Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2017
Publication Date: 9/15/2017
Citation: Bates, J.D., Davies, K.W. 2017. Effects of conifer treatments on soil nutrient availability and plant composition in sagebrush steppe. Forest Ecology and Management. 400:631-644.
Interpretive Summary: Conifer woodland treatments are designed to recover sagebrush-grass plant communities for livestock forage and wildlife conservation and to reduce woody fuel loads to lower wildfire risk in the western USA. We evaluated fuel reduction, soil nutrient availability, and sagebrush grassland recovery after western juniper cutting followed by prescribed fire in early fall, winter and spring. Soil nitrogen and other nutrients increased after juniper control and burning slash and increase were greatest in severely burned slash and litter zones. Native perennials were dominant at the big sagebrush/Idaho fescue association and perennials and annual grass co-dominated the sagebrush/bluebunch wheatgrass association. Invasive annual cover was positively correlated to soil inorganic N concentration.
Technical Abstract: Piñon-juniper woodlands of the western United States have expanded 2 to 10-fold since the late 1800’s. Since the 1950’s woodland control measures using chainsaws, heavy equipment and prescribed fire have been used to reduce woodlands and restore big sagebrush steppe and decrease woody fuel loading. We compared nutrient availability and herbaceous recovery following various cutting and prescribed fire treatments in late succession western juniper woodlands on two sites in southeast Oregon from 2007 to 2012. Sites were a cool, wet big sagebrush-Idaho fescue association (FESCUE), highly resistant to exotic annual grasses and a warm dry big sagebrush-bluebunch wheatgrass association (BLUEBUNCH), moderately resistant to annual grass invasion. Treatments were untreated controls, partial cutting followed by fall broadcast burning (SEP), clear-cut and leave (CUT), and clear-cut and burn in winter (JAN) and spring (APR). Concentrations of soil inorganic N (NO3-, NH4+), phosphate (H2PO4-), potassium (K+), and cover of herbaceous species were measured in three zones; interspace, litter mats around the tree canopy (canopy), and beneath felled trees (debris). Inorganic soil N, P, and K increased in after juniper cutting, slash burning and broadcast burning, Following woodland cutting, the results of the various slash treatments measured significant differences thru time in the availability of inorganic N, P, and K and vegetation composition, though the greatest increases tended to occur within the first two years after treatment. The increases in N, P, and K were greatest in severely burned slash and litter zones of the SEP and APR treatments. Invasive annual grass cover was positively correlated to soil inorganic N concentrations and was greatest in severely burned canopy and debris zones of SEP and APR treatments. However, herbaceous composition at the FESCUE site was generally resistant to juniper treatments with native plants dominating post-treatment even in highly impacted debris and canopy zones of the SEP treatment. The BLUEBUNCH site is lower resistance and resilience and exotic annual grasses was a major component of the understory especially when tree and slash burning occurred at higher fire severities. To mitigate these impacts on these areas requires slash burning be applied from late fall to early spring, when fuel moisture and relative humidity are higher, to maintain an adequate perennial for recovery.