Temporal changes in the dominance of tree functional traits, but no changes in species diversity during woody plant encroachment in a Brazilian savanna

Abstract

AbstractQuestions: Woody encroachment in savannas has been associated with changing taxonomic composition and ecosystem function. Interestingly, there is little understanding of how encroachment impacts plant functional diversity and how those changes relate to plant demography, a crucial mediator between taxonomic composition and ecosystem function.Location: Southeastern Brazil.Methods: Using a landscape scale fire suppression experiment in a diverse Brazilian savanna, we quantify how change in species composition over seven years impacted vegetative and reproductive tree functional diversity as determined by new recruits, dead and surviving trees.Results: Over seven years, tree above‐ground biomass increased by 15%, while total species richness did not change. Despite minor changes, species composition remained overall similar (82%), with few species contributing significantly to plot dissimilarity over time. There were small changes in vegetative traits, where the community‐weighted mean increased in maximum tree height (↑ 2.1%) and specific leaf area (↑ 5.3%), and decreased in wood density (↓ 1.3%) and bark thickness (↓ 9.4%). Changes in reproductive traits were larger than in vegetative traits, with an increase in the prevalence of monoecy (↑ 32.6%), dioecy (↑ 44.2%), large seeds (↑ 20.3%), animal‐mediated seed dispersal (↑ 4.9%) and pollination by very small insects (↑ 45.5%), and a decrease in the prevalence of hermaphroditism (↓ 9%), small seeds (6.8%) and pollination by small insects (12.5%). The overall decrease in bark thickness and increase in monoecy and dioecy were mainly driven by characters of the new recruits, while the overall increase in specific leaf area (SLA) and decrease in small seeds appeared largely determined by the loss of trees possessing those traits.Conclusions: Encroachment leads to changes that are likely increasing ecosystem vulnerability to fire and drought. Further, the compositional changes observed appear to drive marked change in reproductive traits, indicating increasing dependence on animals for dispersal and reproduction. Understanding post‐hoc encroachment impacts in an era of widespread pervasive encroachment is fundamental to reconciling ecosystem functions such as nutrient cycling and pollination services as there is a loss of species with open ecosystem life‐history strategies. Among savannas, there remains an urgent need to understand relationships between woody cover and ecosystem function to determine thresholds in woody cover promoting resilient savanna ecosystems.