Acta CEcologica CEeol. Plant., 1989, Vol. 10, n° 2, p. 147-158.

Seed and seedling populations in two contrasted communities: open grassland and oak

(Quercus agrifolia) understory in California

T. Marañón (*) and J. W. Bartolome Department of Forestry and Resource Management,

University of California, Berkeley, California 94720, U.S.A.

ABSTRACT

Seed production, soil seed reserve, and seedling establishment were studied in two annual cornmuni­ties: open grassland and oak woodland understory in California. The soB seed reserve in the open grassland (15,980 to 40,730 seeds per m2) was composed mostly of annual grasses, whereas the smaller reserve in the oak understory (5,760 to 6,850 seeds per m2) was dominated by Stellaria media. At bi­weekly censuses we recorded timing of emergence, seedling numbers, and survival during a drought period after germination. A variety of species response was found in both communities. The average species seed weight was higher in the shaded understory community (7.80 mg) than in the open grassland (5.01 mg). It is suggested that differences among species in seed size and seedling behaviour would promote coexistence in annual communities.

KEY WORDS: California, mediterranean grassland, seed reserve, seed size, seedling survival.

RÉSUMÉ

On a étudié la production des semences, leur réserve dans le sol et l'établissement des plantules, dans deux communautés de plantes annuelles : paturage auvert et saus rarbre, en Califarnie. La réserve des semences dans le sol de paturage ouvert (15980-40730 sem./m2) est camposée fondamentalement par des graminées annuelles, tandis que sous l'arbre elle est ¡nférieur (5760-6850 sem.fm2

), a prédomi­nance de dicotylédone Stellaria media. On a réalisé des censes chaque 2 semaines, en registrant au moment de l'émergence, les numéros des plantules et la survivance pendant une période courte de sécheresse apres la gennination. On y a trouvé une réponse diversifiée dans les différentes especes des deux communautés. Le poids moyen des semences est plus élevé dans le paturage sous l'arbre (7,80 mg) que dans le paturage ouvert (5,01 mg). Les différences panni les especes, dans le poids moyen des semences et dans la réponse des plan tules, favariseraient la coexistence dans les communautés des plantes annuelles.

MOTS CLÉS : Californie, paturages méditerranéens, réserve des semences, poids des semences, survivance des plantules.

(*) Present address: Instituto de Recursos Naturales y Agrobiología, C.S.LC., Apdo. 1052, Sevilla 41080, Spain.

Acta CEeologieajCEcologia Plantarum, 0243-7651/89/02/147/12/$ 3.20/ © Gauthier-Villars

148 T. MARAÑÓN AND J. W. BARTOLOME

INTRODUCTION

Annual plants dominate the mediterranean-type grasslands of California. Newly produced seeds remain dormant throughout the five warm and rainless months (May to September). First signficant rains in October-November initiate the process of germination and establishment of seedlings (BARTOLOME, 1979).

The seed population must survive the critical period of summer and early autumn. Seeds are equipped with dormancy mechanisms, nutritional reserves, dispersal devices, and morphological-chemical protection against predators. The energy investments for these different functions are subject to evolutionary trade­offs through differential survival.

It is recognized that the fate of the populations in the regeneration·phase (seed germination and seedling establishment) mostly determines the species abundance in the vegetation (GRUBB, 1977; HARPER, 1977). This assertion becomes more certain in a type of community which every autumn "reactivates" as a whole from the seed stage.

The soil seed reserve in the California annual grassland reaches high values at the beginning of tbe growth season, as shown in the few studies available: 61,000-67,000 seed per square meter on ungrazed grassland dominated by Vulpia der­tonensis (BARTOLOME, 1979), and 15,300-270,000 seeds per square meter in a grazed, Bromus mollis dominated, community (YOUNG et al., 1981). Data from herbaceous oak understory, a considerable extension of grassland in California, are lacking.

Several generalizations have been inferred from seedling establishment observed in California' s annual grasslands:

- Dominant annnal grasses seem to have a transient sail seed reserve (BARTOLOME, 1979; YOUNG et al., 1981). Nevertheless, legumes, with hard seeds, show a significant carry over (persistent soil seed reserve) that may be important for after-drought recovery (YOUNG et al., 1981).

- The seedlings emerged represent a fraction of the estimated seed reserve: from 20-30% (YOUNG et al., 1981) to 39-75% (BARTOLOME, 1979).

- Patterns of germination timing will influence the population response to early autumn drought (YOUNG et al., 1981) and the community composition for the remainder of the growing season.

- The fates of seedlings are related to the seed size. Species growing in xeric, shade or competitive situations tend to have higher seed weight (BAKER, 1972). Differential predation by rodents seems to favor smaH-seeded species (BORCHERT & JAIN, 1979). The seed size patterns would reflect adaptive compromises between stored nutritional reserve, dispersability and predator defence.

In this study, we have recorded the soil seed reserve and the seedling establish­ment for aH the species in the comrnunity, in two representative habitats of the California oak savanna: the open grassland and the oak understory. The results wiH allow us to test generalities on population structure found in other mediter­ranean annnal communities, comparing two contrasted, adjacent, environments.

The study of population pararneters of 18 species in two habitats, at the expense of detail, responds to the increasing awareness of the needs for understand­ing the population structure of the plant cornrnunity (HoBBs & MOONEY, 1984; WHITE, 1985). This paper reports the observational approach at the seed and

Acta fEcologicajfEcoJogia P[antarum

SEEOS ANO SEEOLINGS IN ANNUAL GRASSLANOS 149

seedling stages, the experimental phase of the study (transplants) will be published elsewhere.

The ecological significance of the species seed size is enhanced in these annual communities regenerating by seeds every year. The relation between average seed weight and demographic parameters is studied at the community level.

MATERIAL AND METHODS

STUDY SITE

The study site is located on the University of California's Russell Reservation, in Contra Costa County, California, U.S.A., in the Coast Range, 20 km inland from San Francisco Bay. The mediter­ranean-type climate alterna tes cool-humid winter with warm-dry summers. Annual precipitation averages 680 mm (97% falIs between October and April), and mean annual temperature is 14°C, with a maximum monthly mean of 28°C in July and a minimum monthly of 5°C in December. Rainfall in 1982-1983 totalled 1,080 mm, and 520 mm in 1983-1984, far above and slightly below average, respectively. Autumn distribution of rainfall was typical in both years, with a germination-inducing rain in late September followed by significant drought prior to onset of steady winter rains (fig. 1).

120

~ 100 E E

80 z o \ ....

60 "" .... "-

" 40 w a: "-

20

O ~, ¡JI S O N O J F M A M S O N O J F M A

1982 1 9 8 3 1984

T I M E (YEAR ANO MONTH)

FIG. 1. - Weekly precipitation during 1982-1983 and 1983-1984 in Russell Reservation, Contra Costa County, California.

Sandstone overlain by a Millsholm soil (loamy, inceptisol) supports a mosaic of oak woodland, shrubland and grassland on hilIy rolling topography. Introduced annual grass species (Lolium multiflo­rum, Bromus mollis, Bromus diandrus and Avellafatua) dominate the grassland and woodland understory, with occassional patches of native perennial grasses (Stipa pulchra and Elymus triticoides).

Plots were located on a 20° South-facing slope covered by annual grassland and inside an adjacent oak (QlIerclls agrifolia) woodland. The area has remained ungrazed by livestock for the last 20 years.

Vol. lO, n° 2 - 1989

150 T. MARAÑÓN AND J. W. BARTOLOME

SAMPLING

Twenty-five soil eores, 4.2 cm in diameter and 10 cm in depth, were taken from the open grassland in September 1982 and 1983. Under the oak canopy, each sample was obtained by mixing three soil cores (oC the same volume), due to the expected lower seed density.

The soil samples were crushed by hand, mixed, and spread aboye a layer oC vermiculite in plastic pats (15 cm in diameter), watered bi-daily and kept outdoors for 6 months in Strawberry Canyan, Berkeley. The emergent seedlings were identified, counted, and discarded.

During the 1983-1984 seasons, seedlings oC every species were censused bi-weekly in ten randomly located 10 x 10 cm perrnanent plots in each habitat. At the eod oC the growing season, in May, individual plants inside each plot were clipped at the base, ovendried and weighed.

Emergence rates were estimated as the ratio between the total number of seedlings censused in the field and the total number of seedlings emerged in the spread soil samples (soil seed reserve estimate). Those values renect the influences of dormancy, availability of safe sites, seedling mortality, etc., and a1though they must be interpreted with caution, they represent an "expression" of the population seed reserve.

Seed production was estimated in May 1983 for each species by multiplying the average seed production per plant (from ten individuals) by the average density of plants (from ten samples of 25 x 25 cm in each habitat).

Given the relative constancy of the average seed weight of a species (HARPER et al., 1970), we relied on the seed weight data provided by H. G. BAKER (pers. cornm.) for the species there represented. Arithmetic mean of average seed weight for each species was calculated from species lists in both habitats (BAKER, 1972). To renect the relative contribution of species to community seed number, plant number or plant biomass, related to their seed size, weighted means were also calculated for different demographic stages.

Nomenclature of plant species follows MUNZ (1959-1968).

RESULTS

SEED RESERVE IN THE SOIL

Seed1ing emergence from spread, watered, soi! samples was used as a measure of the soi! seed reserve. A total of 15,980 seed1ings/m2 emerged from the open grass1and soi1 samp1es in 1982, and 40,730 seedlings/m2 (2.6 times as many as 1982) in 1983. In both years, annual grasses comprised more than 80% of the germinating seeds (fig. 2). From 1982 to 1983, there was an increasing trend for Hypochoeris glabra (5.6 x), Si/ene gallica (4.7 x) and Bromus mollis (3.5 x), whi!e Lupinus bicolor (0.1 x) and Medicago polymorpha (0.2 x) decreased.

In the adjacent oak-wood1and understory, the seed reserve in the soi1 was estimated at 5,760 seeds/m2 in 1982 and 6,850 seeds/m2 in 1983 (fig. 2). Stel/aria media (55-60% of seed reserve) and Bromus diandrus (24-21%) were the dominant species in both years. No signficant inter-year variation was observed in the number of seeds of any species.

Comparing the two habitats, the soi! seed reserve estimated in the open grass1and was greater (2.8 times in 1982 and 5.9 times in 1983) than in the oak­understory soi!, and their distribution by species was also different, excepting Bromus diandrus, which was re1ative1y abundant in both habitats.

Al! the seeds counted in both habitats belonged to annua1 p1ants.

Acta CEcologicaJCEcologia Plantarum

SEEDS AND SEEDLINGS IN ANNUAL GRASSLANDS

N E '- 40.000 • ~ • • •

w >

30.000 oc w • dicots <n w O grasses oc

'" w 20.000 w <n

~

o <n

10.000

OPEN GRASSLAND MK UNDERSTORY

FrG. 2. - Estimated soil seed reserve in September 1982 and 1983 from two communities: open grassland and oak understory. Vertical bars indicate mean +SE.

SEED GERMINATION AND SEEDLING ESTABLISHMENT

151

In the open grassland, a maximum of 11,210 seedlings/m2 were eensused on 7 Deeember 1983 (fig. 3 a). Bromus mollis (42% of seedlings), Lolium multijlorum (23%) and Hypochoeris glabra (19%) were dominan!.

The first seedlings emerged in October, after late September rains. A subse­quent short drought (fig. 1) eoineided with 77% seedling mortality in the eommu­nity. The effeet of this post-germination drought varied among the different species populations (fig. 3 b).

Early emerged Bromus mollis and Lolium multijlorum seedlings suffered high mortality (74 and 86% respeetively), but germination after the drought reeovered both populations, with the highest seedling density in the early Deeember eensus. The few early emerged seedlings of Hypochoeris glabra died during the drought, but subsequent emergenee resulted in 2,150 seedlings/m2, so that this speeies made up 19% of the seedling eommunity in Deeember.

The less abundant species, Avena fatua, Bromus diandrus, Carduus pycnoce­phalus and Geranium dissectum also showed some emergenee in October, followed by drought-indueed mortality and post-drought germination, whereas the legumes (Lupinus bicolor, Vicia sativa and Medicago polymorpha), Si/ene gallica and Torilis nodosa first emerged in late November, avoiding the early autumn drough!.

Vol. 10, n° 2 M 1989

152 T, MARAÑÓN AND J. W. BARTOLOME

12,000 A N

E "- 10,000 ó e

8.000

'" '" z

" " w w

'" O N O

TI M E (months)

e

1.000

N E "-ó e ,:::: '" 100

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O N O O N O TI M E (months)

FIG. 3. - Seedling germination and establishment during October-December 1983. (A) Total emerged seedlings (± SE) in the open grassland (-0-) and the oak understory (-e-). Seedling establish­ment patterns for different populations, in the open grassland (B) and the oak understory (C). Species symbols are: (-0-) Bi'Omus mollis; (-..6.-) Lolium multiflorum; (-0-) Hypochoeris glabra; (-\7-) Geranium dissectum; (-0-) Avena fatua; (-e-) Bromus diandrus; (-A-) Momia perfoliata; (-.-) Galium aparille; (-T-) Carduus pycnocephalus; and (-+-) Stellaria media.

In the oak-understory (fig. 3 a), a maximum of 1,590 seedlingsjm2 were census­ed on 13 December 1983. Bromus diandrus (55%) and Montia pelfoliata (23%) dominated.

The seedling community in Oetober suffered 37% mortality during the drought, but subsequent rainfall induced new germination. Individual species responses showed a diversity of patterns (fig. 3 e).

Early-emerged Bromus diandrus seedlings suffered a slight mortality (11%) during the drought, followed by new emergenee in November, showing the highest density for the eommunity in the Deeember eensus. Stellaria media and Carduus pycnocephalus, also emerged early in Oetober, but suffered a high mortality during the drought (89 and 63% respeetively), followed by a low post-drought germination.

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SEEDS AND SEEDLINGS IN ANNUAL GRASSLANDS 153

The few seedlings of Montia pe/foliata and Galium aparine that emerged in October, died during the drought, but subsequent rains induced a significant germination in November.

EMERGENCE RA TES

The seedling community established in the open grassland, in December, represented 28% of the estimated soil seed reserve. Hypochoeris glabra (52%) showed higher emergence rate than Bromlls mollis (26%) and Lolium multiflorum (23%). Among the low abundance species, Geranium dissectum showed 100% emerg­ence, whereas Silene gal/ica only 1%.

In the oak-woodland understory, the community emergence rate was 29%, although most species showed higher rates, e. g. Bromus diandrus (60%), M ontia pe/foliata (81%), Galium aparine (51%) and Carduus pycnocephalus (86%). These high values were counterbalanced by the low emergence rate of Stel/aria media (7%), which was the most numerous population in the soil seed reserve. Sisymbrium officinale, which was scarce, but frequent in the soil samples, was not recorded in the field census.

SPECIES SEED SIZE AND DEMOGRAPHIC PATIERNS

The weighted means of seed size calculated for the open grassland community were 1.97 mg for the seed production in May 1983 and 2.93 mg for the seed reserve in October 1983. Considering the size of seeds that became plants, the weighted means were 2.69 mg for the established seedlings (December 1983) and 2.82 mg for the reproductive plants (May 1984). With regards to the seed weight "converted" in plant (aboveground) biomass, the weighted mean was 4.97 mg.

In the oak understory community, the weighted means of seed size were: 5.53 mg for the seed production, 3.22 mg for the seed reserve, 6.83 mg for esta­blished seedlings, 6.99 mg for reproductive plants and 5.90 mg for the aboveground biomass. .

Summarizing (fig. 4), in the open grassland community the weighted mean seed size is below 3 mg for the four demographic stages considered, as a result of the importance of small-seeded species (Lolium multiflorum, Bromus mollis and Hypochoeris glabra). In the oak understory community, except for the soil seed reserve (dominated by the small-seeded Stel/aria media), the rest of the demographic stages have a weighted mean seed size close to or above 6 mg, due to the importance of large-seeded species (Bromus diandrus and Carduus pycnocephalus). These large­seeded species (including Lupinus bicolor) contribute mostly to the above-ground biomass in both communities.

Considering the seed size classification by Baker (1972), the species found belong to the classes 6, 7, 8 and 9 (table 1). Class 6 (.316 to .999 mg) species dominate the seed production in open grassland and the soil seed reserve in the oak understory. Class 8 (3.162 to 9.999 mg) species dominate the rest of the stages in both habitats (fig. 5). However, this class includes two species, Bromus mol/is and B. diandrus, that showed different demographic patterns.

Vol. 10, n° 2 • 1989

~

u,

TABLE 1. - Mean seed weight (data fram Baker, pers. comm.), seed production, soil seed reserve, seedling emergence in autumn, ...

and peak aboveground biomass in two constrasted communities: open grassland and oak understory. Standard errors are shown between brackets.

Soil seed Number of seedlings A boveground

Mean seed weight Estimated reserve recorded per m2- biomass seed (seeds/m') (g/m')

Baker's production October December Species (mg) class) (seeds/m')

Open grassland Lolium multiflorum 2.00 7 18,660 11,200 (1,564) 1,050 (351) 2,580 (309) 80.0 (9.3) ,.. Bromus mollis 3.25 8 12,220 18,110 (2,737) 1,960 (344) 4,700 (1,215) 89.8 (16.6) ¡;: ,. Bromus diandrus 7.00 8 740 2,000 (387) 280 (84) 200 (54) 14.0 (6.3) ¡: Avena Jatua 8.88 8 4,820 1,590 (272) 90 (50) 540 (125) 32.7 (9.1) z· Hypochoeris glabra 0.57 6 17,330 4,140 (955) 160 (67) 2,150 (688) 100.1 (29.4) O'

Z Silene gallica 0.32 6 30,820 2,230 (891) 20 (20) ,. Carduus pycnocephalus 5.49 8 1,220 600 (212) 40 (22) 100 (35) 13.6 (7.7) ~ Geranium dissectum 2.19 7 1,400 700 (140) 20 (13) 710 (170) 16.8 (3.0) ,.. Vicia saliva 16.39 9 1,200 100 (53) 50 (22) 33.7 (16.7) " Lupinus bicolor 8.24 8 30 (30) 100 (47) 102.2 (88.9) '" ,. Lotus subpinnatus 2.07 7 10 (10) 0.2 (0.2)

~ M edicago polymorpha 3.67 8 30 (30) 20 (20) 2.9 (2.9) Erodium cicutarium 2.15 7

t"'

>.. O

¡¡ Torilis nodosa na 30 (30) 5.6 (5.6) ¡;:

'" ¡¡l Community (TOTAL) 88,410 40,730 (2,753) 3,600 (498) 11,210 (1,447) 491.6 (104.5)

e Ooak understory

i Bromus diandrus 7.00 8 6,860 1,480 (202) 830 (189) 880 (161) 63.7 (17.5) Montia perfoliata 2.20 7 3,130 450 (106) 50 (22) 360 (64) 5.2 (2.3)

~ Stellaria media 0.50 6 1,740 4,110 (700) 280 (109) 70 (40) 5.6 (3.7) Galium aparine 22.70 9 400 260 (66) 10 (10) 130 (47) 3.3 (1.7)

~ Carduus pycnocephalus 5.49 8 1,790 350 (93) 300 (110) 120 (49) 194.6 (96.5) E' Avena fatua 8.88 8 670 30 (30) 30 (30) 1.7 (1.7)

? Sisymbrium officinale na 170 (80) , Cornrnunity (TOTAL) 14,590 6,850 (769) 1,470 (294) 1,590 (231) 274.1 (98.8) ¡¡ , ¡¡

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SEEOS ANO SEEOLINGS IN ANNUAL GRASSLANOS

SEEO SEED SEEOLlNG REPROCIJCTIVE BIOMASS PROOUCTlON RESERVE ESTABLISHMENT PLANTS

May 83 Det 83 Dee 83 May 84 Mav 84

155

EOak

understory

Dpen Egrasstand

FIG. 4. - Weighted mean of seed weight for different !ife cycle stages, in the open grassland (-0-) and the oak understory (-e-). Arrows on the right side point the arithmetic means of seed weight for both communities. Arrows on the left side mark the mean seed weight of individual species.

DISCUSSION

80IL SEEO RESERVE

The high number of seeds germinating in the soi1 samples from the open grass1and (15,980 to 40,730 seeds/m2) is typical of Mediterranean annual grassland communities, where the numerous soil seed reserve is mostIy composed by "tran­sient" seed reserves of annual grasses (BARTOLOME, 1979; YOUNG et al., 1981; MARAÑÓN, 1985).

The authors do not know of any other study estimating the seed reserve in soils of oak woodlands in California. Compared to the open grassland community, the main differences were:

- Lower values of seed reserve (5,760 to 6,850 seeds/m2). Similar reduction has been found under isolated oaks in the Spanish "dehesa" (MARAÑÓN, 1985) and in temperate forests (THOMPSON, 1978).

- The dominant species, Stellaria media seems to have a "persistent" seed reserve (see SOBEY, 1981, for the British populations).

- Less inter-year fluctuations, due to lhe absence of those species which showed lhe higher variability belween years in lhe open grassland.

Vol. 10, n° 2 - 1989

156 T. MARAÑÓN AND J. W. BARTOLOME

OPEN GRASSLANO OAK UNDERSTORY

SEED PRODUCTION ~ U. SOIL SEED RESERVE II l--SEEDLING ~ l.L ESTABLISHMENT

1

II REPRODUCTIVE ~ DENSITV %

BIDMASS 67 8 9 SEED WEIGHT CLASS

FIG. 5. - Relative importance (as percentage oC cornmunity value) oC the seed weight classes for different life cyc1e stages, in the opeo grassland (leCt colurno) and the oak understory (right colurno).

SEED GERMINATION AND SEEDLING ESTABLISHMENT

The occurrence of a short dry period after the first significant rainfall in October 1983, allowed us to observe different behaviours in seed germination and seedling establishment. Most of the species showed early germination (in October), but the quantitative importance and sensitivity to drought stress of these seedling cohorts were diverse, as was the quantitative importance of the post-drought (November) cohorts. As a result, the seedling populations in December were predominantly formed by post-drought cohorts of Bromus mollis, Lolium multijlo­rum and Hypoclzoeris glabra, whereas for Bromus diandrus, the predrought (October) cohort was more important.

Acta fEcologica/fEcoiogia Plantarum

SEEOS ANO SEEOLINGS IN ANNUAL GRASSLANOS 157

In other words, Bromus mollis, Lolium multijloJ'um, and Bromus diandrus showed a pre-emptive behaviour, wilh a considerable amount of early germination, but after the drought stress, the response of the two former species was based on high seed nnmbers and delayed germination, while the latter species seems to rely on large nutritional reserves that allowed most seedlings to survive the stress.

What signficance will the germination timing have for the reproductive success of the subsequent early (October) versus late (November) seedling cohorts? Unfortu­nately the phenomenon has not been followed in this study but other evidences suggest an increase in survival and reproductive output for the earlier seedlings (Ross & HARPER, 1977).

In the understory habitat, Ste/laria media, Carduus pycnocephalus and Bromus diandrus showed early germination. The two former species were sensitive to the drought stress and did not recover to the initial numbers, while the latter species exhibited drought tolerance with low mortality. Montia perfoliata and Galium aparine, late emerging, "avoided" the early autumn drought stress.

In general, species composition and relative proportions in seedling populations emerged by December reflected the (estimated) soil seed reserve in both habitats. Although average emergence rates were similar (close to 30%) in both communities, abundant species had lower emergence rates in the open grassland (23 to 52%) than in the oak understory (51 to 86%, excluding Stel/aria media). The results obtained in the open habitat are in accordance with other values found in California grasslands (BARTOLOME, 1979; YOUNG et al., 1981).

A few species, Ste/laria media and the less abundant Silene gallica and Sisym­brium officinale, showed marked discrepancy between soil seed reserve and emerged seedlings, due probably to a disturbance-adapted persistent seed reserve.

SPECIES SEED WEIGHT WITHIN THE COMMUNITY

The mean seed weight is a characteristic relatively constant within the species (HARPER et al., 1970). Based on that assumption, SALISBURY (1942) and BAKER (1972) calculated the average seed weight fram species lists of contrasted communities in Britain and California, respectively. Both authors observed that the average seed weight was higher under shade conditions, and explained that fact as a result of the selection for larger investment in nutritional reserve for the seedling. In our study, the (arithmetic) mean of the species mean seed weight for the herbaceous understory community (7.80 mg) is higher than the mean for the open grassland community (5.01 mg), so supporting that hypothesis. The weighted mean, which is affected by the "weight" or importance of each particular species in the community, however, varied for different demographic stages in both com­munities (fig. 4).

- The weighted means are lower than the arithmetic mean (fram the species list) in both communities. Small-seeded species have relatively more "weight" in the community demography.

- Weighted means in the oak understory community (close to 6 mg) are higher than in the open grassland community (close to 3 mg), but not in the soil seed reserve stage, where the understory community has a low value (3.22 mg) due to the importance of the small-seeded S tel/m'ia media.

Vol. 10, n° 2 - 1989

158 T. MARAÑÓN ANO J. W. BARTOLOME

Small-seeded popu1ations are more numerous in the open grassland, but the eommunity biomass is displaced towards large-seeded speeies (weighted mean of 4.97 mg).

A detailed analysis of the seed and seedling populations within a eommunity normally reveals differenees in speeifie eharaeters: mean seed weight, seed produe­tion, seed reserve in the soil, timing of seedling emergenee, seedling survival of drought, seedling numbers, etc. The broad speetrum of regeneration requirements will favor the eoexistenee of the species within the eommunity (GRUBB, 1977).

The unpredietable pattern of early autumn preeipitation in California savannas will promote the eoexistenee of species with different "regeneration niehes". How­ever, in the understory habitat, the environmental fluetuation is buffered by the eanopy effeet and the speetrum of regeneration syndromes is redueed (MARAÑÓN, 1986).

ACKNOWLEDGEMENTS

We thank R. KELLEY foc ficld assistance, to H. G. BAKER foc kindly providing seed weight data. R. G. BAKER. P. JORDANO, R. W. CRUDEN and t\Vo anonimous reviewers made cornments on the manuscript. T.M. was supported by a FulbrightjMinisterio de Educación y Ciencia Scholarship.

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BARTOLQME J. W., 1979. - Germination and seedling establishment in California annual grassland. J. 01 Ecology, 67, 273-281.

BORCHERT M. l. & JAIN S. K., 1979. - The erfeet of rodent seed predation on four species of California annual grasses. CEcologia, 33, 101-110.

GRUBB P. J., 1977. - The maintenance of species-richness in plant communities: the importance of the regeneration niche. Bio/. Reo., 52, 107-145.

HARPER J. L., 1977. - Population Biology oj Plants. Academic Press, London, HARPER 1. L., LovELL P. H. & MOORE K. G., 1970. - The shapes and sizes of seeds. Ann. Reo, Eco!.

Syst., 1, 327-356. HOBBS R. 1. & MOONEY H. A., 1985. Community and population dynamics of serpentine grassland

annuals in relation to gopher disturbance, CEcologia, 67, 342-351. MARAÑÓN T., 1985. - Reserva de semillas en el suelo de una dehesa en Sierra Morena: relación

con la vegetación. Anales Edaf. Agrobio/., 44, 1805-1816. MARAÑÓN T" 1986. Plant species riehness and eanopy effeet in the savanna-like "dehesa" of S, W. Spain.

Ecol. Medit., 12, 131-141. MUNZ p, A" 1959-1968. - A Calijomia Flora and Supplement. University of California Press, Berkeley. Ross M. A. & HARPER J. L., 1972. - Occupation of biological space during seedling establishment. J.

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