Agriculture Reference
In-Depth Information
2.2.2 Neotropical savannas
The largest neotropical savannas are in the Llanos of Venezuela and Colombia and the
Cerrados of central Brazil. Fortunately the latter have been extensively studied over
the last decades, together with their adjacent seasonally dry tropical forests (SDTF).
This section will thus concentrate on Brazil and much of the information is contained
in a volume edited by Pennington et al. (2006). Both the Cerrados and SDTF are being
extensively converted to agricultural use, including production of soybeans, grasslands
for cattle and, more recently sugar cane for bioethanol production. The Cerrado biome
alone covers 2 million km
2
of central Brazil and is ranked as one of the 25 global hot
spots for biodiversity (Myers et al., 2000). The soils are dystrophic, like some of the
older soils of Africa, being acid, freely draining with high levels of aluminum and low
levels of available magnesium and calcium. The vegetation varies from grassland with
a sparse covering of trees, to almost enclosed woodland (Plate 2.6). Fire is frequent and
much of the woody flora is well adapted to this, for example, by producing xylopodia
from which new shoots are produced after fire (Plate 2.7). The woody vegetation has
been most extensively studied and in all areas the legumes come out highest in terms
of number of species, many of which are endemic (Ratter et al., 2006). Of these,
Mimosa
is the largest genus and appears to be uniformly nodulated (Elliott et al., 2007a; E.K.
James et al., unpublisheddata). However, the lower shrubs and herbaceous angiosperm
flora also have a high proportion of legumes, with
Chamaecrista
being among the most
numerous and also uniformly nodulated (Sprent et al., 1996). Brazil is a centre of
diversity for both these large genera. The endophytes of
Chamaecrista
in Brazil have not
been studied, but those of
Mimosa
in Brazil are almost exclusively
-rhizobia (James
et al., unpublished data; see also Chapter 4). All the available evidence suggests that
nodules are effective. Trying to estimate how much nitrogen is fixed in the field is
fraught with difficulties, with the
15
N methods being the only option. In woody
ecosystems, this has major problems (Boddey et al., 2000), but even so, Sprent et al.
(1996) calculated that
Chamaecrista
nodules and those of various herbaceous legumes
fixed significant amounts of nitrogen. This was supported by work on
Mimosa
nodules
from the field that were shown to have functional nitrogenase protein, using immuno-
labelling for the
nifH
protein (E.K. James, personal communication). Thus the situation
contrasts with that in African savannas, where nitrogen fixation appears to be more
variable. A reason for this may be that Brazil has one of the moister savanna regions,
with rainfall of 800 to 2000 mm. Although there is a pronounced dry season, drought
is less of a problem than in many African savannas. It is also noteworthy that there are
relatively few non-nodulated legumes found in the Cerrados.
2.2.3 Australian savannas
It has been estimated that nearly a quarter of the entire Australian continent consists
of tropical savannas, varying greatly in species composition, geology and climate (Fox
et al., 2001). They may have a very high summer rainfall, followed by extreme drought
conditions. The woody nitrogen-fixing components of many of them are
Acacia
spp.
(sub-genus
Phyllodineae
), often with herbaceous legumes in the wet season. However,
in some areas the role of legumes has been usurped by cycads. In Western Australia,
