Effects of precipitation change on population dynamics of
Bromus tectorum
Janet S. Prevéy and Timothy R. Seastedt, University of Colorado at Boulder, prevey@colorado.edu
Introduction
Population dynamics
Abundance and Biomass
Shifts in precipitation patterns resulting from global climate change are expected to
affect composition and ecosystem function of plant communities. In the Front Range
of Colorado, Bromus tectorum (cheatgrass) and other non-native winter annuals are
invading native grasslands. As the climate warms, yearly precipitation may shift
to a more winter-wet pattern and benefit early-growing winter annuals, such as
cheatgrass, to the detriment of native species.
Wet May
Dry May
Figs 4 and 5. In early spring, B. tectorum was more abundant in winter wet treatments, and
least abundant in the winter-dry treatment. Ambient precipitation and biomass differed greatly
between 2011 and 2012. In 2012, biomass was greatest in winter wet treatments.
Research Question:
How will cheatgrass populations respond to precipitation change?
Fig 1. Life-cycle diagram for B. tectorum. Probabilities of surviving from one life stage to
another were used o calculate the rate of population growth of cheatgrass in the different
treatments.
a
Experimental Design
ab
b
Treatment
Winter precipitation
(Oct – March)
Summer precipitation
(April – September)
control
ambient
ambient
winter wet
50% increase
ambient
↑
B. tectorum
winter wet / summer wet
50% increase
50% increase
↑
B. tectorum
winter wet / summer dry
50% increase
50% decrease
winter dry / summer wet
(historical)
50% decrease
50% increase
ab
ab
Hypotheses
↑
↓
6/23/2011
B. tectorum
B. tectorum
Four precipitation manipulations were established in summer 2010, and population
dynamics of B. tectorum were measured for the following two years.
Fig 2. In 2012, population
growth rates were highest for
B. tectorum growing in the
winter-wet treatment, and
negative (indicating
population decline) in the
winter-dry / summer wet
treatment.
Fig 3. Bromus tectorum was very sensitive to
changes in precipitation timing. Cheatgrass flowered
and senesced earlier in the winter wet treatment (a),
and later in the control (b) and winter dry treatments.
Winter wet / summer dry
Increased winter precipitation caused earlier green-up and senescence of B. tectorum.
Reduced summer precipitation suppressed abundance and production of later-growing
native species.
Conclusions
• Wetter winters increase population growth and abundance of B. tectorum.
(a) Winter wet / summer dry
• Population growth of B. tectorum declined in the winter dry / summer wet
treatment that simulated historical conditions, suggesting that changing
precipitation patterns may already be assisting the expansion of winter annuals in
Front Range ecosystems.
• Increased winter precipitation and reduced summer precipitation caused earlier
green-up and senescence of B. tectorum. This shift in phenology could result in
earlier brown-down of grasslands, and less primary production in the summer.
The field site is located in a grassland in the foothills near Boulder, CO.
Manipulations were created with rainout-shelters and water additions.
(b) Control
Photos taken on 6/22/2011
• Cheatgrass may become more invasive in grasslands along the Front Range
as the climate changes.