OGDEN: PLANT POPULATION STRUCTURE AND PRODUCTIVITY J

PLANT POPULATION STRUCTURE AND PRODUCTIVITY

J. OGDEN

Department of Botany and Zoology, Massey University, Palmerston North

SUMMARY: In many animal populations age distribution ha.~ been studied as a basiccomponent of population structure. In plants, developmental plasticity and vegetative repro-duction may mean that age distribution is less significant than size distribution, particularly inproduction processes occurring within populaJtions. Changes in size distribution with time andat different densities are discussed, with particular reference to herbaceous monocultures. Therelationship between size distribution and 'productive structure' (layering) is considered andexamples of the vertical profile of chlorophyll content have been taken from three herbcommunities. The effects of these population attributes on rates of mortality and turnover,gross production, net production and biomass are discussed and a graphic model is presentedwhich relates aspects of production to density.

IN1RODUCTION

In the population ecology of animals 'popula-tion structure' refers to the age distribution ofindividuals of each sex within the population.Knowledge of the relative proportions of youngand old or males and females, often summariseddiagramatically as a 'pyramid of numbers', isregarded as a preliminary step in the quantitativedescription of that population. This knowledge isan essential prerequisite if reliable predictions offuture population trends are to be attempted.

The concept of 'structure' in this sense has notbeen generally applied to plant populations. Thestudy of plant population dynamics has sufferedparticularly from emphasis given to plasticity andvegetative reproduction. characteristics which ren-der plants awkward as demographic units(Harper, 1967). Vegetative reproduction presentsus with problems of definition, both of the truegenetic individual and of the unit to be regardedas an individual for the purpose of counting.Plasticity may cause individuals of the same abso.lute age to diUer 100-fold or more in size, allowingthem to play very different roles within the popu.lation. The ecologist may, understandably, bereluctant to give such individuals equal weight ina population census. Finally, it is often difficultor impossible to ascertain the absolute age of aplant growing in the field, although careful observ.ation may sometimes make this possible, as whenscars are left by annually-produced inflorescences(Tamm,J 948). Many temperate-forest trees pro-vide notable exceptions to these difficulties. Mosttrees lack vegetative reproduction and form dis~crete countable individuals whose age can beestimated without too much difficulty. Moreover,

particularly in New Zealand, many species havedistinct sexes, so that for tbese species it seemsfeasible to construct pyramids of numbers directlycomparable with those obtained for animal popu-lations.

THE FREQUENCY DIS1RtBUTION OF PLANT

WEIGHTS

In most herbaceous plants age and size are notsynonymous, so that the concept of populationstructure must be expanded to include the weightand height distributions of the individuals withinthe population. Japanese workers associated withKira in the 1950s (Hozumi et al., 1955, Koyamaand Kira, 1956) found that populations in whichweights were initially normally distributed deve.loped a positive skew as the population aged. Thisskewing was exaggerated at high population densi.ties and led eventually to the appearance of L-shaped frequency distribution curves. Koyamaand Kira argued that these distributions are oftenlog-normal, i.e. a histogram of the log of theweights follows a normal curve. This log.norma!-ity they interpreted as arising from the exponentialnature of the growth processes. They argu.ed thatskewing was exaggerated by intraspecific competi-tion, but that it would develop even in non-com.peting, widely spaced populations. Obeid et al.(1967), studying the European fibre flax (Linumusitatissimum L.), have confirmed the Japanesework and, as su~gested by Donald (1963), haveexamined the influence of density on the frequencydistributions of certain yield components.

Log-normal frequency distributions are notrestricted to plants growing in monocultures. In1966 I investigated the frequency distributions of

PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 17, ]970

weight in mixed annual herb communities colonis-ing an arable field in Wales (Appendix]). Thedistributions of the community as a whole, andof the individual species populations. were 311markedly skewed in favour of small individuals(Fig. 1). In four of the seven cases examined therewas no significant departure from log-normality(Fig. 2). It may be a fair generalisation to Con-

to °0

PLANT DRY WEIGHT IG,MS)

FIGURE 1. Frequency distributions of individualplant dry weight in mixed annual weed popula-tion5 colonising an arable field in N. Wales.II is the density of individuals per 6 square feet and

the sample size. The ringed numbers refer to the speciesas follows: l, Graminae, mostly Poa anlllw L 2, Atri-pJex patllia L. 3, Polygol1um aviculare agg. 4, All other1:ipecies (see Table 3, Appendix 1). 5, Stachys arveI1sis L.6, Ste/laria media (L.) ViII. 7, Spergula arvensis L8, Senecio vII/garis L. 9, Polygonum persicaria L. and

P. lapathifolium L

elude that annual (and perennial?) plant popula-tions usually consist of a few large individualsand a relative]y large number of small ones, andthat this skewing increases with increasing densityand the passage of time.

PLANT DRY WEIGHT (LOGS>

FIGURE 2. Frequency distributions of the loga-rithms of individual plant dry weight in mixedannual weed populations colonising an arable field

in N. WalesFor details of derivation of class intervals see Appen.

dix I, For key to species see Figure 1. Goodness of fit tonormal curve tested by chi-square. P values greater than0.05 indicate no significant departure from normality.

THE FREQUENCY DIS'ffimUTION OF PLANT

HEIGHTS (THE HEIGHT PROFILE)

Several sets of data for annual plants growingin pure cultures (Koyama and Kira, ]956; Kuro-iwa, 1960a) indicate that generally the heightfrequency curve is either more-or-Iess normal atall densities or negatively skewed (i.e. it is skewed

2

OGDEN: PLANT POPULATION STRUCTURE ANI) PRODUCTIVITY

in the direction opposite to the weight frequencycurve), Similarly, in perennial herbaceous mono-

cu/tureg highly skewed weight Ji~lJ'ibutiOLJ~ maybe associated with approximately normal heIghtdistributions (Fig. 3). This situation suggests thatat least some of the abundant low weight individ-uals in dense populations tend to maintain theirheight, struggling to reach a canopy providedprimarily by a few large plants. [n Helianthusannuus L. (Kuroiwa, 1960a; Hirai and Mansi,1964) these suppressed plants are lank and etiol-ated, with a relatively increased proportion ofnon-photosynthetic to photosynthetic tissue (stemto leaves). Tezuka (1960) has demonstrated thesame effect in high density stands of Fagopyrumesculentum.

[n more complex vegetation the height fre-quency distribution, or profile, of the communityas a whole may show several distinct modes; forexample, those corresponding with the upperlimits of herb, shrub and tree strata in a forestcommunity. However. if we consider all the indi.viduals present rather than individual species,these strata may not always be as distinct as isfrequently supposed. .Grubb et al. (1963) havepointed out that, in tropical rain forest, the fre.quency distribution of individual tree heights fre-quently approximates a hollow curve, with manysmall trees and few large ones in any limited area.

If seedlings are considered this skewing is evenmore marked. Similarly, highly skewed girth fre-quency distributions ha.ve ~l"n recom..led for aU

individuals in 2S plots m ram forest m Guyana,although girth distributiDns for individual speciesoften deviated from this pattern (Ogden, 1966).Plant communities are so varied in composition

and structure that no general statement concerningtheir height frequency distributions is possible. Inherbaceous monocultures, however, the changesin the distribution of weight and height frequencieswith the passage of time indicate that an initialsituation in which all or most of the individualsreach the canopy develops into one in which amuch smaller proportion of individuals do so.

PRODUCTIVE STRUCTURE (THE BIOMASS PROFILE)

As suggested above, the concept of stratifica-tion, or layering, has generally been applied toforest communities. Japanese workers associatedwith Saeki (Mansi and Saeki, 1953; Saeki andKuroiwa, 1959) using the 'stratified clip' tech-nique, have studied the vertical profile of biomassof leaves and stems within herbaceous monocul.tures. This profile they have called the 'productivestructure' of the population (Fig. 4). 'Productivestructure' is an important aspect of Odum's'organic structure' (Odum, 1966); although theformer term has been applied mainly to popula-tions and the latter to ecosystems.

- - .. ... ' ...,.....

PlANT FRESH WEIGHH10 (GM;J

FIGURE 3. Frequency distributions of weight and height in a natural stand of Elatostema rugosum.

3

- --- --- .."PLANT HEIGHT M-

PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 17, 1970

Although Ihe vertical distribution of leaves andthe relative proportions of 1eaves and stems arecertainly of great importance in controlling popu-lation productivity, we must not forget that largequantities of chlorophyll may be present in stems(Ovington and Lawrence, 1967). This suggeststhat the vertical profile of chlorophyll contentmight be more important than that of leaf distri-bution. The results of some preliminary examina-tions of productive structure in perennial herbsare given in Figures 4, 5 and 6, and further detailsgiven in Appendix 2.Pigment stratification in natural communities of

Elatostema rugosum A. Cunn. Precur. (a largeforest herb spreading by horizontal stolons) wassimilar to that shown by natural 'swards' ofTradescantia fluminensis Veil. (Fig. 5). In both,chlorophyll concentration (optical density jg.measured at 665ml') was greatest in the upper40 per cent of the 'sward', and the total quantityof pigment per unit area was greatest in two dis.tinct layers. The uppermost of these layers wasthe region of vertical leafy stems, where chloro-phyll was present at a high concentration. Thelower layer was composed by horizontal creep-ing stems. where pigments were at a low con-centration, but in which most of the biomass

occurred. In other herbaceous species suchaccumulation layers ('sinks') frequently lackchlorophyll and are located underground, forexample in bulbs, tubers and rhizomes. Note thatthe stolon layer of a Tradescantia sward harbours

DRY WE!GHT GMsjM1I.

FIGURE 4. Productive structure in a natural !J'tand

of Elatostema rugosum.

4

LEAVES

OPTICAL DENSITY ~1OY30c~s;' O.gA3M.X1Q't

FIGURE5. Vertical distribution of pigment and dry matter in a natural stand of Tradescantia flumin-.

enSlS.

TABLE \. Productivity of high density stands ofHelianthus annuus. (From Kuroiwa, 1960a.)

Plant niomassclassification Plants/m.2 g.{m.2 g./m.2Jday

Gross Respir~ Net pro~prodo. ation duction

Dominant 16 105 I\.2 6.4 4.8Large inter-mediate 32 80 8.7 5.5 3.2

Intermediate 58 60 6.6 4.6 2.0Small inter-mediate 64 35 3.0 2.6 0.4

Suppressed 30 8 0.6 0.7 -0.1Totals 200 288 30.1 19.8 10.3

OGDEN: PLANT POPULATION STRUCTURE AND PRODUCTIVITY 5

TOTAL DRY WEIGHT GMS/30crns~

80 60 40 20

OPTICAL DENSITY "c1gY30C,m& (O.o.;GM. WT.) "10'

FIGURE 6.Vertical distribution of pigment and dry matter in a natural stand of Bromus catharticus.

a large and diverse invertebrate fauna, so that thecommunity as a whole possesses an upper pro-ducer zone and a lower heterotrophic zone inwhich matter accumulates in plant structures andis eaten by invertebrates.

In tall Bromus catharticus YaW. grassland, threeintergrading layers can be recognised: (I) anupper in which chlorophyll is at a high concentra-tion, but in which there is little dry matter; (2)a middle with the greatest absolute quantity ofchlorophyll and (3) a lower containing littlechlorophyll but most of the dry matter.

THE INFLUENCE OF THE FREQUENCY DISTRIBU-

TIONS AND PROFILES OF HEIGHT AND WEIGHT ON

POPULATION PRODUCTIVITY AND MORTALITY

Watson (1947) showed that with the growth ofa sward, leaf area index (L.A.I.) increases so thatthe lower leaves gradually become shaded by thoseabove and contribute less and less to growth.Eventually a point is reached when further in-crease in leaf area will not increase the growthrate of the population, because as one new leafcomes into function above, an old leaf passes outof function in the shade below. When this pointis reached, assuming no changes in the grossmorphology of the plants, the photosynthetic sys-tem has reached its maximum extent and further

growth will be restricted to the non-photosyntheticparts. The consequence of this is a gradual de-crease in the photosynthesis/respiration ratiotowards a value of one, at which the plants areat compensation point and no further growth ispossible.It is clear that similar processes operate on

individuals within dense stands. Kuroiwa (1960a)classified individuals in a dense experimentalstand of Helianthus annuus into five classes rang-ing from dominant to suppressed. He examinedhow these different classes shared the total leafarea at different levels in the population profileand how this influenced their energy budgets. His

lnitial Term. b'mass Net prodn. Mortalitydensity (g. dry (g. dry (%Initial(No./m.') wl/m.') wt/m.') density)

6.25 512 530 025.0 1011 1015 044.4 979 1145 16.0100.0 775 1410 50.0400.0 690 1775 66.7

PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 17, 1970

Mathews and Westlake (1969) have recalcul-ated some of Hirano and Kira's (1965) data onproductivity of dense seedling stands of Prunuspersica (L.) (Table 2) and make the importantpoint that yield at final harvest (terminal biomass)may give little indication of the amount of matterwhich has been lost in the form of dead leaves anddying individuals prior to harvest. Total net pro-duction may be as much as two or three times asgreat as biomass in high density populations. I

reached the same conclusion in experiments withthe rhizomatous perennial Tussilago farfara(Ogden, 1968). In low density populations of thisspecies the maximum biomass achieved in a two-year period was over 65 percent of Ihe total netproduction. In high density stands, however, maxi-mum biomass was only between 55 and 60 percentof the total net production. This difference wascaused primarily by increased individual and leafmortality at high density.

DENS I TV .

FIGURE 7. General diagrammatic model showingsuggested relationships between biom@s, net pr(}~

duetion, gross production and density in annualmonocu!tures.

Both scales in arbitrary units. Curve I, total gross pre.-duction up to time of harvest. Curve 2, total net produc~tian up to time of harvest. Curve 3, a or b, biomass at

harvest.

6

results (Table I) indicate that the suppressedplants suffer a progressive decrease in gross pro-duction/respiration ratio, so that the quantity ofnet production (Pg-R) gradually declines. Sup-pression may progress beyond compensation pointwhereupon the plants respire reserves presumablyaccumulated before the development of excessiveskewness. Long (1936) demonstrated that sup-pressed plants of this species possess lower eal./g.values than dominants; so it is certain that thesesuppressed plants have very little food store.Unless they can correct their negative energybalance they must perish.It seems likely that when suppressed plants are

at, or near, compensation point they will be parti-cularly vulnerable. Any reduction in gross produc-tion caused by increased shading by growth ofdominants, shortening of days, attack by herbivor-ous insects or saprophytic fungi may cause rapiddeath. However, Tamm's (1948) observations onperennial herbs showed that small suppressedindividuals in pastures may survive for manyyears. Furthermore, internode measurements andleaf scars on tree seedlings in forest shade suggestthat these may remain close to compensation pointfor long periods. Only an increase in light throughopening up of the canopy is likely to allow thefurther development of such suppressed offspring.In dense populations the skewed frequency distri-bution of plant weights indicates that a large pro-portion of the population falls into the suppressedclass, so that a high rate of mortality is to beexpected. Less dense populations gradually deve-lop skewness so that individuals are being con~stantly recruited to the suppressed class and pass-ing from this class by mortality. It seems possiblethat such a process could explain the steady deathrale of some perennial plants in natural popula-tions. (See Harper (1967) for an analysis of thedata in Tamm (1956».

TABLE2. Productivity and population dynamic,' ofexperimental Prunus persica (L) populations over186 days. (Data from Mathews and Westlake,

1969.)

.

respiration

death of parts

,living biomass

OGDEN: PLANT POPULATION STRUCTURE AND PRODUCTIVITY 7

Kuroiwa's data for Helianthus (Table I, fromKuroiwa, 1960a) and Abies sp. (Kuroiwa, 1960b)indicate that a dense population containing manysuppressed individuals loses a proportionallygreater quantity of its gross production throughrespiration than does a less dense stand. Theevidence given above suggests, also, that more ofthe net production of such dense stands is lost bythe death of parts of the population. An asympto-tic curve is usually found when average yields, interms of biomass weights per unit area, are plottedagainst density. The homeostatic mechanismsimplied by this curve operate through the skewingof the weight frequency distribution. This, in turn,is controlled by the vertical pattern of leaf (orchlorophyll) distribution which develops as theindividual plants compete together for light.Homeostasis of tinal yield is achieved only byadjustments of the energy balances of the individ-uals within the population. If account is taken ofnet production and. even more so. of gross pro-duction, then it appears that high density standswill outstrip those at lower density as productive,though wasteful, systems (Fig. 7).

ACKNOWLLOOEMENTS

I wish to thank Sir William Roberts and the trustees ofthe Sir William Roberts Scholarship fund of the Univer-sity College of North Wales for providing a grant duringthe tenure of which the work reported in Figures 1 and 2and Appendix 1, was done. I also wish to thank Me J. A.Monro of Massey University who did most of the workreported in Figures 4, 5 and 6, and Appendix 2.

REFERENCES

DONALD, C. M. 1963. Competition among crop and pas-ture plants. Advance. Agron. 15: 1-118.

GRUBB, P. J., LLOYD, J. R., PENNINGTON, T. D., andWHITMORE, T. C. 1963. A comparison of montaneand lowland rain forest in Ecuador. 1. The foreststructure, physiognomy and floristics. l. Ecol. 51:567-601.

HARPER, 1. L 1967. A Darwinian approach to plantecology. J. Eeol. 55: 247-270.

HIRANO, S., and KIRA, T. 1965. Influence of autotoxicroot exudation on the growth of higher plants grownat different densities. (Intra-specific competitionamcng higher plants XII.) l. BioI., Osaka Cy. Univ.16: 27-44.

HIROI, T., and MONSI, M. 1964. Physiological andecological analyses of shade tolerance in plants.4. Effect of shading on distribution of photosyn-thesate in Helianrhus annuus. Bot. Mag., Tokyo77 (No. 907). 1-9.

HOZUMI. K., KOYAMA,H., and KlRA, T. 1955. Intra-specific competition among higher plants. IV. A pre-liminary account of the interaction between adja-cent individuals. l. Inst. Poly tech. Osaka Cy. Univ.Series D, 6: 121-130.

KOYAMA,H., and KlRA, T. 1956. Intraspecific competitionamong higher plants. VIII. Frequency distrib~tionof individual plant weight as affected by the mter-action between plants. l. Inst. Poly tech. Osaka Cy.Univ. Series D. BioI. 7: 73-94.

KUROIWA,S. I960a. Intraspecific competition in artificialsunflower communities. Bot. Mag., Tokyo 73: 300-309.

KUROIWA.S. 1960b. EcoloJical and physiological studieson the vegetation of Mt Shimagare. V. Intraspecificcompetition and productivity difference among treeclasses in Abies stand. Bot. Mag., Tokyo 73: 165-174.

LONG,F. L. 1936. Application of calorimetric methods tc.ecological research. Carnegie Inst. Wash. Suppl.Publ. No. 19,91-105. (Reprinted from: PI. Physiol.,Wash. 9: 323-337.)

MATHEWS, C. P., and WESTLAKE,D. F. 1969. Estimationof production by populations of higher plants sub-ject to high mortality. Oikos 20: 156-160.

MONSI, M., and SAEKI,T. 1953. Uber den Lichtfaktor inden Pflanzengesell-schaften und seine bedeutung flirdie Stoffproduktion. lap. l. Bot. 14: 22-52.

OBEID, M.,MACHIN, D., and HARPER, J. L. 1967. Influ~ence of density on plant to plant variation in Fiberflax Linum usitatissimum L. Crop Sci. 7: 471-473.

ODUM, E. P. 1963. Ecology. Modem Biology Series. Holt,Rinehart and Winston.

OGDEN, 1. 1966. Ordination studies on a small area oftropical rain forest. M.Sc. thesis. Univ. Wales.

OGDEN, J. 1968. Studies on reproductive strategy withparticular reference to selected composites. Ph.D.thesis. Univ. Wales.

OVINGTON,J. D., and LAWRENCE, D. B. 1967. Compara-tive chlorophyll and energy studies of prairie,savanna, oakwood and maize field ecc'3ystems.Ecology 48: 515-524.

.

SAEKI,T., and KUROIWA,S. 1959. On the establishmentof the vertical distribution of photosynthetic systemin a plant community. Bot. Mag., Tokyo 72: 27-35.

TAMM, C. O. 1948. Observations on reproduction andsurvival of some cerennial herbs. Bot. Notiser. 3:305-321.

-

TAMM, C. O. 1956. Further observations on the survivaland flowering of some perennial herbs. Oikos 7:273-292.

TEZUKA, Y. 1960. The influence of nutrients on thegrowth of plant populations under different densities.(Relations of plant communities to edaphic factorswith special reference to mineral nutrition. III.) Bot.Mag., Tokyo 73: 7-12.

WATSON, D. J. 1947. Comparative physiological studieson the growth of field crops. Ann. Bot. 11: 41-76.

ApPENDIX IAN ANALYSIS OF THE FREQUENCY DISTRIBU-TIONS OF INDIVIDUAL PLANT WEIGHTS IN AMIXED ANNUAL WEED COMMUNITY COLONIS-ING AN ARABLE FIELD IN NORTH WALES

1. Description of siteThe area investigated was a 6X6 fc.ot plot in mixed

but apparently homogeneous annual vegetation growingon an arable field at the experimental station of theSchool of Plant Biology at the University College ofNort~ ~ales, Bangor, Caernarvonshire, in August 1966.At this time the plant cover was composed of 17 species,

Total Meanbiomass biomass/plant

Species Density % Densily (g.) % Biomass (g.)Stachrs arvensis L. 342 36.9 31.2 27.2 0.09Stellaria media (L.) Vill. 222 23.9 21.5 18.7 0.10Spergula arvensis L. 89 9.6 23.5 20.5 0.26Polygonum persicaria L. ( 82 8.8 16.5 14.4 0.20Pol)'gonam lapathifolium L. \Graminae* 65 7.0 0.9 0.8 0.01Atriplex patula L. 42 4.5 3.6 3.1 0.09Senecio vulgaris L. 38 4.1 4.6 4.0 0.12

Pol.rgonum aviculare agg. 30 3.2 10.6 9.2 0.35Other spp. ** 17 1.8 2.4 2.1 0.14TOTALS 927 99.8 114.8 100.0 0.15

Sample Tradescantia Elatostema BromllsLocality Blcdisloe Park Kahuterawa vaHey. Massey University campus,

Palmerston North, N.Z. Tararua Range, N .Z. Palmerston Nc.rth. N.Z.Description of site Lowland forest remant Montane forest on steep Open grassland on

on flat by stream stony bank waste groundSample size 0.5XO.5 m. 1.0X1.0m. 0.5XO.5m.Number of samples 1 2 1

Ht. veg. sampled (em.) 50 I)U 72No. stratified clips 10 6 10Width of strata (em.). 5 20 8

8 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 17, 1970

mostly annuals (see Table 3) and was about 6 in. high.Most of the plants possessed some ripe seed, so that thesward as a whole might be considered mature. Althoughthe density was high, about ISO individuals per squarefoot seme bare soil was visible because of the generalpaucity of leaves at this time.

2. Methods

An area of 6 sq. ft was marked out within the plotand all individuals within it were removed with theirroots, and individually bagged. They were dried at 70"cfor three days before weighing. The species' data wereclassified into a variety of weight-class groups using acomputer programme which defined the number ofweight classes as max.-min.{n where max. and min. werethe heaviest and lightest plants respectively and n wasany s,Decified integer. In practice, the size of 11is depen-dent on the total population size (see Koyama and Kira.1956). This method of classification is arbitrary. butthere seems to be no justification for using a class intervalrelated to the standard deviation when the population isnot normally distributed (Fig. 1). Subsequently, the datawere logged and reclassified using a class interval deter-mined by 4 s /Il where s is the standard deviation and n

is any specified integer (Fig. 2). This formula gives foreach soedes, different class intervals and a differentnumber of classes.

3. ResultsDensity and biomass data for the whole community

sample are presented in Table 3. The weight-frequencydistributions of the most numerous species are presentedin Figures I and 2. All the species had approximately10J-normal weight-frequency distr,ibutions. In four orthe seven instances in whieh the logged data were testedfor departure from normality using the chiMsquare test.no significant (P < .05) departure from normality wasfound (Fig. 2).

ApPENDIX 2

STUDIES OF PIGMENT STRATIFICATION INNATURAL COMMUNITIES OF TRADESCANTIAFLUMINENSIS YELL., ELATOSTEMA RUGOSUMA. CUNN. PRECUR. AND BROMUS CATHARTICUS

YAHL.

1. Description of sampling sites and size of samplesSee Table 4 (below). The sampling was done during

December 1968.

TABLE 3. Densi(v and biomass of all species per 6 sq. ft. in an annual weed community.

* Mostly Poa anlllw L** Matricaria matricarioides (Less.) Porter. Capsella bursapastoris (L.) Medic. Fllmaria mllralis ssp. bawd (Jord.)Pugs!. Anagalis arvensis L Veronica officilwlis L Geranium molle L. Sonchlls oleracells L RWlIll1CIlIlls repens L.Taraxacum officillale agg.

TABLE4. Description of sampling sites and sample sizes.

TABLE 5. Biomass per sq .m.

SAMPLE Tradescantia Elatostema Bromus

Total fresh weight (g.jm.') 7135 10713 5426Total biomass dry wt. (g.jm.') 815 645 1155

(528*)% dry matter 11.4 6.] 21.3

OGDEN: PLANT POPULATION STRUCTURE AND PRODUCTIVITY

2. Methods

The area to be sam1Jled was marked out with tall pegsand the surrounding vegetation removed. Working fromthe to,> downwards the vegetation in the sample area wasclipped to successive heights using hand shears. Thematerial from each stratum was kept separate andweighed. Sub...sam!,Jes were taken from each stratum fordetermination of dry weight and chlorophyll. WithElatostema seoorate determinations were made for leavesand stems and a second m.2 sample was taken for heightand weight frequency distributions (Fig. 3). For chloro-phyll estimation sub-samples (0.5-1.0 g.) were groundwith pestle and mortar in about 20 rol. of 2: 1 methanol:petroleum ether mixture and the mixture was then :filteredinto a large separation funnel. Extraction was repeatedthree times, the extracts being added to the separationfunnel each time. The contents of the funnel were thenshaken with c. 40 ml. of saturated sodium chloride solu-tion 'and the aqueous methanol separated off and re-extracted with petroleum ether four times. The resultingpetroleum ether extract cO:1taining the chlorophyll was

9

dried with anhydrous sodium sulphate, and its opticaldensity at 665 mil read in an Eel 'Spectra' spectophoto-meter.

3. Results

Chlorophyll and dry matter profiles for the Tradescan-tia and Bromus communities are presented in Figures 5and 6. The results for Elatostema were similar to thosefor Tradescantia, but difficulty arose because the extrac-tion process removed suspended substances and pigmentsother than chlorophyll. Therefore the large total opticaldensity per unit area in the layer of creeping horiwntalstems might not arise entirely from chlorophyll. Totalbiomass weigbrts/m.2 are given in Table 5. With Bromusa considerable quantity of dead leaf bases, culms, etc.,was present in the lower layers of the sward and inTradescantia the sam Die included dead leaves and smalltwigs faUen from the-trees above.It is unwise to draw any generalisations from such few

data, but the remarkable contrast in succulence betweenthe Elatostema and Bromus stands should be noted.

... excluding dead matter and surface rootlets.