Variation in the Composition of Understory Vegetation in a Tropical Rain Forest as a Function of Soil and Topographic Position. 1986 - 1990

Understory plants are a major contribution to the high plant species
diversity of Neotropical rain forests. Shrubs, understory trees,
saplings of overstory trees, and herbs occupy a habitat of generally
low light levels and high humidity in which there seem to be few
obvious mechanisms to support habitat partitioning. Moreover several
plant families are characterized by a high number of co-occurring
understory species. In 1987-1989 we sampled understory vegetation in
18 sites at the La Selva Biological Station of the Organization for
Tropical Studies in Heredia Province, Costa Rica. At each site we used
20 nested quadrats to investigate the effects of soil type on
replicated sites of mapped alluvial and residual volcanic soils (5 map
units) and topographic positions (ridges, midslopes and flats) on
composition, density and diversity of small (1m tall to 5cm dbh, 25 m2
quadrat) and large(5-10cm dbh, 100 m2 quadrat) understory plants. We
also measured fine litter dry mass, extractable P, total organic
matter, percent slope and percent incident light radiation in each
quadrat.

林下植物是新热带雨林植物物种多样性水平极高的重要贡献因素。灌木、下层乔木、上层乔木幼树以及草本植物，共同栖息于光照普遍偏低、湿度偏高的生境中，此类生境似乎鲜有支撑生境分化的明确机制。此外，多个植物科以共存林下物种数量繁多为典型特征。1987年至1989年，我们在哥斯达黎加埃雷迪亚省热带研究组织（Organization for Tropical Studies）下属的拉塞尔瓦生物站（La Selva Biological Station）的18个样地中开展林下植被采样调查。在每个样地内，我们采用20个嵌套样方（nested quadrats），针对已测绘的冲积土与残留火山土（共5个制图单元）的重复样地，探究土壤类型与地形位置（坡脊、中坡与平地）对两类林下植物的群落组成、密度及多样性的影响：一类为株高1米至胸径（dbh，diameter at breast height）5厘米的小型林下植物（对应样方面积25平方米），另一类为胸径5至10厘米的大型林下植物（对应样方面积100平方米）。我们还测定了每个样方内的细枯落物干重、可提取磷含量、总有机质含量、坡度百分比以及入射光辐射百分比。