Résumé
On coastal dunes, both plant community composition and abiotic factors change along the foredune-established dune sequence. However, little is known about changes in belowground biomass, particularly in root mass, along such gradients. Yet, belowground plant structures are important for substrate cohesiveness and coastal dune integrity. Using soil cores, we determined the spatio-temporal dynamics of root mass density, from the embryo dunes to the stabilised dunes, on a coastal dune system in subarctic Quebec, Canada. Root mass density varied along the gradient, but it did not show a monotonic increase as observed in other studies. Roots were concentrated in the upper layer of the substrate (0-20 cm), except on the embryo dunes where most of the roots were found at 20-60 cm below the surface. Root mass density did not differ among the first three sampling dates (18 June, 07 July and 25 July), probably as a consequence of early root growth in the spring. However, root mass significantly decreased in August at all depths. Root mass density was not significantly correlated with soil water content, but it was correlated negatively with soil pH, and positively with total salts. Because root mass is concentrated near the substrate surface, subarctic coastal dunes may be particularly susceptible to human disturbance, such as trampling.