Photosynthetic capacity and nitrogen partitioning among species in the canopy of a herbaceous plant community
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Partitioning of nitrogen among species was determined in a stand of a tall herbaceous community. Total amount of nitrogen in the aboveground biomass was 261 mmol N m−2, of which 92% was in three dominant species (Phragmites, Calamagrostis and Carex) and the rest was in the other eight subordinate species. Higher nitrogen concentrations per unit leaf area (nL) with increasing photosynthetically active photon flux density (PPFD) were observed in all species except for three short species. The changes in nL within species were mainly explained by the different nitrogen concentrations per unit leaf mass, while the differences in nL between species were explained by the different SLM (leaf mass per unit leaf area). Photon absorption per unit leaf nitrogen (ΦN) was determined for each species. If photosynthetic activity was proportional to photon absorption, ΦN should indicate in situ PNUE (photosynthetic nitrogen use efficiency). High ΦN of Calamagrostis (dominant) resulted from high photon absorption per unit leaf area (Φarea), whereas high ΦN of Scutellaria (subordinate) resulted from low nL although its Φarea was low. Species with cylinder-like “leaves” (Juncus and Equisetum) had low ΦN, which resulted from their high nL. Light-saturated CO2 exchange rates per unit leaf area (CER) and per unit leaf nitrogen (potential PNUE) were determined in seven species. Species with high CER and high nL (Phragmites, Carex and Juncus) had low potential PNUE, while species with low CER and low nL showed high potential PNUE. NUE (ratio of dry mass production to nitrogen uptake) was approximated as a reciprocal of plant nitrogen concentration. In most species, three measures of nitrogen use efficiency (NUE, ΦN and potential PNUE) showed strong conformity. Nitrogen use efficiency was high in Calamagrostis and Scutellaria, intermediate in Phragmites and relatively low in Carex. Nitrogen use efficiency of subordinate species was as high as or even higher than that of dominant species, which suggests that growth is co-limited by light and nitrogen in the subordinate species.
Key wordsCanopy structure Nitrogen use efficiency Photon absorption Photosynthesis Species coexistence
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