Abstract: Small, medium and large gaps in three forest types, Tilia amurensis-Pinus koraiensis mixed forest (TP), Picea koraiensis-Abies fabri-Pinus koraiensis mixed forest (PAP), Betula costata-Pinus koraiensis mixed forest (BP), were selected as experimental plots, and its corresponding closed stand around each gap was set up as the controls in Xiaoxing’an Mountains of Northeast China. Soil organic carbon (SOC) and total nitrogen (TN) contents at the depth of 0-10 cm in the different sized forest gaps and their controls of various forest types were determined from June to September in 2012, aiming to illustrate the effects of gap size on the variations in SOC and TN contents in different types of natural P. koraiensis mixed forest. The results showed that gap size had no significant effect on the SOC content in the three kinds of natural Korean pine mixed forest, while had significant effect on TN content only in PAP. The SOC and TN contents varied greatly with time in the three kinds of natural P. koraiensis mixed forest. SOC content was in order of large gap>small gap>medium gap, while TN content was in order of large gap>medium gap>small gap in PAP and BP; SOC and TN contents were in order of medium gap>small gap>large gap in TP. SOC content was higher in the gaps than in the closed stands of the three kinds of forest, while TN content was lower in the gaps than in the closed stands in TP and BP, and was in the opposite order in PAP. SOC and TN contents in the different forest types were ranked in the order of PAP>TP>BP and TP>PAP>BP, respectively. The variations in SOC and TN contents over months mostly presented a unimodal type of curve with the peak values in July or August, but TN peaked in September in PAP. The difference in C/N between different size gaps in the three kinds of natural P. koraiensis mixed forests was not significant. Our study provides basic data for gap regeneration and sustainable management in natural P. koraiensis mixed forest in
Xiaoxing’an Mountains.

Key words: Toona ciliata var pubescens, sexual reproduction, regeneration., natural population, fitnesscoefficient, individual reproductive fitness