Agriculture Reference
In-Depth Information
Legumes occur at high altitude in other areas, of which the Andes have received the
most attention. Here lupins are common (Plate 2.13). These mountains are the most
species-rich of any tropical mountains, and the species there must have colonised the
area over the last 2 to 4 Ma. Hughes and Eastwood (2006) found a monophyletic group
of 81 species of Lupinus , which they estimated to have arisen over the last 1.18 to 1.76
Ma, thus given them the record for the fastest rate of evolution yet found in plants.
Geographic isolation was a major contributory factor to this rapid speciation. Physio-
logically, how nodules, whose operation depends on correct gas pressure (Chapter 5),
operate at such low pressures is completely unknown. Indeed there is little knowledge
of how any plant processes are affected by altitude. High-altitude legumes are usually
nodulated papilionoids, a fewAndean species of Senna , a non-nodulating genus, being
the exception (C.E. Hughes, personal communication).
2.6 Invasive legumes
Invasive plants and animals are receiving a great deal of attention, as the extent of their
occurrence and effects on the environment become clearer. Among plants, there are
numerous cases of invasive legumes. Some such as Ulex europeaus (gorse) and Cytisus
scoparius (broom) were introduced into Australia and New Zealand by early settlers
and have since been classified as noxious weeds. Broom has also invaded large areas
of the Pacific Northwest of the United States, where it acidifies the soil and affects
nutrient cycling (Caldwell, 2006). Gorse is also a major problem in volcanic soils of
Hawaii (Leary et al., 2006b). Both these shrubby legumes produce large quantities
of seed, making eradication difficult. They also appear to nodulate freely wherever
they occur. In wetter areas of Hawaii, the large, fast-growing nodulated tree, Falcataria
moluccana , has transformed the Metrosideros forest so much that it is suggested that it
be considered a completely different ecosystem (Hughes & Uowolo, 2006). A native
annual legume in Hawaii, Chamaecrista nictitans , by enriching the soil in nitrogen after
its death, is thought to facilitate invasion by alien grasses (Carino & Daehler, 2002).
Arguably one of the fastest-growing legumes known, Pueraria montana (Kudzu), was
introduced into the United States in 1876, used to control erosion, but has been so
successful that in 1997 it was classed as a noxious weed. Although it is known to be
efficient at fixing nitrogen, and appears to nodulate freely, very little else is known
as to its effects on native ecosystems (Forseth & Innis, 2004). Similarly invasive Mi-
mosa species may be successful because of their ability to nodulate with local strains
of Burkholderia (Chen et al., 2005). Lupinus nootkanensis , the Alaska lupin, has been
introduced into Iceland where it is variously regarded as a benefit (because of ability
to nodulate and to extract phosphorus form low-phosphorus soils) or as a threat to
natural ecosystems, because it is very invasive (Plate 2.14). Parker et al. (2006) showed
that, in some cases, the invasiveness of legumes (including Cytisus scoparius ) could be
limited by the availability of suitable rhizobial symbionts. In a detailed meta-analysis
of invasive plants, Liao et al. (2007) showed that ability to fix nitrogen had a major
effect on ecosystem carbon and nitrogen cycles. Australia has been exporting acacias
around the world for years and in some places, such as South Africa, they are becoming
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