Vertical root distributions of northern tree species in relation to successional status

Measurements of root biomass, number, diameter, and length by soil depth for northern tree species were assembled from 19 published papers, yielding a total of 123 vertical root distributions. Species were classified into three tolerance classes based on successional status. A nonlinear function, Y = 1 − β d , where Y is the cumulative root fraction from the soil surface to depth d in centimetres, was fit to the data for each excavation. The regression coefficient, β, was considered to be a measure of vertical root distribution and was used as a response variable to test whether significant differences in vertical root distributions existed among tolerance classes. Early successional or intolerant species had a significantly greater proportion of roots occurring deeper than did late successional or tolerant species. Differences in vertical root distributions are presumed to be related to the inherent genetic potential of early successional species for deep exploitation of a more homogeneous substrate, resulting from either geologic deposition or nutrient and water redistribution following forest disturbance. Early successional species are also able to adapt to sites limiting in water and nutrients because of their ability to exploit larger volumes of soil. Late successional or shallow-rooted species are better adapted to sites where resources are concentrated near the soil surface as the result of biocycling and soil development.