How tree species fill geographic and ecological space in eastern North America

Abstract

Background and aims: Ecologists broadly accept that the number of species present within a region balances regional processes of immigration and speciation against competitive and other interactions between populations that limit distribution and constrain diversity. Although ecological theory has, for a long time, addressed the premise that ecological space can be filled to 'capacity' with species, only with the availability of time-calibrated phylogenies has it been possible to test the hypothesis that diversification slows as the number of species in a region increases. Focusing on the deciduous trees of eastern North America, this study tested predictions from competition theory concerning the distribution and abundance of species.

Methods: Local assemblages of trees tabulated in a previous study published in 1950 were analysed. Assemblages were ordinated with respect to species composition by non-metric multidimensional scaling (NMS). Distributions of trees were analysed by taxonomically nested analysis of variance, discriminant analysis based on NMS scores, and canonical correlation analysis of NMS scores and Bioclim climate variables.

Key results: Most of the variance in species abundance and distribution was concentrated among closely related (i.e. congeneric) species, indicating evolutionary lability. Species distribution and abundance were unrelated to the number of close relatives, suggesting that competitive effects are diffuse. Distances between pairs of congeneric species in NMS space did not differ significantly from distances between more distantly related species, in contrast to the predictions of both competitive habitat partitioning and ecological sorting of species.

Conclusions: Eastern deciduous forests of North America do not appear to be saturated with species. The distributions and abundances of individual species provide little evidence of being shaped by competition from related (i.e. ecologically similar) species and, by inference, that diversification is constrained by interspecific competition.

Keywords: Competition; E. Lucy Braun; diversity limits; ecological space; forest trees; plot occupancy; species diversity; species-abundance distributions.

Fig. 1.

Locations of the forest plots included in Braun (1950), with the forest type indicated by the colour of the symbol. Many of the plots were located in the same locality, and their symbols overlap. The few Oak–Pine Forest Region plots are not shown. Map courtesy of Jason Knouft.

Fig. 2.

Distribution of forest plots coded by forest type (see Fig. 1) on the first two axes of a non-metric multidimensional scaling ordination of 293 forest plots.

Fig. 3.

Exponential decline in the estimated number of individuals per species of tree in deciduous forests of eastern North America as a function of rank from most to least abundant. The inset shows the extrapolation of the regression line to an abundance of one individual, presumably representing the rarest species in the region. Analysis based on the forest plots of Braun (1950).

Fig. 4.

The average number of individuals per species within families as a function of the number of individuals per family, based on the compilation of forest inventory analysis data of Xing et al. (2014). The relationship is slightly, but not significantly, positive.

Fig. 5.

Distribution of the Euclidean distances between pairs of species in the space defined by three non-metric multidimensional scaling axes of the reduced Braun (1950) data set. Pairs of species within families are marginally significantly closer on average than pairs of species in different families, as assessed by a contingency test of the expected distribution of within-family species pairs based on the between-family distribution.

Fig. 6.

Relationship between the distribution of species of tree on non-metric multidimensional scaling axes 1–3 in relation to the average local density of each species on plots where it occurs. None of the relationships is significant (all P > 0·05, r² < 0·05).