PHOTOSYNTHETIC CAPACITY AND NITROGEN PARTITIONING AMONG SPECIES IN THE CANOPY OF A HERBACEOUS PLANT COMMUNITY

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 Nm(-2), of which 92% was in three dominant speci es (Phragmites, Calamagrostis and Carer) and the rest was in the other eight subordinate species. Higher nitrogen concentrations per unit le af area (n(L)) with increasing photosynthetically active photon flux d ensity (PPFD) were observed in all species except for three short spec ies. The changes in n(L) within species were mainly explained by the d ifferent nitrogen concentrations per unit leaf mass, while the differe nces in n(L), between species were explained by the different SLM (lea f mass per unit leaf area). Photon absorption per unit leaf nitrogen ( Phi(N)) was determined for each species. If photosynthetic activity wa s proportional to photon absorption, Phi(N) Should indicate in situ PN UE (photosynthetic nitrogen use efficiency). High Phi(N) of Calamagros tis (dominant) resulted from high photon absorption per unit leaf area (Phi(area)), whereas high Phi(N) of Scutellaria (subordinate) resulte d from low n(L) although its Phi(area) was low. Species with cylinder- like ''leaves'' (Juncus and Equisetum) had low Phi(N), which resulted from their high n(L). Light-saturated CO2 exchange rates per unit leaf area (CER) and per unit leaf nitrogen (potential PNUE) were determine d in seven species. Species with high CER and high n(L) (Phragmites, C arex and Juncus) had low potential PNUE, while species with low CER an d low N-L showed high potential PNUE. NUE (ratio of dry mass productio n to nitrogen uptake) was approximated as a reciprocal of plant nitrog en concentration. In most species, three measures of nitrogen use effi ciency (NUE, Phi(N) and potential PNUE) showed strong conformity. Nitr ogen use efficiency was high in Calamagrostis and Scutellaria, interme diate in Phragmites and relatively low in Carex. Nitrogen use efficien cy of subordinate species was as high as or even higher than that of d ominant species, which suggests that growth is co-limited by light and nitrogen in the subordinate species.