ObjectiveForest layer is a kind of structure on vertical scale of stand spatial structure, which is also a research hotspot of stand spatial structure at present. The study of forest layer mostly focuses on the methods of forest layer division and the analysis of the structure of each forest layer. However, there were few studies on the relationship between forest layers. Detailed analysis of stand structure is very helpful for precise forest management. Exploring the distribution of forest layer structure and its relationship with growth can enrich the theoretical basis of structured forest management, and also has certain guiding value for the construction of competition index.

MethodBased on the relationship between neighboring trees, the distribution of target trees and neighboring trees in the upper, middle and lower forest layers was distinguished. Five cases of stand level rate (L) were refined to 45 types of " forest layer structure types”. The distribution of forest layer structure of Q. mongolica forest was deeply analyzed with L and forest layer structure types, then the relationship between them and DBH increment was discussed.

Result(1) The forest layer structure of Q. mongolica forest is more complex: four neighboring trees in the structural unit of the target wood were less in the same layer with the target wood, while cases of two or three neighboring trees distributing in different layers with the target wood were more common. In the target tree structure unit, the five trees tended to distribute in three forest layers or in two adjacent forest layers, and the more trees distributed in the middle layer, the more quantity of this sort of " forest layer structure type” was. (2) The significant difference of DBH increment between the stand level rate type of the Q. mongolica forest was: the lower layer > the upper layer > the middle layer; when the neighboring trees in the same layer ranged from 4 to 0, the growth of the underlying trees had a significant downward trend, the upper trees had a significant upward trend, while the role of the neighboring trees in the middle layer was complicated, resulting in no significant change in the growth. (3) The forest layer factors affecting the growth of upper target trees in Q. mongolica forest were relatively simple, mainly the lateral extrusion of the neighboring trees in the upper layer; while the effects of forest layer factors on the growth of lower and middle forest layers in Q. mongolica forest were more complicated. The middle object wood was mainly affected by the cover of upper layer trees, followed by the lateral extrusion of the middle layer trees; the lower target wood was mainly affected by the cover of the middle layer, followed by the lateral extrusion of the lower layer or the cover of the upper layer.

ConclusionThe " forest layer structure type” constructed in this paper has a good effect in analyzing the quantity distribution of forest layer structure and explaining the size of DBH increment. It can systematically and comprehensively analyze the inter-layer structure.