Light and salinity affect growth of the salt marsh plant Aster laurentianus.

• The effects are reported of substrate salinity and light on the relative growth rate of the annual Aster laurentianus, an endangered species of eastern Canadian salt marshes. • The independent and combined effects of photosynthetically active photon flux density and salinity on the relative growth rate (RGR) and overall plant performance were measured in glasshouse and growth-chamber experiments on seedlings of A. laurentianus. •  Low light availability decreased RGR through its negative effect on unit leaf rate. However, specific leaf area and leaf mass ratio varied inversely with changes in light, such that leaf area ratio did not differ greatly among light levels. High salinity decreased RGR by reducing unit leaf rate and leaf area ratio; a reduction in the latter was brought about by a lower leaf mass ratio rather than by a lower specific leaf area. Low light availability combined with high substrate salinity affected A. laurentianus in a strictly additive manner; there was no significant interaction between the two factors on overall plant performance. • Light and salinity are important factors controlling growth of A. laurentianus, and might explain the distribution pattern of the species in the field.