Evolutionary and biogeographic divergence jointly shape competitive outcomes among Quercus congeners

Darwin proposed that competition should be strongest among close relatives because shared ancestry drives functional similarity and ecological overlap. Empirical support for phylogenetic effects on competition has been mixed, with some studies finding that congeners exclude one another and others documenting stable coexistence. This variation reflects differences in spatial scale and environmental context: At regional scales, demographic stochasticity and habitat heterogeneity can obscure competitive signals, whereas at local scales and under controlled conditions, competitive patterns may become detectable. Geographic distance may therefore provide complementary information about competition by capturing differences in adaptive history not fully reflected in phylogenetic relationships.

I tested three hypotheses linking evolutionary divergence and environmental context to competition among Quercus congeners: (i) distantly related species produce greater overall growth because they use resources differently and interfere less with each other; (ii) species from widely separated geographic regions show greater growth asymmetry because they evolved different competitive abilities under distinct selective pressures; and (iii) both effects weaken under drought stress, where water limitation constrains growth across lineages and reduces the influence of evolutionary history.

Eight Quercus species from Asia, Europe and North America were grown in pairwise combinations under wet and dry soil conditions in a greenhouse experiment. I quantified pair‐level performance using an overall growth response metric (interspecific competition index) and relative performance within pairs using a growth asymmetry metric (competitive asymmetry index). Phylogenetic and geographic distances between species served as continuous predictors of both responses.

Overall growth increased with phylogenetic distance: Distantly related species produced more combined growth than close relatives. Growth asymmetry within pairs increased with geographic distance: Species from widely separated regions showed larger dominance hierarchies, with one species consistently outgrowing the other. Neither relationship was modified by the moisture gradient, indicating that these dimensions of evolutionary divergence influenced seedling competition independently of the soil water availability examined here.

Synthesis . Phylogenetic and geographic distances captured distinct dimensions of competition among Quercus congeners. Evolutionary divergence predicted how much biomass species produced together, consistent with distantly related species using resources differently. Geographic separation predicted which species dominated, consistent with species from different regions evolving different competitive abilities. The persistence of both relationships across moisture treatments suggests these historical dimensions shape seedling competition in ways robust to variation in water availability.