Whole Plant Carbon Gain of an Endangered Herbaceous Species Aster kantoensis and the Influence of Shading by an Alien Grass Eragrostis curvula in its Gravelly Floodplain Habitat

Abstract Aster kantoensis , an endangered plant species, is endemic to gravelly floodplains of a few large rivers in central Japan. In recent years, competitive exclusion by alien perennial grasses in its natural habitat has been suspected to be one of the major factors threatening this species. In the River Kinu, increased shading by the perennial alien grass Eragrostis curvula reduces light availability for A. kantoensis . To reveal the influence of shading on the establishment and growth of A. kantoensis rosettes, the potential carbon gain of A. kantoensis in its natural habitat was estimated using microenvironmental data and whole plant photosynthetic and respiratory responses to light and temperature. Whole plant CO 2 exchange responses were measured with a specifically designed ‘double chamber’, which enabled measurement of the CO 2 gas exchange rates of the foliage (F) and the culm (C; stem and roots) separately. It was demonstrated that in a plant with average C/F ratio, positive carbon gain could be maintained only in the microsites where the relative PPFD (photosynthetically active photon flux density) was above 15 or 30% of unshaded conditions in early- or mid-summer, respectively. Increasing C/F ratio, caused by an increase in root biomass as an adaptive response to drought, resulted in a large reduction in the carbon gain irrespective of microsite, weather and season. The high light requirement of A. kantoensis is interpreted as a cost of the morphological responses necessary to avoid stresses characteristic of this gravelly floodplain habitat.