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Effects of Thinning Intensity on the Growth of Swietenia macrophylla Plantations and Understory Vegetation
The site was located at the plantation No.24 and No.55 in 2nd and 9th compartment of Hsin-Hua forest station of National Chung Hsing University, to discuss different thinning intensity influencing the growth of Swietenia macrophylla plantation and the change of understory vegetation. So accomplishing the thinning treatment of 10%, 20%, 30% and 40% thinning degree and control site by the parameter diameter basal area. After the thinning treatment, the researcher makes the inventory in the first and third year to record the diameter of woodystory and the coverage area of undrestory. Moreover, the researcher investigates the relationship among the environment factors for the reference of the future management.
By operating in different thinning intensity, after three years growth, there is the significant difference in the total species number of woodystory. That shows that thinning can increase the diversity of woodystory species. There is no significant difference in the total species number of understory. It is that there is no difference in the composition of understory species in each plot.
The diversity index of the plant community will increase by raising the thinning intensity. That means the higher thinning intensity can increase the species diversity of stand, promote the stability of stand and help the growth of Swietenia macrophylla. Due to light thinning, the promotion is obvious in the first year after thinning treatment; however, in the third year, the growth is getting declined. Through estimating the growth of Swietenia macrophylla, the stand of the 23-year-old Swietenia macrophylla has not reach maturity and there is still much growing space. Besides, according to the inventory, the researcher discovers that there are more seedlings in each plot. It means that there is no difficulty on natural regeneration of Swietenia macrophylla seedlings. The number of the big diameter structure is not little. Therefore, on the future succession stage, the big diameter structure still dominates the canopy. Moreover, from the result of the investigation, the research also discovers that in higher thinning plot the importance value index of Mikania micrantha is also higher. That shows that after thing the open of the canopy can help Mikania micrantha with growth.
From the correlation analysis between the environment factors and the growth of Swietenia macrophylla, the result shows that the steeper slope is, the worse Swietenia macrophylla grows and the lower pH of the soil is, the worse Swietenia macrophylla grows.
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