Browsing wildlife and heavy grazing indirectly facilitate sapling recruitment in an East African savanna

Abstract

Management of tree cover, either to curb bush encroachment or to mitigate losses of woody cover to over‐browsing, is a major concern in savanna ecosystems. Once established, trees are often “trapped” as saplings, since interactions among disturbance, plant competition, and precipitation delay sapling recruitment into adult size classes. Saplings can be directly suppressed by wildlife browsing and competition from adjacent plants, and indirectly facilitated by grazers, such as cattle, which feed on neighboring grasses. Yet few experimental studies have simultaneously quantified the effects of cattle and wildlife on sapling growth, particularly over long time scales. We used a series of replicated 4‐ha herbivore‐manipulation plots to investigate the net effects of wildlife and moderate cattle grazing on Acacia drepanolobium sapling growth over 10 years that encompassed extended wet and dry periods. We also simulated more intense cattle grazing using grass removal treatments (0.5‐m radius around saplings), and we quantified the role of intraspecific tree competition using neighborhood tree surveys (trees within a 3‐m radius). Wildlife, which included elephants, had a positive effect on sapling growth. Wildlife also reduced neighbor tree density during the 10‐yr study, which likely caused the positive effect of wildlife on saplings. Although moderate cattle grazing did not affect sapling growth, grass removal treatments simulating heavy grazing increased sapling growth. Both grass removal and neighbor tree effects on saplings were strongest during above‐average rainfall years following drought. This highlights that livestock‐driven reductions in grass cover and catastrophic wildlife damage to trees during droughts present a need, or an opportunity, for targeted management of sapling growth and woody plant cover during ensuing wet periods.

Savanna resilience to droughts increases with the proportion of browsing wild herbivores and plant functional diversity

“…In our study, we focussed on a subset of these factors and found that when perennial grass cover decreased significantly or recovered too slowly, shrubs used their competitive advantage in the acquisition of water from deep soil layers and shrub seedlings established on the available space hence limiting the regrowth of grasses. These results are also in line with previous findings showing that grass removal increased sapling growth and woody plant cover (LaMalfa et al, 2021). Additionally, our model showed the stabilising effect of reduced herbivory pressure on grasses initiated by the introduction of browsing herbivores.…”

Demographic consequences of mutualism disruption: Browsing and big‐headed ant invasion drive acacia population declines

“…This effect may be particularly strong at Ol Pejeta Conservancy due to relatively high densities of elephants, black rhinoceroses, and other ungulates, which frequently browse saplings and small trees (Wahungu et al, 2009). Recent work at Mpala Research Centre, a nearby, uninvaded study system with lower elephant densities and no rhinos, found muted effects of herbivory on saplings (LaMalfa et al, 2021). Additionally, and in previous studies on A. drepanolobium , LMH forced the reallocation of energy from reproduction to defensive investment (spines), further reducing λ by limiting reproduction (Goheen et al, 2007; Maclean et al, 2011).…”

“…Interventions to increase survival of small trees and allow them to grow to larger, less vulnerable sizes could be used to bolster population growth rates and stabilize populations. A recent study showed that A. drepanolobium saplings grow rapidly in years of heavy rainfall when protected from browsing pressure (LaMalfa et al, 2021). Targeted fencing of small trees in high rainfall years or with irrigation could help to buffer acacia populations against increasing numbers of mega-browsers.…”

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

“…We have demonstrated here that if grazers replace browsers and reduce grass competition in combination with the loss of fire (a scenario that is occurring due to intensive cattle grazing across many African savanna systems [Hempson et al, 2015; Probert et al, 2019; Veldhuis, Ritchie et al, 2019; Venter et al, 2018]), then seedling establishment may experience a ~7.5‐fold increase. Riginos (2009) found that the negative competitive effects of grasses on tree growth persists across all demographic stages and LaMalfa et al (2021) found that if high grazing pressure persists, even in the presence of browsing, then established saplings have a higher likelihood of recruiting to the next demographic stage. Thus, our results provide strong experimental evidence in this system to support the theory that historic tree cover change in the Serengeti was driven by cascading impacts during the collapse and subsequent recovery of the wildebeest population (Holdo et al, 2009).…”

“…Thus, our results provide strong experimental evidence in this system to support the theory that historic tree cover change in the Serengeti was driven by cascading impacts during the collapse and subsequent recovery of the wildebeest population (Holdo et al, 2009). More broadly, our findings align with previous work showing that savanna tree:grass ratios are sensitive to the direct and indirect effects of grazers and browsers with changes to either herbivore guild capable of driving sudden shifts in system structure and function (Goheen et al, 2010; LaMalfa et al, 2019, 2021; O'Connor et al, 2014; Van Langevelde et al, 2003).…”