Plant Establishment and Soil Microenvironments in Utah Juniper Masticated Woodlands
Young, K. R.. 2012. Brigham Young University; PhD
Abstract
Juniper (Juniperus spp.) encroachment into sagebrush (Artemisia spp.) and bunchgrass
communities has reduced understory plant cover and allowed juniper trees to dominate millions
of hectares of semiarid rangelands. Trees are mechanically masticated or shreddedto decrease
wildfire potential and increase desirable understory plant cover. When trees are masticated after
a major increase in treepopulation density and associated decrease inperennial understory cover,
there is a risk that invasive annual grasses willdominate because they are highlyresponsive to
the increased resource availability that commonly follows removalof the main resource user. To
determine if tree mastication increases resource availability and subsequently favors invasive
annual or perennial grasses, we compared soil temperature, water, and nutrient
microenvironmental conditions and seedling establishment and growth. We used the major
rangeland weed, cheatgrass (Bromus tectorum L.), to represent invasive annual grasses and
Anatone bluebunch wheatgrass (Pseudoroegneria spicata(Pursh) A. Löve), a natural accession
of native bluebunch wheatgrass, to represent the perennial grasses of the sagebrush-bunchgrass
plant community. These comparisons were made betweenand within paired-adjacent masticated
and untreated areas at three locations in Utah dominated by Utah juniper (Juniperus osteosperma
(Torr.) Little). Juniper tree mastication generally increased resource availability with masticated
areas having greater soil temperature, soil water availability, and soil N supply rates than
untreated areas. Prior to juniper tree mastication litter mounds were not found to be resource
islands probably because juniper trees themselves were using subcanopy soil water and nutrients.
After juniper tree mastication and elimination of thesepredominant resource users, litter mounds
served as resource islands with greater soil water availabilityand N supply rates than bare
interspaces during the critical time for seedling establishmentin spring. Plant growth followed in
line with greater resource availability after tree mastication with masticated areas having more
productive although fewer invasive-annual and perennial grass seedlings than untreated areas.
These results suggest that increases in resource availability and warmer spring temperatures
associated with mastication will not necessarily favor invasive annual over perennial grass
seedling establishment. Resilience of the sagebrush-bunchgrass communityto return to
dominance after juniper control will likely be greatly influenced byhow much of the sagebrush-bunchgrass community remains following tree control and the intensity of propagule pressure by
invasive species. If only invasive annuals remain when the trees are treatedthen invasive annuals
would be expected to dominate the post-treatment plant community especially with their ability
to establish inside litter mounds unless they were also controlled and perennial grasses planted at
the time of treatment.
Key Words
Anatone bluebunch wheatgrass, carbon, cheatgrass, degree days, fertilizer, Great Basin, mulch, nitrogen, PRS®, rangeland restoration, sagebrush steppe, SageSTEP, seedling establishment, shred, wet days, wet degree days, woodland, woody debris
