Multitrait successional forest dynamics enable diverse competitive coexistence

An enduring challenge in ecology is to understand how diverse plant species coexist when competing for the same basic resources. Two candidate frameworks for meeting this challenge currently exist in forest ecology. Niche-based approaches succeed in describing successional dynamics in response to recurrent disturbances. Approaches based on Hubbell's neutral theory describe abundance patterns in highly diverse communities. Yet both theories are unsuccessful where their competitor is strong, highlighting an enduring need for reconciliation. In fact, the perception that niche and neutral processes are incompatible may have arisen because both types of models have lacked some critical features of real forests. Here we report a productive reconciliation that arises from extending niche models to include three ubiquitous features of vegetation: (1) plant growth under light competition, (2) multiple trait-mediated tradeoffs in plant function, and (3) evolutionary community assembly. We show that fitness equivalence – which neutral theory controversially takes as an assumption – naturally arises as a mechanistic outcome of niche differentiation, but only within a specific region of trait space, corresponding to shade-tolerant later-successional strategies. In this way, niche diversification can lead, via evolved neutrality, to a proliferation of shade-tolerant strategies, one of the phenomena that motivated the development of neutral theory. Earlier niche models focussed on a single growth-mortality trait and found no evolved neutrality. In contrast, the model introduced here includes a second trait, height at maturation, in addition to a growth-mortality trait in the form of leaf-construction cost. We show that fitness equivalence only emerges when diversifications include this second trait. We also demonstrate that our extended niche model generates markedly different forest types under different environmental conditions, recovering known biogeographic patterns. By evolving forests from first principles, our study constructively resolves the protracted debate about niche versus neutral dynamics for forest communities, and provides a platform for a synthetic theory of forest structure and diversity.