The hare, tortoise and crocodile revisited: Tree fern facilitation of conifer persistence and angiosperm growth in simulated forests

Abstract

Forests in which conifers and angiosperms coexist in the canopy with a well-developed understorey/subcanopy have often been conceptualized as three component systems. In these systems, the coexistence and regeneration of the two canopy tree components may be mediated by a third component in the understory and/or subcanopy, for example, palms, bamboo or tree ferns. However, the effect of this understorey/subcanopy component has only been studied over short time periods relative to the longevities (centuries) of the canopy species. Simulation models provide a way to evaluate the inter-generational consequences of patterns and mechanisms observed over short temporal extents. We extended a spatially explicit individual-based simulation model representing the dynamics of New Zealand mixed conifer–angiosperm forest by including a growth form representing tree ferns (the third component). Using model-based experiments, we evaluated the effects of varying initial densities of tree ferns, the contributions of external propagule sources and the long-term effects of macro-litterfall and suppressed seedling density by tree ferns on forest composition and structure over 2,500 years. The model accurately reproduced observed tree fern growth patterns (height and age distributions) in old-growth forests, patterns of species replacement and dominance hierarchies in forest succession. The outcomes of the model experiments suggested that shade-intolerant conifers persist longer and grow older and taller with increasing tree fern density. Although angiosperm stem density reduced along the same series, their longevities and heights also increased with tree fern density. We suggest that these increases are due to conifer and angiosperm trees experiencing reduced competition from neighbouring trees associated with tree ferns increasingly occupying spaces between trees. Over centuries of simulation, the population-level effect of seedling suppression beneath tree ferns suggests an emergent pattern of negative density dependence in understorey tree ferns. Synthesis. Tree fern understories are important determinants of forest structure, partially releasing shade-intolerant conifers from angiosperm competition, reducing angiosperm density and, therefore, competition intensity in northern New Zealand forests. In many forest ecosystems, conifer–angiosperm forest dynamics can be conceptualized as three component systems.