Savannah trees buffer herbaceous plant biomass against wild and domestic herbivores

Abstract

Questions Given the growing abundance and dominance of domestic herbivores in savannah ecosystems, can trees maintain plant herbaceous standing biomass under increasing herbivore pressure? Are there differences in the capacity of leguminous and non‐leguminous trees in sustaining understory herbaceous biomass? And finally, to what extent does plant community composition underneath trees modulate the effects of herbivore assemblage and abundance on herbaceous biomass? Location Pasturelands and protected areas along the borders of the Serengeti National Park, Tanzania, East Africa. Method Monthly herbaceous biomass was monitored non‐destructively using a calibrated pasture disc. Measurements were taken within a network of paired exclosures and open plots, underneath large leguminous and non‐leguminous tree canopies and outside canopies. Herbivore community assemblage compositions and abundances were characterised using monthly dung counts, and herbaceous plant community composition was surveyed annually every wet season over two years. Results Overall, we found that trees promote herbaceous standing biomass, particularly in the presence of moderate herbivory rather than under herbivore exclusion. Greater herbivore abundance and livestock dominance reduced herbaceous plant biomass, but trees, particularly leguminous trees, limited these negative effects. This capacity for trees to limit the effect of herbivores was related to herbaceous plant species composition. Understory plant communities that were compositionally typical of protected areas sustained the highest plant biomass when found in pasturelands with high herbivore pressure. Conclusion Our findings give greater credence to the importance of preserving large trees in savannah landscapes increasingly dominated by high abundances of livestock. Moreover, our results highlight that park managers and pastoralists need to maintain the specialist herbaceous understory community beneath trees in order to benefit from facilitative tree–understory interactions.

Proximity to high densities of pastoral settlements reduces grassland regrowth in a protected tropical savanna

“…For instance, Belsky (1986a) found a sevenfold variation in regrowth in a clipping experiment on 14 common savanna grasses. High‐intensity, year‐round livestock grazing can act as strong selection pressure on savanna grassland species composition in wildlife grazed savannas, selecting for grassland species tolerant of higher biomass removal and generating distinct communities (Smith et al, 2020; Tegegn et al, 2011; Young et al, 2013). The consequence of changes in grass species composition caused by livestock grazing for plant productivity is less predictable.…”

“…The consequence of changes in grass species composition caused by livestock grazing for plant productivity is less predictable. Livestock‐induced changes in savanna grassland composition variously increase, decrease, or have no effect on savanna plant productivity (Charles et al, 2017; Smith et al, 2020; Veblen et al, 2016). Although pastoral settlements may lead to greater grass dominance closer to settlements, changes in regrowth capacity will likely depend on species‐specific responses to disturbance.…”

Camera traps enable the estimation of herbaceous aboveground net primary production (ANPP) in an African savanna at high temporal resolution

“…We approximated biomass at the onset of each productivity measurement using a calibrated disc‐pasture meter across a 30 × 30 m grid, and a 10 × 5 m grid at each site, matching the spatial resolution of Sentinel‐2 derived NDVI and field camera derived GCC respectively. Disc‐pasture meter measurements were averaged for each site before using a calibration equation ( R 2 = 0.83) specific to the Serengeti to convert the height (cm) of the disc into herbaceous biomass (g m −2 ) (unpublished data Smith et al., 2020).…”

Teatime in the Serengeti: macrodetritivores sustain recalcitrant plant litter decomposition across human-modified tropical savannahs

“…Rainfall for the wet and dry season incubation periods were obtained from satellite-based daily rainfall estimates from NASA's Goddard Earth Sciences Data and Information Services Centre (Huffman 2017), based upon half-hourly measurements of cloud cover retrieved using multi-satellite microwave data at 10 × 10 km resolution. Previous work at the study sites showed significant positive correlation between these remote satellite based estimates of rainfall and soil moisture content (Smith et al 2020). From daily satellite rainfall estimates we calculated cumulative rainfall for each seasonal incubation period averaged at the site-scale.…”