Agriculture Reference
In-Depth Information
precipitation patterns directly (see Aanderud et al. 2011, Robinson 2011), but
long-term data on precipitation and productivity across the broad range of MCSE
plant communities provide insight into how the changes in precipitation pat-
terns predicted for this region may influence their productivity and diversity. For
example, in the untilled successional treatment of the Diversity by N-Fertilization
Experiment, which is dominated by perennial species, MAP is positively related
to ANPP in both unfertilized (
R
2
= 0.25,
p
< 0.025) and fertilized (
R
2
= 0.14,
p
<
0.05) plots. However, in the annually tilled plots where annual species dominate,
there is no relationship, regardless of fertilizer addition. Instead of MAP, one might
expect growing season precipitation to be a better predictor of ANPP in tilled com-
munities because their growth is strongly controlled by tillage, which is a seasonal
event. But it is not—there is no significant relationship between growing season
precipitation (i.e., April-September) and ANPP. Precipitation totals during specific
periods of the growing season prove to be better predictors of aboveground produc-
tivity than either annual and growing season totals (Robinson et al. 2013). It is not
surprising that the amount of precipitation during specific life stages (e.g., germi-
nation) is a key driver for annual communities. Analyses of long-term data and of
short-term manipulation experiments show that precipitation during the first weeks
of the growing season has long-lasting effects on annual community development
(Robinson 2011).
Only in the No-till system, which includes both annual and perennial weed spe-
cies, is weed biomass related to precipitation variation (growing season:
R
2
= 0.44;
annual:
R
2
= 0.14). The lack of a correlation between precipitation and weed bio-
mass in the Conventional, Reduced Input, and Biologically Based cropping systems
may result from differences in weed management (Table 7.1). All three include
tillage as part of their management, although the timing and frequency of tillage
events differ among them, whereas the No-till system relies only on herbicides for
weed control. This difference between systems suggests that management, particu-
larly the timing and implementation of weed control practices, affects the response
of weed communities to external drivers such as variability in the amount and sea-
sonal distribution of precipitation. The response, however, may be due more to
changes in the composition of the weed community than in its total biomass.
That no relationship exists between annual or growing season precipitation
and weed biomass in the row-crop systems or plant biomass in annually disturbed
successional plots stands in direct contrast to the strong relationship observed
in more water-limited systems (deserts and grasslands) (Noy-Meir 1973). In
more mesic systems such as KBS, it is likely that the timing and intensity of
precipitation events, as well as the intervals between them, impact productivity
(Robinson et al. 2013). Climate shifts that affect the timing of snowmelt and the
frequency and intensity of storms (Easterling et al. 2000, Weltzin et al. 2003,
IPCC 2007) will likely affect productivity, as well as composition, of annual
weed communities in agricultural systems. Annual communities may be particu-
larly responsive to the frequency and intensity of precipitation events, as this can
affect the timing and percentage of seed germination in annual species, which
can differ in their response to variability in precipitation (see Pake and Venable
1996, Robinson and Gross 2010, Robinson 2011). Because the germination of
