Whole plant carbon gain of an endangered herbaceous species Aster kantoensis and the influence of shading by an alien grass Eragrostis curvula in itsgravelly floodplain habitat

Aster kantoensis, an endangered plant species, is endemic to gravelly flood plains of a few large rivers in central Japan. In recent years, competitive exclusion by alien perennial grasses in its natural habitat has been suspe cted to be one of the major factors threatening this species. In the River Kinu, increased shading by the perennial alien grass Eragrostis curvula red uces light availability for A. kantoensis. To reveal the influence of shadi ng on the establishment and growth of A. kantoensis rosettes, the potential carbon gain of A. kantoensis in its natural habitat was estimated using mi croenvironmental data and whole plant photosynthetic and respiratory respon ses to light and temperature. Whole plant CO2 exchange responses were measu red with a specifically designed 'double chamber', which enabled measuremen t of the CO2 gas exchange rates of the foliage (F) and the culm (C; stem an d roots) separately. It was demonstrated that in a plant with average C/F r atio, positive carbon gain could be maintained only in the microsites where the relative PPFD (photosynthetically active photon flux density) was abov e 15 or 30% of unshaded conditions in early- or mid-summer, respectively. I ncreasing C/F ratio, caused by an increase in root biomass as an adaptive r esponse to drought, resulted in a large reduction in the carbon gain irresp ective of microsite, weather and season. The high light requirement of A. k atoensis is interpreted as a cost of the morphological responses necessary to avoid stresses characteristic of this gravelly floodplain habitat. (C) 2 000 Annals of Botany Company.