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Plant Establishment and Soil Microenvironments in Utah Juniper Masticated Woodlands

Young, Kert R.. 2012. Brigham Young University; PhD

Abstract

Juniper (Juniperusspp.) encroachment into sagebrush (Artemisiaspp.) 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