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

Aster kantoensis, an endangered plant species, is endemic togravelly floodplains of a few large rivers in central Japan.In recent years, competitive exclusion by alien perennial grassesin its natural habitat has been suspected to be one of the majorfactors threatening this species. In the River Kinu, increasedshading by the perennial alien grass Eragrostis curvula reduceslight availability for A. kantoensis. To reveal the influenceof shading on the establishment and growth of A. kantoensisrosettes, the potential carbon gain of A. kantoensis in itsnatural habitat was estimated using microenvironmental dataand whole plant photosynthetic and respiratory responses tolight and temperature. Whole plant CO₂exchange responses weremeasured with a specifically designed ‘double chamber’,which enabled measurement of the CO₂gas exchange rates of thefoliage (F) and the culm (C; stem and roots) separately. Itwas demonstrated that in a plant with average C/F ratio, positivecarbon gain could be maintained only in the microsites wherethe 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 inroot biomass as an adaptive response to drought, resulted ina large reduction in the carbon gain irrespective of microsite,weather and season. The high light requirement of A. kantoensisis interpreted as a cost of the morphological responses necessaryto avoid stresses characteristic of this gravelly floodplainhabitat. Copyright 2000 Annals of Botany Company Aster kantoensis Kitam., Eragrostis curvula Nees, biomass distribution, carbon gain, gravelly floodplain, respiration, shading, whole plant photosynthesis