Functional analysis of canopy structure of grasslands

1.研究了莱茵河漫滩植被(Amerongen)、牧场(Molenpolder)、白垩草地(林堡)、浮泥草地(Westbroek)和芒草草地(Kawatabi)的冠层结构。在每个植被中，确定了光量子通量密度(PPFD)、叶面积、生物量和氮素在物种之间的分布和分配。冠层中的高大优势种在叶面积上的平均PPFD(PHI<面积>)较高。然而，高大树种的单位地上生物量吸收的PPFD并不高于从属树种。地上生物量被视为获取PPFD(效益)的投资成本。PHI质量作为效益与成本的比率，表明了生物质投资捕获光子通量的效率。与次生物种相比，高大物种在接收大部分入射PPFD方面具有优势，而从属物种在有效利用其生物量捕获PPFD方面具有优势。测定了不同树种的单位叶氮吸收光量子效率(PHI_N)，以反映植物的光合氮素利用效率。高大优势种和矮小优势种都有较高的PHI_N，但过程不同：优势种的高PHI_N是由于PHI_<面积>高，而从属种的高PHI_N是由于单位叶面积的氮浓度n_L低所致。测定了单位叶面积的光饱和CO2交换速率(CER)和单位叶氮的光饱和CO2交换速率(PNUE)。CER高、n_L高的物种PNUE潜力低，CER低、n_L低的物种PNUE潜力大。氮素利用效率(NUE)也是植物氮素浓度的倒数。三个指标的NUE表现出很强的一致性。由此可见，优势种的NUE与优势种的NUE相当，甚至更高。

1. Canopy structure was studied in the following grasslands : flood plain vegetation of River Rhein (Amerongen), pasture (Molenpolder), chalk grassland (Limburg), floating fen (Westbroek) and Miscanthus sinensis grassland (Kawatabi). In each vegetation, distribution and partitioning of photon flux density (PPFD), leaf area, biomass, and nitrogen among species were determined.2. Tall dominant species in a canopy received higher PPFD averaged over leaf area (PHI_<area>). However, the PPFD absorbed per unit aboveground biomass (PHI_<mass>) of the tall species was not higher than that of the subordinate species. Aboveground biomass was regarded as an investment cost to capture PPFD (benefit). PHImass, as a ratio of benefit to cost, indicates an efficiency of biomass investment to capture photon flux. Tall species have an advantage over subordinate species in receiving a large fraction of incident PPFD, while subordinate species have an advantage in efficiently using their biomass to capture PPFD.3. Photon absorption per unit leaf nitrogen (PHI_N) was determined for each species to indicate in situ PNUE (photosynthetic nitrogen use efficiency). Both tall dominant species and short subordinate species could have high PHI_N, but through different processes : high PHI_N of dominants resulted from high PHI_<area>, while high PHI_N of subordinates resulted from low n_L (nitrogen concentration per unit leaf area).4. Light-saturated CO_2 exchange rates per unit leaf area (CER) and per unit leaf nitrogen (potential PNUE) was determined. Species with high CER and high n_L had low potential PNUE, while species with low CER and low n_L showed high potential PNUE. Nitrogen use efficiency (NUE) was also evaluated as a reciprocal of plant nitrogen concentration. Three measures of NUE showed strong conformity. It was concluded that NUE of subordinate species was as high as or even higher than that of dominant species.