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Journal of Arid Environments 100-101 (2014) 72e77

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Journal of Arid Environments

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The effect of fire exclusion on the structure and tree mortality patternsof a caldén (Prosopis caldenia Burkart) woodland in semi-arid CentralArgentina

Radim Matula*, Martin Svátek, Radomír �RepkaDepartment of Forest Botany, Dendrology and Geobiocoenology, Faculty of Forestry and Wood Technology, Mendel University in Brno, Zem�ed�elská 3,613 00 Brno, Czech Republic

a r t i c l e i n f o

Article history:Received 7 November 2012Received in revised form10 June 2013Accepted 17 October 2013Available online 7 November 2013

Keywords:CaldénCentral ArgentinaDensity-dependent mortalityFire frequencyFire-induced stem mortalityProsopis caldeniaStem mortality

* Corresponding author. Tel.: þ420 545 134 058; faE-mail address: radim.matula@mendelu.cz (R. Ma

0140-1963/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.jaridenv.2013.10.014

a b s t r a c t

We studied the effects of long-term fire exclusion on the structure and mortality of a caldén (Prosopiscaldenia Burkart) woodland, a rare vegetation type remaining only in fragments in semi-arid CentralArgentina. We tested the hypothesis that differences in caldén stand structure are consequence of dif-ferences in fire history by quantifying DBH and tree height in 30 plots set in caldén woodlands with fireoccurring at least every 10 y, with fire excluded for 20e30 y and without fire for at least 60 y. Stemheight, density and basal area increased after 20e30 years of fire exclusion. The frequently burnedwoodland had high mortality in all but the largest (>60 cm in DBH and >8 m in height) size class,whereas the fire-excluded plots exhibited high mortality only in the smallest stems (<15 cm in DBH and<3 m in height). Our results showed that the fire exclusion increased stem basal area and density whiledecreasing mortality of larger size classes and reducing resprouting of stems of any size; the probableexplanation for these changes is a shift in the prevalent thinning mechanisms from fire-induced tocompetition-induced mortality.

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1. Introduction

Fire has shaped the structure and functioning of ecosystemsthroughout the world (Scholes and Archer, 1997), but fire regimeshave been significantly relaxed over the last 150 years in many fire-adapted ecosystems. Reductions in fire frequency or intensity wereeither due to direct human efforts to stop fires or due to indirecteffects such as livestock grazing and habitat fragmentation (Briggset al., 2002; Medina, 2007; Peterson and Reich, 2001; Taylor, 2010;Taylor and Skinner, 2003). As a result, structural changes, such asincreases in the woody plant density, basal area and canopy cover,have occurred in many areas (Covington and Moore, 1992; Fulé andCovington, 1998; White, 1985).

Repeated fires are known to control the structure, dynamics andcomposition of woodlands and savannahs by reducing the density,height and biomass of the woody vegetation (Andersen et al., 2005;Peterson and Reich, 2001), thus maintaining a balance between thetrees and undergrowth vegetation (Bond and Keeley, 2005). How-ever, the role of fire in the vegetation dynamics is still not fully

x: þ420 545 212 293.tula).

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understood. A recent experiment of Higgins et al. (2007) examiningeffects of four decades of fire manipulation in four differentsavannah ecosystems showed that, contrary to the results of pre-vious studies, the density of woody individuals was unresponsiveto fire. Furthermore, species vary in their sensitivity to and recoveryfrom fire (Glitzenstein et al., 1995; Peterson and Reich, 2001).

Frequent low- or moderate-intensity surface fires are charac-teristic for caldén (Prosopis caldenia Burkart) woodlands (Bóo et al.,1997) in Central Argentina. These woodlands, locally known as“Caldenal”, occur in the semi-arid phytogeographic region in thesouthernmost part of the Espinal phytogeographical province inCentral Argentina (Cabrera, 1976). The dominant tree, caldén (P.caldenia), is endemic to Argentina, and may form almost purewoodlands, with an admixture of Prosopis flexuosa DC. (algarrobo),Celtis tala Gillies ex Planch. (tala) and Geoffroea decorticans (Gilliesex Hook. & Arn.) Burkart (chañar), all of which are well adapted tofrequent fire (Bóo et al., 1997; Medina, 2007; Willard, 1973).

Before the introduction of livestock in the 1900s, caldénwoodlands were relatively open, with a low tree density (Dussartet al., 1998); however, in recent decades, increasing densities ofboth native and non-native trees and shrubs not previously foundin this ecosystem have been observed (Sarasola et al., 2005), withcaldén clearly being the most rapidly spreading species (Dussart

R. Matula et al. / Journal of Arid Environments 100-101 (2014) 72e77 73

et al., 1998). Because most of the Espinal phytogeographical prov-ince has been transformed into agricultural fields, only scatteredfragments of the original caldén woodlands remain (Bogino andVillalba, 2008). The introduction of livestock is considered themain cause for the recent changes in the caldénwoodland structure(Dussart et al., 1998). However, Bóo et al. (1997) demonstrated thata single fire event could significantly reduce the density and heightof the canopy cover of dominant woody vegetation and increase themortality of all of the principal woody species, suggesting thatfrequent fire may be another mechanism that reduces tree densityand maintains the characteristic low-density structure. Fires, his-torically recurring every 7e10 years (Medina et al., 2000; Sarasolaet al., 2005; de Villalobos et al., 2007), have been absent for de-cades in many caldén woodlands (Morris and Ubici, 1996). Wetested the hypothesis that the shifts in caldén woodland structureare caused by changes in the tree stem mortality patterns resultingfrom duration of fire exclusion. Specifically, we hypothesized thatfire exclusion results in a decrease in stem mortality and that thelack of fire-induced thinning leads to an increase in the density,basal area and height of woody plants in caldén woodlands.

2. Methods

2.1. Study area

The study was conducted in Central Argentina in provinces LaPampa and San Luis (Fig. 1). The woodlands were composed almost

Fig. 1. Geographical location of study area (shown in gray).

purely of P. caldenia; other species represented less than 1% of themeasured trees. Mean annual temperature in this region rangedfrom 15.2 to 16.0 �C and a mean annual precipitation ranged from520 to 580 mm. The soils were Entisoles and Molisoles on sandyand loamy eolic plains (Casagrande et al., 1980).

2.2. Data collection

Square plots of 900 m2 each were placed in 30 caldénwoodlandstands. Twelve plots were set in caldén woodland forests burned atleast every 10 years (hereafter referred to as F10), and 12 plots wereset in caldén woodlands previously regularly burned but with fireexclusion for the last 20e30 years (F30). Six plots were set in “old”caldénwoodland with no fire for at least the last 60 years, hereafterreferred to as unburned woodland (UNB). The number of UNB plotswas smaller because we could not find more stands with docu-mentation of such a long period of fire exclusion. Information onthe last occurrence and the frequency of fires was obtained fromfarmers and landowners and was verified using the age of post-firesprouts, presence/absence of fire scars and char marks. All plotswere set in typical farmland grazed by cattle except for the UNBplots which were located in areas without cattle but with regulardeer browsing. To minimize the edge effect (the differences inecological processes near habitat edges and interior (Donovan et al.,1997)), the plots were situated in stands large enough to maintain aminimum distance of 20 m between the plot boundary and frag-ment border. The boundaries of all of the plots followed the fourcardinal directions.

In each plot, all of the woody plants with a DBH (diameter atbreast height, i.e.,1.3m above ground)� 5 cmwere identified to thespecies level, and their DBH (to the nearest mm using diametertape) and height (to the nearest 0.5 m using laser rangefinder(TruPulse 360B, Laser Technology Inc., Colorado, USA)) weremeasured. In cases where trees had multiple stems >1.3 m inheight, each stemwasmeasured. The DBH and height of dead stemswhich remain standing long after they die (Bóo et al., 1997) was alsorecorded, alongwith the absence/presence of sprouts growing fromtheir base.

2.3. Data analysis

For each plot, we calculated the basal area of all, living and deadstems (BA, BAlive and BAdead, respectively), and the density of all,living and dead stems. For multi-stemmed trees, basal area of eachstemwas summed (BAindiv). The KruskaleWallis test withWilcoxonpaired tests were performed to test for differences in the BA, BAliveand BAdead, density of all, living and dead stems and height betweenthe three fire regimes. We used generalized linear models (GLM)with a Poisson distribution of errors to test the effects of BA andtotal density on the number of dead stems per plot within each fireregime.

To test the effects of the BA, total density, BAindiv, DBH of thethickest stem within an individual tree (DBHmax) and height onprobability of the individual stemmortality within each fire regime,we used generalized mixed-effects models (GLMMs) with binomialdistributions fitted using the Laplace approximation (Bolker et al.,2009). The experimental unit, i.e., plot, was used as the randomvariable to account for potential spatial pseudoreplication becausethe individuals sampled within each plot were not likely to becompletely spatially independent. Using an ANOVA and theAkaike’s Information Criterion (AIC), the GLMMs were compared tosimpler GLMs with binomial distributions of errors using the samefixed effects variables, but without the random effect of the plots.GLMMs were used because including the random effect of the plotled to a significant improvement in all of the models (P < 0.05,

R. Matula et al. / Journal of Arid Environments 100-101 (2014) 72e7774

decrease in AIC > 2). Because BAindiv and DBHmax were stronglycorrelated (r ¼ 0.89, P < 0.0001) as were height with BAindiv andheight with DBHmax (r ¼ 0.61, P < 0.0001 and r ¼ 0.65, P < 0.0001,respectively), we created separate models with the explanatoryvariables height, BAindiv and DBHmax while retaining the explana-tory variables density and BA in all of the models. Because themodels with height and DBHmax had the lowest AICs, the resultsonly for these two variables are presented. Using the equations forfixed effects from the GLMMs, curves showing the probability of astem death in relation to DBHmax and height were created.

Using the GLMswith binomial distribution (link¼ logit), we alsotested the effect of total density, BA, BAtotal, DBHmax and tree heighton the probability of the resprouting of dead stems. In this case,adding a random variable did not significantly improve the model(P > 0.05, decrease in AIC < 2). Because there were too fewsprouting dead stems for an analysis in the UNB, we performed theanalyses only for the F10 and F30 plots.

All of the presented final models were selected using stepwisebackward elimination from the full model (i.e., themodel with all ofthe explanatory variables) and comparing the change in deviancewith the chi-squared for the GLMs and the change in AIC for theGLMs and GLMMs. All of the analyses were performed using the Rstatistical program (R Development Core Team, 2010) and lme4package (Bates et al., 2012).

3. Results

3.1. Structure

There were significant differences in the woodland structureamong the fire regimes. The total stem density was the greatest inthe UNB plots, and lower, but comparable, in the F10 and F30 plots(Fig. 2a). The mean density of dead stems in F10 was nine timesgreater than in F30 and seven times greater than in UNB, whereasthe mean density of living stems was the greatest in UNB, lower inF30 and lowest in F10 (Fig. 2). The BA and BAlive in F10 wassignificantly lower than in the F30 and UNB plots (Fig. 2b). TheBAdead was significantly greater in F10 than in F30 and UNB.

Therewere also significant differences in the average tree heightamong all three fire regimes (P < 0.01). The average height was thegreatest in the UNB plots (6.7 � 0.1 m), lower in F30 (6.2 � 0.1 m)and lowest in F10 (5.4 � 0.1 m).

As shown in Fig. 3, the distribution of BA is more skewed tosmaller size classes in UNB than in F10 and F30, reflecting the highdensity of the stems, with BA <5 dm2 in UNB. Dead stems occurred

Fig. 2. Mean total basal area (BA) and mean density of all, living and dead stems per 900 mregimes for the density and BA of all trees, the lowercase letters for the density of living tr

in all of the BA classes in the F10 plots and were largely confined tothe BA class <5 dm2 in F30 and UNB (Fig. 3).

3.2. Stem mortality

The density of dead stems was significantly correlated with thedensity of all stems in the UNB plots (Table 1). In the F10 and F30,density of all stems had significant effect on the density of deadstems in the interactionwith the BA, indicating that the effect of thedensity of all stems on the density of dead stems became weakerwith an increase in the BA (Table 1).

In almost the entire height and DBHmax range, the probability ofstem death was much higher in F10 than in the UNB and F30 plots(Fig. 4). The probability of stem death significantly decreased withincreases in the height and DBHmax in F10 and F30 (Table 2). In UNB,the height and DBHmax affected the likelihood of stem death in theinteraction with total density (Table 2) indicating that the effect oftotal density on the probability of stem death decreased with in-creases in DBHmax and height.

3.3. Sprouting

There were significant differences in the representation of deadstems that resprouted among the fire regimes (c2 ¼ 226.515,P < 0.0001). Some 73% of the dead stems in F10 resprouted,whereas only 13% and 0.1% of those in the F30 and UNB plotsresprouted.

Within the F10 regime, the probability of sprouting decreasedwith increasing DBHmax (c2 ¼ 78.63, P < 0.0001; Fig. 5), whereasthe total density and BA variables had no effect (P > 0.25). None ofthe explanatory variables was found to affect sprouting in the F30regime (P > 0.05).

4. Discussion

Our study showed that fire exclusion leads to significantchanges in the size structure of woodlands dominated by P. calde-nia. Trees in stands experiencing frequent fire were shorter instature and had significantly lower basal area than trees in stands inwhich fire had been excluded for more than 20 years. The density ofliving stems and the tree height increased with the increase in theduration of fire exclusion. These effects of fire exclusion on thestructure of woody vegetation are in accordance with Bond andKeeley (2005) who described the impact of recurrent fire on anecosystem as similar to the impact of large herbivores in reducingthe density, biomass and height of the woody vegetation.

2 plot. The capital letters above columns signify a difference (a ¼ 0.05) among the fireees and numbers for the density of dead stems.

Fig. 3. Number of living and dead stems in 5 dm2 basal area (BA) classes. The values shown are standardized for the area of 1 plot (900 m2).

Table 1Effects of the total stem density, basal area (BA) and their interaction on the density of dead stems in a fire regimewith frequent fire (F10), fire regime for 20e30 years withoutfire (F30) and fire regime without fire for at least 60 years (UNB). Generalized linear models with Poisson error distribution were used.

Density of dead stems

F10 (N ¼ 12) F30 (N ¼ 12) UNB (N ¼ 6)

Coefficient z P Coefficient z P Coefficient z P

Intercept 1.823 � 0.355 5.14 <0.0001 2.427 � 2.075 1.17 0.242 0.866 � 0.431 2.01 0.0445Density 0.048 � 0.008 6.44 <0.0001 �0.093 � 0.056 �1.66 0.097 0.028 � 0.006 4.55 <0.0001BA 0.160 � 0.217 0.73 0.464 �1.053 � 0.575 �1.83 0.067 1.073 � 0.95 1.13 0.259Density:BA �0.012 � 0.004 �2.75 0.006 0.047 � 0.017 2.74 0.006 �0.021 � 0.016 �1.35 0.178

Bold numbers denote significant effects (a < 0.05).

Fig. 4. Partial effect of stem height and DBH of the largest stem within an individualtree (DBHmax) on the probability that an individual stem will die. The curves are basedon generalized linear mixed effect models with binomial error distribution.

R. Matula et al. / Journal of Arid Environments 100-101 (2014) 72e77 75

The positive correlation between stem density and mortality inthe frequently burned and unburned for at least 60 years caldénwoodland suggests that density-dependent, whether fire- orcompetition-induced, mortality plays an important role in formingthe woodland structure in these two regimes. In the caldénwoodland unburned for many decades, density-dependent mor-tality acts as the principal thinning mechanism most probablydriven by intensified competition caused by the increased stemdensity and basal area. This competition-induced self-thinningresults in a relatively high mortality of the smallest stems (basalarea <5 dm2), but it does not affect the stems in the larger basalarea classes. It is evident that even after more than 60 years of fireexclusion the competitive self-thinning is not sufficient to reducetotal stem density of a stand (Fulé and Covington, 1998). Infrequently burned caldénwoodland, fire is clearly themain cause ofmortality, killing a great proportion of stems of all sizes and causingamuch highermortality of the stems of all BA classes in comparisonwith the woodlands in which fire was excluded. Similar to theunburned woodland, the smallest stems had the highest mortality,as also documented by other studies (e.g., Yu et al., 2009). However,in contrast to the unburned and fire-excluded caldén woodlands,we also found a relatively high mortality in the BA classes greaterthan 5 dm2. In the frequently burned woodland, the significantpositive effect of the density on the stem mortality at the plot levelsuggests that areas with a high stem density have a higher fire-induced mortality (Fulé and Covington, 1998; Yu et al., 2009). Inaddition to the density effect, there was a significant effect of thestem size on the probability of stem death in F10, which signifi-cantly decreased with an increase in the stem size. For example,when a tree stem, either by chance or due to a prolonged periodwithout fire, manages to grow to over 60 cm in DBH and 8 m inheight, it had a probability of death of near 20%, which is in contrastwith the near 100% probability of death for the smallest stems at5 cm in DBH and 2 m in height (Fig. 4). The great resistance of largestems to fire is characteristic for trees in many frequently burnedecosystems (Higgins et al., 2007). Both Fulé and Covington (1998)

Table 2Fixed effects of the total stem density, DBH of the thickest stemwithin an individual tree (DBHmax) and height, including their interactions, on the probability of the death of thestem in a fire regime with frequent fire (F10), fire regime for 20e30 years without fire (F30) and fire regime without fire for at least 60 years (UNB). Generalized linear mixedeffect models with binomial error distribution were used. The plot identity was used as the random variable.

Probability of stem death

F10 (N ¼ 499) F30 (N ¼ 424) UNB (N ¼ 414)

Coefficient z P Coefficient z P Coefficient z P

Intercept 6.599 � 1.966 3.35 <0.001 1.458 � 0.639 2.28 0.023 5.813 � 2.949 1.97 0.049DBHmax �0.130 � 0.048 �2.73 0.006 �0.279 � 0.046 �6.09 <0.0001 �0.350 � 0.236 1.48 0.139Density �0.061 � 0.035 �1.75 0.08 0.001 � 0.026 0.05 0.96 0.138 � 0.049 2.78 0.006Density:DBHmax 0.000 � 0.001 0.37 0.713 �0.004 � 0.003 �1.33 0.182 �0.012 � 0.005 �2.62 0.009

Intercept 8.449 � 1.492 5.66 <0.0001 2.897 � 0.83 3.491 <0.0001 4.671 � 1.726 2.73 0.006Height �1.028 � 0.126 �8.16 <0.0001 �1.225 � 0.175 �7.01 <0.0001 �0.178 � 0.348 0.51 0.609Density �0.036 � 0.025 �1.45 0.147 0.011 � 0.033 0.32 0.746 0.088 � 0.033 2.67 0.008Density:height 0.015 � 0.01 1.55 0.12 �0.001 � 0.01 �0.05 0.961 �0.016 � 0.005 �3.11 0.002

Bold numbers denote significant effects (a < 0.05).

R. Matula et al. / Journal of Arid Environments 100-101 (2014) 72e7776

and Yu et al. (2009) found that the spatial patterns of large trees inrecently burned forest stands and stands not burned for longerdurations did not differ, which suggests that these trees are rarelykilled by surface fires.

Unlike in woodlands with fire exclusion, a great proportion ofthe dead tree stems in frequently burned woodland resprout fromthe base. This good sprouting ability under frequent fire but almostno sprouting in woodlands without fire indicates that the treesoften survive the fire-induced death of their stems by creating newsprouts, whereas the stems that die as a result of competition self-thinning represent whole-plant mortality. In addition, we foundthat the youngest and smallest stems in our study were those thatresprouted best and that the probability of resprouting decreasedwith an increase in the tree size, as has been documented for othertree species (e.g., Bond and Midgley, 2001; Del Tredici, 2001;Matula et al., 2012).

The woodland with fire excluded for 20e30 years displayedstructural and mortality traits of woodlands both frequentlyburned and unburned for a long time, indicating that this woodlandis in a transition from a fire-induced mortality regime to compe-tition self-thinning; however, a time of fire exclusion of 20e30 years is likely an insufficient period to develop the lattermechanism. Due to relatively low stemdensity and basal area, thereis likely to be little competition among woody species. Becausethere is neither disturbance by fire nor intensive competition due tolow stem density and basal area in this fire regime, most of the

Fig. 5. Probability of sprouting after a stem death in relation to DBH of the largest stemwithin an individual tree (DBHmax). The curve is based on a generalized linear modelwith binomial error distribution (link ¼ logit).

young stems can survive and continue their growth, and the den-sity and basal area increase.

The results of our study suggest that fire limits woody plantproliferation and is a key factor for maintaining the low-densitystands, as in similar ecosystems in North America (Brown andCook, 2006; Miller, 1999; White, 1985) or South Africa (Roqueset al., 2001). Fire exclusion may also be a key factor for theexpansion of woody vegetation with P. caldenia as a principalspecies into grasslands observed in some areas of Central Argentina(Distel et al., 1996). This expansion has thus far been explainedmostly by the increased densities of cattle that forage on the P.caldenia fruits and disperse its seed in their dung, which, togetherwith grass elimination by grazing, facilitates the recruitment andearly growth of P. caldenia (de Villalobos et al., 2005). However, insites with frequent fires, much of the regeneration from the seedsspread by cattle is likely to be killed, even in older stages; however,without fire, there is no mechanism to eliminate establishedseedlings and saplings. Frequent fire as a controller of woody spe-cies expansion has been documented in woodlands and savannahsfrom arid ecosystems in different parts of the world (Browninget al., 2008; Roques et al., 2001; Scholes and Archer, 1997).

The results confirmed our hypothesis that frequent fire is animportant element in the Caldén woodlands in Central Argentinaand is necessary for maintaining the low-density open structure ofthis vegetation. It is evident that the frequency of fire must be takeninto account in areas where conservation management is plannedto preserve the last fragments of caldén woodland.

Acknowledgments

We thank Horacio Riesco and Manuel Vetrone for their collab-oration on this study. We are thankful to two anonymous reviewersfor valuable comments on a previous version of the manuscript.This study was funded by an IGA project of the Faculty of Forestryand Wood Technology of Mendel University in Brno titled “Use ofgenetic information in forest botany, tree physiology, dendrologyand geobiocoenology” and by a project “Creation and Developmentof Multidisciplinary Team on the Basis of Landscape Ecology (reg.No. CZ.1.07/2.3.00/20.0004)”.

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