Competitive effect and response rankings in 20 wetland plants : are they consistent across three environments ?
1 There is evidence that plants in natural communities form transitive competitive hierarchies, but the pervasiveness and malleability of hierarchies remain controversial. We constructed three competitive rankings among 20 wetland plant species in con- ditions known to be important in wetlands: a mesic, fertile environment, an infertile environment and a flooded environment. 2 Rankings were constructed using plants grown from seed in pairwise combinations for one growing season in an outdoor compound. The indicator species used to construct the rankings were Carex crinita, Gnaphalium uliginosum and Lycopus amer- icanus. The others represented a wide array of morphologies, habitats and abundances and ranged from the large cosmopolitan Typha angustifolia to the small and rare Sabatia kennedyana. 3 Competitive rankings formed in all three environments. Competitive effect rankings based upon the results for all three indicator species were significantly concordant across the three environments (W = 0.59; P < 0.05), i.e. competitive effect did not change across environment. When calculated separately for each indicator species, rankings across the three environments were significantly concordant for two of the three indicator species. Within any environment the ranking varied among the indi- cator species. 4 Rankings based upon the mean competitive response to all three phytometer species were not concordant across the three environments (W = 0.35; P > 0.3) and were not concordant when calculated separately for each indicator species. Within any environment, response rankings were significantly concordant for two out of the three indicator species. 5 Competitive effect rankings tended to be constant across environments and were sensitive to the kind of neighbour. Competitive response rankings varied across environments and were insensitive to the kind of neighbour.