Influência do calor na germinação de sementes

8/3/2019 Influncia do calor na germinao de sementes

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Plant Ecology 136: 95103, 1998. 95c

1998 Kluwer Academic Publishers. Printed in Belgium.

Influence of heat on seed germination of seven Mediterranean Leguminosaespecies

Jose M. Herranz, Pablo Ferrandis & Juan J. Martnez-Sanchez

Department Plant Production & Agricultural Technology, E.T.S.I. Agr onomos, University of Castilla-La Mancha,Campus Universitario s/n, 02071 Albacete, Spain

Received 23 June 1997; accepted in revised form 24 January 1998

Key words: Hardseedness, Heat, Leguminosae, Seed germination

Abstract

The influence of high temperatures (dry heat and hot water) on germination of seven Mediterranean Leguminosae

species typical of fire-prone ecosystems in southern Spain is analyzed, in order to know the response of seeds to

wildfires and the possible implications in their regeneration after this disturbance. Seeds were heated to a rangeof temperatures (50 150 C) and exposure times (160 min) similar to those registered in the upper soil layers

during wildfires. Germination tests were carried out in plastic Petri dishes over 60 days. In general, the degree

of seed germination promotion by dry heat treatments showed a wide interspecific variation, although the final

germination level was increased in all the studied species exceptfor Scorpiurus muricatus. The thermal pretreatment

of50 C, however, was not effective for germination in any species, and rising the temperature to 70 C only slightly

enhanced the germination in Cytisus patens. The preheatings of 90 C (5 and 10 min), 120 C (5 and 10 min),

and 150 C (1 min) were the most effective in promoting seed germination. Hot water (100 C) scarification also

increased the final germination level in all cases, with the exception of C. patens. The germination rates after

preheating were much lower than in mechanically scarified seeds and closely resembled those of the untreated

seeds, except for C. reverchonii, whose seed germination rate decreased with heat. The response of species to heat

shock had no clear relationship with life trait or with the specific post-fire regeneration strategy (obligate seeder or

facultative resprouter). Those species coexisting in the same habitats had different heat optimal requirements forseed germination, an strategy suggested by some authors as minimizing interspecific competition in the secondary

succession started after fire.

Introduction

Fire has a great influence in maintaining many plant

communities throughout the world, especially in

Mediterranean-type ecosystems(Naveh 1975;Trabaud

1983; Trabaud & Lepart 1981; Arianoutsou & Margar-

is 1982; Mazzoleni & Pizzolongo 1990; Hanes 1971;

Purdie 1977), but also in African savannas, pine plain

communities in southeastern USA, and heathlands inEurope (Bullock & Webb 1995).

After fire, most species in these communities are

able to regenerate by vegetative regrowth, but there are

a few others which regenerate solely from seeds (Tra-

baud 1987; Casal 1987; Tarrega et al. 1992; Martnez-

Sanchez et al. 1996). When plants are killed by fire

(obligate seeders), they regenerate by seedling estab-

lishment. Where adult plants are capable of resprout-

ing from belowground vegetative buds, no seedling

recruitment may be needed to maintain a community

after a single fire. However, there may be some mor-

tality of adult plants during the fire and some sen-

escence in interfire periods, so that some successful

seedling recruitment will be necesary in order to com-

pensate adult plant losses over a long series of fires(Auld & OConnell 1991). Moreover, it must be kept

in mind that although sexual reproduction may not

be quantitatively the most important way of regenera-

tion with regard to biomass contribution, it can be of

great interest since it is the only way of conserving the

genetic variation of species which tend to regenerate


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by sprouting (Tarrega et al. 1992). Therefore, it is cru-

cial to know the factors affecting seed germination of

main species in a community, in order to understand

the post-fire plant dynamics.

Although there are many studies that demostrate

the influence of fire on seed germination of species of

Mediterranean-type ecosystems (Naveh 1974; Arian-outsou & Margaris 1981; Troumbis & Trabaud 1986;

Keeley 1987, 1991; Trabaud & Oustric 1989; Thanos

& Georghiou 1988; Thanos et al. 1992; Corral et al.

1990; Roy & Sonie 1992; Valbuena et al. 1992;

Gonzalez-Rabanal & Casal 1995), those referring to

legumes are scarce (Cushwa et al. 1968; Martin et al.

1975; Cavanagh 1980; Mott et al. 1982; Pereiras et al.

1985; Tarrega et al. 1992; Auld & OConnell 1991).

These studies suggest that the high temperatures gen-

erated in the fire can stimulate germination in species

with a hard seedcover, by inducing the breaking of

seed coats, thereby facilitating the subsequent embryo

imbibition and radicle expansion.

In the present work, heat requirements for seed ger-

mination of seven MediterraneanLeguminosae species

are studied. They are: Cytisus striatus (Hill.) Rothm.,

Cytisus reverchonii (Degen & Hevier) Bean, Cytisus

patens L., Dorycnium pentaphyllum Scop., Argyro-

lobium zanonii (Turra) P.W. Ball, Scorpiurus murica-

tus L., and Psoralea bituminosa L. These species are

an important component in shrublands established in

regularly burnt areas in the southern half of the Iberi-

an Peninsula, and can be frequently found growing

side by side, as in the case of C. reverchonii and D.

pentaphyllum, as well as P. bituminosa, S. muricatus,D. pentaphyllum, and A. zanonii (Martnez-Sanchez

1994; Martnez-Sanchez et al. 1994, 1996).

The three Cytisus species are Iberian endemic

medium-large shrubs (1.53 m), which behave as fac-

ultative resprouters after fire. C. striatus forms mixed

shrublands on siliceous substrata with Cistaceae and

Ericaceae species in the western half of the Iberian

Peninsula, C. patens is a typical plant of humid hab-

itats in the eastern half, whereas C. reverchonii forms

practically pure shrublands in theBetic mountain range

(south Spain). The remaining studied species show a

broad Mediterranean distribution (Tutin et al. 1964

1980). P. bituminosa, S. muricatus, and A. zanonii are

obligate seeders, whereas D. pentaphyllum is a fac-

ultative resprouter. These four species usually form a

dense vegetation during the first post-fire years, not

only in Spain but also in other different Mediterranean

countries, and show notable declines subsequently

(Papavassilious & Arianoutsou 1993; Clemente et al.

1996). This phenomenom has been attributed by these

authors to a fire-induced seed germination, although

until now no test of this hypothesis has been published.

This study tries to make up for the lack of information

in this field on Leguminosae species. Besides, it is

of great interest to know whether those species which

coexist in the same habitats have similar heat require-ments for the promotion of seed germination or not. As

it has been suggested for Cytisus scoparius (L.) Link

and Genista florida L. (Tarrega et al. 1992), differen-

tial requirements for heat could have adaptative value,

as such a mechanism could permit germination of each

species in different points of the ground, according

to the temperature reached with the fire. In contrast,

uniform heat requirements would cause simultaneous

germination of these species and lead to a damaging

competition between seedlings.

Thus, the aim of this study is:

(1) assessing the temperature intervals and dry heat

exposure times which promote seed germination for

the studied species, as well as the effect of hot-water

treatments;

(2) knowing the degree of similarity in heat require-

ments for the promotion of seed germination in those

species tending to share the habitat.

Methods

Plant material

Names of taxa follow the nomenclature of Tutin et al.(19641980).

Seed samples were collected from ripe fruits of

as many plants as possible in healthy looking wild

populations (so that high seed viability was assumed

for all species) in several localities in the province of

Albacete and Ciudad Real (southern half of Spain):

C. striatus: Cabaneros National Park, province

of Ciudad Real, M.T.U.: 30SUJ6860, 840 m of alti-

tude, quartzite substratum, in a mixed maquis shrub-

land with Arbutus unedo L., Erica australis L., E.

arborea L., Phillyrea angustifolia L., Cistus ladanifer

L., and Halimium ocymoides (Lam.) Willk. The area

was last affected by fire in August 1970. Date of seed

collection: 22 July 1994.

C. patens: Banos de Tus, Yeste, province of

Albacete, M.T.U.: 30SWH5246, 820 m of altitude,

siliceous sand substratum, in a mixed maquis shrub-

land with Pistacia lentiscus L., Quercus coccifera L.,

Lonicera implexa Aiton, Arbutus unedo, and Phil-


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lyrea angustifolia. The area was last affected by fire

in August 1979. Date of seed collection: 26 July 1994.

C. reverchonii: Monte Ardal, Yeste, province of

Albacete, M.T.U.: 30SWH5646, 1220 m of altitude,

calcareous substratum, in a mixed shrubland with Ros-

marinus officinalis L., Juniperus oxycedrus L., Cis-

tus monspeliensis L., Brachypodium retusum (Pers.)Beauv., and Festuca nevadensis (Hackel) Markg. The

area was last burnt in August 1985. Date of seed col-

lection: 26 July 1994.

D. pentaphyllum,A. zanonii, P. bituminosa,and S.

muricatus: the surrounding area of Moropeche village,

Yeste, province of Albacete, M.T.U.: 30SWH5560,

900 m of altitude, siliceous sand substratum, in gaps of

a mixed shrubland with Quercus coccifera, Juniperus

oxycedrus, Rosmarinus officinalis, Phillyrea angusti-

folia, and Cistus ladanifer. The area was last burnt in

August 1989. Date of seed collection: 27 July 1994.

The three mentioned localities in Yeste were sub-

sequently affected by a great forest fire, which des-

troyed 13 341 ha between 7 and 19 August 1994.

Until February 1996, when germination tests were

started, seed samples were stored in paper bags in a

5 C cold room.

Temperature treatments

Seeds were heated to a range of temperatures similar

to those registered during wildfires in the upper lay-

er of the soil (depth of 05 cm), where the majority

of seed banks are usually concentrated (Roberts 1981;

Simpson et al. 1989). Measurements in wildfires inthe California chaparral (De Bano et al. 1977), in the

garrigue of southern France (Trabaud 1979), and in

Mediterranean-type ecosystems of southeastern Aus-

tralia (Floyd 1966; Auld & OConnell 1991; Bradstock

& Auld 1995), showed that temperature in the 05 cm

deep soil layer usually ranges between 50150 C,

with very variable heat exposure periods depending on

depth and fire intensity. Therefore, it was decided to

test the treatment temperatures of 90 C, 120 C, and

150 C, for durations of 1, 5, and 10 min. Moreover,

it was considered interesting to test the effect of lower

temperatures during longer heat exposition periods:

50 C for 10, 15, 30, and 60 min, and 70 C for 5, 10,

15 and 30 min. Thus, the temperature interval used in

this study was similar to those tested in other works on

legumes seed germination of the same nature (Cush-

wa et al. 1968; Mott et al. 1982; Pereiras et al. 1985;

Auld & OConnell 1991; Tarregaet al. 1992). The pre-

heating was carried out on dry seeds spread on glass

dishes in a muffle furnace and the temperatures were

maintained stable within a range of

2 C.

There were 4 replicates of 25 seeds for each treat-

ment. Besides, one complete set of 100 untreated

seeds was used as control in each species. Another

set was submitted to mechanical scarification, that was

achievedby abrasion between two pieces of sandpaper.This treatment has a high effectiveness in the remov-

al of physical dormancy imposed by an impermeable

hard seedcoat, both in Leguminosae (Pereiras et al.

1987; Anorbe et al. 1990; Tarrega et al. 1992) and

in Cistaceae (Thanos & Georghiou 1988; Trabaud &

Oustric 1989; Thanos et al. 1992) species (70100%

germination level). It gives valuable information on

seedlot viability and an important reference to com-

pare the effectiveness of thermal treatments on seed

germination promotion.

In those species for which there were enough seeds

(all of them exceptD. pentaphyllum and P. bituminosa),

an additional treatment with hot water was carried

out. This scarification technique has been previously

used to reduce hardseedness in legumes (Cushwa et al.

1968), as well as in Cistaceae species with hard coat

seed (Perez-Garca et al. 1995). Seeds were immersed

in a glass containig 50 ml hot distilled water (100 C),

and then left to cool to room temperature.Treated seeds

were kept in the water for 24 h, and subsequently sown

and incubated in the same conditions as seeds submit-

ted to the other treatments.

Germination conditions

Seeds were germinated in plastic Petri dishes (diam.

7 cm) over 2 caps of filter paper. They were kept satur-

ated with distilled water throughout all the treatments,

and laid at random on a temperature controlled cham-

ber (model Radiber, AGP-600, 9000 LUX,

0.1 C of

precision). Incubation was carried out in photoperiod

conditions: fourteen hours in light at 18 C, and ten

hours in darkness at 13 C.

Radicle emergence was the criterion for deciding if

germination had taken place (Come 1970). Recount of

the germinated seeds was carried out every two days

over a period of 60 days, and germinated seeds were

removed from the plastic Petri dishes.

Statistical analysis

The effect of treatments on seed germination was stud-

ied by the analysis of two parameters: (i) the final

cumulative percentage of germination (i.e., germina-


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tion level), and (ii) the rapidity of germination (i.e.,

germination rate; Keeley 1987; Thanos & Georghiou

1988; Thanos et al. 1992) expressed by the T50 para-

meter (Thanos & Georghiou 1988; Escudero et al.

1997), which can be defined as the time required for

manifestation of half of the final germination level.

For each species, germination level and germin-ation rate data were submitted to a oneway ANOVA

with treatment (defined by temperature and exposure

time, besides mechanical scarification, control, and, in

some cases, hot water scarification) as factor. Tukeys

test (1949) was used to detect any significant differ-

ences ( =

0:

05) in the comparison between the pairs

of treatments. Previously the sampling normality was

checked by the David-test (1954) and the homogeneity

of the variances by the Cochran-test (1941). Data of

final germination (percentages) needed an arcsin trans-

formation.

Results

The germinability of untreated seeds was generally

low, ranging between 615%, with the exception of

P. bituminosa whose control seeds reached a final ger-

mination level of 27% (Figure 1). As deduced from the

T 50 values (Table 1), the germination rate of untreated

seeds was considerably slower than that of mechan-

ically scarified seeds, except for A. zannonii and P.

bituminosa, and similar to the values recorded for pre-

heated seeds. Only in the case of C. reverchonii the

germination rate value of untreated seeds was higherthan those recorded in thermal treatments.

The mechanical scarification of the seed coat res-

ulted in a pronounced increase of germinability, with

final germination levels including between 77.5 and

93% (Figure 1), and theT 50 values decreased to 4.512

days (Tabla 1). Hot water scarification also increased

the final germination level in all tested species, except

in the case of C. patens. The germination rate of hot

water scarified seeds was similar to that recorded by

mechanical scarification in C. patens, A. zanonii, and

S. muricatus, but slower in C. striatus and C. rever-

chonii (Table 1).

Dry heat scarification had also a strong effect on

final germination level, with the exception of S. mur-

icatus. Inthecase ofD. pentaphyllum, germination was

increased significantly over the control merely for one

heat treatment: 120 C for 10 min. It can be said that

germination followed a characteristic response. At low

temperatures(50 C) germination was not enhanced in

any studied species, irrespective of heat exposure dura-

tion, and the 70 C temperatureonly slightly promoted

the germination of C. patens seeds. From this point,

increases in temperature caused significant increases

of seed germination in the majority of cases, until a

constant level. Preheatings at 90 C for 5 and 10 min

enhanced germination in C. striatus, C. reverchonii, A.zanonii, and P. bituminosa. Temperatures of 120 C

and 150 C were also very effective in promoting seed

germination, although 150 C during exposure times

equal and longer than 5 min resulted in a detriemental

effect on seed viability of all the studied species. In

both C. striatus and C. reverchonii there were sever-

al dry heat treatments which promoted germination as

much as mechanical and hot water scarification.

D. pentaphyllum had the highest heat requirements

for promoting germination, and C. striatus showed the

highest heat resistance. So, the preheatingat 120 Cfor

10 minon seeds of this species gave a final germination

value of 76

8:

48%, and even 2% of seeds were able

to germinate after the 150 C for 10 min treatment.

Discussion

All the species included in the study produce a substan-

tial fraction (627%) of softcoated seeds which ger-

minatewithout thenecessity of any particular treatment

(control seeds), similar in magnitude to those obtained

in other Cytisus species and Genista florida (Anorbe

et al. 1990; Tarrega et al. 1992). This softcoated seed

fraction is responsible for the maintenance of popula-tion levels of these species during periods without fire,

and explains their colonizing role in disturbed areas

as abandonned fields and vegetation gaps. The ability

of D. pentaphyllum and the three Cytisus species for

regenerating by vegetative regrowth, however, enables

them to decrease their dependence on reproduction by

seeds.

Nevertheless, in all tested species, the proportion

of hardcoated seeds which can be softened and, thus,

promoted to germinate by mechanical and hot water

scarification or thermal pretreatments has been high-

er. In nature there are many mechanisms producing the

crack of the tegumentary barrier in legumes, as temper-

ature oscillation and the alternance of dry and wet peri-

ods (Quinlivan 1971; Rolston 1978), bacteria and other

soil microrganism action, and the chemical scarifica-

tion suffered throughout the herbivoredigestive system

(Pereiras et al. 1985), although the heat generated by

wildfires is an important trigger factor (Casal 1982;


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Figure 1. Effect of high temperature pretreatments (50 C150 C) upon the germination of Cytisus striatus, Cytisus reverchonii, Cytisus

patens, A. zanonii, S. muricatus, D. pentaphyllum and P. bituminosa seeds. Anova F ratio value for each species is indicated under specific

names (p

0 001 in all cases). Different lower case letters above columns indicate significant differences (p

0 05) between final average

germination percentages in the pretreatments carried out for each species. Vertical lines in the columns indicate standard deviation. C=

control

seeds; S=

mechanical scarified seeds; W=

hot water scarified seeds. 1 0 , 50 , 100 , 15 0 , 30 0 , and 600 are the different dry heat exposure periods

(in min) used in the experiment.


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Table 1. Average T50values ( standard deviation) recorded for untreated (control; C) seeds, mechanically (S) and hot water (W)

scarified seeds, as well as for those thermal treatments which remarkably promoted germination. F = Anova F ratio value ( :

p 0.01; : p 0 001) for each species. Superscripts with different letters denote significant differences ( p 0 05) between

pairs of cases within each species. Blank spaces correspond to absence of germination in a species.

C S W 90 50 90 100 120 10 120 50 150 C 1 0 F

Cytisus 22.00a 6.00b 31.00a 37.00a 40.50a 32.00a 36.50a 40.50a 7.08

striatus (

7.48) (

0.00) (

3.00) (

1.00) (

2.95) (

12.08) (

2.17) (

5.17)

Cytisus 24.00a 5.50b 27.50a 48.50c 37.50d 40.00d 35.00d 43.00c 98.78

reverchonii (

0.00) (

0.96) (

0.86) (

0.86) (

5.17) (

1.41) (

1.00) (

2.23)

Cytisus 37.50ab 12.00c 14.50c 38.00bd 38.00bc 28.50f 48.00c 48.50c 15.67

patens ( 0.86) ( 1.41) ( 0.86) ( 4.47) ( 2.00) ( 3.84) ( 0.00) ( 2.95)

Argyrolobium 8.66a 6.00a 6.50a 15.00ab 15.00ab 22.50bc 13.00ab 7.22

zanonii ( 3.77) ( 0.00) ( 0.86) ( 3.60) ( 1.00) ( 7.92) ( 2.23)

Scorpiurus 28.00a 9.50b 9.00b 32.50a 30.00a 32.00a 35.00a 29.50a 25,84

muricatus (

4.00) (

0.86) (

0.00) (

1.65) (

1.63) (

1.63) (

3.00) (

4.33)

Dorycnium 12.00a 4.50b 11.00a 9.50ab 12.00a 14.50a 11.00a 3.91

pentaphyllum ( 1.63) ( 0.86) ( 1.00) ( 1.65) ( 4.69) ( 0.86) ( 1.73)

Psoralea 6.50 6.50 7.00 7.50 6.50 6.00 7.50 0.77

bituminosa ( 0.86) ( 0.86) ( 1.00) ( 1.65) ( 0.86) ( 0.00) ( 1. 65) (n. s. )

Baskin & Baskin 1989; Tarrega et al. 1992). As other

plant families having hardseedness feature, in legumes

heat cracks the seed coat thus allowing water uptake

and removing those mechanical restrictions to radicle

expansion (Rolston 1978; Vuillemin & Bullard 1981).

Therefore, the hardcoated seed fraction, commonly

associated with a persistent soil seed bank (Baskin &

Baskin 1989), may be mainly responsible for massive

germination of legumes recorded after some wildfires,

since soft seeds and some of the less hard ones are very

likely to be destroyed by fire. So, in several burnt stands

adjacent to Moropeche (Yeste, province of Albacete),

the average cover of P. bituminosa, A. zanonii and D.

pentaphyllum in the first spring after fire occurred in

August 1989 were 82.00%, 3.78% and 4.98% respect-

ively; they showed values of 0.40%, 0.82% and 5.83%

in the spring 1994 (Martnez-Sanchez et al. 1996). A

suitable explanation for these post-fire legume coverfluctuations may be that hardcoated seeds stored in

soil as a seed bank persistent component are promoted

to germinate in a massive way by high temperatures

reached during wildfire, although the presence of A.

zanonii and P. bituminosa tend to be reduced in sub-

sequent years by competition with other bigger size

plants. Similar cover oscillations during the first post-

fire years has been described in Serrada Arrabida (Por-

tugal) for the Leguminosae species P. bituminosa and

Astragalus lusitanicus Lam. (Clemente et al. 1996).

The practically total absence of germination recor-

ded for all the studied species at higher temperatures

during long exposure times (different among species),

must be due to these heating treatments turning out

to be fatal for the embryo, as has been suggested by

Tarrega et al. (1992) for C. scoparius and G. florida.

Results in this direction have been recorded for many

legume species in other fire-prone regions. So, Martin

et al. (1975) showed 100110 C for 4 min as a leth-

al heat treatment for several Lespedeza, Cassia, Des-

modium and Galactia herbaceous species of meadows

in southeastern USA. Auld & OConnel (1991) recor-

ded 120 C for 5 min treatment killing whole seed-

lots of several Acacia, Bossiaea, Daviesia, Glycine,Gompholobium, and Platylobium species of Australi-

an flora. However, seed germination of other Acacia

species, as well as of Pultenaea, and Sphaerolobium

was stimulated, denoting a wide variation in the spe-

cific seed germination requirements.


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The degree of stimulation of germination by

thermal treatments obtained in the present work for

some species (i.e., C. striatus, C. reverchonii, A. zan-

onii and P. bituminosa) show similar values to those

obtained for G. florida and C. scoparius (Tarrega et al.

1992), and for Ulex europaeus L. (Pereiras et al. 1985)

in Spain, for Acacia saligna (Labill.) Wendl., Poda-lyria calyptrata Wild., and Virgilia oroboides (Berg.)

Salter in South Africa (Jeffery et al. 1988), and for

several legume species in eastern Australia (Auld &

OConnel 1991). The most effective temperature in

promoting C. patens seed germination was70 C, coin-

ciding with records for six Acacia species in the east

coast of Australia (Floyd 1966).

Five species in the study had a broad interval of

thermal treatments promoting seed germination. This

interval is within the range of temperature conditions

commonly encountered during fires in the soil surface

horizons (Trabaud 1979; Tarrega 1992), so that any

heat conditions in the soil produced by fire induce

the softening and subsequent germination of a certain

seed quantity. During wildfires the heat conditions in

soil are not uniform in space and time, because of

the ground microrelief, the presence of big stones and

rock emergence, and the uneven distribution of litter

and standing biomass (Trabaud& Oustric 1989). Thus,

when a fire takes place, there is always a relatively

high number of hardcoat seeds released from physical

dormancy, irrespective of the intensity and the duration

of the fire heat (Thanos & Georghiou 1988). The range

of temperatures in the soil allows the seeds of each

legume, whose germination is promoted by thermaltreatments, to find their specific heat requirements.

The high temperatures promoting germination in

those studied species which often share thehabitat were

notably different.Mixed shrublands withD. pentaphyl-

lum and C. reverchonii are frequent in Betic mountain

ranges. Although there is certain risk in using results

from laboratory experiments for the interpretation of

field observations, it may be thought that post-fire ger-

mination inD. pentaphyllum will be only stimulated in

those microhabitats where temperatures reach 120 C

for 10 min, whereas for C. reverchonii there are a

broad range of thermal treatments which promote its

seed germination. A similar interpretation of results

can be made for those communities where D. penta-

phyllum, P. bituminosa,A. zanonii and S. muricatus are

important components in the post-fire vegetation, as

commonly occurs in several localities in the river Tus

valley (Yeste, Albacete province). The heat require-

ments ofP. bituminosa and A. zanonii are similar, but

very different to those for D. pentaphyllum. The pre-

heating at 120 C for 5 min, however, was effective

in promoting germination for P. bituminosa but not for

A. zanonii, and the preheating at 90 C for 5 min pro-

duced a higher final germination level in the second

species than in the former. S. muricatus seed germina-

tion was not stimulated by any dry heat treatment usedin the experiment. Its high post-fire cover and density

values recorded in several locations of river Tus val-

ley (Martnez-Sanchez 1994; Martnez-Sanchez et al.

1996) could be explained taking into account that a

considerable fraction of seeds are able to tolerate the

fire effect (i.e., seeds submitted to 120 C for 10 min

treatment showed a final germination value of 9%),

the high seed production in this species, as well as the

possible softening of its hardcoated seeds in nature.

The strategy of heat requirement differentiation in

coexisting species has been already described both in

Leguminosae species, as G. florida and C. scoparius

(Tarrega et al. 1992), and several Cistus species (Tra-

baud & Oustric 1989), and interpreted as a mechanism

which minimizes the intensity of interspecific com-

petition, since it allows a differential establishment of

species by occupying different points in the ground

after fire.

The present work fails to find a clear relationship

between the response to heat shock and the life trait

or the specific post-fire recovery strategy. Moreno &

Oechel (1991, 1994) have hypotetized and confirmed

for Ceanothus greggii Gray and Adenostoma fascicu-

latum H. & A. in California chaparral, that seeds of

obligate seeders are more resistantto elevate fireintens-ity than facultative resprouters, because the former rely

solely on their seeds to regenerate after fire. Our results

show an opposite pattern in the case of the facultative

resprouter C. striatus (and even for D. pentaphyllum),

which showed a higher resistance to elevate temperat-

ures (120 C) than S. muricatus, A. zanonii and P. bitu-

minosa, the three obligate seeders in the study. For the

other two facultative resprouters, C. reverchonii and C.

patens, no clear trend was detected. Future studies on

the importance of each post-fire regeneration strategy

in these facultative resprouters may help to explain the

different seed response to heat: a more active involve-

ment of seed germination, instead of resprouting, in the

post-fire regeneration ofC. striatus could be predicted,

as observed by Herranz et al. (1996) for the facultative

resprouters C. scoparius and G. florida in Tejera Negra

Natural Park (central Spain).

With the exception of P. bituminosa, the germin-

ation rates after thermal pretreatments were much


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102

slower than in mechanically scarified seeds and almost

similar to those of untreated seeds (for C. reverchonii

was even slower than the control set). This behaviour

has been previously recorded in Leguminosae species

(Tarrega et al. 1992) as well as in Cistaceae spe-

cies (Troumbis & Trabaud 1986; Thanos & Georghiou

1988; Trabaud & Oustric 1989; Thanos et al. 1992).These differences in germination rates has been inter-

preted as a result of the different effect of mechanic-

al scarification and thermal pretreatments on the seed

coat structures in species with hard coat seeds (Thanos

et al. 1992). The slow germination rate of softened

seeds by heat has been previously considered as an

obvious ecological advantage in the summer-dry and

fire-prone Mediterranean climatic conditions (Thanos

& Georghiou 1988; Thanos et al. 1992). It could be

interpreted as a delaying mechanism which prevents

germination under conditions of occasional rainfall

during late summer and early autumn but which do

not supply enough moisture for seedling establishment

and growth.

Acknowledgements

We wish to thank I. Cuenca and R. Garca for their

laboratory assistance, and four anonymous referees for

their suggestions on the original manuscript. This work

forms a part of a more extensive study focused on post-

fire plant recovery processes in Mediterranean forests

of southeastern Spain carried out in the AGF96-1151

project, supported by the Comision Interministerial deCiencia y Tecnologa of Spain.

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