• Emergence date effects on resource partitioning between diffuse knapweed seedlings

      Sheley, R. L.; Larson, L. L. (Society for Range Management, 1996-05-01)
      Diffuse knapweed (Centaurea diffusa Lam.) has reduced forage production, watershed quality, and biodiversity, and increased soil erosion on millions of hectares of rangeland. Diffuse knapweed has evolved mechanisms that allow it to dominate sites in nearly monotypic stands. Understanding these mechanisms may provide useful information in developing weed management strategies. Objectives of this study were to investigate interference, growth rates, and resource partitioning between early and late emerging diffuse knapweed seedlings. Seeds of diffuse knapweed were planted 21 March (early emerging) and 14 April (late emerging) 1993 in addition series mixtures with total stand densities ranging from 1,000 7,000 plants m-2. Shoots were harvested on 1 and 2 June 1993. The greatest interference was among coemerging seedlings. Resource partitioning ratios (51 and 1398) indicated substantial partitioning between seedlings having different emergence dates. Continuous seedling emergence may allow diffuse knapweed to occupy all available safe sites.
    • Environmental effects on picloram uptake and ethylene production by broom snakeweed

      Sterling, T. M.; Lownds, N. K.; Murray, L. W. (Society for Range Management, 1996-05-01)
      Broom snakeweed [Gutierrezia sarothrae (Pursh) Britt. &Rusby] is a rangeland weed widely distributed in the western United States. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) uptake and picloram-induced ethylene production by broom snakeweed grown in the field were determined every 2 to 4 weeks over 36 months. For each collection date, picloram uptake and concentration in the tissue ranged from 1.5 to 46.2% of applied and 0.2 to 4.7 nmol g-1 fresh wt, respectively. Of the measured environmental variables, average precipitation and average minimum temperature 7 days prior to treatment best predicted picloram uptake and concentration in the tissue, suggesting that warmer temperatures and precipitation previous to application contribute to picloram uptake. Average minimum temperature alone also provided a good predictor for picloram concentration in the tissue. For each collection date, picloram-induced ethylene production by total tissue ranged from 50 to 791% of control. Picloram-induced ethylene production by total tissue was best predicted by the precipitation and minimum temperature 7 days prior to treatment and picloram concentration in the tissue. Therefore, the amount of picloram absorbed and the environment prior to application both contribute to the physiological sensitivity of broom snakeweed to picloram. Picloram uptake and picloram-induced ethylene production were greatest in July and August, when plants were in the phenological stages of shoot regreening or flower bud emergence and when temperatures and precipitation were high. Previous field studies have shown broom snakeweed is most responsive to field picloram application in the post-bloom stage from October to December or in April and May with high moisture and soil temperature conditions; therefore, it appears that changes in uptake and physiological sensitivity as measured by picloram-induced ethylene production are not the only factors controlling differential sensitivity to picloram.
    • Fringed sagebrush response to sward disturbances: Seedling dynamics and plant growth

      Bai, T.; Romo, J. T. (Society for Range Management, 1996-05-01)
      Fringed sagebrush (Artemisia frigida Willd.), the most common dicotyledonous species in the Northern Mixed Prairie, often increases dramatically following disturbance. It was hypothesized that the increase could be due to release of established plants, increased recruitment of plants, or both. Experiments were conducted on a sandy range site in central Saskatchewan. Tillage, clipping, litter removal, and a combination of clipping+litter removal were compared to an undisturbed control to determine their effects on emergence and survival of fringed sagebrush seedlings and growth of established plants. In no circumstance was seedling emergence or plant growth greater in the undisturbed control than in the disturbed sward. Emergence of fringed sagebrush seedlings increased almost 80-fold the second year after tillage at 1 site, but emergence was not altered relative to the control by clipping, litter removal, or clipping+litter removal Averaged across treatments, 52 to 98% of the seedlings emerged in May and June, and 47 to 99% of these seedlings survived through the growing season and winter. Plants grew fastest in June when precipitation was highest and temperatures were moderate. Growth of plants was improved 2- to 3-fold by tillage the second year; this stimulation in growth was due to the removal of competition. Activities that reduce or remove vegetation and create bare soil surfaces promote emergence and growth of fringed sagebrush on the Northern Great Plains. Most seedlings of fringed sagebrush emerge in spring and early summer, enabling them to temporally exploit the period for optimal growth. Fringed sagebrush is well adapted to persist in Northern Mixed Prairie in a successional continuum from early to late seral stages.