[Gap features and renewal dynamics in secondary natural Pinus tabulaeformis forest in hilly loess region]

Taking the secondary natural Pinus tabulaeformis forest in hilly loess region as research object, the shape, size structure, distribution, gap-maker features, and renewal dynamics of gaps were investigated. The results showed that the areas of canopy gap (CG) and extended gap (EG) appeared to have a skewed and small gap- dominated distribution. The CGs had an average area of 31.15 m2, and those with an area of 20-40 m2 made up the highest proportions in number and area, accounting for 38.24% and 30.50%, respectively; while the EGs had an average area of 58.04 m2, and those with an area of 30-60 m2 made up the highest proportions in number and area, accounting for 36.77% and 27.79%, respectively. The average CG area accounted for 53.67% of the average EG area. The gaps were mainly elliptical, and their ages were mainly within 10-20 years, which occupied 33.82% of the total. The gaps were mainly with a height of 14-16 m, and those with a height of 18-22 m made up 36.8% of the total. The gaps were mainly formed by the trees being broken at their bases and the standing dead trees, which made up 47. 66% and 23.44% of all gap-makers, respectively, and thinning and unlawful felling were the major factors in gap formation. The tree deaths from lowered resistance due to tree- ageing, drought, and pest and diseases were one of the reasons for gap formation. The gap-makers per gap numbered 1.89 on average, and most of the gaps were formed by two gap-makers. The gap-makers were mainly Pinus tabulaeformis, followed by Populus davidiana, Betula platyphylla, and Quercus liaotungensis. The diameters of the gap-makers appeared to have a remarkably skewed normal distribution, and the diameters commonly ranged in 10-20 cm and 21-30 cm, taking up 25.0% and 45.31%, respectively. This skewed normal distribution agreed with the skewed area distribution of the gaps. In forest gap, trees had a better regeneration condition, and Chinese pine seedings had no age-discontinuity; while in the understory of Chinese pine, seedings had an obvious discontinuity in age-structure.