Neither Wild nor Planted: Essential Role of Giesta …oldsite.econbot.org/webmaster/factoids/10_wild_nor...Role of Giesta (CYTISUS, Fabaceae) in Traditional Agriculture of Beira Alta,

For the past many hundreds of years, agri-cultural technology in Europe changed verylittle until the early 20th century, when tech-nology was developed that consumed largeamounts of (fossil fuel) energy to break thestable bond of N2 to transform atmospheric ni-trogen into ammonia and nitrate. This technol-ogy soon came to be used to manufacture ni-trogen fertilizer, the availability of whichstimulated the development of new varieties of(especially) cereal crops that could use largequantities of soil nitrogen. Machinery and

agro-chemicals to complement this high-energy fertilizer and the new crop varieties thatrespond to it soon followed. By the third quar-ter of the 20th century, yields had increaseddramatically, but in many cases so had contam-ination and erosion of soil, and pollution of theair, the water, and even the crop itself. In somecases, this new technology is not sustainablebecause it depletes or contaminates the soil andwater on which it depends.

By the last quarter of the 20th century, manyagronomists and other scientists had becomeaware of the need to address this situation. Amore ecological approach to agricultural re-search was initiated in which sustainability was

Neither Wild nor Planted: Essential Role of Giesta(Cytisus, Fabaceae) in Traditional Agriculture ofBeira Alta, Portugal1

George F. Estabrook

Estabrook, G. F. (Department of Ecology and Evolution, The University of Michigan, AnnArbor, MI 48109; e-mail: [email protected]). Neither Wild nor Planted: EssentialRole of Giesta (CYTISUS, Fabaceae) in Traditional Agriculture of Beira Alta, Por-tugal. Economic Botany 60(4):307–320, 2006. Giesta (Cytisus) is the dominant or onlywoody plant on about one-quarter of the land area of the high granite mesas of interiorBeira Alta, Portugal, where for at least the past 800 years the main cereal crop has been rye.During this time, a substantial fraction of this crop has been sold and taken to cities to feedthe urban population, carrying with it nitrogen in the proteins of the grain. These farms haveremained sustainable for nitrogen in the face of this outflow because farmers have allowedand even encouraged giesta, which is a nitrogen fixer, to occupy a substantial fraction of theland area, from which it is harvested and buried in the cultivated soil to restore its fertility.Stable N15 analysis and ecological techniques estimate quantitatively the importance of thispractice.

Nem bravia, nem semeada: A função indispensãvel da Giesta (Cytisus, Fabaceae) naagricultura tradicional da Beira Alta, Portugal. As giestas (Cytisus spp.) são asplantas lenhosas dominantes em cerca de um quarto das áreas montanhosas graníticas do in-terior, nomeadamente na Beira Alta (Portugal), onde durante os últimos 800 anos, o principalcereal cultivado era o centeio. Nessa altura, uma fracção substancial deste cereal era ven-dida e enviada para as cidades, para alimentar a população urbana, transportando consigonitrogénio nas proteínas dos grãos. Estes campos cerealíferos mantinham um fluxo susten-tável em nitrogénio, porque os lavradores permitiam e até auxiliavam o crescimento das gies-tas, que sendo fixadoras de nitrogénio e ocupando uma grande parte dessa área, cortavam-nas e enterravam-nas no solo de cultivo, mantendo, assim, a respectiva fertilidade. Análises“Stable” N15 e técnicas ecológicas confirmam quantitativamente a importância destaprática camponesa.

Key Words: Nitrogen, soil fertility, rye, stable isotope, sustainable agriculture, traditionalecological knowledge, Cytisus, Portugal.

Economic Botany, 60(4), 2006, pp. 307–320.© 2006, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.

1Received 27 March 2006; accepted 14 June 2006.

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included explicitly as an objective (Altieri1987; Gliessman 1990; Loomis and Conner1992). Some researchers became aware that tra-ditional agrarian communities contain valuableecological knowledge (Hunn 1999), which isoften expressed in cultural rather than scientificterms (Estabrook 1994). The study of a tradi-tional, solar energy-based technology that hasbeen practiced sustainably for hundreds ofyears in the same place may help us access andpreserve traditional ecological knowledge andcultural diversity (Hunn 2001), and provide un-derstandings of the natural world that may con-tribute to healthier and more sustainable mod-ern agricultural technology.

One approach to the study of sustainableagriculture is to investigate a traditional agricul-tural system that has produced for hundreds ofyears without depleting the resources on whichit depended. This paper reports the essentialrole played by two apparently wild species ofCytisus (Fabaceae) and the way they are man-aged in the traditional agriculture of interiorBeira Alta, Portugal, to achieve sustainability ofnitrogen soil fertility over the hundreds of yearsthis region has served as a “bread basket” forthe populous area of northern interior Portugalnorth of the Serra da Estrela and south of theDouro River.

An earlier quantitative study of nitrogen fluxin traditional agriculture is reported by Loomis(1978). He made very plausible estimates ofcompartment sizes and flux rates based on hisconsiderable experience in ecological agricul-ture. This work remains a useful landmark tocompare and contrast nitrogen pools and fluxesunder more modern technology. Loomis andConner (1992) in Fig. 8.2 show a generalizednitrogen flux diagram for a modern agriculturalsystem, and in Fig. 8.5 give estimated fluxes fora traditional agricultural system based on thework of Loomis (1978). In the farm analyzed inthat work, only minor roles are played by nitro-gen fixation and the maintenance of shrub landsfor the main purpose of bedding animals keptprimarily to maintain soil fertility.

Nitrogen flowed out of the agro-ecosystemsof the farming villages distributed over the highgranite mesas of interior Beira Alta with the ryegrain that was carried away to Viseu, Lamego,Guarda and other smaller cities nearby. Giestasare nitrogen fixers, i.e., the roots of the giesta

308 ECONOMIC BOTANY [VOL. 60

accept Rhizobium to form nodules, where nitro-gen from the air is made biologically available,using energy from the sun captured by thestems and leaves of the giesta. Thus much ofthe nitrogen in the giesta twigs and branchesthat are buried in the cultivated soil to restoreits fertility has recently entered the biospherefrom the atmosphere. This replaces the nitrogenin the protein of the rye seeds carried away tofeed urban populations. Although traditionalfarmers in these villages had no concept of ni-trogen or nitrogen fixation, their traditional eco-logical knowledge and culturally informedtechnology have instructed them to keep aboutone-quarter of the land area that participates ina village agro-ecosystem covered with what ap-pears to be wild, but is in fact deliberately man-aged, giesta.

The results reported here arise from a studyof traditional farming practices of Beira Alta,in the mountainous interior of Portugal, locatedas shown on the map of Fig. 1, where rye hasbeen produced using basically the same tech-nology for at least 800 years (Raposo 1994),even though the New World crops, corn andpotato, joined the rotations in the mid-17thcentury, and the land reforms of the early 19thcentury increased the number of resident own-ers (Oliveira 1980). A 19th century account ofagricultural technology in Beira Alta is givenby Almeida (1882). Two or more generationshave passed since the advent of agriculturaltechnology based on large inputs of fossil-fuelenergy in America and Europe, where few liv-ing people have even witnessed the practice oftraditional agricultural technology. However,during the middle half of the 20th century, Por-tugal was ruled by a dictator who did little todevelop its interior (Bruce 1975). During thelast quarter of the 20th century, traditional agri-culture was still practiced, although by fewerand fewer people, in the mountainous interiorof Portugal. As a consequence of the economicdecline of this region, younger people movedaway and unchecked fires destroyed much ofthe forest cover (Damaso 1992). Most of theolder people who remained just retired fromagriculture instead of replacing traditionaltechnology with modern. Thus, much of theland and equipment, although no longer in use,has been left as it was, and many people stillremember the technology. For these reasons,

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the mountainous interior of Portugal is a goodplace to study traditional Portuguese agricul-ture.

Over the past two decades, I have studiedthe traditional agriculture of this region, livingthere for periods of from a few weeks to sev-eral months during 1984, 1987, 1991, 1993,1996, 1997, 2002, and 2003, talking to life-long residents and observing their practices,and sampling soil, plant, and animal materialto measure quantitatively the effectiveness ofthese practices. For example, Estabrook(1998) described how some of the techniquesof traditional agricultural practiced on theshale hillsides of the parish of Cabril, Portu-gal, maintained the fertility of their cultivatedsoil.

In this paper, I will describe how traditional

2006] ESTABROOK: BROOM IN TRADITIONAL PORTUGUESE AGRICULTURE 309

farmers on the granite mesas and valleys ofBeira Alta manage and use giesta to maintainthe fertility of their cultivated soil, and then es-timate quantitatively the rate at which theseplants and practices bring new atmospheric ni-trogen into the traditional agro-ecosystem.These estimates use two kinds of techniques:those of plant ecology to measure primary pro-ductivity of giestas; and those of mass spec-trometry of stable N15 isotopes to measurepercent total nitrogen in giesta, and to estimatethe fraction of that nitrogen derived from theatmosphere. These quantitative results willdemonstrate clearly the importance of the tra-ditional practices that manage giesta in main-taining the nitrogen sustainability of the agro-ecosystems of Beira Alta over the pastcenturies.

Fig. 1. Beira Alta region of Portugal in the high divide between the Mondego and Douro river drainages.Shaded rectangle of insert shows location within the Iberian peninsula. District capitals shown with large solidcircle. County seat of Trancoso shown with small solid circle. Parish centers of Sebadelhe and Aldeia Novashown with small open circles.

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Management and Use of Giestaby Traditional Farmers

Two species of giesta commonly occur on thegranite mesas and upper erosion valleys of BeiraAlta: giesta branca (Cytisus multiflorus [L’Her.]Sweet); and giesta negral (C. grandiflorus [Brot]DC.). These giestas are bushes from about 0.5 to3.0 meters tall, with long, flexible photosyn-thetic stems that keep their leaves for only a fewmonths in the year. Because of their physical ap-pearance, they are often called “broom.” In Por-tugal, an area dominated by giesta is called a gi-estal. Giesta branca is allowed/encouraged togrow where it would be difficult to plow be-cause the soil is rocky and shallow above thegranite bedrock. Large areas dominated by gi-esta branca include grasses among these shrubs,but rarely any other woody plants. Sheep grazethe grass and farmers cut the giesta to bed thesheep when they are housed in stone sheltersduring the cold, rainy winter.

Giesta branca is cut just above ground level.Farmers allow four years for a cut giesta branca


to regenerate before they cut it again, when ithas reached 70 to 80 cm in height. In the sheepshelters, the giesta is enriched with sheep ex-crement. It is removed and replaced about oncea week. The resulting organic matter is called“estrume” by Portuguese farmers, who trans-port it to their cultivated fields and bury it in thesoil at the rate of 8 to 15 metric tons dry weightper hectare to restore its fertility before plantingrye. Giesta branca produces relatively fewseeds, investing instead in vegetative spread andregeneration. I have looked carefully in all sea-sons of the year and never found a naturally oc-curring giesta branca seedling.

Giesta negral grows around the edges of cul-tivated rye fields. In April and May, its copious,large yellow flowers are conspicuous. These de-velop large numbers of seeds, which are re-leased toward the end of the dry summer. Afterabout six years of cultivation, a rye field is al-lowed to remain fallow. At this time, seeds fromthe surrounding giesta negral, which are abun-dant in the rye field, germinate and grow. Thehabitat of these seedlings is fertile relative to

Fig. 2. Farmers in Sebedelhe parish center, county of Trancoso, Beira Alta, Portugal, transporting estrume(composted giesta with sheep excrement) to cultivated fields in a cart drawn by two cows, yoked by theirhorns.

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that of giesta branca, and the seedlings growrapidly. By their third year they are about 1.5meters tall and producing a few flowers. By thefourth year they may exceed two meters andflower abundantly. Then, in the middle of thedry season in July, they are all cut just aboveground level and removed. Their side branchesare cut off and used, along with the branches ofgiesta branca, to bed sheep and make organicmatter to bury in the soil. The main stems mayhave reached a diameter of up to 5 or even 6 cmnear the ground. These are dried and used tofire the communal bread ovens. Cut giesta ne-gral does not regenerate. During the last coupleof months of the dry season, its roots die andare removed with a large hoe before the field isenriched with estrume and plowed at the onsetof the rainy season when rye is again sown.

Another shrubby nitrogen fixer, called pilro(Genista cineria [Villars] in Lam DC.) grows inwetter areas, such as seeping ground nearsprings or on banks of streams, but occupiesvery little area compared with giestas. Becauseof its high resin content, its primary use is tostart cook fires.

This use of giesta has long been a part of thetraditional technology in Beira Alta. Althoughits role in soil fertility maintenance is essential,farmers do not describe and explain its use forsoil fertility, but instead describe its use in thecare of their animals. Farmers’ explanations donot evoke agro-ecological concepts, which onlyrelatively recently have come to be well under-stood by agronomists and ecologists. These sci-entific explanations serve a scientific purpose.However, the preferred explanations of thesetraditional farmers help them remember the de-tails and timing of their practice, while placingthe real purpose outside the realm of their re-sponsibility, eliminating the need for a boss ordecision-maker to implement it, and protectingit from question during years when yields arelow. Although the effectiveness of traditionalknowledge can be evaluated using modern eco-logical concepts and technology to measure theconsequences of traditional practice, these cul-tural explanations help to encode traditionalecological knowledge and to inform its practice(Estabrook 1994).

MethodsTo estimate the rate at which new, atmo-

spheric nitrogen becomes biologically available


in this traditional system, I measured the rate atwhich giestas grow (primary productivity), howmuch of giesta dry weight is nitrogen, and thefraction of that nitrogen that had been fixedfrom the atmosphere. This estimated amountper unit area can be applied to the amount ofarea maintained in giesta, and compared withthe area under cultivation.

Primary productivity. Because pilro makessuch a small fraction of the nitrogen fixers man-aged by the farmers in a village, I estimatedproductivity only for giesta branca and giestanegral. In conversation with residents of thetown of Sebedelhe da Serra, in the Concelho ofTrancoso (located on the map of Fig. 1), Ilearned that the town has a common hillsidewhere giesta branca can be harvested by dayworkers (who do not own or rent plow-land buthave a small door-yard vegetable patch), and Ireceived permission to cut a small area. Peopleharvest giesta branca after it has been regener-ating for about four years, so in an area whereothers had recently been harvesting giesta, Iclear cut at ground level (as do the residents) anarea 2 m × 3 m. I weighed the cut giesta with ahand-held spring scale, selected a representativehalf-kilo, including the full spectrum of stemdiameters from about 1.5 cm down to 2 mm,and sealed it in a bag, which I took with mewhen I returned to Coimbra a few days later. Inthe plant physiology laboratory of the BotanyDepartment of the University of Coimbra, Iweighed, then oven-dried, then reweighed thesample to estimate the fraction dry weight. Toestimate annual primary productivity, I multi-plied this fraction by the wet weight of the gi-esta removed from the 6 square meter area Ihad cut and divided by the four years it hadbeen growing there.

Giesta negral does not regenerate but growsfrom seed spontaneously in fallowed rye fields.Antonio Sovral Dias, one of the farmers whowas still actively working his land in Sebedelheda Serra, was about to remove some giesta ne-gral from some land he had not cultivated forjust over five years. He collaborated with me toestimate the productivity of this giesta. He usedan axe to cut just above the ground the approxi-mately 5 cm diameter stems of these 3+ metertall bushes. When he had worked for about 45minutes, he allowed me to measure the area hehad cleared. The cut giesta was arranged inseven piles, each small enough to be weighed

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with the hand-held spring scale. About a kilosample, with the full spectrum of stem diame-ters from 5 cm down to 2 mm, was sealed in abag and used as described above to estimatefraction dry weight. Giesta negral germinatesafter the rains start in late October, so in thiscase the time this giestal had been growingfrom seed could be determined, within a monthof accuracy, at five years and six months.

How much new nitrogen? Agronomists havelong recognized the importance of measuringquantitatively the rate of biological nitrogen fix-ation under natural conditions. Hardy et al.(1968) described an assay based on the reduc-tion of the triple bond in acetylene to form thedouble bond in ethylene to measure N2 fixationunder field conditions. Although it required thetransport of heavy and awkward equipment intothe field and several hours to use it, this acety-lene reduction technique was widely and enthu-siastically used during the 1970s. Now its usehas been abandoned for a variety of reasons, ofwhich the most decisive was the developmentof more convenient, accurate, and efficient tech-niques using stable 15N isotope analysis withmass spectrometry.

Essential to the development of these newtechniques was the discovery (Marioti 1983)that the ratio, a = [15N]/[14N], of the concen-trations of 15N and 14N stable isotopes of ni-trogen in air is nearly constant at a = 0.0036765anywhere near the surface of the earth. A massspectrometric analysis measures the ratio, s =15N(sample) / 14N(sample), of the 15N and14N stable isotopes of nitrogen in a sample,compares it to a, the ratio in air, and expressesthe result as


that are the result of chemical transformationsthat take place in living plants and animals, orafter organic matter leaves living plants andanimals. On the ground, in the animal beddingor later within the soil, chemical transforma-tions continue, many mediated by fungus andmicrobes (Harris 1988). Often, these reactionsproduce ammonia and dinitrogen, which enterthe air as gasses. Some of the products that re-main in the soil are leached into ground or sur-face water, others are taken up soon by plantroots, and others become part of the soil humus.These chemical transformations often have apreference to bring forward into the product rel-atively more of one or the other of the stableisotopes of nitrogen, with the result that thestable isotope ratio of the nitrogen in soil or-ganic matter varies from one area to another,and can be quite different from the nearly con-stant ratio for nitrogen in air.

Chalk and Smith (1994) compare Delta val-ues for nitrogen in tissues of two crops, one butnot the other a nitrogen fixer. Plants of the twocrops were grown close together so that theirroots might exploit the same soil. It is assumedthat the nitrogen derived from the soil by plantsof either crop comes from the same pool of soilorganic matter and thus has the same Deltavalue. Under this assumption, the Delta valueof the nitrogen in the tissues of the nitrogenfixer is a mixture of nitrogen with a Delta valueequal to that of the non-fixer plus nitrogen witha Delta value of zero for the nitrogen biologi-cally fixed from the air. Chalk and Smith (1994)then used these Delta values, together withother considerations, to estimate the fraction ofthe nitrogen in a nitrogen-fixing plant that camefrom the air. Alves et al. (2000) confirmed thepotential accuracy of this approach by carefullymeasuring all factors under controlled labora-tory conditions. Boddy et al. (2000) review re-cent studies that apply this basic approach to ni-trogen fixation by woody perennials, and Lajthaand Michener (1994) review a variety of appli-cations of stable N15 technology in ecologicalstudies.

To apply stable N15 technology to the pres-ent study, I looked for a giesta growing so closeto a non-nitrogen-fixing plant of similar sizethat that their branches intertwined. For eachplant, many small amounts of the leaves andstems were taken from a number of differentplaces and combined to make a sample. Figure

Delta is a measure of the fraction of nitrogenin organic matter that is heavy nitrogen relativeto that fraction in air; Delta will be positive iforganic matter has a higher fraction of heavy ni-trogen than air. In this study, values for Deltafall between −3.0 and 6.0.

If chemical nitrogen fertilizer has not beenadded to soil, then organic matter in soil pro-vides virtually all nitrogen to most non-nitrogen-fixing plants. The dynamics of soil or-ganic matter (Jenkinson 1988) and its nitrogenturnover (Groot et al. 1991) are complex. In thissoil organic matter, nitrogen occurs in molecules

Delta(s) � 1000 * (s � a) / a (1)

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3 pictures a giesta branca with such a neighbor-ing sargaco negro (Cistus monspeliensis L. Cis-taceae) that were sampled in this way. Eachsample was dried at 60 C, ground and homoge-nized, and then Delta values were determinedby mass spectrometry at Laboratorio de Isoto-pos Estaveis at the Faculdade de Ciencias of theUniversity of Lisboa, Portugal. Mass spectrom-etry also measured the fraction of dry weight ofeach sample that was nitrogen.

Fraction of nitrogen from atmosphere. To es-timate the fraction of the nitrogen in a giestathat came from air, using the Delta values re-ported by mass spectrometry analysis, I rea-soned as follows. Denote with b the fraction ofthe nitrogen in the tissues of the nitrogen-fixinggiesta that was derived from the soil. Denotewith f the ratio of 15N/14N for the nitrogen inthe tissues of the nitrogen-fixing giesta, andwith n the ratio of 15N/14N for the nitrogen inthe tissues of the non-fixer. We assume that theroots of the nitrogen fixer are sampling the samesoil organic matter as are the roots of the non-fixer. Thus, we assume that n is also the ratio15N/14N for the soil derived nitrogen in the ni-trogen fixer.


Observe that f/(1+f) is the fraction of total ni-trogen in the nitrogen fixer that is heavy. Simi-larly, for the non-fixer that fraction is n/(1+n),and for air that fraction is a/(1+a). The fractionof nitrogen in the fixer that is heavy is a mixtureof the fraction in the soil and the fraction in theair, in which we weight the soil fraction by band the air fraction by (1−b), as shown.

(2)f / (1+f ) � b * (n / (1+n))+ (1�b) * (a / (1+a))

Fig. 3. Giesta branca (Cytisus multiflorus) growing right next to sargaco (Halimium lasianthum) sampledfor stable nitrogen isotope analysis by mass spectrometry, county of Trancoso, Beira Alta, Portugal.

Solving for b gives b = ((n+1) / (f+1)) * ((f−a) / (n−a)).

Now, consider the quotient (n+1)/(f+1). Inthe results reported below, n never exceeds0.005 and f never exceeds 0.0025; thus thisquotient is between 0.992 and 1.007, which dif-fers from 1.0 by far less than the proportionalvariation in repeated mass spectrometric analy-ses of the same sample. So we set this quotientto 1.0 and estimate b = (f−a) / (n−a).

However, the sample analyses are expressedin Delta units, so it would be convenient to ex-press the estimate of b with Delta units.

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Delta(s) = 1000 * (s − a) / a (3)

which implies

(s − a) = a * Delta(s)/1000

for any sample, s. Substituting f for s gives

(f − a) = a * Delta(f)/1000

and substituting n for s gives

(n − a) = a * Delta(n)/1000

Now, b = (f−a) / (n−a) implies that


was chosen to estimate the rates of nitrogen fix-ation by giesta. Although SS is regularlygrazed, the giestas occur fairly densely as doesthe bushy non-fixer sargaco negro, the perennialforb leiteira (Euphorbia characias L. Euphor-biaceae), and the less common nitrogen fixerpilro. At a distance of 4 or 5 kilometers fromthe village center, on the higher exposed ridges(altitude 900m) above Quinta Baralhada (anisolated farm), another giestal, designated QB,not presently very actively grazed but whose gi-esta and pilro are still harvested for fuel or bed-ding, was also chosen to estimate the rate of ni-trogen fixation of pilro, using sargaco branco(Halimium lasianthum [Lam] Spach) that growswith pilro on wetter soils.

Another giestal sampled for this study is inthe parish of Aldeia Nova (altitude 460m). Thisgiestal, designated AN, is actively managed bya farmer with sheep who, during the past 22years, has regularly cut giesta from this giestalto make sheep bedding, which he ultimatelyburies in his cultivated fields. This giestal has agreat deal of frequently grazed grass cover overwhich grow well-spaced giestas. No non-nitrogen-fixing woody shrubs grew here, soplants of the non-nitrogen-fixing perennialforbs, rosmaninho (Lavandula stoechas L.Menthaceae) and dedaleira da serra (Digitalisthapsi L. Scrophulariaceae) growing in giestalAN in close physical association with giestaswere sampled instead.

To complement data from locations activelymanaged with traditional technology, I tooksamples of giestas growing in places wherethey were no longer actively managed, wherenon-fixing close neighbors of comparable size,age, and woodiness had been allowed to grow.During the past decade, a giestal of giesta ne-gral has been growing unmanaged on an aban-doned rye field about 5 kilometers south westof Trancoso. These plants have reached heightof about 2 meters, as have closely associatedplants of urze (Erica arborea L.). Samples fromthis Trancoso site are designated TR. Sampleswere taken from a similar old giestal near thevillage of Mende Gordo (designated MG),about 8 kilometers east of Sebedelhe da Serra,which contains large urze the size of nearby gi-esta negral. Samples were also taken a few kilo-meters south near the village of Terrenho, des-ignated TE, from a six-year-old giesta negraland a nearby flourishing sargaco negro in ayoung pine plantation planted seven years ago;

(4)b � (a * Delta(f)/1000) / (a * Delta(n)/1000)

� Delta(f )/Delta(n)

Thus, we use the quotient Delta(f)/Delta(n)to estimate the fraction of the nitrogen in thetissues of a giesta that was derived from the soilorganic matter.

It is desirable to estimate fixation rates of gi-estas growing under conditions resemblingthose of the traditional agriculture of BeiraAlta. However, giesta usually occurs in the tra-ditional farming systems of Beira Alta as densemonocultures of giesta negral growing rapidlyfor a short time on fallow rye fields, or asdense monocultures of giesta branca growingfor many years as a regenerating, rhizomatousperennial on rocky infertile slopes and hilltops.In these two vegetation types, nitrogen fixationrates could not be measured because the gies-tas are so dominant that no plausible non-fixerwas ever found thriving in close physical prox-imity. Occasionally a giesta is found growingnear enough to entwine branches with anotherlarge non-nitrogen-fixing bush, which can besampled to apply the technique describedabove to estimate nitrogen fixation. Some-times, the only relatively large long-livedplants found growing near giestas are perenni-als, which regenerate their aboveground partsannually from underground storage organs.When nearby bushy non-fixers were not avail-able to estimate nitrogen fixation rates, I sam-pled nearby perennials whose root systemsseemed to be exploring some of the same soilas the nitrogen fixer.

Samples for this study were taken from anumber of actively managed locations in theConcelho of Trancoso, District of Guarda, inBeira Alta, Portugal, shown on the map of Fig.1. In the parish of Sebedelhe da Serra, at 860 maltitude, giestal, designated SS, currently ac-tively grazed and harvested for sheep bedding,

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these samples are near the usual age whenfarmers cut them for animal bedding and cookfires. Samples were also taken a few more kilo-meters south near the village of Castanheira,designated CA, in a wet, sandy meadow nearthe headwaters of the Rio Teja co-dominated bygiesta branca and sargaco branco. Voucherspecimens for all taxa sampled have been de-posited in MICH, the University of MichiganHerbarium, Ann Arbor, Michigan, USA. Theirnames are presented all together in Table 1.

I took two approaches to estimating the areaof giestal relative to the area under cultivation. Iobserved and conversed with three farmers whowere still actively managing their land; AntonioSovral Dias, his cousin Jose of Sebedelhe, andManuel Andrade of Aldeia Nova. In 2002/3,when I did these field studies, only a few tradi-tional farmers were still active. The Diascousins, but especially Antonio, were able tomake credible estimates of land areas used indifferent ways by their uncle Manuel Dias, whoin about 1940 operated the large Quinta Baral-hada, a few kilometers from the village ofSebedelhe da Serra. On many occasions Iwalked with Antonio Dias over the lands of theQuinta Baralhada. I could see and measure ap-proximately how much area Manuel Andradeactively cultivated, so by measuring the rate atwhich he harvested giesta to bed his sheep, andusing the rates of primary productivity of gies-tas, I could estimate the minimum amount ofarea in giesta required to sustain that harvestrate.

In addition, as part of a national rural landsurvey for tax purposes, the government of Por-tugal sent teams into the agricultural villages ofTrancoso in 1974 to estimate for every rural


landowner what area of his or her land wasused to grow each crop, such as rye, potato,olive trees, chestnut trees, or grape vines, andother cover such as pine trees or giesta. Theseestimates were recorded in a series of booksthat are still archived in the tax office of the cityof Trancoso, where Jorge Santos Costa, the re-sponsible agent, generously allowed me tospend many days transcribing them for the vil-lages of Aldeia Nova and Sebedelhe.

Results and DiscussionFrom a 6.0 square meter area, 12.5 kg wet

weight of giesta branca was harvested. Thefraction dry weight was determined to be 0.38.This estimates an average yearly primary pro-duction of about 200 g dry weight per squaremeter, which is about 2 metric tons per hectare.From a 7.5 square meter area, about 91.7 kgwet weight of giesta negral was harvested. Itsfraction dry weight was determined to be 0.41.This estimates an average yearly primary pro-duction of about 900 g dry weight per squaremeter, which is about 9 metric tons per hectare.

It would have improved accuracy to havebeen able to make additional estimates of pri-mary productivity in other giestais, but therewere obstacles that made this impractical. Icannot cut people’s giesta without their permis-sion. It is difficult to discover owners of giestaisand locate them. Many of them do not live lo-cally. Even if they are local, it is difficult to askthem for permission to cut their giesta withoutfirst establishing a personal relationship withthem, which is not always possible and in anycase takes time. Finally, this investment wouldbe worthless unless it were possible to knowwhen the giesta was last cut, which is rarely

Table 1. Common name, scientific name, author, family name and Estabrook’s collectionnumber for taxa discussed in text. All voucher specimens are in the University of Mich-igan Herbarium, MICH.

Common Name Scientific Name, Author, FamilyCollectionNumber

Giesta branca Cytisus multiflorus (L’Her) Sweet, Fabaceae 567Giesta negral Cytisus grandiflorus (Brot) DC, Fabaceae 568Pilro Genista cineria (Villars) DC in Lam., Fabaceae 577Sargaco negro Cistus monspeliensis L., Cistaceae 576Leiteira Euphorbia characias L., Euphorbiaceae 579Sargaco branco Halimium lasianthum (lam) Spach, Cistaceae 578Rosmaninho Lavandula stoechas L., Menthaceae 580Dedaleira Digitalis thapsi L., Scrophulariaceae 575Urze Erica arboria L., Ericaceae 582

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known by owners because most land is nolonger managed in the traditional manner, andmy own serious attempt to discover anatomicalage markers such as growth rings or annualbranching patterns, which are common for realtrees, did not bear fruit. I was fortunate to havebeen present when Antonio Dias was ready toclear cut a stand of giesta negral of known age,and to have established my welcome in a parishwhere a common of giesta branca was stillbeing managed, so that we can have at least oneestimate of the primary productivities of thesetwo giestas, under something resembling tradi-tional management conditions.

Estimates of fraction of nitrogen in giesta


fixed from the air were successfully calculatedfor 12 samples of giesta, and are reported inTable 2. Results are presented in pairs: each gi-esta is followed by the non-nitrogen fixer grow-ing in close proximity. Place is designated asexplained above. Total N is % of dry weight ofplant tissue sampled. Three separate analyses ofmaterial taken from the same dried, ground, andhomogenized sample were made. Because totalN rarely varied by more than 0.2% among thethree repeated analyses of the same sample,only their average is reported. Because Deltavalues were more variable, all three are re-ported. Three estimates of the percent of total Nfixed from the air are reported: the ratio of the

Table 2. Estimates of % N fixed using dilution of 15N/14N.

Name Place Date % N fixed % N Deltas

giesta branca 52 (29, 68) 2.4 0.73 0.53 0.51Pair 1 AN 30 Jan 03rosmaninho non-fixer 2.2 1.58 1.14 1.03

giesta negral 35 (33, 44) 1.3 0.51 0.48 0.43Pair 2 AN 26 Sep 02dedaleira non-fixer 0.9 0.77 0.76 0.76

giesta branca 95 (99, 78) 2.5 0.45 0.11 0.09Pair 3 SS 29 Jun 03sargaco negro non-fixer 2.1 2.21 2.21 2.10

pilro 96 (97, 82) 2.4 0.27 0.08 0.04Pair 4-b SS 20 Mar 03leiteira non-fixer 2.3 1.58 1.53 1.48

giesta negral 47 (34, 51) 1.6 1.03 0.89 0.89Pair 5 MG 7 May 02urze non-fixer 0.8 1.82 1.73 1.54

pilro 92 (93, 86) 2.6 0.15 0.09 0.08Pair 6 QB 29 Jan 03sargaco branco non-fixer 2.2 1.21 1.12 1.06

giesta negral 82 (82, 79) 1.6 -0.50 -0.51 -0.56Pair 7 TR 9 May 02urze non-fixer 0.9 -2.65 -2.75 -2.82

giesta negral 40 (49, 27) 2.8 1.32 1.22 1.11Pair 8 TE 28 Jan 03sargaco negro non-fixer 1.9 2.19 2.03 1.81

giesta branca 88 (91, 84) 1.5 -0.20 -0.33 –Pair 9 CA 7 May 02sargaco non-fixer 1.0 -2.00 -2.11 -2.15

giesta branca 88 (63, 53) 3.0 2.43 2.19 1.93Pair 10 SS 20 Mar 03sargaco negro non-fixer 2.4 5.28 5.27 5.16

giesta branca 73 (77, 71) 3.5 0.70 0.66 0.62Pair 11 AN 21 Mar 03rosmaninho non-fixer 2.8 2.67 2.42 2.42

giesta negral 68 (83, 66) 3.3 0.42 0.42 0.23Pair 12 AN 21 Mar 03rosmaninho non-fixer 2.5 1.37 1.30 1.23

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sums of the two most similar Deltas for eachsample (if the three Deltas were equally spacedthen twice the middle value was used), fol-lowed parenthetically by the maximum andminimum among the results using ratios of anytwo Deltas with one from each sample. In con-sidering these values, it is important to remem-ber that they may not be typical of the fixationrates of most of the giesta actually used tomaintain soil fertility, which grow in giestaiswhere no other comparable non-nitrogen-fixingplant grows.

According to Antonio Sovral Dias and hiscousin Jose, when their uncle Manuel held theQuinta Baralhada in about 1940, it comprisedabout 220 hectares. About half of it was pineforest and about a quarter of it was giestabranca. About half the area of this giestal wouldactually have giesta plants, the rest would berock or grass. Uncle Manuel had about 250sheep. He kept about 25 sheep for every hectarecultivated, which estimates about 10 cultivatedhectares. About 8 would have rye, and the other2 hectares would have fodder crops such as rye-grass (Lolium perenne), beta root (Beta vul-garis) and rutabaga (Brassica napus, or pota-toes (Solanum tuberosum). In smaller plotsnearer the village, vegetables were grown. Be-cause rye fields were fallowed about every eightyears, the 8 hectares with rye would imply an-other 4 temporarily in giesta negral. So we haveabout 50 to 60 hectares of giesta branca produc-ing at about half the rate I measured, and about4 hectares of giesta negral, all supporting about10 cultivated hectares. This estimates about 50metric tons of giesta branca and 36 metric tonsof giesta negral buried every year in 10 culti-vated hectares, which is about 8 to 9 metric tonsper hectare.

Of course, estrume also contains the sheep’scontribution in addition to the giesta. In thepast, farmers kept sheep primarily to lower theC/N ratio of estrume so that it would rot rapidlyenough in the soil to fertilize rye, which wasthe valuable crop. When the ratio of carbon tonitrogen (C/N ratio) in fresh organic matter isabove about 30, it decomposes very slowly be-cause these conditions do not favor large popu-lations of the micro-organisms that rapidly rotfresh organic matter. When the C/N ratio isbelow about 20, fresh organic matter can rotquickly, releasing nutrients to the soil wherethey can be taken up by growing plants. About


45% of the dry weight of giesta is carbon andabout 1.3% in some giesta negral up to 3.5% insome giesta branca is nitrogen (Table 2). ThusC/N ratios for giesta can be estimated betweena high of about 40 down to below 20 in somecases.

In Aldeia Nova, with the help of Manuel An-drade, I buried giesta alone in the soil of a smallarea of a larger plot to be sown in rye. Althoughthere was enough giesta buried to supply all thenitrogen needs of the growing rye, the rye theregrew very poorly compared to the rye growingnearby in soil enriched with estrume. At thetime of rye harvest, I dug up the soil in thissmall area to discover that many of the largersticks and twigs of the buried giesta were stillconspicuous. At the same time, digging nearbyit was rare to find any sticks or twigs at all.About 11 to 13 percent of dry weight of sam-ples of sheep excrement analyzed in the na-tional lab for agricultural chemistry in Lisboa(LQARS) is nitrogen. To be effective, thesheep’s contribution to estrume needs to bringits C/N ratio down to about 20, which would beabout one part in five (dry weight) for the mostnitrogen poor giesta, but typically about onepart in seven or eight (dry weight). Adding aseventh to the 8 or 9 metric tons of giesta perhectare of rye estimated for Manuel Dias in theQuinta Baralhada of Sebedelhe da Serra, bringsit to about 10 metric tons (dry weight) of es-trume per hectare per year.

People in these farming villages in the 19thand early 20th centuries probably did not knowabout nitrogen or C/N ratios, so their culturemust have informed them in some other way tokeep sheep. People in these villages do not eatsheep or lamb nor sell it before it is quite old.The only meat they eat is pork and chicken (andoccasional wild game). When sheep get too old(about six years) they are sold in the markettown about 30 kilometers away, as are old cowsno longer reliable for their primary purpose,which is traction (Fig. 2). Sheep are shorn inthe spring, but their wool is not abundant and oflow quality; it is used mostly for blankets andfloor coverings. Sheep are not well fed so theirbirth rate is low, producing a lamb about everyother year. The small amount of milk, availablefor a short time at weaning, is made into acreamy cheese that is very much appreciated.Cows are milked briefly at weaning, but theirmilk was traditionally fed to the pigs. Because

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the value of these sheep products is not com-mensurate with their cost in labor and space, itseems clear that the primary purpose for thesheep was to help maintain soil fertility by con-tributing about 1/3 of the replaced nitrogen,lowering the C/N ratios of estrume, and prob-ably also replacing other nutrients as well, suchas phosphorous, which is about two to threepercent of the dry weight of estrume.

Manuel Andrade, in Aldeia Nova, had 38sheep. At night he kept them in a shed with gi-esta spread on the floor; he added fresh giestaabout every other day. Every Saturday he re-moved all the giesta from the shed floor andplaced it in a pile outside the shed door. Beforecleaning his shed the next Saturday, he forkedthis pile down the hillside to join a growingheap of estrume, which he would eventuallybury in the soil of his cultivated fields. I mea-sured the volume of a weekly pile, which wasjust over one cubic meter. Two liters of this wetestrume weighed about 700 g and when driedabout 300 g. A week’s worth of estrume from34 sheep is just over 1000 liters, so weighsabout 400 kg wet or about 160 kg dry. If thisrate continues all year, it will fertilize some-what less than a hectare at the usual rate of 10metric tons dry weight estrume per hectare peryear. This give us about 40 sheep per cultivatedhectare. However, Andrade is raising sheep tomake cheese, not to sell rye, which in fact hegrows to feed to his sheep. From Table 2 thepercent dry weight nitrogen in giesta is some-what variable around 2%, with branca running alittle higher than negral on average. Samples ofManuel Andrade’s estrume are just over 4% dryweight nitrogen on average. Samples of thesheep excrement part contain about 12% dryweight nitrogen (some of the nitrogen was lostas volatile ammonia before the sample wastaken). Combining these percentages impliesthat the sheep’s contribution is about one-fifththe dry weight of estrume. Thus, Manuel An-drade is harvesting about 130 kg dry weight(330 kg plant material) of giesta per week, orabout 700 metric tons per year. About 1/5hectare in fallow provides an average of about200 metric tons a year of giesta negral, so hemust have about 2.5 hectares of productive gi-esta branca, which is likely to be actually about5 or 6 hectares about half covered in giestabranca, with the rest rocks and grass.

Tax surveys from 1974 did include Quinta


Baralhada, but accounted for only about 90hectares. Jorge Santos Costa assures me that fora consideration, land owners could easily per-suade members of these teams to under-estimate their holdings. However these recordsare better than nothing, and almost certainly itwould be the area of valuable crops (grapes,olive, grain, and, to some extent, pine) that tax-paying owners would like to have underesti-mated. Areas of giestal would probably be inac-curately estimated because they were not worthenough for tax purposes to take the time to esti-mate accurately. Manuel Andrade moved ontothe land he now holds in the mid-1980s whenhe bought some of the Quinta Pisao from Anto-nio Fonseca of Aldeia Nova when he retired.Thus the correspondence between what land heheld in 2003 and the 1974 tax survey was lost,so I could not directly compare estimates. How-ever, in 1974, the tax survey for another owner-ship parcel of Quinta Pisao reported 1.6 ha gi-estal, 0.135 ha in rye, and 0.390 ha in potato.Potato is a low-nitrogen crop given less thanhalf the estrume as rye, so at most 1.0 ha of gi-estal would fertilize the potato, leaving a ratioof about 5 to 1 for the rye, or slightly higher ifsome of the rye fields are fallow.

Estimates from Table 2 of the percentage ofnew nitrogen from the atmosphere present ingiesta negral vary in five samples from 35% to82%, with an average of about 54%; estimatesof percentages in giesta branca vary in fivesamples from 52% to 95%, with an average ofabout 80%. Applying these rates to the culti-vated rye fields of Quinta Baralhada of Sebe-delhe da Serra gives about 120 kilograms ofnew nitrogen per ha per year. Observing andtalking with Antonio Sovral Dias determinedthat rye seed was sown at a rate of about 80kilograms per ha. Traditionally harvests werequantified in seeds, i.e., the number of seedsharvested for each seed sown. Harvests variedfrom as little as three seeds in poor years to asmuch as 15 seeds in very good years, withabout seven seeds considered satisfactory, and10 seeds good. Thus a rye harvest wouldrarely exceed a metric ton of seed per ha. Ryeseed is at most 3% nitrogen, so a rye harvestwould rarely remove more than 30 kilogramsof nitrogen. Some seed was saved to plant thefollowing year, and some rye was consumedby the farming family; so at most 20 kilo-grams of nitrogen per ha cultivated in rye per

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year would leave the area in exported grain.This leaves a surplus of about 100 kg of newnitrogen per ha per year. Even if the N15stable isotope technology overestimated thepercentages of biologically fixed new nitrogenin giesta by a factor of two, a substantial sur-plus would remain. Nitrogen losses due toother human activity seem probable, and thereare natural soil nitrogen losses as well: usingloss rate estimates given in Loomis and Con-ner (1992) as a guide, perhaps as much as 40kg per ha might be lost to denitrification, an-other 10 kg per ha to volatilization, and an-other 5 kg per ha to leaching (but I capturedand analyzed leach water from a half-meterbelow the soil surface under growing ryethroughout the rainy growing season, and atmost traces of nitrogen was detectable). Imade no attempt to account for the nitrogenlost in the sale of sheep and cows, and in theburning of wood to cook and bake. There alsomay be other natural sources of new nitrogensuch as nitrogen in rain water.

ConclusionIt is important not to overgeneralize these re-

sults. Agricultural sustainability depends onmany factors, not just the sustainability of thenitrogen soil fertility examined in the study re-ported here. Replenishing other nutrients in thesoil, keeping soil erosion rate below soil regen-eration rate, avoiding soil pollution or contami-nation or compaction, maintaining soil watertable, and defending farmland from urbansprawl are examples of other important factors.Indeed, very little agriculture is practiced inBeira Alta at present, largely for economic rea-sons not directly related to soil fertility mainte-nance. Most of the estimates presented herewere not made with high precision, but hope-fully with appropriate accuracy to argue themain points. Clearly, giesta makes a very sub-stantial contribution to the nitrogen sustainabil-ity of the traditional farming practices of BeiraAlta, Portugal, especially in the face of substan-tial 18th, 19th, and early 20th century exportsof nitrogen, in the form of rye seeds. This tradi-tional farming system maintains giesta brancaon soils too poor to cultivate, and giesta negralin the vicinity of cultivated fields, ready to takeover during fallow to grow rapidly for four orfive years. The amount of fallowed rye fieldswith giesta negral, the area maintained with gi-


esta branca in relation to amount of area of cul-tivated in rye, and the number of sheep per cul-tivated hectare all represent strategic balancesthat are a part of the culturally informed tech-nology of this tradition.

Visit the Web site http://www.Personal.umich.edu/~gfe/ to download additional colored pho-tos that illustrate and document this study.

AcknowledgmentsManuel Andrade of Aldeia Nova and Antonio

Sovral Dias of Sebedelhe, both experiencedfarmers raised in the traditional agriculture ofTrancoso, Portugal, were valuable sources ofinformation and help. Maria de ConcecaoAlexandre of Trancoso provided valuable logis-tic support. Dra. M. Regina Martins and JorgeSantos Costa helped me access civil and landuse records in Trancoso. Professor Helena Fre-itas, Department of Botany, University ofCoimbra, Portugal, made available laboratoryspace there for me to prepare specimens. I ap-preciate the welcome as invited professor ex-tended to me by the Anthropology Departmentof the University of Coimbra. I am grateful toProfessor Don Phillips, Department of Agron-omy, University of California at Davis, for in-troducing me to stable isotope techniques forestimating in the field amounts of nitrogen bio-logically fixed by long-lived plants. I thank Pro-fessor Cristina Maguas of the University of Lis-boa, Portugal, who facilitated my use of theMass Spectrometry Laboratory of the Univer-sity of Lisbon, and Rodrigo Maia, her techni-cian, who performed the analysis of my sam-ples, for his skillful, careful, and patient work,and for his advice for their preparation andpreservation in transport. I admire the skill ofartist Vaike Haas who drew the map of Fig. 1. Iappreciate financial support from FundacaoLuso-Americano de Desenvolvimento, and thesabbatical leave program of the University ofMichigan.

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