Functional traits contribute in opposite directions to taxonomic turnover in northeastern US forests over time

Abstract

Aims: Understanding the processes driving forest dynamics requires considering that species within communities do not respond in a coordinated manner to external factors. We argue that temporal turnover in species composition results from contrasting differences in species life history and functional strategies (demography and seed and wood traits) that might simultaneously promote stability and dynamism by operating on distinct subsets of species. Specifically, fast taxonomic turnover should be promoted by species that take advantage of sporadically available resources, while forest stability should be promoted by species with conservative and stress‐tolerant life histories.Location: Northeastern USA.Methods: We combine demographic information (survival, recruitment) over a 14‐year period from temperate tree communities in the northeastern United States with trait information on species seed mass and wood density as a proxy for their reproductive and resource acquisition strategies, to evaluate the differential contribution of species with contrasting ecological strategies (low vs high seed mass/wood density) to rates of compositional turnover in temperate forests.Results: The apparent dynamism of US forests is mostly driven by high mortality and low recruitment of small‐seeded species and by high mortality and recruitment of low wood density species. Simultaneously, species with the opposite traits, i.e., high seed mass and wood density, contribute more to stability. Our findings suggest that forests dynamics in the northeastern United States are the outcome of opposing contributions of life history and plant traits that simultaneously promote forest stability and rapid taxonomic turnover by operating differentially across tree species.Conclusions: Small‐seeded and low wood density species promote faster forest turnover than species with the opposite traits. Not accounting for these functional differences in community‐level analyses is likely to mask the complex dynamics of temperate forests. This study demonstrates the importance of studying forest composition and structure under a dynamic scope that accounts for differences in functional strategies across species.This study investigates the drivers of temporal shifts in community composition for forest across the northeastern United States. We found that tree taxonomic turnover hinders the contribution of two opposed ecological processes, dynamism and stability, that operate differently on groups of species with distinct functional traits. Small‐seeded and low wood density species promote faster forest turnover than species with the opposite traits. Photo legend: Acer rubrum, United States; Photo credit: Hans Reijnen.