Effects of conifer treatments on soil nutrient availability and plant composition in sagebrush steppe
Bates, J.D.and K.W. Davies. 2017.
Abstract
Piñon-juniper woodlands of the western United States have expanded 2 to 10-fold
since the late 1800's. Tree 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 evaluated 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), cut and leave (CUT), and cut and burn in winter (JAN) and spring (APR). Soil
inorganic N (NO3-, NH4 ), phosphorus (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). Following woodland cutting, the results of the various slash
treatments measured significant differences through time in the availability of inorganic N, P,
and K and vegetation composition. Peak nutrient availability tended to occur within the first
two years after treatment. The increases in N, P, and K were greatest in severely burned debris
and canopy zones of the SEP and APR treatments. Invasive annual grass cover was positively
correlated to soil inorganic N concentrations. Herbaceous composition at the FESCUE site was
generally resistant to annual grasses after juniper treatments and native plants dominating
post-treatment even in highly impacted debris and canopy zones of the SEP treatment. The
BLUEBUNCH site was less resistance and resilient, thus, exotic annual grasses were a major
component of the understory especially when tree and slash burning was of high fire severity.
3
To lessen these impacts requires slash burning be applied from late fall to early spring, when
fuel moisture and relative humidity are higher, to maintain an adequate perennial herbaceous
composition for recovery.
Key Words
heatgrass; fuel reduction; nitrogen; phosphorus; prescribed fire; resin probes; western juniper
