Mesobrowser Abundance and Effects on Woody Plants in Savannas

Augustine, David J.; Scogings, Peter F.; Sankaran, Mahesh

doi:10.1002/9781119081111.ch16

Cited by 5 publications

(13 citation statements)

References 102 publications

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“…Browsing negatively impacted seedling survival during both pre‐burn and post‐burn stages. Although browsing impacts on seedlings in savannas is well recognized (Augustine et al, 2019; Moe et al, 2009; Prins & van der Jeugd, 1993; Staver & Bond, 2014), the intensity of this signal in an herbivore community dominated by grazers highlights the strength of the direct browser‐seedling pathway over the indirect grazer‐grass‐seedling pathway. Our simulation results may have overestimated the effects of browsing by ignoring the potential for grass to conceal seedlings from browsers (Riginos & Young, 2007).…”

Section: Discussionmentioning

confidence: 99%

“…Megaherbivores (4 and 6) will be largely unaffected by predators, but if mesoherbivore effects (3 and 5) dominate seedling establishment dynamics then predators (level 3) could impact patterns of seedling establishment via mesoherbivore behavioral responses. Blue and red arrows represent positive and negative effects on other drivers respectively structure (Augustine et al, 2019;Moe et al, 2009;Prins & van der Jeugd, 1993). Conversely, successful establishment of seedlings into the sapling bank (i.e., multiple cohorts of established saplings) can prime the system to transition from open/grassy to closed/woody states over short time periods when limitations to recruitment are temporarily removed (Prins & van der Jeugd, 1993;Staver & Bond, 2014).…”

Section: Introductionmentioning

confidence: 99%

“…The establishment of new seedlings represents a key bottleneck for tree populations in savannas (Higgins et al, 2000; Scholes & Archer, 1997), and limitations to tree seedling establishment can result in sharp boundaries in tree cover (Salazar et al, 2012; Wakeling et al, 2015) and strong effects on long‐term woodland structure (Augustine et al, 2019; Moe et al, 2009; Prins & van der Jeugd, 1993). Conversely, successful establishment of seedlings into the sapling bank (i.e., multiple cohorts of established saplings) can prime the system to transition from open/grassy to closed/woody states over short time periods when limitations to recruitment are temporarily removed (Prins & van der Jeugd, 1993; Staver & Bond, 2014).…”

Section: Introductionmentioning

confidence: 99%

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

Donaldson

Holdø

Sarakikya³

et al. 2022

Ecology

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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.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Donaldson

Holdø

Sarakikya³

et al. 2022

Ecology

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“…exuvialis ) is only 12 per km 2 . These area‐averaged values, however, do not represent the herbivore densities experienced by individual seedlings (Augustine et al., 2020). Locally, browser densities can exceed 50 per km 2 on grazing lawns and herbivore hotspots (Burkepile et al., 2013) and the WRF itself had a density over 30 per km 2 .…”

Section: Discussionmentioning

confidence: 99%

“…Mammalian herbivory pressure varies at both local (Burkepile et al., 2013) and regional (Hempson et al., 2015) scales. Intermediate‐rainfall environments have higher densities of browsers and mixed feeders—average values can reach over 800 kg/km 2 (or ~14 mesobrowsers/km 2 ; Augustine et al., 2020; Hempson et al., 2015). These are the environments where there might be selection for early resprouting.…”

Section: Introductionmentioning

confidence: 99%

Browsing is a strong filter for savanna tree seedlings in their first growing season

et al. 2021

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1. Newly germinated seedlings are vulnerable to biomass removal but usually have at least 6 months to grow before they are exposed to dry-season fires, a major disturbance in savannas. In contrast, plants are exposed to browsers from the time they germinate, making browsing potentially a very powerful bottleneck for establishing seedlings.2. Here we assess the resilience of seedlings of 10 savanna tree species to topkill during the first 6 months of growth. Newly germinated seeds from four dominant African genera from across the rainfall gradient were planted in a common garden experiment at the Wits Rural Facility and clipped at 1 cm when they were ~2, 3, 4 and 5 months old. Survival, growth and key plant traits were monitored for the following 2.5 years.3. Seedlings from environments with high herbivory pressure survived topkill at a younger age than those from low-herbivore environments, and more palatable genera had higher herbivore tolerance. Most individuals that survived were able to recover lost biomass within 12 months, but the clipping treatment affected root mass fraction and branching patterns. 4. Synthesis. The impact of early browsing as a demographic bottleneck can be predicted by integrating information on the probability of being browsed and the probability of surviving a browse event. Establishment limitation through early browsing is an under-recognized constraint on savanna tree species distributions.

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Ungulates mitigate the effects of drought and shrub encroachment on the fire hazard of Mediterranean oak woodlands

Lecomte,

Bugalho,

Catry

et al. 2024

Ecological Applications

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Climate change is increasing the frequency of droughts and the risk of severe wildfires, which can interact with shrub encroachment and browsing by wild ungulates. Wild ungulate populations are expanding due, among other factors, to favorable habitat changes resulting from land abandonment or land‐use changes. Understanding how ungulate browsing interacts with drought to affect woody plant mortality, plant flammability, and fire hazard is especially relevant in the context of climate change and increasing frequency of wildfires. The aim of this study is to explore the combined effects of cumulative drought, shrub encroachment, and ungulate browsing on the fire hazard of Mediterranean oak woodlands in Portugal. In a long‐term (18 years) ungulate fencing exclusion experiment that simulated land abandonment and management neglect, we investigated the population dynamics of the native shrub Cistus ladanifer, which naturally dominates the understory of woodlands and is browsed by ungulates, comparing areas with (no fencing) and without (fencing) wild ungulate browsing. We also modeled fire behavior in browsed and unbrowsed plots considering drought and nondrought scenarios. Specifically, we estimated C. ladanifer population density, biomass, and fuel load characteristics, which were used to model fire behavior in drought and nondrought scenarios. Overall, drought increased the proportion of dead C. ladanifer shrub individuals, which was higher in the browsed plots. Drought decreased the ratio of live to dead shrub plant material, increased total fuel loading, shrub stand flammability, and the modeled fire parameters, that is, rate of surface fire spread, fireline intensity, and flame length. However, total fuel load and fire hazard were lower in browsed than unbrowsed plots, both in drought and nondrought scenarios. Browsing also decreased the population density of living shrubs, halting shrub encroachment. Our study provides long‐term experimental evidence showing the role of wild ungulates in mitigating drought effects on fire hazard in shrub‐encroached Mediterranean oak woodlands. Our results also emphasize that the long‐term effects of land abandonment can interact with climate change drivers, affecting wildfire hazard. This is particularly relevant given the increasing incidence of land abandonment.

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Mesobrowser Abundance and Effects on Woody Plants in Savannas

Cited by 5 publications

References 102 publications

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

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

Browsing is a strong filter for savanna tree seedlings in their first growing season

Ungulates mitigate the effects of drought and shrub encroachment on the fire hazard of Mediterranean oak woodlands

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