Tải bản đầy đủ - 0 (trang)
IV. Types of Interaction Causing Nontransgressive Deviations of Mixture Yields from Mid-Monoculture Values

IV. Types of Interaction Causing Nontransgressive Deviations of Mixture Yields from Mid-Monoculture Values

Tải bản đầy đủ - 0trang

187



BIOMASS PRODUCTIVITY OF MIXTURES



TABLE I1

Distribution of the Relative Yield Totals of Mixtures Based on

Published Data of Biomass of Components in 572 Mixtures

~



~~~



RYT value

0.5 to

0.7



0.7 to

0.9



1



2



Either grasses

or legumesb

Wheat

Rice

Grasses"

A series of

nonlegumesb

Subterranean

clover

Flax and

linseed

2

Grassesa

Barley



1

3



Crop

Grasses



0.9 to

1.1



1.1 to

1.3



6

5



4

9

2

26

41



11



4



8



3



1.3 to

1.5



21.7



4

1



Williams (1962)



3



5

3



Williams (1963)

9

22



5



95

10



3



7



20



9



Grasses.



3



106



4



21



26



21



4



6

6



5

18



2



3



6



-



-



3



Grasses.

Rye

-



6

1.0%



3



3



' L - 2



-



72

378

12.6% 66.1%



13.6%



Author

Ahlgen and

Aamodt (1939)

Aberg et al. (1943)

Donald (1946)

Sakai (1953)

Sakai (1955)

Lampeter (1960)



1

2

1



G rassesa



Grassesa



1.5 to

1.7



95

13

16.6% 2 . 3 %



5



0.9%



3

0.5%



Harper (1965)

England (1965)

NorringtonDavies (1967)

NorringtonDavies (1968)

van den Bergh

(1968)

Whittington and

O'Brien (1968)

Tbomson (1969)

NorringtonDnvies and

Hutto (1972)

= 572



20.3%

~~



. . . . . . . . . . . . . . . . . . . . . . . . .



Data derived from a series of cuts.

Mixtures of leguminous and nonleguminous species have been omitted.



nism and then consider how it may influence the relationship between the

yield of a mixture and the yields of its component monocultures.

The environmental resources for which plants compete are principally

the light, water, and soil-nutrient supplies necessary for growth (Clements

et al., 1929; Harper, 1961; Donald, 1963; Risser, 1969; Rhodes, 1970b).

Although carbon dioxide is required for shoot photosynthesis and oxygen



188



B. R. TRENBATH



is needed for the respiration of the roots, competition for these seems normally unlikely to occur. Competition for C02 is probably absent because

the air within the canopy is generally well mixed (Monteith, 1963; Impens

et al., 1967) ; all leaves are surrounded by air of approximately the same

CO, concentration. Similarly, except in almost waterlogged soils, the diffusion of oxygen in soil is usually fast enough to maintain adequate supplies

to all roots (Greenwood, 1969).

Apart from the relatively small number of instances where chemical secretions by one species are thought to have influenced the growth of

another, it seems that most investigators who have reported results of mixture experiments in English-language journals appear satisfied that the

neighbor effects which they observed were due to competition for light,

water, or one of the major nutrients (nitrogen, phosphate, and potassium),

or a combination of these factors. Since it is relatively easy to measure

and explain the unevenness of the sharing of light between the foliage of

two components in a mixture, the studies involving competition for light

have generally been the most conclusive.

Clements and many others have related differences in success in mixture

to differences in height between the leaves of the components of mixtures,

simply inferring that leaves of the shorter component must be experiencing

some degree of harmful shading. More recent analytical approaches have

been based on methods suggested by Monsi and Saeki (1953). Working

with natural communities, Monsi and Saeki introduced techniques for the

measurement of profiles of light intensity and leaf area index (LAI) . Since

then, these techniques have been used very successfully in experimental

mixtures to relate the differences in growth rates of the two components

to differences in the proportions of the total incident light intercepted by

leaves of the two components (Black, 1958; Iwaki, 1959; Stern and

Donald, 1962a,b; Williams, 1963). The general conclusion from all experiments involving competition for light is that the component with its leaf

area higher in the canopy is at an advantage. It is also likely (Stern and

Donald, 1962a) that, if the leaves are horizontal, the advantage is greater

than if they are erect (this is because horizontal leaves intercept more of

the total downward light flux per unit area of leaf than do erect leaves).

If the taller component has a greater leaf area, its advantage is again correspondingly greater (Iwaki, 1959).

In most of the experiments cited above, the investigators have attempted

to exclude the possibility of competition for soil factors. By providing optimal soil conditions or by separating the roots of the mixture components,

it was intended that neighbor effects would be interpretable in terms of

competition only for light. In most agricultural environments, however,

soil conditions are suboptimal and since root systems usually interpenetrate



BIOMASS PRODUCTIVITY OF MIXTURES



189



each other freely, there is a possibility of competition for water and/or

nutrients. Where the soil is very infertile and the density of planting low,

competition between root systems for soil factors is likely to decide which

component becomes the aggressor because the leaf area produced may

never become great enough to lead to significant competition for light. Indeed, large neighbor effects have been reported in cases where shading

was claimed to have been absent (Pavlychenko and Harrington, 1935;

Myers and Lipsett, 1958). In experiments in containers where competition

for light has been prevented by partitions, competition for soil factors has

similarly been shown to produce large effects (Donald, 1958; Aspinall,

1960; Rhodes, 1 9 6 8 ~Snaydon,

;

1971; Eagles, 1972).

The principles involved in competition between root systems have not

been as well worked out as in the case of competition between shoots,

but Bray (1954) has outlined a general theoretical approach. In contrast

with supplies of light, nutrient supplies are usually not greatly added to

after the beginning of growth of an annual crop although subsequent rainfali or irrigation normally supplements the initial store of soil water.

Whereas success of a species in competition for light depends on it having

a large absorptive area closer to the light source than the leaves of the

other component, the variable and complex geometry of root systems and

the sources of water and nutrients makes it less easy to define the plant

characteristics likely to confer competitive success.

Bray (1954) noted that competition between root systems for nitrogen

is likely to start at lower root densities, i.e. earlier in growth, than competition for phosphorus or potassium. This is because nitrogen is much more

mobile in soil than phosphorus and potassium. Zones of nitrogen depletion

round individual roots will therefore begin to overlap relatively early. To

test this proposition, Andrews and Newman (1970) grew pure and mixed

communities of young wheat plants in soil deficient in nitrogen and phosphorus with root densities between 1.2 and 8 cm/cm3; their resuIts showed

that whereas competition for nitrogen was intense, competition for phosphorus was indeed slight. Since the mobilities of nitrogen and water in

soil are similar (Fried and Broeshart, 1967), the soil resources most likely

to be subject to competition are nitrogen and/or water.

If Bray’s propositions are correct, they allow the identification of the

characteristics that will determine the relative aggressiveness of the components of a crop mixture growing on infertile soil. For one component to

gain an advantage over the other in the early competition for nitrogen and

water, a faster growing root system (i.e., generally a greater length of root)

is required. The limited evidence so far available does indeed suggest a

possible correlation between seedling competitive ability and early root production (see review in Rhodes, 1970b). However, it also seems clear that



190



B. R. TRENBATH



if root lengths are similar, the genotype having the more widely spreading,

less-branched root system will be at an advantage. Theoretical considerations suggest that there will probably also be an advantage in having roots

as thin as possible, thus allowing the available root material to be present

as the maximum length of root (Olsen et al., 1962). Abundant, long root

hairs (see Olsen and Kemper, 1968) and a high root “demand” factor

(Drew et al., 1969) are also likely to contribute to competitive success.

This theoretical consideration of competition for soil factors has treated

competition between root systems (root competition) as independent of

competition for light between shoots (shoot competition). However,

Donald (1958) has suggested that in well developed agricultural crops of

uniform genotype, both shoot and root competition are usually occurring.

The relative importance and time of onset of root and shoot competition

will depend on environmental conditions (Aspinall, 1960) ; the nature of

the genotypes involved will also have an effect. To compare, for a particular set of conditions and genotypes, the contributions of the effects of root

and shoot competition with the overall effect of being grown in mixture,

pot experiments have been designed in which partitions separated either

roots or shoots, or both, or none (Donald, 1958; Griimmer, 1958; Aspinall,

1960; Rhodes, 1968c; Snaydon, 1971; Eagles, 1972).

Of the results of these experiments, those of Snaydon (1971 ) are the

easiest to interpret, for in his experiment alone were there constant

numbers of partitions per treatment and plants per compartment. Using

ecotypes of white clover, Snaydon confirmed the results of the other investigators who had shown that root competition had a greater effect (and probably began earlier) than shoot competition. Although this finding suggests

that dominance-suppression relationships in crop mixtures may depend

more on root characters than on shoot characters, the concentration of

roots at the surfaces of the containers in such experiments must have

hastened the overlapping of zones of depletion around individual roots,

and hence led to an overestimation of the agronomic effect of competition

for soil factors.

Considering a possible interaction between the two types of Competition,

Donald (1958) showed that the proportional reduction in the subordinate’s

yield due to shoot competition was greater when root competition was occurring than when it was absent. Similarly, the proportional reduction due

to root competition was greater when shoots were competing for light than

when they were separated. Donald suggested that the failure of, say, the

root to acquire sufficient nitrogen caused leaf development to suffer. This

in turn reduced the supplies of assimilate to the roots which grew less and

so were less able to compete for nitrogen. Donald proposed a similar set

of causes and effects for a component of a mixture which was unsuccessful



BIOMASS PRODUCTIVITY OF MIXTURES



191



in competition for light. The important conclusion was that the effects of

Competition, once initiated, tend to be magnified by a system of positive

feedbacks if simultaneous shoot and root competition are occurring.

Milthorpe (1961) has discussed the effects of the onset of competition

for soil factors in a mixture where competition for light has already caused

some depression of the growth of one component. Arguing from the depressive effect of shading on root:shoot ratio (e.g., Brouwer, 1966), Milthorpe

considered that any deficiency of soil water or nutrients would cause the

accelerated suppression of the subordinate if any previous shading had reduced its root’s uptake capacity relative to the size of its shoot. Similarly,

the writer has presented results (Trenbath and Harper, 1973) which suggest that the development of a slight deficiency of soil factors where competition for light was already occurring caused an approximately 4-fold increase in the depression shown by a series of subordinates.

While competition for resources has usually seemed able to explain observed differences between per-plant yields in mixture and monoculture,

a number of workers believe that these differences (neighbor effects) are

sometimes caused by other processes. This belief has been expressed by

workers well acquainted with the mechanisms of competition (Warne,

1953), has been implied by other investigators (Sakai, 1955), and has

been used as a convenient explanation of unexpected results by others

(Ahlgren and Aamodt, 1939; Went, 1942).

As an alternative explanation of neighbor effects, allelopathy (Grummer,

1955) has been frequently proposed. The type of allelopathy most likely

to occur in a field crop is that described by Winter ( 1961), where one

or more biologically active substances are liberated from the root or shoot

of a plant, enter a neighboring plant, and there cause a depression of that

plant’s growth. T o allow for cases of stimulation of growth in the affected

plant, the general term “allelochemical” effect (Anonymous, 1971 ) has been

introduced to include both positive and negative effects on growth. Since,

however, the term “allelopathy” is both more convenient and more familiar, in what follows it will be used in a general sense, i.e., allowing the

possibility of either positive or negative effects.

Allelopathy in plant communities has been studied most actively in the

Soviet Union and Germany; the rapidly expanding literature has been reviewed by Griimmer (1955), Rademacher (1959), Grodzinsky (1965,

1971), and Borner (1971). On the other hand, in some other European

countries and in the United States, research on allelopathy has had an uneven history. Several instances of apparent allelopathy have been shown

(sometimes by the original investigators) either to be due to other causes

(Went, 1942; Gray and Bonner, 1948; but see Muller, 1953; Grummer,

1958; but see de Wit, 1960; Sandfaer, 1968; but see Sandfaer, 1970a,b)



192



B. R. TRENBATH



or to be explained by alternative mechanisms (Muller, 1966, but see Bartholomew, 1970). These cases have tended to make investigators cautious

of suggesting allelopathy as the cause of neighbor effects, indeed, they have

inclined the present author to agree with Harper’s (1965) opinion that

research into the role of allelopathic substances in mixtures is beset with extreme technical difficulties. To avoid greatly extending this review no detailed

discussion will be attempted, but three general comments will be made.

First, as Borner (1960) pointed out, although many investigators have

demonstrated allelopathic effects between plants growing in solution culture

or sand culture, instances where the same effects have been conclusively

proved to be significant under field conditions are rather few. Among such

cases reported in English-language and western European literature, the

most convincing seem to be the existence of inhibitory effects on associated

plants due to walnut (Massey, 1925; Bode, 1958) and to Arctostaphylos

species (Hanawalt, 1971) . Also, the demonstration of autotoxicity among

individuals of Grevillea robusta (Webb et al., 1967) grown in both the

field and glasshouse seems conclusive. It should be noted that although

Bonner (1946) demonstrated the autotoxic activity of root exudates of

guayule plants in gravel culture, the active substance was degraded so fast

in field soil that no autotoxic effect was likely under normal agricultural

conditions. A rather different case is that of Hirano and Kira (1965), who

demonstrated apparent autotoxicity among densely planted peach saplings

in an experiment planted in field soil (clay loam) under glass; this result

accorded well with expectations (Hirano and Morioka, 1964; Proebsting

and Gilmore, 1940). However, a similar experiment carried out on a sandy

soil in the open gave no indication of an autotoxicity effect (Hirano and

Kira, 1965).

Second, since crops are normally grown at densities high enough for

competition between neighbors for resources to be intense (Donald, 1963) ,

any allelopathic effect would be either exaggerated (or reversed) through

competition. To emphasize the importance of taking into account both aspects of the total interaction, we may consider an hypothetical example

of a crop mixture growing on a soil poor in nitrogen. If the suppression

of one of two otherwise evenly-matched components was initiated through

some slight allelopathic inhibition of its nitrogen uptake, later competition

for nutrients might result in plants of this component being nitrogen deficient and heavily suppressed. The agronomist would “prove” the suppression to be due to competition for nitrogen since he would find, in an experiment with separated shoot systems, that neighbor effects were strongest

on soils with the lowest nitrogen level; in the meanwhile, the specialized

investigator of allelopathic effects might have discounted some slight inhibitory effect as insignificant in view of the great intensity of the suppression

in the field. Through the lack of an integrated approach, both workers



BIOMASS PRODUCTIVITY OF MIXTURES



193



would have drawn false conclusions. The experimental separation of the

effects of competition and allelopathy is thus an important objective, although it is certain to be difficult to achieve (Welbank, 196l ) .

Third, providing (a) that an allelopathic inhibition takes effect early

enough in the life of the crop for compensatory growth to occur in the

other component (see Section V ) , and (b) that the inhibition reduces the

uptake of growth factors rather than the efficiency with which they are

used, the total quantity of environmental resources intercepted by the mixture may not be much affected. Consequently, the yield may scarcely be

less than that of another mixture with the same values of P, and P , in

which a similar degree of suppression has been brought about by competition acting alone. Where, however, the allelopathic effect reduces the efficiency with which growth factors are used in dry-matter production, the

RYT will probably be lowered and the mixture could underyield (see Section V) .

Whether caused by competition alone or by a combination of competition and allelopathic effect, the unequal sharing of resources between components generally affects the yields of mixtures. If the higher-yielding component in monoculture is the aggressor, then with RYT = 1 the mixture

yield will lie between f' and P I . The greater the depression of the loweryielding component, the closer the approach of M to P,; with its complete

suppression, M = PI. Similarly, if the lower-yielding component in monoculture is the aggressor in the mixture (the so-called Montgomery effect,

Gustaffson, 1951), then M lies between P and P,. With complete suppression of the subordinate, M = P,.

The tendency for mixture yields to lie above P (Table I) combined with

the closeness of RYT's to unity (Table 11) suggests that there is a positive

correlation between aggressiveness in mixtures and biomass production in

pure stands. The association is not strong, however, for the value of the

correlation coefficient has been estimated from published data to be only

about 0.3 (Trenbath, 1972). The existence of a positive correlation is

nevertheless in agreement with the ideas discussed earlier, namely, that

a large leaf area displayed at a sufficient height gives an advantage in competition for light (e.g., Donald, 1961) ; the close dependence of productiv;

ity in monoculture on LA1 is well established (Watson, 1952).

A possible reason for the looseness of the correlation between aggressiveness and monoculture yield is found in a suggestion by Iwaki (1959).

Iwaki postulated that a species would be aggressive in mixture if it diverted

a particularly large share of photosynthetic product into building tall stems.

Associated species would be shaded and suppressed. In monoculture, however, the tall species would grow relatively slowly because of the low proportion of its dry matter invested in productive leaves. A negative correlation between aggressiveness and monoculture grain yield in rice (Jennings



194



B. R. TRENBATH



and Aquino, 1968) was similarly related to the tallness of the stems of

one type of rice, but in this case the low monoculture yield of the tall

type was mainly the result of its tendency to lodge. The bending over of

the tips of long grass leaves (“flagging”) will also lead to greater aggressiveness (see above) and lower growth rates in monoculture (Alberda,

1966).

The aggressiveness of a species is well known to depend on environmental conditions and such a dependence will affect the relative values

of M ,P I , and P,. The dominance-suppression relationship between genotypes has been reversed by changing the temperature regime (Eagles,

1972) and by aItering the soil conditions (van Dobben, 1955; Sakai and

Iyama, 1959; Stern and Donald, 1962a; van den Bergh and Elberse, 1962;

Snaydon, 1971). Aggressiveness often depends on the stage of growth of

the plants (e.g., de Wit and van den Bergh, 1965; van deli Bergh, 1968;

Rhodes, 1968a; Nguyen Van, 1968), but in field experiments, the concurrent changes in developmental stage and in meteorological factors usually

make it difficult to identify the factor responsible for any change of aggressiveness with time. The hard-to-define differences between growing seasons

have marked effects on aggressiveness at least where the index used to

measure it is based on seed yields (Laude and Swanson, 1942; Sakai and

Oka, 1955; Allard and Workman, 1963; Workman and Allard, 1964; Lin

and Torrie, 1968). Allard et al. (in Edwards and Allard, 1963) showed

from the data of Suneson (1949) that taking the value of ATLAS as 100,

the selective advantage of VAUGHN ranged from 40 to 173 in 13 seasons.

Transfer from the field to the greenhouse may also cause reversals of dominance (Aberg e f al., 1943; Syme and Bremner, 1968).

If aggressiveness is so sensitive to environmental conditions, it might

be asked how this sensitivity affects mixture yields over a series of sites

or seasons. Unfortunately, although diallel experiments have been carried

out using a range of environments or treatments (Harper, 1965; England;

1965; Whitehouse et al., 1967; Bell et al., 1968; Norrington-Davies, 1968;

Sandfaer, 1970b; Schutz and Brim, 1971), no systematic attempt has yet

been made to study the effect of associated changes of aggressiveness on

mixture yields. Presumably connected in some way with such changes, a

greater stability of seed yields has often been found in varietal mixtures

of cereals and soybeans grown over a range of environments [Allard, 1961;

Simmonds, 1962; Pfahler, 1965; Frey and Maldonado, 1967; Byth and

Weber, 1968; Qualset and Granger, 1970; Schutz and Brim, 1971; but

see Rasmusson (1968) and Clay and AIIard (1969)l. Taking together

the results of three reports (Allard, 1961; Pfahler, 1965; Qualset and

Granger, 1970) in which an index of yield stability was given for each

mixture and monoculture, 5 out of a total of 12 binary mixtures were more



BIOMASS PRODUCTIVITY OF MIXTURES



195



stable than their more stable component monoculture; the remaining mixtures showed stabilities between those of their component monocultures.

In a comprehensive study involving 16 environments, Schutz and Brim

(1971) calculated, for each type of stand, indices of stability with respect

to several types of environmental variation (seasons, locations, replicates,

etc.) . The lack of agreement between the several stability indices of individual types of stand suggests a complex situation with no clear-cut stability

advantage for mixtures. To add to difficulties of interpretation, stability

parameters are much less accurately estimated than parameters related directly to yield (Allard, 1961;Marshall and Brown, 1973) .

With reference to biomass rather than to seed yield, very few data are

available that allow the comparison of any index of yield stability of individual mixtures with those of their component monocultures. While two

studies (England, 1968; Sechler and Chapman, 1967) suggested that mixture yields were collectively more stable than monoculture yields, data of

individual mixtures from two other studies (Pfahler, 1965; Thomson,

1969) showed stabilities to be intermediate between those of the monocultures in 9 out of a total of 10 cases. These limited data seem to indicate

that the stability of mixture yields is, like yield itself, usually nontransgressive, and that in seed yield, but not in biomass yield, there is a tendency

for mixture stability to be close to that of the more stable component.

In a theoretical study of mixture stability, Marshall and Brown (1973)

suggested that varietal mixtures were most likely to show agronomically

useful stability in highly variable environments to which the available genotypes were not individually well adapted. Perhaps it is partly the lack of

crop species with sufficiently wide adaptation that has led to the extensive

culture of multispecific mixtures in India (Aiyer, 1949). The markedly

differing adaptation characteristics of the components of such mixtures ensure that even in the worst season there will be something to harvest.

The principal conclusions from this and the previous sections may now

be summarized. In field-crop mixtures, competition for both light and soil

resources will usually be occurring; allelopathic effects (if present) will

operate in conjunction with this competition; competition for resources

and/or allelopathy will usually cause per-plant yields of each genotype in

mixture to differ from that in monoculture; the differences of per-plant

yields are usually of a compensating type; if RYT = 1, M will lie between

PIand P,;any positive correlation between aggressiveness and monoculture

yield will produce a tendency for M > P; there are some indications that

the biomass yield of a mixture having sufficiently different components is

likely to be more stable than that of the more stable of its components

in monoculture. We continue now by turning to the more varied and subtle

forms of interaction that may lead to transgressive yielding.



196



B. R. TRENBATH



V.



Mechanisms Capable of Causing Transgressive Yielding by Mixtures



The results reviewed in Section I1 show that mixtures have often been

recorded as apparently yielding transgressively. Furthermore, the data indicate that records of mixtures overyielding are significantly more frequent

than records of underyielding. Two contrasting interpretations of this situation could be proposed.

1 . It might be suggested that if RYT could be measured without experimental error, its value might always be close to unity; the observed skewness of its distribution (Table 11) could be a consequence of basing RYT

on two ratios of random normal variables (Fieller, 1932). Since there is

a weak, positive correlation between aggressiveness and monoculture yield

(Section IV) , the preponderance of mixture yields greater than P could

be the result of this correlation. If this were the case, theoretical error-free

experiments would show mixture yields lying usually between P, and p,

less frequently between P and P,, and never outside the range P, to P2.

The experimental error found in actual experiments would be expected

to disperse the observed results about their “true” values, so that overyielding mixtures would be recorded more frequently than underyielding ones.

This view implies that observed cases of transgressive yielding are due only

to experimental error.

2. An alternative interpretation might be that at least some of the observed cases of transgressive yielding do represent real effects. If the correlation between aggressiveness and monoculture yield were discounted as

being too weak to have appreciable effect, the tendency for mixture yields

to exceed P could be attributed to apparent mutual stimulation of the mixture components, as indicated by the preponderance of RYT’s greater than

unity (Table 11).

These two interpretations are only preliminary attempts to explain the

observations; they represent extreme, but not necessarily incompatible, approaches. Having discussed in Section I1 the evidence in favor of transgressive yielding being a real phenomenon, we consider here the mechanisms

that could lead to transgressive yielding. If they appear likely to operate

under the conditions normally used for mixture experiments, this would add

credibility to the second interpretation given above.

In connection with overyielding, the findings and theory of animal

ecology may be relevant since they help to define conditions that might

lead to this kind of transgressive yielding. Gause (1934) quoted Formozov’s observations of natural mixed populations of tern in which four species coexisted, apparently because they exploited the environment in different ways. In such a case, it seems likely that a mixed population would



BIOMASS PRODUCTIVITY OF MIXTURES



197



be more productive than any monospecific population. Gause (1934)

summed up his own experimental results in the principle that species of

very similar requirements tend not to coexist. Using this principle, Harper

(1967) and others argued that the species diversity found in natural communities implies that the component species occupy differing niches. Together, therefore, they must exploit the environment more completely than

a community of few species. Providing support for this view, Beardmore

et ul. (1960) and Seaton and Antonovics (1967) reported overyielding

of mixtures of Drosophilu strains in controlled environments. Seaton and

Antonovics suggested that in their mixtures the two genotypes were avoiding competition by occupying different niches.

In the botanical literature, genotypes believed to be avoiding competition

while they share a habitat are said to be “complementary” (Woodhead,

in Salisbury, 1929). Woodhead first used the term in connection with natural woodlands where, by the sharing of environmental resources in either

time or space, the various types of species seem to escape, at least partially,

the effects of competition. His concept of the complementary use of resources would seem to be applicable to crop mixtures; the possible ways

in which resources may be used in such a manner will now be considered.

Van den Bergh and de Wit (1960) reported an example where temporal

sharing of the environment may have been responsible for a case of apparent mutual stimulation in mixture. In a mixture of two grass species which

differed markedly in time of development, plants of both components had

more tillers (53% and 3 6 % ) than did plants in the corresponding monocultures; although biomasses were not reported, it may be noted that the

RYT based on tiller number was 1.49. Syme and Bremner ( 1968) reported

a series of experiments involving oats and barley varieties chosen to differ

in flowering time; the results of an experiment performed under glass (data

given in Syme, 1963) showed that in all four oats-barley mixtures, both

components showed higher per-plant shoot weights than in monoculture.

While overyielding of dry matter did not occur, the RYT based on shoot

weights averaged over the four (replicated) mixtures and two densities

was 1.12. Such an RYT value in a mixture of which the two monoculture

yields were very similar would be associated with the mixture outyielding

the monocultures by 12%. Sechler and Chapman (1967) also reported

an experiment with cereal genotypes (oats) which differed greatly in

flowering date. Again no mixture overyielded. Unfortunately, the data

given were insufficient to allow the calculation of RYT’s and so no detailed

comparison is possible with the experiment of Syme and Bremner ( 1968).

In corn-rice mixtures in the Philippines, the corn flowers and matures

before the rice begins to flower; this phenological difference probably explains the reported overyielding of grain by such a mixture (International



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

IV. Types of Interaction Causing Nontransgressive Deviations of Mixture Yields from Mid-Monoculture Values

Tải bản đầy đủ ngay(0 tr)

×