Agriculture Reference
In-Depth Information
Chapter 2
Global Distribution of Legumes
Legumes are found in all areas of the world except for open seas; in other words,
wherever other angiosperm families are found. However, the proportion of legumes
versus other families varies greatly. There are also large variations in the proportions
of legumes that are nodulated or not in a particular environment and, where they are
nodulated, the efficiency with which their nodules fix nitrogen. What are the prop-
erties of legumes generally that enable them to colonise particular environments so
successfully, and why, in some environments, is the added benefit of nitrogen fixation
an advantage? This chapter will discuss these questions for the major geographical
regions of the world, in the general context of the four biomes discussed in Schrire
et al. (2005), but with some deviations. These biomes are outlined in Table 2.1. Many
areas have intermediate features; for example, the Cerrado region of Brazil has both
seasonally dry tropical forest (SDTF) and savanna characteristics. Some areas, such
as the Miombo woodlands of Africa, are not included, as they have been covered
elsewhere (Hogberg & Alexander, 1995; see also Sprent, 2001). Almost every topic on
tropical ecology defines savannas in a slightly different way. In this chapter particular
examples will be given, including soil and climate features where these appear in part
to define the functional groups of flora present. Because of current concerns, a section
on invasive legumes will also be included. The biogeographical sections start with
deserts and proceed via increasing moisture to rain forests, with the temperate areas
at the end. As there are major problems in estimation of nitrogen fixation at the field
scale in natural ecosystems, there is no systematic attempt to do this here.
2.1 Deserts
Althoughnodulated legumes are known to grow indeserts, there has beennopublished
systematic study of their ability to nodulate and fix nitrogen there. One of the most
drought and saline tolerant genera is Prosopis (Mimosoideae)
Several species are native
to Chile and can be grown in extremely dry and saline conditions. They may absorb
water from mist through their leaves and some can bring up water from deep in the
.
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