Kirkman L K, Mitchell R J, Helton R C, Drew M B
Joseph W. Jones Ecological Research Center, Ichauway, Route 2, Box 2324, Newton, Georgia 31770 USA.
Am J Bot. 2001 Nov;88(11):2119-28.
The fire-dependent longleaf pine-wiregrass (Pinus palustris Mill.-Aristida beyrichiana Trin. & Rupr.) savannas of the southeastern United States provide a unique opportunity to examine the relationship between productivity and species richness in a natural ecosystem because of the extremely high number of species and their range across a wide ecological amplitude (sandhills to edges of wetlands). We used a natural gradient to examine how plant species richness and plant community structure vary with standing crop biomass (which in this system is proportional to annual net productivity) as a function of soil moisture and nitrogen mineralization rates in a frequently burned longleaf pine-wiregrass savanna. Highest ground cover biomass and highest species richness were found at the same position along the gradient, the wet-mesic sites. Relative differences in species richness among site types were independent of scale, ranging from 0.01 m(2) to 100 m(2). Nitrogen availability was negatively correlated with species richness. Dominance of wiregrass (in terms of biomass) was consistent across the gradient and not correlated with species richness. Regardless of site type, the community structure of the savannas was characterized by many perennial species with infrequent occurrences, a factor in the low temporal heterogeneity (percent similarity between seasons and years) and high within-site spatial heterogeneity (percent dissimilarity of vegetation composition). The coexistence of numerous species is likely due to the high frequency of fire that removes competing hardwood vegetation and litter and to the suite of fire-adapted perennial species that, once established, are able to persist. Our results suggest that soil moisture is an important factor regulating both the number of species present and community production within the defined gradient of this study.
美国东南部依赖火烧的长叶松-线茅草（Pinus palustris Mill.-Aristida beyrichiana Trin. & Rupr.）稀树草原提供了一个独特的机会，可用于研究自然生态系统中生产力与物种丰富度之间的关系，这是因为该生态系统中物种数量极多，且分布在广泛的生态幅度范围内（从沙丘到湿地边缘）。我们利用一个自然梯度来研究在频繁火烧的长叶松-线茅草稀树草原中，植物物种丰富度和植物群落结构如何随现存作物生物量（在这个系统中与年净生产力成正比）而变化，该变化是土壤湿度和氮矿化率的函数。在梯度上的同一位置，即湿润-中生地带，发现了最高的地面覆盖生物量和最高的物种丰富度。不同地点类型之间物种丰富度的相对差异与尺度无关，范围从0.01平方米到100平方米。氮的有效性与物种丰富度呈负相关。线茅草（就生物量而言）的优势在整个梯度上是一致的，且与物种丰富度无关。无论地点类型如何，稀树草原的群落结构都以许多多年生但出现频率低的物种为特征，这是导致时间异质性低（季节和年份之间的相似百分比）和地点内空间异质性高（植被组成的不同百分比）的一个因素。众多物种的共存可能是由于频繁的火烧消除了竞争性的硬木植被和枯枝落叶，以及一系列适应火烧的多年生物种，这些物种一旦建立就能持续存在。我们的结果表明，在本研究定义的梯度范围内，土壤湿度是调节现存物种数量和群落生产力的一个重要因素。
