半干旱黄土丘陵沟壑区柳枝稷（Panicum virgatum）的生物质形成

为深入认识半干旱黄土丘陵沟壑区引种能源植物柳枝稷生物质生产的开发潜力及其约束机制，调查了农田、植丛尺度上早熟和晚熟柳枝稷年度生命周期内生物量累积、分株建成动态，以及土壤水分供求平衡过程。研究发现，植丛尺度早熟柳枝稷抽穗比例近100％，分株生殖发生大小阈值依赖基本丧失，高度大小分布近似正态，种群内光资源竞争强度明显弱化，与晚熟类型形成明显差异。农田尺度晚熟柳枝稷生物质产量可以达到15t／hm^2，高出早熟类型近1倍，但是其立地80～400cm土层的含水量稳定在7％以下，土壤干旱已经发生，早期干旱胁迫导致的生长停滞，以及生长中后期的成片倒伏现象在两年的观测周期内连续出现。早熟柳枝稷立地则形成相对稳定的白草、柳枝稷复合优势植被结构，深层土壤水分含量稳定在10％以上，实现了跨年度的土壤水分供求平衡。植丛尺度的生物质形成在一定程度上取决于分株生殖发生的大小依赖程度和分株间竞争关系格局，基于植丛尺度普遍的生殖发生和明显弱化的光资源竞争，早熟柳枝稷表现出更为高效的生物质形成机制。农田尺度晚熟柳枝稷尽管在雨热同步期的降水资源利用效率上存在明显比较优势，但是在降水资源利用分配策略和效应上，早熟柳枝稷表现出了综合的生态适宜性优势。保证雨热同步期降水资源利用和保蓄的平衡，是半干旱黄土丘陵沟壑区生物质生产应该遵循的基本原则之一。

Biomass formation for switchgrass (Panicum virgatum) in the semiarid loess hilly-gully regions

In order to deeply understand biomass formation for Panicum virgatum in the semiarid loess hilly-gully regions, we investigated the biomass production, ramet generation, and resource supply and demand at the plot and tussock scales during the yearly life cycles of two switchgrass species, i.e. early-maturing and late-maturing ones, respectively. Here, the reproduction ratio for early-maturing switchgrass at the scale of tussock was nearly 100% with the absence of size-dependent reproduction, and the height-size pattern was nearly normal with obviously relaxed competition among those ramets. However, the later-maturing switchgrass represented significantly different ramet-population ecology. At the scale of plot, the biomass yield in a year for the later-maturing switchgrass was 15 t / hm2, nearly one time more than that of the early-maturing one. However, the soil water content in the soil profile of 80-400 cm was lower than 7% for the later-maturing switchgrass, therefore deeper-profile drought had happened in 2005-2006, with phasic growing stop in June and extensive lodging in August or September. Contrarily, multiple vegetation, Panicum virgatum and Pennisetum flaccidum as the dominant species, had been formed in the early-maturing switchgrass plot, with the balance of supply and demand for soil water, while the water content of above 10% can be kept and extended to the next year. In conclusion, the efficiency of biomass formation at the tussock scale was decided in certain degree with the size-dependent reproduction and competitive relation pattern among ramets. And the early-maturing switchgrass presented higher efficiency of biomass production based on its size-independent reproduction and relaxed competitive relationship. At the farm scale, the early-maturing switchgrass also represented its better ecological adaptability than later-maturing one, for the balance between utilization and storage of limited rainfall, although the later achieved more biomass yields by the fully utilization of rainfall during the rainfall season. The balance between utilization and storage of limited rainfall during the rainfall season should be considered as a basic rule for the sustainable production of biomass in the semiarid loess hilly-gully regions.