西双版纳热带森林树种幼苗的组成、空间分布和旱季存活

COMPOSITION, SPATIAL DISTRIBUTION AND SURVIVAL DURING THE DRY SEASON OF TREE SEEDLINGS IN A TROPICAL FOREST IN XISHUANGBANNA, SW CHINA

林下树种幼苗是森林生态系统的一个重要组成部分,在森林的自然更新过程中发挥着重要作用。于2007年10月底在西双版纳20hm2热带森林动态监测样地内设置了450个幼苗样方(1m2),对样方内的树种幼苗(胸径<1cm)进行了调查,并观测了当年的实生苗(H≤30cm)在2007～2008年旱季期间的存活变化。结果表明,该区森林林下树种幼苗的组成比较丰富,但个体数在物种间的分布极为不均匀。除4.17%的幼苗个体未鉴定外,调查到的已知树种幼苗共计6367株,隶属于49科116属161种,并且Simpson、Shannon-Wiener和Pielou指数分别达到了0.69、2.29和0.45。个体数少于5株的幼苗贡献了50.31%的物种数;在1m2的样方尺度上,幼苗密度和种丰富度两者的平均值分别为(14.15±0.72)株·m–2和(3.73±0.10)种·m–2(Means±SE,n=450);优势幼苗为望天树(Parashorea chinensis)、短刺锥(Castanopsis echidnocarpa)和假海桐(Pittospor opsis kerrii)等,它们的空间分布格局与地形、海拔和母树分布三者之间存在着密切的相关性;旱季期间,幼苗的死亡比例较大(26.49%),主要集中在优势幼苗。幼苗在干旱胁迫环境下的死亡格局表现出了明显的种间差异:香花木姜子(Litsea panamanja)和望天树两优势幼苗的死亡率分别为62.65%和35.87%,而假海桐和蚁花(Mezzettiopsis creaghii)等7种优势幼苗的死亡率较低(<9%),除望天树等13种优势幼苗外,其余692株幼苗(共129种)仅死亡了10.40%,而且有91种幼苗均没有发生死亡。幼苗间的存活差异有待于从种子大小、幼苗的形态和生理特性及其所在空间位置的生境特征等多方面开展深入的研究。

Aims Tree seedlings are a vital component of forest ecosystems because they determine natural regeneration; however, they are poorly understood, especially in tropical areas. Our objectives were to examine the characteristics of seedling composition, spatial distribution of the dominant seedlings and changes in survival of newly established seedlings during the dry season in a tropical forest in Xishuangbanna, China. Methods In late October 2007, we established 450 1-m2 seedling plots in a 20 hm2 tropical forest dynamics plot and completed the first survey, in which tree seedlings with DBH 〈1 cm were tagged, measured and identified to species. We re-censused the seedlings in late February and again in early May 2008. We selected newly established seedlings （height ≤30 cm） in 2007 to analyze survival of seedlings during the dry season from 2007 to 2008. Important findings The plot had relatively high tree seedling richness and differences in numbers of seedlings among species. The 6 367 tree seedlings identified and recorded represented 49 families, 116 genera and 161 species （277 other seedlings were unidentified）. The seedlings had values of Simpson, Shannon-Wiener and Pielou indexes of 0.69, 2.29 and 0.45, respectively. Species with 〈5 individuals accounted for 50.31% of the total number of species. At the 1-m2 scale, the seedling density and species richness were （14.15± 0.72） ind.m^-2 and （3.73 ± 0.10） sp·m-2 （Mean ± SE, n=450）, respectively. Dominant seedlings included Parashorea chinensis, Castanopsis echidnocarpa and Pittosporopsis kerrii, which displayed different spatial distribution patterns closely associated with the topography, altitude and spatial patterns of their parent trees. During the dry season, 26.49% of the seedlings died, mainly dominant species. Mortality patterns varied widely among seedlings, e.g., mortality rates were 62.65% and 35.87% for Litsea panamanja and Parashorea chinensis, respectively. However, low mortality rates （〈9%） occurred in the seven dominant seedlings （Pittosporopsis kerrii, Mezzettiopsis creaghii, etc.）. In addition, except for the 13 dominants （Parashorea chinensis, etc.）, seedlings of the other 129 species had a combined mortality of 10.40%, and 91 of these 129 species lost no individuals.