the effect of land parameters on vegetation performance and … · 2009-07-17 · land parameters,...

Ž .Catena 40 2000 3–17www.elsevier.comrlocatercatena

The effect of land parameters on vegetationperformance and degree of erosion under

Mediterranean conditions

C. Kosmas a,), N.G. Danalatos b, St. Gerontidis a

a Agricultural UniÕersity of Athens, Laboratory of Soils and Agricultural Chemistry, Iera Odos 75, Athens11855, Greece

b Department of Agriculture, Crop and Animal Production, UniÕersity of Thessaly, Pedion Areos, 38334Volos, Greece

Abstract

Important land parameters such as soil texture, soil depth, topography, parent material, andclimatic conditions were studied in a semi-detailed soil survey conducted in the island of Lesvos,and were related to the vegetation performance and degree of erosion. Rainfall data indicated that

Ž .the island could be divided into two major climatic zones: a semi-arid western part and a dryŽ .sub-humid eastern part . A 45% rainfall gradient has been recorded in the above two climatic

zones. The great reduction in rainfall combined with the higher evapotranspiraton demands in thesemi-arid zone has significantly affected vegetation performance and degree of erosion in the area.Due to the general lack of available water, the semi-arid part of the island is dominated by poormaquis vegetation, while olive trees, oak and pine forests prevail in the dry sub-humid part undersimilar topographical and geomorphological conditions. Vegetation cover increases with increas-ing rainfall and soil depth. Slope grade has a variable effect on soil erosion in the various climaticzones. Erosion is decreasing with increasing rainfall for the same slope classes. Severely erodedsoils prevail in the semi-arid zone with slopes greater than 12%, while slightly to moderatelyeroded soils are found in the dry sub-humid zone under similar slope classes. The highest rates ofland degradation have been measured in areas with soils formed on pyroclastic formations. Areaswith soils formed on basic igneous rocks, shale and lava are well vegetated and protected from

) Corresponding author

0341-8162r00r$20.00 q 2000 Published by Elsevier Science B.V. All rights reserved.Ž .PII: S0341-8162 99 00061-2

( )C. Kosmas et al.rCatena 40 2000 3–174

erosion under a broad range of slopes and weather conditions. q 2000 Published by ElsevierScience B.V. All rights reserved.

Keywords: Parent material; Erosion; Vegetation; Mediterranean climatic conditions

1. Introduction

The Mediterranean region has experienced a serious decline in environmental re-Ž .sources resulting in land degradation Grove, 1996; Thornes, 1996 . Since ancient times,

the extensive deforestation and intensive cultivation of the sloping lands has already ledto soil erosion and degradation through the progressive inability of the vegetation and

Ž .soils to regenerate themselves Kosmas and Danalatos, 1993 . Mediterranean landscapesare characterised by fragile natural ecosystems, long-period human exploitation andinsufficient rainfall for fast vegetation recovery after each degrading event. Soil erosionby water is the main process leading to land degradation under these conditions.

The rate of soil degradation is dependent upon the rate of vegetation degradation,Ž .which in turn is influenced by both adverse climate Langbein and Schumm, 1958 and

Žland use management changes Newson, 1985; Bryan and Campbell, 1986; Grainger,.1992; Kosmas et al., 1997 . Vegetation cover plays a key role on land degradation

Ž .Francis and Thornes, 1990 , and in fact, reduction in the perennial cover is regarded asŽ .an indicator of the onset of desertification Thornes, 1996 . Natural and semi-natural

ecosystems dominated by evergreen sclerophyllic species are well adapted to Mediter-ranean climate and landscape conditions. Extreme climatic conditions and human actioncan cause disequilibrium of these systems. Vegetation cover may be altered radically byMan within a short time, but physical and biological changes within the soil, affecting

Žerosion rates, may take longer periods Ruiz-Flano et al., 1992; Martinez-Fernandez et.al., 1995, 1996 .

The Mediterranean region is characterised by a seasonal climate and specific ecologi-cal and pedological conditions which make its ecosystems the most vulnerable in Europedue to the fragility of the environment and the low rates of biomass production during

Ž .dry periods. Lavee et al. 1991 have shown that infiltration rates become lower and thesoil erodibility increases as climatic conditions become drier. Adverse climatic condi-tions, irregular terrain with steep slopes, parent material and long periods of land misuseare the main factors responsible for land degradation in the Mediterranean.

Soils derived from different parent materials react differently to vegetation, soilŽ .erosion and desertification Yassoglou and Kosmas, 1990 . For example, limestones

Žproduce shallow soils with a relatively dry moisture regime Kosmas and Danalatos,.1993 , characterized by high erodibility and slow vegetation recovery. Many Mediter-

ranean landscapes on limestone are desertified with the soil mantle eroded and thevegetation completely removed. Similarly, acid igneous materials produce shallow soilswith high erodibility and desertification risk.

Soils on stable landscape surfaces and under good plant cover conditions mayimprove with time by accumulating organic material, increasing floral and faunalactivity, enhancing soil aggregate stability, increasing infiltration capacity, and decreas-

( )C. Kosmas et al.rCatena 40 2000 3–17 5

Ž . Ž .ing erosion potential Trimble, 1990 . Soeiro de Brito et al. 1993 demonstrated thatsoil erosion in hilly areas of Portugal is markedly greater under wheat or even underfallow, than in abandoned fields. Furthermore, natural vegetation such as Cistus maquis

Ž .is more protective of the soil than wheat Kosmas et al., 1997 . However, the dominanceof this over other annual species is responsible for the absence of low bushes leavinglarge patches of bare soil.

Considering the above, the objective of the present work is to evaluate the effect ofland parameters, such as soil texture, soil depth, parent material, topography and climateon vegetation performance and degree of erosion in an extensive Mediterranean area. As

Ž .a typical example, the island of Lesvos eastern Greece is chosen, characterized by alarge diversity in these land parameters and great susceptibility to environmentaldegradation.

2. Materials and methods

2.1. The study area

Ž .Lesvos island Fig. 1 is characterized by a variety of landscapes, lithological unitsand climatic conditions, and is under a number of land-uses representative of theMediterranean region, e.g., semi-natural forests and shrubland, and agricultural landwhich is largely being abandoned. The island is situated within the Mediterraneanclimatic zone with great variations in climatic conditions due to the regional effect ofmountains and atmospheric circulation patterns. Lesvos is to a great part already badlydegraded and desertified, and the rest is undergoing a slow but constant deterioration ofits natural resources.

2.2. Mapping of soil parameters

Ž .An integrated study on land degradation in the Lesvos island 163,429 ha wasconducted in the period February 1995–June 1997 by means of a semi-detailed surveyof land parameters. The soil mapping system used by the Greek National Soil Survey

Žwas adopted, modified for the local conditions and the purpose of the study Yassoglou.et al., 1982 . This system is based on soil texture, depth to bedrock, degree and direction

of soil development, topography, erosion of the soil surface, and parent material. TheseŽland parameters were studied in a dense network of 13,600 field observations soil auger

holes and road cuts to the depth of bedrock or to a maximum depth of 150 cm in case of. Ž .deep soils , and were recorded on each mapping unit Fig. 2 . The boundaries of the

Ž .mapping units were drawn on six topographic maps scale 1:50,000 supplied by theGreek Army Geographical Service.

ŽIn particular, the soil textural classes were defined according the USDA system Soil.Survey Staff, 1975 , and were grouped into the following textural classes: very coarse

Ž . Ž . Ž . Ž .S, LS , coarse SL , medium L, SiL, Si , moderately fine SCL, CL, SiCL and fineŽ .SC, C, SiC . The parent material was defined according to the geological map of the

Ž .area scale 1:50,000 supplied by the Greek National Institute of Geology and Mineral


ŽFig. 1. Location and climatic zones of the Lesvos island meteorological stations and rain gauges locations are.indicated .

Ž .Exploitation IGME . The main parent materials mapped in the study area were theŽ . Žfollowing: a acid igneous rocks volcanic lava, vitrophyric lava, pyroclastics, ign-

. Ž . Ž . Ž . Ž . Ž .imbrite , b basic igneous rocks peridotite , c metamorphic schist–marble , d

Fig. 2. Mapping unit code used in the soil survey of the island of Lesvos.


Table 1Slope classes and corresponding mapping symbols

Ž .Slope class % Description

0–2 nearly level2–6 gently sloping6–12 moderately sloping12–18 strongly sloping18–25 moderately steep25–35 steep)35 very steep

Ž . Ž .sedimentary shale, marl , and e alluvial deposits. Soil depth to unconsolidated bedrockŽwas measured in the auger holes. The following classes were used: very shallow depth

. Ž . Ž .0–15 cm , shallow 15–30 cm , moderately shallow 30–50 cm , moderately deepŽ . Ž . Ž . Ž .50–100 , deep 100–150 cm and very deep )150 cm . Slope gradient Table 1 wasdescribed using the topographic maps. The soils were classified according to the Soil

Ž . Ž .Taxonomy Soil Survey Staff, 1975 into three broad categories Soil Orders , viz.Entisols, Inceptisols, and Alfisols.

The degree of erosion was assessed qualitatively during soil survey. In all mappingŽ .units, the present situation of erosion was characterized according to a the presence of

Ž . Ž .an A-horizon; b the percentage of eroded areas; c the degree of exposure of theŽ . Ž .parent material on the soil surface; and d the presence of erosion gullies Table 2 .

Subsequently, the erosion degree was quantitatively assessed in most mapping units, bymeasuring the length of current distinct erosion areas and rock outcrops along catenas.In each measurement site, three 30-m-long transects were chosen perpendicular to thecontour lines, and the length of the erosion areas and the rock outcrops were measured.As distinct erosion areas were defined areas in which current erosion features could beeasily distinguished, such as washing away or deposition of soil materials. The degree of

Table 2Erosion classes and description

Erosion class Description

No no erosion features are presentSlight Parts of the A horizon have been eroded, so that usually less than 20 of the initial A

horizon present current scattered spots of erosion.Moderate Soils that present an intricate pattern of current spots of erosion ranging on the

average from 20 to 50% on the original A horizon.Severe Soils that shows an intricate pattern of eroded spots ranging from 50% to 80%

of the original A horizon. In most areas of class 3 erosion, the parent materialis exposed at the surface.

Very severe Soils that have lost more than 80% of the A-horizon and some or all the deeperhorizons throughout most of the area. Original soil can be identified only in spots.Some areas may be smooth, but most have an intricate pattern of gulliesand the parent material is exposed at the soil surface.


erosion was then assessed by the ratio of the total length of the eroded areas plus rockŽoutcrops over the total length of the study catena, expressed on a percentage basis Table

.2 .

2.3. Mapping of Õegetation

Vegetation was defined on the basis of the dominant species such as maquisŽ . ŽSarcopoterium spinosum, S. Õerucosum , shrubs Quercus sp., Pyrus amygdaliformis,

. Ž .Pistacia sp., etc. , olive trees, pine trees Pinus nigra, P. brutia , deciduous oakŽ . Ž .Quercus pubescens, Q. aegilops, etc. , evergreen oaks Q. coccifera , annuals and bareland. The percentage cover was defined in classes according to its relation to soil erosionand land degradation as determined from aerial photo-interpretation and ground data at ascale of 1:30,000. The following classes of plant cover were used: 0–5%, 5–25%,25–50%, 50–75%, 75–90% and 90–100%.

2.4. Climate characteristics

Long-term daily weather records were supplied by the meteorological station ofŽMytilene, the capital of Lesvos, representing the eastern part of the island 45-years

.records — Greek National Meteorological Service , and the station of Antissa represent-Žing the western part of the island 29-years records — Agricultural Research Service,

.Greek Ministry of Agriculture .Additionally, three automatic meteorological stations were installed in the western,

Ž .central and eastern part of the island at the beginning of the project Fig. 1 . Airtemperature and wind speed were measured every 15 sec and averaged every hour.Precipitation was measured via an automatic rain gauge at intervals of 5 min. Open

Ž .pan-evaporation rate E was automatically recorded every 6 h. All data were recordedo

Fig. 3. Cumulative amount of rainfall measured in the three climatic zones of Lesvos for 16 monthsŽ .SA ssemi-arid, TRs transtitonal, and SHsdry sub-humid zone .


ŽFig. 4. Dominant vegetation present in the three climatic zones of Lesvos A ssclerophylous, Osolives,.Bsevergreen oak, Qsdeciduous oak, Pspines, Csannual crops .

on Campbell CR10 data loggers. Furthermore, 15-rain gauges were installed in variousŽ .sites throughout the island Fig. 1 , and the amount of rainfall was measured immedi-

ately after each shower for 32 months.

3. Results and discussion

3.1. Climate and Õegetation

The climate of Lesvos is characterized by strong seasonal and spatial variations inrainfall and high oscillations between minimum and maximum daily temperatures. Long

Table 3Ž .Distribution of the various parent materials present in the three climatic zone of the island area in hectares

Parent material Semi-arid Transitional Sub-humid Total

Volcanic lava 6762 10,849 29,968 47,579Vitrophyric lava 0 749 2932 3681Magmatic conglomerates 16,254 2.032 1294 19,580Ignibritic layer 48 180 18,273 18,501Peridotite 0 0 18,459 18,459Marble-schist 2571 1 23,594 26,175Marl 103 11 1677 1791Unconsolidated terraces 0 0 2876 2876Shales 0 0 9576 9576Alluvial deposits 1873 533 11,436 13,842Total 27,611 14,355 120,085 162,051


weather records of the Mytilene meteorological station indicate the existence of a hotand dry summer period from April to October with a mean temperature of 26.18C, and acool-wet winter period from November to March with a mean temperature of 10.48C.During the hot-dry period, rainfall is rare, with an average minimum of 2.7 mm fallingin July, and a total rain of 14.7 mm from June to August. Rainfall is concentrated in thewet-cold period, with a peak of 152.7 mm in December. The average annual rainfallfluctuates from 725 mm in the eastern part to only 415 mm in the western part of theisland. The long-term rainfall records in Mytilene demonstrate a decrease in rainfall forabout 35% in the last 20 years.

Fig. 5. Probability distribution of soil depth classes in various parent materials measured in two climatic zonesof Lesvos.


Rainfall data measured at various sites throughout Lesvos for 32 months indicatedthat the western part of the area received only 65% of the rainfall occurring in the

Ž .central and eastern parts of the island Fig. 3 . The reduced rainfall in western Lesvos ismainly attributed to the high-speed winds blowing in this area, having average velocitiesabout twice as those in the eastern and central part of the island. The existing long-term

Ž .weather data of Mytilene and Antissa Karra, 1986 and the conducted rainfall measure-ments suggest that the island can be divided into two climatic zones, according to

Ž . Ž .FAO-UNESCO 1977 climatic regimes classification system Fig. 1 : a semi-arid zoneŽ . Ž .western part , and a dry sub-humid zone central and eastern part with an annualrainfall of 414 and 725 mm, respectively. A transitional semi-arid zone, about 10-km

Ž .wide, can also be distinguished with an average rainfall of 487 mm Fig. 1 .The great variation in rainfall, occurring at a relatively short distance, can be used for

interrelation of soil characteristics with vegetation establishment and degree of erosion.ŽThe greatly reduced rainfall combined with the higher evapotranspiraton demands 1533

.mm per year, Antissa meteorological station , due to much greater wind speed, hasgreatly affected the vegetation performance in the western part of the island. Due to thelack of available soil water and the moderate grazing, this semi-arid part of Lesvos is

Ž .dominated by poor maquis vegetation 75.9% of this area , while olives and oak treesŽ . Ž . Ž .Quercus sp. cover only l7.1% of the area Fig. 4 . The remaining land 6.7% isconfined to the recent alluvial plains, which are cultivated mainly with annual crops.

Contrarily, the wetter climatic conditions in the dry sub-humid and the transitionalzones, favour the growth of olives, oak and pine forests under topographic andgeomorphology conditions similar to those prevailing in the semi-arid zone. Oliveplantations are widely extended in the hilly areas and cover 39.4% of the dry sub-humidzone. Pine forest exists mainly in the central part, covering about 28.9% of thesub-humid zone. Oak trees combining with other sclerophyllous vegetation occupy12.1% of the sub-humid zone, while annual crops are restricted to the plains.

3.2. Soil and land degradation

3.2.1. Parent material and soil depthSedimentary and volcanic materials are the main geological units in Lesvos. Volcanic

Ž .lava is present in all climatic zones Table 3 covering an area of 47,620 hectares, or29.3% of the island. Pyroclastics are widespread but they are dominant in the western

Table 4Ž .Distribution of soil depth classes in hectares in the climatic zones of Lesvos

Ž .Soil depth cm Semi-arid Transitional Sub-humid Total

0–15 12,499 2 0 12,50115–30 10,523 2867 2355 15,74530–50 927 4776 34,475 40,17850–100 1704 5569 61,924 69,197100–150 83 608 9895 10,586)150 1875 533 11,436 13,844


Fig. 6. Probability distribution of vegetation cover in various soil depth classes measured in three climaticzones of Lesvos.


Ž .part Table 3 covering 58.9% of the semi-arid zone. Narrow alluvial plains are presentin the lower coastal areas, with new deposits supplied by the main streams. Basic

Ž .igneous rocks peridotite , ignimbrite, marble–schist, shale, and unconsolidated terracedeposits are mainly found in the dry sub-humid zone.

Soil texture and soil depth has been greatly affected by parent material and climaticconditions. Great variation in depth and texture were related to the parent material of

Ž .soils in the same climatic zone. As Fig. 5 shows, very deep soils )100 cm of fineŽ .textures clay, silty clay were found in areas with vitrophyric lava and dry sub-humid

climatic conditions. Soils on ultrabasic rocks such as peridotite were moderately deepŽ . Ž .30–50 cm to deep 50–100 cm , with clay, and clay loamy textures. Soils on

Ž . Ž .metamorphic rocks schist–marble were moderately deep to deep Fig. 5 , coarse- tomoderately fine-textured under sub-humid climatic conditions.

As Fig. 5 shows, climate has a great impact on soil depth development through theŽ .processes of weathering and erosion. In the semi-arid zone, very shallow 0–15 cm to

Ž . Ž .shallow 15–30 cm soils mainly occur Table 4 . Contrarily, in the sub-humid zone,Ž . Ž .moderately deep 30–50 cm to deep 50–100 cm soils prevail. The effect of climate on

soil depth becomes more pronounced for soils formed in the same parent material. Thesoils formed in volcanic lava are moderately deep to deep under dry sub-humid

Ž .conditions, whereas shallow soils prevail under semi-arid conditions Fig. 5 . Soils onpyroclastic formations show the highest erosion degree, especially in the semi-arid zone,and very shallow soils were found. In the dry sub-humid zone, shallow to moderatelydeep soils prevail in the same parent material.

3.2.2. Soil depth and plant coÕerA clear correlation between surface cover percentage and soil depth occurs in all

climatic zones. Fig. 6 illustrates the probability distribution of the various classes ofvegetation cover present in the climatic zones of Lesvos. Vegetation cover increaseswith increasing rainfall and soil depth. A progressive increase in vegetation cover was

Ž .measured as the rainfall changed from 415 to 725 mm Fig. 6 . Soils with vegetationcover less than 50% were almost absent in the dry sub-humid zone, whereas soils

Ž .covered by 25–50% were mainly found in the semi-arid zone probability 92.6% .Soil depth has a great effect on vegetation cover, of course in relation to the climate.

Ž .Vegetation cover of 25–50% has the maximum probability of appearance 69.8% in

Table 5Ž .Distribution of slope classes in hectares in the climatic zones of Lesvos

Ž .Slope % Semi-arid Transitional Sub-humid Total

0–2 1873 533 11,863 14,2692–6 754 570 9089 10,4136–12 2062 467 13,060 15,58912–18 1801 1137 9888 12,82618–25 1754 3423 16,239 21,41625–35 1929 1681 13,074 16,684)35 17,439 6544 47,096 71,079


soils of the semi-arid zone ranging in depth from 15 to 30 cm. Contrarily, soils ofsimilar depth in the dry sub-humid zone supported a higher vegetation cover rangingfrom 75–90% to 90–100%, with probability of appearance from 64.2% to 34.4%,respectively. The obtained data indicate that a soil depth of about 30 cm is critical fornatural vegetation performance and soil protection from erosion under semi-arid condi-tions. Soils deeper than 30 cm are well vegetated with a surface cover normally greaterthan 75%. This reflect the great importance of soil depth and climate as key indicatorsfor defining environmentally sensitive areas to degradation and desertification.

Fig. 7. Probability distribution of erosion degree classes under different slope grades in the semi-arid and dryŽsub-humid climatic zones of Lesvos NEsnon eroded, SEsslightly eroded, MEsmoderately eroded,

.SEsseverely eroded, VSEs very severely eroded soils .


3.2.3. Slope grade and erosionŽ .Lesvos is rather hilly with a highest peak at 967 m ASL. Very steep slopes )35%

Ž . Ž .prevail throughout, covering 43.9% of the island Table 5 . Moderately steep 18–25%Ž .to steep 25–35% slopes occupy 13.2% and 10.3% of the area, respectively. Nearly flat

Ž . Ž .to gently sloping soils 0–6% and moderately to strongly sloping soils 6–18% cover15.3% and 17.5% of the area, respectively. A relatively uniform distribution of slope

Ž . Ž .grades was found along the various climatic zones Table 5 . Very steep )35% , steepŽ . Ž .25–35% and moderately steep 18–25% are the major slope classes in the semi-arid,the transitional and the dry sub-humid zones covering 76.4%, 81% and 63.5% of thearea, respectively.

Due to the difference in the amount of rainfall, slope grade has a variable effect onerosion in the different climatic zones. The probability of appearance of high erosion

Ž .degree decreased with increasing rainfall for the same slope classes Fig. 7 . Severelyeroded soils are present in the semi-arid zone with slopes greater than 12%, whereasslightly to moderately eroded soils were found in the sub-humid zone under the sameslope classes. Similar results have been reported by Davidson and TheocharopoulosŽ . Ž .1992 for the semi-arid region in central Greece Viotia , in which severely eroded soilssubstantially dominated in slopes steeper than 18%, whereas only moderately to severelyeroded soils were found and equally distributed in slopes ranging from 12 to 18%. AsFig. 7 shows, the probability of finding severely eroded soils on moderately steep tovery steep slopes was rather high in the semi-arid zone. In contrast, moderately erodedsoils had the highest probability of occurrence under similar slopes in the dry sub-humidzone. In the transitional climatic zone, intermediate conditions for erosion degreebetween semi-arid and dry sub-humid climatic zones were found.

4. Conclusions

The long-term consequences of the interactions of climate, soils and human activityhave greatly affected vegetation performance and degree of erosion on Lesvos island.The distinguished semi-arid zone is badly degraded, characterised by very shallowŽ . Ž .depth 0–15 cm to shallow 15–30 cm , severely to very severely eroded, and poorlyvegetated soils. This area constitutes a critically sensitive area to land degradation.Burning and overgrazing this climatically and topographically marginal zone comprisesa degradation-promoting land use, further deteriorating the existing land resources. Thisarea is very sensitive to low rainfall and extreme events.

The transitional zone is a rather sensitive area to land degradation. Any change to thedelicate balance of climate and human activity is likely to enhance reduction inbiological potential, and further degradation of the remaining vegetative cover resultingin greater erosion rates. This area will be a threat to the environment of the surroundingareas by generating great runoff and sediment losses, causing sedimentation andflooding downstream. Under drier climatic conditions in the future, the already highlydegraded semi-arid zone is expected to expand towards this transitional area.

The central and eastern part of Lesvos climatically comprises a dry sub-humid zone.This area is threatened by higher rates of degradation under significant climate change


and if the existing land use of the well adapted olive trees is replaced and the pine forestis burned. Due to the relative good vegetative cover, the soils in this zone are

Ž . Ž .moderately shallow depth 30–50 cm to moderately deep 50–100 cm , well vegetatedwith olive trees, pine or oak forests, and slightly to moderately eroded.

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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