Effects of Different Gaps on the Form Quality and Growth of Taxus chinensis var. mairei in Pinus massoniana Plantation

In order to evaluate the effects of forest gap size on crown morphology, growth and form quality of Taxus chinensis var. mairei, clarify crown morphological regulation techniques, 30 sample plots were established in Pinus massoniana plantation gaps in Mingxi County, Fujian, China to measure the indices of the crown morphology, growth and stem form indexs of T. chinensis var. mairei seedlings. The relationship between the gap size and crown morphology, growth and stem form were investigated. The 30 sample plots were located at 3 microhabitats which were classified in gap size as follows: Class I, II and III, which had a gap size of 50-100 m², 100-150 m², 150-200 m², respectively. The 3 classes of forest gap were evaluated comprehensively by using the multi-objective decision making method. The effects of crown size, crown shape rate and crown rate on the growth, form quality and comprehensive performance of T. chinensis var. mairei among the forest gap types were analyzed by path analysis, clarified the key points and directions of morphological regulation and verified. The results showed that gap size significantly affected 4 indices, i.e. height, DBH, individual volume index, length of non-knot timber and comprehensive evaluation value, but it had no significant effects on taperingness; classⅠand II both significantly promoted the growth of individual volume, DBH, length of non-knot timber and comprehensive evaluation value; classⅠ significantly promoted the height growth, gap size significantly affected crown morphological characteristics of T. chinensis var. mairei, such as crown width, crown shape rate and crown rate; classⅠ significantly promoted the crown width growth, classⅠ and II both significantly promoted crown shape rate and crown rate; crown width, crown shape rate had significant positive effects to individual volume, DBH, length of non-knot timber, crown rate had significant negative effects to individual volume, DBH, length of non-knot timber of all forest gap types; crown shape rate had significant negative effects to taperingness, but crown width, crown rate had no significant effects to taperingness in class Ⅲ forest gaps; crown width, crown shape rate, crown rate had no significant effects to taperingness in classⅠ and II forest gaps. The influence and the relative importance order of forest gap types and canopy characteristics on those growth and form quality indicators of T. chinensis var. mairei were different, the key point and direction of crown morphology regulation for promoting volume growth and improving stem-form quality were different; crown width, crown shape rate had significant positive effects to the growth and form quality comprehensive performance, but crown rate had significant negative effects to the growth and form quality comprehensive performance, the order of relative importance of factors is crown width > crown shape rate > crown rate of all forest gap types. This study suggests that controlled cutting intensity can be used to create forest gaps of 50-150 m², which improves the precious timbers cultivating process of T. chinensis var. mairei in P. massoniana plantation. The key point and direction of crown morphological regulation of T. chinensis var. mairei at the present stage was to promote the crown width. It has been proved that the volume, DBH and sapwood length of T. chinensis var. mairei after crown morphological regulation had ideal gain, and the comprehensive performance gain was 16.83-35.31%.