Agriculture Reference
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level may stimulate photosynthesis in some
weeds, leading to higher growth of rhizomes and
other storage organs in perennial weeds, higher
seed production in annual weeds, etc., and make
their control diffi cult concurrently or over the
seasons/years. Thus, an appropriate technology
towards controlling increasingly more diffi cult
perennial weeds will be the important paradigm
in future weed research. Climate change is likely
to trigger differential growth in crop and weeds
and may have more implications on weed con-
trol across crops and cropping systems. Some
emerging areas of weed research relevant under
climate change scenarios are discussed in this
chapter.
Climate change may affect invasive plants
through:
• Increased disturbance due to fi re, fl oods, and
other extreme climatic events
• Potential range shifts (e.g., movement towards
cooler latitudes or higher elevations)
• Higher temperatures and reductions in frost
events
• Changes in rainfall timing, frequency, and levels
(including humidity and evapotranspiration)
• Reduced stream and river fl ows (exposing
well-watered riparian areas)
• Changes in coastal and estuarine habitat due
to rising sea levels
• Increased carbon dioxide fertilization (and
resultant increases in weed growth)
• Changes in pathogen pressures, including
serious impacts on biological control
programs
• Changes to fl owering and fruiting times
• Changes to species interactions (e.g., between
plants and pollinators, weed vectors, etc.)
Climate change is expected to increase the
range or “damage niche” (also called “invasion
niche”) of many weed species. Research suggests
that the composition of invasive weed communi-
ties will be fundamentally altered by the end
of the century under increasing temperature
scenarios, with new weed species entering com-
munities as a result of geographic range shifts
(McDonald et al. 2009 ). For example, the range
of the yellow star thistle, a California weed, is
expected to increase to more northern parts of
California and Nevada due to climate change
(Bradley et al. 2009 ).
The effects of climate change on weed-plant
interactions are likely to vary by region and crop
type. Understanding of the underlying physiolog-
ical mechanism responses to such factors is
needed in order to address these effects. Because
the interactions between crops and weeds are
“balanced” by various environmental factors,
local changes in these factors may tip the scale
towards either crop or weed. Furthermore, as the
geographic distribution of weed species changes,
so will the community composition, posing both
challenges and opportunities for invasion control.
If the invasion of new weed species can be
detected, efforts can be made in advance to pre-
vent and control their establishment.
Changes in temperature and carbon dioxide
are likely to have signifi cant direct (CO 2 stimula-
tion of weed growth) and indirect effects (climatic
variability) on weed biology. In spite of the
importance of weed biology in both the environ-
ment and in farms, very little is known regarding
the impact of these environmental changes on
either the reproductive success of agronomic or
invasive weeds or the potential consequences for
their management. Yet, given what is known, it is
clear that the agricultural, environmental, and
health costs of not understanding the impact of
CO 2 on weed biology may be substantial. It is
hoped therefore that the current chapter may
serve to both emphasize the critical nature of this
topic and to serve as an initial guide to those who
wish to recognize the ramifi cations of rising CO 2
beyond the polemic of global warming.
10.3
CO 2 Enrichment
Higher CO 2 will stimulate photosynthesis and
growth in C3 weeds and C3 crops and reduce
transpiration and increase water-use effi ciency in
both C3 and C4 weeds and crops (Fig. 10.1 ). Due
to their different types of photosynthesis, C4
and C3 plants react very differently to elevated
atmospheric CO 2 . Basically elevated CO 2 does
not directly stimulate C4 photosynthesis and
growth. Nonetheless, drought stress can be
 
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