塔里木河上游荒漠河岸胡杨林树洞型空心树发生过程与形成机制

Characteristics of hollow⁃bearing tree in Populus euphratica forest in the upper reaches of Tarim River

空心树是森林生态系统的重要组成部分,它在维系森林生态系统的动物或微生物多样性方面起着重要作用,但树木空心的发生过程和形成机制仍是一个悬而未决的科学问题。通过野外实调,解析了塔里木河上游肖夹克荒漠林区胡杨树洞型空心树数量特征、类型与分布格局,探讨了空心树的发生与形成过程。结果表明:1)胡杨幼林空心树发生频率较低,为10.34%,但成熟林空心树发生频率达到了83.78%;2)空心树树洞分为主干型和分枝型两大类,其中主干和分枝断口造成树洞是树洞型空心树形成的两种主要表现方式,分别占树洞类型的41.94%和19.35%;3)推断树洞发生过程为“外伤⁃分泌物外泄⁃木质部被腐蚀、消融⁃伤口扩大⁃形成树洞”;4)树木发生外伤的主要起因可能为长期的风沙侵扰,造成树木小枝或者分枝折断。

Hollow⁃bearing tree is a part of forest ecosystem,which plays an important role for maintaining species diversity of animals and microorganisms in forest.However,occurrence process and formation mechanism of hollow⁃bearing trees are still outstanding.As the main constructive species of desert riparian forest,Populus euphratica has importantly ecological significance in ecological balance maintenance,desertification control,biodiversity protection and natural protection of oasis ecosystem.In this study,the quantitative characteristics and distribution pattern of hollow⁃bearing trees of Populus euphratica in the desert riparian forest in the upper reaches of Tarim River were studied through field investigation and statistical analysis.We set up a sample plot of 30×200 m in young Populus euphratica forest and mature Populus euphratica forest,respectively.A total of 996 Populus euphratica trees were investigated,including 892 young trees and 104 mature trees.The hollow⁃bearing tree was defined that there was at least one visible hole with the diameter≥5 cm.For the holes lower apart from the ground,we directly measured the length and width of the holes with a ruler;for the holes distributed at the high trunk or branches,we estimated the size of the holes with a telescope(10×25).The distance from the ground,the position in the trees,hole orientation,and the number of holes,as well as the diameter at breast height(DBH)of each tree was recorded when the field investigation was conducted.Based on the field investigation results,the types,occurrence process of hollows of Populus euphratica were discussed.The results showed that:1)the occurrence frequency of hollow⁃bearing tree in young Populus euphratica forest was relatively low(10.34%),but that of mature forest was very high,which was up to 83.78%.2)Hollow⁃bearing trees could be divided into two categories:hollows in main stems and hollows in branches,in which hollows in twigs of main stem or hollows in branches of trees accounted for 41.94%and 19.35%of all the hollows,respectively.3)The formation process of tree hollows might be“injury⁃nutrients and moisture discharge⁃rotten of xylem and phloem⁃expansion of the wound⁃hollows formation”.4)The long⁃term wind and sand invasion was the possibly main driver of these injuries,which broke the twigs and branches of trees to form the injuries.