International Journal of Wildland Fire (14[4], 2005), in press
The influence of vegetation recovery on soil hydrology and erodibility following fire: an elevenyear investigation
Artemi CerdàA,B* and Stefan H. DoerrC
A
Department of Geography, Universitat de València. Blasco Ibáñez, 28, 46010-Valencia, Spain ([email protected])
Instituto Pirenaico de Ecología, Consejo Superior de Investigaciones Científicas. Aula Dei, 202, 50080-Zaragoza, Spain
C
University of Wales Swansea, Department of Geography. Singleton Park, Swansea SA2 8PP, UK ([email protected])
B
*Corresponding Author. Telephone +34 6 3864237; Fax +34 6 3864249
Suggested running heading title: Vegetation recovery effects on post-fire soil hydrology
Additional keywords: Soil Erosion, Overland flow, Wildfire, Vegetation Recovery, Mediterranean Environments,
Rainfall Simulator, Soil Hydrophobicity, Water Repellency.
Short summary:
The effectiveness of different types of vegetation in reducing runoff and soil losses following a severe wildfire in the
eastern Mediterranean was examined over an 11-year period. In addition to vegetation density, vegetation type was also
important, with herbs and shrubs being more effective compared to dwarf shrubs and particularly trees.
Abstract
This study investigates long-term changes in soil hydrological properties and erodibility during the regrowth of different
types and densities of vegetation following a severe wildfire in the eastern Mediterranean (Serra Grossa Range, eastern
Spain). Twelve plots of similar slope and soil characteristics, naturally re-colonized by four different vegetation types
(trees, herbs, shrubs and dwarf shrubs) were examined using rainfall simulations during an 11-year period.
The mean erosion rate was 80 g m-2 h-1 six months after the fire under wet-winter conditions, declining to 30 g m-2
h-1 in the following summer and reaching < 10 g m-2 h-1 after 2 years. Considerable variation under the different vegetation
types was observed. Herbs and shrubs reduced erosion and overland flow coefficients to negligible values 2 years after
fire, whereas under trees and dwarf shrubs appreciable overland flow and soil loss still occurred after 5 years. On tree
covered plots (Pinus halepensis), overland flow actually increased over time in association with the development of topsoil
hydrophobicity, reaching a coefficient of 27 % ten years after burning. Rates of post-fire overland flow and erosion
reduction were not only strongly influenced by vegetation coverage, but also by the type of cover and its effects on soil
hydrophobicity.
International Journal of Wildland Fire (14[4], 2005), in press
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1. Introduction
Runoff and soil erosion tend to increase following wildfire due to the removal of vegetation and litter and fire-induced
changes in soil properties such as hydrophobicity (water repellency) (DeBano et al. 1970; Doerr et al. 2000), porosity
(Imeson et al. 1992), fertility (Anderson 1949; Brown 1972; Úbeda and Sala 1998; Andreu et al. 2001) and aggregate
stability and distribution (Llovet López 2005), which in turn affect soil erodibility (i.e. the soil’s susceptibility to
detachment and transport by erosion; Bryan 1968-1969). Vegetation cover is viewed as the key factor on the ground in
controlling soil erosion (Morgan 1986; Shakesby et al. 2000) and its recovery normally leads to a decline in post-fire
runoff and soil erosion rates as demonstrated by many authors at different spatial scales and under different ecological
conditions (Cerdà 1998a,b; Benavides-Solorio and MacDonald 2001; Moody and Martin 2001a,b; Shakesby et al. 1993).
Data on post-fire runoff and erosion, however, have often been collected only for the first months or a few years
following burning, although effects on overland flow and erosion can be more persistent associated with longer-term fire
impacts on flora, fauna, geomorphology, and soil properties (crusting, aggregate stability, porosity, hydrophobicity) (Neary
et al. 1999; Doerr et al. accepted subject to revision). Long-term studies on the effect of fire and other disturbance events
are thus comparatively rare, despite their importance in the understanding of ecosystem functioning (Wolman and Miller
1960). Furthermore, much of the research carried out on the relationship of vegetation to soil erosion has focussed on the
overall vegetation cover and its biomass, but comparatively little is known about the specific effects of different types of
vegetation or specific plant species on soil erosion (Hester et al. 1997). Recent research on the spatial distribution of
vegetation unrelated to fire has demonstrated important differences in the effects of different plant species on soil erosion
processes (Cerdà 1997; Reid et al. 1999).
Examining vegetation recovery effects under Mediterranean environmental conditions
Compared to many other environmental conditions, vegetation recovery after forest fire in the Mediterranean can be rather
rapid due to the adaptation of vegetation to disturbance by fire, and, following burning, the low competition for sunlight,
increased nutrient availability and reduced water losses by transpiration (Naveh 1974; Trabaud 1981). The reliable
determination of associated temporal changes in soil hydrological and erosional processes, however, is notoriously difficult
to achieve under Mediterranean climatic conditions due to the high temporal variability of the rainfall, which results in
large differences in inter-annual responses. For example, rainfall events of 600 mm during two days were recorded in the
western Mediterranean basin (Olcina 1994), and daily rainfall events exceeding 300 mm are not unusual (López Bermúdez
1990; Pérez Cueva 1994). On the other hand, prolonged drought periods with rainfall lower than 200 mm y-1 are also
recurrent and their frequency is expected to increase with predicted future climate change (De Luis et al. 2000, 2001;
Ceballos et al. 2004). The importance of the temporal variability of climatic conditions in soil hydrology and erosion has
been highlighted in a number of studies in the Mediterranean. For example, a ten-year study on 15 x 4 m plots on a southfacing slope in the Ebro Valley, Spain, demonstrated that during wet years the erosion rate can be three orders of
magnitude higher that during dry years with, annual sediment yield during 1991 being 24.8 Mg ha-1 compared to 0.06 Mg
ha-1 in 1994 (Desir 2000). Similar findings have been reported from fire-affected land. For example, during a study using
4 x 20 m plots under a Matorral cover near Alicante burnt in September 1989 (see Figure 1), a very high variability in
rainfall resulted in runoff coefficients of between 20 % and 80 % during the three years prior to, but only 3.4 % in the year
after fire (Sánchez et al. 1994). In a study in burnt Pinus halepensis woodland using 2 x 8 m plots, Bautista (1999) found
that soil loss on one day (7.2 Mg ha-1; September 30th 1997) was higher than the total soil eroded during the 3 previous
years (5.7 Mg ha-1; 1993-1996). Thus, in addition to vegetation and soil related parameters being key controls associated
soil hydrological and erosional responses can remain highly variable when driven by a few extreme events each decade
(Wolman and Miller 1960). Data collected during a period dominated by high frequency-low magnitude rainfall events
can result in below-average overland flow and erosion, whereas the occurrence of one or several high magnitude-low
frequency events will have the opposite result.
The problems in determining the effects of vegetation recovery under the highly variable Mediterranean climatic
conditions can be circumvented using simulated rainfall. While this approach provides data only on soil hydrological and
erosional responses collected for the selected rainfall characteristics, with measured rates that may not necessarily represent
those actually occurring under natural rainfall, the standardisation of rainfall characteristics allows the detailed study of the
effects of vegetation and soil characteristics independent of the variations in natural rainfall (Meyer 1988). In the present
study, we report on rainfall simulations and associated investigations carried out over a 11-year period following a wildfire
in the Serra Grossa Range in eastern Spain with the aim to examine the effects of (i) different types of vegetation and their
coverage, and (ii) temporal changes in vegetation and soil hydrophobicity characteristics, on soil hydrological and
erosional responses following fire, covering a period that extends well beyond those typically examined in previous studies
carried out on the effects of fire on soil hydrology and erosion in the Mediterranean and elsewhere.
2. Study sites
A wildfire in August 1989 burnt an area of 87 ha in the Serra Grossa Range in La Costera district southwest of Valencia in
eastern Spain (Figure 1). The north-facing slope, covered by an Aleppo pine open forest (Pinus halepensis) with a dense
understorey matorral (Quercus coccifera, Pistacia lentiscus, Erica multiflora, Rosmarinus officinalis) was completely
burnt. Fire severity was high associated with the hot daytime temperature (38 ºC), low relative humidity (35 %), low soil
moisture (< 5 % at 0-2 cm depth) and the dense biomass of the P. halepensis and scrubland. The presence of white ash
International Journal of Wildland Fire (14[4], 2005), in press
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after the fire confirmed the high fire severity (Bentley and Fenner 1958). The dense vegetation cover of the north-facing
slopes generally results in comparatively high burn temperatures as recently demonstrated by Ruiz Gallardo (2004) for a
nearby area farther inland. This vegetation cover is typical of non-cultivated calcareous soils in eastern Spain where the
felling of trees for charcoal production and of shrubs for fuel, as well as grazing over millennia has resulted in dramatic
changes in floristic composition. As a consequence, the climax vegetation, dominated by Quercus ilex, is now uncommon.
The studied slope is north facing and has an elevation range of 150-299 m. The experimental plots were located
on a slope of 14º at 200 m elevation. The bedrock comprises homogeneous Cretaceous limestone with a 5-50 cm deep soil
cover and occasional rock outcrops. The soils are Leptosols and Luvisols with Lithic Leptosols occurring at exposed
locations with depths of < 5 cm. Where the experiments took place, the soil depth is 15-50 cm. Soils are of loamy sand
texture with a typical composition of 69 % sand, 16 % silt and 15 % clay at 0-2 cm depth. A discontinuous clay horizon
(Bt) is found in some locations at ca 20 cm depth. Organic matter content averages about 4.8 % at 0-2 cm depth and 2.7 %
at 4-6 cm depth and calcium carbonate content about 4.4 % at 0-2 cm depth. Bulk density at 0-2 cm depth was about 1.1 g
cm-3. Surface cracking, which could enhance preferential flow, was not observed.
The climate is typically Mediterranean, with dry and hot summers and warm winters. Mean annual rainfall was
688 mm y-1 at the nearest meteorological station 15 km to the east, with our records at the study area further inland
showing a mean of 505 mm y-1. October is the wettest month, with daily rainfall in excess of 100 mm day-1 occurring on
average once a decade (Elias and Ruiz 1979) although in the study area, daily rainfall amounts of > 300 mm occurred at
least twice during last 20 years. Drought periods with precipitation < 300 mm y-1 also occur, the most recent being 19801982 and 1993-1994. Rainfall is often also spatially highly variable as for example demonstrated by a thunderstorm in
summer 1998, when 19 mm fell on the north-facing study slope while only 3.9 mm were measured on the south-facing
slope of the same hill. Monthly rainfall shows the highest amounts during September. In the year 1989, shortly after the
fire, 50 mm fell at the end of August and 392.7 mm during September (Table 1). However, no rills or other visible erosion
features were found after these events. In the post-fire period, no other external disturbance events, such as grazing or
human interference, occurred on any of the plots.
3. Methods
Twelve plots with the same aspect, comparable slope angle (9-14 º) and with similar soil characteristics were selected at
mid- to lower slope positions six months after the fire in early 1990. To determine temporal changes in post-fire soil
hydrological and erosional characteristics, rainfall simulation experiments were carried out on these plots (Figure 2)
following dry summer conditions (> 15 days with no rainfall; volumetric soil moisture at 0-2 cm: 2-6 %) and wet
autumn/winter conditions (following one week without rainfall to avoid the effect of saturated soil conditions affecting
runoff generation; volumetric soil moisture at 0-2 cm: 12-21 % in 1990, 1991, 1992, 1995, 1997 and 2000. Thus, twelve
rainfall simulations were performed twice a year during six selected years between 1990 and 2000, resulting in 144
individual simulations.
During initial plot selection, vegetation had not been considered, but it soon became clear that different plots
became dominated by different plant communities, which provided the opportunity to examine any differences the effects
of contrasting vegetation types on hydrological and erosional response. Some vegetation sprouted after the fire (e.g.
Quercus coccifera, Pistacia lentiscus, Erica multiflora), while other vegetation grew from seedlings after successful
germination (Brachypodium retusum, Cistus albidus, Pinus halepensis, Ulex parviflorus). The emerging differences in
vegetation type and coverage were recorded using repeat plot photographs, and plots were classed into four main types:
trees (2 plots), herbs (2 plots), shrubs (3 plots) and dwarf shrubs (5 plots).
Simulated rainfall of 55 mm over one hour duration was produced with deionized by means of a sprinkler rainfall
simulator (Cerdà et al. 1997) (Figure 2) over a 1 m2 plot surface (0.25 m2 were used as a rainfall simulation plot surface
and 0.75 m2 as a border surface). The plots were initially left permanently in place, but after the experiments of 1995 they
had to be removed and re-inserted in directly adjacent comparable locations for the experiments of 1997 and 2000 (Fig. 3)
to avoid sediment exhaustion, which could otherwise have been responsible for a reduction in measured erosion rates.
Overland flow (i.e. surface runoff) was collected at 1 to 2 minute intervals. At least 3 samples were collected in bottles and
the sediment content was measured in the laboratory following complete evaporation of the water. These measurements
allow determination of overland flow discharge and coefficient, sediment concentration, sediment yield and erosion rates.
As an additional indicator of soil hydrological behaviour, we also measured ‘time to ponding’ (tp), where tp is the time from
the onset of the rainfall initiation to the development of ponding on ca 40 % of the total plot surface (Imeson et al. 1992).
Soil surface hydrophobicity was also measured in situ prior to every simulation using the Water Drop Penetration
Time (WDPT) test (Letey 1969). The test comprised placing ten drops (0.05 ml) on a representative soil surface close to
the rainfall simulation plot following careful removal of any litter, and measuring the time until complete infiltration for
each drop. The average of these ten values was taken as the respective WDPT. Following the widely-adopted
classification of Bisdom et al. (1993), WDPTs ≤5 s were taken as indicative of wettable soil conditions, 5-60 s as slight,
60-600 s as strong, 600-3600 s as severe and >3600 s as extreme hydrophobicity.
Plot removal and reinstallation in 1995 and 2000 allowed excavation of parts of the plots for visual examination of
the wetting front.
International Journal of Wildland Fire (14[4], 2005), in press
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4. Results and Discussion
4.1 Post-fire vegetation development
Vegetation recovery was relatively rapid as is typical of many Mediterranean environments (Naveh 1974; Trabaud 1981)
with some plots under herbs or dwarf shrubs plots reaching > 100 % coverage (i.e. consisting of several layers provided by
moss, litter, herbs and shrubs) within 2 years after burning. At this stage, recovery was so distinct that plots could be
grouped by vegetation type and classified by their dominant plant species: trees (two plots with Pinus halepensis), herbs
(two plots with Bachypodium retusum), shrubs (three plots with, Quercus coccifera, Juniperus oxycedrus and Pistacia
lentiscus) and dwarf shrubs (five plots with, Ulex parviflorus, Erica multiflora, Rosmarinus officinnalis, Cistus albidus and
Thymus vulgaris). The post-fire vegetation cover present at the time of each rainfall simulation for each plot is given in
Table 2 (total vegetation cover). The herb-dominated plots showed the fastest vegetation growth, reaching a dense cover of
> 50 % within only six months of the fire (1990). Amongst shrubs and dwarf shrubs, the increase in vegetation cover
varied widely. The lowest coverage was by Ulex parviflorus and Erica multiflora with values of < 50 % after 3 years
(1992), whereas Quercus coccifera, Cistus Albidus and Thymus vulgaris exceeded 100 % cover at this stage. Vegetation
coverage by trees was intermediate between these, reaching 60-80 % in 1992. It is notable that the increase in cover during
1990 and 1991 exceeds the variation caused by the seasonal reduction in cover during the dry season (Table 2). In later
years, however, a reduction in cover during the dry season becomes evident, particularly for the herb and dwarf shrub
dominated plots (Table 1). Eight years after burning (1997), vegetation cover was well above 100 % at all plots (109-269
%). In comparison, at long unburned (>10 years since a fire) sites nearby, vegetation cover, comprising mainly shrubs with
some herbs patches is typically above 100 %
4.2 Post-fire soil hydrophobicity
Soil surface hydrophobicity data, obtained using the WDPT test, is summarised in Table 3. None of the plots exhibited
hydrophobic soil surface conditions (WDPT > 5 s) six and twelve months after fire. Exploratory measurements carried out
in similar unburnt vegetation (unpublished data collected in 1998) suggest that before the fire low levels of hydrophobicity
may have existed here under pine. The wettable soil condition after the fire suggests that any pre-fire surface soil
hydrophobicity has been eliminated during burning. The high severity of the burn, as indicated by the presence of white
ash, suggests that surface soil temperatures may have been sufficiently high to eliminate any pre-existing surface
hydrophobicity. If sufficient oxygen for combustion of hydrophobic compounds would have been available, this threshold
for soil hydrophobicity elimination would have been around 300 ºC (Doerr et al. 2004), rising to 500 ºC under oxygendepleted conditions (Bryant et al. this issue). All except the tree-covered plots retain wettable surface soil conditions
(WDPT ≤ 5 s) during the entire 11-year period of investigation, which supports the notion that surface soil hydrophobicity
is not a feature of calcareous soils under these vegetation types in the region (Mataix-Solera and Doerr 2004).
Hydrophobic soil conditions did, however, develop from year two onwards under tree cover (Pinus halepensis plots),
supporting the view that any pre-fire surface hydrophobicity at the plots was eliminated during the burn. During the
following years, hydrophobicity under pine increased until the eighth year after fire, but did not exceed slight
hydrophobicity (WDPT 5-60 s). The slight hydrophobicity measured here matches hydrophobicity levels reported from
calcareous soils under long unburnt (>30 years) P. halepensis in south-east Spain reported by Mataix-Solera and Doerr
(2004), but contrasts with the often much higher hydrophobicity levels found under shrub or pine vegetation in some other
regions with a Mediterranean type climate (e.g. Giovannini and Lucchesi 1983; Soto et al. 1994; Doerr et al. 1998) and it is
suggested that the calcareous soil conditions present here do not promote the natural development of hydrophobicity
compared with the acidic conditions typically associated with more severe levels of hydrophobicity. A potential reason
may be that fungal activity, which has been implied in the development of hydrophobicity (Jex et al. 1985; Hallett et al.
2001), is more prominent under alkaline conditions (Paul and Clark 1996). Although hydrophobicity was only measured at
the soil surface, the fact that it increased naturally under pine in the post-fire period supports the notion that it may well
have been present also to some depth in the soils before and after the fire as observed in studies conducted under the same
and other pine species elsewhere (e.g. Doerr et al. 1996; Scott 2000; Mataix-Solera and Doerr 2004).
Throughout the experimental period, hydrophobicity was always higher under dry summer compared to wet
autumn/winter conditions (Table 3). This seasonal pattern is typical of hydrophobic soils and is thought to be related to
soil moisture. Hydrophobicity tends to be absent above, and present below a critical moisture threshold zone. Within this
zone, hydrophobicity has the potential to be present, but may not be fully expressed. The limits of this zone differ between
soils and are thought to depend on soil texture and organic matter characteristics (Dekker et al. 2001). In the present study,
volumetric surface soil moisture (0-2 cm depth) was consistently 4-5 % for dry season measurements and 15-17 % in the
wet season (average of 3 samples per plot). Hydrophobicity was reduced, but always present for wet season
measurements, which suggests that the 15-17 % moisture content measured here is within the soil moisture threshold zone
for hydrophobicity expression. The data obtained here do not allow determination of the upper soil moisture threshold, but
studies under pine (Doerr and Thomas 2000) and other vegetation (Soto et al. 1994) conducted elsewhere in the
Mediterranean suggest an upper limit in the region of 20-28 %.
4.3 Post-fire soil hydrology and erodibility
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Overland flow at all burnt plots (Table 4) was initially much higher than that recorded for unburnt scrub or grass covered
terrain of otherwise similar characteristics near the study area (Cerdà, 1998b). From the initially highest levels measured
six months after the fire, all twelve plots show a strong decline in overland flow and soil loss under simulated rainfall
during the first 3 years with superimposed variations in overland flow (and sediment concentration) between wet
autumn/winter and dry summer conditions (Figure 4). Thus average wet and dry season overland flow coefficient decline
from 44.6 % and 11.5 % six and twelve months after the fire to 5.0-6.6 % and 7.6-4.6 % respectively from year three
onwards (Table 4). Average wet and dry season soil loss decline sharply from 79.7 and 30.0 g m2 h-1 in months six and
twelve to 4.8 and 2.8 g m2 h-1 in months 30 and 36 respectively. Thereafter, soil loss declines further, reaching 0.33 and
0.24 g m2 h-1 ten years after the first respective measurement (Table 4). Although greater overland flow was generally
associated with higher erosion rates, the latter declined much more sharply than the former (Figures 5 and 6). This is a
pattern typically observed under recovering vegetation and it is thought that the increase in vegetation coverage, in addition
to reducing eroding overland flow, also resulted in a lower rate of entrainment of soil particles for a given rate of overland
flow as evident from the sediment concentration data presented in Table 6 and Figure 7. The effects of the various
vegetation types are discussed in more detail in the following Section 4.4.
The considerable variations in overland flow and soil loss between wet and dry seasons are thought to be caused
primarily by the higher antecedent soil moisture content in the wet season, leading to a more rapid development of
saturation overland flow. The average overland flow coefficient over the whole period of investigation was 24 % during
wet, but only 11 % during dry periods with the differences between wet and dry seasons declining over the years.
Sediment concentration in the overland flow is somewhat higher during the dry compared to wet periods of the
first three post-fire years (Figure 7). This may be due to a number of reasons including sediment exhaustion, the dilution
effect associated with the greater overland flow volume generated during the wet season rainfall simulations and aggregate
behaviour under wet conditions. During the wet periods, any overland flow generated from natural rainfall (and during
simulated rainfall) would have caused soil particle and aggregate wash, resulting in the partial exhaustion of the available
sediment. During the dry and hot conditions dominant in the Mediterranean summer, surface wash does not take place,
allowing uninterrupted particle accumulation on the soil surface and air trapped in dry aggregates makes them more
vulnerable to breakdown (i.e. slaking) during rapid wetting. As a consequence, the sediment available during a rainfall
simulation following prolonged dry conditions is greater. Soil erosion rates in similar terrain, though not affected by recent
fires, showed comparable seasonal trends with a similar experimental layout 1995 (Cerdà 1998b). Similar results were also
found by Simanton and Emmerich (1994) under natural vegetation and simulated rainfall in USA.
The above data demonstrate (i) the increase of the overland flow and erosion rates after the fire in comparison to
soils not recently affected by fire, (ii) the rapid reduction in overland flow generation and soil erodibility within three years
of burning, and (iii) the high seasonal variability of the hydrological and erosional response typical of the Mediterranean
ecosystems.
4.4 The influence of vegetation type and soil hydrophobicity on soil hydrology and erodibility
Average overland flow generation at the plots following wet antecedent conditions shows a steep decline with increasing
vegetation until ca 50 % cover was reached in 1995 (Table 4 and 5). As overland flow generation levels off thereafter, the
further increase in vegetation cover appears not to have an effect on wet season overland flow generation. Following dry
antecedent conditions, the decline in overland flow with increasing vegetation recovery continues throughout the whole
study period. Overland flow generation during the dry season, however, is much lower than during the wet season,
resulting in a lower rate of decline (Table 4). Erosion rates decline much more sharply over the study period than overland
flow as already pointed out. This is an expected outcome as increasing vegetation cover tends not only to reduce overland
flow totals, but also the potential of a given amount of overland flow to entrain soil particles (Morgan 1986).
As regards vegetation type affecting overland flow and erosion some important differences emerge between
vegetation types (Tables 2-6 and Figures 5, 6). The reduction in overland flow coefficient measured for both wet and dry
antecedent conditions over the whole study period was greatest for plots recovering with herbs, followed by shrubs and
dwarf shrubs (Figure 5). The plots recovering with pine showed the weakest reduction in overland flow under wet
conditions, but, in contrast to all other vegetation types, an overall increase during the dry conditions compared to year 1.
The same ranking of different vegetation types applies to their effectiveness in reducing erosion, except that in contrast to
overland flow, erosion also continued to decline under trees with increasing cover.
More specifically, the vegetation in herb-dominated plots recovered most rapidly (96-142 % cover after 1 year),
reducing dry season overland flow to around 5-6 % after one year, 0.5-1 % after two years and no overland flow occurring
three years later. Shrub-dominated plots recovered less rapidly (38-69 % after 1 year) and overland flow was accordingly
reduced at that time to only 9-10 %. However, when shrubs exceeded coverage of 100 % in year six, dry-season overland
flow was also reduced to zero. Plots dominated by dwarf-shrubs had a similarly rapid vegetation recovery, exceeding on
average 100 % in year 6, however, here overland flow never reached zero. Vegetation recovery rate under tree covered
plots was similar to shrubs and dwarf-shrubs, also exceeding 100 % in year 6. However, dry-season overland flow showed
the aforementioned overall increase from 15-18 % in year 1 to a maximum of 36-45 % in year 6, followed by a decline to
24-29 % in year 11.
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As regards erosion, within two years erosion rates at the herb plots fell below what has been classified as
negligible for forested terrain under natural rainfall under western Mediterranean conditions (0.02 g m2 per mm rainfall,
equivalent to 1.1 g m2 h-1 here; Shakesby et al. 2002). The reduction to such negligible erosion rates was reached during
the next measuring campaign six months later under shrubs and by year 6 under dwarf-shrubs. Under trees, despite a
progressive decline, erosion rates reached levels close to this threshold (1.1-1.4 g m2 h-1) only during year 11 (Table 5,
Figure 6). The overland flow exceeding 24 % during each campaign allowed significant erosion to occur until year 10
despite the vegetation cover exceeding 100 % from year 6 onwards.
From the results presented above, it is evident that the changes in post-fire erosion and, to a greater extent,
overland flow rates, are not related to vegetation recovery alone. For example, despite showing very similarly trends in
vegetation recovery, both overland flow and erosion remained generally higher under dwarf shrubs compared to shrubs
(Table 2 and Figures 5, 6) despite the overall similar morphology of these plants. Also, dry season overland flow under
herbs showed a significant reduction between 1991 and 1995 (Table 4) despite negligible differences in vegetation cover
during the same period (Table 1). Soil surface hydrophobicity for all these plots shows consistently low values (WDPT < 5
s), and the same applies to dry season soil moisture (≥ 5 %), effectively ruling out their influence. Determining the reasons
for the differences in the effects of similar coverage within and between vegetation types clearly warrants further
investigation.
For the pine-dominated plots, the relationship between vegetation recovery and post-fire overland flow and
erosion is more complex than under other vegetation types. Here, however, soil hydrophobicity increases to significant
levels with vegetation recovery, as already discussed in more detail in Section 4.2, which would be expected to counter
some of the effects of vegetation cover increase. The data in Table 3 demonstrate that soil hydrophobicity was present at
the soil surface for both wet and dry season rainfall simulations and could thus have contributed to the higher overland
flow and associated erosion rates on the pine compared to all other plots, where significant hydrophobicity (WDPT > 5 s)
was absent throughout. Evidence for the potential contribution of soil hydrophobicity here is confirmed by the fact that
higher overland flow rates during summer compared to winter conditions from 1995 onwards coincide with higher
hydrophobicity levels. If soil had been wettable throughout, greater overland flow would be expected during winter due to
more rapid saturation of the already moist or wet soil, as observed for nearly all rainfall simulations except under pine
plots. For the latter, the higher hydrophobicity levels, that often accompany drier soil conditions (Doerr et al. 2000),
appeared to be more influential on overland flow generation and associated erosion than the higher soil moisture contents
during winter. Thus, from 1995 onwards, the further increase in vegetation cover between wet and subsequent dry seasons
appears to have been insufficient to counter the effects of increased dry-season hydrophobicity. Between the fire and 1995,
soil hydrophobicity is also likely to have affected overland flow generation, however, during that period, the higher
vegetation recovery rate is considered to have been more influential here than the slowly increasing hydrophobicity levels.
Overland flow also showed the effect of soil hydrophobicity development associated with the recovery of Pinus
halepensis cover. Figure 8 shows the hydrographs of the 1990 and 2000 experiments under dry conditions on Pinus
halepensis covered soils. The shape of the overland flow curve for the measurements in 2000 shows a sharp increase
during the first 5 minutes of rainfall before reaching a steady-state infiltration rate thereafter, whereas the 1990 post-fire
rainfall simulation did not show this response. The response in 2000 is similar to that typically observed for hydrophobic
soil conditions following fire (Robichaud 2000).
Given the otherwise comparable soil conditions and the increase in vegetation cover observed at all plots, it is
reasonable to conclude that increasing soil surface hydrophobicity is the key factor in increasing, or maintaining the
occurrence of, dry-season overland flow under pine. In-newly burnt eucalyptus stands in Portugal (Leighton-Boyce et al.
2005), the influence of hydrophobicity has been isolated under simulated 30 minute rainstorms (~100 mm h-1) by using
wetting agents in some simulations to impart wettable soil conditions. Despite the high rainfall intensity, no overland flow
occurred when wetting agents were used, whereas overland flow ranged between 48.4 and 92.5 % under hydrophobic soil
conditions. At their study site, however, soil hydrophobicity levels were predominantly extreme (Leighton-Boyce et al.
2005), as classified by the Critical Surface Tension test, which equates to WDPTs > 1 h (Doerr et al. 1998). Given the
comparatively low hydrophobicity levels measured in the current study, it is perhaps remarkable that hydrophobicity
appears nevertheless to have such a marked effect on overland flow generation.
Additional information on the potential importance of various factors on overland flow generation and associated
erosion can be gained from the time to ponding (tp) data (Figure 9); i.e. the time taken from the onset of the rainfall
initiation to the development of ponds on ca 40 % of the total plot surface. In addition to WDPT data, which is only
indicates the degree of hydrophobicity at the very soil surface within the contact area of the respective water droplet, tp will
be affected by soil surface micro-topography and the infiltration capacity of the topsoil layer, which in turn will be
influenced by the spatial distribution of any existing hydrophobicity and the effectiveness of more wettable soil patches
and of macropores in allowing infiltration to bypass less wettable soil areas (Doerr et al. 2003). As one would expect, tp,
averaged for each of the vegetation types, showed inverse trends to overland flow, with herbs having the longest and trees
shortest times. At the pine plots, where significant hydrophobicity was present, tp lengthened with increasing WDPTs,
suggesting that hydrophobicity is considerably more influential here than the interception capacity of the vegetation.
Finally, useful insight into the effects of vegetation and soil hydrophobicity can be gained from the shape of the
wetting front present after simulated rainfall during the dry season (Figure 10). Perhaps most striking is the observation
that the wetting fronts were homogeneous following the first simulation after fire (1990), indicating that matrix flow
dominated the infiltration process. Six years after burning (1995) the wetting fronts are heterogeneous and patchy due to
International Journal of Wildland Fire (14[4], 2005), in press
6
the influence of macropore flow associated with cracks, rootholes, and faunal burrows. The soil horizon boundaries were
not unusually discontinuous and are thus though to have had only a minor, if any, influence on the shapes of the wetting
fronts. Except for pine, there was an increase in the depth of the wetting front over the years as more rainfall infiltrated
and less was lost as overland flow. Under Pinus halepensis, however, the wetting front reached only shallower depths as
less infiltration and more overland flow occurred except that here some preferential flow to a depth of 20-30 cm was
observed. Enhanced preferential flow is a typical feature of hydrophobic soils (see review by Doerr et al. 2000) and
hydrophobicity is also viewed as promoting its occurrence here. Some preferential flow was also present within the root
zone of other plots in 2000 (Figure 10) and, given the absence of surface hydrophobicity at these plots and the fact that
natural (i.e. not fire-induced) hydrophobicity tends to decrease with depth (see review by Doerr et al. 2000), it is speculated
here that the root channel developed during re-vegetation and cracks have promoted preferential flow here.
4.5 Implications for post-fire terrain recovery and land management under Mediterranean environmental conditions
A number of wider implications for post-fire terrain recovery under Mediterranean environmental conditions arise from the
results of this study.
(i) Overland flow and erosion rates are enhanced following fire associated with the removal of vegetation and
changes to the soil as a result of heating. The recovery of the terrain to pre-fire erosion rates requires 2-4 years under the
shrub and herb vegetation examined. Under Pinus halepensis, however, soil heating during the fire appears to have been
sufficient to reduce soil hydrophobicity and increase the dry-season infiltration rates. Hydrophobicity re-establishment
associated with pine regrowth increases dry-season overland flow during the first few years of vegetation recovery, which
appears to be the cause of the considerable delay in the reduction of erosion rates to pre-fire conditions under pine. Had
soil temperatures remained lower during the burn, maintaining or enhancing pre-fire soil hydrophobicity, overland flow
and erosion would be expected to have been even higher during the first few years following the fire. These findings
demonstrate that where vegetation growth is coupled with soil hydrophobicity development, an increase in vegetation
cover does not necessarily reduce overland flow as might otherwise be expected.
(ii) In Spain, most of the past and current afforestation is carried out with Pinus halepensis due to his fast growth
and the comparatively high value of the timber. When mature, these forests are undoubtedly just as, if not more, effective
in preventing soil erosion as a dense grass or shrub cover. The results presented here, however, suggest that, following
disturbance events such as fire, shrubs and herbs are more efficient for soil erosion control. Thus, where disturbance
events are frequent, as is increasingly the case for many parts of the Mediterranean, shrubs and herb vegetation appears
also to be more effective in preventing erosion in the long-term compared to P. halepensis.
(iii) The results suggest that where soil erosion control measures after fire are envisaged, they would be most
effective in terms of encouraging recovery of herbs as a result of their fast growth followed by sprouting shrubs such as
Quercus coccifera, Pistacia lentiscus and Juniperus oxycedrus. For herbs, aerial sowing can be an efficient technique as it
reduces trampling and related soil disturbance. The resulting patchy distribution of shrubs and presence of herbs between
them is a typical feature of Mediterranean rangelands, which also helps to reduce the connectivity of future fires.
5. Conclusions
The rainfall simulations carried out over a period of 11 years following a severe wildfire in Mediterranean eastern Spain
demonstrate different rates of plant re-establishment and in their effectiveness in reducing overland flow and erosion,
resulting in different responses throughout the 11 years experimental period. Overland flow and soil losses reached a peak
in the winter following the fire in summer 1989. Overland flow was then reduced most rapidly under herbs, followed by
shrubs and dwarf shrubs, whereas under a tree cover (Pinus halepensis), the re-development of a hydrophobic surface layer
during the post-fire regeneration period resulted in an increase in overland flow rates, especially during summer. Post-fire
erosion rates showed a more rapid decline than overland flow responses, with less distinctive differences between herb,
shrub and dwarf-shrub covered plots. Only under trees did re-development of surface soil hydrophobicity appear to have
delayed erosion rate reduction. The differences in the effectiveness in reducing overland flow and soil erodibility are
clearly not only related to the degree of vegetation cover, but also to the type of cover, with herbs and shrubs being more
efficient in reducing overland flow and erosion than dwarf shrubs and particularly trees. The influence of vegetation type
on the hydrophobic behaviour of the soil is thought to be a key factor here. The findings demonstrate that where vegetation
growth is coupled with soil hydrophobicity development, an increase in vegetation cover does not necessarily reduce
overland flow as might otherwise be expected. The results of this study also demonstrate that, while significant terrain
recovery towards pre-fire conditions has taken place during the first two years of investigation, major changes to soil
hydrology and erodibility have occurred much later in this investigation. This highlights the importance of long-term
monitoring approaches in study of fire effects. Shorter-term studies may not always be sufficient in allowing a thorough
determination of (i) the changes to soil hydrology and erodibility caused by a fire and (ii) the effectiveness of various
vegetation types or management measures in reducing accelerated post-fire overland flow and soil erosion responses.
International Journal of Wildland Fire (14[4], 2005), in press
7
Acknowledgements
This work was supported by the REN2002-00133/GLO (Spain) project and a NERC Advanced Fellowship NER/J/S/2002/00662 (UK).
We thank Anna Ratcliffe for drawing Figure 1, and Richard Shakesby and the anonymous referees constructive comments on a previous
version of the manuscript.
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Table 1. Monthly rainfall at the nearest meteorological station at Xàtiva (see Figure 1). The wildfire took place prior to the 1.5 mm rainfall event
in August 1989. Values are rounded to the nearest mm.
A
Year
Jan.
Feb.
March
April
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
Total
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
Average
102
372
347
20
2
14
0
56
104
77
89
47
38
1
111
213
304
1.5
48
41
0
8
15
0
33
158
77
131
37
39
5
95
53
6
0
97
15
54
36
102
43
6
25
89
24
13
170
7
24
20
47
52
95
13
209
36
10
2
55
15
40
9
13
61
15
0
17
123
42
0
6
3
45
7
1
28
14
0
6
4B
6
65
7
0B
2
8
0
12
0
10
5A
1B
38
0B
0
7
163
0
0B
22
0
0B
4
398
58
12
0
13
129
8
289
438
31
68
4
107
27
177A
377A
71
205
115
105A
17
31
1
65
451
69
164
26
6
0
124
39
12
56
11
33
95
9A
64
310
33
70.2
288B
10
2
124
89
149A
23
17
58
1301
949
1164
975
866
420
588
676
979
646
548
Sampling carried our under wet conditions; B sampling carried out under dry conditions.
International Journal of Wildland Fire (14[4], 2005), in press
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Table 2. Post-fire evolution of total vegetation and litter cover (plant, trees, moss and
herbs) in percent at each individual plot over the whole period of investigation
measured for predominantly wet (autumn, winter) and dry (summer) conditions.
Primary species
on plot
Season
Trees
Pinus halepensis
Pinus halepensis
Herbs
Brachypodium retusum
Brachypodium retusum
Shrubs
Pistacia lentiscus
Juniperus oxycedrus
Quercus coccifera
Dwarf shrubs
Ulex parviflorus
Erica multiflora
Rosmarinus officinalis
Cistus albidus
Thymus vulgaris
Average
StDev
1990
wet
dry
1991
wet
dry
1992
wet
dry
1995
wet
dry
1997
wet
dry
2000
wet
dry
15
16
30
19
40
25
42
38
80
60
78
56
120
120
110
121
135
178
125
168
149
195
156
168
60
57
142
96
163
143
175
162
185
150
165
132
250
241
168
169
269
265
175
158
287
286
268
245
24
11
53
38
35
69
69
45
95
75
58
110
97
78
130
79
86
125
124
130
165
112
130
145
135
145
178
125
140
165
169
168
210
174
135
198
16
3
37
41
39
31.0
18.6
30
25
45
54
84
55.6
34.8
35
31
65
70
110
74.3
43.1
45
56
72
89
115
86.4
43.8
35
42
74
95
160
98.8
45.6
42
48
75
110
145
95.1
38.1
78
98
115
115
158
142.8
51.0
85
89
114
110
158
125.9
27.3
109
124
128
165
158
165.8
49.7
102
136
135
145
158
144.3
20.4
165
125
135
199
165
187.8
50.1
156
154
125
189
154
176.8
40.8
Table 3. Water Drop Penetration Times (WDPT in seconds; average values of ten
drops) at each plot measured over the whole period of investigation for predominantly
wet (autumn, winter) and dry (summer) conditions. Cells with WDPTs > 5 s (i.e.
hydrophobic conditions) are highlighted with dark shading.
Primary species
on plot
Season
Trees
Pinus halepensis
Pinus halepensis
Herbs
Brachypodium retusum
Brachypodium retusum
Shrubs
Pistacia lentiscus
Juniperus oxycedrus
Quercus coccifera
Dwarf shrubs
Ulex parviflorus
Erica multiflora
Rosmarinus officinalis
Cistus albidus
Thymus vulgaris
1990
wet
dry
1991
wet
dry
1992
wet
dry
1995
wet
dry
1997
wet
dry
1.36
1.25
2.36
2.05
5.25
4.23
5.36
6.35
6.35
7.32
10.25
9.65
7.52
7.36
15.65
18.25
8.65
8.25
24.65
25.32
10.21
9.36
23.51
24.65
0.50
0.62
2.32
2.30
1.25
1.14
3.25
3.15
1.36
2.25
2.32
3.01
2.25
1.25
2.65
2.01
1.25
1.25
2.32
1.85
2.32
1.25
2.03
1.96
1.02
0.59
0.58
3.25
2.15
2.58
1.26
1.25
1.54
4.25
2.15
3.35
1.24
1.85
1.96
2.65
2.36
2.45
1.36
2.25
1.45
2.36
2.14
2.65
2.32
1.52
1.96
1.58
1.95
1.65
0.99
2.25
1.23
1.75
1.45
1.78
0.69
0.85
0.75
1.25
1.36
2.41
2.32
2.14
2.35
2.74
0.95
1.26
1.38
1.56
1.25
4.25
4.32
3.25
2.65
3.25
2.35
2.48
2.65
2.58
3.25
2.48
2.35
2.14
3.10
2.65
1.69
2.85
1.96
2.35
2.98
2.48
2.35
2.31
2.45
2.15
1.85
1.75
2.35
1.25
1.25
1.48
2.35
2.14
1.24
1.65
1.08
1.47
2.32
0.98
0.58
2.02
1.36
2.45
1.98
1.65
International Journal of Wildland Fire (14[4], 2005), in press 12
2000
wet
dry
Table 4. Overland flow coefficient (% of rainfall) after the forest fire (August 1989) for
each plot over the whole period of investigation measured for predominantly wet
(autumn, winter) and dry (summer) conditions.
Primary species
on plot
Season
Trees
Pinus halepensis
Pinus halepensis
Herbs
Brachypodium retusum
Brachypodium retusum
Shrubs
Pistacia lentiscus
Juniperus oxycedrus
Quercus coccifera
Swarf shrubs
Ulex parviflorus
Erica multiflora
Rosmarinus officinalis
Cistus albidus
Thymus vulgaris
Average
StDev
1990
wet
dry
1991
wet
dry
wet
1992
dry
70.00 17.65 54.60 32.00 42.56
47.80 15.32 32.70 36.24 65.32
1997
2000
dry
wet
dry
wet
dry
30.25
24.35
35.26
34.26
45.00
36.50
25.60
35.65
33.15
36.90
25.35
28.15
29.35
24.30
1.90
11.30
1.02
0.35
2.32
3.56
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
35.60 10.25 21.10
49.10 16.35 32.50
29.00 9.00 10.40
2.36
5.69
1.03
6.35
6.89
4.65
0.36
1.05
0.65
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
12.36
15.65
10.25
11.32
8.36
11.52
4.02
10.25
10.36
12.54
11.36
9.65
11.07
11.67
15.36
16.35
18.56
17.98
15.36
17.94
18.47
3.50
3.45
3.26
4.65
2.63
6.18
10.07
2.23
1.35
2.65
3.65
0.25
6.64
13.20
2.35
1.26
2.35
2.65
1.05
7.60
15.62
1.25
1.39
2.14
2.25
0.56
5.74
11.85
0.65
1.05
0.98
0.87
0.48
6.17
13.51
2.32
1.24
0.98
0.97
1.25
5.02
10.19
0.87
0.05
0.35
0.59
0.25
4.65
10.42
12.00
30.00
49.20
60.50
53.10
55.70
43.70
44.64
15.80
5.36
6.35
1995
wet
43.90
30.00
44.30
34.70
23.20
28.38
15.54
Table 5. Post-fire soil erosion rates (g m-2 h-1) during 55 mm h-1 rainfall simulations for
each plot over the whole period of investigation measured for predominantly wet
(autumn, winter) and dry (summer) conditions.
Primary species
On plot
Season
Trees
Pinus halepensis
Pinus halepensis
Herbs
Brachypodium retusum
Brachypodium retusum
Shrubs
Pistacia lentiscus
Juniperus oxycedrus
Quercus coccifera
Dwarf shrubs
Ulex parviflorus
Erica multiflora
Rosmarinus officinalis
Cistus albidus
Thymus vulgaris
Average
StDev
1990
wet
dry
1991
wet
dry
1992
wet
1995
1997
2000
dry
wet
dry
wet
dry
wet
dry
164.0 52.23 37.54 37.66 14.75
65.73 42.13 18.34 25.11 18.68
16.97
9.24
6.79
7.91
5.69
3.01
4.51
2.75
1.64
1.62
2.09
1.55
1.45
1.07
16.37 12.85
58.08 16.90
0.26
2.24
0.13
0.13
0.03
0.04
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
47.97 22.44 11.84
98.57 48.74 18.41
41.15 18.27 3.89
3.01
7.67
1.12
0.91
0.57
0.23
0.07
0.32
0.07
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
31.07
36.58
21.37
35.18
22.90
30.05
12.96
13.30
11.45
16.62
16.56
16.35
12.43
11.25
2.96
5.85
8.68
3.46
1.61
4.81
6.20
1.25
1.69
1.70
2.15
0.78
2.85
5.12
0.39
0.19
0.99
0.30
0.06
1.39
2.81
0.16
0.02
0.41
0.16
0.01
0.79
1.76
0.18
0.11
0.56
0.11
0.11
0.69
1.43
0.01
0.01
0.01
0.01
0.00
0.28
0.63
0.06
0.08
0.08
0.04
0.10
0.33
0.70
0.17
0.00
0.03
0.08
0.03
0.24
0.49
63.86
114.8
68.63
139.7
78.11
79.75
42.47
30.42
22.28
32.41
24.81
15.82
18.19
12.11
Table 6. Sediment concentration in the overland flow (g L-1) for each plot over the
whole period of investigation measured for predominantly wet (autumn, winter) and dry
(summer) conditions.
International Journal of Wildland Fire (14[4], 2005), in press 13
Primary species
on plot
Season
Trees
Pinus halepensis
Pinus halepensis
Herbs
Brachypodium retusum
Brachypodium retusum
Shrubs
Pistacia lentiscus
Juniperus oxycedrus
Quercus coccifera
Swarf shrubs
Ulex parviflorus
Erica multiflora
Rosmarinus officinalis
Cistus albidus
Thymus vulgaris
Average
StDev
wet
1990
dry
wet
1991
dry
wet
1992
dry
1995
1997
2000
wet
dry
wet
dry
wet
dry
4.26
2.50
5.38
5.00
1.25
1.02
2.14
1.26
0.63
0.52
1.02
0.69
0.35
0.42
0.23
0.15
0.32
0.14
0.09
0.08
0.15
0.10
0.09
0.08
2.48
3.52
4.36
4.84
0.25
0.36
0.23
0.65
0.02
0.02
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2.45
3.65
2.58
3.98
5.42
3.69
1.02
1.03
0.68
2.32
2.45
1.98
0.26
0.15
0.09
0.36
0.56
0.20
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2.36
3.45
2.35
4.56
3.25
3.12
0.78
4.57
4.25
3.79
5.65
4.98
4.66
0.66
1.26
1.35
1.33
1.30
1.24
1.01
0.38
2.36
2.01
2.41
2.65
3.08
1.96
0.84
0.35
0.65
0.85
0.35
0.19
0.34
0.27
0.65
0.89
0.95
0.84
0.54
0.56
0.35
0.32
0.25
0.68
0.15
0.45
0.22
0.23
0.13
0.03
0.32
0.11
0.01
0.08
0.11
0.26
0.15
0.48
0.09
0.36
0.15
0.17
0.02
0.02
0.01
0.02
0.02
0.02
0.03
0.05
0.12
0.14
0.08
0.14
0.07
0.06
0.35
0.02
0.15
0.26
0.21
0.10
0.12
International Journal of Wildland Fire (14[4], 2005), in press 14
Figure 1. Location map of the study area in eastern Spain.
International Journal of Wildland Fire (14[4], 2005), in press 15
Figure 2. View of the rainfall simulation equipment. The rainfall is provided by a nozzle
2 m above the ground. The tent prevents wind from distorting the rainfall distribution.
International Journal of Wildland Fire (14[4], 2005), in press 16
Figure 3. View of the four types of vegetation: (a) trees (understorey of Pinus
halepensis) (b) herbs (Brachypodium retusum), (c, d) shrubs (Juniperus oxycedrus and
Pistacia lentiscus), and (e, f) dwarf shrubs (Erica multiflora and Cistus albidus).
a
b
c
d
e
f
Figure 4. Average overland flow coefficient, sediment concentration and erosion rates
expressed as a percentage of the maximum value measured, following the fire in 1989
(first measurement in 1990) up to 2000 (11 years later) for dry (summer) and wet
(winter) conditions.
International Journal of Wildland Fire (14[4], 2005), in press 17
Percentage
the maximum
measured
Percentage
ofto
maximum
valuevalue
measured
120
100
Sediment concentration
80
Overland flow coefficient
60
40
20
0
Erosion rate
0
2
4
6
8
Years since burning
International Journal of Wildland Fire (14[4], 2005), in press 18
10
12
Figure 5. Post-fire overland flow coefficient (% of rainfall) evolution for dry and wet
antecedent conditions over the study period distinguishing between plots covered by
trees, herbs, shrubs and dwarf shrubs.
70
Overland flow coefficient (%)
60
Wet conditions
50
40
30
Trees
Dwarf shrubs
20
Shrubs
10
Herbs
0
Dry conditions
Overland flow coefficient (%)
60
40
20
0
1990
1992
1994
International Journal of Wildland Fire (14[4], 2005), in press 19
1996
1998
2000
Figure 6. Post-fire erosion rate evolution for dry and wet antecedent conditions over the
study period distinguishing between plots covered by trees, herbs, shrubs and dwarf
shrubs.
120
Trees
Wet conditions
Dwarf shrubs
2
-1
Erosion
m 2h
Erosionrate
rate(g(g/m
/h))
100
80
Shrubs
60
40
20
Herbs
0
Dry conditions
2
-1
Erosion rate (g m 2h )
Erosion rate (g/m /h)
60
40
20
0
1990
1992
1994
International Journal of Wildland Fire (14[4], 2005), in press 20
1996
1998
2000
Figure 7. Sediment concentration in overland flow for dry and wet antecedent
conditions over the study period distinguishing between plots covered by trees, herbs,
shrubs and dwarf shrubs.
Sediment concentration (g l-1)
4
Wet conditions
3
2
1
0
6
Dry conditions
Sediment concentration (g l-1)
Trees
5
4
3
2
Dwarf shrubs
1
Shrubs
Herbs
0
1990
1992
International Journal of Wildland Fire (14[4], 2005), in press 21
1994
1996
1998
2000
Figure 8. Overland flow curves for rainfall simulations (55 mm h-1) on Pinus
halepensis covered plots following dry antecedent conditions in 1990 and 2000.
Overland flow coefficients were 17.65 % and 29.35 % 1 and 11 years after the fire
respectively.
30
Runoff curves on Pinus halepensis plot
Overland flow (mm h-1)
25
2000 (11 years after the fire)
20
15
10
5
1989 (1 year after fire)
0
0
10
20
30
40
Time (minutes)
International Journal of Wildland Fire (14[4], 2005), in press 22
50
60
70
Figure 9. Time to ponding (TP) for dry and wet antecedent conditions over the study
period distinguishing between plots covered by trees, herbs, shrubs and dwarf shrubs.
4000
Time to ponding (seconds)
Wet conditions
3000
Herbs
Shrubs
2000
Dwarf shrubs
1000
Trees
0
4000
Time to ponding (seconds)
Dry conditions
3000
2000
1000
0
1990
1992
1994
International Journal of Wildland Fire (14[4], 2005), in press 23
1996
1998
2000
Figure 10: Shape and position of the wetting front (summer experiments) as observed
after rainfall simulations in 1990, 1995 and 2000 for one plot per vegetation type.
.
.
.. .. .
.... . .
.
.
.. .. .
..
International Journal of Wildland Fire (14[4], 2005), in press 24
. .. . . . . . ... . .... . . ... ... .
.... ....... . . . . . . .. .. .. ... ... .....
.......
... ..
.
..
.......
2000
.
.
. . .... . . ... . . ... .. .... .... .. . ... .. .
...... ... .. ... .. ........... .. .. .. ........ ..... .
.. .... . ..... . ..... .. . .. .. ..... .... .. ..
.. .
.. .
.
.
.
... .
..... ..
. . .....
. .... ...
.
..
10 cm
.... . .
2000
1995
10 cm
... ... .. ...... . .. ... ... . .... ..
...... ......... ...... .. ............ .... ...............
. .. . . . . . . . . . . .. .... ...
. .... ....... ....... ... ......... ... .... .................
.. .. . . . .. .. .. . .. . . . . ... ... .
. . .. ..
.. .
..
.....
.
.
.
.
.
..
.. .
. .. . . . . . ... . .... . . ... ... .
.... ....... . . . . .... .. .. .. ... ... ....
.......
... ..
.
..
.
........
.. . .... . . ... . . ... .. .... .... .... ... ... .
. .... .. .. ... . . ......... .. .. .. ....... .......
. .... ...... . ..... ... .. .. ... .... ... .
... .... .. ... ... . .. ... ... . .... ..
....... ......... ...... ................ .... ... ............
. .. . . . . . . . . . .. .... ...
. .... .............. .. ......... .... .... ........ .........
. . . . ... ... .
... . . .
. .. .. .... . . ... .. .. . . . .
..
1990
10 cm
1995
. . .... . . .. . . ... .... . .... . . ... ... .
...... .. .. ... .. . .. ....... .. .. ... ... .....
.
..
1990
.................. .. .. .... .... ... .. ... .... ....
.
........
. .. .. .
wet soil
.
wetting
front
.. . .... . . ... . . ... . .. . .... . . ... .. .
..... .. .. ... .. ............... .. .. .... ... ..... .
. . .. . .
. .. . .
.
... .... .. ...... .... ... ... . .... ...
....... ......... ..... ....... ....... .... ................
. .. . . . . . . . . .. .. .. ...
Dwarf shrubs
(various species)
. .. .. .
.................. .. ... .... ... ... .. ... .... ......
. . . . . . ... .. ... .. .... ..
Shrubs
(various species)
.
Herbs
(Brachypodium retusum)
.
Trees
(Pinus halepensis)