An experimental investigation, using stomatal conductance and fluorescence, of the flood sensitivity of Boltonia decurrens and its competitors

Summary Boltonia decurrens , a perennial floodplain species endemic to the Illinois River Valley, USA, is threatened with extinction due to destruction of habitat. Extant populations of B. decurrens have become isolated and restricted to human‐disturbed, alluvial soil habitats. In sites that are not disturbed, B. decurrens is replaced by competing vegetation after 3–5 years. If further decline in size and number of populations is to be avoided, strategies for management of the species must be developed. To assess the feasibility of using controlled flooding to reduce competition in sites where B. decurrens occurs, we compared the effects of 10 weeks of root‐zone saturation on mortality, biomass, stomatal conductance and the ratio of variable to maximum fluorescence (the fluorescence ratio) in B. decurrens and three of its competitors: an annual, Conyza canadensis ; a non‐rhizomatous perennial, Aster pilosus ; and a rhizomatous perennial, Aster ontarionis . Boltonia decurrens was unaffected by the 10‐week treatment, but all other species showed increases in mortality, reduction or re‐allocation of biomass, and decreases in stomatal conductance. There was no effect of flooding on the fluorescence ratio. Because reductions in stomatal conductance were accompanied by changes in mortality and biomass, we concluded that measurement of stomatal conductance could be used as a non‐destructive technique to determine differential flood tolerance. The order of tolerance among species in this study was B. decurrens > A. ontarionis > A. pilosus > C. canadensis ; therefore, controlled flooding may be an effective management tool in preserving populations of B. decurrens .