Responses of African Grasses in the Genus Sporobolus to Defoliation and Sodium Stress: Tradeoffs, Cross-Tolerance, or Independent Responses?

Abstract

In the Serengeti ecosystem of East Africa, grazing ungulates prefer areas with elevated grass Na, suggesting that some grasses tolerate both high soil Na and defoliation. We performed a factorial Na-by-defoliation greenhouse study with five abundant Sporobolus congeners to explore whether Serengeti grasses possess traits which: (i) confer tolerance to both Na and defoliation (cross-tolerance); (ii) display a tradeoff; or (iii) act independently in their tolerances. Our expectation was that related grasses would exhibit cross-tolerance when simultaneously subjected to Na and defoliation. Instead, we found that physiological tolerances and growth responses to Na and defoliation did not correlate but instead acted independently: species characterized by intense grazing in the field showed no growth or photosynthetic compensation for combined Na and defoliation. Additionally, in all but the highest Na dosage, mortality was higher when species were exposed to both Na and defoliation together. Across species, mortality rates were greater in short-statured species which occur on sodic soils in heavily grazed areas. Mortality among species was positively correlated with specific leaf area, specific root length, and relative growth rate, suggesting that rapidly growing species which invest in low cost tissues have higher rates of mortality when exposed to multiple stressors. We speculate that the prevalence of these species in areas of high Na and disturbance is explained by alternative strategies, such as high fecundity, a wide range of germination conditions, or further dispersal, to compensate for the lack of additional tolerance mechanisms.

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

“…Herbivory results in higher rates of turnover (by ramet) for the most Na‐tolerant plant species, indicating that Na tolerance interacts with plant defoliation response in the field (Fig. ; Griffith & Anderson ). Furthermore, the interaction between species Na tolerances and herbivory leads to a biotic gradient from low‐turnover patches of Andropogon greenwayi on one end to high‐turnover, low‐richness communities (i.e.…”

“…Therefore, the costly nature of many Na tolerance adaptations results in plant species that must tolerate chronic herbivory through stoloniferous growth (Hester et al. ; Wilsey ), high fecundity, germination ability or other high‐turnover strategy (Griffith & Anderson ). This interplay of herbivory and Na appears to influence grass distributions in the Serengeti Plains by limiting Na‐tolerant species to areas of high soil EC.…”

“…Given the expected energetic costs of specific Na tolerance traits (e.g., Na exudation, osmotica; see Munns & Tester ; but not Na accumulation in old leaves; see Bailey & Scholes ), previous work has sought to understand plant responses to simultaneous defoliation and Na stress (Hamilton ). In general, species’ growth and photosynthetic activity following defoliation are inhibited by Na tolerance (Griffith & Anderson ). However, we lack a complete understanding of the community‐level consequences of how species respond to the increased grazing that occurs at sites that experience high soil Na concentration in grazing ecosystems (McNaughton ).…”

“…Given the costs associated with both defoliation tolerance and Na adaptation it is expected that species should exhibit a trade‐off between defoliation tolerance and Na tolerance traits (Griffith & Anderson ). However, because herbivores preferentially graze many grass species that must also tolerate high‐Na conditions it is unclear what mechanisms allow plants to persist in the field where they experience these pressures simultaneously (Coughenour ; Fokkema et al.…”

Increased resistance to a generalist herbivore in a salinity-stressed non-halophytic plant

“…In most cases, biotic stressors like herbivory have not been considered when studying salt stress (cf. Griffith and Anderson 2013 ), even though plants growing in saline environments are not immune to herbivore attack.…”