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ABSTRACT. Urbanisation has led to increasing homogenization of plant communities across cities. However, it is unclear whether these patterns extend to cosmopolitan plant species at the genetic level. We examined genome‐wide genetic patterns in six widespread plant species (three Poaceae and three Asteraceae) across five cities in the USA (Boston, Baltimore, Minneapolis‐St. Paul, Phoenix, and Los Angeles) using reduced‐representation sequencing. We assessed genetic structure, differentiation, and patterns of isolation by distance (IBD) and environment (IBE) to determine if species were genetically homogeneous or differentiated by city, percentage of impervious surface, or both. Most species exhibited limited population structure overall, withPoa annua(annual bluegrass),Taraxacum officinale(dandelion), andCynodon dactylon(Bermuda grass) showing no significant genetic differentiation among cities, a pattern consistent with high gene flow mediated by human activity. Notable exceptions included city‐level differences inErigeron canadensis(horseweed) andLactuca serriola(prickly lettuce), especially in Phoenix. We also observed low genetic diversity inDigitaria sanguinalis(crabgrass) from Phoenix, suggesting recent founder effects or selection via environmental filtering.Erigeron canadensis,the only native species studied, displayed stronger differentiation by city, along with significant isolation by temperature and distance. Among all species, we found no evidence for population structure by impervious surface. Our findings indicate that widespread population genetic structure patterns of cosmopolitan plants are likely to depend more on species attributes (e.g., self‐compatibility) and human‐mediated dispersal than on urbanisation per se. 

During the 1930s Dust Bowl drought in the central United States, species with the C 3 photosynthetic pathway expanded throughout C 4 -dominated grasslands. This widespread increase in C 3 grasses during a decade of low rainfall and high temperatures is inconsistent with well-known traits of C 3 vs. C 4 pathways. Indeed, water use efficiency is generally lower, and photosynthesis is more sensitive to high temperatures in C 3 than C 4 species, consistent with the predominant distribution of C 3 grasslands in cooler environments and at higher latitudes globally. We experimentally imposed extreme drought for 4 y in mixed C 3 /C 4 grasslands in Kansas and Wyoming and, similar to Dust Bowl observations, also documented three- to fivefold increases in C 3 /C 4 biomass ratios. To explain these paradoxical responses, we first analyzed long-term climate records to show that under nominal conditions in the central United States, C 4 grasses dominate where precipitation and air temperature are strongly related (warmest months are wettest months). In contrast, C 3 grasses flourish where precipitation inputs are less strongly coupled to warm temperatures. We then show that during extreme drought years, precipitation–temperature relationships weaken, and the proportion of precipitation falling during cooler months increases. This shift in precipitation seasonality provides a mechanism for C 3 grasses to respond positively to multiyear drought, resolving the Dust Bowl paradox. Grasslands are globally important biomes and increasingly vulnerable to direct effects of climate extremes. Our findings highlight how extreme drought can indirectly alter precipitation seasonality and shift ecosystem phenology, affecting function in ways not predictable from key traits of C 3 and C 4 species.