Effects of pretreatment tree dominance and conifer removal treatments on plant succession in sagebrush communities

Encroachment;Expansion;Infilling;Sagebrush;Fuel control;Resistance;Resilience;Pinyon;Juniper

Williams, Rachel E ; Matthews ; Jeffrey W.

关键词: Encroachment;    Expansion;    Infilling;    Sagebrush;    Fuel control;    Resistance;    Resilience;    Pinyon;    Juniper;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/92773/WILLIAMS-THESIS-2016.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

In sagebrush (Artemisia tridentata Nutt.) communities, the expansion and infilling of conifers decreases perennial vegetation cover and density, and lowers ecosystem resilience and resistance of the shrub-grass dominated state. Successional trajectories following disturbance are highly dependent upon residual species abundance, composition, and resulting structure. Understanding how tree dominance and tree-reduction treatments interact over time will help inform state-and-transition-models to guide management. Juniper (Juniperus spp. L.) and pinyon (Pinus spp. L.) trees were reduced by prescribed fire and cutting at 10 sites across the western United State. Vegetation cover and density were measured on untreated and treated plots across a gradient of tree dominance index (TDI, defined as tree cover / (tree + shrub + tall grass cover)) 3 and 6 years after treatment. I analyzed responses by functional group using mixed model analysis of covariance, with TDI treated as a covariate. As tree cover increased and TDI approached 0.5 (22% tree cover), shrub cover declined to 25% of the maximum. Three years after treatments, prescribed fire reduced both shrub and perennial herbaceous cover. Although total shrub cover returned to pre-burn percentages 6 years after treatment, it was still much lower than on the unencroached reference state and sagebrush cover was still < 1%. Six years after cut treatments, total shrub cover increased by 7% and sagebrush cover increased by 2.2% compared to no treatment. Tall perennial grasses are especially important in resisting dominance by invasive species such as cheatgrass (Bromus tectorum L). By 6 years after treatment, tall grass and cheatgrass cover both increased on prescribed fire and cut treatments, especially at higher pretreatment TDI. However, ratios of cheatgrass to tall grass cover were much lower on cut than on burn plots. This outcome suggests that system resistance to cheatgrass dominance is best supported by tree cutting. To retain the shrub, and especially sagebrush, components on a site and increase ecosystem resilience and resistance through increases in tall grasses, I recommend treating at low to mid TDI and using mechanical methods, such as cutting or mastication. Differential effects of prescribed fire compared to mechanical tree reduction, when implemented at different phases of tree dominance, should be incorporated into state-and-transition-models to clarify transitional effects and state outcomes.