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

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.