Agriculture Reference
In-Depth Information
increase, trends are not clear and will depend on
the local conditions. Optimal temperatures for
growth in C4 plants are generally higher than
optimal temperatures for C3 plants, but with
higher CO 2 , the optimum temperature of many
C3 plants also increases (Bunce and Ziska 2000 ).
However, looking at photosynthesis and tem-
perature alone might be insuffi cient. The barn-
yard grass ( Echinochloa crus-galli ) in
combination with a mycorrhiza also benefi ts
from elevated CO 2 levels. In drought situations,
C4 weeds might also have advantages over C3
crops under elevated CO 2 .
by one research team to be +31 % in wheat, +30 %
in barley, +27 % in rice, +39 % in soybean, +57 %
in alfalfa, and +84 % in cotton. In contrast, a survey
of experimental results on 27 non-crop C3 species
revealed that biomass accumulation increased
from 79 to 272 % compared to ambient CO 2
(Patterson 1995 ). An experiment, which investi-
gated seven C3 crop and three C3 weeds at 350
and 700 ppm CO 2 , showed similar growth rates
and mass of C3 crops and C3 weeds (Bunce 1997 ).
10.3.4
C4 Weeds in C4 Crops
Since all C4 plants (weeds and crops) have the
same photosynthesis path, they may react to
changes in the same ecosystem in a similar way.
However, research on impact of climate change
in this combination has not been done.
10.3.2
C3 Weeds in C4 Crops
The benefi t of elevated CO 2 under suffi cient water
condition will lead to higher C3 weed competitive-
ness in C4 crops. An experiment with sorghum and
a C3 and C4 weed showed what the potential impli-
cations increased CO 2 level may have on the crops.
Under ambient CO 2 , the presence of the C3 weed
velvetleaf ( Abutilon theophrasti ) had no signifi cant
effect on either sorghum seed yield or total aboveg-
round biomass; however, at elevated CO 2 , yield and
biomass losses were signifi cant. The additional
loss in sorghum yield and biomass was associated
with a threefold increase in velvetleaf biomass in
response to increasing CO 2 (Ziska 2003 ).
Elevated CO 2 alone might not only lead to an
increase of pure biomass of C3 weeds. The
dandelion ( Taraxacum offi cinale ) produced more
fertile seeds and eventually larger seedlings.
However, C4 crops might outcompete better
growing C3 weed in drought situations and at
higher temperatures utilizing mycorrhiza.
10.4
Elevated Temperatures
Higher temperatures can possibly offset some of
the benefi ts of elevated CO 2 for both weeds and
crops. High temperatures sometimes limit repro-
ductive development and global warming may
decrease reproductive output in such situations
despite an increase in CO 2 . It is unclear whether
this is more likely to occur in C3 than C4 species,
but if it were, it could alter weed community
compositions and affect crop-weed interactions
(Bunce and Ziska 2000 ).
In temperate regions, global warming will
affect the growth and marginally affect phenol-
ogy and infl uence the geographical distribution
of weeds. Weed species of tropical and subtropi-
cal origins, currently restricted to the southern
regions, may expand northward (Patterson 1995 ).
Warmer seasonal temperatures and milder win-
ters will extend the distribution of invasive weeds
(Kudzu and Ragweed).
Increasing temperatures may mean an expan-
sion of weeds into higher latitudes or higher alti-
tudes. Very aggressive weeds that are currently
found in the south are limited in the northern
states by low temperatures. Many C4 grass weeds
are serious problems in the southern USA but do
10.3.3
C3 Weeds in C3 Crops
Logic would imply that the same type of plants
(with regards to photosynthesis) in the same eco-
system would react to changes in a similar way.
This is only partly true, while C3 crops and C3
weeds both benefi t from elevated CO 2 , it seems
that the magnitude varies. Stimulation of biomass
accumulation from CO 2 doubling was estimated
 
Search WWH ::




Custom Search