Precipitation and invasive winter annual grass data for the Great Plains

Aim: Climate change is predicted to increase spatial extent and
equilibrium abundance of many invasive species, and there is evidence this
may already be happening.  In North American grasslands, the most
concerning invaders are winter annuals.  Understanding winter
annual responses to climate change is challenging because these species
are regulated by weather during multiple seasons, unlike perennials that
are overwhelmingly regulated by growing season precipitation.  We
quantified downy brome (Bromus tectorum L. (ITIS)) and Japanese brome
(Bromus japonicus Thunb.) responses to precipitation and
temperature.  These functionally similar invasive winter annual
grasses are destroying wildlife habitat, reducing livestock production,
and increasing wildfire risks across vast portions of the western
U.S.  Location: Great Plains, U.S.A. Methods: Using Bayesian
methods to integrate experimental and long-term (30 yr) monitoring data,
we estimated the effects of precipitation and temperature on biomass
production of bromes and native species.  Results:
Unsurprisingly, brome biomass increased with the current growing season
(spring) precipitation.  Alternatively, brome biomass declined
with previous growing season precipitation, perhaps because previously wet
conditions strengthened perennial competitors of bromes.  These
positive and negative effects of growing season precipitation largely
cancelled out and left mean brome biomass unchanged.  This
suggests bromes will be insensitive to changes in growing season
precipitation.  Conversely, bromes proved highly sensitive to
fall precipitation.  Fall precipitation is necessary for
germination, and brome biomass in the current growing season increased
with precipitation the previous fall (p<0.0001), two falls ago
(p<0.001), and likely even three falls ago (p=0.09). 
 Conclusions: Fall precipitation is projected to increase in much
of the western U.S., and a mere 5% (3.0 mm) increase would cause an 11%
(7%, 14%) [mean(95% confidence interval)] increase in brome
biomass.  These results should urge increased focus on fall
weather to understand winter annual plant responses to climate
change.