林窗对长苞冷杉自然更新幼苗存活和生长的影响

 长苞冷杉(Abies georgei)林是我国西南亚高山针叶林的重要类型之一，分布于海拔3 200～4 200 m。目前对于该森林林窗对树苗更新的调节还很少了解。通过1997～2000年对20个林窗的连续观测调查，研究了滇西北白马雪山自然保护区西坡亚高山长苞冷杉林林窗大小和林窗位置对自然更新幼苗存活和生长的影响。长苞冷杉针叶林林窗大小分布为，面积大于100 m2的大林窗占20%左右，中等林窗面积为50～100 m2，占35%左右，小林窗面积小于50 m2，占45%左右。4个生长季节的连续观测结果表明：林窗与林下非林窗内的幼苗大小和幼苗存活数量差异明显。林窗由小到大，单位面积内的自然更新苗木数量逐渐增加，大林窗中更新苗为小林窗的1.5倍左右，而林下的更新苗很少，0.5 ind.·10 m-2。中等林窗和小林窗内的幼苗数量在从南到中心到北的位置上几乎没有明显的差异；大林窗中存在由南到北的位置差异，更新幼苗数量逐渐增加。从更新幼苗的生长来看，中等林窗内的幼苗，高度最大、生长最快，定居阶段的平均年高生长为(7.8±0.5) cm·a-1，小林窗次之，大林窗和林下幼苗个体最小，生长最慢。更新幼苗的基径随林窗大小的变化与高度变化相似。进一步从林窗位置来看，中、小林窗幼苗大小和年平均高生长量几乎无位置差异，大林窗则由南到北，幼苗由大变小，年高生长量逐渐减低。从幼苗存活数量、生长大小来看，中等林窗大小是长苞冷杉幼苗更新的适宜面积，这为该类型退化亚高山针叶林恢复提供了一定的参考。

THE EFFECTS OF GAP SIZE AND WITHIN GAP POSITION ON THE SURVIVAL AND GROWTH OF NATURALLY REGENERATED ABIES GEORGEI SEEDLINGS

The survival and growth of Abies georgei seedlings growing in forest canopy gaps was investigated over a 4-year period on the Baima Snow Mountain in northwest Yunnan Province, China. Of the total gaps surveyed in A. georgei forests, 45％ were small (gap area ＜ 50 m2), 35％ were medium sized (50-100 m2)and 20％ were large ( ＞ 100 m2) gaps. Seedling growth rates, survivorship, abundance and size were measured and compared among the three different sized gaps and under the closed forest canopy and within gaps according to their position (north, south and center) in each gap. The seedlings all originated from natural seed rain. The results showed that there were significant differences in the size and number of seedlings among the different sized gaps and understorey. There were 1.5 times more seedlings in larger gaps than in smaller ones and very few seedlings (0.5 ind. · 10 m-2) were found in the understory. The survival of A. georgei seedlings within gaps tended to increase from south to north positions within the larger gaps, whereas survivorship did not vary within the small and medium sized gaps. Seedling size, in terms of both height and basal di ameter, in the different sized gaps was as follows: medium ＞ small ＞ large ＞ understorey. Relative growth rates showed a sinilar pattern. The average annual growth in height was (7.8 ± 0.5) cm in medium gaps.Furthermore, size and annual relative height growth of seedlings did not differ significantly among positions within the medium and small gaps, but seedling height peaked in the southern position of large gaps and declined towards the center and north edges. Seedling establishment was greater in gaps than under the closedcanopy. Due to adaptation to light conditions, relative growth rates of seedlings were greatest in medium sized gaps but were relatively low in regions of the large gaps that had greater exposure to direct solar radiation as well as in small gaps that were shaded and received little direct solar radiation. Finally, based on observed changes in survival and growth of seedlings over four growing seasons, the author suggests that medium sized forest canopy gaps provide the best habitat for regeneration of A. georgei seedlings.