successively by oil pine and spruce forest.
The saplings density and D_EG/H were analysed based on the linear model, the logarithm model and the second-order multinomial distribution model. The results showed that the regression relationship of the three kinds of model are very significant for two significance levels 0.05 and 0.01, thus all of the three models can be used to fit the relationship between saplings density and D_EG/H in those three kinds of forest. The solution of the logarithm model is the best by using the correlation indicators of the coefficient and the P-value for each model.