Agriculture Reference
In-Depth Information
contrast, corn grain yield has steadily declined in the monoculture treatment over
time (
R
2
= 0.56). A severe drought in 2007 reduced yields in all treatments, but
yields in the two highest diversity systems rebounded to predrought levels the next
year (Fig. 7.7). This suggests that more diverse cropping systems (four to six spe-
cies) may be more resilient (
sensu
Scheffer et al. 2001) to drought (and presumably
other environmental perturbations) than continuous monocultures.
What causes these differences in overall yield and in capacity to recover from
stress? That remains to be determined. Smith et al. (2008) proposed that the higher
grain yield in corn, but not in other crops, may result from greater reliance on
spring soil N levels, which tend to be higher in the more diverse cropping systems.
Spring soil N levels and cropping system diversity are positively correlated, and
the strength of the relationship depends on the number of legumes (grain crops and
cover crops) in the rotation (Smith et al. 2008). Parker (2011) confirmed that soils
from the more diverse corn treatments had higher N-mineralization rates, but in
a greenhouse experiment detected no effect of N fertilizer on corn grown in soils
from these different treatments, suggesting that some factor other than N must be
responsible for reduced yields in less diverse cropping systems. Although disease
and/or pest buildup can be a major concern in continuous monocultures, to date
we have seen no evidence that pathogens and/or pests are higher in the less diverse
systems. Instead, it may be that changes in the diversity and/or composition of
the soil microbial community—and its ability to process carbon and nitrogen—are
important determinants of corn grain yield across these treatments.
Other Factors Affecting Diversity and Productivity of
Agricultural Landscapes
Landscape Structure and Community Composition
Landscape structure, past land use, and management history are increasingly rec-
ognized as important drivers of local species diversity that affect successional tra-
jectories (e.g., Myster and Pickett 1993, Foster and Gross 1999), the restoration of
native ecosystems (Suding et al. 2004, Gross and Emery 2007), and weed composi-
tion in crop fields (Poggio et al. 2010). Overcoming seed limitation may be as or
more important than reestablishing natural disturbance regimes for the successful
restoration of a native plant community (Suding et al. 2004, Suding and Gross
2006b, Houseman and Gross 2006, 2011). Intentionally seeding restoration areas
with native species may be necessary to overcome their dispersal limitations and to
increase the ratio of native to nonnative plants in these communities (Suding and
Gross 2006b).
Past land use, the absence of fire, and changes in surrounding landscape diver-
sity have all been shown to influence the composition and diversity of restored
and successional grasslands in the U.S. Midwest. Although seed addition and fire
are often used in grassland restoration (Leach and Givnish 1996), experimental
studies in degraded grasslands near KBS found that neither fire nor seed addition
alone increased native species richness. In some sites, fire increased the number
