Contrasts among cationic phytochemical landscapes in the southern United States

Abstract

Understanding the phytochemical landscapes of essential and nonessential chemical elements to plants provides an opportunity to better link biogeochemical cycles to trophic ecology. We investigated the formation and regulation of the cationic phytochemical landscapes of four key elements for biota: Ca, Mg, K, and Na. We collected aboveground tissues of plants in Atriplex, Helianthus, and Opuntia and adjacent soils from 51, 131, and 83 sites, respectively, across the southern United States. We determined the spatial variability of these cations in plants and soils. Also, we quantified the homeostasis coefficient for each cation and genus combination, by using mixed-effect models, with spatially correlated random effects. Additionally, using random forest models, we modeled the influence of bioclimatic, soil, and spatial variables on plant cationic concentrations. Sodium variability and spatial autocorrelation were considerably greater than for Ca, Mg, or K. Calcium, Mg, and K exhibited strongly homeostatic patterns, in striking contrast to non-homeostatic Na. Even so, climatic and soil variables explained a large proportion of plants' cationic concentrations. Essential elements (Ca, Mg, and K) appeared to be homeostatically regulated, which contrasted sharply with Na, a nonessential element for most plants. In addition, we provide evidence for the No-Escape-from-Sodium hypothesis in real-world ecosystems, indicating that plant Na concentrations tend to increase as substrate Na levels increase.

Keywords: Random Forest; calcium; homeostasis; magnesium; potassium; sodium; spatial autocorrelation.

FIGURE 1

Geographic locations and aboveground phytochemical landscapes of (a) Ca, (b) K, (c) Mg, and (d) Na (log₁₀ ppm) across the southern United States. Triangle, square, and circle shapes depict sites where Atriplex, Helianthus, and Opuntia were sampled, respectively. A color gradient demonstrates plant Ca, K, Mg, and Na concentrations, with darker shades indicating higher concentrations and lighter shades indicating lower concentrations.

FIGURE 2

Concentrations and variation in concentrations of cations for aboveground plant tissues (purple) and adjacent soils (gold) for each genus across all sites sampled. Responses for (a) Ca, (b) Mg, (c) K, and (d) Na are shown in boxplots and the coefficients of variation are shown in bar graphs for each genus considered. All responses are log‐transformed. Asterisks (***) indicate a significant difference (p < .0001).

FIGURE 3

Mantel correlograms for plant cation spatial structure for Atriplex, Helianthus, and Opuntia. Mantel r values are plotted on the y‐axis. Closed circles indicate significant correlations (p < .05) after 9999 permutations. Positive correlations indicate positive spatial autocorrelation among sites.

FIGURE 4

Mantel correlograms for soil cation spatial structure for Atriplex, Helianthus, and Opuntia. Mantel r values are plotted on the y‐axis. Closed circles indicate significant correlations (p < .05) after 9999 permutations. Positive correlations indicate positive spatial autocorrelation among sites.

FIGURE 5

Importance values of potential abiotic drivers of plant concentrations of (a) Ca (b) K (c) Mg and (d) Na for Atriplex, Helianthus, and Opuntia. Abiotic drivers are ranked by relative variable importance calculated for each random forest model; variable importance represents the increase in mean error across trees when a predictor is permuted. Black points and error bars are mean and 95% confidence values of relative importance values and were calculated across 50 iterations of the same random forest model. For visualization purposes, the relative importance scores of spatial predictors were excluded.

FIGURE 6

Responses of plant concentrations of (a) Ca, (b) Mg, (c) K and (d) Na for Atriplex, Helianthus, and Opuntia to mean annual temperature (MAT). Each line represents a response curve estimated by a random forest model, which was fitted for each combination of plant genus and cation and repeated 50 times. Plant Na is on a log₁₀ scale.

FIGURE 7

Responses of plant concentrations of (a) Ca, (b) Mg, (c) K and (d) Na for Atriplex, Helianthus, and Opuntia to soil cation concentrations (ppm). Each line represents a response curve estimated by a random forest model, which was fitted for each combination of plant genus and cation and repeated 50 times. Plant and soil Na are on a log₁₀ scale.