Ungulate grazing drives higher ramet turnover in sodium‐adapted Serengeti grasses

Abstract

Questions: Ungulate grazers requiring sodium (Na) will preferentially graze in areas with Na-accumulating forage grasses (i.e. many saline soils with high NaCl). Given that high-Na conditions necessitate specific adaptations in plants, the mechanisms allowing plants to persist under combined herbivory and Na stress are unclear. Has adaptation to soil salinity in East African grasses modified the evolution of grazing tolerance, leading to plants that are susceptible to defoliation? Does grazing differentially impact the survivorship of plants with different tolerances to Na? Finally, can species traits that confer salinity and herbivory tolerances help to explain the presence of mosaic structure in high salinity regions?Location: Short-grass plains of Serengeti National Park, Tanzania, East Africa. Methods:We tracked the fate of 836 individuals of 26 grass species, over an entire growing season, in the short-grass plains using a network of 41 quadrats (0.25 m 2 ). We investigated the impact of soil properties (e.g., electrical conductivity [EC], bulk density, clay), ungulate herbivory and grass species identity on patterns of composition and turnover. Finally, we linked plant leaf Na and leaf palatability data to species environmental responses.Results: Under comparable levels of grazing pressure, grass species known to accumulate Na have much higher levels of turnover compared to low-Na species. These Na accumulators were generally short, highly palatable and clonal species.Conclusions: Herbivory most affects turnover (of 'ramets') of species with the highest tolerance to Na, suggesting that Na tolerance ability interacts with defoliation response in Serengeti grasses. As a result, spatial heterogeneity in soil EC produces high-turnover communities that overlay high-EC soil adjacent to more stable communities composed of Na-intolerant species. While Na-intolerant grasses in Serengeti range from short and palatable to taller and less palatable species, grasses that accumulate Na tend to be short and palatable, a likely result of the grazing pressure they experience.

Lineage‐based functional types: characterising functional diversity to enhance the representation of ecological behaviour in Land Surface Models

“…By contrast, we found that the C 4 LFTs had higher SLA than the C 3 LFT, but SLA did not differ between the two dominant C 4 grass lineages (Atkinson et al ., 2016). SLA can be highly variable within lineages in grasses, probably due to the importance of herbivore pressure as a competing demand on leaf economics (Anderson et al ., 2011; Griffith et al ., 2017b) as well as intraspecific variation. As a result, SLA highlighted that some traits are harder to generalise than others using the LFT approach and suggested that a range of values may be more appropriate than a single value for constraining LFT parameters.…”

The ‘plantspec’ r package: A tool for spectral analysis of plant stoichiometry

“…The C:N ratio is a strong indicator of palatability (e.g. Griffith et al., ) and shows significantly more variation than expected by a Brownian model of trait evolution ( K = 0.62, p = 0.003), indicating that distantly related grasses have evolved similar levels of palatability. This result is unsurprising giving the high level of variation in herbivory‐related traits such as Leaf Mass per Area in these grasses (Anderson et al., ).…”

“…For this analysis, above‐ground biomass samples for each species were collected from each of 10 sites (20 × 50 m plots) spanning the entire rainfall gradient in Serengeti (see Griffith, Quigley, & Anderson, and Griffith, Anderson, & Hamilton, for details of the data collection). We aggregated individual samples by species at each site because many samples had low biomass.…”

A handbook for the standardised sampling of plant functional traits in disturbance-prone ecosystems, with a focus on open ecosystems

“…All animals require sodium (Robbins 1994), but it is not considered an essential nutrient for plants and can be toxic at high concentrations (Kronzucker et al 2013). Because of their sodium requirements, grazers seek out plants with high sodium concentrations (Griffith et al 2017), and, consequently, by concentrating sodium in their leaves, grasses cannot only get rid of excess sodium, but promote regular grazing to maintain high light conditions close to the soil level (i.e. high sodium concentrations can facilitate the persistence of short-statured grasses in communities that might otherwise grow tall and shade them out).…”

“…high sodium concentrations can facilitate the persistence of short-statured grasses in communities that might otherwise grow tall and shade them out). Grasses with high FS concentrations should, thus, also have traits conferring high resistance or tolerance to grazing (Griffith et al 2017).…”