Infiltration, runoff and soil loss in Andisols affected by forest fire (Canary Islands, Spain)

HYDROLOGICAL PROCESSESHydrol. Process. (2012)Published online in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.9403

Infiltration, runoff and soil loss in Andisols affected by forestfire (Canary Islands, Spain)

J. Neris,* M. Tejedor, J. Fuentes and C. JiménezDepartamento de Edafología y Geología, Facultad de Biología, Universidad de La Laguna, Av. Astrofísico Fco. Sánchez s/n, 38071La Laguna, Tenerife,

Canary Islands, Spain

*CLaE-m

Co

Abstract:

Depending on the severity of the fire, forest fires may modify infiltration and soil erosion processes. Rainfall simulations wereused to determine the hydrological effects of fire on Andisols in a pine forest burned by a wildfire in 2007. Six burned zoneswith different fire severities were compared with unburned zones. Infiltration, runoff and soil loss were analysed on slopesof 10% and 30%. Forest floor and soil properties were evaluated. Unburned zones exhibited relatively low infiltration (23 and16 mm h�1 on 10% and 30% slope angles, respectively) and high average runoff/rainfall ratios (43% and 50% on 10% and30% slope angles, respectively), which were associated with the extreme water repellency of the forest floor. Nonetheless, thislayer seems to provide protection against raindrop impact and soil losses were found to be low (8 and 16 g m�2 h�1 for 10%and 30% slope angles, respectively). Soil cover, soil structure and water repellency were the main properties affected bythe fire. The fire reduced forest floor and soil repellency, allowing rapid infiltration. Moreover, a significant decrease wasnoted in soil aggregate stabilities in the burned zones, which limited the infiltration rates. Consequently, no significantdifferences in infiltration and runoff were found between the burned and the unburned zones. The decrease in post-firesoil cover and soil stability resulted in order-of-magnitude increases in erosion. Sediment rates were 15 and 31 g m�2 h�1

on the 10% and 30% slope angles, respectively, in zones affected by light fire severity. In the moderate fire severityzones, these values reached 65 and 260 g m�2 h�1 for the 10% and 30% slope angles, respectively. Copyright © 2012John Wiley & Sons, Ltd.

KEY WORDS Andisols; forest fire; infiltration; soil loss; forest floor

Received 23 January 2012; Accepted 8 May 2012

INTRODUCTION

Wildfires alter the water infiltration process in soils,while also modifying the hydrological behaviour of thewatersheds in which they occur. Most studies identifymarked increases in runoff and in erosive processes after afire (White and Wells, 1982; Cerdà et al., 1995; Robichaudet al., 2000; Larsen et al., 2009), with at times catastrophicconsequences (Shakesby and Doerr, 2006).Fire severity describes the ecological effect of a forest

fire (Ryan, 2002). According to Keeley (2009), most fireseverity metrics are focussed on the changes in above-ground vegetation and soil organic matter. Fire severity isdirectly related in most ecosystems to fire intensity, ameasure of the time-averaged energy flux. However, fireseverity also depends on other factors such as heatingduration, soil and plant moisture, vegetation composition,stand age, topography, substrate, climate, etc. (Keeley,2009). The effect of fire on soil properties dependsdirectly on the fire severity (Certini, 2005). Ryan (2002)shows that the effects of fire on vegetation and on theforest floor vary substantially according to whether thefires are crown or active surface fires and also according

orrespondence to: Jonay Neris, Edafología y Geología, Universidad deLaguna, Av. Astrofísico Fco. Sánchez s/n, 38071 La Laguna, Spain.ail: [email protected]

pyright © 2012 John Wiley & Sons, Ltd.

to the fire severity. Studies of the effect of fire on soilproperties, such as structural stability (e.g. Giovannini andLucchesi, 1997; DeBano et al., 1998; Benavides-Solorioand MacDonald, 2001) and water repellency (e.g. Horneand McIntosh, 1998; Doerr et al., 2000) also highlightconsiderable dependencies on the temperatures reachedby the soil surface and the residence time, both of whichare main factors affecting fire severity. As a result, waterinfiltration and soil loss are, unsurprisingly, dependent onthe fire severity due to the considerable effect of theaforementioned properties (structural stability and waterrepellency) on these processes. Despite the foregoing,results have proven contradictory in some cases. Robichaud(2000) noted a 40% reduction in infiltration whencomparing pre-fire and post-fire conditions, whereasShakesby et al. (1993) found decreases of up to 60%.Benavides-Solorio and MacDonald (2001) and Rulli et al.(2006) note that, although no major variations in runoffare observed, erosive processes increase by up to twoorders of magnitude in soils affected by high severity fire.However, Cerdà and Doerr (2008) indicate that thepresence of a substantial ash and needle layer couldfavour water infiltration and reduce erosion temporarily,provided the layer remains on the surface. Similarfindings have been noted by other authors (Leighton-Boyce et al., 2007; Onda et al., 2008; Woods andBalfour, 2008).

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Under natural conditions, Andisols (Soil Survey Staff,1999) are characterised by high structural developmentand porosity, which account for their high waterinfiltration rate (Harden, 1991; Nanzyo et al., 1993; Perrinet al., 2001). According to most authors, their singularmineralogical properties and high organic carbon contentsplay a decisive role in their structural properties (Warkentinand Maeda, 1980; Hoyos and Comerford, 2005). In soils ofthis type, intense drying processes, such as those occurringafter a forest fire, trigger total and irreversible changesin their characteristic noncrystalline constituents (Nanzyoet al., 1993). As a consequence of these changes, theirphysical structures suffer serious alterations which couldhave a major effect on the soils’ hydric properties(Poulenard et al., 2004). Although many studies have beenpublished on the effects of fire on soil properties and waterinfiltration (e.g. Lavee et al., 1995; Martin and Moody,2001; Larsen et al., 2009), few of these focus on Andisolsand consider their peculiar mineralogical characteristics.The present work aims to (i) analyse the effect of forest

fires on infiltration, runoff and soil loss in Andisols on theisland of Tenerife (Canary Islands, Spain); (ii) study therelationship between the modification of the soil proper-ties affected by fire and the soils’ hydrological behaviour;and (iii) establish the effects of fire severity on infiltrationand soil loss processes.

MATERIALS AND METHODS

Study area and forest fire description

The island of Tenerife, part of the archipelago of theCanary Islands (Spain), lies between 27�550 and 28�350

north latitude and 16�050 and 16�550 west longitude(Figure 1). The island is 2057 km2 and has a highest pointof 3718 m a.s.l., factors that, along with others such as theinfluence of the trade winds and aspect, account for itswide variety of climatic situations. The north side, whichis exposed to the trade winds, is cool and humid, whereasthe leeward side is dry and warmer. The following soil

Figure 1. Location of the island of Tenerife; A

Copyright © 2012 John Wiley & Sons, Ltd.

moisture and soil temperature regimes have been definedaccording to elevation in the north: ustic, udic and xeric,and hyperthermic, isothermic, isomesic and mesic(Tejedor et al., 2007; Rodríguez Paz et al., 2010). Theregimes defined for the south side are aridic and xeric, andhyperthermic and thermic (Tejedor et al., 2007). Suchenvironmental variability is reflected in the diversity ofthe vegetation and soils present. The island is volcanicwith different lithological materials of varying ages. Thefollowing Soil Taxonomy (Soil Survey Staff, 1999) soilorders are represented: Alfisols, Andisols, Aridisols, Entisols,Inceptisols, Ultisols and Vertisols.The study zone is located on the north side of the

island, between 950 and 1250 m a.s.l., where averageannual precipitation is between 600 and 1000 mm and issupplemented by water from condensation, which canamount to as much as five times the amount of rainfall oreven higher (Marzol Jaén, 2005). Bedrock consists ofbasaltic pyroclasts and lava flows (0.7–0.01 Myr) withsubsequent rejuvenations by analogous ashes (<0.01Myr). The vegetation is mainly pine and green forest andthe terrain is dominated by moderately steep (20%–50%)hillsides, according to the Soil Survey Division Staff(1993) classification. The soils are mostly Andisols andare classified as Ustands and Udands, depending on theirmoisture regime (Soil Survey Staff, 1999). Althoughoccupying much smaller areas, soils of the Inceptisol andEntisol orders (Soil Survey Staff, 1999) are also found.A forest fire broke out in the north of the island on 30

July 2007. According to data from ISTAC (InstitutoCanario de Estadística, 2010), a total area of nearly17 000 ha was affected by the blaze, which had aperimeter of 90 km (Figure 1). Thirteen thousand fivehundred hectares corresponded to woodland, occupiedmainly by pine forest (Pinus canariensis; 13 000 ha) and,to a lesser extent, green forest (Erica scoparia, Ericaarborea, Myrica faya, Laurus azorica, among otherspecies; 500 ha). Approximately 3500 ha correspondedto non-wooded areas. The elevation range affected by thefire was 500 to 2000 m a.s.l.

ndisols (Udands and Ustands) and study sites

Hydrol. Process. (2012)

INFILTRATION, RUNOFF AND SOIL LOSS IN BURNED ANDISOLS

The fire conditions were of high temperature, low relativehumidity and winds occasionally exceeding 50 km h�1

(weather); moderately sloped hillsides (landform terrain);and alternation of vegetal formations and silviculturaltreatments (vegetation). Under these conditions, the firebehaviourwas heterogeneous. Explosive crownfire affected30% of the pine forest, whereas 70% was affected by activesurface fire. Shrubs and green forest zones were alsoaffected by active surface fire.

Research design

Five sites were selected (Figure 1), all with Andisols,which were either Ustands or Udands (Soil Survey Staff,1999). Pine forest was the dominant vegetation, althoughit varied in terms of undergrowth and silviculturaltreatment. For comparison purposes, each site consistedof an unburned zone (U1–U5) and a burned zone (B1–B5).In the case of site 2, two burned zones (B2a and B2b) withdifferent fire severities were selected. In all, a total of 11zones were studied (five unburned and six burned). In eachzone, three plotswere selected for each study slope (10%, 33plots; and 30%, 33 plots). Following the Soil SurveyDivision Staff (1993) slope classification, we will use theterms gentle for 10% slope angle and moderately steep for30% slope angle.

Pre-fire and post-fire forest floor characteristics

Bearing in mind the influence of forest floor on thehydrological behaviour of forest soils (Guevara-Escobaret al., 2007; Keith et al., 2010), a complete description wasundertaken in each unburned zone. The thickness andcovering of litter and duff were examined visually, as wasthe presence of fungi hyphae in the latter. Duff coherencewas determined by hand in moderately dry samples usingrupture resistance classes (Soil Survey Division Staff, 1993)and water repellency was determined by the water droppenetration time test (WDPT; Doerr, 1998). After the fire,severity was determined in the burned zones using thecriteria proposed by Ryan (2002) relating to the changes inaboveground vegetation and soil organic matter.

Infiltration, runoff and soil loss

Infiltration, runoff and soil loss were studied using arainfall simulator similar to that developed by Nacci andPla (1991) and consisting of a gravity drip simulatorplaced at a height of �2 m over a 0.1-m2 rectangularbounded parcel. The parcel boundaries were establishedusing a 0.27� 0.37� 0.25 m frame which was nailed0.05 m into the ground. Plastic sheeting was wrappedaround the simulator to minimise the variability in rainfalldue to wind. Bearing in mind the high infiltration capacityof Andisols, high rainfall intensity (60 mm h�1) was usedto guarantee runoff phenomena. Rainfall intensity wascalculated before and after each event by measuring thevolume of water collected during a 5-min interval in aplastic calibration pan placed over the plot frame. Instantrainfall intensity was determined by linear regressionbetween the intensity calculated before and after the


experiment. The experiments were of 35-min duration.Demineralised water was used, given that electrolyteconcentrations could affect rainfall simulation results(Agassi and Bradford, 1999; Borselli et al., 2001).Once the rainfall event had commenced, the volume to

runoff (VR; mm)—the amount of rainwater needed togenerate runoff—was measured. All the runoff water wasmeasured at 5-min intervals and collected at alternateintervals in 500 ml bottles for sediment analysis. Theinfiltration rate (IR: mm h�1) was determined until thesteady state was reached (steady-state infiltration rate,SIR: mm h�1) and the runoff/rainfall ratio (RR, %)—therelationship between the total runoff volume and thevolume of rainfall during the rainfall event—was alsodetermined. Runoff samples were dried (105�C) until thewater had completely evaporated and were then weighedto determine the mass of eroded soil. Soil loss wasrepresented as the soil loss rate (SR: g m�2 h�1) andsediment concentration (SC: g l–1).

Soil analysis

Three bulk soil samples were collected from the upper5 cm of each zone. Soil surface characteristics aredecisive in infiltration processes (Lavee et al., 1995;Cerdà and García-Fayos, 1997) and are representative ofthe effects of forest fires (Kutiel and Naveh, 1987). Themain soil properties relating to infiltration and soil losswere analysed. Soil organic carbon was determined bydichromate oxidation (Walkley and Black, 1934). Bulkdensity was measured on an oven-dried weight basis of a96.6-cm3 core sample collected at field-moisture condi-tions (Blake and Hartge, 1986). Water repellency wasdetermined using the WDPT (Doerr, 1998). Three indicesrelating to soil structure— soil aggregation (SA), wet soilstability (WSS) and wet aggregate stability (WAS)—werestudied, in accordance with Bartoli et al. (1991).

Statistical analysis

SPSS version 17.0.0 was used for the statistical analysisof the results obtained. Given that the data did not fulfil therequirements for normality and homogeneity of variance,the analyseswere performed using non-parametricmethods.The Wilcoxon signed-rank test, which establishes thedifferences between the groups defined for dependentsamples (burned and unburned), was used to test thehypotheses. A significance level of 0.05 was set.

RESULTS

Forest floor characteristics, fire behaviour and severity

Table I sets out themain forest floor properties. The forestfloor in all the study zones consisted of litter formed byloose, fresh pine needles on duff comprising decomposingorganic material. The litter lacked coherence in all the zonesand hence the calculation of its water repellency was notrelevant. The duff presented continuity andmoderately hardconsistency in zones U1, U2 and U4, forming a continuous


Table I. Forest floor (litter + duff) characteristics

Litter Duff

Site Depth Coverage Depth Coverage WDPT Fungi frequency Consistency

cm % cm % s %

1 2 100 1 100 4080 100 Moderately hard2 1 85 1.5 70 2160 33 Moderately hard3 1 75 0.5 30 ND 0 ND4 2.5 100 1 100 3420 100 Moderately hard5 1 70 0.5 20 ND 0 ND

ND: not determined.

J. NERIS ET AL.

and coherent structure on the soil surface. This organicmaterial tends to be rich in fungi hyphae. WDPT test resultsindicate that the duff in these zones is extremely repellent. Incontrast, in zones U3 and U5, the duff covering on the soilsurface was minimal and hence estimation of its waterrepellency was unnecessary.Figure 2 shows the fire environment, behaviour, severity,

post-fire soil surface characteristics, study zones and photosillustrating each situation. The fire severity of four burnedzones (zones B1, B2a, B4 and B5) was classified as light inaccordance with the criteria proposed by Ryan (2002). Thecanopy trees were scorched; the original forest floor wascharred or consumed but the soil organic layers remainedintact. After the fire, the mineral soil was covered withlitter comprising needles from defoliation of the scorchedcrowns. In these cases, a thin ash layer was also seen onthe surface. These zones coincided with pine forest areaswith little or no undergrowth, in which active surfacefires constituted the main fire behaviour. The fire severity oftwo burned zones (zones B2b and B3) was classified asmoderate. The undergrowth was charred, the forest floorand crown needles were largely consumed and the soil

Figure 2. Fire environment, fire seve


organic layers were deeply affected. The mineral soil wasbare or partially covered in ash after the fire. Thesezones corresponded to pine forest areas with abundantundergrowth in which explosive crown fire was the main firebehaviour.

Effect of fire on soil properties related to infiltration anderosion

Table II provides the results of the soil analyses carried outin the burned and unburned zones. Typically of unburnedAndisols, organic carbon content was high, bulk density lowand the soils were highly developed structurally (SA, WASand WSS). All the natural Andisols studied presented somedegree of water repellency: extreme in the case of U1 andU2, severe in U3, strong in U5 and slight in U4.After the fire, organic carbon generally decreased by

between 3% and 11%, the difference being statisticallysignificant. Soil structure was also affected (Table II): SAdiminished by between 2% and 8%, and WAS and WSSby between 2% and 20%. Once again, the differenceswere statistically significant. The biggest decreases in the

rity and surface dynamics after fire


Table II. Soil characteristics of unburned and burned zones

Site State OC BD �33 kPa �1500 kPa SA WSS WAS

% mg m�3 % % % % %

1 U 19.2� 1.8 0.50� 0.08 62.3� 8.8 32.1� 7.2 53.0� 7.9 46.5� 10.8 85.7� 8.9B 8.3� 3.1 0.55� 0.05 76.8� 7.8 27.1� 10.6 44.7� 8.1 29.0� 9.3 62.5� 10.1

2 U 10.0� 4.6 0.77� 0.11 54.5� 8.9 20.3� 8.9 43.2� 5.7 29.4� 5.0 66.9� 2.0Ba 6.0� 1.1 0.64� 0.07 41.6� 9.9 17.7� 7.6 40.1� 6.0 19.2� 6.5 48.7� 21.3Bb 6.9� 1.9 0.45� 0.03 68.0� 5.4 19.1� 0.9 40.8� 2.1 27.5� 5.1 65.1� 9.8

3 U 16.7� 4.8 0.52� 0.07 73.6� 1.8 26.6� 4.6 45.0� 5.6 30.9� 8.9 65.2� 11.5B 9.1� 1.5 0.47� 0.07 74.7� 11.7 17.3� 2.4 39.9� 4.9 21.5� 7.9 51.5� 15.1

4 U 6.7� 2.2 0.85� 0.02 38.0� 16.3 17.7� 7.1 55.0� 9.4 55.5� 9.3 99.8� 13.1B 7.9� 1.4 0.71� 0.06 47.4� 4.4 28.5� 3.8 48.1� 3.3 34.2� 5.1 68.8� 10.0

5 U 7.8� 4.7 0.79� 0.13 46.3� 5.4 23.6� 6.6 47.8� 6.2 38.8� 5.1 78.2� 2.4B 5.3� 1.4 0.49� 0.16 48.7� 6.1 16.4� 1.8 42.7� 0.6 29.2� 9.9 67.2� 21.3

U, Unburned zone; B, burned zone; OC, organic carbon; BD, bulk density; SA, soil aggregation; WSS, wet soil stability; WAS, wet aggregate stability.


stability parameters were recorded in zone B1 and thesmallest in B2b. In all zones except B1, bulk destinyvalues also fell (Table II), with the decrease rangingbetween 0.05 and 0.30 mg m�3. Burned zones B2band B5 saw the biggest post-fire decreases. Thedifferences in this parameter were found to be statisticallysignificant. With regard to water repellency, a markedfall in values was observed in some of the burned zones,with the biggest decreases noted in B1 and B2b (Table II).In zones B4 and B5, however, repellency increased inthe post-fire period. The differences between burnedand unburned zones were not statistically significant forthis property.No clear relationship was observed between fire

severity and the effects of fire on the properties of thesoil. As Figure 3 shows, although certain differencesaccording to fire severity were found in parameters suchas bulk density and water repellency, in no case were theystatistically significant.

Figure 3. Influence of fire severity on organic carbon (OC), SA, WSS,WAS, bulk density (BD) and water repellency (logWDPT)


Pre-fire and post-fire infiltration, runoff and soil loss

Figure 4 shows the results of the simulated rainfall eventsin the unburned and burned zones for the two study slopes(10% and 30%). Bearing in mind that the soils are Andisols,the unburned zones have relatively low VR and SIR valuesand high RR values. VR ranged from 1.5 to 5.3 mm for

Figure 4. VR, SIR and RR of burned and unburned soils (box plot: thickbar =median; upper and lower limits of the 75th and 25th percentiles,

respectively)


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gentle slopes and 1.0 to 1.9 mm for moderately steep slopes.SIR ranged from 16 to 37 mm h�1 for gentle slopes and 7 to25 mm h�1 for moderately steep slopes. RR was between22% and 56% for gentle slopes and 37% and 63% formoderately steep slopes. ZoneU5 stands out from the rest inthat it produced the highest SIR and lowest RR values. Norelationship was observed between WSS and RR for thesoils unaffected by the fire. Similarly,WDPTwas not foundto have any clear effect on RR.The soil loss values were relatively low in the unburned

zones (Figure 5). SC ranged from 0.1 to 1 g l�1 both for thegentle andmoderately steep slopes. In no case did SR valuesexceed 15 and 33 g m�2 h�1 for the two slope angles,respectively. The biggest soil losses were recorded in zonesU3 and U5, although these produced the highest SIR valuesand lowest RR values of the overall data set. Despite its highrunoff values, zone U2 had the lowest sediment rate andconcentration values.Figure 4 also illustrates the effect of the fire on infiltration

and runoff processes. Post-fire increases in VR werebetween 0.1 and 1 mm for gentle slopes and between 0.3and 0.9mm formoderately steep slopes, although a decreasewas noted in B2b and, in particular, in B5 for the 10% slopeangle. The VR differences between the burned andunburned zones were not statistically significant. In general,no clear post-fire tendency as regards SIR was seen(Figure 4). SIR values for the gentle slopes increased inzones B1, B3 and B4, although the magnitude of theincrease was of relevance only in B1 (17 mm h�1). Incontrast, a decreasewas recorded for the same slope angle inzones B2a, B2b and B5 (11, 4 and 5 mm h�1, respectively).SIR behaviour in the moderately steep slopes also proveduneven. Whereas SIR increased in zones B1, B2a and B2b(3, 3 and 8 mm h�1, respectively), it fell by 9 mm h�1 in B3

Figure 5. Soil loss (SR and SC) of burned and unburned soils (box plot:thick bar =median; upper and lower limits of the 75th and 25th percentiles,

respectively)


and did not change noticeably in B4 and B5. The SIRdifferences between burned and unburned zones were notstatistically significant. The behaviour of the post-fire runoffvalues was variable, reflecting infiltration (Figure 4). Inzones where SIR increased because of the fire, a slightdecrease in RR values was noted (gentle slopes, B1 and B3;moderately steep slopes, B1, B2a and B2b). The oppositeoccurred in zones B2a, B2b and B5 for gentle slopes and inB3 for moderately steep slopes: here, the fire caused a slightincrease in runoff processes because of the fall in SIR. Onlythe 34% decrease observed in the RR of zone B1 for gentleslopes was of relevance. The RR differences betweenburned and unburned zoneswere not statistically significant.Fire severity was not found to be clearly related to post-fireinfiltration and runoff behaviour (Figure 6).Post-fire erosive processes were more pronounced and

presented a much clearer behaviour pattern than the otherparameters studied (Figure 5). In general, SR and SCvalues increased because of the fire, considerably so insome of the study zones. The soil loss processes werefound to be closely linked to fire severity and the state ofthe surface soil (Figures 2 and 7). Zones affected by lightfire severity (B1, B2a, B4 and B5) had moderately highersoil loss values compared with unaffected zones. AverageSC and SR values doubled in these zones for both gentleand moderately steep slopes. SC averaged 0.8 and 1.2 g l–1,respectively, for the two slope angles. Within this group,zone B5 produced the highest SC values for both gentleand moderately steep slopes (1.5 and 2.9 g l–1,

Figure 6. Influence of fire severity on soil infiltration (VR and SIR), runoff(RR) and soil loss (SR and SC)


Figure 7. Influence of runoff (RR) and sediment concentration (SC) onsediment rate (SR)

Figure 8. Examples of erosion type and sediment transportation affectedby fire severity: (a) rainfall simulation in light severity burned plot, (b)rainfall simulation in moderate severity burned plot and (c) floating

aggregates on the runoff water


respectively). SR averaged 16 and 32 g m�2 h�1 forgentle and moderately steep slopes, respectively, withthe highest values found once more in zone B5. In thezones affected by moderate fire severity (B2b and B3),the effect of fire on soil loss was of a greater magnitudebecause the soil surface was either bare or covered withash due to the behaviour of the fire. The average SCvalue increased by a factor of 6.5 compared withunaffected zones on the gentle slopes and reachedapproximately 2.5 g l–1 in the burned zones. On themoderately steep slopes, post-fire SC was, on average, 15times higher compared with pre-fire SC, reaching 8.7 and6.3 g l–1 for B2b and B3, respectively. As a result, the SRvalue increased 8-fold for gentle slopes and 16-fold formoderately steep slopes, reaching values of 65 g m�2 h�1

on the 10% slope angle and 260 g m�2 h�1 on the 30%slope angle. Fire severity was also found to influence theeffect of slope on soil loss processes (Figure 6). The zonesaffected bymoderatefire severity saw the biggest rises in bothSC and SR with the slope increase from gentle to moderatelysteep. In zones B2b and B3, the rise in SC with increasingslope was approximately 6 and 4 g l–1, respectively. The SRvalue for these zones was approximately 195 g m�2 h�1

lower for the gentle slopes than for themoderately steep ones.No relationships were found between the soil properties

(structural stability and water repellency) and SC in eitherthe unburned or burned zones (Figure 5). Moreover, SRdid not correlate directly with RR but did with SC,irrespective of the effect of fire and for both study slopes(Figure 7). This result suggests that the runoff valuesalone do not account for the soil loss values obtained. Thefindings point to other factors that control soil erodibilityand which may be related to the characteristics of theforest floor and soil cover and protection. Observation ofthe rainfall events indicate that the importance of splasherosion in unburned zones and in light fire–severityburned zones was very minor (Figure 8a). In contrast, thistype of erosion was relevant in moderate fire–severity


burned zones with bare soil surfaces (Figure 8b).Furthermore, the bulk of the material transported in thesezones was mobilised by floating and was not dispersed inthe runoff water, as is usually the case (Figure 8c).

DISCUSSION

It is widely acknowledged that forest fires promote runoff(Imeson et al., 1992; Cerdà et al., 1995; Rulli et al.,2006). As Doerr et al. (2000) note, this behaviour mightbe related to (i) the appearance of or increase in initial soilhydrophobicity or increased effectiveness thereof and,(ii) the effect of the fire on infiltration-related soilproperties. With respect to water repellency, most studiesnote a post-fire increase in hydrophobic substances inthe soil (Horne and McIntosh, 1998; Doerr et al., 2000).Furthermore, sealing processes acquire importance after afire (Wells et al., 1979; Woods and Balfour, 2008). Theappearance of this phenomenon is associated with a totalor partial loss of raindrop impact protection (Chartres andMucher, 1989; Cerdà, 1998), as well as with the reductionin aggregate stability in medium–high severity fires(Giovannini and Lucchesi, 1997; Ketterings et al., 2000;Mataix-Solera et al., 2011) or the clogging action of ash inmacropores (Wells et al., 1979; Lavee et al., 1995).Despite this, the results of this study indicate that no

general behaviour is seen in the soil water intake processafter a fire. In addition, the effects of fire on the infiltrationprocess were found to be independent of fire severity. Thehigh runoff and low infiltration observed in the unburnedzones—related to the duff characteristics of the soils (Neriset al., 2012)—mean that no dramatic effects on thesehydrological properties can be expected during the post-fireperiod (Benavides-Solorio and MacDonald, 2001).


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Moreover, as some authors have noted (Benavides-Solorioand MacDonald, 2001; Rulli et al., 2006), the large numberof soil and environmental factors modified by the fireand their different effects on infiltration may account for thelack of major variations in water intake. The present studyhas found that fire affects both the forest floor and soilproperties such as organic carbon, structural stability, bulkdensity and water repellency, which can have contrastingeffects on infiltration. The effect of fire on soil structure isvariable and depends on several factors (Mataix-Soleraet al., 2011). Some authors have found that heating has noeffect on aggregate stability (Llovet et al., 2008; Zavalaet al., 2010), whereas others report an increase (Giovanniniand Lucchesi, 1997; Llovet et al., 2009) or a decrease inthis property (Badía and Martí, 2003; Are et al., 2009).According to Mataix-Solera et al. (2011), soil properties(e.g. organic matter, water repellency and texture) andenvironmental factors (e.g. fire severity) directly affect theaggregate stability of soils. Fire usually promotes soilstructure enhancement in clay-rich hydrophilic soils withcalcium carbonate as the main binding agent. On theother hand, soils that are rich in organic matter and havehigh repellency and low–moderate clay content (such asthe soils studied here) show a noticeable decrease inaggregate stability after a medium–high severity fire. Thisis mainly due to the negative effect of moderate-highseverity fires on the organic matter, the principal cementingsubstance of these soils, and on the soil water repellency,whichmay also act as a binding agent in strong hydrophobicsoils. These effects on soil structure could negatively affectinfiltration due to porosity occlusion by non-aggregatematerial (Terry and Shakesby, 1993; Assouline andMualem, 2000; Mataix-Solera et al., 2011). Similarly, thepresence of surface ash may also be a factor due to itscontribution to soil pore clogging (Wells et al., 1979; Laveeet al., 1995; Woods and Balfour, 2010). With regardto promoting water intake, the effects of fire on theforest floor were such that the floor was eliminated entirelyin all the study soils and replaced by a litter of scorchedneedles and ash in zones affected by light fire severity, nocover or an ash layer in zones where the fire was ofmoderate severity. Duff combustion results in the elimin-ation of the runoff promotion effect of this layer, which isattributed to its high continuity and water repellency (Neriset al., 2012). Moreover, the presence of an ash layer on thesoil surface leads to an increase in the water infiltration ratedue to the high water retention capacity of the ash (Cerdà,1998; Cerdà and Doerr, 2008; Woods and Balfour, 2008;Woods and Balfour, 2010). An additional contributingfactor is the reduction in water repellency observed on thesurface of some of the burned zones. Although it isacknowledged that fires promote hydrophobicity in soils(DeBano, 2000; Horne and McIntosh, 1998; MartínezZavala et al., 2009), authors such as Doerr et al. (2006) havenoted a reduction in this parameter at the surface level inburned soils with high pre-fire water repellency. Thedisparate effect of fire on the soil properties controllingwater intake account for the lack of major post-firedifferences in runoff.


Turning to erosion, the results highlight the consider-able effect of fire on soil loss, despite its very minor effecton infiltration. This finding coincides with the observa-tions of Rulli et al. (2006), Benavides-Solorio andMacDonald (2001) and Johansen et al. (2001). The soilloss process is conditioned not just by the volume ofsurface water flow but by a combination of factors whichare also modified by the fire (Rulli et al., 2006). Of thesefactors, the most salient are (i) a reduction in soilprotection against raindrop impact (Scott et al., 2009)and, (ii) the susceptibility of the soil constituents todetachment and transportation by runoff (Wells et al.,1979; Terry and Shakesby, 1993), due primarily to thesoil’s diminished structural stability. Although, in thisstudy, infiltration and runoff phenomena were largelyunaffected by the fire, the results are in agreement withhypotheses concerning soil cover and stability. The highdependence of the sediment rate (SR) on the sedimentconcentration (SC) and the fact that it is independent ofthe runoff rate (RR) indicate that erosion is dominated bythe erodibility of the soil and not by the volume of surfacewater flow. Moreover, the results reflect the importantnegative effect of fire on structural properties. However,the negligible relationship observed between WSS andSC in the rainfall events indicate that, alone, the variationin this parameter does not account for the soil lossbehaviour. Soil cover and the fire effects on forest floormay be the decisive factors in the erosion behaviour seenin the simulated rainfall events. The fire effects on soilsurface cover depend on the fire severity (Ryan, 2002).According to Scott et al. (2009), the forest floor protectsthe soil against the impact of raindrops by reducing theirkinetic energy and limiting splash. The loss of the soilcover because of fire exposes the soil surface to theeffects of erosive forces such as water and leads to order-of-magnitude increases in erosion (Scott et al., 2009).Johansen et al. (2001), de Luis et al. (2003) and Cerdàand Doerr (2007) indicate that the amount of bare groundcorrelates positively with the degree of erosion. In thepresent study, fire severity was found to have a majorinfluence on soil cover characteristics and on the soilerosion processes that followed the fire studied. All theburned zones lost their original forest floor and their coverdepended on the severity of the fire. Consequently,although in the soils affected by light and moderate fireseverity the effects on soil structure were similar andno major differences were observed in runoff, soil lossresults were much higher where soil cover was lowerowing to the greater fire severity. The total elimination ofthe forest floor after a moderate severity fire reducesprotection against raindrop impact and water detachmentforces. These results are in agreement with those obtainedby Benavides-Solorio and MacDonald (2001), for whomthe direct relationship between fire severity and sedimentproduction is due fundamentally to the magnitude of thealteration which the fire causes to the soil protectionafforded by the vegetation.With respect to erosion, the observations of the present

study highlight the importance of both splash erosion and



the transportation of soil particles by flotation in runoff.Larsen et al. (2009) indicate that, in situations in whichincreased sediment yield was found to be independent ofthe increase in runoff, splash erosion was the main causeof soil loss. This hypothesis is in agreement with theindependence of SR and RR found in our study data andwith observations noted during the rainfall events whichsuggest that the effect of the raindrop impact plays amajor role in the erosive processes in moderate fireseverity burned zones. Regarding sediment transporta-tion, the observations are in agreement with the results ofPoulenard et al. (2001) and Rodríguez Rodríguez et al.(2002) for Andisols. These authors suggest that theerosive process is dominated by the transportation byflotation of low bulk–density and highly water-repellentaggregates, as opposed to the general model of soilparticle mobilisation by dispersion.Lastly, despite the marked increase in soil loss in the

burned zones, the results of erosion are relatively low ifcompared with those obtained in similar studies. In mostsuch studies, the SR and SC values are at least oneorder of magnitude higher than those given here, even ifthey are for different soil types (e.g. Cerdà et al., 1995;Benavides-Solorio and MacDonald, 2001). Our resultsare, however, comparable with those obtained forAndisols by Poulenard et al. (2003), who published SRvalues of around 15 g m�2 h�1 for slopes of 25% subjectedto periodic burning before cultivation. These resultsunderline Andisols’ high stability and low susceptibility towater erosion (Nanzyo et al., 1993; Poulenard et al., 2001;Rodríguez Rodríguez et al., 2002).

CONCLUSIONS

Thewater repellency and continuity of forest floor duff havea major influence on water infiltration in the unburnedzones. The water intake–limiting characteristics of thisorganic horizon seem to prevail over the high infiltrationcapacity that is attributed to soils of the Andisol order and isusually associated with their high soil aggregation andstability. Although promoting runoff phenomena, the soilprotection afforded by the forest floor against raindropimpact results in soil loss values that are manifestly low forthis type of soil.No major variations are seen in infiltration and runoff

after a fire. The low infiltration values observed in theunburned zones, the effect of the fire on the forest floor andon the soil properties and its disparate effect on infiltrationand runoff mean that no clear post-fire behaviour pattern isdetected in these two processes. A post-fire reduction in theWSS of the soils is seen but this is offset by the decrease insurface water repellency found in most of the study soils, aswell as by the disappearance of the forest floor and the lossof its runoff promotion effect.The extent of the effect of the fire on erosion processes is

greater than on infiltration or runoff. In some cases, the soilloss values in the burned zones exceed those recorded in theunburned zones by two orders of magnitude. Fire severity


heavily influences soil loss processes, even though norelationship is found between this parameter and soilproperties such as soil stability or water repellency. Soilcover characteristics are, however, found to be clearlyrelated to fire severity. Post-fire soil cover ultimatelydetermines soil losses. Erosive processes are most severein themoderate fire–burned zones inwhich the soil is bare orcovered with a thin ash layer. In such cases, splash is themost common soil erosion mechanism. Where scorchedneedles form a post-fire litter, erosive phenomena do notreach such high values, even if the values are still higherthan in natural conditions.

ACKNOWLEDGMENTS

The research for this study was undertaken as part of aproject entitled ‘Forest fire impact on soil hydrologicalproperties’, funded by the Consejo Insular de Aguas deTenerife (Tenerife Water Council).

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Documents

Infiltration, runoff and soil loss in Andisols affected by forest fire (Canary Islands, Spain)