Fire, grazers, and browsers interact with grass competition to determine tree establishment in an African savanna

Abstract

In savanna ecosystems, fire and herbivory alter the competitive relationship between trees and grasses. Mechanistically, grazing herbivores favor trees by removing grass, which reduces tree-grass competition and limits fire. Conversely, browsing herbivores consume trees and limit their recovery from fire.Herbivore feeding decisions are in turn shaped by risk-resource trade-offs that potentially determine the spatial patterns of herbivory. Identifying the dominant mechanistic pathways by which fire and herbivores control tree cover remains challenging, but is essential for understanding savanna dynamics. We used an experiment in the Serengeti ecosystem and a simple simulation driven by experimental results to address two main aims: (1) determine the importance of direct and indirect effects of grass, fire, and herbivory on seedling establishment and (2) establish whether predators determine the spatial pattern of successful seedling establishment via effects on mesoherbivore distribution. We transplanted tree seedlings into plots with a factorial combination of grass and herbivores (present/absent) across a lion-kill-risk gradient in the Serengeti, burning half of the plots near the end of the experiment. Ungrazed grass limited tree seedling survival directly via competition, indirectly via fire, and by slowing seedling growth, which drove higher seedling mortality during fires. These effects restricted seedling establishment to below 18% and, in conjunction with browsing, resulted in seedling establishment dropping below 5%. In the absence of browsing and fire, grazing drove a 7.5-fold increase in seedling establishment. Lion predation risk had no observable impact on herbivore effects on seedling establishment. The severe negative effects of grass on seedling mortality suggests that regional patterns of tree cover and fire may overestimate the role of fire in limiting tree cover, with regular fires representing a proxy for the competitive effects of grass.

Soil geochemistry and constraint of tree seedlings immediately after germination on Macrotermes termite mounds in the Kruger National Park, South Africa

“…Second, the grass sward on mounds is likely to be considerably more nutritious than the grass sward off the mounds in the savannas, which would increase the herbivory pressure on the mounds. Such pressure is likely to constrain tree seedlings more than it would constrain the grass sward (Donaldson et al., 2022 ; Muvengwi et al., 2019 ; Voysey et al., 2021 ). Lastly, notwithstanding the greater herbivory pressure on termite mounds, relatively fast grass growth on the mounds in wetter years is likely to result in higher‐intensity fires on the mounds than off them (Figure 1c ).…”

Tree and grass competition in savannas: Interactive effects of soil moisture, grass defoliation and grass species identity on the growth and survival of tree seedlings

“…We expected sunlight interception to vary between grass species, and those seedlings grown in tall (i.e. unclipped), high‐biomass grasses would experience reduced growth and survival due to higher rates of grass transpiration and lower light levels (Bhadouria et al., 2020; Donaldson et al., 2022; Tomlinson et al., 2019). However, we predicted that tree seedling species that dominate more mesic, productive sites in Serengeti would have higher growth and survival when grown with unclipped grasses compared to tree species dominating drier sites (Anthelme & Michalet, 2009).…”

Effects of long‐term fixed fire regimes on African savanna vegetation biomass, vertical structure and tree stem density

“…While resources, such as mean annual precipitation (MAP), can set limits on vegetation growth, disturbances such as fires can constrain savanna vegetation structure. Fires consume above‐ground biomass (AGB), alter tree‐grass coexistence and modify the size class distribution of trees (Donaldson et al, 2022; Higgins et al, 2007; Murphy et al, 2014), preventing vegetation from reaching its resource driven potential and creating a substantial discrepancy between observed and climatically defined potential vegetation biomass (Bond, 2005; Bond & Keeley, 2005). However, vegetation structural responses to fire over large spatial extents are difficult to ascertain, due in part to a lack of spatially expansive fine‐scale data on vegetation structure.…”