Functional analysis of canopy structure of grasslands

1.研究了莱茵河泛滥平原植被（Amerongen）、牧场（Molenpolder）、白垩草地（林布尔格）、漂浮沼泽（Westbroek）和芒草草地（Kawatabi）的冠层结构。在每种植被中，测定了光子通量密度（PPFD）、叶面积、生物量和氮在物种间的分配和分配.在冠层中，高的优势种获得较高的PPFD平均叶面积（<area>PHI_1）。但高大树种单位地上生物量吸收的PPFD<mass>并不高于下位树种。生物量被视为获取PPFD（效益）的投资成本。PHImass，作为收益与成本的比率，指示生物质投资捕获光子通量的效率。高大树种在接收大部分入射PPFD方面优于下位树种，而下位树种在有效利用其生物量捕获PPFD方面具有优势。测定了每种植物的单位叶氮光吸收量（PHIN），以反映植物的光合氮利用效率（PNUE）。无论是高的优势种还是矮的从属种，都能产生高的PHI_N，但过程不同：优势种的高PHI_N来源于高的PHI_N，<area>而从属种的高PHI_N来源于低的n_L（单位叶面积氮浓度）.测定了单位叶面积光饱和CO_2交换速率（CER）和单位叶氮（PNUE）。高CER和高n L的物种PNUE潜力低，而低CER和低n L的物种PNUE潜力高。氮利用效率（NUE）也被评价为植物氮浓度的倒数。NUE的三个指标表现出较强的一致性。结果表明，各亚种群的氮素利用效率与优势种群相当，甚至高于优势种群。

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.