How did changing climates during the Cenozoic Era contribute to the evolution of scrubland plants?

During the Cenozoic Era, the evolution of scrubland plants was primarily driven by the gradual cooling and aridification of global climates. Many ancestral plant species, originally adapted to moister, more tropical environments, underwent convergent evolution to survive these shifting conditions. As world climates became drier, these plants developed similar characteristics—such as a shrubby habit—to cope with the resulting moisture stress, even though they were often not closely related.12

This climatic transition forced vegetation to adapt to environments where conditions were less than optimal for trees. For instance, the elimination of reliable summer rainfall in regions like California, which occurred approximately six million years ago, was a significant factor in the development of local scrubland flora. These evolutionary pressures led to the emergence of ecological equivalents across different regions, where diverse plant species independently developed similar strategies to thrive in semiarid or Mediterranean climates characterized by hot, dry summers and mild, moist winters.21