Understory dynamics in cut and uncut western juniper woodlands

Expansion of western juniper (Juniperus occidentalis spp. occidentalis Hook.) woodlands in the sagebrush steppe has the potential to change composition, structure, and productivity of understory vegetation. Cutting of western juniper woodland can potentially restore understory productivity and diversity. Understory responses were assessed after cutting a juniper woodland in southeastern Oregon in 1991. The experimental design was a randomized complete block with eight, 0.8 ha sized blocks and 2 treatments, cut and uncut woodland. Understory cover, density, diversity, biomass, and nitrogen (N) status were compared between treatments after cutting. Plants were separated into 5 functional groups: bluegrass (Poa spp.), perennial bunchgrass, perennial forb, annual forb, and annual grass. Cutting of juniper reduced belowground interference for soil water and N. Leaf water potentials were less negative (P<0.01) and understory N concentration and biomass N were greater (P<0.05) in the cut versus woodland treatment. Cutting of juniper trees was effective in increasing total understory biomass, cover, and diversity. In the second year post-cutting total understory biomass and N uptake were nearly 9 times greater in cut versus woodland treatments. Perennial plant basal cover was 3 times greater and plant diversity was 1.6 times greater in the cut versus woodland treatments. In the cut, perennial bunchgrass density increased by 1 plant m(-2) in both duff and interspace zones and bluegrass increased by 3 plants m(-2) in interspaces. Plant succession was dominated by plants present on the site prior to juniper cutting suggesting that pre-treatment floristics may be useful in predicting early successional understory response. Early plant dynamics on this site supports the multiple entrance point model of succession as perennial grasses and bluegrass made up the majority of total herbaceous biomass and cover.