Quantifying the impacts of tree densification on the grassy understorey: a trait-based approach

Abstract

Understanding how trees limit light will advance theories of bush encroachment and tree-grass coexistence in savannas and it will enable us to explore how we can best conserve fire maintained, fire-dependent vegetation systems in South Africa and how our biome boundaries may shift. In order to understand how fire maintains the boundary between closed canopies and open grassy environments, the aim of this research was to test the hypothesis that; tree traits affect light that passes through the canopies of trees to the ground, by in uencing the threshold tree density at which grass stops growing under trees. Using the Point-Centered-Quarter method I estimated tree density using a proxy, mean PCQ distance, to refl ect changes in inter-tree distance across the gradient of tree density. At the same plots I used dry-weight ranking to record changes in grass cover, species composition and abundance, a balance scale to weigh wet and dry grass biomass, in tandem with densiometer measurements and hemispherical images to obtain canopy cover and the amount of sunlight received by the ground, in addition to the tree density data. To elucidate the interaction between tree traits and sunlight (correlation, boxplots, ANOVA, post-hoc test, MRD, quartile-regression analyses and linear modelling) as well as response of grass species to light (regression analysis, boxplots, ANOVA and GAMs), respective passable analyses were employed using R. The results show that biomass declines with changes in grass species composition and abundance, that are in turn driven by decline in light transmittance; when horizontal and compound leafed trees, especially on short, umbrella shaped trees with horizontal spheroids prevail. Consequently reducing light loving grass species with characteristics that lead to high fire frequency and intensity; high cover, height and biomass, that prevail when tall trees with vertically angled leaves and spheroids dominate, leading to higher light transmittance. This succinct understanding highlights variation in the tree-grass relationship across savanna ecosystems, suggesting site-specific interventions and recommendations to bush encroachment. Findings reveal value in monitoring tree canopy cover across savanna ecosystems; using it as an early warning proxy for changes in primary production and fire regimes. Conclusions challenge assumptions about the minimum light tolerance of grass, and provide important clues to help disantangle mechanisms by which grasses may persist at low light levels. Key words: Savanna, South Africa, light transmittance, trees, grass, bush encroachment