Environmental Engineering Reference
In-Depth Information
heavy browsing of woody phytomass and toppling of trees help keep savannas open,
is a key reason for the regrowth of shrubs and trees.
Permanent cropping replaces natural vegetation with domesticated species and
creates new agroecosystems that are nearly always much less diverse in comparison
with the original communities. The contrast is obviously greatest where the domi-
nant cultivation takes the form of monocultures planted over vast areas. Crop
rotations offer a more rational management approach, but even they will under-
perform compared with their natural predecessors. Agriculture's effects on the
standing phytomass and on primary productivity i t a clear pattern of overall loss
in all instances where mature and rich natural ecosystems were replaced with a
low-yielding annual crop in a climate whose short vegetation period makes only
one harvest possible.
Converting a patch of a boreal forest to a wheat i eld, a transformation common
in medieval Europe or nineteenth-century Canada, replaced an ecosystem storing
several hundred tons of aboveground phytomass per hectare with an agroeco-
system whose peak preharvest aboveground phytomass was at best 3 t/ha. Bondeau
et al. (2007) modeled this effect globally and found that global vegetation has been
reduced by about 24% as a result of agriculture, and that the annual phytomass
harvest was reduced by 6-9 Gt C during the 1990s. They also displayed their i nd-
ings in a map grading the phytomass storage and net primary productivity (NPP)
difference between the existing farmland and previous natural vegetation.
For most of the world's cultivated land the difference between carbon stocks in
cultivated plants and in the vegetation of natural ecosystems that used to occupy
their place is more than 2 kg/m
2
(200 kg C/ha). Moreover, and somewhat counter-
intuitively, the difference is greatest (more than 1 t C/ha) in some of the world's
most productive agricultural regions, including the Corn Belt in the United States
and parts of Europe, Russia, and China, as even high wheat, corn, and rice yields
remain far below the levels of carbon stored in mature forests or rich grasslands
that were converted to i elds. The only area where agriculture accumulates more
carbon than natural vegetation would have done in the same places is where irriga-
tion boosts productivity in arid regions, such as Egypt or Pakistan and northern
India: there the crops will store more than the short-grass communities they replaced.
On a small scale, such gains are possible even in areas where the losses are domi-
nant. Converting temperate grassland into a crop i eld producing an annual harvest
of corn with alfalfa as a winter crop can result in an overall phytomass and pro-
ductivity gain. The NPP of a good European meadow may be 10-15 t/ha, while a
modern corn crop will yield 8-10 t/ha of grain and an identical mass in stover, and
