Gregg M. Riegel

Competition for Resources Between Understory Vegetation and Overstory Pinus Ponderosa in Northeastern Oregon

The objective of this research was to determine which environmental resources, light, water, and/or nutrients, control understory plant production in a Pinus ponderosa forest in northeastern Oregon. A split-plot experimental design, with three 5.0-ha blocks, four treatments, and 44 plots, was established in the summer of 1985. Twenty plots (4 X 4 m) were trenched (root-reduction treatment) °1 m in depth, and 24 non-trenched plots (root-control treatment) were used to assess the effects of root competition of overstory trees on understory plants. Trees were commercially thinned (canopy-reduction treatment) in half of each block (2.5 ha) during the winter and early spring of 1986, from a density of 345 to 148 trees/ha to increase light levels to the understory. Thinning significantly increased photosynthetically active radiation, decreased midday relative humidity, and increased midday air temperatures. Xylem potential of the dominant graminoid (Carex geyeri), soil water potential, mineralizable nitrogen, and pH were increased within the root-reduction vs. the root-control treatments. Micro- and macronutrients in C. geyeri and Symphoricarpos albus, the dominant shrub, were influenced in both treatments. Increasing light did not increase understory biomass production. Reducing root competition for soil water and nutrients increased understory aboveground biomass by 53 and 94% in 1986 and 1987, respectively. This research demonstrated that belowground resources were the primary controlling factors of understory production in P. ponderosa forests in northeastern Oregon.

Spatial pattern of gas exchange for montane moist meadow species

. Montane moist meadows of western North America are relatively unique in that they are characterized by annual cycles of spring inundation followed by summer drought. Thus, plants adapted to these ecosystems must cope with a wide range of environmental conditions. We compared gas exchange and water relations of upper canopy leaves and leaf area index (LAI) of three dominant montane meadow graminoids growing at both streamside and mid-meadow locations (2.5 and 20.0 m, respectively from the stream edge). The two locations were environmentally very different in spite of their close proximity. The water table at the streamside location was ca. 40 cm closer to the surface compared to the mid-meadow. When averaged over season and species, photosynthetic rates were 12.5 % (p 0.05) The similarity in stomatal response was surprising given that streamside plants had midday xylem potentials that averaged 0.72 MPa less negative (less water-stressed) than that of mid-meadow plants (p < 0.05). There was no clear relationship between xylem potential and conductance for the species studied. Total LAI for the three species combined was nearly three times higher at the streamside compared to mid-meadow location. At the streamside location, Carex nebrascensis had a LAI over twice as high as that of the other species (p 0.05) at the mid-meadow location. Carex nebrascensis had a more conservative gas exchange strategy (lower photosynthesis and transpiration) than Juncus balticus or Deschampsia cespitosa, yet developed more leaf area than either of these species. Juncus had higher photosynthesis and transpiration compared to the other species, yet maintained less negative midday xylem potential (p < 0.05). Apparently, Juncus balticus maintained a greater supply of moisture to above-ground tissues than the other species, which is probably an indication of higher root length per unit leaf area, or a more efficient water conducting system. The other two species - Carex nebrascensis in particular, maintain relatively conservative gas exchange rates, but under favorable growing conditions (such as occurred at the streamside) can develop high leaf area indices. Wetland species with similar growth forms (graminoid) and distributions can have contrasting patterns of gas exchange and plant water relations.

Soil Attributes in a Sierra Nevada Riparian Meadow as Influenced by Grazing

Abstract Data on the effects of livestock grazing on soil nutrient availability are virtually nonexistent for meadow systems. We measured the effect of livestock grazing on soil, emphasizing soil-solution chemistry, in a Sierra Nevada riparian meadow. Treatments were livestock exclusion (begun in 1989) and grazing to leave 1 000 kg·ha−1 of vegetation. Ceramic tension lysimeters were placed in the treatments (2 replicates) by landscape position (stream edge, midfloodplain, and forest edge), and by depth (approximately 0.1, 0.6, and 1.2 m below the soil surface). Lysimeter water was extracted twice monthly in April, May, and June of 1990 through 1993, and cations and anions were quantified. In addition, KCl-extractable NO3− and NH4+; bicarbonate-extractable ortho-P; available Mn, Cu, Fe, and Zn; and root-length density (RLD) were quantified in soils by treatment, landscape position, and soil depth in July 1991 and September 1993. RLD was not affected by grazing. Significant (P ≤ 0.05) treatment effects were largely limited to the forest edge. The grazed treatment had greater lysimeter-extractable Na+, Ca+2, Mg+2, and NO3−; higher pH; and less K+ and NH4+ than the excluded treatment. Compared with corresponding excluded treatments, bicarbonate-extractable P was significantly greater on the grazed forest edge, and available Mn was significantly greater at the grazed stream-edge position in 1991. Extractable NO3− was significantly higher in the 0–25 cm depth increment of the grazed treatment, and available Zn was significantly greater on the grazed midfloodplain position in 1993. Grazing did not result in more anoxic soil conditions than the excluded treatment. Grazing effects were most pronounced at the forest edge, possibly as a result of spatial transfer of nutrients via cow urine and feces. Management goals to sustain high-elevation meadows should emphasize maintenance of high RLD to sequester soil nutrients.

Soil genesis and morphology of a montane meadow in the northern Sierra Nevada range

Given the importance of riparian areas in the western United States, knowledge about the spatial distribution, properties, and genesis of these soils is surprisingly limited. In conjunction with an interdisciplinary study of the impacts of grazing on soils and vegetation, we characterized three pedons along a hydrologic gradient on a montane meadow of the northern Sierra Nevada range. Radio-carbon dating of charcoal indicates that meadow pedogenesis began approximately 3600 years B.P., after a catastrophic valley erosional event. Since that time, nearly 1 meter of soil has accumulated over a basal glaciolacustrine unit. Critical factors and processes influencing soil genesis and morphology include : seasonal variation in soil redox status, frigid soil temperatures, additions of volcanic tephra, wildfires, and polygenesis related to Holocene climatic, hydrologic, and vegetation changes. Argillans are present on ped faces of certain soil horizons, which suggests extended dry periods at which time clay pervection occurred. Clay mineralogy is disjunct ; surface horizons are dominated by kaolinite and underlying horizons by smectite. The high clay content of such youthful soils suggests rapid primary mineral weathering. Charcoal-containing strata attest to frequent wildfires during the Holocene epoch. The spatial complexity of soil patterns and their properties infers that these riparian areas are dynamic, and their character may have been shaped by previous climatic patterns.