Vegetation responses to climate change: lessons from the past 1 Ma of Earth history

Records of vegetation and environmental change during the Late Quaternary period are numerous and globally distributed, and provide information on actual climate changes and ecosystem responses that have no parallel in recent times. Plants have shown remarkably little macroevolution, and apparently few extinctions, over the past 10⁵–10⁶ years – despite the Earth experiencing alternating warm and cold periods, the latter punctuated by multiple episodes of rapid (decadal to centennial) climate change accompanied by almost equally rapid biome shifts. The persistence of tree taxa in both temperate and tropical regions through multiple climatic cycles indicates considerable resilience to large changes in climate. Phylogenetic niche conservatism has favoured geographic or topographic range shifts, rather than adaptive evolution, as the principal mode by which plants have responded to climate changes on the glacial-interglacial scale. Nevertheless, a pervasive feature of the palaeorecord is the frequent appearance of “novel” communities and disappearance of others: biomes may shift, but community composition is transient. Past vegetation changes also record the effects of atmospheric CO₂ concentration on photosynthetic physiology: high CO₂ favoured forests and low CO₂ favoured C₄-dominated grasslands, due to the positive effect of CO₂ on the water use efficiency of C₃ plant leaves.

This “palaeoperspective” has several, under-appreciated implications for nature conservation in the face of continuing climate change. (1) Novel ecosystems are normal; the preservation of existing assemblages is unlikely to succeed.  (2) Rapid migration of plant species (including trees) is possible, likely facilitated by long-distance dispersal, and may be much faster than currently assumed. (3) Rising CO₂ has likely been a primary cause of “woody thickening” in savannas, and will continue to promote the colonization of open vegetation by trees.