Calamagrostis canadensisis present in the understory of most boreal forest mixed-wood stands and can rapidly dominate an area once the overstory is removed. The nature of the photosynthetic response to light was studied by investigating the stomatal response to cycles of high and low fluence light, and by developing photosynthetic light response curves for sun- and shade-grown plants. Photosynthetic light response curves did not differ between sun- and shade-grown plants. However, higher stomatal conductances, at all light intensities, were found in shade-grown plants. Stomatal conductance tracked changes in light under intermittent high and low fluence light. Average rates of stomatal opening after a change from low to high light were greater than closing rates (23 vs. 17 mmol m−2s−1/min). The faster opening response was negated by a 1 – to 2-min time lag between the onset of high light and the initiation of stomatal opening. Thus the time to reach 80% of maximal response did not differ between opening or closing. The photosynthetic system ofC.canadensisis not particularly well adapted to intermittent light conditions. The poor performance in the understory of this apparently meadow-adapted photosynthetic system is offset by the gain in the ability to rapidly recolonize the site after disturbance.Key words: gas exchange, stomatal conductance,Calamagrostis canadensis, non-steady state photosynthesis, variable light.