Rangeland resilience and resistance: annual and perennial grass stable states

Authors: Harmon, Daniel - Dan; Clements, Darin - Charlie; Blank, Robert - Bob

Submitted to: Society for Range Management Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 12/14/2016
Publication Date: 1/29/2017
Citation: Harmon, D.N., Clements, D.D., Blank, R.R. 2017. Rangeland resilience and resistance: annual and perennial grass stable states. Society for Range Management Meeting Abstracts. 70:190.

Interpretive Summary:

Technical Abstract: The concept of resilience, the ability to resist a shift to an alternative vegetation state, has become an important topic in range management. To quantify the degree to which a plant community is resilient, we experimentally manipulated communities dominated by either the invasive annual grass cheatgrass (Bromus tectorum) or a perennial grass. We hypothesized that an interaction between soil moisture and seed availability controls resilience in these communities. To test resilience, we controlled cheatgrass and at separate sites removed perennial grass using herbicides and observed the establishment of the alternate vegetation state. We seeded 270 perennial grass seeds/m2 and 1,000 cheatgrass seeds/m2 into their alternate state (annual grass dominant and perennial grass dominant, respectively). Removal of the annual grass resulted in an increase of soil moisture (June gravimetric mean: 15cm depth removal 4.91%, non-removal 2.69%, 3cm depth removal 1.7%, non-removal 0.3%) and subsequent shift to perennial grass state after seeding. Without annual grass removal, perennial grasses failed to establish. Removal of the perennial grass resulted in a two fold increase in annual grass (mean: removal 42 cheatgrass/m², non-removal 20.45 cheatgrass/m2) however, small sample sizes limited detection of soil moisture differences. These results indicate a less resilient stable state for perennial grass than annual grass dominated states, since cheatgrass was able to establish (20.45 cheatgrass/m²) in the presence of perennial grass. Our results support the hypothesis that soil moisture is the direct threshold maintaining annual grass stable states and that annual plant presence or absence is the indirect threshold limiting the shift to an alternate state. For perennial states, a combination of perennial presence and annual seed abundance interact to limit alternate states. We conclude that density of the dominant vegetation is an acceptable indicator of the resilience of these two stable states.