Old and ancient trees are life history lottery winners and act as evolutionary buffers against long-term environmental change

Abstract

Trees can live many centuries with sustained fecundity and death is largely stochastic. We use a neutral stochastic model to examine the demographic patterns that emerge over time, across a range of population sizes and empirically observed mortality rates. A small proportion of trees (~1% at 1.5% mortality) are life-history lottery winners, achieving ages >10-20x median age. Maximum age increases with bigger populations and lower mortality rates. One quarter of trees (~24%) achieve ages that are 3-4 times greater than median age. Three age classes (Mature, Old, and Ancient) contribute unique historical diversity across complex environmental cycles. Ancient trees are an emergent property of forests that requires many centuries to generate. They radically change generation time variance and population fitness, bridging infrequent environmental cycles. These life-history lottery winners are vital to future forest dynamics and invaluable data about environmental history and individual longevity. Old-growth forests contain trees that cannot be replaced through restoration or regeneration in the near future. They simply must be protected to preserve their unique diversity.