濒危树种望天树大量结实后幼苗的生长和存活

 在云南西双版纳州勐腊自然保护区补蚌村的灰阴河望天树(Shorea wantianshuea)林， 选择群落结构均一的100 m×200 m区域，设置3条垂直于 沟谷并沿谷底到坡顶纵向延伸的20 m×100 m样带 (间隔50 m)，在每一样带内每隔20 m设置谷底、低坡、中坡、次坡顶和坡顶5个均匀分布的 观测样地，每一样地内设置两个3 m×3 m的小样方。从2004年12月初到2005年9月底每隔2 ～3个月重复记录幼苗株高、基径和叶片数，统计幼 苗死亡数和被动物捕食数，计算幼苗死亡率、动物捕食率和存活率；同时测定各样地土壤含水量。实验结束收获全部现存幼苗，测定和计算幼 苗单株叶面积、比叶面积、总生物量和根冠比等生长参数。结果表明：在干旱季节，从谷底到坡顶，各样地土壤含水量依次降低，而幼苗死亡 率则依次增大，这种趋势一直持续到实验结束，此时幼苗在谷底、低坡、中坡、次坡顶和坡顶的死亡率分别为55.0%、54. 3%、39.4%、35.9%和 26.5%。动物捕食导致望天树幼苗的大量损失，但各样地间差异不显著，幼苗的“丢失”也可能是动物捕食的结果。水分胁迫对幼苗生长的影响 在干旱季节更为明显，实验结束时幼苗株高在谷底和低坡较大，而在坡顶和次坡顶较小，其它生长参数也都表现出类似的特点，但在实验结束 时这些差异并不显著。该文就各种环境因素对望天树幼苗生长与存活的影响进行了讨论。研究的结果可为探寻望天树的致危原因、有效地保护 这一珍稀濒危树种和退化次生林的恢复等提供幼苗生态学方面的理论依据。

SEEDLING GROWTH AND SURVIVAL OF THE ENDANGERED TREE SPECIES SHOREA WANTIANSHUEA AFTER A MAST-FRUITING EVENT

Aims Shorea wantianshuea is an endangered tree species of Dipterocarpaceae in Xishuangbanna seasonal rain forest, and is listed in the Plant Red Book under Grade Ⅰ in China. This species regenerates exclusively from seeds. Our objective was to determine the fate of seedlings (including their growth and survival) following a mast fruiting event. Results will be useful in restoring this species and degraded natural forests dominated by it. Methods In early December 2004, after mast fruiting of S. wantianshuea, we established three parallel 20 m×100 m transects spaced 50 m apart within an area of 100 m×200 m spanning from valley bottom to ridge top. Each transect was stratified into five contiguous plots of 20 m and near the center of each plot two 3 m×3 m quadrats were established. We labeled all newly established S. wantianshuea seedlings in each quadrat, recorded seedling height, basal stem diameter and number of leaves every 2-3 months over a 10-month period and calculated death rate, herbivory rate and survival rate of the seedlings. At the end of the survey, we destructively sampled seedlings and determined leaf areas, total dry weight, root-shoot ratio and specific leaf area.Important findings Many S. wantianshuea seedlings established after the mast-fruiting event did not recruit into its natural population because of high mortality rate and herbivory rate caused by seasonal drought stress, small mammals and other predators. The density of seedlings was as high as 2.76 plants&;#8226;m^-2 in December 2004 (first census), but was less than 0.26 plants&;#8226;m^-2 in September 2005 when the study ended. Seedling mortality rate was high during the dry season, particularly after the first two months. The final seedling mortality rate on ridge-top and lower-ridge sites, 55% and 54%, respectively, were higher than those on other sites, with a gradually increase from valley bottom to ridge top. Herbivory rate in the first census was low and in 8 of 15 sites no seedlings were gnawed or pulled. At the end of the study, mean seedling height was 17.9, 19.7, 18.4, 13.0 and 12.1 cm in valley-bottom, lower-slope, medium-slope, lower-ridge and ridge-top sites, respectively; however, no significant differences were observed in seedling height and other growth parameters.