黄土丘陵区油松天然次生林林窗特征与更新动态

研究了黄土丘陵区油松天然次生林林窗的形状、大小结构、分布、形成木特征及其更新状况.结果表明:在油松天然次生林中，林冠林窗(CG)和扩展林窗(EG)面积均呈以小林窗为主的偏态分布.CG平均面积为31.15 m²,以20～40 m²林窗的数量比和面积比最大,分别为38.24％和30.50％;EG平均面积为58.04 m²,以30～60 m²的数量比和面积比最大,分别为36.77％和27.79％,且CG的平均面积占EG平均面积的53.67％;林窗形状多呈椭圆形,高度多在14～16 m;林窗形成年龄以10～20年为主,占33.82％.林窗中基折和枯立木分别占形成木总数的47.66％和23.44％.林窗形成的主要因素是人为间伐或盗伐,树木衰老等引起的抗性下降、干旱、病虫害等也是导致树木死亡的原因之一;每个林窗中平均有1.89个形成木,其中以2株形成木的林窗最多.林窗形成木主要是油松,其次为山杨、白桦和辽东栎等.形成木的径级呈明显的偏态分布,以10～20 cm和21～30 cm径级最为普遍,分别占总数的25.0%和45.31%,与林窗面积偏态分布吻合;林窗内林木的更新状况好于林下,且油松幼苗不存在断层,而油松林下幼苗在年龄结构上有明显的断层.

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.