Revision of a state‐and‐transition model to include descriptions of state functional attributes

Abstract

AbstractSuccessful conservation of ecosystems and ecosystem services requires understanding the structural–functional relationships underlying resilience to disturbance pressures. State‐and‐transition models (STMs) are box‐and‐arrow diagrams conceptualizing system resilience, but many STMs lack functional descriptions. For a cattle‐grazed sagebrush steppe ecosystem in northwestern Colorado, we aimed to update an existing STM by incorporating functional descriptions of the states to inform future hypotheses. In this STM, a diverse set of native perennial understory plants define a Diverse state, but long‐term exclusion of disturbances to sagebrush results in a shift to a Depauperate state with increased bare ground interspaces and sagebrush dominance. We hypothesized that herbaceous species were inferior competitors to sagebrush over the long term for soil nitrogen. To examine evidence for this hypothesis, develop functional descriptions of the Diverse and Depauperate states, and identify more specific hypotheses for future studies, we measured the following: the mass, C:N, and distribution of the litter layer; nitrogen mineralization rates; nitrate pools; and soil attributes on plots classified as Diverse (n = 5) or Depauperate (n = 5). Plots in the Depauperate state had ~20% higher soil bulk density, half the clay and soil moisture concentrations, and 30% lower percent soil nitrogen compared to Diverse state plots; litter layer patchiness was nearly three times greater on the Depauperate plots than on the Diverse plots, but there was no significant difference between states in mean litter layer mass or C:N. We detected no significant differences between states in nitrogen mineralization rates or nitrate pools. This may be due to our sampling design, which examined nitrogen transformation at the plot level rather than at microsites within the plots, or it may be that another resource such as soil moisture is more limiting than nitrogen. Differences in soil texture between plots in the Diverse and Depauperate states may indirectly influence the competitive interactions between sagebrush and native herbaceous understory by controlling available soil resources. These results suggest that the vulnerability of sagebrush steppe ecosystems to state changes may be dependent on soil texture, which is highly variable across this broadly distributed ecosystem. Based on these insights, we present a revised STM and propose future work.