Gradient studies of wetland forests have inferred that competition from upland tree species confines waterloggingtolerant tree species to hydric environments. Little is known, however, about competition effects on individual-tree growth along stress gradients in wetland forests. We investigated tree growth and competition in mixed-species stands representing a waterlogging stress gradient in Fraxinus nigra
Marsh. (black ash) forests in Minnesota, USA. Using competition indices, we examined how F. nigra
basal area increment (BAI) responded to competition along the gradient and whether competition was size-asymmetric (as for light) or size-symmetric (as for soil resources). We modeled spatial distributions of F. nigra
and associated tree species to assess how variation in species mixtures influenced competition. We found that although F. nigra BAI did not significantly differ with variations in site moisture, the importance of competition decreased as waterlogging stress increased. Competition across the gradient was primarily size-asymmetric (for light). Variation in species mixtures along the gradient was an important influence on competition. Some segregation of tree species occurred at all but the most upland site, where waterlogging stress was lowest and evidence of competition was greatest, confirming that competition from upland tree species confines F. nigra
and potentially other waterlogging-tolerant species to hydric environments.
environmental stress gradient
black ash wetlands
point pattern analysis
Looney, Christopher E.; D'Amato, Anthony W.; Fraver, Shawn; Palik, Brian J.; Frelich, Lee E. 2018. Interspecific competition limits the realized niche of Fraxinus nigra along a waterlogging gradient . Canadian Journal of Forest Research. 48(11): 1292-1301. https://doi.org/10.1139/cjfr-2018-0023.