MILLER: STUDY OF ECOSYSTEMS 49

The following papers were included in a symposium on "Processes within Pasture andCrop Ecosystems" at the 1965 Conference.

THE STUDY OF ECOSYSTEMS

R. B. MILLER

Soil Bureau, Lower Hutt

The term "ecosystem" may be defined as"an open system comprising plants, animals,organic residues, atmospheric gases, water andminerals which are involved together in the flow ofenergy and the circulation of matter".

It is an all.embracing system and this givesit its greatest value---that it is treated as awhole, not as a number of separate parts.As papers on soils, papers on climate and

others on plants and animals, even if theypenetrate some way into neighbouring fields,do not add up to a symposium on ecosystemsI thought I would resist the inclination todiscuss soils and nutrient cycling, and insteaddeal more generally with the study ofecosystems. This might provide a wider per-spective and encourage discussion to rangemore freely.

SOILS AND ECOSYSTEMS

There is no great difficulty for a soil chemistto talk about ecosystems. The father of soilscience, Dokuchaev, sixty-five years ago wroteof the need for a new science ("an inde-pendent and honourable one") to study thecomplex correlations and interactions of sur-face rocks, the earth's sculpture, soils, waters,climate, vegetation, animals and men (Rode1962). He thought it would come soon andthat pedology would be the nucleus of it. Hewas wrong on both counts, as it took manyyears to emerge, and from its essence no oneaspect of it can be considered nuclear, butcome it certainly has. '

Dokuchaev well understood the interrelation-ships of soil and its environment but it wasnot until 1941 that Jenny defined the factorsof soil formation clearly in the equation

soil = f(cl, 0, r, p, t . . . .)

where cl = climate, 0 = organisms, r = relief,p = parent material and t= time. Withoutstraining definitions this could as easily beecosystem = f(cl, 0, r, p, t. . .) (Krajina 1961)In other words the forces moulding the soilare the forces moulding the ecosystem as awhole.

This is confirmed by our experience. Wheresoil is studied in relation to some factor of theenvironment the whole ecosystem needs to betaken into account. There seems little pointin studying the ability of soils to supply plantnutrients and ignoring animals dying ofhypomagnesaemia, or of studying fine detailsof nutrient cycles in beech forests and ignoringthe effects of moisture or the slow rates of treegrowth.

It is, then, a reasonable transition fromthinking in terms of soils to thinking in termsof ecosystems. The next stage also follows:

In soils one is familiar with the idea ofdefining soils in terms of processes, and indeed,soils are often more significantly expressed inthese terms than in terms of the soil itself.There are many examples of this in New Zea-land soil classifications and Jenny (1961) haswritten:

"Dynamics and energy flow per unit area ofland. . . will become as fundamental in conceptualimportance to soil science as are free energy andentropy in chemistry."

It is the same with ecosystems. As Ovington(1962) has suggested, processes seem likely togive more significant expression to theecosystem than any particular part of it, suchas trees.

ECOSYSTEM PROCESSES

There are so many processes going on inan ecosystem that one must start by sub-dividing them into groups. This is done withregret since they are all closely interrelated,but the advantages gained outweigh the dis-advantages, providing we remember that weare doing it purely for our convenience and thatthe ecosystem doesn't know about it!Four groups of processes as set out by

Ovington (1962) are:I The Production of Matter~OrganicMatter Dynamics

II The Flow of Energy - EnergyDynamics

50 MILLEn: STUDY OF ECOSYSTEMS

III The Flow of Water-Water DynamicsIV The Cycle of Nutrients-Circulation of

Chemical ElementsAnother one--the changes in the numbers ofplants and animals, or Population Dynamics-will be considered in the next section.The benefits of working in terms of processes

are tha t(a) the factor of time is brought in at an

levels-one studies rates, not quantities,(b) there is a clearly formulated framework

of thought which (while not restrictive)enables one to work on even small partsof the system and not lose sight of thewhole,

(c) the processes are sufficiently wen definedto allow quantitative measurement.

Organic Matter Dynamics

A forest ecosystem such as the hard beech(Nothofagus truncata) site being studied in theHutt Valley (Miller 1963) is a massive thing.It contains 125 tons/acre of plant biomassand 20 tons/acre of soil organic matter. How-ever, it is lovi' on the animal side with certainlymuch less than 100 lb.jacre.Compare this with a pasture ecosystem

which might have a plant biomass of less than1 ton/acre and a similar biomass of animals.If we consider processes rather than the

ecosystems themselves the differences are less.The forest each year adds a ton/acre of woodto thc trees and drops 3 tons of litter back tothe soil. The pasture produces a roughlysimilar amount of plant biomass, some of whichis retained in the animals and the rest returnedto the soil.Of the many measurements that might be

made the net primary productivity (the in-crease in biomass/acre/year) is possibly themost useful for, using it, ecosystems of allkinds may be compared factors limiting growthfound, improvements evaluated and, if nece'5-sary, the performance of the system expressedin economic terms. Other important aspectsinclude litter formation and decomposition,animal returns, pasture consumption by otherorganisms, the whole complex of seasonalchanges and the effects of management(Brougham, this volume).Changes in organic matter are governed by

the other groups of processes.

Energy Dynamics

The flow of energy from the sun and itsutilisation and dissipation in the ecosystem is

fundamental to ecosystem dynamics. Theenergy is used for evaporation of water andfor heating, some is reflected, some retrans-mitted and a very important fraction is trans-formed into chemical energy by photosynthesisand stored in plant tissues.

In forests much energy is stored in timberand when the timber is harvested this isimmediately available. In a pasture ecosystemthe plant material is eaten by an animalwhich in turn is eaten by human consumerswith very large (say 90%) losses at each stage.The efficiency of utilisation of energy is there-fore very much lower, aJthough not nearly aslow as in, say, a marine ecosystem where theenergy pathway may have many more steps.

As with other processes in ecosystems energyflow lends itself to quantitative treatment. Inour hard beech project measurements are madeof solar radiation, light reflected and trans-mitted by the canopy, energy content of thevegetation, respiration and photosynthesis, soilrespiration, litter breakdown, temperature-allof which help to build up the picture. Inpasture systems a great deal more work hasbeen done than in forests (Mitch en, thisvolume).

Water Dynamics

Thousands of tons of water move into eachacre every year from the atmosphere and areevaporated, transpired, run off or run throughthe soil into the drainage system. Water isan important carrier of nutrients, and a majoruser of energy. Again, the components of thecycle may be measured. There is an upsurgeof interest in this kind of work with theapproach of the International HydrologicalDecade and the projects to be undertaken inNew Zealand should make a real contributionto ecosystem studies.

At the Taita Experimental Station we areat present designing two flow recorders forpasture catchments to compare water flowswith those occurring in native forest and exoticforest catchments. These and associated equip-ment will supply information about the largescale flow of water-precipitation, evaporation,transpiration and run off.

For pasture studies, of course, much moreis needed than this. In particular measure-ment of water movement within the soil andwithin the pasture canopy is essential. Somework of this kind is described elsewhere in thissympoSIum.

MILLER: STUDY OF ECOSYSTEMS 01

Circulation of Minerals

To a soil chemist mineral circulation is ofparticular interest. Its study involves estimat-ing (i) the quantities of plant nutrients sup-plied by the soil to plants and returned to it byplants, (ii) the contribution from the atmos-

phere, (iii) from weathering and (iv) thelosses to drainage. As this kind of work goeson it becomes more and more obvious that itis as impossible to study soils without the restof the ecosystem as it would be for, example,to study aerial parts of plants but not the rootsand soil.In studying the circulation of minerals the

forest ecologist has the advantage over thepasture ecologist in that the return mechanismis much more clearly defined. In our beechforest site by measuring the movement ofelements from the atmosphere in leaf wash (therainwater reaching the forest. floor) and litterfall, in the trees, and in drainage water out ofthe soil, we are starting to learn something ofthe needs of plants and of the capacity of thesoil to supply them.In pasture, of course, this kind of study pn'-

sents real problems. It is, for example, 60years since the importance of leaf wash in thechemical composition of pasture was shown (IeClerk and Breazeale 1908) but so far, I do notthink there have been any field measurements.When the grazing animal is included theproblems become even more complex. How-ever, they are being tack]ed. A great deal canbe learned, for example, from regular samplingand analysis of pasture and soils.Saunders et al. (1963) have shown that

although Taranaki yellow-brown loams arequite capable of supplying sufficient phosphorusfor pasture during the spring flush the supplymust be supplemented to keep growth rateshigh in other periods.In Hawkes Bay Metson (pers. comm.) has

found that different soils have differentcapacities for maintaining element balances inpasture. In places, spring and autumn pasturesmay have abnormally high levels of someelements and low levels of others, so much sothat they appear at times to be implicated inthe ill health or even death of stock.This sort of work emphasises the importance

of current interest in pasture quality. Foroptimum growth animals as well as plantsrequire food of the right composition through-out the year. This is particularly so in timesof stress such as lactation. Then the soil, also,

is under stress to supply rapidly growing plants;and limitations are likely to make themselvesfelt. The ecosystem must be considered in allits aspects and element movements thoroughlyunderstood so that imbalances may be correctedby additions of nutrients to the cycles at theright time and in the right amounts.Another aspect of the mineral cycle was

discussed by Healy at this meeting. Referencemight also be made to a study in which mea-surements were made of both chemical andbiological aspects of a pasture developmentsequence after scrub burning (Miller, Stout andLee 1955-1962). This showed that the unpro-ducti ve energy released on burning was accom-panied by a large and very productive releaseto the mineral cycle of large amounts of solubleplant nutrients. At the same time there weresubstantial changes in the soil fauna and micro-flora and these continued to change as the newpasture ecosystem was established. One thingwas clear and that was that the soil biologyand chemistry were closely related to eachother and to the changing vegetation and thatstudy of only one aspect would have given avery incomplete picture of the changes takingplace.

THE ECOSYSTEM ITSELF

What is present?

Although ecosystem processes, like soil pro-cesses, have proved a fruitful field of study,the ecosystem itself-the soil, the plants andthe animals that are there-must also be takeninto account. As Southern (1965) has written:

". . . the smoothed picture of energy flow Hndproductivity studies gives no indication of the com~plexity and pattern of a community's organisationand, therefore, no information about the componentsof the production and the paths through whichenergy flows, nor about their reactions to inter-ference or exploitation".

How is it changing?

eonsideration of the stability of ecosystems,or of any changes in them that come aboutnaturally or by management, introducesanother facet, that of population dynamics. Atany given point in time it is "what is present"that is significant, but over a period changesin numbers and species of plants and animalsmust be taken into account.In a managed pasture ecosystem with changes

occurring all the time in animals and plants,studies of population dynamics are essential iffull understanding is to be achieved. And it is

52 MITCHELL: VEGETATION STRUCTURE AND ENERGY EXCHANGE

not only changes in the main components thatare important: not only should earthwormsand grass grubs be studied as well as sheep, butweeds are involved in the system as well asgrass and clover.

CONCLUSIONS

a. For studies of pasture production and itsutilisation the ecosystem is the natural unit ofstudy.

b. A particularly significant expression of theecosystem is found in ecosystem processes.

c. However, the nature of the soils and thespecies of plants and animals present in theecosystem are also ilnportant.

d. So are the changes that take place in thenumbers and species of organisms present.

e. These three, the ecosystem itself, thedynamics of its populations and the rates of itsvarious processes provide a conceptual frame-work which allows the whole system to bekept in view while detailed studies are carriedout.

£. The compenents and processes may bemeasured quantitatively and it is possible totake into account not only natural changes butalso changes introduced by man.

STRUCTURE OF VEGETATION IN RELATION TO

ENERGY EXCHANGE

K. J. MITCHELL

REFERENCES

JENNY, H., 1941. Factors of soil formation. McGraw Hill& Co., New York.

JENNY, H., 1961. Comparison of soil nitrogen and carbonin tropical and temperate regions. Univ. Miss. ColI.Agrie. Res. Bull. 765: 5-31.

KRAJINA, V. 1., 1961. Ecosystem classification of forests:

A 5UI11111aI'Y.Rec. Adu. Bul. II, 1597-1603.

LE CLERK, J. A., and BREAZEALE, J. F., 1908. Plant foodremoved from growing plants by dew and rain.U.S. Dept. Agrie. Yrbk Agrie. 389-408.

MILLER, R. B., STOUT, J. D., and LEE, K. E., 1955-62.Biological and chemical changes following scrubburning on a New Zealand hill soil I-IV. N.Z. J.Sei. Teeh. B 37, 290-313, N.Z. J. Sei. 2, 17t-81,4,740-'>2, 5, 259-68.

l\thU.ER, R. B., 1963. Plant nutrients in hard beech, III.The cycle of nutrients. N.Z. J. Sei. 6; 388-413.

OVINGTON, J. D., 1962. Quantitative ecology and thewoodland ecosystem concept. Adv. Ecol. Res. 1:103-203.

RODE, A. A., 1962. Soil Science. Israel Program forScientific Translations, Jerusalem.

SAUNDERS,W. M. H., TAYLOR,W. n., and GOLD, B.,1963. Residual effect of phosphate topdressing on ayellow-brown loam from andesitic ash. N.Z. J. Agrie.Res. 6: 484--507.

SOUTHERN, H. N., 1965. The place of ecology in scienceand affairs. Proc. N.Z. Eeal. Soc. 12: 1-10.

Plant Physiology Division,Department of Scientific and Industrial Research,

Palmerston North

New Zealand has a proud ecological history.Particularly as far as our vegetation is con-cerned, ecology has gone through phases similarto those in most countries of the world. Thepioneers described the general patterns of thecommunities and set in perspective the picturefor the country as a whole. Then came moredetailed description of particular situations, andthe introduction of quantitative measurementtechniques. The latter allowed standardiseddefinition of what is present at a site andmeasurement of differences between sites. Theresult has been a generally satisfactory answerto the question of "what species are presentand what trends are occurring"?

Next came the question of why certainspecies are present and why the trends areoccurring. It is with the treatment of thesequestions of "why" that vegetation ecology,and probably many other branches of ecology

as well, has become notable for its imaginativespecula tion and for the low quota of hard factson which those speculations are based.Remembering Kelvin's dictum that an issuedoes not become science until you can measureit, we have here a reason for the relativedecline in the status of plant ecology, withinthe scientific community within the last 30years.