Local Coexistence of Deciduous‐Forest Groundlayer Species Growing in Different Seasons

Abstract

In 10 stands of deciduous forest, evidence was sought for competitive inhibition between groundlayer species photosynthetically active in different seasons. In each stand, the forest floor had probably not been disturbed catastrophically for >50 yr, thus favoring detection of cumulative competitive effects. The underlying hypothesis was that long—lived species occupying the same microsites but not competing directly for light may, nevertheless, interfere with each other's growth by seasonal depletion of labile soil nutrients. For a plot size that holds, on average, two species, coefficients of rank correlation were computed, stand by stand, between the cover values of (1) species that are in leaf only through early spring, and (2) species that leaf out later in the growing season. Fewer than 5% of the correlations were significantly negative at the 5% level, indicating that competition between phenologically distinct plants may be too weak or sporadic to affect local species patterns. The magnitude of negative correlations, or the percentage of significant negative correlations, was, however, greater than average between certain abundant species thought to be liberal in uptake of labile nutrients. This was most pronounced for correlations between Erythronium americanum, a spring—ephemeral herb, and Parthenocissus quinquefolia, a summer—active shrub with relatively indeterminate shoot growth. Another result suggesting nutrient—based competitive inhibition was a 50% lower mean cover for two nutrient—demanding summer—active species (Acer saccharum and Ulmus rubra, both abundant as groundlayer plants) in plots with an exceptionally high density of shoots in early spring. In contrast, the pooled cover sums for all summer—active species were unaffected by locally high densities of shoots in early spring.