Autumnal photosynthesis is extended in nitrogen-fixing European black alder compared with white basswood: possible adaptive significance

Net photosynthesis was measured from late September to mid-November on leaves of actinorhizal Alnus glutinosa and nonactinorhizal Tilia heterophylla growing in C Illinois. Black alder retained its leaves and continued to photosynthesize for a month longer than white basswood. Maximum photosynthetic rates occurred in September and declined during the autumn for both species, although weekly values fluctuated widely. Mean maximum CO₂ fixation rates for black alder and white basswood were 11.88 and 4.62 μmol CO₂ . m^-2.s^-1, respectively, at the initial measurement in late September. On each subsequent weekly measurement date black alder had a significantly higher rate of CO₂ fixation than basswood. Night temperatures of 0°C and below caused a sharp decline in photosynthesis for black alder on the following day. Stomatal conductance was highly correlated with photosynthesis, but as these 2 parameters declined over the autumn, intercellular CO₂ concentration increased. Thus, nonstomatal limitations are thought to be involved in the temperature-induced reduction in photosynthesis. Prolonged photosynthesis in autumn may give black alder a competitive growth advantage over other deciduous species, compensate for the high energy cost associated with N fixation, or be associated with black alder's inefficient conservation of foliar N via autumnal retranslocation. -Authors