Agriculture Reference
In-Depth Information
Howe 2000) and grazing intensity and the type of grazer (Collins et al. 1998,
Knapp et al. 1999, Burns et al. 2009) have all been shown to affect the diversity,
composition, and productivity in prairies. Reduction in fire frequency has been
linked to the conversion of grasslands to woodlands and the loss of native species
(Anderson and Bowles 1999, Packard and Mutel 1997, McPherson 1997). As
a result, fire and the reintroduction of grazing are important management tools
for restoring and maintaining native diversity in grasslands (Suding and Gross
2006a, b, Martin and Wilsey 2006).
In cropping systems, tillage and herbicide applications are disturbances that,
like fire and grazing, affect not only the composition and diversity of existing weed
communities but also those of the subsequent emergent weed community (“emer-
gent” refers to a germinated and established weed in a crop field, as opposed to the
potential weed community in the seed bank; Johnson et al. 2009, Hilgenfeld et al.
2004, also Mortensen et al. 2012). While such management changes can alter the
composition and diversity of weed communities (see Smith and Gross 2006, 2007;
Smith et al. 2010), growers are generally less interested in how management affects
diversity and more interested in the effect on crop yield. Nevertheless, to manage
for ecosystem services from agriculture, we need a better understanding of how the
disturbance from agronomic practices affects the diversity and productivity of the
overall plant communities—weeds as well as crops—in agricultural landscapes.
Effects of Disturbance on Weed Communities
The MCSE annual cropping systems provide the opportunity to compare the impact
of tillage and herbicides on weed community structure under four different man-
agement regimes (Smith and Gross 2007). However, in these four systems, it is
difficult to distinguish the effect of tillage alone because herbicides and fertilizer
are also included in the management (see Table 7.1). The annually tilled plots
in the Disturbance by N-Fertilization Experiment and the Biodiversity Gradient
Experiment (Table 7.2) thus serve as reference communities to examine the effects
of tillage alone (Smith and Gross 2007), or of tillage plus N fertilizer, on weed com-
munities (Grman et al. 2010), and to relate long-term changes in species composi-
tion and dominance not only to annual disturbance (tillage), but also to longer-term
drivers such as climate change (Robinson 2011, Cleland et al. 2013, Dickson and
Gross 2013).
Despite major differences in management—including chemical inputs and till-
age (Table 7.1)—differences in weed biomass and composition among the four
annual cropping systems of the MCSE have been relatively small (Davis et al.
2005). This suggests that disturbance, whether created by tillage or herbicide, has
similar effects on the emergent weed community. Ordinations of aboveground
weed biomass and composition over the first 13 years of the study (1990-2002) did
not show a strong association with management, although overall weed biomass
was lower in the Conventional and No-till systems than in the Reduced Input and
Biologically Based systems (Davis et al. 2005). There is, however, considerable
interannual variation in weed biomass (Table 7.3) and weed species composition
(Fig. 7.1) in the four annual cropping systems. This may reflect differences in what
