Soil Water Repellency Dynamics in Pine and Eucalypt Plantations in Portugal - a High-Resolution Time Series

land degradation & developmentLand Degrad. Develop. (2013)

Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.2251

SOIL WATER REPELLENCY DYNAMICS IN PINE AND EUCALYPTPLANTATIONS IN PORTUGAL – A HIGH-RESOLUTION TIME SERIES

Juliana M. Santos1, Frank G. A. Verheijen1*, Filipa Tavares Wahren2, Andreas Wahren2, Karl-Heinz Feger2,Léonard Bernard-Jannin1, María E. Rial-Rivas1, Jacob J. Keizer1, Joao P. Nunes1

1Department of Environment and Planning, Centre for Environmental and Marine Studies (CESAM), University of Aveiro, 3810-193 Aveiro, Portugal2Institute of Soil Science and Site Ecology, Technische Universität Dresden, Pienner Str. 1901735 Tharandt, Germany

Received: 8 August 2013; Revised: 26 August 2013; Accepted: 27 August 2013

ABSTRACT

During the 20th century, afforestation resulted in plantations of Pine and Eucalypt becoming the main crops in north-central Portugal withassociated and well-known soil water repellency (SWR). The aim of this study was to improve the insights in the temporal dynamics andabrupt transitions in water repellency of the topsoil and the mechanism that determine the behaviour of SWR. Topsoil water repellencywas monitored in the Caramulo Mountains (north-central Portugal) between July 2011 and June 2012. The intensity of SWR was measuredin situ at soil depths of 0, 2.5 and 7.5 cm using the ‘molarity of an ethanol droplet’ test. Volumetric soil moisture content was monitoredin situ using a Decagon ECH2O EC-5 probe. SWR behaviour broadly followed five alternating dry and wet periods during the 12-monthperiod, with more pronounced differences in the Pine site than in the Eucalypt site. SWR under Eucalypt was substantially more tem-porally dynamic than under Pine, with double the number of moderate and large SWR changes at the 7.5 cm depth. Soil moisture contentand antecedent rainfall were better correlated to SWR under Pine than under Eucalypt, although in both cases insufficient to predict thetemporal variations. Copyright © 2013 John Wiley & Sons, Ltd.

key words: hydrophobicity; monitoring; transition zone; soil moisture content; rainfall

INTRODUCTION

Soil water repellency (SWR) is understood as a keyhydrological, geomorphological and pedological processnecessary to understand Earth surface processes (Doerr,2000; Shakesby & Doerr, 2006; Leighton-Boyce et al.,2007; Cammeraat et al., 2010; Jordán et al., 2013). Water-repellent layers have been found in fire-affected forestsoils (Doerr et al., 1998; Keizer et al., 2008; Fernándezet al., 2012) and agricultural land (Keizer et al., 2007;Blanco-Canqui & Lal, 2009; González-Peñaloza et al., 2012;García-Moreno et al., 2013). These examples show thathigh organic matter contents in soils under organic farming,or the impact of fire (Pereira et al., 2013), can triggerSWR. However, water-repellent soils are also found inforest and scrublands not affected by fire where they affectsoil fertility, triggering land degradation processes (Shakesbyet al., 2000).Soil water repellency has been reported to alter the infiltration

of water and solutes into the soil with important implications forplant growth, surface runoff and soil erosion (Doerr, 2000; Doerr& Thomas, 2000; Shakesby et al., 2000; Benito et al., 2003;Coelho et al., 2005; Malvar et al., 2013; Mataix-Solera et al.,2013; Prats et al., 2013). For this reason, SWR is consideredan important factor in hydrological modelling (Ferreira et al.2000; Doerr et al. 2003). In the case of Portugal, several studies

*Correspondence to: F. G. A. Verheijen, Department of Environment andPlanning, Centre for Environmental and Marine Studies (CESAM), Univer-sity of Aveiro, 3810-193 Aveiro, Portugal.E-mail: [email protected]

Copyright © 2013 John Wiley & Sons, Ltd.

have highlighted the effects of SWR on the hydrologicbehaviour of unburnt Eucalypt and Pine plantations. Ferreiraet al. (2000) and Keizer et al. (2005b) studied the consequencesof SWR on runoff generation in Eucalypt stands in Portugal:runoff processes, and their drivers varied radically betweenwater-repellent phases (dry conditions) and wettable phases(wet conditions). Prats et al. (2012) observed that extremeSWR significantly enhanced overland flow generation in amulching experiment under Eucalypt. Doerr & Thomas (2000)identified the difficulties in isolating the erosional impact ofSWR from that of other factors and in identifying the exact roleof these factors and of their interactions in the development offlow pathways through repellent soils as major research gaps.This paper aims to study the seasonal evolution of SWR,

which will be helpful to develop sustainable landmanagementand to develop better and more efficient soil conservationstrategies (Prats et al., 2012; Prats et al., 2013). Two speciesexpanded rapidly in Portugal in the last century to supportthe wood-based industry (lumber, pulp and paper): Pinuspinaster since the 1900s and Eucalyptus globulus since the1970s. By 2005, they constituted the dominant forest types(Louro et al., 2010), covering 8.9% and 7.5% of the country’sland area, respectively (Rego et al., 2013). Both tree speciesare associated with SWR, especially during dry summerconditions (Doerr & Thomas, 2000; Ferreira et al., 2000; Benitoet al., 2003; Ferreira et al., 2005; Keizer et al., 2005a).Several studies observed that spatial and temporal variability

of SWR are related to soil moisture contents (SMCs) inP. pinaster and E. globulus forest soils (Leighton-Boyce et al.,2005; Keizer et al., 2008; Rodríguez-Alleres & Benito, 2011;

J. M. SANTOS ET AL.

Rodríguez-Alleres & Benito, 2012). In the same area and foresttypes as this study, Doerr & Thomas (2000) studied the effect ofsoil moisture on SWR and observed that SWR was destroyedwhen the SMC exceeds 28% (ww�1), but showed that afterwetting, SWRwas not necessarily re-established during drying,suggesting that short-term and seasonal changes in SWR arenot simply a function of variations in SMC. Fresh input ofwater-repellent substances or microbial activity have beensuggested as mechanism for the re-establishment of SWR indrying cycles as commonly observed under field conditions(Doerr 2000; Doerr & Thomas, 2000; Rillig et al., 2010). Alsoin the same area as this study, Leighton-Boyce et al. (2005)investigated SWR and SMC over 16months (at 2monthintervals) in four different E. globulus plantations and identifieda soil moisture transition zone, that is, when volumetric SMCwas less than 14% soils were repellent and wettable when itexceeded 27%. Keizer et al. (2008) investigated SWR andSMC for burnt E. globulus plantations in Portugal over10months at bi-weekly intervals and concluded that SMCexplained a part of the SWR and that even biweekly samplingmight not capture the full dynamics of topsoil repellency.The present work aims to improve our insights in the temporal

variations and, in particular, abrupt transitions in SWR in the twoprincipal forest plantation types in north-central Portugal, that is,E. globulus andP. pinaster. To this end, SWRat and near the soilsurface was monitored in situ in a paired-field comparisonbetween soils under Eucalypts and Pines for a 1-year period, witha high temporal resolution of one or two weeks. The observedSWRpatternswere comparedwith those of potential explanatoryvariables, in particular SMC and antecedent rainfall. Finally, theimplications of the present findings are discussed as they relatedto our current understanding of how soil moisture controls SWRand how they influence hydrological modelling.

MATERIALS AND METHODS

Study Area and Study Sites

Two sites under Eucalypt (E. globulus) and maritime Pine(P. pinaster) were selected for this study in the Caramulo

Figure 1. Location of the Eucalypt and Maritime Pine study sites near/in the experis available in colour online at wiley


Mountains in Portugal (Figure 1) at the Lourizela and Serrade Cima experimental catchments, respectively. The climateof the study area is wet Mediterranean (Csb in the Köppenclassification), with a long-term mean annual rainfall of1,379mm at 200masl and with long-term average monthlytemperatures ranging from 19.8 °C in August to 5.8 °C inJanuary (Leighton-Boyce et al. 2005). Soils are UmbricLeptosols and Leptic Cambisols (humic) (IUSS, 2006)developed on weathered schist on moderately steep slopesbetween 300 and 500masl (Table I).Eucalypt plantations cover 73% of the forested area of the

Serra de Cima catchment and Pine plantations cover 62% ofthe forested area of the Lourizela catchment. The Eucalyptstands are managed as short rotation coppices with a fullrotation cycle involving 2–3 cuts at 10–12 year intervals,after which a new stand is planted following soil preparationoperations. The Eucalypts in this study site were plantedafter a wildfire in 1986 and cut in 2005. The bulk of the Pinestands in the area have experienced wildfires in the past twodecades, showing natural regeneration. The Pine plantationin this study was burnt in 1991 and, thus, approximately20 years old. Further vegetation characteristics are given inTable I. The two study sites were selected on environmentalsimilarity and proximity to an existing meteorologicalstation: Pousadas, located approximately equidistant fromthe study sites (Figure 1). The most important limitation inenvironmental similarity is the slope angle variation:35–40% in the Pine site and 2–20% in the Eucalypt site(Table I).Rainfall was measured at the two study sites using a

storage gauge and at the Pousadas meteorological station,using both a storage gauge and an automatic rainfall gauge(Pronamic Professional Rain Gauge) connected to thestation’s data logger (Campbell CR1000), which wasinstalled on 22 July 2004 and replaced by a CampbellCR800 on 22 September 2011. The rainfall accumulated inthe storage gauges was measured during each field trip, witha periodicity of 1/2weeks, in presence of rainfall in theantecedent period.

imental catchments of Serra de Cima and Lourizela, respectively. This figureonlinelibrary.com/journal/ldr.

LAND DEGRADATION & DEVELOPMENT, (2013)

wileyonlinelibrary.com/journal/ldr

Table I. General characteristics of the study area and description of soils and vegetation

Eucalypt Pine

TerrainCoordinates 40°36′29″N; 8°19′59″W 40°37′51″N; 8°18′30″WElevation (masl) 478 323Slope section length (m) 80 25Slope angle (degrees) 2–20 35–40Aspect N–NW N–NE

SOILSoil type (IUSS, 2006) Leptic Cambisol (Humic) Umbric LeptosolClay (%) 24 12Silt (%) 47 45Sand (%) 29 43Soil organic matter (%) 12·1 7·4Dry bulk density (g/cm3) 0·84 1·01Θ_pF2 – Field capacity (vol.%) 29 35Θ_pF4.2 – Wilting point (vol.%) 8 6

VegetationDominant understory shrubs (%) Genista tridentate (35) Pteridium aquilinum (20)

Erica sp. (35) Genista tridentata (10)Ulex europaeus (20)

Litter layer 10% litter cover – 5–15 cm thick 70% litter cover – 8–20 cm thick

SOIL WATER REPELLENCY DYNAMICS UNDER PINE AND EUCALYPT

Soil Water Repellency and Soil Moisture ContentMeasurements

Soil water repellency and SMCmeasurements were made onceevery 1 or 2weeks at the Eucalypt site (37 times) and the Pinesite (36 times) between 7 July 2011 (Eucalypt) and 14 July2011 (Pine) until 13 July 2012. In the Pine site, sampling wascarried out along transects of 25m (approximately 5m distancebetween adjacent transect points) and in the Eucalypt site alongtransects of 80m (15–20m between adjacent transect points).Transects were parallel to the slope, and transect points wereroughly representative of shrub density and distance to trees.Sampling points were selected immediately adjacent (1m) tothe points used previously. From the start of the experimentuntil the end of September 2011, five points were sampled foreach transect on each field visit. From October 2011 onwards,three transect points were sampled because of restraints on timeresources and because spatial variability in SWR was limitedduring that period.Thus, SWR and SMC were measured at three to five

transect points at three soil depths (soil surface after removalthe litter layer, 2.5 cm and 7.5 cm depths) for three patches afew centimetres apart (3–8 cm), so that between 27 and 45readings were made at every sampling occasion. In total,794 and 710 SMC readings and 1,220 and 1,152 SWRmeasurements were carried out under Eucalypt and Pine,respectively. SWR was determined in situ using the molarityof an ethanol droplet (MED) test (King, 1981), whichprovides an indirect measure of the surface tension of thesoil and reflects the intensity of how a water drop is repelledby soil. It was preferred over the water drop penetration time(WDPT) test, as the WDPT measurements would haverequired considerably more time because of the expected highlevels of SWR, and methodological difficulties associatedwith evaporation of the applied droplets. The SWR tests


involved applying three droplets of increasing ethanolconcentrations to the soil until infiltration of two of the threedroplets of the same concentration occurred within 5 s(Crockford et al., 1991). Test results are given as medianethanol concentrations (vol.%) and associated medianconcentration classes. Ethanol classes were divided over 10repellency intensity classes as follows: 0, very wettable(0%); class 1 and class 2, wettable (1% and 3%, respectively);class 3, slightly water-repellent (5%); class 4, moderatelywater-repellent (8.5%); class 5, strongly water-repellent(13%); class 6 and class 7, very strongly water-repellent(18% and 24% respectively); and class 8 and class 9, extremelywater-repellent (36% and more than 36%, respectively)(Doerr et al., 1998). The ‘integrated area below the water-dependent repellency curve’ was calculated by multiplyingthe average MED class for two consecutive sampling datesby the number of days for that period and summing allperiods for the 1 year study duration. The integrated areais expressed as a percentage of the maximum possible area(MED class 9 for the whole study period).Volumetric SMC was measured by inserting a hand-held

probe (Decagon ECH2O EC-5) within 5 cm of the SWRmeasurements (‘SMC point’ in Table II). SMC (v v�1) wasalso monitored (at both sites) at 15min. intervals using fourDecagon ECH2O EC-5 probes connected to a Decagon Em5bDatalogger, inserted at two depths around the midpoint of theslope section length (‘SMC cont.’ in Table II).

Data Analysis

The relations of SWR with SMC, rainfall and the dailytemperature were assessed by means of Spearman’s rank corre-lation coefficients, computed using SigmaPlot. Spearman’scoefficient was preferred because the ethanol concentrations(classes) utilised in the MED test did not correspond to a scale


Table II. Spearman rank correlation coefficients of median soil water repellency values with precipitation and soil moisture variables, in aEucalypt and Pine plantation

Sites under Pines Sites under Eucalypts

SWR Surface 2·5 cm 7·5 cm Surface 2·5 cm 7·5 cm

PrecipitationP POU �0·53* �0·51* �0·48 �0·05 �0·14 �0·13P LO/SC �0·56* �0·54* �0·5 �0·08 �0·13 �0·1N (0·1–1) mm (days) 0·1 0·27 0·07 0·2 �0·14 �0·03N (1–10) mm (days) �0·34 �0·45 �0·21 �0·06 �0·1 �0·05N> 10mm (days) �0·53* �0·41 �0·41 �0·12 �0·19 �0·21N total (days) �0·38 �0·35 �0·23 �0·08 �0·24 �0·13

Soil moistureSMC point (2·5 cm depth) �0·74* �0·32SMC point (7·5 cm depth) �0·73* �0·72*SMC cont. (2·5 cm depth – mean value day before) �0·76* �0·13SMC cont. (2·5 cm depth – mean weekly value) �0·76* �0·3SMC cont. (7·5 cm depth – mean value day before) �0·62* �0·41SMC cont. (7·5 cm depth - mean weekly value) �0·69* �0·45

P, precipitation; POU, Pousadas; LO, experimental catchment under Pines; SC, experimental catchment under Eucalypts; N, number of days with rainfall; SMCpoint, soil moisture content measured by hand; SMC cont., soil moisture content monitored in continuous in the sites under Pines and Eucalypts; SWR, soilwater repellency.n= 37 and n= 36 for sites under Eucalypt and Pine, respectively.Data was collected from July 2011 to July 2012.Underlined coefficients were statistically significant at α= 0·050.*Statistically significant at α= 0·001.

J. M. SANTOS ET AL.

with a constant measurement unit and were also not normallydistributed. The correlation between SWR values (determinedat 0, 2.5 and 7.5 cm depths in soils under Eucalypts and Pines)and potential explanatory variables (SMCby in situmonitoring,SMC by hand-held probe at SWR measurement, antecedenttotal rainfall and number of rainfall events of different sizes)was explored.

RESULTS

Temporal Patterns in Soil Water Repellency

Soil water repellency expressed as the annual median MEDclass showed relatively small differences between the two

Table III. Indices of soil water repellency status and changes during a 1

Soils un

0 cm 2.5

Annual indicesMedian ethanol class 7Median SMC NANumber of wettable days (ethanol class = 0) 141 10Percentage of wettable days (% of the year) 39 2Integrated area (%) 49 5

SWR changes (number of ethanol classes)Small (x = 2) 1Moderate (3≤ x< 5) 1Large (5≤ x< 9) 8Total changes 10 1

SMC, soil moisture content; SWR, soil water repellency; x, difference in ethanoln= 37 and n= 36 for sites under Eucalypt and Pine, respectively.Data collected from July 2011 to July 2012.Wettable days correspond to days with ethanol class 0.SWR changes refers to difference in ethanol classes between two consecutive sam


study sites (Table III), class 7 for Pine and class 8 forEucalypt, the number of wettable days (MED class being0). SWR expressed as the ‘integrated area below the water-dependent repellency curve’ revealed substantial differences(Table III). The difference in annual median SWR fromMED class 8 to 7 corresponds to a 14% lower surfacetension. The difference in the integrated area was of a similarmagnitude at 7.5 cm depth (�13%) but was considerablygreater at 2.5 cm (�34%). The number of wettable dayswas also higher for the Pine site, that is, 3–7% at the Eucalyptsite and 28–33% at the Pine site.The SWR and SMC dynamics during the 12months were

clearly different for the two sites (Table III and Figure 2).

2month period in a Eucalypt and Pine plantation

der Pines Soils under Eucalypts

cm 7.5 cm 0 cm 2.5 cm 7.5 cm

7 7 9 8 89·1 9·3 NA 8·2 12·34 123 0 11 288 34 0 3 75 55 94 83 63

4 2 1 5 42 1 0 3 45 6 0 4 101 9 1 12 18

classes; NA, not available.

pling occasions.


Figure 2. Daily rainfall amounts (mm) and mean daily temperature recorded at the meteorological station of Pousadas (A), during the study period and tem-poral patterns in median molarity of an ethanol droplet classes (B and C) and median moisture contents (D and E) (n= 37 and n= 36) at 2.5 and 7.5 cm depth in

a Eucalypt and Pine plantation, respectively. This figure is available in colour online at wileyonlinelibrary.com/journal/ldr.


The proportion of very wettable (MED class 0) measure-ments was around 30% at both depths in the Pine site asopposed to 5% and 11% at 2.5 and 7.5 cm depth in theEucalypt site, respectively. The corresponding medianSMC at both sites was between 15% (2.5 cm) and 17%(7.5 cm). The frequency of extreme water repellency


(classes 8 and 9) was around 40% at both depths in Pineand 70% (2.5 cm) and 50% (7.5 cm) in Eucalypt. Thefrequency of very strong water-repellent conditions wasaround 20% at both sites and at both depths.Soil water repellency was more dynamic at the Eucalypt

than at the Pine site, when expressed by SWR changes of


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J. M. SANTOS ET AL.

2 or more ethanol classes, particularly at the 7.5 cm depth(total changes are 18 vs. 9, respectively; Table III). Largerchanges in SWR, between 5 and 9 classes in either direction,also occurred more frequently in the Eucalypt site than in thePine site, with 10 vs. 6 occasions, respectively (Table III).The temporal patterns in SWR (median MED classes) andSMCs (median of three readings) over the 1-year studyperiod are depicted in Figure 2. Five different wet or dryperiods could be identified, on the basis of rainfall occur-rence (Figure 2):From July 2011 to mid-October 2011, the sampling sites

exhibited very strong to extreme SWR (ethanol classes 6–9),coinciding with a dry period (120mm rainfall in 98 days).From mid-October 2011 to mid-January 2012 (450mm ofrainfall in 90 days), the first drop in SWR was observed on 4November 2011, with the soils at the Pine site becoming verywettable within 2 and 4weeks and remaining wettable until 26January 2012 with relatively high SMCs at both samplingdepths (12–23%); at the Eucalypt site, the behaviour of bothSWR and SMC was more dynamic, exhibiting more changesthan at the Pine site (SMC ranging from 4% to 23%; SWRranging from class 0 to 9) and only reaching very wettableconditions at one depth (7.5 cm) and one occasion, whereasthe soil at the 2.5 cm depth remained strongly to extremelywater-repellent. From February to mid-April 2012 (72mmrainfall in 63 days), at the Pine site, the previously wettablesoils increased quickly in SWR before gradually becomingdrier and extremely water-repellent (by the 13th of April); atthe Eucalypt site, SWR continued with a similar dynamic pat-tern as in the previous period. From mid-April to 4th week ofJune 2012 (371mm of rainfall in 36 days), SWR wasdestroyed earlier at the Pine site (26 April) than at the Eucalyptsite (4 May) after a very rainy period (180mm in 10 days). Atthe Pine site, the transition from repellent to wettable wassmooth, changing from extreme (class 8) to moderate tovery strong SWR (class 4 to class 6) in 1week and towettable in the subsequent weeks; afterwards, the Pine soilremained wettable for 5 weeks. At the Eucalypt site, thetransition from water-repellent to wettable was moreerratic, with a sharp increase in SWR in the 2nd week ofMay, after 3 dry days, and only reaching wettableconditions on the 25th of May. From the 4th week of Juneto July 2012 (68mm of rainfall in 42 days, with 5 weeksrecording less than 20mm), large variations were observedin both SMC and SWR, including between samplingdepths, particularly at the Eucalypt site. Nevertheless, theSMC and SWR results of July 2012 closely matched thoseof July 2011, with extreme SWR and low SMCs (between5 and 10 vol.%).At the Pine site, SWR was generally largest at the 7.5 cm

depth, intermediate at the 2.5 cm depth and smallest at thesurface – indicating that SWR broke down in the topsoilfrom top to bottom (Figure 3). In contrast, in the Eucalyptsite, SWR was generally largest at the surface, intermediateat the 2.5 cm depth and smallest at the 7.5 cm depth –indicating that SWR broke down in the topsoil from bottomto top (Figure 3).


Soil Water Repellency, Soil Moisture and Rainfall

Figure 4 shows the lower and upper SMC limits or transitionzones from water-repellent to wettable. At the Pine site, soilswith SMCs lower than 12% (vol.) were always strongly toextremely water-repellent (MED class 5 or greater), whereassoils with SMCs greater than 16% (vol.) were typicallywettable (MED class 1 or lower), excluding one point at2.5 cm. At the 7.5 cm depth, a range of SWR intensities atSMCs between 12% and 16% did occur, even though in afew instances only (MED classes 2–5 were never recorded).At the Eucalypt site, soils with SMCs less than 14% werealways strongly to extremely water-repellent (MED class5 or greater). The upper SMC limit for wettable soilconditions could not be established robustly, because wettableconditions were reached on only two occasions and only at the7.5 cm depth at SMCs greater than 19% (vol.).Significant correlations were more commonly found in

the Pine than Eucalypt plantation for SWR and precipitationand SMCs at different depths (Table II). SWR at the Pinesite was significantly negatively correlated (correlation coef-ficients ranging from �0.34 to �0.56; p≤ 0.05) with ante-cedent rainfall and with SMC at the same soil depth. SWRat all three depths of the Pine soil was significantly nega-tively correlated with the number of heavy rainfall events(>10mm) during the week preceding the SWR measure-ments, whereas SWR at the surface and at the 2.5 cm depthwas also significantly negatively correlated with the numberof medium-sized rainfall events (>1.0–10mm). By contrast,SWR at the Eucalypt site was not significantly correlatedwith any of the meteorological or SMC variables, except atthe 7.5 cm depth. SWR at the 7.5 cm depth in the Eucalyptsoil was negatively correlated with SMC at the same depth,especially when considering the in situ SMC measurementsrather than the automatic recordings.

DISCUSSION

Annual Soil Water Repellency Values

Considering that the precipitation at the Pine and Eucalyptsites was very similar, the key question is what factorscaused the observed differences in SWR between the twosites. Doerr et al. (1996) found SWR to be associated morewith the finer than coarser soil fraction. De Jonge et al.(1999) observed that the finest fraction (<0.063mm) oftwo water-repellent soils presented the highest degree ofSWR and suggested that this could partly be explained bythe higher organic matter content of the finest fraction. How-ever, Leelamanie & Karube (2011) experimented withSWR and SMC in model soils – that is, without soil organicmatter (SOM) – and found soils with 20% kaolinite clay toexhibit an increase of about 28% in SWR compared withsoils with 10% clay, SWR being measured by the integratedarea below the water-dependent repellency curve. Becausethe difference in clay content between the Eucalypt site(24%) and the Pine site (12%) studied here is of a similarorder, this difference in soil texture could explain most of


Figure 3. Total antecedent rainfall (A) and relative frequency of molarity of an ethanol droplet classes at the soil surface (B) and at 2.5 (C) and 7.5 cm (D) in a Eucalyptand Pine plantation (n=37 and n=36, respectively) from July 2011 to July 2012.This figure is available in colour online at wileyonlinelibrary.com/journal/ldr.


the observed difference in SWR. However, other work hasshown SWR to developmore easily in coarse than in fine-sandparticles (González-Peñaloza et al., 2012), which couldcounter any potential clay effect, or no apparent relationshipbetween texture and SWR (Vogelmann et al., 2013). Differ-ences in SOM content and/or quality are likely to have played

Figure 4. Relationship between median soil moisture content (in vol.%) and med(n= 36) site.This figure is available in colour on


a role as well, possibly an additional one. Many authors havereported positive relationships between SOM content andSWR (Bisdom et al., 1993; Kawamoto, 2007; Barton &Colmer, 2011) although a clear relationship has remainedobscure, suggested to be caused by confounding effects of dif-ferences in SOMquality (Doerr, 2000;Müller &Deurer, 2011),

ian ethanol classes at 2.5 and 7.5 cm for the Eucalypt (n= 37) and the Pineline at wileyonlinelibrary.com/journal/ldr.


J. M. SANTOS ET AL.

and soil structure is known to interact with SWR (Urbaneket al., 2007; Vogelmann et al., 2013). Further research intoSOM quality and soil structure is required at our research sitesto explain the differences in SWR.The Eucalypt site demonstrated a greater spatial heteroge-

neity of SWR at the soil surface than at the 2.5 cm depth andat the 2.5 cm than at the 7.5 cm depth. In contrast, the Pinesite had basically the same spatial heterogeneity at the threesampling depths. These differences in spatial heterogeneitybetween the two sites could be related to the greaterresistance of the soil under Eucalypt than Pine to the break-ing of SWR, in combination with the greater resilience inre-establishing SWR. Rodríguez-Alleres & Benito (2011)also found that, during spring, SWRwas re-established fasterunder Eucalypt than Pine.

Intra-annual Soil Water Repellency Dynamics

The results showed clear intra-annual SWR patterns in bothtypes of forest plantations, for the 1-year study period. Thisagreed well with the findings of prior studies in the same partof Portugal (Doerr & Thomas, 2000; Leighton-Boyce et al.,2005; Leighton-Boyce et al., 2007; Keizer et al., 2008).SWR behaved more dynamically under Eucalypt than Pine.Figure 4 shows that the lower SMC limit of the transitionzone is marginally lower for the Pine site than the Eucalyptsite (12–16% vs. 14–19%). Both ranges fit in with thereported critical SMC thresholds for A-horizons, rangingfrom 5% to 38% (Dekker & Ritsema, 1996; Doerr &Thomas, 2000; Verheijen & Cammeraat, 2007). However,Rodríguez-Alleres & Benito (2011) studied SWR underthe same tree species in north-west Spain and found differentSMC ranges for the transition zones, that is, 21–50% for Pineand 17–36% for Eucalypt, making their transition zoneswetter and wider than those observed in our study. A potentialcause for the relatively large difference in the SMC range of thetransition zone may be that Rodríguez-Alleres & Benito (2011)measured SWR on disturbed soil samples in the laboratory,whereas our SWR measurements were made in situ inundisturbed soil. The greater SMCs of the lower limits ofthe transition zones observed by Rodríguez-Alleres & Benito(2011) could be caused by differences in several soil variables,including SOM quantity and quality and soil texture and/or bydifferences in SWR assessment: SWR intensity measured bythe MED test in our study and SWR severity measured by theWDPT test in the study of Rodríguez-Allares & Benito (2011).At the Eucalypt site, a relatively strong SWR gradient

with depth was observed on some sampling dates (e.g. in18 November and 16 December 2011). Keizer et al.(2008) observed a similar pattern under Eucalypt, wherethe median SWR was extremely water-repellent at 2–3 cmand wettable at 7–8 cm. Furthermore, the SWR gradientswith soil depth were sometimes opposed for the two studysites, that is, with SWR decreasing from the surface to7.5 cm depth at the Eucalypt site as opposed to increasingat the Pine site. The temporal patterns at the Eucalypt sitealso suggested that SWR was broken first at the greater soildepth. SWR at the 7.5 cm depth did, in fact, reveal the


highest spatial variability throughout most of this study,with wettable conditions being observed for the bulk of thesampling occasions. Spatial variability in SWR was mark-edly less at both 2.5 cm depth and the soil surface, with verystrong to extreme SWR prevailing during most occasions.The Pine site revealed marked differences in repellency

compared with the Eucalypt site. First, spatial variability inSWR tended to be similar for the three sampling depths.Second, the soil under Pine was sometimes less repellentat the surface and at the 2.5 cm depth than at the 7.5 cmdepth. These observations could suggest that SWR was bro-ken bottom-up from the 7.5 cm depth to the surface underEucalypt and top-down under Pine. Bottom-up repellencybreaking patterns are not necessarily common in repellentsoils. Ganz et al. (2013), for example, found persistentSWR in the subsoil (to 120 cm depth). It is possible thatthe patterns reported here under Eucalypt (and possiblyunder Pine less than 10 cm depth) were associated withmacro-pores and preferential flow paths. Both subsoilSWR and within-soil distribution and alignment of stonesare recommended topics for further research.

Implications for Hydrological Modelling

Several authors working in forests in Portugal and north-west Spain (Doerr et al., 2003; Fernández et al., 2010;Esteves et al., 2012) have pointed out the potential thataddressing SWR in hydrological models has for improvingmodel predictions of water-repellent conditions. In light ofthe results presented here, one potential process to improvewould be infiltration and wetting-up of the soil duringrepellent periods to determine whether and how SWR isbroken. Recent efforts to address this issue have focusedon the impacts of SWR on soil wettability. On a physicallybased level, Moody et al. (2009) proposed a modified repre-sentation of the function linking soil sorptivity and initialmoisture content, in which sorptivity would decrease formoisture contents below a certain threshold. Simplifiedapproaches could be considered, in which some limitationto soil wetting is introduced during soil repellency periods,as defined by a soil moisture threshold. Nunes et al. (2012)presented a first approximation to such an approach. Thisand subsequent works can provide valuable information forboth simple and complex modelling approaches, by studyingthe relationship between soil moisture and SWR for differentsoils or by characterizing infiltration and soil wettingprocesses in repellent and non-repellent soils among others.Such information could then be used to either determineparameters or modify soil water equations to represent bettersoil moisture processes in water-repellent conditions.

CONCLUSIONS

The main conclusions from the evidence collected in this studyconcerning in situ topsoil SWR dynamics and infiltration in apaired-site comparison of Pine and Eucalypt plantations innorth-central Portugal, in a high-resolution time series overthe course of 1 year, were the following. First, the annual



comparison of in situ SWR depended to a large extent on theSWR metrics used: 7 vs. 8 (median MED class), 55 vs. 83(integrated area) and 28 vs. 3 (% wettable days) at the 2.5 cmdepth in Pine and Eucalypt soil, respectively. Therefore, thechoice of SWR metric needs careful consideration in studiesassessing the hydrological behaviour of water-repellent soilsunder Pine and Eucalypt. Second, over the course of 1 year,both forest types exhibited the entire range of SWR values intheir topsoils (MED classes 0–9); however, temporal patternsin SWR were substantially more dynamic under Eucalypt thanunder Pine, with twice as many moderate and large changes inSWR at the 7.5 cm depth. Third, SMC and antecedent rainfallwere better correlated to SWR under Pine than under Eucalypt.Both forest types revealed similar values for the lower limit ofSMC, below which the soil always exhibits some degree ofwater-repellency, the upper limit of SMC, however, could onlybe estimated robustly for the Pine soil. Nevertheless, the scopefor predicting the degree of SWR from soil moisture dataseemed rather limited. Fourth, the breakdown of SWR in thetopsoil (ca. 0–10 cm) seemed to occur top-down under Pineand bottom-up under Eucalypt.

ACKNOWLEDGEMENTS

This work was funded by the European Regional DevelopmentFund (through COMPETE), the European Social Fund and thePortuguese Republic (through FCT) within the framework ofprojects HIDRIA and ERLAND, as well as the post-doctoralfellowships attributed to F. G. A. Verheijen (SFRH/BPD/74108/2010), M. E. Rial-Rivas (SFRH/BPD/64425/2009) andJ. P. Nunes (SFRH/BPD/39721/2007) and the doctoral fellow-ship attributed to F. Tavares Wahren (SFRH/BD/61451/2009).Additional funding was provided by the Portuguese Republic(through CRUP) and the Federal Republic of Germany (throughDAAD), within the framework of the bilateral collaborationaction ‘Impact of Mediterranean forestry practices on soil hydro-logical properties: measurement and modelling’ (A-27/11). Wewould like to thank two anonymous reviewers for helping toimprove the manuscript.

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Soil Water Repellency Dynamics in Pine and Eucalypt Plantations in Portugal - a High-Resolution Time Series