The comparative analysis of ecosystems along climatic gradients offers valuable insights into potential future forest dynamics under climate change. Space-for-time substitution provides a powerful tool to investigate vegetation responses to warming and shifting precipitation regimes by using spatial variation as a proxy for temporal change. Considering longitudinal, latitudinal, and elevational gradients is essential to capture the complexity of forest–climate relationships. While general mechanisms may apply globally, hemispheric and regional peculiarities strongly influence vegetation patterns. By comparing temperate forests in Central Europe and Northern Patagonia, we demonstrate how climate gradients, disturbance regimes, and land-use histories shape species composition, tree vitality, and successional dynamics. These transhemispheric comparisons highlight both shared processes and regional contrasts, offering a broader understanding of climate-driven forest change. We conclude that space-for-time substitution, despite its limitations, provides a practical and empirically grounded approach for anticipating ecological shifts, guiding silvicultural strategies, and informing adaptive forest management in a warming world.