Abstract:
Cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea
soistitialis L.) have invaded over 3 million ha of grasslands in the western U.S.
Future management will require an understanding of the conditions,
mechanisms, and processes which control species dominance, community
dynamics, and desirable plant establishment in these alien dominated annual
rangelands. Life-history models of cheatgrass and yellow starthistle were
developed. Sensitivity analysis indicated that cheatgrass and yellow starthistle
seed output was most sensitive to population reductions during the transition
from juvenile to adult and adult survivorship phase of their life history. Key
processes associated with these transition phases are interference (competition),
growth rates and duration, and reproductive allocation. The objective of this
research was to investigate those key processes.
Several addition series experiments, with total densities ranging from 20-
20000 plants m⁻², were conducted to quantify the intensity of interference
between cheatgrass and yellow starthistle. In unrestricted soil depths,
intraspecific interference was approximately twice as important as interspecific
interference for both species with respect to predicting plant weight throughout
the growing season and seed production. Resource partitioning via rooting
depth was evident. The ecological importance of strong intraspecific
interference was associated with being strong competitors, the advantages of
self-thinning, and regulating community composition. Decreasing soil depth
altered the competitive balance toward the relatively shallow and fibrous rooted
cheatgrass.
In a growth chamber study, isolated seedling growth rates of both species
were similar, however yellow starthistle roots grew more geotropically than
those of cheatgrass. In field experiments, yellow starthistle grew more rapidly
than cheatgrass after the seedling stage. We believe that rapid and geotropic
growth allowed deeper soil penetration, continued growth and increased seed
output of yellow starthistle over that of cheatgrass. As densities increased
and/or soil depth decreased, the growth rates and maturation dates of
cheatgrass and yellow starthistle became increasingly similar and lower.
