Response to natural and simulated browsing of two Mediterranean oaks with contrasting leaf habit after a wildfire

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Response to natural and simulated browsing of two Mediterranean oaks with contrasting leaf habit after a wildfire
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  Ann. For. Sci. 63 (2006) 441–447 441c  INRA, EDP Sciences, 2006DOI: 10.1051  /  forest:2006024 Original article Response to natural and simulated browsing of two Mediterraneanoaks with contrasting leaf habit after a wildfire Josep Maria E  *, Abdessamad H  , Javier R  Centre de Recerca Ecològica i Aplicacions Forestals (CREAF) i Unitat d’Ecologia, Universitat Autònoma de Barcelona,08193 Bellaterra (Barcelona), Spain(Received 31 May 2005; accepted 14 December 2005) Abstract –  In this study, we explore the response to browsing of two co-occurring Mediterranean oaks, the evergreen  Quercus ilex   and the deciduous Quercus cerrioides , resprouting in areas a ff  ected by large wildfires in central Catalonia (NE Spain). We tested three hypotheses: (i) di ff  erences in thepreference of browsers will cause a higher impact of browsing on the deciduous oak, (ii) the deciduous oak will show a lower response to browsing thanthe evergreen one, and (iii) the response to browsing of   Q. ilex   and  Q. cerrioides  will di ff  er depending on the season of the year when browsing occurs.To test the first hypothesis, we undertook the monitoring of the degree of browsing on resprouting evergreen and deciduous oaks after fire, while thesecond and third hypothesis were tested by mean of an experiment with di ff  erent intensities of simulated browsing in di ff  erent seasons of the year. Theresults indicate that  Q. cerrioides  individuals were more heavily browsed than  Q. ilex   ones. Moreover, browsing matched the two species in most of thesize variables considered, cancelling the advantage in height and crown projection of the deciduous oak at the beginning of the resprouting process. Inthe experiment of simulated browsing,  Q. ilex   and  Q. cerrioides  showed a similar response to the di ff  erent intensities of simulated browsing applied,but di ff  erences between species occurred depending on the season of the year when browsing occurred:  Q. ilex   showed a higher growth rate of crownprojection than  Q. cerrioides  when it was browsed in autumn and winter, while the opposite pattern was obtained when stumps browsing occurred inspring and summer. deciduous  /   evergreen  /   post-fire regeneration  /   Quercus ilex  /   Quercus cerrioides  /   successionRésumé – Réponse à l’abroutissement après un incendie de forêt de deux chênes méditerranéens à feuillages contrastés.  Dans cette étude, ona exploré la réponse de deux chênes méditerranéens, se rencontrant ensemble  , Quercus ilex   sempervirent et  Quercus cerrioides  caducifolié, rejetantdans de grandes zones a ff  ectées par des incendies de forêt dans le centre de la Catalogne. On a testé trois hypothèses : (i) des di ff  érences de préfé-rence des animaux broutants causeront un impact très important sur le chêne caducifolié, (ii) le chêne caducifolié montrera une plus faible réponse àl’abroutissement que le chêne sempervirent, et (iii) les réponses à l’abroutissement de  Quercus ilex   et  Quercus cerrioides  di ff  èreront en relation avec lasaison pendant laquelle l’abroutissement se produit. Pour tester la première hypothèse, nous avons entrepris de suivre le degré d’abroutissement sur leschênes sempervirents et les chênes caducifoliés rejetant après incendie, tandis que la deuxième et la troisième hypothèse étaient testées au moyen d’uneexpérimentation avec di ff  érents niveaux d’un abroutissement simulé pendant di ff  érentes saisons de l’année. Les résultats indiquent que  Quercus cer-rioides  était plus fortement abrouti que  Quercus ilex  . D’ailleurs, l’abroutissement a assorti les deux espèces dans la plupart des variables de dimensionsconsidérées, annulant les avantages de hauteur et de projection des couronnes du chêne caducifolié au début du processus de rejet. Dans l’expérimenta-tion d’abroutissement simulé  Quercus ilex   et  Quercus cerrioides  ont montré une réponse similaire aux di ff  érentes intensités d’abroutissement simuléesappliquées, mais des di ff  érences entre espèces se sont produites en relation avec la saison où a été appliqué l’abroutissement :  Quercus ilex   a montréun plus fort taux de croissance de la projection des couronnes que  Quercus cerrioides  lorsqu’il a été abrouti en automne et hiver, tandis que le modèleopposé a été obtenu quand l’abroutissement des souches intervenait au printemps et en été. caducifolié  /   sempervirent  /   régénération après incendie  /   Quercus ilex  /   Quercus cerrioides  /   succession 1. INTRODUCTION The response of Mediterranean-type ecosystems to firehas been extensively studied, especially in the MediterraneanBasin [33,44]. Despite a long history of interaction of di ff  er-ent disturbance sources (e.g. wildfires, forest coppicing, over-browsing), less attention has been paid to the consequencesof secondary disturbances, such as the impact of browsing, onpost-fire succession in these communities. The e ff  ect of fire onpopulations of Mediterranean species has been reported to beusually moderate, because plants have life-history traits thatallow a very e ff  ective post-fire recovery, either by resprout-ing from fire-resistant structures or by germination of fire- *Corresponding author: javier.retana@uab.es protected seeds ([15, 27], but see [41]). However, resilienceof Mediterranean communities after a wildfire may be con-strained by the impact of herbivores, which may lengthen thetime required for the vegetation to return to the pre-fire condi-tions or even divert this return [38].Resprouting after fire has been viewed as an e ffi cient life-history trait by which woody plants can recover lost biomassafter disturbance [2, 36]. However, resprouting involves alarge mobilization and consumption of below-groundreserves[8]. Therefore, it has been argued that repeated browsingon resprouting individuals may further deplete below-groundreserves and compromise the success of the regeneration pro-cess [9,45]. The impact of browsing in the community of re-sprouting plants may be expected to vary depending on the Article published by EDP Sciences and available at http://www.edpsciences.org/forestor http://dx.doi.org/10.1051/forest:2006024  442 J.M. Espelta et al. interactionamong:(i)preferencesoftheherbivores,(ii)seasonof browsing and (iii) di ff  erences in life history traits amongthe plant species a ff  ected. One of the plant traits that maybe expected to determine specific di ff  erences in the extent of browsing is the leaf habit (evergreen vs. deciduous). On theone hand, the preference of herbivores for deciduous speciesinstead of con-generic sclerophyllous-evergreens [21,29] hasbeen often suggested because of di ff  erences between thesespecies in leaf texture and thickness, nitrogen content, indi-gestible substances (cutin, lignin) and secondary compounds(e.g. tannins [42], but see [22]). Moreover, distinct seasonalpatterns of renewal of leaves in deciduous and evergreenspecies involve di ff  erences in carbon and nutrient allocation inthe plant that may have important implications, depending onthe season of browsing, both for palatability and vulnerabilityto herbivores [7,28]. Finally, it has been frequently presumedthat evergreenspecies would be more resilient to repeated dis-turbances because their lower resource-loss ratios [1] wouldallow them to be better adapted to harsh environments (e.g.,high water stress, temperature and light intensity) occurring inrepeatedlydisturbedsites([32,37],butsee[5]).Takingintoac-count the abovementioneddi ff  erences,some authorshavesug-gested the idea that the greaterdominanceof evergreenoaksinthe Mediterranean Basin in comparison to deciduous ones hasbeen partially favored by occurrence of repeated disturbances,including the impact of domestic herbivores [12]. However,we do not know at present any study specifically aimed totest whether deciduous and evergreen oaks resprouting afterfire di ff  er in their sensitivity to browsing and to test whetherdi ff  erences between these species depend on the season whenbrowsing occurs.In this study, we explore the response to browsing of twoco-occurring Mediterranean oak species resprouting after awildfire in central Catalonia (NE Spain), the evergreen  Q. ilex  L. and the deciduous  Q. cerrioides  Willk. et Costa.  Quercusilex   is a widespread species, present over a large area ex-tending 6000 km longitudinally from Portugal to Syria and1500 km latitudinally from Morocco and Algeria to France[43], whereas the distribution of   Q. cerrioides  – a speciesfrom the  Quercus humilis  group with several probable intro-gressions from other deciduous  Quercus  – is also abundant inthe NE Iberian Peninsula [10]. In this study we have testedthree hypotheses. The first one is that the deciduous oak willbe more a ff  ected by browsing than the evergreen one, becausebrowsers prefer deciduous than evergreen species [21,29]. Totest this hypothesis, we have undertaken the monitoring of thedegree of browsing on resprouting evergreen and deciduousoaks after fire. The second hypothesis refers to the response tobrowsing (how plants respond to, and are able to compensatefor browsing).We hypothesizethat this response will be largerin the evergreen than in the deciduous species due to di ff  er-ences in sproutingability (numberof resprouts) after recurrentdisturbances [17] and because the lower resource-loss ratiosof the evergreen species allows them to better overcome thecritical conditions during post-disturbance regeneration [32].The third hypothesis predicts that the response to browsing of  Q. ilex   and Q. cerrioides  will di ff  erdependingonthe season of the yearwhenbrowsingoccurs[6].Thetwooaksstudieddi ff  erin the phenology of leaf production:  Q. cerrioides  mostly re-news its foliar tissues in early spring (i.e. April), while partialrenewal of leaves and shoots of   Q. ilex   stars later, in May–June, although a second flush of shot growth and leaves forthis speciesis usually producedin earlyautumn[14].Thus,weexpect that the negative e ff  ects of browsing will be higher inearly spring for the deciduous than for the evergreen species,because the former renews completely its foliar tissues in thatseason. To test the second and third hypotheses, we have car-ried out an experiment with di ff  erent intensities of simulatedbrowsing on the two species in di ff  erent seasons of the year. 2. MATERIALS AND METHODS2.1. Study area This study was carried out in the regions of Bages and Berguedà(Catalonia, NE Spain; 41 ◦ 45’ to 42 ◦ 6’ N; 1 ◦ 38’ to 2 ◦ 1’ E; 350 to950 m above sea level). Climatic conditions vary from dry-subhumidto subhumid Mediterranean (according to the Thornwaite index),with mean annual temperature of 10–13  ◦ C and mean annual pre-cipitation of 550–700 mm. This area was a ff  ected in July 1994 bythe largest wildfire historically recorded in Catalonia. The fire burned24 322 ha, of which 71% was forested. According to the data pro-vided by the Forest Ecological Inventory of Catalonia [24], the mainforest tree species before the fire was  Pinus nigra  (75%) while  Q. ilex  and  Q. cerrioides  were very common in the understory of these pineforests. Due to the nil regeneration of   P. nigra  after the fire, forestschanged to woodlands dominated by resprouted  Q. ilex   and  Q. cer-rioides  stumps [16]. These two  Quercus  species resprout vigorouslyafter disturbances [5,30,31], with sprouts occurring by activation of dormant buds located at the stump level, the root-crown or to a lesserextent on roots [19,39].  Quercus cerrioides  shows a more rapid fo-liage recovery than  Q. ilex   [5], probably because growth in  Q. ilex  could be constrained by a higher investment in longer lasting, sclero-phyllous leaves [1].The large extent of the burned area and the need to replace thetraditional exploitation of wood in the pine forests, have promotedextensive land use changes involving the conversion of 2365 ha of post-fire woodlands (9.7% of the total burned area) to rangelands[16]. In these areas, domestic cattle, mainly cows, eat grasses andresprouting  Quercus  stumps. Cows mainly prefer grasses from latewinter to early summer, and eat foliage, shoots and acorns of oaks insummer and autumn [20]. We have carried out two di ff  erent studiesin these rangelands to assess the e ff  ect of post-fire natural and simu-latedbrowsing on Q. ilex  and  Q.cerrioides  resprouting stumps. Theseobservations and experiments were carried out in di ff  erent locationswithin the burned area (Fig. 1). 2.2. Degree of browsing on the two species This study was carried out in three sites, Socarrada, Alzina Grossaand Cal Teixidor (Fig. 1), in 1998, i.e., 4 years after the fire. In eachsite, an area intensively browsed by cows since the first year afterfire was chosen, together with a nearby enclosure area protected fromcowbrowsing throughout thistime. Browsingwas estimatedasexten-sive (following [21,47]) because of the consumption of many tips and  Browsing ot two Mediterranean oaks 443 Figure 1.  Geographical location of the sites sampledin this study along the area of Bages and Berguedà re-gions a ff  ected by the large wildfire of 1994 (shaded).Study sites: 1, Socarrada; 2, Alzina Grossa; 3, CalTeixidor; 4, Cal Barraler; 5, L’Obaga d’Antius.leaves of   Q. ilex   and  Q. cerrioides  stumps and the heavy consump-tion of less palatable plants (e.g.  Rosmarinus o  ffi cinalis, Cistus  ssp.)present in the area. Fifteen browsed stumps and fifteen control (un-browsed) stumps of   Q. ilex   and  Q. cerrioides  were sampled in eachsite. Stump surface was used as a measure of the size of the individualbefore sprouting, and no initial di ff  erences in this variable were foundamong sites or species before browsing (two-way ANOVA,  p  >  0 . 09in all cases).We determined browsing preference by using a categorical clas-sification of the impact of cows on resprouting stumps of the twospecies in the area intensively browsed of each site. The followingcategories were identified: 0, unbrowsed; 1, few shoot tips browsed;2, most shoot tips browsed; 3, few shoots of the previous yearbrowsed; 4, most shoots of the previous year browsed; 5, completelybrowsed and defoliated. We have compared the number of individu-als of each species in each browsing category with a  χ 2 test. The threesites have been analyzed separately.We have also measured the following variables from each stump:survival, number of resprouts, total height and crown projection(measuring two perpendicular diameters of the crown and computingthe projection as an ellipse). The e ff  ects of site, species and treatment(browsed, not browsed) on thesevariables wereanalyzed by ANOVA.Allvariables except thenumber of resprouts werenormalized byalogtransformation. The sequential Bonferroni method was employed tocontrol the group-wide type I error rate [40], considering together allstatistical tests of this study. The Fisher’s protected least significantdi ff  erence post-hoc test was used to analyze di ff  erences among levelsof each main factor. 2.3. Response of the two species to di ff  erent intensitiesand seasons of simulated browsing This experiment started in winter 1998, i.e. 4 years after the fire,in two areas that had been totally excluded from browsing during thefirst years after the fire: L’Obaga d’Antius and Cal Barraler (Fig. 1).In each site, two 0.25 ha plots were established. At the beginning of the experiment, 96  Q. ilex   and 96  Q. cerrioides  individuals were ran-domly chosen and numbered in each plot. These tagged individualswere randomly assigned to one of the three treatments with di ff  er-ent intensity of simulated browsing. The treatments simulated brows-ing by cows by randomly clipping a certain proportion of all annualshoots from the previous season: 0% (control, stumps were left un-touched), 25% (low-intensity browsing) and 50% (higher-intensitybrowsing). This experimental procedure was repeated with di ff  erentindividuals at the beginning of each season: summer (June), autumn(September), winter (late November) and spring (April). For eachcombination of species, treatment and season, there were 8 samplingindividuals per plot.The experiment lasted one year, i.e., individuals clipped in eachseason were sampled again one year later, that is, in spring, summer,autumn and winter of the following year. In each individual, the fol-lowing variables were measured at the beginning and at the end of theexperiment: total number of resprouts larger than one cm of basal di-ameter, height and crown projection of theindividual. Threeresproutsper individual were marked at the beginning of the experiment andwere also monitored one year later to determine mean annual shootgrowth per individual. Relative growth rate (RGR) of number of re-sprouts, height and crown projection was calculated as RGR  =  (ln(X i -X i − 1 )  /  X i − 1 ), where X i was the value of the variable at the end of the experiment, and X i − 1 was its value at the beginning.Thee ff  ectsof species ( Q.ilex  , Q.cerrioides ), thesimulatedbrows-ing treatment (0%, 25%, 50% browsing), season (winter, spring, sum-mer, autumn), site and plot (nested within site) on these variableswere analyzed by ANOVA’s. In all cases, inspection of residuals wascarried out to check for normality and homoscedasticity. Data of crown projection were normalized by a log transformation. The se-quential Bonferroni method was employed considering all ANOVAtests together to control the group-wide type I error rate [40]. TheFisher PLSD post-hoc test was used to compare the di ff  erent levelsof each variable.At the start of the study, there were significant di ff  erences betweenthe two species in the number of resprouts (ANOVA, F 1 , 714  =  93 . 8,  p  <  0.0001) and height (F 1 , 714  =  19 . 7,  p  <  0.0001) but not in stumpsurface (  p  >  0.20) or crown projection (  p  >  0.35).  Quercus ilex   hadmore resprouts per stump than  Q. cerrioides  (respectively, 7.0  ±  0.3vs. 4.1 ± 0.14) but  Q. ilex   individuals attained a lower height (respec-tively, 157  ±  2 cm vs. 171  ±  3 cm). No di ff  erences in these morpho-logical variables existed between sites, and individuals assigned tothe di ff  erent browsing treatments or browsing seasons (  p  >  0 . 10 inall cases).  444 J.M. Espelta et al. Table I.  Browsing intensity by cows on resprouting stumps of   Q. ilex  and  Q. cerrioides  in the area intensively browsed of the three sitesused to analyze the sensitivity of the two species to browsing. Thefollowing browsing categories were identified: 1, few shoot tipsbrowsed; 2, most shoot tips browsed; 3, few shoots of the previousyear browsed; 4, most shoots of the previous year browsed; 5, com-pletely browsed and defoliated.  N   =  15 individuals per species andsite. Browsing intensitySpecies 1 2 3 4 5(a) Cal Teixidor Quercus cerrioides  0 0 46.7 46.7 6.6 Quercus ilex   13.3 60.0 13.3 13.3 0(b) Alzina Grossa Quercus cerrioides  0 0 26.7 73.3 0 Quercus ilex   6.7 33.3 60.0 0 0(c) Socarrada Quercus cerrioides  0 0 13.3 53.3 33.3 Quercus ilex   40.0 20.0 33.3 6.6 0 3. RESULTS3.1. Intensity of browsing on the two species Most  Q. cerrioides  individuals present in the browsingarea were heavily browsed, while  Q. ilex   individuals weresubjected to considerably lower degree of browsing (Tab. I).Di ff  erences between species were significant in the three sites(  χ 2 =  17 . 4, 19.0 and 22.4 in Cal Teixidor, Alzina Grossa andSocarrada,  p  <  0 . 001 in the three cases). Despite the di ff  er-ent levels of browsing intensity recorded we did not observestump mortality of either  Quercus  species.None of the factors or their interactionsa ff  ected the numberof resproutsperstool(Tab.II).Therewas a significant e ff  ectof species and a very important e ff  ect of browsing in total heightand crown projection (Tab. II). These variables attained highervaluesin Q. cerrioides thanin Q. ilex   stoolsandincontrolthanin browsed stools. However, there was an interaction betweenboth factors, so there were di ff  erences between species in thecontrol treatment, but not in the browsed one (Fig. 2). Thee ff  ect of site was not significant, while the interaction betweensite and browsing treatment was only lightly significant fortotal height. 3.2. Response of the two species to di ff  erent intensitiesand seasons of simulated browsing None of the levels of simulated browsing led to stump mor-tality of either  Quercus  species. Intensity of browsing did nota ff  ect the number of resprouts or the height attained by indi-viduals, but it had a significant e ff  ect in their crown projec-tion (Tab. III). Control stumps showed a lower relative growthrate in crown projection than those in the two browsing treat-ments (control: 0.00  ±  0.04; low-intensity browsing: 0.14  ± 0.3;high-intensitybrowsing:0.22 ± 0.05).BothRGRincrown Table II.  F values from ANOVA tests of e ff  ects of species ( Q. ilex  , Q. cerrioides ), site and browsing treatment on di ff  erent morphologi-cal variables of sprouting stools in the study of the sensitivity of thetwo species to browsing. Significant coe ffi cients (at  α  =  0 . 05 whenthe sequential Bonferroni method is employed) are indicated in bold.All variables except number of stems were log-transformed. Source df Number Total Crownof resprouts height projectionSpecies (SP) 1 0.4  20.0 8.4 Browsing treatment (B) 1 4.2  138.1 91.1 Site (S) 2 0.9 0.1 0.6SP ×  B 1 0.0  8.7 19.6 SP ×  S 2 0.4 2.3 1.9B × S 2 1.0  8.0  0.5SP ×  B ×  S 2 1.0 0.3 1.9Residual 169 Figure 2.  Total height (A, in cm) and crown projection (B, in m 2 ) of sprouting  Q. cerrioides  (solid bars) and  Q. ilex   stumps (open bars) incontrol and in browsed plots in the study of the sensitivity of the twospecies to browsing. Vertical bars extend over  + 1 SE of the mean. projectionandheightvariedaccordingtotheseasonwhensim-ulated browsing was applied (Tab. III). In both cases, stumpsbrowsed at the beginning of the winter period showed lowerrelative growth rate than stumps browsed in the other periodsof the year, and even did not recover in the case of crown pro- jection (Fig. 3). Concerning the comparison between  Q. ilex  and  Q. cerrioides , RGR of number of resprouts and height didnot di ff  er according to species (Tab. III). Thus, di ff  erences ob-served at the beginning of the experiment persisted at the endof the study, in spite of the treatment of intensity of brows-ing or season.  Quercus ilex   individuals maintained more re-sprouts per stump than  Q. cerrioides  (respectively, 7.5  ±  0.3resprouts per stump vs. 4.1  ±  0.1 resprouts per stump), while  Browsing ot two Mediterranean oaks 445 Table III.  F values from ANOVA tests of e ff  ects of species ( Q. ilex  , Q. cerrioides ), browsing treatment (not browsed, low- intensity andhigh-intensity browsing), season of the year (winter, spring, summerand autumn), site and plot (nested within site) on the relative growthrate in number of resprouts, height and crown projection in the exper-iment of simulated browsing. Significant coe ffi cients (at  α  =  0 . 05 af-ter applying the sequential Bonferroni method) are indicated in bold. Source df RGR number RGR total RGR crownof resprouts height projectionBrowsing (B) 2 1.5 0.3  11.4 Species (Sp) 1 0.2 3.7 0.1Season (Se) 3 2.7  6.6 60.4 Site (S) 1  7.8  1.2 0.6Plot (Site) 2 0.3 4.0  14.1 B × Sp 6 1.1 0.6 0.4B × Se 6 1.8 1.2 3.5B × S 2 1.9 0.1 0.2Sp ×  Se 3 0.3 1.6  7.6 Sp ×  S 1 0.1 0.1 0.2Se ×  S 3 0.2 0.1  14.6 B × Sp  × Se 6 0.8 0.6 0.9B × Sp  × S 2 0.3 1.4 0.6B × Se  × S 6 0.1 1.0 1.0Sp ×  Se ×  S 3 1.3 0.2 3.7B × Sp  × Se  × S 6 1.2 0.1 1.0Residual 714 Q. cerrioides  had a higher height than  Q. ilex   (167 ± 2 cm vs.186 ± 3cm).Theinteractionofspeciesandseasonofbrowsingwas only significant for the relative growth rate of crown pro- jection (Tab. III). Thus,  Q. ilex   showed a higher growth rate of crown projection than  Q. cerrioides  when browsed in autumnand winter (especially in winter, when none of the speciesrecovered after browsing), while the opposite was obtainedwhen stumps were browsed in spring and summer (Fig. 4).Few di ff  erences were obtained between the sites where theexperimental treatments were applied (Tab. III): the highestgrowth rate in number of resprouts was observed in Cal Bar-raler, while the highest growth rate in crown projection wasobtained in Cal Barraler in summer, autumn and winter, but inspring in L’Obaga d’Antius (data not shown). 4. DISCUSSION Neither the e ff  ect of natural browsing, nor the di ff  erent lev-els and seasons of simulated browsing, led to stump mortalityof either  Quercus  species, even though browsing was carriedout on individuals just recovering from a recent severe distur-bance event (i.e. fire). These results support previous findingson the role of repeated disturbances in other Mediterraneanresprouting species which have pointed out the occurrence of major architectural and physiological changes (e.g. resproutnumber, height, nutrient content, photosynthesis rates), butfew mortality e ff  ects (see among others, [8,13,35,46]). Re-sprouters show high resilience to repeated disturbances and itis di ffi cult to observe mortality processes due to the exhaus- Figure 3.  Relative growth rate of (A) total height and (B) crown pro- jection of stumps browsed in the di ff  erent seasons of the year in theexperiment of simulated browsing. Vertical bars extend over  + 1 SEof the mean. Di ff  erent letters indicate significant di ff  erences amongperiods according to the Fisher PSLD post-hoc test. tion of belowground reserves after repeated disturbances [9].In our case, of course, it should be taken into account that thetime interval when individuals su ff  ered browsing (four years)is short compared to the expected longevity of oaks.Although browsing did not cause mortality in either of thetwo species of oaks analyzed, in our study, as has been de-scribed for other  Quercus  species [23,34], the negative e ff  ectsof browsing were evident: the results of browsing on plantsof the two species support our first hypothesis of a preferen-tial browsing on the deciduous oak in comparison to the ev-ergreen one. Most  Q. cerrioides  individuals in the browsingarea of each site were heavily browsed, while  Q. ilex   individ-uals were subjected to considerably lower degree of browsing.Moreover, browsing matched the two species in most of thesize variables considered, canceling the initial advantage inheight and crown projection exhibited by the deciduous oak.It remains unclear whether this preferential browsing of thedeciduous oak is caused by the behavior of herbivores or bystructural or chemical di ff  erences between species that reducebrowsingintensity(i.e. lowernitrogencontentand higherscle-rophylly of leaves of evergreen oaks compared to deciduousones; [7,12,21,26,29].Some authors have noted that domesticherbivores are less selective than wild ones, and they browsemore of the larger ( Q. cerrioides  in our study) or dominantplants [25,48]. The fact that stumps of the two species werebrowsed until they attained a similar size and shape seems toindicate that browsing activity proceeds until animals can noteasily consume them, because the branching pattern of theseresprouting stumps, with numerous and lignified resprouts,protects a fraction of the leaves from browsing.
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