Agriculture Reference
In-Depth Information
with a high pH also impose their own constraints. However there are some species,
for example the mimosoid
Leucaena leucocephala
, that grow well in these conditions
(Plate 2.4).
There are some herbaceous annual legumes such as species of
Kennedia
(tribe Phase-
oleae) that germinate, nodulate and grow rapidly after rain events in desert areas.
Other, perennial herbs may grow the wet season and store water and nutrients in sub-
terranean tubers in the dry season. Several species of
Vigna
do this, mainly in Africa
and Asia, with other genera also occurring there and in the New World (Anon, 1979;
see also Chapter 6 of the present topic). Tubers of these species rival legume seeds in
nutrient quality, but most of themare only used at a local scale and have not been devel-
oped for agriculture. Possibly the largest tubers are produced by
Tylosema esculentum
,
which also produces edible seed, both parts being used by bushmen in the Kalahari.
Unfortunately this caesalpinioid species does not nodulate. There are a number of cae-
salpinioid trees that are very drought tolerant and of economic use, but which do not
nodulate. Examples are
Parkinsonia aculeata
and
Cordeauxia edulis
.
The Thar Desert of Western Rajastan in India supports a large number of legumes,
some perennial, some annual and some invasive species from outside the region.
A programme of research on this interesting system has recently been started and
is already producing significant new data (H.S. Gehlot, personal communication).
Thirty-three nodulated legumes, including some from each subfamily, have so far been
recorded. These include six species of
Indigofera
, seven of
Tephrosia
,twoof
Crotalaria
and one of
Rhynchosia
as well as an endemic
Mimosa
. Some of these genera have species
that are known to nodulate with
-rhizobia (Chapter 4). With
global change, such studies are likely to become increasingly important.
Large areas of northern Eurasia are deserts, coupled with very low temperatures.
Many of these are also at high altitude and will be considered in section 2.5.2.
-rhizobia and unusual
2.2 Savannas
Savannas will be consideredmainly on a continental basis as there aremajor differences
in the role of nodulated legumes in these areas. In geological terms they are relatively
recent, developing at the timewhen C
4
grasses first evolved, about 10million years ago.
Fires occur at varying intervals and are an important part of the functioning ecosystems.
Although Houlton et al. (2008) suggest that in phosphorus-limited tropical savannas
nodulated legumes can afford to use nitrogen to synthesise phosphorus-acquisition
systems, this may not always be the case, since nodules are not always effective in
some regions.
2.2.1 African savannas
Scholes and Walker (1993) give a map of Africa where almost half the continent has
some savanna, but designate 'core' savanna areas as those where over 80% of the
vegetation is savanna, the rest having 20% to 80% of savanna, with transition into
forests and arid shrublands. Even in the core areas, there is great variation, both in soil
