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Reproductive Effort and Seed Establishment in Grazed Tussock Grass Populationsof PatagoniaAuthor(s): Gabriel Oliva , Marta Collantes , and Gervasio HumanoSource: Rangeland Ecology & Management, 66(2):164-173. 2013.Published By: Society for Range ManagementDOI: http://dx.doi.org/10.2111/REM-D-11-00121.1URL: http://www.bioone.org/doi/full/10.2111/REM-D-11-00121.1

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Rangeland Ecol Manage 66:164–173 | March 2013 | DOI: 10.2111/REM-D-11-00121.1

Reproductive Effort and Seed Establishment in Grazed Tussock Grass Populationsof Patagonia

Gabriel Oliva,1,2 Marta Collantes,3 and Gervasio Humano1

Authors are 1Researchers, Instituto Nacional de Tecnologıa Agropecuaria, Estacion Experimental Agropecuaria Santa Cruz, Rıo Gallegos, Santa Cruz,Argentina; 2Professor, Universidad Nacional de la Patagonia Austral, Rıo Gallegos, Santa Cruz, Argentina; and 3Researcher, CONICET Museo Argentino

de Ciencias Naturales, Buenos Aires, Argentina.

Abstract

The importance of sexual reproduction in tussock grasses that regenerate through vegetative growth is unclear. Festucagracillima Hook. f. was studied as a model because it is a perennial tussock-forming grass that produces abundant seed butrarely regenerates through seedlings. The Study area was the Magellanic Steppe, Patagonia, Argentina (182 mm rainfall),managed with sheep-grazing regimes of 0.65 (high), 0.21 (low), and 0 (exclosure) ewe equivalents � ha�1 � yr�1. Tussock size andspikelet production of 358 individuals were recorded over 5 yr. Yearly models of reproductive effort in relation to plant sizewere tested using a maximum likelihood procedure. Seed was collected and soil cores were tested for germination and viability.Survival and growth of cohorts of seedlings sown in nylon bags were recorded. Eighteen experimental plots were cleared, andseed establishment under protected and grazed conditions was registered. Reproductive effort varied with years and plant size,with a mean of 2.41%. Florets were produced at mean density of 544 6 217 �m�2. Predispersal losses reduced viable seedproduction to 187 6 48 seeds �m�2. Seed weighed 2–2.5 mg, with 65–95% germination. Postdispersal losses reduced the seedbank in spring to 33 6 1.3 seeds �m�2. Seedling survival curves were negatively exponential, with 95% mortality in the first year.Up to 5% of resources were used for sexual reproduction in favorable years and a recruitment of 1–3 new seedlings �m�2 � yr�1

was expected. These new plants were not observed in undisturbed plots, but established naturally in cleared plots and reached adensity of 1 plant �m�2 after 10 yr, together with 44 plants �m�2 of other species. Competition might block the finalestablishment in these grasslands. Grazing does not appear to interfere in any stage of seed reproduction. Seed production maynot maintain population numbers but could enhance genetic variation in these clonal plant populations and enable dispersal andrecolonization of disturbed areas.

Key Words: demography, Festuca gracillima, rangelands, sexual reproduction, sheep, soil seed bank

INTRODUCTION

The significance of sexual reproduction for the regeneration ofperennial tussock-forming grasses is not well established. Theselong-lived plants usually produce abundant viable seed, andsome authors consider the absence of seed recruits to be anearly degradation clue in Patagonia (Soriano 1956; Bertiller1992; Bertiller and Coronato 1994), and in rangelands of theUnited States, seedlings are considered an indicator ofrangeland health. Some demographic models show that tussockpopulations can subsist based on vegetative growth (Oliva1996), and others point out that dispersal and populationgrowth are dependent on sexual reproduction (Lord 1993;Guardia et al. 2000).

The role of seeds in population demography is a concernbecause grass steppes in southern continental Patagonia andnorthern Tierra del Fuego, that used to be dominated by thetussock grass Festuca gracillima Hook. f. (Borrelli et al. 1997;Collantes et al. 1999), have given way to shrublands ofNardophyllum bryoides (Lam.) Cabrera, and Nassauvia ulicina

(Hook. f.) Macloskie, both dwarf shrubs of family Asteraceaewith prostrate growth and low palatability (Borrelli et al. 1984,1988). The process is slow but the relative increase in shrubsand reduction of grasses has been described in grazing trials(Oliva et al. 1998) and probably involves interference inregeneration, either vegetative or sexual. Direct death fromintense defoliation of tussocks is rarely seen because plants areonly lightly grazed in winter when other palatable short grassesare covered with snow (Posse et al. 1996; Pelliza et al. 1997).The transitions also seem to be irreversible because tussockcover is not reestablished with reduced grazing pressures orexclosures (Borrelli 2001). Tussocks provide resource sinks andincrease soil stability in these wind-eroded rangelands devoid oftaller life forms and also provide habitats for wildlife, andrefuges for palatable plant species (Faggi 1985). Plant diversityin the Magellan steppes is directly related to tussock cover(Oliva et al. 2009).

The relative importance of sexual reproduction in the lifecycle can be inferred from reproductive effort (RE). Thismeasure, based on the ratio between reproductive tissues andtotal vegetative weight, has also been named as ‘‘reproductiveallocation’’ and provides an estimate of the relative allocationof resources to seed and associated structures (Reekie andBazzaz 2005). RE is not a fixed value because it can change inrelation to the physiological status of the population and alsocan differ among size classes of a single population. Theimportance of RE plasticity has been pointed out by Weiner et

Research was funded by the National Institute of Agricultural Technology, Rıo Gallegos,

Santa Cruz, Argentina.

Correspondence: Gabriel Oliva, Chacra 45 A 9400 Rıo Gallegos, Santa Cruz, Argentina.

Email: goliva@correo.inta.gov.ar

Manuscript received 23 July 2011; manuscript accepted 25 October 2012.

ª 2013 The Society for Range Management

164 RANGELAND ECOLOGY & MANAGEMENT 66(2) March 2013

al. (2009), and this variation could influence tussock popula-tions in Patagonia because grazing modifies the size structure ofpopulations, creating a population with smaller tussocks (Olivaet al. 2005).

A second way of evaluating the importance of seedreproduction is the experimental approach to analyze differentsteps of seed establishment and the final seedling output. Thesequential nature of this process has been pointed out bydifferent authors (e.g., Fenner and Thompson 2005) because asuccessful recruitment requires a sequence of favorableconditions. In the first place, adult plants with adequatephysiological status will produce flowers, cross pollinate, andfill the seeds. Secondly, the immature seeds need to escapepredispersal predation by grazers, insects, and birds. In thethird step, the mature seeds have to disperse and survivepostdispersal predation in the soil by rodents, birds, andinsects, and avoid pathogens. In the fourth step, the seeds mustgo through a period of adequate temperature and humidity inthe soil in order to abandon dormancy and complete thegermination process. Finally, the seedlings must survivedrought and sheep grazing and trampling until they aredefinitively established. Failure to meet any of these sequentialconditions can prevent seed establishment. Given that rainfall isvariable in these semiarid habitats, experiments designed toevaluate the steps of sexual reproduction should involve morethan one growing season.

This paper summarizes different experiments carried out inthe last 20 yr on populations of F. gracillima in southernPatagonia, combining direct observation, sampling of soil,sowing, and removal experiments. The main hypothesis wasthat the reproductive effort in F. gracillima is low, variesbetween years and between size classes, and that seedestablishment takes place only in favorable years.

METHODS

Experimental Site and Plant SpeciesFestuca gracillima is a 30-cm-high tussock grass with clumpedfoliage and caespitose growth, with no rhizomes or stolons.The panicles of this species, with 3–7 spikelets, each onebearing 4 to 6 florets, emerge in December, and dispersal takesplace in January. The species is dominant in the Magellanicsteppe, in south continental Argentina and Chile and northernTierra del Fuego Island (Boelcke et al. 1985) between lat498300S and 558000S (Moore 1983). Its habitat is restricted towell-drained soils on plateaus and hill slopes.

Moy Aike Chico is 60 km N-NW from Rıo Gallegos, SantaCruz, Argentina (lat 518470S, long 688470W). The climate ismaritime, with 182 mm of rainfall (Burgos 1985). Rain isdistributed evenly in the year, with 90% of the rainfalloccurring in less than 5-mm amounts (Ferrante 2011)Temperatures are low, with mean values of 12.78C in summer1.48C in winter (De Fina et al. 1968). The soil remains frozenduring a variable period in June–August. Winds are constant,with a yearly mean of 27 km � h�1 and frequent storms in springand summer. The landscape consists of flat plateaus 100 m to150 m above sea level (Anchorena 1985). Soils are Borolichaplargids (Salazar Lea Plaza and Godagnone 1990), sandy

and rich in organic matter in the top 10 cm; deeper horizons areargilic and abundantly pebbled.

Treatments and DesignTussock populations were studied from 1989 to 2010 in twoadjacent 40-ha paddocks, and a 0.5-ha exclosure set in a plantcommunity dominated by F. gracillima with patches of dwarfNardophyllum bryoides shrublands. In 1987, a controllednonreplicated grazing trial started using Corriedale (sheep)wethers with stocking rates (ewe equivalents � ha�1 � yr�1) of0.65 (high), 0.21 (low), and 0 (exclosure). An ewe equivalent isthe annual requirement of forage of a 49-kg live-weight ewethat produces a 20-kg lamb, estimated in 513 kg � yr�1 (Borrelli2001) and is approximately 0.12 animal units (AU) used in theUnited States (SRM 1989). This experiment continued until2000, when the fences were taken away and the site wassubjected to the normal grazing rates of about 0.50 eweequivalents � ha�1 � yr�1.

Table 1 shows the series of experiments that were carriedout in the field. The seed experiments were performed on three0.25-ha plots within these treatments that were chosen in1989. The vegetation was assessed through 500-point lineintercept evaluation of cover by species. Thirty-six plantspecies with 44.6% total vegetation cover and 11.4% F.gracillima cover were recorded. A Sorensen similarity indexwas used to compare the cover data with the typicalcommunities of different conditions proposed by (Borrelli etal. 1988) for their Santacrucense site. The sampling sitesshowed high similarity with the ‘‘fair–good’’ conditioncommunities of this study.

Seed production was estimated in three 431.20 m plots ineach treatment by counting spikelets and culms between 1989and 1995. Tussocks were mapped on photographic platesobtained at 2-m height, using a methodology described inOliva et al. (2005). Any tiller or group of tillers that showed atleast one green leaf and was separated from other groups by 2-cm or more of bare soil, litter, or other plants was consideredto be an individual tussock and was marked in the field withengraved nails. Photographic maps of tussocks were preparedyearly in autumn, and any new individuals were identified andmarked.

The vegetative weight (V) was estimated nondestructively ona yearly basis by estimation from aerial cover of floweringtussocks. A regression was obtained from a sample of 100tussocks, whose outlines were drawn on a photographic map ofan independent plot in 1989. These tussocks were clippeddown to 1-cm height and dried to constant weight. The drymaterial was partitioned into ‘‘green’’ and ‘‘dry’’ components.After logarithmic transformation, the linear regression of aerialcover and weight of green material was highly significant(R2¼0.85; P , 0.001).

Number of florets per spikelet was estimated in a 100-spikelet sample of each grazing treatment in 1989 and 1990.No significant differences were observed between treatmentsor years, so that the general mean of 2.84 florets � spikelet�1

was used as an estimator for all cases. The proportion offlorets that fructified and produced caryopses was estimated inthree samples of 100 florets in each treatment in 1989. Nosignificant differences between treatments were observed, so

66(2) March 2013 165

that a single estimation from nine samples of 100 florets was

obtained from a population close to the grazing treatment inthe subsequent years. Weight of seed was estimated yearly

from 30 10-seed samples. Weight of culms was estimated

yearly from a sample of 100 dry culms without spikelets butincluding glumes.

The reproductive effort RE¼R �V�1 (where R is reproductive

output and V vegetative plant weight) was estimated in two

different ways in order to compare the figures with thoseprovided in the literature: RE(s)¼R(s) �V�1 estimated using only

weight of the seed (s) produced, and R(sþc)¼R(sþc) �V�1, that

included also the weight of culms (c). Weight of seed �m�2 (R[s])

was obtained as: R(s)¼Sp � F � Fr �Cw, where Sp¼number ofspikelets �m�2, F¼number of florets per spikelet, Fr¼propor-

tion of florets that produced caryopses, and Cw¼yearly mean

weight of a caryopsis in grams. R(sþ c) was obtained asR(sþc)¼R(s)þCu � 0.031, where Cu¼number of culms, and

0.031 g is the mean weight of one culm (estimated to be

constant).

In order to evaluate the allometric response of reproductiveeffort (RE), the relation between R and V in the tussock

population was analyzed. If these two variables are linearly

related in the range of tussock sizes, RE is constant in all the

range of size classes. If they are not, the RE will differ. The

relation was explored using a general model proposed by

Klinkhamer et al. (1992) to test linearity and the existence of

Ri ¼ aðVi � bÞc þ Eiði ¼ 1 . . . nÞ; 1½ �

where R is reproductive output, V is the vegetative plant

weight, a is the slope of the relation, b is the intercept with the

V-axis, and c determines the degree of nonlinearity. Starting

with the simpler model that assumes c¼1, b¼0, and R¼a(V),

the hypothesis H0 c¼1 against H1, c„1, assuming b¼0 was

tested using the statistic.

K ¼ nlogðs02s�2Þ; 2½ �

where s02 was calculated under H0 (c¼1 and the relation is

linear) and s2 under H1, (c„ 1 and the relation is non-

linear).The statistic was obtained using SAS procedure NLIN

Method marquardt (SAS Institute 1998), converge¼10�12 and

compared to a v2 distribution with 1 df (Klinkhamer et al.

1992). In case of a significant K we assumed that the relation

was nonlinear R¼a � (V)c .

Table 1. Diagram of the experiments.

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Spikelet count x x x x x x

Spikelet harvest

and

floret � spikelet�1

count

x x x

Floret visual

analysis to

separate

caryopses

x x x x x x

Germination and

viability lab tests

on caryopses

x x x x x x

Germination tests

in seed bags in

the field

x x x x

Seedlings survival

counts in seed

bags in the field

x x x x x x x x x x

Bimonthly soil core

sampling for

seeds

x x x x

Germination and

viability tests in

seeds recovered

from soil

x x x x

Clearing of

removal plots

x

Count of natural

seedling

recruitment in

removal plots

x x x

166 Rangeland Ecology & Management

The hypothesis H0 b¼0 against H1 b„0 was tested in asimilar way. If K was again significant, we concluded that thecomplete model R¼a � (V�b)c was necessary to explain therelation (Klinkhamer et al. 1992). In this analysis, R wasestimated as seed output, without considering weight ofinflorescences (Rs).

Germination tests of seed collected each year were performedfrom 1989 to 1992 with three samples of 100 seeds that wereincubated in Petri dishes at room temperature for 60 d.Germination in the field was studied for the same period using300 seeds � grazing treatment�1 that were placed in groups of 10in 535 cm soil filled nylon mesh bags (7 threads � cm�1). Bagswere put in place in May at 10-cm intervals along two 15-mtransects in each treatment, secured in place with nails andcovered with 1 cm of soil, leaving copper wire markers at thesurface. Bags were recovered bimonthly in four groups.Germinated and damaged seeds were separated visually, andthose remaining were incubated in Petri dishes for 60 d in orderto test germination potential. Those that did not germinatewere put through the tetrazolium test (Smith 1952) in order todetermine embryo viability.

Seedling survival and growth in the field was studied from1989 to 1992 using 450 seeds � treatment. They were includedin groups of 10 in a separate set of nylon mesh bags. Bags wereput in place at 10-cm intervals along three 15-m transects ineach treatment. They were observed monthly from 1989 to1992, and number of live seedlings and green leaves perseedling was recorded. After the first year, the remains of thenylon mesh bags were removed using scissors, leaving nailmarkers in place.

Soil seed bank was sampled in three 990375 cm plots pertreatment. The plots were divided into 330 cells by means of aremovable grid. Twenty cells were randomly selected pertreatment and a 10-cm-deep soil core was obtained using a 5-cm diameter steel cylinder. Sampling was performed in spring(October–November). Empty, damaged, and germinated F.gracillima propagules were separated from soil by sieving.Those in good conditions were incubated at room temperatureand darkness, and germination was estimated after 14 d. Theremaining caryopses were treated with 2,3,5-triphenyl-2H-tetrazolium chloride (Smith 1952) to assess if the embryos werestill alive and dormant.

For a final quantification of the recruitment, the sequentiallosses involved in sexual reproduction were estimated as: 1)failed pollination and abortion, estimated from fruit-fillingobservations; 2) unviable seed embryos, from germinationtests; 3) seed predation in soil, estimated based on differencesbetween the magnitude of the seed production in summer andthe actual density of propagules found in the soil coring in thesubsequent spring; 4) attack of soil pathogens and diseases,estimated from the difference of the proportion of viable seed atdispersal and the sum of germinated and dormant seed inspring sampling; 5) seedling mortality in the first year, fromsurvival observations 12 mo from sowing; and 6) seedlingmortality in the second yr, from survival observations 24 mofrom sowing. When annual estimations for coefficients werenot available, they were replaced by means.

Natural recruitment was studied between 1989 and 1999 inthe permanent photographic plots with yearly examination ofphotographs, identification, and field marking of the existing

tussocks using a methodology already described in Oliva et al.(2005), that detects new recruits over 232 cm.

Recruitment in cleared plots was studied in six 1-m�2 plots ineach treatment that were randomly chosen in 2000 in thevicinity of the permanent plots, and subjected to completemanual removal of vegetation cover. Three of them wereprotected with wire net and three remained unprotected.Recruitment was followed using photographs. In 2010, arevision of the plots was made, and all the recruits wereidentified and counted.

Data analysis was done using descriptive statistics includedin SAS/STAT. Differences between years were tested usingrepeated measures ANOVA and contrasts, using grazingtreatments as blocks.

RESULTS

Annual precipitation ranged between 108 mm (1989–1990)and 293 (1990–1991). Although rain is distributed throughoutthe year, spring was driest and most variable, with rainfallsranging between 1.8 (1993–1994) and 60.5 mm (1994–1995)(Fig. 1).

Grazing EffectNo differences associated with grazing were observed in floretproduction, seed filling, or germination. Although 20% of theyoung panicles were grazed in the high stocking treatment, thefinal production of florets was similar to that of low grazingand exclosure populations that suffered only 1.77% and 0.93%of panicle consumption respectively. Trampling was neverobserved to be the cause of mortality for the young tussocks,and the survival of seedlings was similar in the grazingtreatments. The data are hereby reported as means, and thedifferent grazing treatments are considered blocks in the yearlycomparisons.

Seed Production and Reproductive EffortOnly 3% of the ovules achieved pollination and formedcaryopses after the dry 1993–1994 spring (Fig. 1), but about

Figure 1. Seed weight (100%¼2.5 mg), germination (% of caryopses thatgerminated in the laboratory) and fructification (% of the florets that producecaryopses) in relation to spring rainfall (October–November). Means withthe same letter do not differ significantly between years.

66(2) March 2013 167

66% did so after favorable rains in 1990–1991 and 1994–1995, and fruit filling was correlated with spring rain(R 2¼0.76, P , 0.01). Mean seed weight was about 2 mg, butreached 2.5 mg in the favorable 1990–1991 year, correlatingbetter with annual (R2¼0.53, P , 0.01) than spring rain(R 2¼0.15, P , 0.05). In the dry 1993–1994 season, virtuallyno viable seeds were found and germination and seed weighttests were not performed.

Floret production was variable (Fig. 2) and correlated closelywith spring precipitation (R2¼0.71) but not with yearlyrainfall. This figure reflects the high proportion of nonpolli-nated or aborted ovules, and the low prevalence of nonviablecaryopses. Germinable seed production varied from zero toover 500 caryopses �m�2.

Seed production, R(s), ranged from 0.14 to over 1.6 g ofseed �m�2 (Table 2), with a mean of about 0.5 g �m�2. Totalreproductive tissues, R(sþ s), including culms and rachises ofinflorescences, weighted about five times more, indicating thatcostly structures are necessary to insure pollination anddispersal. Mean vegetative biomass (V) of flowering plantsexceeded 100 g �m�2, and the relation rendered reproductiveefforts of 0.45% if only seeds are considered (RE[s]), and2.41% when all the reproductive tissues are also included(RE[sþc]), with variations of about an order of magnitudebetween years.

With the addition of b parameter, the relation R¼a(V�b)c

between the reproductive output R(s) of individual plants and

their size V was not significant, indicating that no minimum

size for sexual reproduction can be established by this analysis.

The exponential parameter c was significant in rainy and dry

years. A plot was drawn relating estimated RE(s) to V for a

typical range of tussock sizes. RE(s) (Fig. 3) shows similar mean

RE(s) (Table 2), fluctuating between 0.1 for the dry years and

close to 1 for the rest. Linear models R¼a �V for 1990–1991

and 1992–1993 imply that the reproductive effort remains

constant in the range of tussock sizes. Exponential factors in

models R¼a � (V)c for years 1991–1992 and 1994–1995 were

smaller than 1, showing increased RE(s) for smaller tussocks.

On the contrary, the exponential factor of the model for dry

1989–1990 year implies that RE(s) diminishes in the small

tussocks.

Table 2. Reproductive effort RE¼R � V�1 expressed in % (g of seed produced � 100 g�1 of vegetative weight) in 6 yr.

Parameter estimated

Years

Mean1989–1990 1990–1991 1991–1992 1992–1993 1993–1994 1994–1995

R(s): Seed weight (g caryopsis �m�2) 0.139 0.864 1.360 0.159 —1 1.268 0.478

R(sþ c): Total reproductive weight (g caryopsisþ g culms) �m�2 0.92 2.55 5.86 1.74 — 5.11 2.57

V: vegetative weight of flowering plants (g �m�2) 113.8 114.2 112.3 139.0 56.5 102.4 106.4

Reproductive effort (seed): RE(s) ¼ s � V�1 (%) 0.12 0.76 1.21 0.11 — 1.24 0.45

Reproductive effort (seedþ culms): RE(sþ c) ¼ (sþ c) � V�1 (%) 0.81 2.23 5.22 1.25 — 4.99 2.411Indicates that no estimation was done because not enough seed was produced for testing.

Figure 2. Floret production (florets �m�2) and status of the ovuledevelopment found in the laboratory analysis by years.

Figure 3. Plots of reproductive effort RE(s)¼R � V�1 expressed in % (g ofcaryopses produced � 100�1 g of vegetative biomass) for the range oftussock sizes. Equations were estimated using yearly data by the maximumlikelihood method. Rainfall of the year is shown in brackets. Parameterswere: R1989–1990¼0.003 � V1.40; R1990–1991¼0.012 � V; R1991–

1992¼0.076 � V0.84; R1992–1993¼0.082 � V; R1994–1995¼1.46 � V0.69. Plotsfor years 1992–1993 and 1991–1992 are not shown for clarity.

168 Rangeland Ecology & Management

Germination and Seedling SurvivalGermination percentages in the laboratory ranged between65% and 95%, and correlated with rainfall of the spring inwhich the seeds were produced (R2¼0.71, P , 0.01). In fieldtests, they were equal or higher than those obtained inlaboratory incubations. Seed sown in the field in nylon meshbags in 1989 (Fig. 4) entered the soil with 68% forceddormancy (viable seed that did not germinate becauseappropriate conditions of temperature and humidity are notmet), but this fraction gave way to 80% of germinated seed byMay. Seed under induced dormancy (that remains viablejudging by the tetrazolium test but does not germinate undernormal conditions of incubation) constituted 14% duringautumn and winter, but gradually diminished in spring,reaching only 1% after a year in the soil. Dead seed (thatseems untouched but does not germinate or react positively totetrazolium test) was initially 18% and diminished, whereasthe predated fraction (seed that was visually attacked byinsects), reached 23% at the first year. After a year in the soil,about two-thirds of the seed had been lost by germination andthe remaining fraction was predated or dead. The results weresimilar in the 2 subsequent yr. Not a single germinable seed wasrecovered after 12 mo in the soil. Because seed in buried nylonmesh bags are protected from birds, rodents, and some insects,these estimations underscore the actual predation on the seedbank.

The density of propagules (lemma and palea enclosing thecaryopses or incompletely developed ovaries) found in soil core

samples in spring correlated significantly with propagule rain insummer (R2¼0.97, P , 0.01), but about 60% of the seed werelost in this period, probably due to soil-surface predation (Table3). In addition, only 23.4% of the seed recovered from the soilin spring was still under induced, forced dormancy, or hadgerminated (Table 4). Because about 79.3% of the seed wasviable at dispersal, the difference (79.3 – 23.4¼55.9%) couldreflect the fraction that was damaged by fungus or otherpathogens without completely disappearing from the soil.

After autumn germination, the recruits remained inactivedue to low temperatures. The soil remained frozen from June toJuly, and thawing was observed in late August (G. E. Oliva,personal observation). Winter apparently did not inducemortality of the seedlings, even though temperatures of�208C were recorded. Recruitment, measured as the emergenceof coleoptiles in the surface, ranged between 40% in 1990 and3% in 1989 (Fig. 4). Drought during spring was critical, and allthe seedlings died in 6 wk during the dry year 1989. Moistersprings such as 1992 allowed between 4% and 6% of survivaluntil summer, when rains reduced the mortality. Establishedseedlings endured the second spring drought with survivalpercentages close to 61%. Survival curves were negativelyexponential, with decreasing mortality rates that allowed onlybetween 1.4% and 4.5% of the population to reach 24 mo ofage. Recruits after 2 yr showed a mean number of eight greenleaves, and 3 yr later they presented only 15 leaves. None ofthem had flowered after 5 yr.

A mean of 45 seed recruits �m�2 were found in 2010 in plotsthat were cleared in 2000 (Table 5). Only 1 recruit �m�2

established by sexual reproduction was a F. gracillima tussock.On the other hand, short grasses and graminoids were verysuccessful in colonizing the bare ground, and established 33recruits �m�2. No significant differences were found in relationto the protection from grazing in the 2000–2010 period.

Mean population numbers expected from ovules to 24-mo-old seedlings, taking into account the sequential losses, wereestimated (Table 6). These results indicate that up to 3seedlings �m�2 were expected to survive the heavy sequentiallosses of the sexual reproduction process.

Table 3. Propagule1 rain density in summer, compared with propagule density (propagules �m�2) estimated by soil core sampling the subsequent spring.

Parameter estimated

Years

Mean1989–1990 1990–1991 1991–1992 1992–1993

Propagule rain (propagules �m�2) 735 174 536 1 185 657.5

Propagules in soil (propagules �m�2) 226 57 189 709 295.3

Loss (%) 69 67 65 40 60.31Propagules are lemma and palea that include ovules, aborted ovules, or filled caryopses.

Figure 4. Survival of the seedlings emerging from the nylon mesh bags inthe field in the 1989, 1990, 1991, and 1992 cohorts (shown as % of totalseed sown). The curves start from the maximum emergence counting.

Table 4. Condition of caryopses (%) recovered from soil cores in thesubsequent spring. Means of years 1989 to 1992.

Source of seed

Condition of caryopsis (%)

Germinated

Forced

dormancy

Induced

dormancy

Predated

or dead

Seed production

late summer

0 79.3 14.0 6.7

Soil bank in spring 19.6 2.8 1.0 76.6

66(2) March 2013 169

DISCUSSION

Reproductive effort, floret production, ovule maturation, seed

filling, germination, and survival of seedlings were all tightly

correlated with spring rainfall, which is variable in the

Magellanic steppe. Water controls fecundity as in other

semiarid lands, probably because plants close stomata and

interrupt photosynthetic fixation when leaf water potentials fall

(Noy-Meir 1973), and shortage of assimilates limits seed

production and filling.

The total reproductive effort RE(sþc) of F. gracillima, at

2.41% (Table 2) is low and probably reflects ecological

constraints of the habitat within a general tendency of perennials

to produce small seed outputs. The potential RE(sþc) of the

species is likely to be close to 5.2%, the value obtained during

the rainy 1991–1992 year, which compares closely to the 6.9%

estimated for the tussock grass Festuca ovina, under glasshouse

conditions by Wilson and Thompson (1989). These authors

tested many species and found that five tufted perennials with

caespitose habit showed low (less than 10%) and 10 moderate to

high (11% to 30%) RE(sþc). In comparison, annuals presented

uniformly high RE(sþc) that ranged from 41% to 66%.

No evidence of a minimum size for reproduction was found

in F. gracillima in the maximum likelihood analysis. Similar

results were obtained by a different statistical procedure by

Weiner et al. (2009) for 71% of 44 herbaceous plant species in

studies they reviewed. This probably reflects the modular

nature of the tussock grass, and the fact that a single tiller iscapable of reproduction.

Reproductive effort in this species is clearly plastic anddepends on the environmental conditions. Weiner et al. (2009)postulate that the total R–V relationship is, along with meansize of seeds produced by an individual, one of the least plasticplant attributes. According to this hypothesis, at a given size, aplant’s potential reproductive output is relatively fixed. Theseauthors also point out that the generalization holds for annualand monocarpic plants, which should allocate all availableresources to reproduction at maturity, but that iteroparousperennial species will show much more plasticity in their totalR–V relationship. In fact, this review found evidences ofplasticity in 37% of the species that were analyzed, andpresented different models for this relation. Interestingly, F.gracillima shows examples for the three of them (Fig. 3): Type‘‘a’’ with a size-independent RE was obtained in 1990–1991, arainy year following a very dry period, and also in thefollowing year, 1992–1993, that was intermediate in rain (notshown for clarity). An example of Type ‘‘b’’ relation, with anincreasing reproductive effort with size was found in 1989–1990, the driest year, where the small tussocks showed reducedreproductive output. Type ‘‘c’’ relation, where the reproductiveeffort decreases with size, was found in 2 rainy yr: 1994–1995and 1991–1992 (not shown). Groups of tillers in small tussocksare more productive in favorable conditions, probably becausethey are less shaded, suffer less competition and might haveaccess to the resources in the intertussock patches of bare soil.Conversely, in dry years the small tussocks show lower RE,probably because they are more exposed to evaporativedemand. Grazing can act indirectly by modifying the sizestructure of the population that becomes dominated by smalltussocks. Some management practices that aim to rejuvenatethe population, such as intense grazing and short-durationgrazing might be successful in inducing more productivity andseed output in rainy years. Conversely, these subdivided plantsdisproportionately reduce the reproductive output underdrought, reflecting their poor physiological condition; otherstudies such as Oliva et al. (2005) found that most of the

Table 5. Spontaneous seed recruits (plants �m�2) of different vegetationlife forms present in 2010 in plots that were originally cleared in 2000.Protected plots were enclosed in removable exclosures in this period.

Grazed Protected Mean

Festuca gracillima tussocks 0.6 1.3 1.0

Herbaceous dicots 7.6 9.4 8.6

Short grasses and graminoids 35.9 30.2 32.9

Dwarf shrubs 3.5 1.4 2.4

Total seed recruits 47.6 42.4 44.9

Table 6. Numbers (individuals �m�2) expected in each sequential stage of seed reproduction, from ovules to 24-mo-old seedlings. Each row is multipliedby the coefficients (in italics) that represent estimated losses between stages.

Parameter or process estimated

Years

Mean1989–1990 1990–1991 1991–1992 1992–1993 1993–1994 1994–1995

Ovules 174 536 1 185 429 39 903 544

Failed pollination, abortion 0.410 0.648 0.547 0.195 0.319 0.660 0.415

Seed 71.2 347 647.5 83.4 1.3 595.5 225.8

Nonviable caryopsis 0.680 0.930 0.867 0.696 — 0.957 0.826

Germinable seed production 48 323 561 58 0 570 187

Losses from seed predation in soil 0.310 0.330 0.350 0.600 — 0.397 0.397

Seed in spring 15 106 196 35 0 226 74

Losses from soil pathogens and diseases 0.442 0.442 0.442 0.442 0.442 0.442 0.442

Viable or germinated seed in spring 7 47 87 15 0 100 33

Seedling mortality (first year) 0.000 0.051 0.065 0.033 — 0.050 0.050

Expected seedlings first year 0 2 6 1 0 5 2

Seedling mortality (second year) 0.000 0.894 0.519 0.426 0.000 0.613 0.613

Expected seedlings second year 0 2 3 0 0 3 1

170 Rangeland Ecology & Management

mortality in this F. gracillima population is concentrated in thesmallest tussock size classes.

Production of florets varied markedly between years, rangingfrom 39 to 1 185 florets �m�2 (Fig. 2; Table 6). Maximumnumbers were obtained in 1991–1992, that was not the mosthumid year, but showed adequate rainfall and followed the1990–1991 rainfall maximum. This fact underlines theimportance of the favorable runs of years of adequateprecipitation on the population. Haase et al. (1995) observeda similar high production of florets in Stipa tenacissima L.,counting up to 1 260 caryopses �m�2 on the soil surface afterparticularly favorable 1993 year and interpreted it as a mastingevent (heavy synchronous flowering of the population).

Overall, a mean number of 544 florets �m�2 were producedyearly, suggesting that seed establishment is not limited in thestage of ovule production, except in the drier years. About 58%of the ovules failed to produce caryopses, and losses werealmost total in dry years (Figs. 1 and 2). These predispersallosses are common and Wiens (1984) estimated that approx-imately half of the florets of perennial plants are sterile in awide range of habitats mainly because of incomplete pollina-tion or abortion following resource shortage. In other studies,Lord and Kelly (1999) found that 46.2% of ovules of Festucanovae-zelandeae (Hack.) Cockayne failed to produce a matureseed for reasons other than predation, and Stephenson (1980)found that only the early-pollinated ovules developed inCatalpa speciosa Warder, acting as sinks of resources, and theremaining ovules aborted prematurely in a proportion relatedto the availability of assimilates. In F. gracillima, immatureovules also were observed to be predated by insects and some ofthem might not have developed fruits because of lethalcombinations of genes (Wiens 1984). A relatively smallpercentage of developed caryopses seemed to bear dead ornonviable embryos.

Postdispersal losses of seed involved predation and soilpathogens, subtracting an additional 60% (Table 3). Highlosses were also found in Patagonia by Bertiller and Coronato(1994), who estimated up to 90% of losses in Festucapallescens Host., and in deserts of North America, Brown etal. (1979) found that seeds were damaged by rodents and antsat percentages ranging between 30% and 95%.

Few recruits were lost for failed germination (17%) becauseseeds germinated readily both in the field and in the laboratory(Fig. 1), and virtually no seeds were reserved for futuregermination by dormancy mechanisms (Fig. 5). Similar resultswere found in Festuca pallescens incubated in simulatedautumn field conditions, and in F. gracillima and otherdominant Patagonian species in the laboratory by Soriano(1960). The soil seed bank of this type can be classified asTransient type I following Thompson and Grime (1979), and istypical of habitats with seasonal and predictable drought whereseeds are completely lost by germination and remain viable inthe soil only during the summer and early autumn. The strategyis common among perennial grasses that rely on vegetativegrowth instead of dormant seed to ensure populations (Fenner1985); this poses a problem for regeneration of disturbedpatches because reestablishment depends on the dispersal ofpropagules from adjacent flowering plants.

The heaviest toll for the seedlings came during the firstspring, with mortalities that ranged from 100% (1989) to 94%

(1990) with a mean of 95%, depending on the rainfall ofOctober–November period (Fig. 4). In the second year,mortality of seedlings was only 39%, but the young plantsdid not show the usual exponential growth period of youngplants. They vegetated incorporating one new leaf every 2 mo.In contrast, Agropyron desertorum (Fisch. ex Link) Schult. andBouteloua gracilis (Willd. ex Kunth) Lag. ex Griffiths sown infield conditions developed four and six leaves, respectively, bythe 45th day in North Dakota (Ries and Svejcar 1991). Similarextended vegetative periods and death of nonfloweringindividuals have been observed in many perennial plants(Antonovics 1972), and Harper (1980) considered them to bea result of stress in their natural environment. Existing adultplants probably compete, restricting growth and recruitment,as found in Bouteloua gracilis, where young plants onlydeveloped when roots of adult tussocks were excluded(Aguilera and Lauenroth 1993).

Even after these heavy losses in the sequential process ofestablishment, a small number of recruits ranging from 1 to 3seedlings �m�2 � yr�1 were expected (Table 6), but they were notapparent in photographic maps and after thorough examina-tion of plots from 1989 to 1999 (Oliva et al. 2005). A similarsituation was observed by Bertiller et al. (1996), that found65% of survival in seedlings of Festuca pallescens undersimulated drought, but total mortality in natural conditions.The removal experiment, in cleared plots (Table 5) shows thatabout 1 seedling �m�2 of F. gracillima established after 10 yr,indicating that sexual recruitment of tussocks is possible indisturbed patches with relaxed competition. Humano et al.(2005) also found some recruitment when F. gracillima seedswere artificially sown in cleared plots in a nearby site. Theseresults underline that even though the recolonization of barepatches is possible, the species suffers from competition innatural communities, and new swards become dominated byshort grasses and graminoids, with a low prevalence of tussocksafter 10 yr (Table 5).

Sexual reproduction appears to be an inefficient method tosustain population numbers in contrast with vegetative growth.Why do tussocks engage yearly in the production of up to 1 185seed �m�2 that are likely to die completely? In the first place,even with great numbers of seed produced, the reproductive

Figure 5. Condition (% of total) of seeds sown in nylon mesh bags inMarch 1989 and recovered monthly until March 1990 and analyzed in thelaboratory for germination and viability.

66(2) March 2013 171

effort seems conservative when compared to other species.Secondly, seed are not exclusively produced to sustainpopulation numbers because they also maintain geneticvariation in clonal plant populations and provide means ofdispersal and recolonization of disturbed areas. Continuousincorporation of a large number of seedlings should not beexpected in mature tussock populations that show a density of13.2 plants �m�2 and a yearly mortality of 1.48% (Oliva et al.2005). In these conditions, recruitment of about 1 tussock �m�2

every 5 yr would check mortality losses. Nevertheless, if thepopulation is limited to vegetative reproduction of long-established genets, a few plants could eventually dominate,reducing the genetic diversity of the population. Tamm (1972)observed that a population of Primula veris L. lost 18% ofgenets, and incorporated only 6% by seed, loosing geneticvariation in a 28-yr period. An occasional incorporation byseed could be a mechanism to check this genetic erosion, but itapparently is not taking place in the Moy Aike populations.

Seed establishment seemed to be inhibited in this site, butbecause a few millimeters of rain in spring showed a greatinfluence in survival rates, it might be possible in moister areasof the Magellanic grasslands. Actual climate in Moy Aikecorresponds to a transition between steppe and semidesert orshrubby vegetation according to Budyco (1963), Thornwaiteand Hare (1955), and Burgos (1959), and the grasslands thatcover them might be relicts established under different rainfallregimes.

IMPLICATIONS

The establishment from seeds in F. gracillima is subject to heavylosses that only enable recruitment in favorable years. The seedbanks are transient, and the species relies on the yearly input ofviable propagules to produce seedlings. The irreversibletransitions that have been documented in the MagellanicSteppe under sheep grazing are probably explained becauseadult tussocks are lost under poor management, and there is alack of viable seeds in long-term storage in the soil. When thegrazing pressure is relaxed, regeneration relies on seed dispersalfrom nearby flowering tussock plants, which show lowreproductive effort and only produce considerable seed outputson favorable years. Recruitment becomes increasingly difficultin degraded rangelands that show low tussock cover and arepatchy. In these conditions, other species seem to be morecapable of dispersion and colonization of bare soil areas. Oncethese species are established, competition can preclude thetussock recruitment by seedlings and give way to alternatestates, dominated by short grasses. Careful management ofadult tussock populations and preservation of their vegetativereproduction processes seem to be crucial to ensure populationsustainability.

ACKNOWLEDGMENTS

This research was supported by INTA. Two anonymous reviewers and the

Editor of Rangeland Ecology & Management helped to arrive to the

definitive version of this paper. The authors wish to thank Mr. Jorge

Jamieson, owner of Moy Aike Chico, who allowed the installation of the

grazing trial on his farm and Alberto Battini for his technical help. Daniela

Ferrante, Diego Suarez, Vanessa Torres, Horacio Castro Dassen, Carlos

Bartolomei, Eugenia Vivar, and Juan Carlos Kofalt assisted in the field work.

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