Contrasting intra‐annual population dynamics of two codominant species are consistent across spatial and temporal scales.

Despite asymmetric competition and a wide array of functional similarities, two ecologically important C4 perennial grasses, Andropogon gerardii and Sorghastrum nutans, frequently codominate areas of the mesic tallgrass prairie of the US Great Plains. A subtle difference in their vegetative reproduction strategies may play a role in preventing the exclusion of S. nutans, the presumed weaker competitor in such regions.While A. gerardii vegetative tiller densities peak in the early growing season and decline thereafter (determinate recruitment), those of S. nutans may continue to increase throughout the growing season (indeterminate recruitment), providing a potential avenue for recovery from more intensive early season competition. However, until now these patterns have only been informally observed in the field.We examined the year‐to‐year consistency of growing season vegetative tiller dynamics (measured as seasonal change in tiller densities) of each grass species from an intact tallgrass prairie in Kansas – a site within the core of both species' distributions – over a period of 8 years. Then, to investigate environmental effects on these dynamics, we examined whether they differ across a Kansas landscape varying in topography, fire management regimes, and the abundances of the study species. Finally, we expanded the investigation of environmental effects on growing season tiller dynamics by observing them at the periphery of the species' distributions in central Colorado, where climatic conditions are dryer and the study species' abundances are reduced.Synthesis. We found that the tiller densities of A. gerardii decline within seasons with striking consistency regardless of spatio‐temporal scale or environmental factors (topography and fire regimes). In contrast, we found the seasonal dynamics of S. nutans tiller densities were dependent on environmental factors, with seasonal tiller density increases occurring only within the Kansas populations but not consistent between years. These observations lay the groundwork for establishing differences in tiller recruitment determinacy as a potentially important yet underappreciated mechanism for promoting coexistence and codominance among perennial plant species. [ABSTRACT FROM AUTHOR]