sus ainabili y
A icle
Can Sh ub Flammabili y be A ec ed by Goa G azing?
Flammabili y Pa ame e s o Medi e anean Sh ub Species
unde G azing
Juan Manuel Mancilla-Ley ón1, Ca men He nando 2,3, Jesús Camb ollé1, Sa a Muñoz-Vallés1,
Ra ael Pino-Mejías 4and Ángel Ma ín Vicen e 1,*


Ci a ion: Mancilla-Ley ón, J.M.;
He nando, C.; Camb ollé, J.;
Muñoz-Vallés, S.; Pino-Mejías, R.;
Vicen e, Á.M. Can Sh ub
Flammabili y be A ec ed by Goa
G azing? Flammabili y Pa ame e s o
Medi e anean Sh ub Species unde
G azing. Sus ainabili y 2021,13, 1555.
h ps://doi.o g/10.3390/su13031555
Academic Edi o : Geo ge K. Symeon
Recei ed: 8 Decembe 2020
Accep ed: 29 Janua y 2021
Published: 2 Feb ua y 2021
Publishe ’s No e: MDPI s ays neu al
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Copy igh : © 2021 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
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A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
1Depa amen o de Biología Vege al y Ecología, Facul ad de Biología, Uni e sidad de Se illa,
41012 Se illa, Spain; [email p o ec ed] (J.M.M.-L.); [email p o ec ed] (J.C.); [email p o ec ed] (S.M.-V.)
2INIA, Fo es Resea ch Cen e, Depa men o Fo es Dynamics and Managemen , Fo es Fi e Labo a o y,
C a. Co uña Km 7,5, 28040 Mad id, Spain; [email p o ec ed]
3iuFOR, Uni e si y Ins i u e o Sus ainable Fo es Managemen , UVA-INIA, 28040 Mad id, Spain
4Depa amen o de Es adís ica e In es igación Ope a i a, Uni e sidad de Se illa, 41013 Se illa, Spain;
[email p o ec ed]
*Co espondence: [email p o ec ed]; Tel.: +34-954557063
Abs ac :
In his s udy, we e alua ed changes in he po en ial lammabili y o di e en Medi e anean
sh ub species in a pine (Pinus pinea) o es in he Doñana Na u al Pa k (o SW Spain) as a esul o
goa g azing. Plan heigh , o al biomass, ine uel biomass and lea es/wood a io we e measu ed in
indi idual plan s o each species in bo h g azed and ung azed a eas. Mois u e con en , mean ime
o igni ion, mean ime o combus ion, g oss hea o combus ion (GHC) and lammabili y class o
he s udied sh ub species we e de e mined in he labo a o y. The esul s o his expe imen showed
ha g azing in luenced he lammabili y cha ac e is ics o he s udied sh ub species. Howe e , he
s eng h o his e ec was insu icien o modi y he lammabili y index o hese plan s, excep in he
case o My us communis, in which g azed plan s p esen ed a lowe lammabili y index. Acco ding o
Vale e’s classi ica ion, Cis us sal ii olius,Halimium halimi olium and Pis acea len iscus a e lammable
species, Rosma inus o icinalis is a lammable-highly lammable species, and M. communis is non-
lammable. The GHC alues ob ained we e gene ally “in e media e”, excep o hose o R. o icinalis,
which we e classi ied as “high”. The lammabili y pa ame e s o he s udy species did no show a
e y ma ked end in ela ion o g azing, bu he e ical s uc u e o plan s did change by p esen ing
educed biomass o lea es and ine wigs. This change al e ed he physical cha ac e is ics o hese
plan s and possibly ac ed o educe he inhe en i e isk o he sh ublands.
Keywo ds:
goa g azing; phy o olume; wild i e; Doñana Na u al Pa k; Rosma inus o icinalis;My us
communis
1. In oduc ion
O e ecen decades, he dynamics o land use change (abandonmen o u al a eas
and ag icul u al ac i i ies, exclusion o li es ock om o es s and expansion o u ban-wild
in e ace a eas, among o he s) ha e agg a a ed i e haza ds and he po en ial o disas e in
Medi e anean coun ies [
1
]. In ac , he incidence o wild i es in many Eu opean coun ies
has inc eased compa ed o p e ious decades, eaching an annual a e age o 14,000 wild i es
in Spain (2009–2019) and a ec ing an a e age o 74,000 ha pe yea [2].
One o he main me hods o p e en ing wild i e is o educe he uel load and con inu-
i y o o es s ands (by sh ub clea ing, p uning o hinning), using ei he mechanical means
o p esc ibed bu ning [
3
]. Howe e , li es ock g azing ep esen s a less agg essi e and/o
expensi e al e na i e o con olling sh ub enc oachmen in o de o educe he isk o o es
i es [
4
–
6
]. In some pa s o he wo ld, i eb eaks in he ege a ion a e al eady main ained
by con olled g azing, and p e en ion o o es i es by g azing is a well-acknowledged
Sus ainabili y 2021,13, 1555. h ps://doi.o g/10.3390/su13031555 h ps://www.mdpi.com/jou nal/sus ainabili y
Sus ainabili y 2021,13, 1555 2 o 13
egula ing ecosys em se ice [
6
,
7
]. P ac ices such as g azing combined wi h p esc ibed
bu ning and/o mechanical ea men s ( hinning and sh ub clea ance/ emo al) o p e en
uel accumula ion a e beginning o show excellen esul s and ha e he added ad an age
o p o iding se e al posi i e ex e nali ies o u al li elihoods and he en i onmen ha
con ibu e o sus ainable u al de elopmen [6,8,9].
Goa s a e sui able o he pu pose o uel con ol because o hei capaci y o b ows-
ing; i has been shown ha , a adequa e s ocking a es, hey can educe woody biomass
( educing he isk o wild i e), a oid he incidence o monospeci ic sh ubs, p omo e g ass,
e c. [
10
–
12
]. Plan esponses o single-e en o epea ed g azing a y g ea ly depending
on he a ailabili y o lea biomass, me is ems, s o ed nu ien s and soil esou ces, as well
as he equency and in ensi y o de olia ion [
13
]. Th ough hei modula s uc u e and
de elopmen , nu i ional alues and an i-nu i ional compounds, plan s a e o a ce ain
ex en p e-adap ed o compensa e o losses caused by g azing [
13
,
14
]. The e olu ion o
nu i ional, chemical and phenological de ences in g azing plan s, as well as he ad e se
e ec s o hese changes on he he bi o e die , ha e been s udied [
8
–
11
]; howe e , he e
a e e y ew s udies analysing Medi e anean sh ubland lammabili y in ela ion o uel
managemen by g azing.
Plan lammabili y ( he abili y o a species o igni e and sus ain i e) is a complex
phenomenon, he di ec measu emen o which unde labo a o y condi ions is bo h di -
icul and equi ocal, gi en he lack o s anda d me hodologies. Acco ding o Whi e and
Zippe e [
15
], o es uel cha ac e is ics ha e hinde ed he s anda diza ion o me hods and
p o ocols. In addi ion, se e al de ini ions o ‘ lammabili y’ ha e been p oposed by di e en
au ho s. The mos commonly accep ed de ini ion is ha p oposed by Ande son [
16
], which
conside s ou componen s o lammabili y: igni abili y, combus ibili y, sus ainabili y and
consumabili y [
12
,
17
–
19
]. The e a e many s udies on his opic, adop ing se e al me hods
and conduc ed o a ious pu poses, bu mos o hese me hods a e based on measu ing
he ime o igni ion o a gi en plan sample [
20
]. The lammabili y componen s desc ibed
a e in luenced in di e en ways by a mul i ude o ac o s: o e all uel a angemen , s uc-
u al cha ac e is ics, mois u e con en and chemical composi ion (lignins, ca bohyd a es,
mine als and isop enoids) [
21
]. In his sense, uel lammabili y depends on he local ci -
cums ances a a gi en ime and unde pa icula wea he condi ions [
19
]. The aim o his
s udy was o e alua e, du ing he c i ical summe pe iod, changes o lammabili y pa ame-
e s associa ed wi h he igni ion, combus ion and i e p opaga ion p ocesses o di e en
Medi e anean sh ub species caused by goa g azing in a pine o es in sou he n Spain.
2. Ma e ials and Me hods
2.1. S udy A ea and Species Selec ion
The expe imen al si e was es ablished in he Doñana Na u al Pa k, one o he mos
impo an na u e ese es in Eu ope due o i s high biological di e si y (SW Spain,
37◦140N
,
6◦200W).
The s udied o es s and is domina ed by Pinus pinea (100 ha), wi h an a e age densi y
o 217 ees/ha and a e age diame e a b eas heigh (dbh) o 26.92 cm. The clima e is
Medi e anean, wi h a mild and ainy win e (mon hly a e age empe a u e is 10
◦
C in
Decembe and Janua y) and a long d y summe , (mean empe a u e o 25
◦
C in July and
Augus ). Mean annual ain all is a ound 540 mm, wi h 80% o p ecipi a ion occu ing om
Oc obe o Ma ch.
The s udy a ea is used o imbe p oduc ion, hun ing ( abbi , pa idge) and g azing
(domes ic ungula es); howe e , wild he bi o es (dee ) we e elimina ed in 1970 and domes-
ic goa s we e excluded om 2002 o 2007. Du ing his 5-y pe iod, g azing was comple ely
excluded om he s udy a ea and he na u al ege a ion was no subjec ed o any o m o
managemen . As a consequence, i g ew and accumula ed apidly. In he sp ing o 2007, a
he d o adul domes ic goa s (a e age weigh 40–45 kg) was he e o e in oduced in o he
a ea a a s ocking a e o 2.7 ungula es ha
−1
y
−1
(cha ac e ized as mode a e g azing) [
10
].
This li es ock managemen can be conside ed semi-ex ensi e, al hough in o de o exploi
Sus ainabili y 2021,13, 1555 3 o 13
he 100 ha in a uni o m manne , he goa s we e closely con olled and mo ed a ound by a
goa he d. In 1990, i e plo s (2–3 ha each) we e chosen a andom wi hin he s udy a ea
and enced o in o de o exclude hem om goa g azing and o e alua e he e ec s o
changes in sh ubland composi ion. Since 2007, he ege a ion om bo h a eas (ung azed
and g azed) has been sampled [
22
]. The s udy a ea hus comp ised hese i e enced plo s
es ablished in 1990 (ung azed a ea, 5.5 ha
−1
po en ial g azing biomass) and he emaining
un enced a ea, which has been unde con inuous g azing om 2007 (3.2 ha
−1
po en ial
g azing biomass) by a he d o adul domes ic goa s.
Fi e unde s o y species (Cis us sal ii olius L., Halimium halimi olium L., My us com-
munis L., Pis acia len iscus L. and Rosma inus o icinalis L.) we e used o s udy a ious
pa ame e s o lammabili y. These species we e chosen based on hei abundance wi hin
he s udy a ea (join ly ep esen ing 80% o he unde s o y co e ) and high a es o con-
sump ion by he goa s [22].
2.2. P e ious Cha ac e iza ion
In sp ing o 2011, plan heigh , o al abo eg ound biomass, ine uel biomass (lea es
and wigs < 6 mm) and lea es/wood a io we e measu ed in 30 plan s pe species (15 inside
and 15 ou side each exclusion plo ) in o de o cha ac e ize he s uc u e o he selec ed
species. These a iables we e conside ed ele an because o hei po en ial in luence on
species lammabili y.
2.3. Field Sampling
In June 2011, i e plan s o each species, g owing unde simila mic oclima ic condi-
ions (mean daily empe a u e and ligh in ensi y), we e andomly chosen and ma ked in
each plo in o de o enable he collec ion o da a om he same plan s a e e y sampling
session. Ou side each enced plo , i e o he plan s o each species, wi h cha ac e is ics
simila o hose o he adjacen ung azed plo , we e andomly selec ed and ma ked in
he g azed a ea. Vege a ion sampling ook place on h ee occasions du ing he season o
highes i e isk, be ween June and Sep embe [a he beginning (10 June–12 July), middle
(13 July–15 Augus ) and end (16 Augus –20 Sep embe ) o summe ], p oducing i een pai s.
Figu e 1p esen s a e age mon hly empe a u e and ain all alues eco ded in he di e en
sampling imes: i s (a e age max. empe a u e = 32.37
◦
C,
mean ain all = 6.41 mm
),
second (a e age max. empe a u e = 34.10
◦
C, mean ain all = 0 mm) and hi d (a e age
max. empe a u e = 31.97
◦
C, mean ain all = 17.01 mm) collec ions. In each pe iod, in
o de o es ima e he lammabili y o he li e esh uel, 10 cm samples o e minal wigs,
wi h hei lea es, we e collec ed in a homogeneous manne om ma u e plan s o each
species and a ea, and immedia ely placed in la ge sealed plas ic bags and s o ed in a coole
o anspo a ion o he labo a o y. A subsample, o app oxima ely 160 g o each species,
plo and a ea (inside/ou side o he ence), was immedia ely aken in he labo a o y and
sepa a ed in o h ee se s o i y 1 g samples: (i) he i s se o samples was subjec ed
o a lammabili y es (Vale e’s p o ocol, [
23
]), (ii) he second se was used o mois u e
con en de e mina ion, in which samples we e placed in an o en a 100
±
5
◦
C o 48 h,
a e which he mois u e con en was exp essed as he pe cen age o o en-d y weigh (%
ODW), and (iii) he hi d se was used o G oss Hea Con en es s. Fo mois u e con en
de e mina ion, six alues we e calcula ed o each species, plo and a ea (inside/ou side o
he ence); he a e age o hese six alues was aken as he inal es ima e. In his way, o
each species, i e pai s o alues inside/ou side o he plo s we e compu ed.
Sus ainabili y 2021,13, 1555 4 o 13
Figu e 1.
Mon hly empe a u e and ain all a e age eco ded in Doñana Na u al Pa k (SW, Spain)
du ing 2011.
2.4. Desc ip ion o Labo a o y P ocedu es
2.4.1. Flammabili y Tes ing Me hod
The labo a o y lammabili y es was pe o med using an elec ic adia o , an igni ion
appa a us wi h an elec ic hea ing esis ance (500 wa s o hea capaci y) benea h a
10 cm
diame e ce amic pla e loca ed 4 cm below a pilo lame [
23
,
24
]. The lammabili y es
me hod desc ibed by Vale e [
23
], was applied as ollows: when he elec ic adia o
eached he equi ed empe a u e, we placed a 1 g sample om each species on o he
hea ed ce amic su ace and simul aneously s a ed a ch onome e in o de o measu e
ime o igni ion (TI) and ime o combus ion (TC), in seconds. Fo each species, plo ,
ea men and sampling ime, 50 lammabili y es s we e pe o med and he esul s used
o calcula e mean ime o igni ion (MTI) and mean ime o combus ion (MTC). The o al
numbe o posi i e igni ion es s was used o calcula e he igni ion equency (IF), which
was exp essed as a pe cen age [
23
]. Igni ion equency and ime o igni ion we e bo h
used o calcula e he lammabili y index, acco ding o he able p oposed by Vale e [
23
]
(Table A1).
2.4.2. G oss Hea o Combus ion Tes ing Me hod
The g oss hea o combus ion (GHC), also known as high calo i ic alue, o he ine
li e uel (lea es and pa icles <6 mm in diame e ) was de e mined ollowing he Spanish
S anda d UNE 164001 EX (acco ding o he Spanish Associa ion o S anda diza ion and
Ce i ica ion) [
19
]. Fo each species, ea men and sampling ime, a uel sample was
g ound in a mill. Pelle s o app oxima ely 1 g we e p oduced om he g ound ma e ial
using a hand p ess, o en-d ied a 100
±
5
◦
C o 24 h and weighed. Measu emen s we e
aken using an adiaba ic bomb calo ime e equipped wi h a pla inum esis ance senso
(PT 100). The calo i ic alue o benzoic acid (26.44 kJ
·
kg
−1
) was used o calib a e he
calo ime e . Two o h ee measu emen s o calo i ic alue we e aken om each sample,
and any alues di e ing by mo e han 2% om o he alues ob ained wi h he same sample
we e elimina ed.
2.5. Da a Analysis
Da a ob ained om he lammabili y es s we e s a is ically analysed. A epea ed-
measu es ANOVA model was i ed o each dependen a iable ( lammabili y (mean ime
o igni ion and mean ime o combus ion), g oss hea o combus ion and uel mois u e
Sus ainabili y 2021,13, 1555 5 o 13
con en ). The model included wo wi hin-uni ac o s: ea men (g azed and ung azed),
and ime ( h ee sampling imes; beginning, middle and end o summe ). We conside ed
ea men ype as a wi hin-uni ac o since he sampling uni s a e pai ed (plan s inside and
ou side each exclusion plo ). The ANOVA model included ea men , ime and in e ac ion
e ms. The linea model used o each pa ame e was as ollows: Yijk =
µ
+ FGi + Mj +
(FG ’ M)ij +
ε
ijk; whe e Yijk = obse a ions o dependen a iables;
µ
= o e all mean;
FGi = ixed e ec o eeding g oup (i = g azing o ung azed); Mj = ixed e ec o ime (j =
beginning, middle and end o summe ); FG ’ M = in e ac ions be ween and among hese
ac o s, and
ε
ijk = andom e ec o esidual. The compound symme y s uc u e o he
co a iance ma ix was es ed wi h he Mauchly p ocedu e. To es each e ec ( ea men ,
ime and in e ac ion) he co esponding Mauchly es o sphe ici y was i s pe o med. In
he case o ejec ion o he null-hypo hesis (conside ing 0.05 o be he p obabili y o ype I
e o ), he adjus ed es based on Huynh-Feld co ec ions o depa u e om sphe ici y
was used; o he wise, he no mal pa ame ic es was used. A pai ed - es was used o
de e mine he mean di e ence in species biome ic cha ac e is ics be ween g azed and
ung azed a ea. IBM SPSS 20.0 o Windows (SPSS Inc., Chicago, IL, USA) was used in all
s a is ical analyses. Finally, he ela ionships among mois u e con en , ime o igni ion, and
ime o combus ion we e explo ed h ough Pea son co ela ion.
3. Resul s
3.1. P e ious Fuel Cha ac e iza ion
Table 1p esen s he di e en cha ac e is ics o he species in he g azed and ung azed
a eas. Since 2007, o e a pe iod o ou yea s, goa g azing con inuously dec eased he o al
phy o olume o he moni o ed sh ubs loca ed in g azed a eas. The esponse o each species
o goa g azing was di e en ; M. communis was he species mos a ec ed by he p esence
o goa s, wi h only osema y (R. o icinalis) p esen ing simila alues. To al accumula ed
abo eg ound biomass, lea /wood a io and e ical s uc u e (heigh ) we e signi ican ly
lowe in g azed compa ed o ung azed plan s (p
≤
0.05). Biomass o highly lammable uel
(lea es and wigs <6 mm) was also signi ican ly lowe in he g azed sh ubs (Table 1).
Table 1.
Cha ac e iza ion o each s udy species p esen in he g azed (GA) and ung azed a ea (UA) o a Medi e anean pine
o es unde s o y in Doñana Na u al Pa k, SW Spain, p io o beginning he expe imen s (sp ing 2011). Di e en le e s
indica e signi ican di e ences be ween ea men s (p< 0.05).
To al Abo eg ound
Biomass (g m−2)
Folia Biomass
(g m−2)
Lea es + Twigs
< 6 mm
(g m−2)
Lea es
/Wood
Heigh
(cm)
2B owsing P essu e
Cis us sal ii olius
UA
712 a 1155 a 334 a 0.44 a 97 a –
GA
517 b 46 b 150 b 0.08 b 47 b High
Halimuim halimi olium
UA
896 a 126 a 536 a 0.34 a 106 a –
GA
611 b 3 b 225 b 0.14 b 88 b High
My us communis
UA
1374 a 269 a 717 a 0.37 a 126 a –
GA
1004 b 49 b 305 b 0.04 b 84 b Ve y high
Pis acia len iscus
UA
2049 a 485 a 500 a 0.46 a 193 a –
GA
1753 b 196 b 264 b 0.14 b 148 b Medium
Rosma inus o icinalis
UA
1100 a 333 a 483 a 0.62 a 98 a –
GA
1152 a 321 a 390 a 0.49 a 82 a Low
No e: 1Di e en le e s indica e signi ican di e ences be ween ea men s (p< 0.05); 2Values ob ained om Mancilla-Ley ón e al. [22].
3.2. Flammabili y Tes Pa ame e s
Means o mois u e con en , ime o igni ion (MTI), ime o combus ion (MTC), as well
as lammabili y index (FI) alues, a e p esen ed in Table 2. The co esponding leas squa e
means o each g azing ea men , s anda d e o o he mean and p- alue, acco ding o he
ANOVA esul s, a e p esen ed in Table A2.

Sus ainabili y 2021,13, 1555 6 o 13
Table 2.
Mean alues o mois u e con en and he lammabili y es pa ame e s obse ed o o es uel samples collec ed
in 2011 a di e en imes o he summe (ea ly, mid and la e) and a eas [g azed (GA) and ung azed (UA)] om di e en
Medi e anean species in Doñana Na u al Pa k (SW, Spain).
Species Sampling Time
o Summe T ea men Mois u e Con en
[%]
Time o Igni ion
[s]
Time o Combus ion
[s] 1FI
Min. Mean Max. Min Mean Max.
Cis us sal ii olius
Ea ly UA 130 13 25 36 2 12 21 3
GA 136 20 23 25 9 11 14 3
Mid UA 88 16 20 24 10 15 18 3
GA 89 16 18 22 13 16 18 3
La e UA 160 22 27 34 6 9 13 3
GA 155 22 26 32 9 13 17 3
Halimiun halimi olium
Ea ly UA 148 22 27 31 6 9 12 3
GA 146 21 24 27 8 10 12 3
Mid UA 104 16 19 22 12 15 20 3
GA 97 12 16 21 11 17 22 3
La e UA 150 17 20 24 9 12 15 3
GA 138 17 20 25 8 11 14 3
My us communis
Ea ly UA 167 22 32 45 1 3 9 1
GA 103 14 26 51 2 24 65 0
Mid UA 133 13 22 36 3 9 14 1
GA 79 11 23 44 11 38 57 0
La e UA 125 12 19 28 7 12 21 3
GA 97 15 29 53 3 24 58 0
Pis acia len iscus
Ea ly UA 144 10 17 29 5 9 14 1
GA 129 11 16 32 5 9 17 1
Mid UA 122 5 10 16 9 15 23 3
GA 103 5 12 21 9 16 25 3
La e UA 141 9 16 34 6 11 21 3
GA 100 9 17 28 5 11 21 3
Rosma inus o icinalis
Ea ly UA 140 19 25 33 3 8 15 3
GA 107 16 24 33 5 10 17 3
Mid UA 93 9 16 22 7 12 17 4
GA 70 11 15 19 6 12 20 4
La e UA 149 20 26 35 5 7 14 3
GA 135 20 24 28 5 10 17 3
No e:
1
In o de o unde s and he e ec o mois u e on he emaining a iables, and o allow es ima ion o na u al expe imen al a iabili y,
maximum and minimum alues a e also included in his able. Flammabili y Index (FI) classi ica ion scale p oposed by Vale e [
23
]:
0–1 = Non- lammable; 2 = Sligh ly lammable; 3 = Flammable; 4 = Highly lammable.
The clima ic di e ences ( empe a u e and p ecipi a ion, Figu e 1) we e e lec ed in he
uel mois u e con en (FMC) o he samples (Table 2). The FMC did no show a signi ican
in e ac ion be ween ea men s and ime in any o he species, bu a signi ican in e ac ion
was ound wi h ime; he alues o ea ly and la e summe being signi ican ly highe han
hose o mid-summe (FMC anged be ween 70–167%). Wi h he excep ion o C. sal ii olius,
signi ican di e ences we e ound in FMC be ween g azed and ung azed plan s in all o
he species. The FMC also signi ican ly dec eased in g azed plan s (Tables 2and A2).
The sho es MTI was eco ded o P. len iscus, while M. communis p esen ed a sho
MTC ha inc eased as summe p og essed, especially in ung azed plan s (Table 2). Re-
ga ding MTI, all species, excep o C. sal ii olius, showed signi ican in e ac ions be ween
ea men s and ime o sampling. In he p esence o goa s, MTI dec eased signi ican ly
in
C. sal ii olius
,H. halimi olium and R. o icinalis (p= 0.000, 0.000 and 0.041, espec i ely,
Table 2)
bu ended o signi ican ly inc ease in M. communis and P. len iscus (p= 0.043 and
0.032, espec i ely). All species showed a signi ican in e ac ion wi h ime, and he lowes
MTI was eco ded in he second sampling e en (midsumme ) (Tables 2and A2).
Rega ding MTC, he esul s showed a signi ican in e ac ion be ween ea men s and
ime in all species (Table A2). Unde goa g azing, MTC signi ican ly inc eased in all species
Sus ainabili y 2021,13, 1555 7 o 13
( his e ec was e y p onounced in M. communis), excep in he case o P. len iscus, which
did no di e signi ican ly be ween ea men s (Tables 2and A2). Rega ding MTC, all
species showed a signi ican in e ac ion wi h ime; he highes alues we e eco ded in he
second sampling e en (mid-summe ) (Tables 2and A2).
Based on he lammabili y index classi ica ion p oposed by Vale e [
23
], almos all o
he species es ed in his s udy we e ound o be lammable (FI = 3) (Table 2). C. sal ii olius,
H. halimi olium
,P. len iscus and R. o icinalis p esen ed lammabili y index alues equal o o
g ea e han 3. Al hough di e ences in MTI and MTC we e s a is ically signi ican in mos
species, goa g azing did no a ec he lammabili y index o any o he s udied species,
excep o M. communis. Mos di e ences in hese alues we e small (a ew seconds) and hus
p oduced li le change in he lammabili y index o he ege a ion samples. O e all, he species
lammabili y index emained cons an in g azed plan s (index = 0) du ing he s udy pe iod bu
inc eased ( om 1 o 3) o e ime in he ung azed plan s (Table 2). The Pea son co ela ions
showed ha changes in mois u e con en we e nega i ely co ela ed wi h ime o combus ion
( = −0.69, p≤0.05) and posi i ely co ela ed wi h ime o igni ion ( = 0.44, p≤0.05).
3.3. G oss Hea o Combus ion
Figu e 2shows he esul s o he g oss hea o combus ion (GHC) alues o he s udied
species in he di e en (g azed and ung azed) sampling a eas and e en s. Fo all species,
excep H. halimi olium, he esul s showed a signi ican in e ac ion be ween ea men s
and sampling e en s (Table A2). The species R. o icinalis p esen ed he highes GHC
alue (
F = 39.433
,p
≤
0.01) (Figu e 2). Only P. len iscus and R. o icinalis showed signi ican
di e ences be ween ea men s; calo i ic alues we e signi ican ly lowe in he ung azed
han in he g azed samples (p
≤
0.000) (Table A2). Finally, all species showed a signi ican
in e ac ion wi h ime; in gene al, he calo i ic alues o he species inc eased as summe
p og essed (maximum alues in la e summe , Figu e 2and Table A2).
Figu e 2.
Mean calo i ic alues (MJ Kg
−1
) o o es uel samples collec ed om di e en a eas (g azed
(GA) and ung azed (UA)) o a Medi e anean pine o es unde s o y in Doñana Na u al Pa k (SW,
Spain) (n = 3).
Sus ainabili y 2021,13, 1555 8 o 13
4. Discussion
Medi e anean sh ubland is ex emely he e ogeneous in e ms o lammabili y, and
he combus ible p ope ies o di e en species, among o he a iables, a e de e mined
mainly by hei py ophy ism (e.g., R. o icinalis), which can make ce ain plan communi ies
mo e i e-p one han o he s [
17
,
23
,
25
]. Any modi ica ion o he ege a ion, such as g azing,
can modi y hese p ope ies. The esul s o his expe imen show ha g azing a ec ed
he lammabili y cha ac e is ics (MTI and MTC) o he s udied sh ub species. Howe e ,
he change was insu icien o al e he lammabili y index o hese sh ubs, apa om M.
communis, in which g azed plan s p esen ed a lowe lammabili y index han ung azed
plan s. The esul s he e o e sugges ha he species did no change hei lammabili y cha -
ac e is ics in esponse o he bi o y, bu a he main ained hese cha ac e is ics. Acco ding
o Vale e’s classi ica ion, which akes igni ion equency and ime o igni ion in o accoun ,
he s udied species we e anked om 0 o 4 acco ding o iming and g azing (Table 2). C.
sal ii olius, H. halimi olium and P. len iscus we e classi ied as lammable species, R. o icinalis
as lammable-highly lammable and M. communis as non- lammable [17,24,26].
As s a ed abo e, de e mina ion o lammabili y is no s aigh o wa d and classi ica-
ion can be made only in a gene al sense [
23
]. The mos impo an pa ame e in anking he
lammabili y o o es species is TI [
17
,
27
]; howe e , Vale e [
23
] combined he mean ime
o igni ion (MTI) classes wi h hose o he pa ame e IF unde a c osslink able ha helped
o de e mine lammabili y index alues. This p ocedu e diminishes he impo ance o TI.
The e is no s anda d me hod o es ing o es species lammabili y, since he pa ame e
o lammabili y is a plan p ope y ha has no uni in eg a ing all o he h ee compo-
nen s ci ed abo e (igni abili y, combus ibili y and sus ainabili y) [
16
]. Recen s udies ha e
he e o e included o he pa ame e s ha we e no included in he classi ica ion me hod
p oposed by Vale e, such as he dimensions o lames (FH), an indica o o he ola ilized
essen ial oils con ained in ege al lea es, and a sus ainabili y componen (TC), which cha -
ac e izes he abili y o uel o sus ain combus ion o e a p olonged pe iod. Un o una ely,
hese pa ame e s we e no measu ed in he p esen s udy, bu he in o ma ion ob ained
is s ill aluable in e ms o ini ia ing his line o esea ch. Moni o ing how he chemi-
cal componen s o s udy species change h oughou he season and/o in he p esence
o g azing may p o ide u he aluable in o ma ion and should be a conside a ion o
u u e esea ch.
The s uc u al p ope ies o each plan species a e conside ed majo ac o s in hei
lammabili y [
15
,
16
]. Physical s uc u e and componen s (e.g., canopy a chi ec u e, ine
uel biomass, lea size and shape and e ained dead ma e ial) and physiological o cellu-
la elemen s (e.g., ola ile oils and esins, mois u e con en , mine al con en , lignin and
waxes) usually a ec he lammabili y cha ac e is ics o a species [
19
,
28
]. F om a phys-
ical pe spec i e, he su ace a ea- o- olume a io o uel pa icles is o en conside ed a
signi ican ac o in lammabili y [
29
]. Howe e , o whole plan s, lammabili y depends
mainly on he physical a angemen o he plan biomass [
30
]. I has been ecognized ha
ine o es uels play an impo an ole in wild i e ini ia ion and p opaga ion [
12
]. Fine
pa icles igni e mo e eadily and elease hei hea quicke han hicke pa icles o an
equi alen o al weigh . A long- e m absence o dis u bance in sh ublands leads o an
accumula ion o la ge amoun s o dead plan ma e ial [
31
]. G azing animals ha e been
hough o inhibi he accumula ion o dead biomass h ough consump ion o oliage [
32
].
Whe e g azing p essu e is su icien o de elop g azing lawns, plan s a e main ained in a
s a e o con inuous egene a ion [
33
] and he p opo ion o accumula ed dead ma e ial can
be e y low, hus educing i e isk [
18
]. In he s udy a ea, om 2007 o 2011, con inuous
and mode a e goa g azing had signi ican e ec s on he phy o olume and heigh o he
s udied sh ubs. A p e ious s udy by Mancilla-Ley ón e al. [
10
] showed ha , a e 42
mon hs, goa g azing signi ican ly educed species phy o olume by 34%, inc eased ba e
soil by 51% and dec eased he lammabili y o he a ea by 22%. The signi ican educ ion
o he combus ible unde s o ey biomass and he ho izon al ege a ion co e ound in
his s udy a ea ansla es in o a educed i e isk. All o he s udied species, excep R.
Sus ainabili y 2021,13, 1555 9 o 13
o icinalis, p esen ed signi ican ly lowe phy o olume and heigh when g azed, while he
pe cen age o ine uel was also signi ican ly educed. The deg ee o which g azing educes
uel load is de e mined by he densi y o g aze s, hei a e o ood in ake and he g ow h
a es o he plan s [
14
,
34
]. The impac o g azing on sh ubland uel load a ies among he
ege a ion componen s due o a ia ions in eeding p e e ences and in he beha iou al,
mo phological and physiological ai s ha in luence ood in ake [
22
]. Thus, he e ec
o g azing was mos e iden in M. communis (highly g azed), bu less p onounced in R.
o icinalis ( e y ligh ly g azed) (Table 1). The e ec o ligh o mode a e g azing in ensi ies
on ege a ion (b eaking he con inui y be ween sh ubs and c ea ing ‘ egene a ion gaps’ o
he implan a ion o g assland) con ols he combus ible biomass, hus educing he i e isk
and allowing he co-exis ence o species ha we e p e iously supp essed by he densely
packed dominan species.
F om a physiological pe spec i e, ola ile o ganic compounds a e p oduced by many
Medi e anean plan species and a e ela ed o seconda y me abolism p ocesses [
35
]. These
compounds ha e a low igni ion empe a u e and, when he ambien empe a u e inc eases
as a esul o exposu e o he sun o adia ion om a lame on , hey c ea e a lammable
gas mix u e. The g oss hea o combus ion o he es ed species was gene ally lowe han
ha o common Medi e anean o es uels [
12
]. Acco ding o he classi ica ion p oposed by
El i a and He nando [
26
], he GHC alues ob ained we e gene ally “in e media e” (18.81–
20.90 MJ kg
−1
), excep o hose o R. o icinalis (Figu e 2). The calo i ic alue o plan s
inc eases as s ems a e ligni ied (due o he highe calo i ic alue o lignin in compa ison o
cellulose). The mos ligni ied species and hose wi h a highe con en o ola ile o ganic
compounds ( esins, e penes and essen ial oils) (e.g., R. o icinalis) p esen a high calo i ic
alue. The inc eased calo i ic alues ound in he g azed species can be a ibu ed o he
ac ha he goa s emo e he ende shoo s, hus inc easing he p opo ion o he mos
ligni ied pa s in each plan .
Bo h he physical and physiological ac o s in ol ed in he lammabili y o he s udied
plan species a e ime-dependen . These p ope ies may a y wi hin he same species due
o changes in plan s a us (in lowe , unde going eg ow h, woody, e c.) o en i onmen al
condi ions ( empe a u e, humidi y, ain all, e c.). This means ha hei lammabili y
cha ac e is ics (MTI, MTC, e c.) may change depending on he s a us o hese ac o s,
which is in u n la gely de e mined by he ime o yea [
26
]. Fo ins ance, he season
o highes i e isk is dic a ed by he co-occu ence o high igni ion alues and low uel
mois u e [
25
]. This is usually he d ies ime o he yea , which a ies wi h egional clima e.
In Medi e anean- ype sh ublands, summe is he season o highes i e isk [
10
]. While we
ha e e alua ed only one season in his s udy, he FMC alues di e ed among he h ee
summe - ime poin s examined, e lec ing changes in he empe a u e and p ecipi a ion
(Tables 1and 2). Fu he mo e, hese changes in FMC we e nega i ely co ela ed wi h MTC
and posi i ely wi h MTI. The e o e, empo al changes could play a ela i ely g ea e ole
in he analysed lammabili y cha ac e is ics han he p ac ice o goa g azing.
As consume s o ine uels, he ole o na i e and in oduced domes ic he bi o es
in educing i e isk has been well documen ed [
4
,
11
,
25
]. He bi o es may selec plan s
wi h pa icula chemical o mo phological ai s, al e compe i i e hie a chies and di ec ly
modi y ege a ion s uc u e in ways ha can ei he p omo e o educe po en ial wild i e
ac i i y [
25
,
36
]. Whi e and Zippe e [
15
] showed ha he lammabili y cha ac e is ics o a
pa icula species we e in luenced no only by he species i sel bu also by i s en i onmen .
I should also be no ed ha igni ion and sp eading o i e could be di icul whe e he e is
discon inui y in he ege a ion. Unde such a scena io, e en highly lammable species may
no p esen a high isk [
37
]. The opposi e is also ue: less lammable species, p esen ed in
ho izon al and/o e ical con inui y, ep esen a highe isk by complica ing i e igh ing
and supp ession condi ions/p ac ices.
The speci ic cha ac e is ics (i.e., ecological, economic and/o social aspec s) o each
o es a ea should se e as he basis o selec ing he mos app op ia e uel ea men in
each case (mechanical ea men s, p esc ibed bu ning o con olled g azing). Howe e ,