Gaps among perennial plants: Spatial relationships, root biomass and cheatgrass cover

Background/Question/Methods - Recent estimates indicate that only 55% of the sagebrush biome is occupied currently by shrubs of this genus. A significant portion of the remaining area has been displaced by invasive annual grasses, such as Bromus tectorum (cheatgrass) following wildfires. Intact sagebrush communities often have interspaces among perennial plants occupied by cheatgrass, thus creating a continuous fuel for wildfires. Fire managers are treating ecosystems to reduce fuel loads, but the type of treatment may foster increases in cheatgrass. As a precursor to applications of three fuel reduction treatments, we identified seven sites in five western states (WA, OR, ID, NV & UT) with intact sagebrush – bunchgrass communities where native plants dominated communities. We wanted to determine if cheatgrass foliar cover was related to the dominance of native plants in the community or rather to spatial relationships and/or sizes of gaps among bases of native plants. Foliar cover was measured using line-point intercept with 300 points dispersed regularly across 5, 30-m transects. We used median and quantile regressions to relate native plant foliar cover and gaps to foliar cover of cheatgrass. As a follow-up mechanism to these spatial relationships, we described root biomass at varying distance from perennial grasses. Root cores at two depths were taken at regular intervals away from the native plants up to 95 cm and root biomass was modeled as a response to distances from nearest plants, depth and species.

Results/Conclusions – Preliminary results indicate that perennial grass cover was negatively related to increases in mean gap size among perennial plants (r2 = 0.62). Median quantile regressions showed no relationship between cheatgrass cover and perennial plant cover or measures of gaps (mean or proportion of the line occupied by gaps) due to the increasing variation in cheatgrass cover as perennial plant cover declined or as gaps became proportionally larger. We were, however, able to fit the 99% quantile regression to the distribution of cheatgrass cover and perennial grass cover and to cheatgrass cover and the proportion of the line occupied by gaps among perennial plants. We believe this relationship may represent the maximum potential cheatgrass cover for sites with corresponding perennial plant cover or gaps of corresponding sizes. Small gaps among perennial plants have greater rooting biomass and thus less potential for cheatgrass to dominate a site should disturbance occur. Quantile regression may provide a method of measuring resilience of the community to invasive plant dominance.