Climatic and ecological determinants of leaf lifespan in polar forests of the high CO2 Cretaceous 'greenhouse' world

Polar forests once extended across the high‐latitude landmasses during ice‐free ‘greenhouse’ intervals in Earth history. In the Cretaceous ‘greenhouse’ world, Arctic conifer forests were considered predominantly deciduous, while those on Antarctica contained a significantly greater proportion of evergreens. To investigate the causes of this distinctive biogeographical pattern, we developed a coupled model of conifer growth, soil biogeochemistry and forest dynamics. Our approach emphasized general relationships between leaf lifespan (LL) and function, and incorporated the feedback of LL on soil nutrient status. The model was forced with a mid‐Cretaceous ‘greenhouse’ climate simulated by the Hadley Centre GCM. Simulated polar forests contained mixtures of dominant LLs, which reproduced observed biogeographical patterns of deciduous, mixed and evergreen biomes. It emerged that disturbance by fire was a critical factor. Frequent fires in simulated Arctic ecosystems promoted the dominance of trees with short LLs that were characterized by the rapid growth and colonization rates typical of today's boreal pioneer species. In Antarctica, however, infrequent fires allowed trees with longer LLs to dominate because they attained greater height, despite slower growth rates. A direct test of the approach was successfully achieved by comparing modelled LLs with quantitative estimates using Cretaceous fossil woods from Svalbard in the European Arctic and Alexander Island, Antarctica. Observations and the model both revealed mixed Arctic and evergreen Antarctic communities with peak dominance of trees with the same LLs. Our study represents a significant departure from the long‐held belief that leaf habit was an adaptation to warm, dark winter climates, and highlights a previously unrecognized role for disturbance (in whatever guise) in polar forest ecology.