Ž .Catena 40 2000 51–68www.elsevier.comrlocatercatena

The effect of land use change on soils andvegetation over various lithological formations on

ž /Lesvos Greece

C. Kosmas), St. Gerontidis, M. MarathianouLaboratory of Soils and Agricultural Chemistry, Agricultural UniÕersity of Athens, Iera Odos 75, Botanikos,

Athens 11855, Greece

Abstract

The effect of land use change, from arable to abandoned pasture, on soil properties andvegetation establishment was studied in hilly areas of the island of Lesvos. The main purpose ofthis study was to establish key indicators or land parameters that can be used for definingenvironmentally sensitive areas to desertification. 106 soil sites were selected in fields cultivatedor non-cultivated for 40–45 years and measurements related to protection from land degradation

Žafter abandonment, such as fertility status organic matter content, pH, cation exchange capacityŽ . . ŽCEC , exchangeable potassium and sodium , water storage capacity soil water retention charac-

. Ž .teristics, soil depth , erosion resistance soil aggregate stability , and vegetation characteristicsŽ .plant species, extent of vegetation cover were conducted. The selected sites were located on avariety of parent materials such as volcanic lava, pyroclastics, ignimbrite, schist-marble, and shale.The obtained data indicated that soil pH, and CEC were slightly affected after abandonment ascompared to the cultivated soils. Levels of exchangeable sodium and potassium were higher incultivated soils. Organic matter content and soil aggregate stability were greatly enhanced in mostof the abandoned soils. Physical characteristics of the parent material greatly influenced theestablishment of the natural vegetation. A critical minimum soil depth of 25–30 cm wasmeasured. On more shallow soils, the associated natural perennial vegetation cover was rapidlyreduced, under the prevailing climatic conditions of the study area. Reduction of vegetation coverwas related to the parent material. Perennial vegetation was not supported at all on soils below acrucial depth, ranging from 4 to 10 cm, depending on the parent material. This study showed thatsoil depth is the most important parameter which has to be considered in planning land use change

) Corresponding author. Fax: q30-1-5294097

0341-8162r00r$20.00 q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0341-8162 99 00064-8

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from arable to unmanaged pasture under the soil and climatic conditions of the study area. q 2000Elsevier Science B.V. All rights reserved.

Keywords: Land use change; Soil properties; Soil depth; Parent material; Desertification

1. Introduction

The particular nature of the typical Mediterranean relief, with slopes subject tointensive cultivation since ancient times, has led to soil erosion and the formation ofshallow skeletal soils. As the soil is eroded, land use is usually shifted from agricultureto pasture due to increasingly poor yields from the various agricultural crops. Most

Žpasture land in the Mediterranean region is defined as abandoned land today Martinez-Fernandez et al., 1995; Lopez-Bermudez et al., 1996; Puigdefabregas et al., 1996; Roxo´ ´

.et al., 1996 . Previous authors have simultaneously used the terms ‘abandoned land’ and‘grazing land’, but grazing or hunting of an abandoned land is considered as a traditionaluse in the Mediterranean region. Only a few areas which have been fenced or strictlycontrolled by land owners remain ungrazed. Almost all the natural vegetation in theMediterranean basin, apart from a few forests, is grazed to some extent by migrating or

Ž .permanent flocks and herds Clark, 1996 . Therefore, the term ‘abandoned land’ is usedin this study to include areas previously cultivated but now abandoned, and the naturalvegetation has been allowed to grow under various intensities of grazing.

The effects of land abandonment on land quality may be positive or negativedepending on the soils and climatic conditions of the area. Soils under favourableclimatic conditions that sustain plant cover may improve with time by accumulatingorganic materials, increasing floral and faunal activity, improving soil structure, increas-

Ž .ing infiltration capacity, and therefore, decreasing erosion potential Trimble, 1990 .Ž .Martinez-Fernandez et al. 1995 have reported a positive effect of land abandonment

after a period of over 10 years in which characteristics of abandoned soils approachedthose seen before cultivation. The abandonment of this area resulted in improvement ofsoil characteristics such as the organic matter content, water retention capacity, aggrega-

Ž . Ž .tion and structural stability. Jaiyeoba 1995 and Unger 1997 reported a deteriorationof soil fertility under cropping and concluded that the soils under various types ofagricultural land uses contained less organic matter content, total nitrogen, exchangeable

Ž .bases and cation exchange capacity CEC than similar soils under natural vegetation.Ž . Ž . Ž . Ž .Greenland and Nye 1959 , Aweto 1981 , Jaiyeoba 1988 , and Ekanade 1989 showed

that under forest fallow soil organic matter content and nutrients increased. Lopez-´Ž .Bermudez et al. 1996 , in a study along a hillslope under various conditions of´

abandonment and cultivation in southern Spain, showed that soils on fallow, orabandoned for 4–10 years, showed a progressive recovery of vegetation cover and asignificant increase in organic matter content, aggregate stability, water holding capacityŽ .WHC , and hydraulic conductivity.

Where vegetation cover is sparse, erosional processes may be very active and thedegeneration of the abandoned lands may be irreversible. Many authors have demon-strated that in a wide range of environments both runoff and sediment loss decrease

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Žexponentially as the percentage of vegetation cover increases Elwell and Stocking,. Ž .1976; Lee and Skogerboe, 1985; Francis and Thornes, 1990 . Garcia Ruiz 1991 found

that only 3.5% of fields abandoned for a period of less than 10 years in the SpanishPyrenees did not suffer from erosion, and 60% of the fields suffered severe sheet erosion

Ž .without sufficient protection from a vegetation cover. Kosmas et al. 1997 showed thatin general on hilly Mediterranean shrublands runoff and sediment loss increased with

Ž .decreasing annual rainfall above a threshold of 280–300 mm , and attributed this to adecrease in vegetation cover. For areas with rainfall below this threshold, erosion

Ž .decreased with increasing rainfall. Martinez-Fernandez et al. 1996 demonstrated thatthe post-abandonment land use is of major importance for the evolution of favourableland characteristics after abandonment.

A major aim in studying the ability of an ecosystem to return to the original stateŽ .after disturbance resilience is the prediction of the response of such a system to a

Ž .variety of natural and human-induced disturbances Dell et al., 1986; Westman, 1986 .These include drought, fire, grazing, vegetation clearing and cultivation. Moderategrazing pressure on abandoned agricultural land may lead to partial rejuvenation of

Žvegetal communities with a high diversity index Fox and Fox, 1986; Martinez-Fernan-.dez et al., 1996 . The decline in vegetation by overgrazing can include the loss of

Ž .particular herbaceous families Leguminosae, Gramineae which help to maintain soilstructure. Plant species from these families can both protect the soil surface fromraindrop splashing and reduce erosion rates by increasing soil aggregate stability.Disturbance by grazing does not result in the complete removal of vegetation as anintense fire does. The impact of fire is greatest in those areas with the lowest fire

Ž .frequencies Fox and Fox, 1986 . An increase in fire frequency leads to fewer plantspecies, caused by the loss of those which cannot persist when fires are too frequentŽ .Fox and Fox, 1986; Grove and Rackham, 1996 .

Parent material is considered as a soil-forming factor affecting soil properties, plantŽ .growth Kosmas et al., 1993 , soil erosion and ecosystem resilience. Hilly soils formed

on consolidated parent materials such as limestone, sandstone, volcanic lava, etc. usuallyhave a restricted effective rooting depth as the soils are eroded and shallow. Soil depthdefines the root space and the volume of soil from where the plants fulfil their water andnutrient demands. Under hot and dry climatic conditions, rain-fed vegetation may not besupported in such areas, leading to desertification. The number of studies on the effectof parent material on regeneration of natural vegetation and protection of abandonedagricultural land are limited. Furthermore, there are few studies which address the issueof when a land has to be abandoned under certain soil and climatic conditions and howsufficient regeneration of natural vegetation and protection of the soil from erosiondevelop. An attempt has been made in this study to evaluate the effect of parent materialon soil evolution and vegetation recovery following abandonment of agricultural land inhilly areas of the island of Lesvos. In more detail, the objective of the present work isŽ .a to evaluate the effect of land abandonment for a long period on soil properties andvegetation establishment in hilly areas cultivated for centuries with rain-fed cereals

Ž .olives and vines and b to define critical soil characteristics for changing land use fromarable to abandoned pasture for soils formed on different parent materials so thatregeneration of natural vegetation under given climatic conditions can be monitored.

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2. Materials and methods

2.1. Description of the study area

The study was conducted at various sites throughout Lesvos. This island is located inthe northeast part of the Aegean sea and covers an area of 163,429 hectares. The soils ofthe study sites were previously cultivated, mainly with rain-fed crops such as cereals,vines and olives, and due to low productivity the majority of sites were abandoned40–45 years ago. After abandonment, the area was moderately grazed and the growingshrubs were occasionally cleared by using fire. The study sites were cultivated using

Ž . Ž .animal labour once cereals or twice vines, olives per year before abandonment. Nopesticides were used and the only fertiliser applied was animal manure. The presentcultivation practice in the cultivated areas has not significantly changed in the studysites. The land is mainly cultivated with animal labour or small two-wheel tractors. Fullmechanisation of cultivation is not easy in the area due to steep slopes and the presenceof narrow terraces restricting the use of heavy machinery. The only significant recentchange is the application of inorganic fertilisers in combination with manure. Use ofpesticides is greatly restricted to protect grazing animals from poisoning. Therefore, thesoil state at the time of abandonment was different under the various crops, but after aperiod of 40–45 years under natural vegetation and moderate grazing, an equilibriumunder the prevailing soil and climatic conditions of the area has probably been attained.

The study sites are located in hilly areas with slopes ranging from 18–25% forabandoned sites and from 14–21% for cultivated sites. The soils of the study sites are

Ž . Ž .usually shallow depth ranging from 5 to 30 cm to moderately deep depth 30–65 cm ,Ž .well-drained, slightly stony to stony rock fragments content 5–25% , sandy loam to

sandy clay loam, developed on various lithological formations such as shale, schist-mar-ble, volcanic lava, pyroclastics, and ignimbrite. They are classified as Typic Xerochrept,

Ž .Lithic Xerochrept, or Lithic Xerorthent Soil Survey Staff, 1975 .The climate of the study sites is characterised by strong seasonal and spatial

variations of rainfall and high oscillations between minimum and maximum dailytemperatures, typical of the Mediterranean region. A gradient in rainfall occurs acrossthe island with the average annual rainfall in the study sites ranging from 677 mmŽ . Ž .eastern part to 415 mm western part . The mean annual air temperature is 17.78Cwithout any significant gradient across the island.

In the study sites, the dominant natural vegetation growing after abandonmentŽ .belongs mainly to the family Gramineae 55–62% of the existing species . Common

species of this family are: AÕena barbata, Briza maxima, Aegilops neglata, Brizamaxima, Bromus intermedius, Hordeum murinum, Phalaris canariensis, Bromus dian-drus, and Lagurus oÕatus. 15–21% of species belong to the Compositae. The followingspecies are common: Anthemis arÕensis, Carduus nutans, Anthemis cretica, Crepisrudra, Filago sp., Lactuca seriola, and Thagopogon porrifolius. The remaining com-mon plant species belong to families such as Cruciferae, Labiatae, LeguminosaeŽ .Trifolium sp. , and Rosaceae. Perennial shrubs of Sarcopoterium spinosum, andSarcopoterium Õerucosum are wide-spread in the area, covering much of the landsurface. The vegetation cover has been affected by the parent material ranging from bare

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Ž . Ž .to fully covered 100% Table 1 . Vegetation cover was generally lower for soilsŽ .formed on pyroclastics mean 50.5%, Table 1 , while the highest mean vegetation cover

Ž .81.4% was measured in soils formed on shale. Vegetation cover had intermediateŽ .mean values in sites with soils formed on schist-marble, lava, and ignimbrite Table 1 .

2.2. Field measurements and sampling

ŽA total of 106 sites were selected for soil analysis along catenas in hilly areas Fig.. Ž1 . The selected sites were located on soils formed on igneous rocks such as lava 24

. Ž . Ž .sites , pyroclastics 17 sites , and ignimbrite 17 sites ; sedimentary and metamorphicŽ . Ž .rocks such as shale 24 sites , and schist-marble 24 sites , respectively. The study sites

were located along north-facing slopes on a distinct landscape position which wasŽ .identified as backslope, classified according to the scheme of Ruhe 1960 scheme. At

each study site, a transect 24 m long was defined perpendicular to the contour lines and12 disturbed soil samples were collected at an equal distance of 2 m and a composite

Table 1Ž .Vegetation cover % measured at the various study sites classified according to the parent material

Study site Parent material

Pyroclastics Lava Schist-marble Shale Ignimbrite

1 84.5 73.1 85.7 65.9 75.92 75.2 100 55.3 97.9 40.63 100 71.5 84.3 72.4 85.94 5.6 84.0 83.8 85.7 20.45 0 100 62.3 87.2 82.76 0 95.7 65.1 96.7 1007 83.1 100 92.3 100 1008 52.3 94.1 84.5 88.9 72.19 55.1 97.2 93.1 75.8 81.910 65.0 8.9 68.9 73.2 70.511 45.0 82.3 55.0 95.0 71.312 83.2 100 85.4 100 73.913 62.5 87.6 98.1 55.8 68.314 53.5 0 95.2 85.6 10015 72.1 53.3 93.1 74.2 74.216 64.3 82.2 15.3 70.2 88.917 62.7 100 5.2 87.8 95.218 74.2 92.1 0 95.5 92.419 15.8 57.7 35.6 99.0 60.820 43.0 24.3 25.9 87.4 65.221 53.5 47.0 18.1 64.6 92.122 55.8 82.2 17.6 74.9 35.423 0 5.8 84.3 54.6 024 15.3 35.2 92.2 65.8 40.1Mean 50.5 69.7 62.3 81.5 70.3Standard deviation 29.6 32.9 32.7 14.2 26.1

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Fig. 1. Location and distribution of the study sites on the island of Lesvos.

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sample was prepared. Three separate undisturbed soil samples were also collected ateach site. Soil samples were taken mainly from the A-horizon because major alterationshave occurred in this horizon after the land use change. At each study site, the followingmeasurements were conducted: dominant plant species, percentage vegetation cover,shrub diameter, soil depth, thickness of the A-horizon, and slope gradient. Profile or soilsurface stoniness was not determined because surface rock fragment distribution on a

Žhillslope is greatly affected by animal trampling along favoured routes Oostwoud.Wijdenes et al., 1998 . For a comparative analysis of the effect of land abandonment on

soil properties, 23 of the sites were located in cultivated fields. Study sites in cultivatedfields for soils formed on pyroclastics and ignimbrite could not be included due to theabsence of cultivated fields in the area during this study.

ŽThe degree of erosion no erosion, slightly eroded, moderately eroded, severely.eroded, and very severely eroded was estimated by measuring the length of visually

current distinct erosion spots and rock outcrops along the study transects. At each studysite, three transects of 30 m each were located perpendicular to the contour lines and thelength of distinct erosion spots and positions of the rock outcrops were measured. Thedegree of erosion was then assessed as the ratio of the length of erosion spots and rockoutcrops over the total length of the study transect expressed on a percentage basisŽ .Kosmas et al., 1998 . In addition, the rate of exposure of the bedrock was measured at12 sites in areas overlying pyroclastics. Stainless steel pins were permanently installed

Žinto the bedrock defining the exact size of the erosion spots area of spots ranging from2 2 .0.8 m to 22 m where the soil had washed out in the past and bedrock has been

exposed at the surface.Meteorological data were collected from two stations installed at Vatousa and

Antissa. Precipitation was measured via an automatic rain gauge at intervals of 5 min.Ž .Open pan-evaporation rate E was automatically recorded every 6 h. Surface watero

Ž 2 .runoff was recorded at two plots area 20 m automatically by using dipping buckets.All data were recorded on Campbell CR10 data loggers.

2.3. Laboratory measurements

The selected soil samples were analysed for properties that affect nutrient and wateravailability to the plants and erosion resistance. The particle-size distribution of the

Ž-2-mm fraction was determined by the Bouyoucos hydrometer method Gee and.Bauder, 1986 . The organic carbon content was measured using the modified Walkley–

Ž .Black wet oxidation procedure Nelson and Sommers, 1982 . pH was measured in aŽ .soilrwater ratio 1:1 McLean, 1982 . CEC was measured after extraction with ammo-

Ž .nium acetate Rhoades, 1982 . Exchangeable sodium and potassium were determinedŽ . Ž .after extraction with ammonium acetate pHs8.5 Thomas, 1982 . Water retention at

y33 kPa was measured in undisturbed soil samples and at y1500 kPa in disturbedsamples. The volume of available water to plants was then determined as the difference

Ž .between water retained at y33 and at y1500 kPa Klute, 1986 . The wet sievingŽ .technique of Yoder 1936 was used for the determination of the mean soil weight

diameter of the soil aggregates.

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3. Results and discussion

3.1. Land use change effects on soil properties

Textural analysis showed that soils formed on ignimbrite had the highest average clayŽ .content 20.0%, standard deviation, SDs9.7% . Soils formed on volcanic lava and

Žpyroclastics had slightly lower clay contents 17.8%, SDs5.9%; 17.1%, SDs6.3%,.respectively than soils formed on ignimbrite. The lowest clay content was measured in

Ž .soils formed on schist-marble 12.4%, SDs5.3% . A multiple comparison analysis ofthe clay content among the soils formed on various parent materials showed that only

Ž .soils formed on schist-marble are significantly different at the 0.05 level . The highestŽ .sand content was measured in soils formed on pyroclastics 63.7%, SDs6.6% , while

on other parent materials sand content averaged 49.7% to 55.5%. As was expected, nostatistically significant differences have been detected between the abandoned andcultivated soils studied. While no great ranges in clay and sand content have been foundin the various study sites, the present vegetation cover was greatly affected by the parent

Ž .material Table 1 . Soils formed on shale had generally higher vegetation cover ascompared to soils formed on pyroclastics.

Ž .The soils formed on schist-marble were moderately alkaline average pHs7.2 ,Ž .while all the other soils were slightly acid Table 2 . The acidity of the cultivated soils

was slightly lower than that of the abandoned soils formed on the same parent materials,but not statistically significantly different at the 0.05 level. This is mainly attributed tothe distribution of the cultivated soils in the landscape. They are usually found at loweraltitude favouring cation enrichment by overland or sub-surface water flow from thesurrounding areas.

The abandonment of the land has greatly increased soil organic matter content andsoil aggregate stability, especially for the soils formed on lava and schist-marble, as

Ž .compared to the soils under cultivation Table 2 . The greatest change in organic mattercontent was measured in soils formed on volcanic lava. As Table 2 shows, the cultivated

Ž .soils formed on lava had the lowest organic matter content 0.92% under the existingcultivation practices and climatic conditions of the study area. This may be attributed tofrequent disturbance of these soils by occasionally removing rock fragments andboulders to aid mechanical cultivation. The removal of boulders from the fields ismainly achieved by using bulldozers resulting in mixing of the A-horizon with sub-surface soil materials.

Soils formed on shale did not show significant differences in organic matter contentŽ .after abandonment Table 2 . This may be due to the physical structure and mineralogy

of the bedrock. The existence of easily weathered, layered silicates, feldspars, mica, andhydrated mica favours the partial weathering of the consolidated rock and the adsorptionof water, permitting root penetration from the growing plants along fracture planes ofthe bedrock. Therefore, plant roots can penetrate deeper than just through the shallowsoil, and the organic matter is more uniformly distributed in deeper layers. Measure-ments of organic matter in sub-surface layers of selected sites showed that the highest

Ž .content was measured in soils formed on shale means0.78% , followed by soils

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Table 2Ž .Mean and standard deviation SD of soil properties measured in abandoned or cultivated areas with soil formed on various parent materials

Soil Property Land use Soil formed on

Lava Schist-marble Shale Ignimbrite Pyroclasticsa Ž . Ž .n s16 n s8 n s17 n s7 n s16 n s8 n s17 SD n s17 SDc a c a c a c c

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .mean SD mean SD mean SD mean mean

pH abandoned 6.0 1.52 7.23 1.29 6.38 1.53 6.12 0.94 6.14 0.82cultivated 6.12 0.71 7.68 0.59 6.80 1.29

Ž .Organic matter % abandoned 2.36 1.19 3.00 1.10 2.39 1.76 2.87 1.01 2.10 0.57cultivated 0.92 0.48 1.87 0.97 2.25 1.01

Ž .CEC cmolrkg abandoned 14.1 8.8 12.7 10.6 19.3 4.10 19.9 6.70 16.1 7.30cultivated 15.5 5.6 14.1 3.40 16.6 3.80

Exchangeable Na abandoned 48.5 15.3 41.8 9.40 29.1 9.40 57.3 24.7 73.9 17.6cultivated 60.2 27.1 65.1 16.5 42.9 10.5

Exchangeable K abandoned 218.7 105.8 127.2 52.9 128.6 48.5 219.4 61.7 185.4 64.5cultivated 277.5 70.5 265.5 119.1 177.7 79.4

Ž .Aggregate size mm abandoned 6.70 1.10 5.70 3.30 5.90 2.80 6.6 2.2 3.00 2.10cultivated 1.40 0.90 0.50 0.50 0.80 0.50

a n , n snumber of measurements in abandoned and cultivated sites.c a

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Ž . Ž . Ž .formed on schist-marble 0.65% , ignimbrite 0.54% , pyroclastics 0.45% , and lavaŽ .0.42% .

Soils formed on shale have the greatest ability to support perennial or annualvegetation compared to the other parent materials studied in the area due to their high

Ž .WHC Fig. 2 and water storage capacity in the dense network of fracture planes in thebedrock. The higher potential of these soils to support natural vegetation or agriculturalcrops resulted in relatively small differences in the average values of organic mattercontent in abandoned and cultivated soils under the prevailing climatic conditions of thearea. Field observations of root distribution in soil profiles showed that roots of plantsgrowing in soils formed on lava, ignimbrite, and pyroclastics are highly restricted in theexisting shallow soil, significantly increasing the organic matter content in the A-horizonafter abandonment. Root growth into these parent materials is sometimes favoured byexisting cracks or dikes but they do not penetrate the consolidated, hardly weathered,parent material.

All the soils under cultivation had an average aggregate size ranging from 0.5 mm to1.4 mm, independent of the parent material. Abandoned soils formed on pyroclastics had

Ž .the lowest aggregate size 3.0 mm, Table 2 . Soils formed on the other parent materialshad higher average values of aggregate size in the A-horizon, ranging from 5.7 to 6.7

Ž .mm Table 2 . Therefore, aggregate stability was greatly affected by the abandonment ofarable land.

Erosion data indicated that soils formed on pyroclastics usually had the highestdegree of erosion, in agreement with the aggregate stability data. As Fig. 3 shows, 84%of the study soils formed on pyroclastics were severely to very severely eroded. In

Fig. 2. Mean values of WHC of soils formed on various parent materials.

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Fig. 3. Distribution of the various degrees of erosion assessed for sites on different parent materialsŽ .WEsslight erosion, MEsmoderate erosion, SEssevere erosion, VSEs very severe erosion .

contrast, the majority of the soils formed on the other parent materials were moderatelyŽ .eroded Fig. 3 . Of course, other parameters such as vegetation, topography, and

intensity of land use have affected soil erosion, affecting the results in Fig. 3.Ž .The difference in CEC was not statistically significant at the 0.05 level between the

Ž .abandoned and cultivated soils formed on the same parent material Table 2 . Themoderate increase in organic matter content measured in the abandoned soils did notsignificantly affect CEC. Abandoned soils formed on ignimbrite rocks or shale depositsŽ . ŽTable 2 had the highest values of CEC in the A-horizon 19.9 and 19.3 cmolrkg soil,

.respectively . Soils formed on volcanic lava, pyroclastics and schist-marble had loweraverage values, ranging from 12.7 to 16.3 cmolrkg soil.

ŽExchangeable Na can affect aggregate stability and therefore soil erodibility Imeson,.1995 . The obtained data indicated that exchangeable sodium was lower in the aban-

Ž .doned soils than in soils under cultivation formed on the same parent material Table 2 .The highest average amount of exchangeable Na was measured in soils formed onpyroclastics. This can be attributed to the restricted leaching of cations occurring inthese soils due to the almost impermeable underlying bedrock. In all cases, the amountof exchangeable Na was less than 3% of the CEC. This value is not expected to have

Žany significant effect on clay dispersion and reduction of aggregate stability Hillel,. Ž . Ž .1980 . As Lavee et al. 1991 , and Imeson 1995 pointed out, below a rainfall of about

300 mmryear, soil degradation is related more to the dynamics of sodium than toorganic matter content, while organic matter dynamics dominate over sodium effectswhere annual rainfall exceeds about 600 mm. The obtained data indicated that organicmatter content dynamics are more important in the study soils than sodium content,under the climatic conditions of the study area.

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Similar trends to sodium were found for exchangeable potassium. Generally, culti-vated soils had higher amounts of K as compared to the corresponding abandoned soilsŽ .Table 2 due to the addition of K-fertilisers. Cultivated soils formed on schist-marblecontained the highest amount of K.

3.2. Threshold soil characteristics for changing land use

Semi-arid landscapes such as this study area are water-limited by definition andtherefore are potentially sensitive to environmental change and to reduced plant growth.

Ž .The potential evapotranspiration ETo , estimated by meteorological data measured attwo stations, Vatousa and Antissa, was 1206 and 1533 mm during the period from May1996 to May 1997, while the total precipitation during that period was 502 and 526 mm,respectively, indicating a high water deficit for the growing plants in the study area.

The water storage capacity of a soil is defined by the WHC of each soil layer.According to the average values of WHC, the study soils can be classified into the

Ž .following two groups: a soils formed on pyroclastics, lava, and ignimbrite with aŽ . Ž .relatively low WHC 13.1–14.8 mm of water per 10 cm soil, Fig. 2 , and b soils

Žformed on schist-marble, and shale with a relatively high WHC 22.1–22.7 mm of water.per 10 cm soil . The presence of cracks in the bedrock of lava, ignimbrite, schist-marble

and fractures in shale increases soil water storage capacity allowing the plant roots topenetrate them and to absorb the water. The efficiency of bedrock to support vegetationwith water stored in cracks or fractures depends on the extent to which these features arepresent into the bedrock. In addition, the presence of rock fragments on the soil surfacein these soils restricts soil water evaporation. Therefore, the establishment of naturalvegetation in the study area after abandonment was highly favoured by soil propertiesincreasing water storage capacity such as parent material, soil depth, stoniness, presenceof cracks, and degree of bedrock weathering.

As Fig. 4 shows, there is a minimum soil depth, depending on the parent material,below which no shrubby vegetation can be grown under the prevailing climaticconditions in the study area. The diameter of shrubs, such as Sarcopoterium sp., wasreduced as the soil depth was diminished down to a minimum value at a full stage ofplant development. The rate of reduction in diameter accompanied by reduction invegetation cover of these soils was greater on soils formed on pyroclastics than on theother parent materials. Under the existing soil and climatic conditions, a minimumdiameter of about 24–25 cm of the growing shrubs of Sarcopoterium has been measured

Ž .in the study sites Fig. 4 . Therefore, no data appear in Fig. 4 for Sarcopoterium ofdiameters less than 25 cm.

In this study, two soil depths very important for land protection have been distin-guished, the critical and the crucial depth. The critical depth has been defined as thedepth below which the recovery of the natural perennial vegetation is very low andwhere the erosional processes may be very active, resulting in further degradation anddesertification of the land. Crucial depth has been defined as the soil depth below whichperennial vegetation cannot be supported and where the remaining soil is rapidly washedaway by wind or water erosion. The importance of these two depths for land protectionbecomes apparent in the following discussion.

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Fig. 4. Relationship of the average diameter of Sarcopoterium sp. shrubs and soil depth measured inabandoned soils formed on pyroclastics.

As Fig. 5 shows, there is a crucial minimum depth of 10 cm for soils formed onpyroclastics for perennial vegetation to grow in the area. In contrast, soils formed on

Fig. 5. Relationship of percentage vegetation cover of Sarcopoterium sp. shrubs and soil depth measured inareas with soils formed on pyroclastics and schist-marble.

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Fig. 6. Relationship of percentage vegetation cover of Sarcopoterium sp. shrubs and soil depth measured inareas with soils formed on volcanic lava and ignimbrite.

schist-marble, lava and ignimbrite have a greater vegetation cover for the same soilŽ .depth Figs. 5 and 6 . The measured crucial soil depth for these soils was around 4–5

cm. The rate of the area of increase of plant cover with increasing soil depth was higherin these soils than in the soils formed on pyroclastics.

If soil depth is less than 10 cm in landscapes with pyroclastics, then only someannual plant species can survive under the prevailing climatic conditions of this areaŽ .Fig. 7 and erosion is high. Twelve runoff events have been measured in the study siteswith pyroclastics ranging from 0.1 to 3.4 mm during the period from May 1997 to May1998. The area of bedrock exposed in the study spots during that runoff events rangedfrom 3.3 to 12.5% of the initial area. Soil has been washed out by a combination of

Ž .sub-surface above impermeable bedrock and surface water flow and the bedrock hasbeen exposed on the surface.

In addition to the crucial soil depth of 4–10 cm, the critical soil depth is veryŽ .important for supporting plant cover by perennial shrubs Fig. 7 . A value of 40%

vegetation cover is considered critical, below which accelerated erosion dominates onŽ .sloping land Thornes, 1988 . If the vegetation cover covers a greater area than 40%,

then it will act as a factor of resilience or protection of the land. The data obtainedshows that areas with soils formed on schist-marbles and having a depth of at least 25cm had more than 80% plant cover, while areas with soils formed on pyroclastics had

Ž .about half of the plant cover for the same soil depth and climatic conditions Fig. 5 .Therefore, the nature of the parent material becomes important in vegetation establish-ment and land protection as soil depth is reduced due to erosion.

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Fig. 7. Abandoned area with soil formed on pyroclastics showing distinct patches with different vegetationŽcover due to change in soil depth patch with soil having the critical depth is mainly covered with

Sarcopoterium sp., while patches with soil having the crucial depth are partially covered only with annual.vegetation .

Fig. 8. Relationship of soil depth and thickness of A-horizon developed under Sarcopoterium sp. shrubs insoils formed on pyroclastics.

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The thickness of the A-horizon in the study area was also related to soil depth. Underthe existing climatic conditions, the thickness of the A-horizon of soils formed onpyroclastics increased linearly with increasing soil depth until a maximum value of

Ž .about 30 cm and then remained almost constant with increasing soil depth Fig. 8 . SoilsŽ .with a deep A-horizon had a good vegetation cover greater than 65% and were

relatively well-protected from erosion. The potential for biomass production of thesesoils is high and they must be considered as having low risk for degradation under thepresent land use and management practice. Similar trends of soil depth and thickness ofthe A-horizon were found for soils formed on the other parent materials, with themaximum thickness of the A-horizon ranging from 28 to 33 cm. It can be inferred fromthe above data that a depth of 25–30 cm must be considered as a key indicator forabandonment of hilly cultivated areas. Under the existing soil and climate conditions ofthe study area, this depth can support a natural vegetation cover sufficient to protect theland. It has to be emphasised that this critical depth can be used for land use planningand protection of the environment only under the prevailing soil and climate character-istics of the area. Generalisation of this conclusion may be risky for other areas.

4. Conclusions

The following conclusions can be inferred from the obtained data on land abandon-ment and protection from desertification. The abandonment of the land may lead todeteriorating or improving conditions of plant cover depending on the soil and climatecharacteristics of the area. Soil fertility status may slightly increase after abandonment.The most significant soil improvement after a period of 40–45 years of abandonment isrelated to the increase in organic matter content and aggregate stability of the A-horizon.Soils formed on pyroclastics have lower capacity to regenerate natural vegetation, andthis leads to higher erosion rates. Soils formed on shale, ignimbrite, schist-marble, andvolcanic lava have a higher capacity for at least partial regeneration of naturalvegetation. The nature of the parent material becomes increasingly important in vegeta-tion establishment and land protection as soil depth is reduced due to erosion. Under thesoil and climatic conditions of the study area, a cultivated hilly landscape must beabandoned before the soil is depleted to a critical depth of 25–30 cm. The recovery ofthe natural vegetation is very low in soils of lesser depth, and erosional processes maybe very active resulting in further degradation and desertification of the land. If a soil iseroded to a depth of about 10 cm or less, depending on the parent material, then theperennial vegetation cannot be supported and the remaining soil is rapidly removed bywind or water erosion. Degradation and desertification of this land is ultimately anirreversible process.

Acknowledgements

This work was partially financed by the European Union Project ‘‘MediterraneanŽ .Desertification and Land Use’’, MEDALUS III contract number, ENV4 CT95-0119 .

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References

Aweto, A.O., 1981. Organic build-up in fallow soil in a part of south-western Nigeria and its effects on soilproperties. Journal of Biogeography 8, 67–74.

Clark, S.C., 1996. Mediterranean ecology and an ecological synthesis of the field sites. In: Brandt, J., Thornes,Ž .J. Eds. , Mediterranean Desertification and Land Use. Wiley, pp. 271–299.

Ž .Dell, B., Hopkins, J.M., Lamont, B.B., 1986. Introduction. In: Dell, B., Hopkins, A.J.M., Lamont, B.B. Eds. ,Resilience in Mediterranean Ecosystems. Dr. W. Junk Publishers, Dordrecht, Netherlands, pp. 1–3.

Ekanade, O., 1989. An assessment of temporal variation of soil properties under cocoa and fallow towardincreasing cocoa production in Nigeria. Soil Technology 2, 171–184.

Elwell, H.A., Stocking, M.A., 1976. Vegetal cover to estimate soil erosion hazard in Rhodesia. Geoderma 15,61–70.

Fox, B.J., Fox, M.D., 1986. Resilience of animal and plant communities to human disturbance. In: Dell, B.,Ž .Hopkins, A.J.M., Lamont, B.B. Eds. , Resilience in Mediterranean Ecosystems. Dr. W. Junk Publishers,

Dordrecht, Netherlands, pp. 39–64.Francis, C.F., Thornes, J.B., 1990. Runoff hydrographs from three Mediterranean vegetation cover types. In:

Ž .Thornes, J.B. Ed. , Vegetation and Erosion, Processes and Environments. Wiley, Chichester, pp. 363–384.Garcia Ruiz, J.M., 1991. Consecuencias amientales del abandono agricola. Communitations to the Desertifica-

Ž .tion Course. Universidad Internacional Menendez Pelayo, Nalencia policopiado .Ž .Gee, G.W., Bauder, J.W., 1986. Particle size analysis. In: Klute, A. Ed. , Methods of Soil Analyses, Part 2,

2nd edn. Agronomy Monograph 9. ASA and SSSA, Madison WI, pp. 383–411.Greenland, D.J., Nye, P.H., 1959. Increase in the carbon and nitrogen contents of tropical soils under natural

fallows. Journal of Soil Science 10, 284–299.Grove, A.T., Rackham, O., 1996. Physical, biological and human aspects of environmental change. In:

MEDALUS II Project 3, Final Report, Managing Desertification. EV5V-CT92-0165, pp. 39–64.Ž .Hillel, D., 1980. Movement of solutes and soil salinity. In: Hillel, D. Ed. , Fundamentals of Soil Physics.

Academic Press, New York, pp. 233–257.Imeson, A., 1995. The physical, chemical and biological degradation of the soil. In: Fantechi, E., Denis, D.P.,

Ž .Balabanis, P., Rubio, J.I. Eds. , Desertification in a European Context: Physical and Socio-EconomicAspects. European Commission, Directorate-General Science, Research and Development, EUR 151415EN,pp. 153–168.

Jaiyeoba, I.A., 1988. Build-up of organic matter and nutrients under fallow in a tropical rainforest environ-ment, Nigeria. Malaysian Journal of Tropical Geography 18, 10–16.

Jaiyeoba, I.A., 1995. Changes in soil properties related to different land uses in part of the Nigerian semi-aridSavannah. Soil Land Use and Management J. 11, 84–89.

Ž .Klute, A., 1986. Water retention: laboratory methods. In: Klute, A. Ed. , Methods of Soil Analyses, Part 2,2nd edn. ASA, SSSA Agronomy Monograph, Vol. 9, pp. 635–653.

Kosmas, C., Danalatos, N., Moustakas, N., Tsatiris, B., Kallianou, Ch., Yassoglou, N., 1993. The impacts ofparent material and landscape position on drought and biomass production of wheat under semi-aridconditions. Soil Technology 6, 337–349.

Kosmas, C., Danalatos, N., Cammeraat, L.H., Chabart, M., Diamantopoulos, J., Farand, R., Gutierrez, L.,Jacob, A., Marques, H., Martinez-Fernandez, J., Mizara, A., Moustakas, N., Nicolau, J.M., Oliveros, C.,Pinna, G., Puddu, R., Puigdefabregas, J., Roxo, M., Simao, A., Stamou, G., Tomasi, N., Usai, D., Vacca,A., 1997. The effect of land use on runoff and soil erosion rates under Mediterranean conditions. Catena29, 45–59.

Kosmas, C., Danalatos, N., Gerontidis, St., 1998. The Effect of Land Parameters on Vegetation PerformanceŽ .and Degree of Erosion under Mediterranean Conditions submitted for publication to Catena .

Lavee, H., Imeson, A.C., Parientes, S., Benjamini, Y., 1991. The response of soils to simulated rainfall along aclimatological gradient in the arid and semi-arid region. Catena Supplement 19, 19–37.

Lee, C.R., Skogerboe, J.G., 1985. Quantification of erosion control by vegetation on problem soils. In: Swaify,Ž .Al., Moldenhauer, W.C., Lo, A. Eds. , Soil Erosion and Conservation. Soil Conservation Soc. of America,

pp. 437–444.Lopez-Bermudez, F., Romero-Diaz, A., Martinez-Fernandez, J., Martinez-Fernandez, J., 1996. The El Ardal´ ´

( )C. Kosmas et al.rCatena 40 2000 51–6868

Ž .field site: soil and vegetation cover. In: Brandt, J., Thornes, J. Eds. , Mediterranean Desertification andLand Use. Wiley, pp. 169–188.

Martinez-Fernandez, J., Lopez-Bermudez, F., Martinez-Fernandez, J., Romezo-Diaz, A., 1995. Land use andsoil-vegetation relationships in a Mediterranean ecosystem: El Ardal Murcia, Spain. Catena 25, 153–167.

Martinez-Fernandez, J., Martinez-Fernandez, J., Lopez-Bermudez, F., Romero-Diaz, M.A., Belmonte-Serrato,F., 1996. Evolution of vegetation and pedological characteristics in fields with different age of abandon-

Ž .ment: a case study in Murcia Spain . Soil degradation and desertification in Mediterranean environments.Geoforma Ediciones, pp. 279–290.

Ž .McLean, E., 1982. Soil pH and lime requirement. In: Page, A.L. Ed. , Methods of Soil Analysis, Part 2,Chemical and Microbiological Properties. Agron. 9. ASA, SSSA. Madison, WI, USA, pp. 199–223.

Nelson, D.W., Sommers, L.E., 1982. Total carbon, organic carbon, and organic matter. In: Page, A.L., et al.Ž .Eds. , Methods of Soil Analysis, Part 2, 2nd edn. Agronomy Monograph 9. ASA and SSSA, Madison,WI, pp. 539–579.

Puigdefabregas, J., Alonso, J.M., Delgado, L., Domingo, F., Cueto, M., Gutierrez, L. Lazaro, R., Nicolau,M.J., Sanchez, G., Sole, A., Vidal, S., Aguilera, C., Brenner, A., Clark, S., Incoll, L., 1996. The RamblaHonda field site: Interactions of soil and vegetation along a catena in semi-arid southeast Spain. In: Brandt,

Ž .J., Thornes, J. Eds. , Mediterranean Desertification and Land Use. Wiley, pp. 137–168.Ž .Rhoades, J.D., 1982. Cation exchange capacity. In: Page, A.L. Ed. , Methods of Soil Analysis, Part 2,

Chemical and Microbiological Properties, 2nd edn. Agronomy 9. ASA, SSSA. Madison, WI, USA, pp.149–157.

Roxo, M.J., Cortesao Casimiro, P., Soeiro de Brito, R., 1996. Inner Lower Alentejo field site: cereal cropping,Ž .soil degradation and desertification. In: Brandt, J., Thornes, J. Eds. , Mediterranean Desertification and

Land Use. Wiley, pp. 111–135.Ruhe, R.V., 1960. Elements of soil landscape. Trans. Int. Congr. Soil Sci., 7th, Vol. 4, 165–170.

Ž .Thomas, G.W., 1982. Exchangeable cations. In: Page, A.L. Ed. , Methods of Soil Analysis, Part 2, Chemicaland Microbiological Properties, 2nd edn. Agronomy 9. ASA, SSSA. Madison, WI, USA.

Ž .Thornes, J.B., 1988. Erosional equilibria under grazing. In: Bintliff, J., Davidson, D., Grant, E. Eds. ,Conceptual Issues in Environmental Archaeology, Edinburgh University Press, pp. 193–210.

Trimble, S.W., 1990. Geomorphic effects of vegetation cover and management: some time and spaceŽ .considerations in prediction of erosion and sediment yield. In: Thornes, J.B. Ed. , Vegetation and Erosion

Processes and Environments. Wiley, pp. 55–66.Unger, P.W., 1997. Management-induced aggregation and organic carbon concentrations in the surface layer

of a Torrertic Paleustoll. Soil and Tillage Research 42, 185–208.Oostwoud Wijdenes, D.J., Poesen, J., Vandekerckhove, L., Kosmas, C., 1998. The use of tracers to determine

Ž .transport of rock fragments by sheep trampling on Lesvos, Greece to be submitted for publication .Soil Survey Staff, 1975. Soil taxonomy: a basic system of soil classification for making and interpreting soil

surveys. USDA-SCS Agric. Handb. 436. U.S. Gov. Print. Office, Washington, DC.Ž .Westman, W.E., 1986. Resilience: concepts and measures. In: Dell, B., Hopkins, A.J.M., Lamont, B.B. Eds. ,

Resilience in Mediterranean Ecosystems. Dr. W. Junk Publishers, Dordrecht, Netherlands, pp. 5–19.Yoder, R.E., 1936. A direct method of aggregate analysis and a study of the physical nature of erosion losses.

J. Am. Soc. Agron. 28, 337–351.