Characteristics of canopy disturbance for a typical broadleaf-Korean pine mixed forest in Xiaoxing'an Mountains, Liangshui, northeastern China.

To analyze the characteristics and formation causes of expanded gaps due to canopy disturbance in a broadleaf-Korean pine (Pinus koraiensis) mixed forest in Xiaoxing'an Mountains, Liangshui, northeastern China, we investigated all expanded gaps (≥50m2) in a 6ha permanent plot by using the quadrat survey and dendrochronology methods. Results showed that the expanded gaps, with a speed of 0.08ha per year, occurred by an average density of 7.67 gaps per hectare. The frequency and return period of canopy disturbance was 0.42% per year and 240 years, respectively. The main formative tree species of forest gaps were P. koraiensis (50.2%), A. nephrolepis (9.78%), Betula costata (7.78%) and other trees (10.44%) which cannot be identified to species level due to serious decay. Death modes of trees for a gap were mainly breakage at trunk base (BB, 54.89%). Forest gaps were formed mainly by more than three ways of tree death, which accounted for 70.83% of all gaps. The average number of gap makers in a gap was 9.38 individuals, and most of them were in a higher decay level. Small-diameter fallen trees were often the victims rather than makers of gap formation, and the contribution of conifers to a gap formation was much greater than broadleaf trees. The average diameter at breast height in a gap border was 46.68cm (range 11.4-126.5cm) and height of trees 23.6m (range 5.6-42.9m), and the major tree species was Pinus koraiensis (63.08%). Abies nephrolepis grew fastest among all tree species in gap borders, while Picea koraiensis and Acer mono grew slowest. The species and compositions of gap makers and border trees were similar in gaps of different sizes, but the definition standard of gap makers has significant influences on the causes of gap formation. The accuracy of reconstruction of disturbance history and determination of formation causes can be greatly improved by surveying large and medium-sized gaps as well as large diameter makers using several comprehensive methods. In addition, the development and formation process of large and medium-sized gaps in which two or more disturbances occurred can be clarified, which is crucial for understanding the dynamics of canopy disturbance and succession of forest communities.