Agriculture Reference
In-Depth Information
vegetation is evergreen or deciduous shrubland with a high degree of endemism.
Nodulated legumes are prominent. Legumes in the Mediterranean Basin, particularly
the drier areas, are well researched and will not be considered in detail here. Readers
should consult the publications of ICARDA (the International Centre for Agricultural
Research in the Dry Areas) with headquarters in Aleppo, Syria, and the outputs from
Fabamed, based in France (www.montpelier.inra.fr), which holds regular meetings
with publications. All of the regions listed in Table 2.4 have had their original extent
reduced greatly, mainly due to agriculture; all have protected areas and it is possible
to get an idea of the significance of legumes from these.
Southwest of Western Australia
The Southwest Australian Floristic Region (SWAFR), together with the Cape Floristic
Region of South Africa, are different from other Mediterranean ecosystems in that they
are on old, highly weathered, nutrient-poor soils, which have been subject to oceanic
influence since the Jurassic period. The plant biodiversity of these two regions has
been well described by Hopper and Gioa (2004) and Goldblatt and Manning (2002).
Lambers et al. (2007) have put forward a scheme to showhowplant nutrient-acquisition
strategies changewith soil age. Althoughmainly concernedwith phosphorus, they also
show that nitrogen content of soils is low in very young soils, for example after volcanic
eruptions, and in very old, highly weathered soils, peaking in soils of intermediate
age. Thus it might be expected that SWAFR has plants with a diversity of nitrogen
as well as phosphorus acquisition strategies. Most people know about the unique
family Proteaceae that occurs in both SWAFR and the Cape flora. Recently the legume
flora has been extensively studied from a taxonomic point of view, but unfortunately
there are few studies on nodulation and nitrogen fixation. Within the SWAFR, the
Leguminosae is second only in terms of number of species to the Proteaceae, but
perhaps even more significant is that only mimosoids and papilionoids are present
and all appear to have the ability to nodulate (Plate 2.9). That they do so on soils with
very low phosphorus levels goes against the generally perceived wisdom that nitrogen
fixation has an inherently high demand for phosphorus. Indeed some legumes (and
other plants) from this region cannot tolerate levels of phosphorus that are found in less
nutrient-poor soils and Lambers et al. (2007) suggest that these plants may have lost the
ability to down-regulate their phosphate uptake systems so that toxic quantities can be
taken up. In a detailed global analysis, Houlton et al. (2008) found that the rhizospheres
of nodulated plants (legumes and actinorhizas) have high levels of phophatases and
suggest that this enables them to access soil phosphorus.
Plants, including legumes in the SWAFR, have many sclerophyll characteristics and
are generally drought resistant. Indeed the endemic Australian tribes Mirbelieae and
Bossiaeae have recently (3-4 Ma) radiated into arid areas (Crisp et al., 2004; see also
Chapter 3). The ability to nodulate and fix nitrogen may be one of a number of char-
acteristics that enable legumes to survive in these rather hostile environments. Many
of them produce highly toxic compounds, such as fluoroacetate, to deter herbivores,
as well as having phenol-rich, poorly digestible foliage. It has been argued that their
rapid accumulation of nutrient-poor biomass, coupled with the low rates of herbivory,
provides fuel for the intense fires that rage through the area at intervals (Orians &
