The primary broad-leaved Pinus koraiensis forests in China are almost completely lost due to human and natural disturbances in recent years. Hence, it is critical to quantify disturbance regimes in their typical distribution areas. The aims of this study were to: (1) develop the disturbance chronology in a typical broad-leaved P. koraiensis forest in Xiaoxing’an Mountain; (2) investigate the disturbance characteristics of forest gaps; and (3) explore the possible mechanisms of disturbances. Methods
A total of 461 incremental cores in P. koraiensis and 145 cores in Abies nephrolepis were collected from 44 forest gaps in a 6 hm2 permanent monitoring plot. Two disturbance chronologies were developed respectively for P. koraiensis and A. nephrolepis by detecting growth release with boundary-line release criteria. The significant disturbance period was identified by the multi-taper method (MTM) of spectral analysis. In addition, the disturbance mechanisms were evaluated by the superposed epoch analysis (SEA) between percentage growth changes in the two tree species and wind speed, extreme temperatures and sunspot numbers by using the EVENT program.Important findings
The variations of percentage growth changes (GC) in P. koraiensis and A. nephrolepis at the edges of forest gap were similar to those in closed canopy. However, there are apparent differences in GC among different gaps; the forest gap disturbance and its impact varied greatly. The strong growth release in P. koraiensis occurred in the periods 1733-1738, 1748-1752, 1769-1771, 1798-1801, 1827-1833, 1841-1844, 1935-1939, and 1968-1973, with significant disturbance peaks in 1752, 1770, 1800, 1830, 1842, 1937, and 1970. The growth release in A. nephrolepis occurred in the periods 1889-1904, 1932-1938, 1947-1973, and 1986-2005, with significant disturbance peaks in 1894, 1934, 1951, 1968 and 1990. The disturbances occurred at intervals of 2.0 a, 3.5 a, 3.8 a, 7.3-7.9 a, and 9.1-18.2 a in P. koraiensis, and of 3.5-3.6 a, 7.5-48.8 a, and 65-85 a in A. nephrolepis. Wind was a major mode of disturbances for producing forest gaps and resulting in tree growth releases in the primary broad-leaved P. koraiensis forest in the Xiaoxing’an Mountain. In addition, extreme temperatures could also affect the regime of tree growth release in this region. Solar activity may be another important mechanism of forest gap disturbance and tree growth release in the primary broad-leaved P. koraiensis forest; it affects the forest gap dynamics by changing local wind speed, air temperature, precipitation, and other large-scale climate patterns in the Xiaoxing’an Mountain.