3.2 Environmental Adaptation of Forage Tree Legumes

H.M. Shelton

Introduction
Climatic Adaptation
Edaphic Adaptation
Conclusions
References

Introduction

Agriculturalists require forage tree legumes which are adapted to a wide range of environments. Species are needed which are suited to a variety of climates such as the hot humid environments of some Pacific island countries, the seasonally dry tropics, the cooler drier regions of the subtropics where frosts may occur, and the cool environments of the high altitude tropics. Similarly, adaptation to the great range of edaphic conditions that occur in each of these climatic environments is sought.

This section reviews our knowledge of the climatic and edaphic adaptation of the principal forage tree legumes and presents some new information from recent work by the University of Queensland and other research groups.

A tabular summary of the environmental tolerances of the principal tree legume species is given in Table 3.2.1.

Climatic Adaptation

Temperature

The majority of the forage tree legume species considered in this book are tropical in origin and therefore exhibit generally poor cool season growth with little frost tolerance. Nevertheless, tolerances of such stresses are important qualities in both the subtropics and high altitude tropics.

Of the main species considered (Table 3.2.1), tagasaste (Chamaecytisus palmensis), Faidherbia albida and Acacia aneura have moderate frost tolerance although a number of the lesser known species such as Leucaena retusa and Robinia species also exhibit frost tolerance (Table 3.2.2). Albizia lebbeck, Calliandra calothyrsus, Leucaena diversifolia and Sesbania sesban are known to have some cool season growth potential. Work of Swasdiphanich (1993) demonstrated the outstanding growth of S. sesban at mean temperatures of 18.5-23°C compared with L. leucocephala (Table 3.2.3). Unfortunately, field experience with S. sesban at other sites suggests that it is not tolerant of frost. Both L. diversifolia and L. pallida have shown outstanding cool season growth in southeast Queensland and at the high altitude site at Mealani (1,000 m) on the island of Hawaii. These species originate from high altitude sites in Mexico. Leucaena leucocephala accessions performed poorly at the Mealani site due to the low temperatures.

Table 3.2.1. Environmental and edaphic tolerances of some fodder tree legumes.

Table 3.2.2. Frost tolerance of three tree legume species measured in terms of dieback and survival 1 day and 1 month following treatment (Long 1989).

	% Dieback after 1 day				% Tree survival
Species	0°	-5°	-10°	-15°	0°	-5°	-10°	-15°
Leucaena leucocephala	9a	87b	100b	100b	100a	20b	0b	0a
Leucaena retusa	0a	26a	43a	81a	100a	100a	90a	0a
Robinia neomexicana	17a	17a	67a	100b	83a	83a	33c	0a

Different lower case letters denote values that are statistically different (P < 0.05)

Scope exists for selection of ecotypes within species for improved cold and frost tolerance. Kendall et al. (1989) compared the frost tolerance of three populations of L. leucocephala from northeastern Mexico and found greater tolerance among populations originating from higher altitudes (800-2,000 m). However, L. retusa is the only Leucaena species demonstrating significant frost tolerance.

Table 3.2.3. Effect of varying temperature regime on the dry weights of tree legume species expressed as a percentage of their maximum weights after 16 weeks of growth (Swasdiphanich 1993).

	Temperature regimes (day/night °C)
Species	20/17	25/21	30/27	34/32
	(% of maximum value within species)
Acacia villosa	1	4	61	100
Albizia chinensis	1	3	63	100
Calliandra calothyrsus	10	19	100	93
Gliricidia sepium	5	20	69	100
Leucaena diversifolia	2	22	76	100
Leucaena leucocephala	6	11	46	100
Sesbania grandiflora	4	13	75	100
Sesbania sesban	36	67	88	100
Stylosanthes scabra cv. Seca	1	9	100	83

Identification of new germplasm which has both cold and frost tolerance should receive priority in research programmes especially within highly nutritious species such as leucaena. The presently used cultivars of leucaena provide little cool season growth and shed their leaves, the most palatable and nutritious part of the plant, even after light frosts (Figure 3.2.1). High quality leaf material is therefore unavailable for grazing ruminants during the period of the year when it is most needed. Work at the Universities of Hawaii and Queensland has shown that hybrids of L. leucocephala with L. pallida (4n) and L. diversifolia (4n) (designated KX2 and KX3 respectively) have better cool tolerance than L. leucocephala.

The tree legume Gliricidia sepium loses all its leaf in southeast Queensland when temperatures drop to 15°C and needs to be grazed in late summer and autumn before leaf shedding commences (Whiteman et al. 1986). In contrast, Faidherbia albida sheds its leaf during the moist growing season making it an excellent agroforestry species.

Fig. 3.2.1. Seasonal production of Leucaena leucocephala in southeast Queensland (Isarasenee et al. 1984). Frosts occurred in June.

Moisture

The generally accepted rainfall tolerances of tree legumes are shown in Table 3.2.1. A characteristic of tree legumes is that they may be found growing in a very wide range of rainfall environments. For instance, Faidherbia albida can be found growing in regions receiving as little as 300 mm and as much as 3,000 mm rainfall p.a. The accepted rainfall range for L. leucocephala (650-3,000 mm p.a.) is also very wide. Other species such as S. sesban and Flemingia macrophylla are less well adapted to drier environments. The lower drought tolerance of C. calothyrsus and Gliricidia sepium may be partly related to their shallow root systems but also to their inability to survive moisture stress. Swasdiphanich (1993) found that these species, when severely drought stressed, did not have the capacity to endure low leaf relative water contents, were less able to extract moisture from the soil profile and responded to drought stress with a higher proportion of fallen leaf. Species such as L. leucocephala and Stylosanthes scabra (Table 3.2.4) were more drought tolerant.

The Sesbania species may tolerate low precipitation levels provided they are planted in poorly drained 'run-on' areas subjected to periodic waterlogging.

Light

Tree legumes are often planted in the shade of taller plantation crops usually to provide physical support for cash crops such as vanilla and pepper. Leucaena and gliricidia are commonly used in this way. Increasingly, forage tree legumes will be incorporated into plantation systems to improve feed supply to ruminants and therefore an ability to grow at reduced light intensities will be a desirable characteristic. The work of Benjamin et al. (1991) demonstrated that gliricidia is the most shade tolerant of the principal tree legume species (Table 3.2.5). This was confirmed by the work of Liyange and Jayasundera (1989) who also demonstrated potential to select provenances of gliricidia which are specially well adapted to shaded environments. Flemingia macrophylla is also reported to be tolerant of light shade (Anon. 1989).

Table 3.2.4. Drought tolerance indices of some tree legumes after severe water stress (Swasdiphanich 1993).

Species	Av. weekly WUE prior to stress (g/kg)	Drought tolerance parameters after severe stress
		Soil moisture (%)	RWC of young leaf (%)	Fallen leaf (%)
Calliandra calothyrsus	1.6	13.0	49.5	90
Gliricidia sepium	4.2	12.1	43.3	55
Leucaena leucocephala cv. Cunningham	2.6	12.0	33.0	25
Sesbania sesban	1.6	12.5	38.2	55
Stylosanthes scabra cv. Seca	0.6	11.9	22.8	2

WUE = water use efficiency
RWC = relative water content

Edaphic Adaptation

Soil fertility

Forage tree legumes can be found on a wide variety of soil types, including moderately infertile soils, although best production is obtained on fertile soil. For instance, Cooksley et al. (1988) found that highest yields of L. leucocephala were obtained on deep alluvial and colluvial soils rather than on shallow basaltic and andesitic soils even though frosts were more frequent on the former soil types.

One of the advantages of tree legumes is their deep root systems, a characteristic which confers persistence even on infertile soils. By contrast, many herbaceous legumes usually will not persist without the addition of inorganic fertilisers to correct nutrient deficiencies.

Much less work has been done on the nutrient requirements of tree legumes than on the herbaceous legumes. The most detailed study is that of Ruaysoongnern et al. (1989) who report the critical concentrations and external rates of application required by leucaena for a range of nutrients (Table 3.2.6). Many of these data are comparable to those required by other forage legumes; however, the critical concentration of potassium in index leaves (2.0%) and the external requirements of nodulated leucaena plants for phosphorus (225 kg P/ha) and calcium (230 kg Ca/ha) are relatively high, indicating greater requirements for these elements. A detailed review of the establishment requirements of leucaena to ensure effective nodulation is provided in Section 3.3.

Table 3.2.5. Shade tolerance of some tree legume species (Benjamin et al. 1991).

Species	Yield at high light (g/pot)	Yield at low light (g/pot)	Proportional reduction (%)
Acacia villosa	45.0	13.5	70
Albizia chinensis	42.2	25.9	39
Calliandra calothyrsus	38.6	23.2	40
Gliricidia sepium	32.2	24.6	23
Leucaena leucocephala	28.6	12.6	56
Sesbania grandiflora	26.5	16.4	38

Table 3.2.6. Estimated critical concentrations in index leaves and rates of application to achieve 90% maximum yield of leucaena for deficiency of the nutrients N, P, K, Ca, S and for toxicity of Mn (Ruaysoongnern et al. 1989).

Nutrient applied	Special conditions	Critical conc'n	Rate of application for 90% max. yield (kg/ha)
			(dry matter)	(nodule weight)
N	N-supplied	4.1%	645	>90 and <175
P	(a) Inoculated	0.25%	225	no plateau reached
	(b) N-supplied	0.21%	250	no plateau reached
K	(a) N-supplied with Ca(H2PO4)2	2.0%	35	no nodules formed
	(b) N-supplied with NaH2PO4	2.0%	180	no nodules formed
Ca	(a) Inoculated	0.49%	230	2550
	(b) N-supplied with CaSO4	0.38%	175	no nodules formed
	(c) N-supplied with CaCO3	0.38%	140	no nodules formed
S	N-supplied	0.24%	40	no nodules formed
Mn	(a) Inoculated	325 mg/kg	35	136
	(b) N-supplied	325 mg/kg	55	no nodules formed

Acidity

Adaptation to acidity varies quite markedly among the tree legume species. Some species such as Flemingia (Budelman 1989) and Gliricidia (Hughes 1987) have been reported to be extremely well adapted to acidic soils as low as pH 4.5. In general, most species tolerate mildly acid soils (> pH 5.5) while some such as L. leucocephala, G. sepium and Desmanthus virgatus can tolerate highly alkaline soils.

Leucaena leucocephala is native to alkaline Mexican soils and is commonly regarded as being intolerant of soil acidity. However, studies at the University of Queensland have shown that growth of leucaena is not significantly reduced until pH falls below approximately 5.2 (1:5 in H₂O) or 4.2 (1:5 CaCl₂). This was confirmed by field trials in which significant responses to lime were obtained only at the lowest pH of 4.2 (1:5 CaCl₂). The nodulation process in legumes is often more sensitive to the effects of low pH than plant growth per se. Work of Ruaysoongnern et al. (1989) showed that nodulation was severely affected at concentrations of 5 m M monomeric aluminium in solution culture whilst substantial reductions in plant weight did not occur until concentrations exceeded 20 m M (Figure 3.2.2).

Trials in Hawaii have indicated that varieties such as Cunningham, K29, K132 and K420 may tolerate acidity better than others (Brewbaker et al. 1985). However, there are no varieties for the very acid oxisols of tropical South America.

Fig. 3.2.2 The influence of solution concentrations of monomeric aluminium on nodule and whole plant dry weight of Leucaena leucocephala cv. Cunningham (Ruaysoongnern 1990).

Poor drainage

Soils vary in their drainage characteristics and those which are either flat or have subsurface impermeable clay layers are vulnerable to intermittent waterlogging regardless of annual precipitation levels. Waterlogging may result in a number of effects which are injurious to plant growth. These include:

· reduced O₂ content and rate of diffusion in the soil,

· reduced redox potential which will lead to reduction of stable Mn and Fe forms in the soil to more available and toxic ionic species, and

· an increase in concentrations of CO₂, ethylene and some organic compounds which may be harmful.

Tree legumes which are tolerant of these effects are needed for both occasional and regularly waterlogged sites. The Sesbania species are particularly noted for their tolerance of waterlogging. A glasshouse study at the University of Queensland in a soil high in manganese showed S. sesban to be the most tolerant of the species tested. Acacia villosa also showed good tolerance while L. leucocephala, C. calothyrsus and A. chinensis were relatively intolerant (Table 3.2.7). Codariocalyx gyroides is also reported to be tolerant of waterlogged soils (Skerman 1977).

Leucaena is widely reputed to be intolerant of poorly drained sites (Jones et al. 1982) and growth studies at the University of Queensland have confirmed this view (Figure 3.2.3). However, we have found that whilst leucaena seedlings are indeed vulnerable, well established plants are able to transpire excess soil water very quickly are therefore much less susceptible to waterlogging. In the seedling phase, L. leucocephala is susceptible to manganese toxicity and has a toxicity threshold of around 325 mg/kg (Table 3.2.6). Unlike aluminium, manganese appears to be more detrimental to plant growth than to Rhizobium and nodulation processes (Ruaysoongnern et al. 1989).

Table 3.2.7. Waterlogging tolerance of eight leguminous shrubs and trees (M.C. Galang, unpublished data).

Waterlogging tolerance group	Species	% Reduction in yield as moisture increased from 90 to 140% FC
Excellent tolerance	Sesbania sesban	1
Good tolerance	Acacia villosa	14
Moderate tolerance	Sesbania grandiflora	25
	Aeschynomene americana	29
	Gliricidia sepium	31
Moderate intolerance	Leucaena leucocephala	47
	Calliandra calothyrsus	47
Intolerant	Albizia chinensis	75

FC = held capacity

Other species reported to exhibit some tolerance of poorly drained soils are Flemingia macrophylla (Budelman 1989) and Faidherbia albida (Hocking 1987).

Salinity and alkalinity

Tolerance of salinity and alkalinity are important attributes sought in tree legumes. Species which tolerate these stresses can be used to make productive use of naturally alkaline sites, such as the uplifted coral line terraces frequently found around shore-lines of Pacific islands, or to rehabilitate areas salinised as a result of man's activities. Vast areas in India (Abrol and Bhumbla 1971) and Australia, for instance, have become salinised and more productive agriculture and forestry use could be made of these areas if suitable salt tolerant tree legumes were available.

Sesbania species are particularly tolerant of both salinity and alkalinity. Ghai et al. (1985) showed that seed of S. aegyptica (pseudonym S. sesban) was able to germinate without reduction at electrical conductivity levels up to 11 mmho/cm while S. grandiflora and S. glabra showed reductions in germination percentage of 23 and 37% respectively. However, the germination process is more sensitive to salinity than subsequent plant growth and other studies have shown that both S. sesban and S. grandiflora exhibit good tolerance of NaCl up to concentrations of 100 mM (Hansen and Munns 1985). Acacia ampliceps is one of the most salt tolerant of the Australian acacias. It is prominent close to tidal zones and in and around inland salt lakes (Turnbull 1986). It appears to be a useful forage and is often heavily grazed by cattle.

Fig. 3.2.3. The response of Leucaena leucocephala to waterlogging at nil and high rates of phosphorus application (N. Brandon, unpublished data).

Conclusions

A wide range of climatic and edaphic adaptations is exhibited by tree legumes. Although their seedling phase can be relatively more sensitive to various stresses, mature plants can be surprisingly tolerant. These attributes may permit the use of tree legumes on marginal soils and in rehabilitation situations, where herbaceous species would not survive, with minimal use of costly soil amendment practices.

References

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Anonymous (1989) Key NFT species appropriate for given climatic ranges. Nitrogen Fixing Tree Research Reports 7, 153-154.

Benjamin, A., Gutteridge, R.C. and Shelton, H.M. (1991) Shade tolerance of some tree legumes. In: Shelton, H.M. and Stür, W.W. (eds), Forages for Plantations Crops. ACIAR Proceedings No. 32, pp. 83-88.

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Brandon, N.J. (1992) Establishment requirements of Leucaena leucocephala (Lam.) de Wit cv. Cunningham. PhD thesis, The University of Queensland, 251 pp.

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