**5. Discussion**

As pointed out by several authors (García-Ruiz, 2010; Kosmas et al., 1997; Mitchell, 1990), land use and soil cover are considered the most important factors affecting the intensity and frequency of overland flow and surface wash erosion. The results obtained agree with those observed by different authors in varied environments, who consider that runoff and sediment yield decrease with an increase in soil cover with vegetation (Bochet et al., 1998; De Ploey, 1989; Durán Zuazo et al., 2006; Elwell & Stocking, 1976; Francis & Thornes 1990; Roxo 1994). As can be seen in Figure 5, increasing vegetation cover leads to an exponential decrease in runoff, but only when this exceeds a threshold value of over 40%. Correspondingly, similar behaviour can be observed with regard to the relationship between sediment loss and vegetation cover.

However, wide variations in the percentage of plant cover were presented as critical between studies. Studies carried out in natural Mediterranean environments have shown that when vegetation cover drops below 30% soil erosion and runoff increase dramatically (Francis & Thornes, 1990; Gimeno-García et al., 2007). Thornes (1988) suggests that a value of 40% vegetation cover is considered critical, below which accelerated erosion dominates on sloping land. If the vegetation cover covers an area of more than 40%, it will act as a resilience or protective factor for the land. Molinillo et al. (1997) observed an increase in runoff and soil erosion in up to 60% shrub-cover and only above this value a reduction in

Axis 3 shows both the expected opposition between soil porosity and resistance to soil penetration, and its positive relationship to the existence of lichens and mosses, which means less disturbance of the soil surface. Obviously, antecedent soil moisture does not favour water repellency, hence the opposition shown in Axis 4. These two components

Soil parameters Component 1 Component 2 Component 3 Component 4

0.512 0.315 -0.265 0.854

8.65E-02 -0.411 -0.616 -0.321




4.72E-02 7.13E-02 -0.833


7.35E-03



9.41E-02 0.297 0.595

0.892

8.16E-02


5.99E-02 -0.331 -0.288

Extraction Method: Principal Component Analysis; Rotation Method: Varimax with Kaiser

As pointed out by several authors (García-Ruiz, 2010; Kosmas et al., 1997; Mitchell, 1990), land use and soil cover are considered the most important factors affecting the intensity and frequency of overland flow and surface wash erosion. The results obtained agree with those observed by different authors in varied environments, who consider that runoff and sediment yield decrease with an increase in soil cover with vegetation (Bochet et al., 1998; De Ploey, 1989; Durán Zuazo et al., 2006; Elwell & Stocking, 1976; Francis & Thornes 1990; Roxo 1994). As can be seen in Figure 5, increasing vegetation cover leads to an exponential decrease in runoff, but only when this exceeds a threshold value of over 40%. Correspondingly, similar behaviour can be observed with regard to the relationship

However, wide variations in the percentage of plant cover were presented as critical between studies. Studies carried out in natural Mediterranean environments have shown that when vegetation cover drops below 30% soil erosion and runoff increase dramatically (Francis & Thornes, 1990; Gimeno-García et al., 2007). Thornes (1988) suggests that a value of 40% vegetation cover is considered critical, below which accelerated erosion dominates on sloping land. If the vegetation cover covers an area of more than 40%, it will act as a resilience or protective factor for the land. Molinillo et al. (1997) observed an increase in runoff and soil erosion in up to 60% shrub-cover and only above this value a reduction in

explain about 24 % of the total variance observed.

0.254 0.795 0.874 -0.134

7.16E-02

0.399

6.65E-02 -8.79E-02 0.405

> 0.859 -0.757 -0.556

Table 6. Results of Rotated Component Matrix

between sediment loss and vegetation cover.

Slope (%) Soil cover (%) Litter cover (%) Herbs+ shrubs (%) Height of vegetation

(cm)

(%) Runoff (%)

Antecedent soil moisture (%) Resistance to penetration (g m-2) Total of porosity (%) Water repellency (%) Soil organic matter

Soil erosion (g m-2 h-1)

Normalization.

**5. Discussion** 

runoff and erosion processes. Sauer & Ries (2008) consider that only plant cover exceeding 60% can significantly reduce soil erosion in semi-arid environments.

Land use and the type of management applied to each site explain, to a large extent, the variability in annual plant cover and, therefore, the occurrence of overland flow and soil erosion processes (De Luna et al., 2000; Francia Martínez et al., 2006; Gómez et al., 2004). Annually, as it involves mobilisation of the top layer (laying fallow and afforested land), ploughed soil erodes more easily and causes great soil loss. Cereal growth, mainly when it offers a good vegetal protection for the surface of the soil, enhances infiltration and reduces erosion rates. However, the results for soil erosion were greater in comparison with recent abandonment and were one hundred times greater if compared to land cover after verylong abandonment (Table 7). These results also enable us to conclude that traditional cereal cultivation, in particular ploughing in preparation for cereal crops, is a very negative land management practice due to the high runoff and water erosion response. Organic matter content, probably the most important component of soil quality, is also strongly influenced and registered very low figures of less than 1% for arable land. A limit of 1.7 per cent of soil organic matter content is considered an indicator of the pre-desertification stage (Pardini et al., 2002). The gradual depletion of nutrients, which reduces soil fertility and creates a high level of soil degradation, are further reasons for abandoning agricultural plots in the changing cultivation process (Paniagua et al., 1999). Planting trees, according the CAP measures for afforestation of marginal fields, with deep ploughing and bare soil has resulted in very high erosion rates during both rainfall seasons, as observed in other agro-ecosystems in Mediterranean Europe (Shakesby et al., 2002; Ternan et al., 1997; Van-Camp et al., 2004).

Fig. 5. Relationship between runoff and soil erosion and average percentage of plant cover

However, in large parts of marginal areas of the country farmland abandonment has enhanced plant colonisation, replacing historically highly erosive cereal fields with dense

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 77

vegetation and the accumulation of fuel due to fire exclusion policies are cited as some of the major causes of changes to the forest fire regime in Mediterranean Europe (Moreno, 1996). In fact, Portugal's burnt area has increased chiefly during the last three decades. This rising trend, although including some periods of lower burnt area, distinguishes Portugal from the other southern Member States with the highest burnt areas, particularly in the central and northern regions. It is commonly accepted that fire increases runoff and soil erosion (Benavides-Solorio & MacDonald, 2005; Cerdà & Doerr, 2005; Cerdà & Lasanta, 2005; Coelho et al., 2004; Ferreira et al., 2005, 2008; Shakesby et al., 1993, 1996, 2002). Increased erosion after forest fire stems primarily from the destruction of vegetation and changes in the soil physical and hydrologic properties that reduce infiltration rates and increase availability of loose sediment (Ferreira et al., 2005, 2008). The loss of vegetation and other ground cover due to wildfire reduces rainfall interception and attenuation, rainfall storage, and flow resistance (Martin & Moody, 2001). Rainfallgenerated runoff therefore accelerates more quickly and less is retained as pounded water, resulting in reduced residence times and reduced total infiltration (Ferreira et al., 2008). In the Mediterranean basin, potential for post-fire soil erosion is very high as heavy

Converting arable land into pasture can be positive, negative, or without impact depending on management practices. Several studies have evaluated how different grazing intensities affect both plant cover and water infiltration into the soil. These studies are consistent in showing that as grazing intensity is increased and soil cover is depleted, water infiltration declines and soil erosion increases (Rauzi & Smith, 1973). In fact, the direct impact of cattle hooves reshapes the land. Compaction is, perhaps, the strongest direct impact of force which leads to indirect consequences in terms of overland flow and soil erosion. Direct measurements of overland flow and soil loss rates from the pasture plots were more pronounced after the dry, hot season. During wet season, runoff and sediment yield

Despite these negative effects, the conversion of arable land into extensive grassland had a strong, positive effect on surface runoff and erosion in the area monitored. A permanent plant cover of over 50%, in the form of vegetation or ground litter, provides a cushion between raindrops and the soil, preventing the "splash effect" or dislodging of soil particles by rain drops (Molinar et al., 2001). Research findings indicate a threshold at which removing the plant cover and volume have little effect on infiltration rates and soil protection. This threshold is generally about 50% of the current year's growth, which corresponds to the old adage of "take half and leave half" for sustainability (Wood, 2001). The existence of marked seasonal dynamics related to the dominance of annual vegetation creates cycles and temporal differences in protecting the soil against geomorphic processes. At the end of the summer, with exception of ploughed land, the largest amount of runoff and sediment export from soils occurred, chiefly due to the greater erodibility of the soil surface after a warm, dry season and poor soil cover (Romero Díaz et al., 1999). The development of microcrusts in some plots encouraged runoff and, therefore, sediment transport. The water repellency observed in such soils (with shrub and tree species) after a long period without precipitation did not have a significant impact on overland flow generation and erosion rates. Soils with a higher organic matter content appear to offer good protection against runoff and soil erosion. This is provided by litter, from recovering

autumn rainfalls commonly occur after summer wildfires.

*Quercus pyrenaica* and *Cytisus multiflorus*, for example.

decrease significantly.

shrub and woodland communities. Herbaceous cover, the first successional stage in vegetation recuperation after land abandonment, seems not to enhance soil fertility after a crop cycle, but positively influences runoff and sediment yield, in addition to the higher values presented after a hot, dry period. These results stress the importance of nonploughing as a soil protection measure.

When a permanently vegetated cover is dominant, as a result of a long plant succession, with shrub communities and oak-trees, even in recuperation, both soil erosion and surface runoff are very well controlled. Trees and shrub cover also ensure soil conservation and improve some of the soil characteristics, mainly the organic matter content, which registered a significant increase (Table 7). This hydrological and erosional behaviour, together with the soil properties, is closely interrelated and well understood in terms of the dynamics of plant and litter cycling. Vegetation and litter reduces direct raindrop impact on the soil, prevents the formation of mechanical crusts, enhances infiltration capacity and reduces soil erodibility (Nunes et al., 2010). In these soils, long-term spatially structured vegetation patterns play an important role in addition to cover, by increasing the stability and resilience of the system (Boer & Puigdefabregas, 2005; Cammeraat & Imeson, 1999). In general, our data agrees with other results obtained for the Mediterranean basin, in which land abandonment followed by natural vegetation regeneration is improving soil properties such as organic matter content, soil structure and infiltration rates, resulting in more effective protection against soil erosion (Cammeraat & Imeson, 1999; Francis & Thornes, 1990; Grove & Rackham, 2001; Kosmas et al., 2000; Morgan, 1986; Trimble, 1990).


Table 7. Benefits detected in the comparison of cultivated land and the different vegetation cover/stages of abandonment.

Although abandoned land in humid and sub-humid regions self-regulates the development of natural vegetation (grass, weed, bushes, and later woodland) and normally does not need support except in the first years of abandonment, the cessation of traditional management practices, the creation of large homogeneous patches of

shrub and woodland communities. Herbaceous cover, the first successional stage in vegetation recuperation after land abandonment, seems not to enhance soil fertility after a crop cycle, but positively influences runoff and sediment yield, in addition to the higher values presented after a hot, dry period. These results stress the importance of non-

When a permanently vegetated cover is dominant, as a result of a long plant succession, with shrub communities and oak-trees, even in recuperation, both soil erosion and surface runoff are very well controlled. Trees and shrub cover also ensure soil conservation and improve some of the soil characteristics, mainly the organic matter content, which registered a significant increase (Table 7). This hydrological and erosional behaviour, together with the soil properties, is closely interrelated and well understood in terms of the dynamics of plant and litter cycling. Vegetation and litter reduces direct raindrop impact on the soil, prevents the formation of mechanical crusts, enhances infiltration capacity and reduces soil erodibility (Nunes et al., 2010). In these soils, long-term spatially structured vegetation patterns play an important role in addition to cover, by increasing the stability and resilience of the system (Boer & Puigdefabregas, 2005; Cammeraat & Imeson, 1999). In general, our data agrees with other results obtained for the Mediterranean basin, in which land abandonment followed by natural vegetation regeneration is improving soil properties such as organic matter content, soil structure and infiltration rates, resulting in more effective protection against soil erosion (Cammeraat & Imeson, 1999; Francis & Thornes,

1990; Grove & Rackham, 2001; Kosmas et al., 2000; Morgan, 1986; Trimble, 1990).

Benefits compared to ploughing land

Fallow land or short-term abandonment Not detected -32.7% -68.2% Shrub land or long-term abandonment +247.3% -89.0% -99.5%

abandonment +265.5% -99.5% -99.9% Afforested land +52.3% +41.0% +73.5% Pastureland +32.7% -47.9% -92.5% Benefits compared to cereal crop Fallow land or short-term abandonment Not detected +49.2% -24.4% Shrub land or long-term abandonment +247.3% -76.4% -98.8%

abandonment +265.5% -98.8% -99.9% Afforested land +52.3% +212.8% +312.6% Pastureland +32.7% +15.6% -82.1% Table 7. Benefits detected in the comparison of cultivated land and the different vegetation

Although abandoned land in humid and sub-humid regions self-regulates the development of natural vegetation (grass, weed, bushes, and later woodland) and normally does not need support except in the first years of abandonment, the cessation of traditional management practices, the creation of large homogeneous patches of

Organic matter content (%)

Runoff (mm h-1) Sediment loss (g m-2 h-1)

ploughing as a soil protection measure.

Vegetation cover/stages of

Recovering oak or very long-term

Recovering oak or very long-term

cover/stages of abandonment.

abandonment

vegetation and the accumulation of fuel due to fire exclusion policies are cited as some of the major causes of changes to the forest fire regime in Mediterranean Europe (Moreno, 1996). In fact, Portugal's burnt area has increased chiefly during the last three decades. This rising trend, although including some periods of lower burnt area, distinguishes Portugal from the other southern Member States with the highest burnt areas, particularly in the central and northern regions. It is commonly accepted that fire increases runoff and soil erosion (Benavides-Solorio & MacDonald, 2005; Cerdà & Doerr, 2005; Cerdà & Lasanta, 2005; Coelho et al., 2004; Ferreira et al., 2005, 2008; Shakesby et al., 1993, 1996, 2002). Increased erosion after forest fire stems primarily from the destruction of vegetation and changes in the soil physical and hydrologic properties that reduce infiltration rates and increase availability of loose sediment (Ferreira et al., 2005, 2008). The loss of vegetation and other ground cover due to wildfire reduces rainfall interception and attenuation, rainfall storage, and flow resistance (Martin & Moody, 2001). Rainfallgenerated runoff therefore accelerates more quickly and less is retained as pounded water, resulting in reduced residence times and reduced total infiltration (Ferreira et al., 2008). In the Mediterranean basin, potential for post-fire soil erosion is very high as heavy autumn rainfalls commonly occur after summer wildfires.

Converting arable land into pasture can be positive, negative, or without impact depending on management practices. Several studies have evaluated how different grazing intensities affect both plant cover and water infiltration into the soil. These studies are consistent in showing that as grazing intensity is increased and soil cover is depleted, water infiltration declines and soil erosion increases (Rauzi & Smith, 1973). In fact, the direct impact of cattle hooves reshapes the land. Compaction is, perhaps, the strongest direct impact of force which leads to indirect consequences in terms of overland flow and soil erosion. Direct measurements of overland flow and soil loss rates from the pasture plots were more pronounced after the dry, hot season. During wet season, runoff and sediment yield decrease significantly.

Despite these negative effects, the conversion of arable land into extensive grassland had a strong, positive effect on surface runoff and erosion in the area monitored. A permanent plant cover of over 50%, in the form of vegetation or ground litter, provides a cushion between raindrops and the soil, preventing the "splash effect" or dislodging of soil particles by rain drops (Molinar et al., 2001). Research findings indicate a threshold at which removing the plant cover and volume have little effect on infiltration rates and soil protection. This threshold is generally about 50% of the current year's growth, which corresponds to the old adage of "take half and leave half" for sustainability (Wood, 2001).

The existence of marked seasonal dynamics related to the dominance of annual vegetation creates cycles and temporal differences in protecting the soil against geomorphic processes. At the end of the summer, with exception of ploughed land, the largest amount of runoff and sediment export from soils occurred, chiefly due to the greater erodibility of the soil surface after a warm, dry season and poor soil cover (Romero Díaz et al., 1999). The development of microcrusts in some plots encouraged runoff and, therefore, sediment transport. The water repellency observed in such soils (with shrub and tree species) after a long period without precipitation did not have a significant impact on overland flow generation and erosion rates. Soils with a higher organic matter content appear to offer good protection against runoff and soil erosion. This is provided by litter, from recovering *Quercus pyrenaica* and *Cytisus multiflorus*, for example.

Soil Erosion Under Different Land Use and Cover Types in a Marginal Area of Portugal 79

and increasing ground coverage, which was shown to be one of the basic approaches to

On the basis of the experiment results, pastureland should be encouraged, particularly for the degraded soils that are used to produce cereals in order to minimize the amount of soil loss by erosion, thus avoiding slumping and promoting stability. Accommodating pasture management with a weighted number of grazing animals per area unit and extending pasture rotation times could reduce soil erosion processes effectively and ongoing land degradation could be prevented according to the 'Directive of the European Parliament and of the Council Establishing a Framework for the Protection of Soil' (Commission of the European Communities, 2006). Additionally, is important to emphasise that extensive grazing is the main focus of landscape management in marginal areas of the Mediterranean region with very low population densities, only small resident communities, little

In addition to better pasture management, another possible consideration may be management of native shrub land and recovering oak. Land afforestation should be supported by a set of measures to minimise the impact of site preparation techniques, forest management and fire prevention on soils. Plough use as a tool in preparation of soils for sowing seeds in dry farming systems should be replaced in favour of other less pernicious

However, in the study area, as well as in the majority of Portuguese rural areas, key problems remain and are complex to solve. They include: a) How to stop the demographic exodus that took place in the middle of the last century and continues nowadays? b) How to supply an appropriate income to attract young farmers to depopulated areas where the great majority of the population are elderly? c) How to improve farm structures that consist of small scattered plots? d) How to manage systems that favour soil conservation and combat land degradation but maintain economic viability? e) How to adjust Mediterranean

This research was supported by the project ''Land use changes in inland Central and Northern Portugal" (POCTI/GEO/49371/2002) funded by the Science and Technology

Benavides-Solorio, J. de & MacDonald, L. (2005). Measurement and prediction of postfire

Bochet, E., Rubio, J. & Poesen, J. (1998). Relative efficiency of three representative matorral

Spain). *Geomorphology*, vol.23 (June 1998), pp. 139-150 ISBN 0169-555X Boer, M. & Puigdefábregas, J. (2005). Effects of spatially-structured vegetation patterns on

*Fire*, vol. 4 (November 2005), pp. 457–474, ISSN 1049-8001

erosion at the hillslope scale, Colorado Front Range. *International Journal of Wildland* 

species in reducing water erosion at the microscale in a semi-arid climate (Valencia,

hillslope erosion rates in semiarid Mediterranean environments: a simulation study. *Earth Surface Processes and Landforms*, vol.30 (February 2005), pp. 149-167,

controlling soil erosion in all land use types.

mechanised agriculture and poor communications.

agriculture to climate change scenarios for the 21st century?.

tillage techniques.

**7. Acknowledgment** 

**8. References**

Foundation – Portugal (FCT).

ISSN 1096-9837

As pointed out by Imeson (1990), the main characteristics affecting the vulnerability of the Mediterranean area to erosion are intense rainfall after a very dry summer. In fact, autumn is the most water erosive season as a consequence of the heaviest concentration of rainfall and rainfall erosivity (Nunes et al., 2011). Recent research into climate change in Portugal (the SIAM Project, Miranda et al., 2006) for the 21st century (including 3 greenhouse gas emission scenarios used by many global and two regional climate models), are homogeneous predicting a reduction in annual rainfall in mainland Portugal (within the range of 20 to 40% of the current value), as a result of a decline in the duration of the wet season. Predictions for temperature changes agree on an overall increase in the annual mean, with a much more pronounced maximum summer temperature particularly affecting inland areas of Portugal. This climatic trend will extend the long, dry, hot summers in the Mediterranean region and lead to more frequent and intense extreme weather events, which could increase the rates of erosion and the risk of desertification that is threatening substantial areas of Portugal (Nunes & Seixas, 2003; Nearing et al., 2005).
