How and why plant ionomes vary across North American grasslands and its implications for herbivore abundance

Abstract

Plant elemental content can vary up to 1,000‐fold across grasslands, with implications for the herbivores the plants feed. We contrast the regulation, in grasses and forbs, of 12 elements essential to plants and animals (henceforth plant‐essential), 7 essential to animals but not plants (animal‐essential) and 6 with no known metabolic function (nonessential). Four hypotheses accounted for up to two thirds of the variation in grass and forb ionomes across 54 North American grasslands. Consistent with the supply‐side hypothesis, the plant‐essential ionome of both forbs and grasses tracked soil availability. Grass ionomes were more likely to harvest even nonessential elements like Cd and Sr. Consistent with the grazing hypothesis, cattle‐grazed grasslands also accumulated a handful of metals like Cu and Cr. Consistent with the NP‐catalysis hypothesis, increases in the macronutrients N and P in grasses were associated with higher densities of cofactors like Zn and Cu. The plant‐essential elements of forbs, in contrast, consistently varied as per the nutrient‐dilution hypothesis—there was a decrease in elemental parts per million with increasing local carbohydrate production. Combined, these data fit a working hypothesis that grasses maintain lower elemental densities and survive on nutrient‐poor patches by opportunistically harvesting soil nutrients. In contrast, nutrient‐rich forbs use episodes of high precipitation and temperature to build new carbohydrate biomass, raising leaves higher to compete for light, but diluting the nutrient content in every bite of tissue. Herbivores of forbs may thus be particularly prone to increases in pCO2 via nutrient dilution.

Plant functional groups and phylogenetic regularity control plant community bioelement composition through calcium and magnesium

“…Thus, the use of community concentrations of Ca and Mg may be a viable strategy to infer whether plant communities are bioelement‐rich or poor. Differences in the proportions of Ca‐ and Mg‐poor graminoids and Ca‐ and Mg‐rich forbs between quadrats possibly underlie this close relationship between PCBC and plant community Ca and Mg. Aboveground tissues of grasses tend to be generally bioelement‐poor compared to forbs (Han et al 2011, Mládková et al 2018, Kaspari et al 2021), leading to low PCBC in quadrats dominated by graminoids. Since tissue Ca and Mg, i.e.…”

“…Plant community bioelement composition (PCBC, community‐scale aggregated bioelement composition in plant tissues) and plant community bioelement stoichiometry (PCBS, community‐scale aggregated bioelement stoichiometric ratios) are increasingly recognised as important ecosystem functions influencing biogeochemical cycling (Elser et al 2010, Chen and Chen 2021, Sardans et al 2021a, Fernández‐Martínez 2022), decomposition (Makkonen et al 2012), digestibility (Mládková et al 2018) or herbivores (Kaspari et al 2021). The concentrations of bioelements in plant communities can simply reflect soil nutrient availability (‘you are what you root in'; Vitousek et al 1995, Aerts and Chapin 1999, Elser et al 2010, Kaspari et al 2021). However, community aspects, such as biodiversity or the presence of functional groups, have been shown to influence PCBC or PCBS as well (Reich and Oleksyn 2004, Novotny et al 2007, Townsend et al 2007, Han et al 2011, Abbas et al 2013, Guiz et al 2016, Schaller et al 2016, Di Palo and Fornara 2017, Chen and Chen 2021).…”

“…Plants require bioelements in specific quantities and proportions in their tissues to survive and grow (Sterner and Elser 2002, Elser et al 2010, Peñuelas et al 2019, Sardans et al 2021a. More than 20 elements can be found in plant tissues (Baxter 2015), thus, to study plant n-dimensional biolement composition and its wide interspecific differences, concepts, such as the plant ionome (Lahner et al 2003, Baxter 2015, Kaspari et al 2021, multidimensional stoichiometric niche (González et al 2017), biogeochemical niche (Peñuelas et al 2019), stoichiometric niche volume (Ågren and Weih 2020) or elementome (Fernández-Martínez 2022) have been developed. These multi-element concepts are expected to reflect species strategies, the demands for bioelements or the individual investments in biomolecules, such as proteins rich in nitrogen (N) or nucleic acids rich in phosphorus (P), or structural resources, such as cellulose and lignin rich in carbon (C) (Sterner and Elser 2002, González et al 2017, Sardans et al 2021a.…”

“…One of the key aspects of plant communities is the presence of certain plant functional groups. In this respect, distinguishing between graminoids, legumes and forbs has proven to be useful in ecological stoichiometric studies in grasslands (Abbas et al 2013, Guiz et al 2016, Schaller et al 2016, Kaspari et al 2021), sand dunes (Di Palo and Fornara 2017) or across habitats (Han et al 2011). For instance, legumes tend to increase plant community N, graminoids increase plant community C:element ratios (Abbas et al 2013, Guiz et al 2016) or forbs could be related to plant community P, K, Ca and Mg concentrations (Han et al 2011, Mládková et al 2018, Kaspari et al 2021).…”

Electrolytes on the prairie: How urine‐like additions of Na and K shape the dynamics of a grassland food web

“…If true, then co‐fertilization with K and either Mn or Cu + Zn (which are cofactors in the same dismutase) should yield more plant growth than adding K, Mn, Cu, or Zn alone. In one relevant study of 54 grasslands, increases in N and P—long known to limit plant biomass production (Elser et al, 2007; Fay et al, 2015)—generated increased demand for K, Mn, Cu, and Zn (Kaspari et al, 2021). This cascading effect of elemental demands has become the signature of ionomics and multiple element limitation (Fan et al, 2021; Kaspari & Powers, 2016; Salt et al, 2008).…”