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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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