Threshold-dependent plant community composition change across a dryland shrub encroachment gradient results in variable post-herbicide responses

Authors: REICHENBORN, MOLLY - New Mexico State University; SCHROEDER, RYAN - University Of Montana; FAIST, AKASHA - University Of Montana; Webb, Nicholas; LEHNHOFF, ERIK - New Mexico State University

Submitted to: Ecological Society of America Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 4/1/2025
Publication Date: 8/15/2025
Citation: Reichenborn, M., Schroeder, R., Faist, A., Webb, N.P., Lehnhoff, E. 2025. Threshold-dependent plant community composition change across a dryland shrub encroachment gradient results in variable post-herbicide responses. Ecological Society of America Abstracts. Abstract.

Interpretive Summary:

Technical Abstract: Woody plant species have progressively encroached into many grass-dominated ecosystems worldwide. Woody plant encroachment (WPE) can fundamentally change the structure and function of grasslands, jeopardizing the ecological services provided by these ecosystems. Managing woody plant species often does not restore the plant communities observed prior to WPE, likely due to a “tipping point” or ecological threshold in the transition between grass and woody-plant dominated ecological states. Identifying where this threshold exists along the grassland to shrubland state change continuum, and how it affects the plant community response to restoration efforts, can inform the effective application of WPE management. We established twenty sets of paired herbicide-treated and untreated plots across a semiarid grassland to shrubland gradient to examine threshold-dependent plant community responses to shrub management in southern New Mexico, USA. We conducted annual vegetation surveys to characterize plant community composition and the size and abundance of plant canopy gaps before and after herbicide application. Increasing abundance of large all-plant canopy gaps (>100cm) is associated with increased wind-driven soil in drylands, a key ecogeomorphic process expected to drive and reinforce grassland to shrubland transitions. Because of this, we used the abundance of all-plant canopy gaps as a structural indicator of composition change across the encroachment gradient. We hypothesized that plant community composition would change abruptly after crossing a critical threshold between the grassland and shrubland ecosystem state along the encroachment gradient. Following herbicide, we expected that the plots that had not yet crossed this threshold to the shrubland state would increase in herbaceous plant cover. We identified a community-wide shift in composition when the abundance of all-plant canopy gaps exceeded 59%, indicating an abrupt ecosystem state shift. While annual species largely increased in response to herbicide regardless of the encroachment gradient, the response of perennial species varied and were not clearly dependent on the ecosystem state change threshold. The historically dominant grass species, Bouteloua eriopoda, decreased regardless of herbicide application, while the subdominant grass genus Sporobolus spp. increased on herbicide-treated plots. Our results highlighted the need to characterize taxa-specific responses that could be leveraged to achieve grassland restoration objectives, even if the resulting community does not match the historical ecological state. Using an indicator tied to a key process known to drive and reinforce ecosystem state transitions, we captured informative responses that could be used to preempt forthcoming plant community change and apply WPE management strategically.