Methodology for mapping Environmentally Sensitive Areas (ESAs) to desertification

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In book: In 'The Medalus project – Mediterranean desertification and land use. Manual on key indicators of desertification and mapping environmentally sensitive areas to desertification', Publisher: European Commission - Office for Official Pubolications of the European Communities, 1999, Editors: Constantinos Kosmas, Mike Kirkby, Nichola Geeson, pp.31 - 47
Cite this publication
The different types of ESAs to desertification can be analysed in relation to various parameters such as landforms, soil, geology, vegetation, climate, and human action. Each of these parameters is grouped into various uniform classes with respect the its behaviour on desertification and weighting factors are assigned in each class. Then the following four qualities are evaluated (a) soil quality, (b) climate quality, (c ) vegetation quality, and (d) management quality. After the computation of four indices for each quality, the ESAs to desertification are defined by combining them (Fig. 14). All the data defining the four qualities are introduced in a regional geographical information system (GIS), and overlaid in accordance with the developed algorithm and maps of ESAs to desertification are compiled. This approach includes parameters which can be easily found in existing soil, vegetation, and climate reports of an area.
European Commission
Community Research
Project Report
The Medalus project
Mediterranean desertification and
land use
Manual on key indicators of desertification and
mapping environmentally sensitive areas to
EUR 18882
European Commission
The Medalus project
Mediterranean desertification and
land use
Manual on key indicators of desertification and
mapping environmentally sensitive areas to
Edited by
Laboratory of Soils Chemistry, Agricultural University of Athens, Greece
M. Kirkby
School of Geography, University of Leeds, United Kingdom
N. Geeson
Medalus Project Office, Thatcham, Berkshire, United Kingdom
Project ENV4 CT 95 0119
European environment and climate research programme
Theme: Land resources and the threst of desertification and soil erosion
in Europe
Head of Unit: Anver Ghazi
Scientific Officer: Denis Peter
Contact: Mr Denis Peter. Address: European Commission, Rue de la Loi
200, B-1049 Brussels; Tel(32-2) 29-58446; fax (32-2) 29-63024
Science, Research and Development
1999 EUR 18882
Neither the European Commission nor any person acting on behalf of the
Commission is responsible for the use which might be made of the
following information.
A great deal of additional information on the Euroepan Union is available
on the Internet. Ita can be accessed throught the Europa server
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Pubolications of the European
Communities, 1999
ISBN 92-828-6349-2
© European Communities, 1999
Reproduction is authorized provided the source is acknowledged.
Introduction……………………………………………………………………….. 1
M.Kirkby and C. Kosmas
Introduzione………………………………………………………………………. 3
Εισαγωγή………………………………………………………………………….. 5
Introdução……………………………………………………………………….... 7
Introducción……………………………………………………………………….. 8
C. Kosmas, J. Poesen and H. Briassouli
1. Soil quality indicators………………………………………………………... 13
1.1 Parent material……………………………………………………………….. 14
1.2 Rock fragments………………………………………………………………. 14
1.3 Soil depth…………………………………………………………………….. 16
1.4 Slope gradient………………………………………………………………… 17
1.5 Soil structure decline…………………………………………………………. 18
1.6 Salinization…………………………………………………………………… 18
2. Climate quality………………………………………………………………... 19
2.1 Precipitation…………………………………………………………………... 19
2.2 Aridity………………………………………………………………………… 21
2.3 Aspect………………………………………………………………………… 21
3. Vegetation quality……………………………………………………………. 22
3.1 Fire risk and ability to recover……………………………………………….. 23
3.2 Soil erosion protection……………………………………………………….. 23
3.4 Plant drought resistance……………………………………………………… 25
3.5 Plant cover……………………………………………………………………. 26
4. Management quality and human factors…………………………………… 27
4.1 Land use and intensity of land use…………………………………………… 27
4.2 Overgrazing…………………………………………………………………... 28
4.4 Fires…………………………………………………………………………… 30
C. Kosmas, A. Ferrara, H. Briassouli and A. Imeson
1. Definition of ESAs…………………………………………………………….. 31
1. Definizione delle ESAs………………………………………………………… 33
1. Ορισµός των ΠΕΠ……………………………………………………………. 34
1. Definição de ESAs…………………………………………………………….. 35
1. Definición de AMS…………………………………………………………….. 36
2. Data collection………………………………………………………………… 37
2.1 Soil……………………………………………………………………………. 38
2.2 Vegetation…………………………………………………………………….. 39
2.3 Climate……………………………………………………………………….. 39
2.4 Management characteristics…………………………………………………... 39
3. The assessment procedure…………………………………………………… 42
3.1 Soil quality indicators………………………………………………………… 42
3.2 Climate quality………………………………………………………………... 43
3.3 Vegetation quality…………………………………………………………….. 44
3.4 Management quality and human factors……………………………………… 45
4. Matching the results………………………………………………………….. 47
M. Kirkby
1. Rationale for Estimating Total Potential Erosion Rate…………………… 50
2. Factors controlling water erosion……………………………………………. 50
2.1 Climate………………………………………………………………………... 51
2.2 Vegetation…………………………………………………………………….. 53
2.3 Soil Properties………………………………………………………………… 56
2.4 Topography…………………………………………………………………… 57
3. The Integrated Soil Erosion Indicator………………………………………. 57
4. Implementation………………………………………………………………. 59
5. Salinisation Indices…………………………………………………………… 62
6. Conclusions……………………………………………………………………. 65
DEFINING ESAs…………………………………………………………….. 66
1. The Lesvos island (Greece)……………………………………………….… 66
C. Kosmas, St. Gerontidis, V. Detsis, Th. Zafiriou and M. Marathianou
1.1 Application of the derived methodology……………………………………. 66
1.2 Description of ESAs to desertification……………………………………………….. 71
1.3 ESAs and soil erosion………………………………………………………………… 72
2. The Agri basin (Italy)………………………………………………………………….. 74
F. Basso, A. Belloti, S. Faretta, A Ferrara, G. Mancino, M. Pisante, G. Quaranta and M.
2.1 The ESAs estimate in the Agri basin…………………………………………………. 74
2.2 An example of application of estimated ESAs for land use management…………….. 77
3. The Alentejo region (Mertola municipality, Portugal)……………………………… 80
M. Roxo, J.M. Mourao, L. Rodrigues amd P. Casimiro
3.1 General characteristics of the area…………………………………………………….. 80
3.2 Mapping ESAs in Mertola…………………………………………………………… 81
3.3 Evaluation of the results……………………………………………………………… 84
REFERENCES…………………………………………………………………………… 85
1. Introduction.
M. Kirkby1 and C. Kosmas2
1University of Leeds, School of Geography
2Agricultural University of Athens, Laboratory of Soils and Agricultural Chemistry
Desertification is the consequence of a set of important processes which are active in arid and
semi-arid environments, where water is the main limiting factor of land use performance in
ecosystems. In the context of the EC MEDALUS (Mediterranean Desertification and Land
Use), the focus here is primarily on European Mediterranean environments where physical
loss of soil by water erosion, and the associated loss of soil nutrient status is identified as the
dominant problem. In more arid areas, there is greater concern with wind erosion and
salinisation problems, but these are considered to be less significant than water erosion for the
northern Mediterranean area.
Environmental systems are generally in a state of dynamic equilibrium with external
driving forces. Small changes in the driving forces, such as climate or imposed land use tend
to be accommodated partially by a small change in the equilibrium and partially by being
absorbed or buffered by the system. For example, an increased rate of soil erosion commonly
leads to an increase in soil stoniness both at the surface and within the soil profile. These
changes lead to both a greater resistance to erosion due to surface armouring, and to improved
water retention as organic matter in concentrated within the fine fractions of the soil. Both of
these changes tend to offset and buffer the effects of increased erosion. In many cases, the
effects of an external change are also reversible, so that, for example a reduction in erosion
will allow coarse material to weather slowly back to finer fractions.
Desertification of an area will proceed if certain land components are brought beyond
specific thresholds, beyond which further change produces irreversible change. For example,
soils may eventually become so stony that they can only degrade towards scree or bare
bedrock. Climate change cannot bring a piece of land to a desertified state by itself, but it
may modify the critical thresholds, so that the system can no longer maintain its dynamic
Indicators of Desertification may demonstrate that desertification has already
proceeded to its end point of irreversibly infertile soils, for example as rocky deserts or highly
sodic soils. The most useful indicators, however, are those which indicate the potential risk of
desertification while there is still time and scope for remedial action.
For a European Strategy of Action against desertification, it is essential to adopt a
nested approach so that limited resources are applied in a cost effective manner. At the
coarsest scales it is essential to adopt a uniform, objective and scientifically based
methodology which identifies regions where the risk of desertification is highest. At this
scale, it is impossible to identify single fields or communities precisely, but only to identify
the regions for which more detailed work is required. These Regional Indicators should be
based on available international source materials, including remotely sensed images,
topographic data (maps or DEM’s) climate, soils and geological data, at scales of 1:250 000 to
1 000 000). At these scales the impact of socio-economic drivers is expressed mainly through
patterns of land use. Regional Indicators may be used as a base-line for allocation of funds
and expertise between countries and between regions within a country. Each Regional
Indicator or group of associated indicators should be focused on a single process, for example
water erosion. In this way planners and policy makers are able to make informed decisions
about the processes in which they seek to intervene.
Once Regions at Risk have been identified, the second nested scale of investigation
must lie within each Region. At this second scale, applied to a Province or River catchment
(500 - 5 000 km2), much of the data may still be obtained from maps at 1:25 000 to 1:50 000,
but these will need to be substantially supported by field survey. Such intensity of research
effort is only justified within Regions at Risk. The proposed methodology at this scale is
through the identification of Environmentally Sensitive Areas (ESA’s) through a multi-
factor approach based on both a general and a local knowledge of the environmental processes
acting. At this scale it is appropriate and possible to pay much more attention to detailed soil
and vegetation properties, and to local topographic factors such as gradient and aspect.
The final nested scale is of local remediation or mitigation action plans. At this scale,
the interplay of physical needs and socio-economic possibilities becomes dominant, and can
only be carried to a successful conclusion with the full participation of local communities.
This work focuses on the choice of appropriate indicators at the European/National
(RDI) and Regional (ESAs) scales; and illustrates their application to identifying ESAs for
three target areas defining during the execution of MEDALUS Project and located in Greece
(the island of Lesvos), Italy (the Agri basin in Basilicata), and Portugal (Alentejo region).
1. Introduzione*
La desertificazione è la conseguenza di una serie d’importanti processi che sono attivi in
ambienti aridi o semi-aridi, dove l’acqua è il fattore limitante principale per il rendimento del
suolo. Nel contesto del Progetto UE Medalus (Mediterranean Desertification And Land Use),
l’attenzione è rivolta principalmente agli ambienti del Mediterraneo dove la perdita fisica di
suolo, causata dall’erosione idrica e, la conseguente perdita d’elementi nutritivi sono i
problemi dominanti. In aree più aride, c’è più interesse per l’erosione eolica e per i problemi
di salinizzazione, ma questi sono considerati meno significativi dell’erosione idrica per l’area
del nord mediterraneo.
Generalmente i sistemi ambientali sono in uno stato d’equilibrio dinamico con i fattori
esterni. Piccoli cambiamenti di questi fattori, come il clima o l’uso del suolo tendono ad
essere parzialmente contenuti da un piccolo cambiamento nell’equilibrio e parzialmente
assorbito o tamponato dal sistema. Per esempio, un incremento del processo d’erosione di
solito porta ad un incremento della pietrosità del suolo conseguente al trasporto delle
particelle fertili di suolo superficiale. Questi cambiamenti portano di solito ad una maggiore
resistenza dello strato superficiale all’erosione, dovuta alla presenza in superficie di
materiale grossolano e alla migliore ritenzione idrica. Entrambi cambiamenti tendono a
compensare e tamponare gli effetti dell’aumentata erosione. In molti casi, gli effetti di un
cambiamento esterno sono anche reversibili, in modo che, una riduzione dell’erosione
permetterà, nel tempo ad una maggiore disgregazione in frazioni più fini del materiale
La desertificazione di un’area verificherà se certe componenti del suolo sono portati
via oltre specifiche soglie, che potranno produrre cambiamenti irreversibili. Per esempio,
suoli possono diventare, a seguito di processi d’erosione tanto pietrosi essi potranno
degradare solo verso ghiaioni o verso la roccia madre. Cambiamento nel clima, da se, non
può portare alla desertificazione di una superficie di terreno, ma può modificare le soglie
critiche in modo che il sistema non può più mantenere il suo equilibrio dinamico.
Gli indicatori di desertificazione potranno dimostrare che la desertificazione è già
arrivata al punto d’irreversibilità di alcuni terreni non fertili, per esempio deserti, zone
rocciose o suoli con alto contenuto sodico. Comunque, gli indicatori più utili sono quelli che
indicano il potenziale rischio di desertificazione quando c’è ancora tempo per azioni di
E’ essenziale, per gli interventi comunitari contro la desertificazione, di adoperare un
approccio chiaro per limitare le risorse da applicare. Sarebbe opportuno adoperare una
metodologia uniforme, ed un obiettivo scientifico basato su larga scala che individui regioni
dove il rischio di desertificazione è più alto. E’ impossibile di identificare campi singoli o
comunità in modo preciso a scala ampia, ma sarebbe possibile identificare le regioni dove è
richiesta un lavoro più dettagliato. Questi Indicatori Regionali (RDI) dovrebbero essere
basati su materiali internazionali disponibili, includendo immagini satellitari, dati topografici
(mappe e DEMs), dati climatici e geologici e del terreno, alle scale uno: 250 000 a 1 000 000.
L’impatto degli aspetti socio-economici, a queste scale, è principalmente espresso attraverso
schemi d’utilizzazione del suolo. Gli Indicatori Regionali potranno essere usati come base
per l’allocazione dei fondi ed esperienze tra nazioni o tra regioni dentro la nazione. Ognuno
degli Indicatori Regionali o gruppi d’indicatori associati dovrebbero essere focalizzati su un
singolo processo, per esempio erosione idrica. Questo metterà in grado i pianificatori o
politici di prendere decisioni informate sui processi che si vogliono limitare.
Una volta che le Regioni a rischio sono state identificate, il secondo approccio dovrà
essere fattodentro ogni Regione. A questa seconda scala, applicata a livello di bacino o
Provinciale (500 – 5 000 km2), molti dati potranno essere ancora presi da mappe 1: 25 000 a
1:50 000, anche se ci sarà bisogno di supportare il tutto tramite rilievi in pieno campo.
L’intensità di un tale sforzo di ricerca può solo essere giustificata dentro Regioni a rischio.
La metodologia proposta, a questa scala, è attraverso l’identificazione d’Aree Ambientali
Sensitive tramite un approccio multifattoriale basato sia sulla conoscenza generale sia su
quella locale dei processi ambientali in atto. A questa scala è appropriato e possibile
rivolgere più attenzione alle proprietà del suolo e della vegetazione in modo dettagliato, ma
anche sui fattori topografici locali.
La scala finale a livello locale è per favorire azioni di mitigazione. L’interazione tra
fattori fisici e possibilità socio-economiche diventa dominante; questa scala, può essere
portata ad un successo solo con la piena partecipazione delle comunità locali.
Questo lavoro focalizza l’attenzione sulla scelta d’indicatori appropriati a scala
Europea/Nazionale (RDI) e Regionale (ESAs) e illustra la loro applicazione per identificare
ESAs per le tre aree target, definite durante l’esecuzione del Progetto MEDALUS, localizzati
in Grecia (Isola Lesvos), Italia (bacino dell’Agri), e Portogallo (regione d’Alentejo).
* The translation into Italian has been made by: Dr. Achille Mastroberti, Universita degli
Studi della Basilicata, Dipartimento di Produzione Vegetale.
1. Εισαγωγή*
Η απερήµωση είναι το αποτέλεσµα µιας σειράς σηµαντικών διεργασιών που λαµβάνουν χώρα
σε ξηρά και ηµίξηρα περιβάλλοντα, όπου το νερό είναι ο κύριος περιοριστικός παράγοντας
απόδοσης της γης. Στα πλαίσια του προγράµµατος της Ευρωπαϊκής Ένωσης MEDALUS
(Μεσογειακή Απερήµωση και Χρήση Γης), η απερήµωση µελετήθηκε κυρίως στα Ευρωπαϊκά
Μεσογειακά περιβάλλοντα όπου η απώλεια εδάφους λόγω διάβρωσης και η συνακόλουθη
απώλεια εδαφικών θρεπτικών στοιχείων χαρακτηρίζεται σαν η κυριότερη αιτία απερήµωσης. Σε
ιδιαίτερα ξηρές περιοχές υπάρχει επίσης το πρόβληµα της αιολικής διάβρωσης και η αύξηση της
αλατότητας των εδαφών.
Τα περιβαλλοντικά συστήµατα είναι γενικά σε µια κατάσταση δυναµικής ισορροπίας µε
τις εξωτερικές επιδράσεις που δρουν πάνω τους. Μικρές αλλαγές στις επιδράσει αυτές, όπως
το κλίµα ή η χρήση γης τείνουν να εξισορροπηθούν µερικώς µε µια µικρή αλλαγή στο σηµείο
ισορροπίας και µερικώς απορροφούνται ή ρυθµίζονται από το σύστηµα. Για παράδειγµα ένας
αυξηµένος ρυθµός διάβρωσης συχνά οδηγεί σε αύξηση του πετρώδους, τόσο στην επιφάνεια
όσο και στο εδαφικό προφίλ. Αυτές οι αλλαγές οδηγούν σε µεγαλύτερη αντίσταση στη διάβρωση
λόγω της «θωράκισης» της επιφάνειας και στη βελτίωση συγκράτησης της υγρασίας καθώς η
οργανική ουσία συγκεντρώνεται στα λεπτόκοκκα κλάσµατα του εδάφους. Και οι δύο αυτές
αλλαγές τείνουν να αντισταθµίσουν και να ρυθµίσουν τις συνέπειες της αυξηµένης διάβρωσης.
Σε πολλές περιπτώσεις οι συνέπειες µιας εξωτερικής µεταβολής είναι αναστρέψιµες έτσι ώστε
για παράδειγµα µια µείωση της διάβρωσης να επιτρέψει σε χονδρόκοκκο υλικό να
αποσαθρωθεί σταδιακά σε πιο λεπτόκοκκα κλάσµατα.
Η απερήµωση µιας περιοχής θα προχωρήσει εάν συγκεκριµένες παράµετροι ξεπεράσουν
συγκεκριµένα όρια πέρα από τα οποία κάθε επιπλέον µεταβολή προκαλεί µη αναστρέψιµες
αλλαγές. Για παράδειγµα εάν το έδαφος γίνει πολύ πετρώδες τότε η περαιτέρω µεταβολή είναι η
µετατροπή σε «σάρες» ή γυµνό µητρικό πέτρωµα. Η αλλαγή του κλίµατος δεν µπορεί να φέρει
ένα κοµµάτι γης σε κατάσταση απερήµωσης από µόνη της, αλλά µπορεί να µεταβάλει τα
κρίσιµα όρια, έτσι ώστε το σύστηµα να µη µπορεί να διατηρήσει πλέον τη δυναµική του
Η χρήση των δεικτών απερήµωσης µπορεί να αποδείξει ότι η απερήµωση έχει ήδη
προχωρήσει στο τελικό της σηµείο των µη αναστρέψιµα άγονων εδαφών, για παράδειγµα
πετρώδεις ερήµους ή πολύ αλκαλιωµένα εδάφη. Οι πιο χρήσιµοι δείκτες πάντως είναι αυτοί που
δείχνουν τον δυνητικό κίνδυνο της απερήµωσης ενώ υπάρχει ακόµα χρόνος και λόγος για
δράσεις αντιµετώπισης.
Για µια Ευρωπαϊκή Στρατηγική ∆ράσης ενάντια στην απερήµωση είναι απαραίτητο να
υιοθετηθεί µια απλή προσέγγιση έτσι ώστε η χρήση περιορισµένου αριθµού δεδοµένων να
είναι αποτελεσµατική στο συγκεκριµένο θέµα µελέτης. Σε µεγαλύτερες κλίµακες είναι
απαραίτητο να υιοθετηθεί µια ενιαία, αντικειµενική και επιστηµονικά τεκµηριωµένη
µεθοδολογία που να αναγνωρίζει περιοχές όπου ο κίνδυνος της απερήµωσης είναι µέγιστος. Σ
αυτή την κλίµακα είναι αδύνατο να αναγνωριστούν µεµονωµένα πεδία ή κοινότητες µε ακρίβεια
αλλά µόνο να αναγνωριστούν οι περιοχές όπου χρειάζεται πιο λεπτοµερής δουλειά. Αυτοί οι
Περιφερειακοί ∆είκτες (RDI) θα πρέπει να βασίζονται σε διεθνώς διαθέσιµο υλικό,
συµπεριλαµβανοµένων δορυφορικών εικόνων, τοπογραφικών δεδοµένων (χάρτες ή ψηφιακά
υψοµετρικά µοντέλα – DEM), κλιµατικών, εδαφολογικών και γεωλογικών δεδοµένων σε
κλίµακες 1:250.000 έως 1:1.000.000. Σαυτές τις κλίµακες η επίδραση των κοινωνικό-
οικονοµικών δυνάµεων εκφράζεται κυρίως µέσω µοντέλων χρήσης γης. Περιφερειακοί δείκτες
µπορούν να χρησιµοποιηθούν σαν βάση για τη κατανοµή κονδυλίων και τεχνογνωσίας µεταξύ
χωρών και µεταξύ περιοχών της ίδιας χώρας. Κάθε Περιφερειακός ∆είκτης ή οµάδα
σχετιζόµενων µεταξύ τους δεικτών θα πρέπει να εστιάζονται σε µια µοναδική διαδικασία, για
παράδειγµα η υδατική διάβρωση. Μαυτόν τον τρόπο οι σχεδιαστές διαχείρισης και οι
πολιτικοί θα είναι σε θέση να λαµβάνουν ορθές αποφάσεις για τις διαδικασίες στις οποίες
επιδιώκουν να παρέµβουν.
Από τη στιγµή που αναγνωριστούν οι περιοχές που κινδυνεύουν η επόµενη κλίµακα της
έρευνας πρέπει να βρίσκεται µέσα στην περιοχή. Σαυτή τη δεύτερη κλίµακα που εφαρµόζεται
σε µια περιφέρεια ή µια λεκάνη απορροής (500 – 5.000 km2), πολλά από τα δεδοµένα µπορούν
ακόµα να εξαχθούν από χάρτες κλίµακας 1:25.000 έως 1:50.000 αλλά αυτά θα πρέπει να
υποστηριχτούν ουσιαστικά από έρευνες πεδίου. Τέτοια εντατική έρευνα µπορεί να δικαιολογηθεί
µόνο στα όρια των περιοχών που κινδυνεύουν. Η προτεινόµενη µεθοδολογία σαυτή την
κλίµακα είναι η αναγνώριση των Περιβαλλοντικά Ευαίσθητων Περιοχών (ESAs) µέσα από µία
πολυπαραµετρική προσέγγιση βασισµένη τόσο σε µια γενική όσο και σε µια τοπική γνώση των
περιβαλλοντικών διαδικασιών που λαµβάνουν χώρα. Σαυτή την κλίµακα είναι καταλληλότερο
και δυνατό να δοθεί περισσότερη προσοχή σε λεπτοµερείς ιδιότητες του εδάφους και της
βλάστησης και σε τοπικούς τοπογραφικούς παράγοντες όπως η κλίση και η έκθεση.
Η τελευταία κλίµακα είναι αυτή των τοπικών σχεδίων δράσης αποκατάστασης ή
αντιµετώπισης. Σαυτή την κλίµακα η διαπλοκή των φυσικών αναγκών και των κοινωνικο-
οικονοµικών δυνατοτήτων γίνεται κυρίαρχη και µπορεί να οδηγηθεί σε ένα επιτυχές αποτέλεσµα
µόνο µε την πλήρη συµµετοχή των τοπικών κοινοτήτων.
Η παρούσα εργασία εστιάζεται στην επιλογή των κατάλληλων δεικτών σε
Ευρωπαϊκή/Εθνική (RDI) και τοπική (ESAs) κλίµακα και παρουσιάζει την εφαρµογή τους στην
αναγνώριση Περιβαλλοντικά Ευαίσθητων Περιοχών (ESAs) για τρεις πιλοτικές περιοχές που
καθορίστηκαν κατά την εκτέλεση του προγράµµατος MEDALUS και βρίσκονται στην Ελλάδα
(Λέσβος), την Ιταλία (η λεκάνη του Agri στα Βασιλικάτα) και την Πορτογαλία (η περιφέρεια
* The translation into Greek has been made by: Dr. V. Detsis, Agricultural University of
Athens, Laboratory of Soils and Agricultural Chemstry.
1. Introdução*
A Desertificação é a consequência de um conjunto de importantes processos que actuam em
ambientes áridos e semi-áridos, onde a água é o principal factor limitante para diferentes
usos do solo e nos ecossistemas. No contexto do Projecto da União Europeia – MEDALUS
(Mediterranean Desertification and Land Use), a abordagem deste problema centra-se
sobretudo nos diferentes ambientes da Europa Mediterrânica, onde o processo físico de
perda de solo por erosão hídrica e os processos de perda de nutrientes e fertilidade
associados, são identificados como os problemas fundamentais. Nas áreas mais áridas existe
uma preocupação crescente com os problemas da erosão eólica e da salinização dos solos,
que são, no entanto, menos significativos que a erosão hídrica para a Europa Mediterrânica.
Os sistemas ambientais estão geralmente em estado de equilíbrio com as forças
externas que os condicionam. Pequenas mudanças nestas forças externas, tais como o clima
ou usos de solo impostos pelo Homem, tendem a ser acompanhadas, por um lado, por
pequenas mudanças no equilíbrio do sistema e, por outro, por uma absorção ou assimilação
dessa perturbação no interior do sistema. Por exemplo, um aumento no coeficiente de perda
de solo por erosão hídrica conduz a um aumento da percentagem de fragmentos rochosos no
solo («quando o solo morre as pedras nascem…!») tanto à superfície como ao longo do perfil
dos horizontes do solo. Estas mudanças conduzem a uma maior resistência à erosão, devido à
protecção dada pelas pedras («armadura» do solo) e a um aumento da capacidade de
retenção de água já que a matéria orgânica se concentra na fracção mais fina do solo.
Ambas estas mudanças tendem a atenuar os efeitos de um aumento da erosão. Em muitos
casos, os efeitos provocados por uma perturbação exterior são também reversíveis, de forma
que, por exemplo, uma redução na erosão permitirá que o material mais grosseiro possa ser
meteorizado lentamente até integrar de novo as fracções mais finas do solo.
A desertificação de uma área continuará se determinadas componentes do sistema
bioprodutivo terrestre forem sistematicamente ultrapassadas nos seus limites, para além dos
quais, qualquer mudança produzirá alterações irreversíveis. Por exemplo, os solos podem
eventualmente tornar-se tão pedregosos que apenas se podem degradar no sentido de um
rególito (solo pedregoso) ou da rocha-mãe. A alteração climática não pode por si só tornar
uma área desertificada, mas pode modificar os limiares críticos de mudança e absorção, de
forma que o sistema não pode mais manter o seu estado de equilíbrio.
Os indicadores de desertificação podem demonstrar que este fenómeno atingiu já um
ponto final, tornando-se os solos irreversivelmente inférteis, como por exemplo os desertos
rochosos ou solos altamente sódicos. Os indicadores mais úteis são, no entanto, aqueles que
indicam o risco potencial de desertificação, enquanto existe tempo e possibilidades de
empreender acções para remediar o processo.
Para uma Estratégia Europeia de Acção de luta contra a desertificação é essencial a
adopção de uma abordagem concentrada na génese do problema, de forma que recursos
limitados possam ser aplicados de uma maneira efectiva em termos de custos. A uma escala
mais alargada é essencial adoptar uma metodologia uniforme, objectiva e de base científica,
que identifique as regiões onde o risco de desertificação é mais elevado. A esta escala é
impossível identificar pequenas áreas ou comunidades afectadas de forma precisa, sendo
apenas possível a identificação de regiões para as quais é necessário um diagnóstico mais
detalhado. Estes Indicadores Regionais de Desertificação (RDIs) devem basear-se em fontes
de informação disponíveis internacionalmente, incluindo imagens de satélite, informação
topográfica (mapas e Modelos Digitais de Terreno – DEMs), clima, solos e informação sobre
a geologia, coberto vegetal e uso do solo, a escalas entre 1:250 000 e 1:1 000 000). A estas
escalas, o impacto dos factores socioeconómicos é expresso sobretudo através dos padrões de
usos do solo. Os indicadores regionais podem ser utilizados como uma base para a alocação
de fundos e recursos técnicos entre países e entre regiões dentro do mesmo país. Cada
Indicador Regional ou grupo de indicadores associados deverá reflectir um determinado
processo, como por exemplo a erosão hídrica. Desta forma, as autoridades com
responsabilidades no planeamento e na decisão política, poderão ser capazes de tomar
decisões fundamentadas acerca dos processos nos quais desejam intervir.
Uma vez que estejam identificadas as regiões em risco, a segunda escala de
investigação tem de basear-se em cada região. Nesta segunda escala, aplicada à Província
ou bacia hidrográfica (500 – 5 000 km2) grande parte da informação pode ainda ser obtida
dos mapas à escala 1:25 000 ou 1:50 000, mas esta tarefa deverá ser substancialmente
complementada com trabalho de reconhecimento de campo. Um tal esforço de investigação
só é justificado para Regiões em Risco. A metodologia proposta a esta escala baseia-se na
identificação de Áreas Ambientalmente Sensíveis (ESAs), através de uma análise multi-
factorial baseada simultaneamente num conhecimento geral e local dos processos actuantes.
A esta escala é apropriado e possível prestar muito mais atenção e analisar com maior
detalhe as propriedades do solo e da vegetação e a aspectos da topografia local como o
declive ou a exposição das vertentes.
A última escala de análise integrada concentra-se em planos locais de acção de
combate e mitigação da desertificação. A esta escala as interacções entre as necessidades
físicas e as possibilidades socioeconómicas tornam-se centrais e podem apenas ser
conduzidas e concluídas com sucesso com a total participação das comunidades locais.
Este trabalho concentra-se na escolha de indicadores apropriados, à escala
Europeia/Nacional (RDI) e Regional (ESA); e ilustra a sua aplicação na identificação de
ESAs para três áreas de estudo definidas durante a execução do Projecto MEDALUS,
localizadas na Grécia (ilha de Lesvos), Itália (a bacia do Agri, Basilicata), e em Portugal
(região Alentejo, concelho de Mértola).
* The translation into Portuguese has been made by: Dr. Jorge Mourao, Universidade Nova de
Lisboa, Departamento de Geografia e Planeamento Regional, Faculdade de Ciencias e
1. Introducción*
La desertificación es la consecuencia de un conjunto de importantes procesos activos en
ambientes áridos y semiáridos, dónde el agua es el principal factor limitante de la
productividad en los ecosistemas. En el contexto del Proyecto MEDALUS (Desertificación y
Uso del Suelo en el Mediterráneo) de la UE, el punto de atención básico son los ecosistemas
de la Europa mediterránea, dónde la pérdida de suelo por erosión hídrica, y la pérdida
asociada de nutrientes se identifica como el problema dominante. En áreas más áridas hay
una mayor preocupación en la erosión eólica y la salinización, que se consideran menos
significativas que la erosión hídrica en el área del Mediterráneo norte.
Los sistemas medioambientales están generalmente en un estado de equilibrio
dinámico con las fuerzas conductoras externas. Ccambios pequeños en estas fuerzas
conductoras, tales como el clima o un uso del suelo impuesto tienden a ser ajustados en parte
por un pequeño cambio en el estado de equilibrio y en parte son absorbidos o amortiguados
por el sistema. Por ejemplo, un incremento en tasa de erosión del suelo lleva a un incremento
de la pedregosidad tanto en la superficie como en el perfil. Estos cambios conducen a una
mayor resistencia a la erosión debido al encostramiento, y a una retención mejorada del
agua ya que la materia órganica se concentra en las fracciones finas del suelo. Ambos
cambios tienden a compensar y amortiguar los efectos del incremento de erosión. En muchos
casos, los efectos de un cambio externo son también reversibles, de modo que, por ejemplo,
una reducción en la erosión permitirá que el material grueso se meteorice lentamente en
fracciones más finas.
La desertificación de un área ocurrirá si ciertos componentes del sistema son llevados
más allá de umbrales específicos, más allá de los cuales un cambio posterior es irreversible.
Por ejemplo, el suelo puede finalmente llegar a ser tan pedregoso que sólo se puede degradar
hacia un canchal o roca madre desnuda. El cambio del clima no puede llevar a un área al
estado desertificado por sí mismo, pero pudiera modificar los umbrales críticos, de manera
que el sistema no puede mantener su equilibrio dinámico.
Los indicadores de desertificación pueden demostrar que la desertificación está ya
operando hacia suelos irreversiblemente infértiles, su punto final, por ejemplo en forma de
desiertos rocosos o suelos altamente sódicos. Los indicadores más útiles, sin embargo, son
aquellos que indican el riesgo potencial de desertificación mientras que aún haya tiempo y
oportunidad para acciones de rehabilitación.
Para una Estrategia Europea de Acción contra la desertificación, es esencial adoptar
una aproximación anidada de modo que los recursos limitados sean utilizados de manera
rentable. A las escalas de grano grueso es esencial adoptar una metodología, uniforme,
objetiva y sustentada científicamente que identifique las regiones dónde el riesgo de
desertificación es más elevado. A esta escala, es imposible identificar campos individuales o
comunidades con precisión, pero es posible identificar las regiones de las que se requiere un
trabajo más detallado. Estos Indicadores Regionales (IRD) deberían estar basados en
materiales disponibles internacionalmente, incluyendo imágenes de teledetección, datos
topográficos (mapas o MDE), clima, suelos y datos geológicos a escalas de 1:250.000 a
1:1.000.000. A estas escalas el impacto de las fuerzas socioeconómicas se expresa
principalmente en los patrones de uso del suelo. Los Indicadores Regionales pueden usarse
como línea de base para la distribución de fondos y conocimientos técnicos entre países y
entre regiones dentro de un país. Cada Indicador Regional o grupo de indicadores asociados
debería enfocarse sobre procesos individuales, por ejemplo erosión hídrica. De este modo los
planificadores y decisores políticos podrán tomar decisiones bien informadas sobre los
procesos en los que pretenden intervenir.
Una vez que la Regiones en Riesgo han sido identificadas, la segunda escala anidada
de investigación debe ser cada Region. A esta segunda escala, una Provincia o cuenca
hidrológica (500-5.000 km2), muchos de los datos se pueden obtener aún de mapas, a escalas
1:25.000 a 1:50.000, pero se necesitará un apoyo sustancial de la investigación de campo.
Tal intensidad en el esfuerzo investigador sólo se justifica dentro de las Regiones en Riesgo.
La metodología propuesta a esta escala es la identificación de Areas Medioambientalmente
Sensibles (AMS) a través de una aproximación multifactorial basada tanto en conocimientos
generales como locales de los procesos ambientales actuantes. A esta escala es apropiado y
posible prestar gran atención a propiedades detalladas del suelo y la vegetación, y a los
factores topográficos locales tales como la pendiente y la exposición.
La última escala anidada es la de los planes locales de rehabilitación o mitigación. A
esta escala el juego entre las necesidades físicas y las posibilidades socioeconómicas llega a
ser dominante y sólo puede ser llevado a cabo con éxito con la participación plena de las
comunidades locales.
Este trabajo está enfocado en la elección de indicadores apropiados a las escalas
Europea/Nacional (IRD) y Regional (AMS); e ilustra su aplicación para identificar las AMS
para tres áreas objetivo definidas durante la ejecución del Proyecto MEDALUS, y
localizadas en Grecia (la isla de Lesvos), Italia (la cuenca del Agri en Basilicata) y Portugal
(la región de Alentejo).
*The translation into Spanish has been made by: Dr. Gonzalo Gonzalez-Barbera, Universidad
de Murcia, Area de Geografia Fisca.
C. Kosmas1, J. Poesen2, and H. Briassouli3
1Agricultural University of Athens, Laboratory of Soils and Agricultural Chemistry
2Katholieke Universiteit te Leuven, Laboratory for Experimental Research
3University of the Aegean, Department of Human Geography
Desertification is the consequence of a set of important degradation processes in the
Mediterranean environments, especially in semi-arid and arid regions, where water is the
main limiting factor of land use performance on ecosystems. Environmentally Sensitive
Areas (ESAs) to desertification around the Mediterranean region exhibit different sensitivity
to desertification for various reasons. For example there are areas presenting high sensitivity
to low rainfall and extreme events due to low vegetation cover, low resistance of vegetation
to drought, steep slopes, highly erodible parent materials, etc. High sensitivity can be also
related to the type of land use for the cases that it promotes desertification in climatically and
topographically marginal areas. For example cereals cultivated in hilly areas with soils
formed on marl present a serious threat for desertification. Furthermore, there are areas
which are sensitive to desertification for special reasons, such as fire risk, which is likely to
generate runoff and erosion problems for some years; rambla and flood plain environments,
where fluctuating phreatic levels may show salinization and toxicity problems; and exotic
tree plantations, where poor ground cover and autotoxicity may lead to higher runoff and
sediment yields.
The various types of ESAs to desertification can be distinguished and mapped by
using certain key indicators for assessing the land capability to withstand further
degradation, or the land suitability for supporting specific types of land use. The key
indicators for defining ESAs to desertification, which can be used at regional or national
level, can be divided into four broad categories defining the qualities of soil, climate,
vegetation, and management (stressor indicators). This approach includes parameters which
can be easily found in existing soil, vegetation, and climate reports.
La desertificazione è la conseguenza di una serie d’importanti processi di degradazione del suolo in
ambiente Mediterraneo, specialmente nelle regioni aride e semi-aride, dove l’acqua è il fattore
limitante principale per il rendimento dell’uso del suolo. Aree Ambientali Sensitive (ESAs) alla
desertificazione nella regione del Mediterraneo mostrano una diversa sensibilità alla desertificazione
per vari motivi. Per esempio, ci sono aree che presentano un’elevata sensibilità alla bassa piovosità
ed ad eventi piovosi di elevata intensità che favoriscono il processo erosive di aree a scarsa copertura
vegetale, con pendenze elevate, e su suolo molto erosivo, ecc. L’alta sensibilità può anche essere
riferita al tipo di utilizzazione del suolo che porta alla desertificazione in aree climaticamente e
topograficamente marginali. Per esempio, cereali coltivati in aree collinari con suoli formati su
marna presentano una minaccia seria alla desertificazione. Inoltre, ci sono aree che sono sensibili
alla desertificazione per ragioni speciali, come il rischio d’incendi, che sicuramente genera problemi
di deflusso e d’erosione per alcuni anni; ambienti rambla e pianure alluvionali, dove il variare del
livello freatico potrebbe mostrare problemi di salinizzazione e di tossicità; piantagioni d’alberi
esotici, dove la scarsa copertura del suolo e l’auto-tossicità potrebbero portare a deflussi e sedimenti
più alti.
I vari tipi d’ESAs alla desertificazione possono essere distinti e mappate usando degli
indicatori chiavi per la stima della capacità del suolo a resistere a processi di degradazione, oppure
l’idoneità del suolo di supportare specifici usi. Gli indicatori per definire ESAs alla desertificazione
sia a livello regionale sia a livello nazionale possono essere divisi in quattro categorie definendo la
qualità del suolo, la qualità del clima, la qualità della vegetazione e la qualità della gestione
(indicatori di stress). Quest’approccio include parametri che possono essere facilmente trovati nelle
relazioni esistenti sul suolo, sulla vegetazione e sul clima.
Η απερήµωση είναι η συνέπεια µιας σειράς από σηµαντικές διαδικασίες υποβάθµισης στα µεσογειακά
περιβάλλοντα, ειδικά στις ηµίξηρες και ξηρές περιοχές, όπου το νερό είναι ο κύριος περιοριστικός
παράγοντας του δυναµικού των οικοσυστηµάτων. Οι Περιβαλλοντικά Ευαίσθητες Περιοχές (ΠΕΠ) στην
απερήµωση στην περιοχή της Μεσογείου παρουσιάζουν διαφορετική ευαισθησία στην απερήµωση για
διάφορους λόγους. Για παράδειγµα υπάρχουν περιοχές που παρουσιάζουν υψηλή ευαισθησία στη χαµηλή
βροχόπτωση και τα ακραία φαινόµενα εξαιτίας της µικρής φυτοκάλυψης, της µικρής αντοχής της
βλάστησης στην ξηρασία, των απότοµων κλίσεων, της υψηλής διαβρωσιµότητας του µητρικού υλικού
κλπ. Η υψηλή ευαισθησία µπορεί επίσης να σχετίζεται µε τον τύπο της χρήσης γης στην περίπτωση που
υποβοηθά την απερήµωση σε κλιµατικά και τοπογραφικά οριακές περιοχές. Για παράδειγµα σιτηρά που
καλλιεργούνται σε λοφώδεις περιοχές µε εδάφη σχηµατισµένα από µάργα είναι ένας σοβαρός κίνδυνος
για απερήµωση. Επίσης υπάρχουν περιοχές που είναι ευαίσθητες στην απερήµωση για ειδικούς λόγους,
όπως ο κίνδυνος πυρκαγιάς, που είναι πιθανό να δηµιουργήσει προβλήµατα επιφανειακής απορροής και
διάβρωσης για µερικά χρόνια, υποβαθµισµένες περιοχές και περιοχές που πληµµυρίζουν περιοδικά, όπου
η διακύµανση του υδροφόρου ορίζοντα µπορεί να οδηγήσει σε προβλήµατα αλάτωσης και τοξικότητας,
φυτείες εξωτικών ειδών όπου η φτωχή κάλυψη του εδάφους και η αυτοτοξικότητα µπορεί να οδηγήσει σε
υψηλότερη επιφανειακή απορροή και παραγωγή ιζήµατος.
Οι διάφορες κατηγορίες των ΠΕΠ για την απερήµωση µπορούν να διακριθούν και να
χαρτογραφηθούν χρησιµοποιώντας συγκεκριµένους δείκτες-κλειδιά για να εκτιµήσουµε την ικανότητα
ενός τόπου να αντισταθεί στην παραπέρα υποβάθµιση ή την καταλληλότητά του για διάφορους τύπους
χρήσεων γης. Οι δείκτες κλειδιά για την διάκριση των ΠΕΠ για την απερήµωση, οι οποίοι µπορούν να
χρησιµοποιηθούν σε τοπικό ή εθνικό επίπεδο, µπορούν να διαιρεθούν σε τέσσερεις ευρείες κατηγορίες
που προσδιορίζουν τις ιδιότητες του εδάφους, του κλίµατος, της βλάστησης και της διαχείρισης (δείκτες
πίεσης). Αυτή η προσέγγιση περιλαµβάνει παραµέτρους που µπορούν να βρεθούν εύκολα σε διαθέσιµες
εκθέσεις για το έδαφος, τη βλάστηση και το κλίµα.
A Desertificação é consequência de um conjunto importante de processos de degradação em
ambientes mediterrâneos, especialmente em regiões áridas e semi-áridas, onde a água é o principal
factor limitante nos diferentes usos do solo e nos ecossistemas. As Áreas Ambientalmente Sensíveis à
desertificação ao longo das regiões mediterrâneas, apresentam diferentes sensibilidades à
desertificação por várias razões. Por exemplo, existem áreas que apresentam uma elevada
sensibilidade à escassez de precipitação e à ocorrência de fenómenos climáticos extremos, devido a
uma escassez do coberto vegetal, baixa resistência da vegetação à secura, vertentes declivosas,
grande erodibilidade dos materiais rochosos, etc. Uma elevada sensibilidade pode também estar
relacionada com o tipo de uso do solo que em determinados casos promovem a desertificação em
áreas marginais do ponto de vista climático e topográfico. Por exemplo, os cereais cultivados em
áreas de relevo movimentado, com vertentes declivosas e solos margosos, apresentam elevado risco
de desertificação. Por outro lado existem áreas que são bastante sensíveis à desertificação por razões
especiais, tais como áreas de risco de incêndio, com problemas de erosão e drenagem; vales e
planícies aluviais, onde as flutuações no nível freático podem fazer surgir problemas de salinização e
contaminação dos solos; e plantações de essências arbóreas exóticas, onde a pobreza do sub-coberto
e problemas de auto-toxicidade podem conduzir a valores mais elevados de escorrência superficial e
perda de solo.
Os vários tipos de ESAs no contexto da desertificação podem ser identificadas e
cartografadas, mediante a utilização de indicadores-chave para o diagnóstico das capacidades dos
recursos naturais para resistir à degradação, ou ainda a adequação das terras para suportarem
determinados usos de solo. Os indicadores-chave na definição das ESAs no contexto da
desertificação, que podem ser utilizados à escala regional ou a nível nacional, podem dividir-se em
quatro grandes categorias que são definidas pelas qualidades do solo, do clima, da vegetação e pelas
qualidades de gestão (indicadores de pressão). Esta abordagem inclui parâmetros que podem ser
facilmente acessíveis em estatísticas e fontes de informação sobre solos, clima e vegetação.
La desertificación es la consecuencia de un importante conjunto de procesos de degradación en lo
ecosistemas mediterráneos, especialmente en las regiones semiáridas y áridas, dónde el agua es el
principal factor limitante en la productividad de los ecosistemas. Las Areas Medioambientalmente
Sensibles (AMS) a la desertificación situadas en la región mediterránea exhiben una diferente
sensibilidad a la desertificación por varias razones. Por ejemplo, hay areas que presentan alta
sensibilidad a la baja precipitación y los eventos extremos debido a una baja cobertura vegetal,
escasa resistencia de la vegetación a la sequía, altas pendientes, materiales altamente erosionables,
etc. La sensibilidad alta se puede relacionar también con el tipo de uso del suelo en los casos que este
uso promueva la desertificación en áreas climática y topográficamente marginales. Por ejemplo, los
cereales cultivados en areas montañosas sobre margas presentan una seria amenaza de
desertificación. Más aún, hay áreas que son sensibles a la desertificación por razones especiales, tales
como el riesgo de incendios, proceso que puede generar problemas de escorrentía y erosión durante
algunos años; ecosistemas de rambla y llanuras de inundación, dónde la fluctuación de niveles
freáticos puede inducir problemas de salinización y toxicidad; y la plantación de árboles exóticos,
dónde una pobre cobertura y la autotoxicidad pueden llevar a mayor escorrentía y producción de
Los diversos tipos de AMS a la desertificación pueden ser distinguidas y cartografiadas
mediante el uso de ciertos indicadores clave para evaluar la capacidad de la tierra para resistir más
degradación, o su potencialidad para soportar tipos específicos de usos del suelo. Los indicadores
clave para definir AMS a la desertificación, que se pueden utilizar a nivel regional o nacional, se
pueden dividir en cuatro amplias categorías que definen la calidad del suelo, la calidad del clima, la
calidad de la vegetación y la calidad de la gestión (indicadores de estrés). Esta aproximación incluye
parámetros que se pueden encontrar fácilmente en los informes existentes sobre suelo, vegetación y
1. Soil quality indicators
Soil is a dominant factor of the terrestrial ecosystems in the semi-arid and dry sub-humid
zones, particularly through it’s effect on biomass production. Desertification will proceed, in
a certain landscape, when the soil is not able to provide the plants with rooting space and/or
water and nutrients. In the semi-arid and the sub-humid zones, the land becomes irreversibly
desertified when the rootable soil depth is not capable to sustain a certain minimum
vegetation cover. There are cases that desertification proceeds in deep soils, when their water
balance is incapable to meet the needs of the plants. In these cases the phenomenon is
reversible. Nutrient supply to plants seldom becomes critical in the two climatic zones
mentioned above.
Soil quality indicators for mapping ESAs can be related to (a) water availability, and
(b) erosion resistance. These qualities can b evaluated by using simple soil properties or
characteristics given in regular soil survey reports such as soil depth, soil texture, drainage,
parent material, slope grade, stoniness, etc. The use of these properties for defining and
mapping ESAs requires the definition of distinct classes with respect to degree of land
protection from desertification. The definition of classes requires the study of relations such
as: (a) soil depth and plant cover under various climatic, lithological, and topographical
conditions, (b) parent material and water availability, (c) soil water holding capacity and soil
1.1 Parent material
Soils derived from different parent materials react differently to soil erosion, vegetation and
desertification. Limestone produces shallow soils with a relatively dry moisture regime. In the
opposite, as Plate 1 (left photograph) shows, soils formed in flysch are deep, well vegetated.
Areas with soils in limestone are characterised by high erodibility and slow vegetation
recovery (Plate 1). Several areas on limestone formations in the Mediterranean region are
already desertified with the soil mantle eroded, and the vegetation cover completely removed.
Under Mediterranean climatic conditions, regeneration of soils and vegetation is impossible,
and desertification is irreversible. Similarly, acid igneous parent materials such as pyroclastics
(Plate 1) produce shallow soils with high erodibility and high desertification risk.
Plate 1. Areas highly eroded and desertified with soils formed in limestone (upper part,
left photograph) and pyroclastics (right photograph) and areas well vegetated with deep
soils formed in flysch (lower part, left photograph) (Photo by C. Kosmas).
Extensive areas on hilly agricultural lands in the semi-arid zone of the Mediterranean
region are cultivated with rainfed cereals. Areas with soils formed in marl are very susceptible
to desertification. Such soils cannot support any annual vegetation in particularly dry years,
despite their considerable depth and high productivity in normal and wet years (Kosmas et al.,
1993). On the contrary, soils formed on shale-sandstone, conglomerates, basic igneous rocks,
etc. despite their normally low productivity in wet years, may supply appreciable amounts of
previously stored water to the stressed plants and to secure a not negligible biomass
production even in dry years.
The presence of cracks or fractures and faults into the bedrock favours the soil
formation by weathering or the removal of soil aggregates into the cracks by gravity. The
formed ‘tube’ type soils are well protected from erosion and the percolating water can be
stored into and protected from evaporation. The presence of deep soils in cracks and faults is
of great ecological importance, supporting relatively well the natural vegetation under
Mediterranean climatic conditions and preventing large hilly areas from desertification (Plate
1.2 Rock fragments
Rock fragments have a great but variable effect on runoff and soil erosion (Poesen et al.
1994; Danalatos et al., 1995), soil moisture conservation (van Wesemael, et al., 1995;
Moustakas et al., 1995) and biomass production (Poesen and Lavee, 1994), so playing an
important role on land protection in the Mediterranean region. Generally, runoff and
sediment loss are greater from stony than stone-free soils, apart from soils rich in coarse
gravel (Fig. 1) on the surface subjected to heavy and prolonged showers. Bunte and Poesen
(1993) found that interill sediment loss increased with increasing rock fragment percentage
up to about 20%. Beyond this value, the limited space between fragments prevents
development of scour holes and thus limits soil loss. For sheet and rill erosion, however, rock
fragments cover always reduces sediment production in an exponential way (Poesen et al.,
1994) (Fig. 2).
Plate 3. Cracks (hilly area in Sardinia) and dikes (hilly area in Lesvos) present into the
bedrock favouring the growth of natural vegetation under adverse climatic conditions
(photo by C. Kosmas).
Despite increasing runoff and erosion, cobbles have a beneficial effect on soil
moisture conservation under conditions of moderate water stress such as those prevailing in
spring and early summer, the most crucial periods for the productivity of winter crops. The
presence of cobbles can be very valuable, particularly in dry years, by conserving appreciable
amounts of water stored in previous times or adsorbed at night, thus protecting arge areas
from desertification (Kosmas et al., 1998).
Fig. 1. Annual water run-off measured in bare plots covered with cobbles (L2AO),
coarse gravel (M1AO), and vegetated but stone-free soil (OV) (source: C. Kosmas).
Stony soils along slope catenas of parent materials rich in rock fragments such as
conglomerates, shale-sandstone, etc., despite their normally low productivity, may supply
appreciable amounts of previously stored water to the stressed plants and ensure an adequate
biomass production in dry years (Kosmas et al., 1993). As Fig.3 illustrates, the biomass
production of wheat growing under water- limiting conditions was reduced by 10-30% in
plots in which the rock fragments were removed from the soil surface during cultivation, as
compared with the stony plots of the same soils along hillslope catenas. Soils formed on marl
are free of rock fragments and despite their considerable depth and high productivity in
normal and wet years, they are susceptible to desertification in particularly dry years. In such
dry years, they are unable to support any vegetation due to adverse soil hydraulic properties
and the absence of gravel and stone mulching.
Fig. 2. Effect of rock fragment cover at the soil surface on relative interill and rill
sediment yield (Poesen et. al., 1994).
1.3 Soil depth
Dryland soils on hilly areas are particularly vulnerable to erosion, especially when their
vegetation cover has been degraded. Soils on Tertiary and Quaternary consolidated
formations usually have a restricted effective soil depth due to erosion and limiting
subsurface layers such as petrocalcic horizon, gravely and stony layer, and/or shallow
bedrock. Therefore, the tolerance of these soils to erosion is low and, under hot and dry
climatic conditions and severe soil erosion, rainfed vegetation can no longer be supported,
leading to desertification.
Fig. 3. Wheat biomass production measured along catenas in nearby plots with and
without rock fragments in the soil surface (source: C. Kosmas).
Soils formed in various parent materials show different ability to support a
considerable vegetation cover for erosion protection under given climatic conditions. Soils
formed in pyroclastics (Fig. 4) are the most sensitive in supporting adequate macchia
vegetation with a crucial depth of 10 cm under which the existing vegetation can not longer
survive (Kosmas et al., 1998). Below that crucial depth, all the perennial vegetation
disappears and only some annual plant species can survive. The erosion rates below that
critical depth are very high, favouring the appearance of the underlying bedrock on the soil
surface. Soils formed in schist-marble metamorphic rocks have a higher ability to support
perennial vegetation under the same climatic conditions with crucial depth around 4-5 cm.
Fig. 4. Relation of percentage vegetation cover of Sarcopoterium sp and soil depth
measured in areas with soils formed in pyroclastics (magmatic conglomerates) and
schist-marble (Kosmas et al., 1998).
Given certain physical characteristics and underlying parent material, two soil depths,
very important for land protection, can be distinguished, the critical and the crucial depth
(Fig. 4). The critical depth can be defined as the soil depth in which plant cover achieves
values above 40%. On soil less than that depth the recovery of the natural perennial
vegetation is very low and the erosional processes may be very active resulting in further
degradation and desertification of the land. When a hilly landscape of marginal capability is
cultivated, agriculture should be abandoned before the soil reaches the critical depth. While
the critical depth is a limit to cultivation, the crucial depth can be defined as a lesser soil
depth on which the perennial vegetation can no longer be supported, and the whole soil
structure is rapidly washed out by wind or water erosion. This is an irreversible process.
1.4 Slope gradient
Slope angle and generally topography are undoubtedly important determinants of soil
erosion. Erosion becomes acute when slope angle exceeds a critical value and then increases
logarithmically. Soil survey data of the island of Lesvos shows that slope grade has a variable
effect in the different climatic zones, depending on annual rainfall. The probability of
appearance of high erosion degree decreased with increasing rainfall for the same slope
classes (Fig. 5). Severely eroded soils are present in the semi-arid zone with slopes greater
than 12%, while slightly to moderately eroded soils are found in the dry sub-humid zone
under the same slope classes. As Fig. 5 shows, the probability of finding severely eroded
soils on moderately steep to very steep slopes is rather high in the semi-arid zone. On the
opposite, moderately eroded soils have the highest probability of occurrence under similar
slopes in the dry sub-humid zone.
Fig. 5. Probability of appearance of various degrees of erosion under different slope
classes in the semi-arid and dry sub-humid climatic zones of the island of Lesvos
(NE=no erosion, WE=slight erosion, ME=moderate erosion, SE=severe erosion,
VSE=very severe erosion) (Kosmas et al., 1998).
1.5 Soil structure decline
Soil structure stability is affected by various factors such as change in organic matter content,
use of heavy machinery, irrigation with poor quality of water, etc. Large scale deforestation
of hilly areas around the Mediterranean, intensive cultivation, and burning of the vegetation
results in a drastic reduction of the organic matter content and the aggregate stability of the
surface soil horizon. Cultivation of the Thessaly hilly areas (Greece) brought about a
decrease of organic matter content to less than 2.5% as compared with an excess of 5%
occurring some 40 years ago due to water and wind erosion and frequently burning of the
crop residues (Danalatos, 1993).
Land use change greatly affects organic matter content and aggregate stability. For
example the shift from olive trees to vine cropping had a degrading effect on the organic
matter content and the aggregate stability. Data collected along two catenas in Attica
demonstrated that the organic matter content decreased by about 33% and the aggregate size
by about 10 times in 12-years period of cultivation with vines as compared with the soils
under olives (Kosmas et al., 1995).
1.6 Salinization
The transport and distribution of salts within a landscape and in a soil profile reflect the
prevailing water balance conditions, and the depth of the groundwater. Therefore,
precipitation and evapotranspiration together with soil profile characteristics are important
for the distribution of salts in a landscape and in a soil profile. A general decrease in
precipitation and/or an increase in evapotranspiration will cause an increase of soils affected
by saline or sodic conditions around the Mediterranean region. This is because in those
regions with high evaporation rates, capillary rise is accelerated and salts accumulate
residually, where drainage is nearly absent. The extent to which this will happen at a local
scale will depend on various factors controlled by the water balance, soil type, and by the
total salt and sodium inputs. Salinity problems will be most severe in areas receiving rainfall
between 300 and 600 mm. The increasing concentrations of salts result in radical changes in
the water economy of the soil, creating a potentially adverse ecological environment for
native vegetation or agricultural crops leading to desertification (Plate 3).
Plate 3. Flat area located along the seashore of Lesvos with very poorly drained soils
and very shallow water table facing severe problems of salinization and desertification
(photo by C. Kosmas, August 1997).
2. Climate quality
The uneven annual and interannual distribution of rainfall, the extreme events and the out of
phase of rainy and vegetative seasons in the semi-arid and arid zones of the Mediterranean
are the main climatic attributes that contribute to the degradation of land. Land in the above
two climatic zones is unstable and desertification processes are triggered only if the other
land components cross specific thresholds. Global climate change is expected to widen the
present geography of the vulnerable zones in the Mediterranean. In a number of years, the
prevailing weather conditions during the growing period of annual crops may be so adverse
that the soils remain bare, creating favourable conditions for overland flow and erosion. Any
loss of volume from these marginal lands greatly reduces the potential for biomass
production, ultimately leading to desertification. Desertification at present threatens only the
shallow and severely eroded soils. Global change may threaten the majority of them.
2.1 Precipitation
The atmospheric conditions that characterise a desert climate are those that create large water
deficits, that is, potential evapotranspiration (ETo) much greater than precipitation (P). These
conditions are evaluated by a variety of indices. One of these is the FAO-UNESCO (1977)
bioclimatic index: P/ETo. Areas which are sensitive to desertification can be divided into the
following categories:
The arid zone : 0.03<P/ETo<0.20
The semi-arid zone : 0.20<P/ETo<0.50
The sub-humid zone: 0.50<P/ETo<0.75.
An area becomes naturally desertified when the ratio: P/ETo acquires values below a certain
threshold, regardless of the other components. In contrast, when the ratio exceeds an upper
threshold, desertification does not advance (FAO-UNESCO, 1977). The following scheme is
proposed for the threat of desertification induced by the climate:
Erosion data collected in various sites along the Mediterranean region shows that the
amount of annual rainfall of 280-300 mm is very crucial (Fig. 6). There is a tendency of
increasing runoff and sediment loss with decreasing rainfall in hilly Mediterranean
shrublands, especially in the region where rainfall is greater than 300 mm/year. Below to that
limit, runoff and sediment loss decreases with decreasing rainfall.
Fig. 6. Runoff-annual precipitation relationships measured in four Mediterranean sites
under shrubland (Kosmas et al., 1997).
Rainfall amount and distribution are the major determinants of biomass production on
hilly lands under Mediterranean conditions. Decreasing amounts of rainfall combined with
high rates of evapotranspiration drastically reduce the soil moisture content available for
plant growth. Reduced biomass production, in turn, directly affects the organic matter content
of the soil and the aggregation and stability of the surface horizon to erosion. Studies on the
effect of diminishing soil moisture on soil properties and biomass production of rainfed
wheat in rainfall exclusion experiment showed that the total above ground biomass
production was proportionally reduced with the amount of rainfall excluded (Kosmas et al.,
1993). Reductions in biomass of 90%, 71.4%, and 53.4% were measured in the experimental
plots in which rainfall was reduced by 65%, 50% and 30%, respectively (total amount of rain
falling in the open field during the growing period R=361 mm). As Figure 7 shows, the leaf
area index (LAI) of the crop was also greatly affected throughout the growing period. The
maximum LAI-values measured in the plots of 100%, 70%, 50% and 35% rain interception
were 5.2, 3.7, 2.9 and 1.6, respectively.
Fig. 7. The change in leaf area index of rainfed wheat grown in plots with 35%, 50%,
70% and 100% rain interception (Kosmas et al., 1993).
2.2 Aridity
Aridity is a critical environmental factor in determining the evolution of natural vegetation by
considering the water stress which may occur reducing vegetation cover. However, the
existing Mediterranean vegetation presents a great capacity of adaptation and resistance to
dry conditions, which most of these species can survive under prolonged droughts with soil
moisture content below the theoretical wilting point for many months.
The effect of aridity on vegetation characteristics can be clearly demonstrated by the
distribution of vegetation in the various climatic zones of Lesvos. The climate of the island of
Lesvos can be divided into two major climatic zones defined as semi-arid and dry sub-humid.
The great reduction in rainfall for about 45% combined with the high evapotranspiraton
demands has greatly affected the vegetation performance. Due to the lack of available soil
water, the semi-arid part of the island is dominated by poor maquis vegetation (Fig. 8), while
olive trees, oak and pine forests prevail in the dry sub-humid part under similar
topographical and geomorphological conditions with the previous zone. Vegetation cover
increases with increasing soil depth and decreasing aridity.
In a comparative analysis of the Agri basin (Italy) and the Lesvos island indicated that
the greatest part of the island (74%) is characterised as very dry with a Bagnouls-Gaussen
bioclimatic aridity index greater than 150. In the opposite 48% of the Agri basin is
characterised as moist with an aridity index less than 50. The rest of the Agri basin is
characterised as dry with an aridity index ranging from 50 to 125. The greater aridity index
of climate in Lesvos resulted in vegetation of higher resistance to drought than the vegetation
existing in the Agri basin. Extensive pine (Pinus sp) forests and olive groves are dominant
in the island. In contrast, 52% of the Agri basin is covered with vegetation of low to very
low resistance to drought such as deciduous forest.
2.3 Aspect
Slope aspect is considered an important factor for land degradation processes. Aspect affects
the microclimate by regulating isolation. The angle and the duration at which the sun rays
strikes the surface of the soil depends on the slope aspect. In the Mediterranean region lands
with southern and western aspects are warmer and have higher evaporation rates and lower
water storage capacity than northern and eastern aspects. So a slower recovery of vegetation
is expected in southern and western aspects and higher erosion rates than in northern and
eastern aspects. As a consequence, southern exposed slopes usually have a lower vegetation
cover than northern exposed slopes (Poesen et al., 1998)higher As Fig. 9 shows, the degree of
erosion measured along north- and south-facing hillslopes is twice as much as or even
higher in the south-facing slopes under various types of vegetation cover.
Fig. 8. Dominant vegetation present in the three climatic zones of Lesvos
(A=sclerophylous, O=olives, B=evergreen oak, Q=deciduous oak, P=pines, C=annual
3. Vegetation quality
The dominant biotic land component in terms of desertification is the vegetative cover of the
land. Vegetation cover is very crucial for run-off generation and can be readily altered along
the Mediterranean hilly areas depending on the climatic conditions and the period of the year.
In areas with annual precipitation less than 300 mm and high evapotranspiration rate, the soil
water available to the plants is reduced drastically and the soil remains relatively bare
favouring overland water flow. Key indicators of desertification related to the existing natural
or agricultural vegetation can be considered in relation to: (a) fire risk and ability to recover,
(b) erosion protection offered to the soil, (c) drought resistance, and (d) percentage plant
Fig. 9. Distinct erosion patches measured along hilslopes with various types of
vegetation located at north and south-facing slopes (P=pines, B=evergreen oak,
O=olives, Q=deciduous oak, A=perennial shrubs).
3.1 Fire risk and ability to recover
Forest fires is of the most important cause of land degradation in hilly areas of the
Mediterranean region. Fires have become very frequent especially in the pine dominated
forests (Fig. 10) during the last decades with dramatic consequences in soil erosion rates and
biodiversity losses. The frequency of fire occurrence is lower in grasslands, and mixed
Mediterranean macchia with evergreen forests. Also, Mediterranean pastures are frequently
subjected to man-induced fires in order to renew the biomass production. The Mediterranean
vegetation type is highly flammable and combustible due to the existence of species with
high content of resins or essential oils.
Plants react to fire in very different ways. They possess numerous fire-related
adaptations. For any given species, there is a range of fire-resistance possibilities which vary
according to fire intensity (Trabaud, 1981). These possibilities may vary with growing season
and maturity. For example, observations show that winter or spring fires do not harm
subsequent development of sprouts of Quercus ilex or Quercus coccifera, while fires in
summer or autumn which are more intense, decrease sprouting ability.
Fig. 10. Average fire frequency occurring in areas covered mainly with pine forests,
mixed deciduous and evergreen oak forests, and pastures measured in Greece in the
last three decades (Greek Ministry of Agriculture).
The Mediterrananean vegetation is known to have a high ability to recover after fire
(Trabaud, 1993) and the environmental problems related to fire normally last for only a
limited number of years after the fire occurred. Mediterranean ecosystems are well-adapted to
fire, which can even be beneficial under certain conditions. There is no succession, in the
strict sense of the term, sfter a fire incidence, since all the species, which constitute the
mature plant community, are present already in the first year after the fire. There is an initial
increase in plant diversity followed by a decrease as the regeneration process advances.
(Trabaud, 1980). All this is true if no mismanagement takes place, which unfortunately is the
case more often than not.
There are several parameters, which affect the process of recovery, apart from the fire
and site characterisitics, which can be both natural and anthropogenic. Years of unusual
drought (Mazzoleni and Esposito, 1993) or sites that can’t be affected from the moist sea
winds during summer (Saracino and Leone, 1993) ahow a slower rate of recovery. Human
interference such as livestock grazing or change in the land use pattern may damage
irreversibly the recovering vegetation (Clark, 1996). Particularly important are the time
intervals between two subsequent fires. The ability of the ecosystems to recover is not
unlimited and a fire frequency beyond a certain threshold can also lead to a degraded stage
(Trabaud, 1980). This can be due both to the nutrient and seed bank depletions and to
increased erosion. These processes have already led to severe degradation and desertification
of extensive hilly areas in the Mediterranean region.
There are two main strategies of survival for plants in fire-prone ecosystems:
resprouting and germinating. In the first case the plants resprout from the undergraound parts
of them, which survive the fire due to soil protection. In the second case the seeds are bale to
withstand the high temperatures and in some cases the germnation rate is drastically
increased. part of the soil seed bank is destroyed by fire and the propostion becaomes greater
with increasing fire intensity (Cancio et al., 1993). The same is true for resprouters; the more
severe the fire the deeper down are the intact plant parts that must resprout (Clark, 1996).
3.2 Soil erosion protection
Vegetation and land use are clearly important factors controlling the intensity and the
frequency of overland flow and surface wash erosion (Bryan and Campbell, 1986; Mitchell,
1990). Extensive areas cultivated with rainfed crops such as cereals, vines, almonds and
olives are mainly confined to hilly lands with shallow soils very sensitive to erosion. These
areas become vulnerable to erosion and desertification because of the decreased protection by
vegetation cover in reducing effective rainfall intensity at the ground surface (Faulkner,
1990). Perennial crops such as almonds, and olives have largely expanded in Mediterranean
hilly areas, while vines have declined during the last decades (Grove, 1996). These crops
require frequent removal of annual vegetation using pesticides or by tillage. Actually, such
soils remain almost bare during the whole year, creating favourable conditions for overland
flow and soil erosion.
Erosion data measured in eight sites along the northern Mediterranean region and the
Atlantic coastline located in Portugal, Spain, France, Italy and Greece in a variety of
landscapes and under a number of land-uses representative of the Mediterranean region, such
as agricultural land with rainfed cereals, vines, olives, eucalyptus plantation or natural
vegetation (shrubland) showed that the greatest rates of runoff and sediment loss were
measured in hilly areas under vines (Fig. 11). Areas cultivated with wheat are sensitive to
erosion, especially during winter, generating intermediate amounts of runoff and sediment
loss especially under rainfalls higher than 380 mm per year. Olives grown under semi-natural
conditions, as for example with an understory of vegetation of annual plants greatly restrict
soil loss to nil values. Erosion in shrublands increased with decreasing annual rainfall to
values in the range 280-300 mm and then it decreased with decreasing rainfall.
Several hilly areas under natural forests around the Mediterranean region have
reforested with exotic species such as eucalyptus. Such soils are undergoing intense erosion
as compared with soils left under natural vegetation (Aru and Barrocu, 1993). However, the
measured rates of erosion under eucalyptus are relatively lower than those measured under
vines, almonds and cereals.
Soil erosion data measured under various types of vegetation and certain
physiographic conditions in the island of Lesvos showed that the best protection from erosion
was measured in areas with a dominant vegetation of evergreen oaks, pines and olive trees
under semi-natural condition (Fig. 9). Pines have lower ability to protect the soils in southern
aspects due to the higher rate of litter decomposition and restrict growth of understory
vegetation. Deciduous oak trees offered relatively low protection from erosion in cases that
the falling leaves do not cover the whole soil surface.
Fig. 11. Average annual erosion rates measured in various types of land uses in runoff
plots located along the northern Mediterranean region (C. Kosmas et al., 1997).
3.4 Drought resistance
The main factors affecting the evolution of the Mediterranean vegetation, in the long term,
are related to the irregular and often inadequate supply of water, the long length of the dry
season, and perhaps fire and grazing (Clark, 1996). According to the types of leaf
generation, the following two major groups of vegetation can be distinguished (Clark, 1996):
(a) deciduous and drought avoiding with a large photosynthetic capacity but no resistance to
desiccation; and (b) evergreen (sclerophyllous) and drought enduring with low rates of
photosynthesis. The main response of the plants to increased aridity is the reduction in leaf
area index. Severe droughts that cause a reduction in leaf area index may be beneficial in the
short term as it reduces transpiration, but such drought will increase the probability of
enchanced soil erosion when rain eventually falls, as protective vegetation cover is reduced.
The various ecosystems present in the Mediterranean region presents a great capacity
of adaptation and resistance in aridity which most of the species existing under
Mediterranean climatic conditions have to survive under long droughts and soil moisture
contents below the theoretical wilting point for many months (Table 1). Probably the
expected changes in the vegetation performance, resulted from a gradual precipitation
decrease, could be only noticed after a critical minimum number of years.
Table 1. Classification of the dominant Mediterranean vegetation according to a
decreasing rate of drought resistance.
Types of vegetation
1 Mixed Mediterranean macchia/evergreen forests, Mediterranean macchia
2 Conifers, permanent grassland
3 Evergreen perennial agricultural trees
4 Deciduous perennial agricultural trees
5 Deciduous forests
6 Annual agricultural crops, annual grasslands
Among the prevailing perennial agricultural crops in the Mediterranean, olive trees present a
particularly high adaptation and resistance to long term droughts and support a remarkable
diversity of flora and fauna (Plate 5) even higher than some natural ecosystems (Margaris,
1995). Under these conditions, annual vegetation and plant residues form a high soil surface
cover, preventing surface sealing and minimising the velocity of the overland run-off water
(Plate 5). In the case that the land is intensively cultivated, then high erosion rates are
expected. The olive groves can be considered as a natural forest highly adapted in dry
Mediterranean conditions, with lower vulnerability to fires as compared to pine forests,
protecting hilly areas from desertification.
Plate 5. Olive groves (a) with understorey of annual vegetation (Lesvos), well-protected
from erosion (left) and (b) intensively cultivated (Cordoba), and severely eroded (right)
(photo by C. Kosmas, spring 1997).
3.5 Plant cover
Many studies showed that the variation in runoff and sediment yields in drainage basins is
attributed to the vegetation cover and land use management changes (Douglas, 1969; Reed,
1971; Williams and Reed, 1972; Patton and Schumm, 1975; Newson, 1985; Bryan and
Campbell, 1986). Many authors have demonstrated that in a wide range of environments both
runoff and sediment loss decrease exponentially as the percentage of vegetation cover
increases (Elwell and Stocking, 1976; Lee and Skogerboe, 1985; Francis and Thornes, 1990).
A piece of land is considered desertified when the biomass productivity drops below a certain
threshold value. A value of 40% vegetative cover is considered critical below which
accelerated erosion dominates in a sloping land (Thornes, 1988). This threshold may be
modified for different types of vegetation, rain intensity and land attributes. It shows,
however, that degradation begins only when a large portion of the land’s surface is denuded,
then it proceeds with an accelerated mode, that cannot be arrested by land resistance alone.
Deep soils on unconsolidated parent materials show slow rates of degradation and loss of
their biomass production potential. In contrast, shallow soils with lithic contact on steep
slopes have low productivity, and low erosion tolerance if they are not protected by
Fig. 12 Probability of appearance of percentage cover in various soil depth classes
measured in two climatic zones of the island of Lesvos
Soil and vegetation survey data of the island of Lesvos clearly indicated that the
percentage cover was greatly affected by the soil depth in the various climatic zones (Kosmas
et al., 1998). Fig. 12 shows the frequency of appearance of the various classes of vegetation
cover present in the three climatic zones of the island of Lesvos. Vegetation cover increased
with increasing soil depth and decreasing drought. In the soil depth class of 15-30 cm, the
vegetation cover class of 25-50% had the maximum frequency of appearance (93%) in the
semi-arid zone, whereas areas with soils having the same soil depth class had a higher
vegetation cover with a 64% maximum frequency of appearance of the cover class 75-90%
cover in the dry sub-humid zone.
4. Management quality and human factors
The definition of ESAs to desertification requires both key indicators related to the physical
environment and to the human-induced stress. A piece of land, irrespective of its size, is
characterised by a particular use. This use is associated with a given type of management
which is dictated by and changes under the influence of environmental, social, economic,
technological and political factors. Depending on the particular type of management, land
resources are subject to a given degree of stress. Moreover, the existence of environmental
policies which apply to a certain area moderate the anticipated impacts of a given land use
type compared to the situation where no such policies are in effect.
4.1 Land use and intensity of land use
The extensive deforestation of hilly areas and intensive cultivation with rainfed cereals in the
Mediterranean has already led to accelerated erosion and degradation in the last century. The
erosion risk is especially high in areas cultivated with rainfed cereals (Plate 6). For one or
two months after sowing winter cereals the land remains almost bare, and the erosion risk is
high considering that rains of high intensity and occasionally long duration occur during that
period. The sloping lands of the Thessaly plain, the greatest lowland of Greece, were for
centuries under grazing especially in winter by transhuman flocks and herds. The rapid
increase in population due to immigration in early 1920's resulted in the sharp increase of the
areas which were brought under wheat cultivation. Erosion experiments and estimations from
the exposure of tree roots demonstrated that erosion on these areas has proceeded at rates of
1.2-1.7 cm soil per year since the introduction of wheat. The hilly soils on Tertiary and
Quaternary hilly landforms usually have limiting subsurface layers, such as petrocalcic
horizons or bedrock, and under high erosion rates and hot and dry climatic conditions, growth
of cereals produces increasingly poor yields and the cultivated land is abandoned.
Plate. 6. Hilly areas (a) cultivated with cereals and subjected to accelerated erosion (left,
Cordoba Spain) and abandoned after long period cultivation with wheat highly
degraded (right, Thiva Greece) under high desertification risk (photo by C. Kosmas).
Many hilly areas around the Mediterranean are experiencing abandonment at an increasing
Land abandonment may lead to a deteriorating or improving phase of the soils,
depending on the particular land and climatic conditions of the area. Hilly areas that can
support sufficient plant cover may improve with time by accumulating organic materials,
increasing floral and faunal activity, improving soil structure, increasing in infiltration
capacity and therefore, causing a decrease in the erosion potential (Kosmas et al. 1995). In
cases of poor plant cover, the erosional processes may be very active and the regeneration of
these lands may be irreversible. In cases of land partially covered by annual or perennial
vegetation, the remaining bare land with soils of low permeability (clays) creates favourable
conditions for overland flow, soil erosion and land degradation (Plate 6).
4.2 Overgrazing
Wheat production in hilly Mediterranean areas has drastically declined during the last few
decades and the intensity of grazing has increased at the same time (Fig.13). Shepherds
usually damage the natural vegetation by deliberately setting fires to eradicate the vegetation
and encourage the growth of grass, which they then overgraze. Once the land is bare of its
vegetative cover and the soil is loosened, the torrential rains of autumn and winter begin to
wash away the topsoil. The process of land degradation can be greatly accelerated by high
densities of livestock which lead to vegetation degradation and, in turn, to soil compaction.
An obvious consequence of overgrazing is the increase in soil erosion, since the gradual
denudation of the landscape exposes the soil to water and wind erosion. Under such
management conditions and hot and dry climatic conditions, soils of these areas cannot
economically support a sufficient vegetative cover, leading to desertification (Plate 7).
Overgrazing of this climatically and topographically marginal areas, accompanying by fires,
constitutes a desertification-promoting land use, further deteriorating the existing land
Fig. 13. Change in the number of sheep grazing in the island of Lesvos and the total
area cultivated with cereals during the last 90 years (Kosmas, 1997).
4.3 Abandonment of terraced land
In the last decades, favourable soil and climatic conditions and the availability of ground or
surface water has resulted in intensive farming of the lowlands of the Mediterranean region.
The development of high input agriculture in the plains provided much higher net outputs
than those obtained from terracing agriculture. Furthermore, in the last decades, the value of
such terraces has markedly decline because of low accessibility by tractors. At present, most
of these areas have been abandoned, and the terraces have been collapsed causing a rapid
removal of the soil by the runoff water, apart from some cases that the stone walls are
protected by the roots of fast growing shrubs and trees. Maintaining such terraces appears a
very expensive practice comparing to most other alternatives for soil erosion control (Plate
8). Considering that such terraces protect very valuable soil for preserving the natural
vegetation, these agricultural structures should be ameliorated with the aid of the national
consolidation schemes, particularly in the environmentally sensitive areas.
Plate 7. Badly degraded area (Lesvos) due to adverse climatic conditions, intensive
cultivation in the past and overgrazing accompanying with fires today (photo by C.
Kosmas, October 1996).
4.4 Fires
The recent number and the extent of forest fires occurring in the Mediterranean region are
amongst the most serious environmental problems. In addition to the loss of vegetation, forest
fires induce changes in physico-chemical properties of soils such as water repellency, loss in
nutrients and increased runoff and erosion. They also extinguish wildlife habitat, cause loss of
human life and damage infrastructure. The loss of vegetation after fire and the progressive
inability of soils to regenerate adequate vegetation cover due to erosion has already led to
severe degradation and desertification of extensive hilly areas in the Mediterranean region.
Plate 8. A typical example of an abandoned terraced area (Peloponnesus, Greece), in
which terraces have collapsed resulting washing out of the soil (photo by C. Kosmas).
Fires have become frequent in the pine dominated forests during the last fifty years.
Most of the fires can be attributed to the people carelessness. The majority of fires occur in
areas with high xerothermic indices and moisture deficits. Soil dryness and wind speed are the
principal factors of fire evolution. The areas affected by forest fires are increasing
dramatically throughout the Mediterranean basin. In the period from1960 to 1975, the average
rate of burned area was 200 000 ha/yr, from 1975 to 1980 470 000 ha/yr, and 660 000 ha/yr
from 1981 to 1985 (Conacher and Sala, 1998). Since 1960s, 1 144 710 hectares have been
burned in Greece. Considering that the forest land in Greece covers about 8 200 000 ha,
therefore 14% of that area has been affected by fires in the last 38 years. Similar conditions
have been reported for other areas of the Mediterranean region. For example 9.4% of the
forested area has been affected by fires in Spain during the last 12 years.
Erosion rates seem to be enhanced after fires. The increased erosion rates are only
partly due to the removal of vegetation. More important seems to be the forming of an
impermeable subsurface layer, which decreases infiltration rates, while causing a quick
saturation of the upper layers leading to overland flow and erosion (Giovannini and Lucchesi,
1993). In contrast aggregate stabiloity increases after fire and that increase is more
pronoiunced after severe burns (Molina, 1993).
C. Kosmas1, A. Ferrara2, H. Briassouli3, and A. Imeson4
1Agricultural University of Athens, Laboratory of Soils and Agricultural Chemistry
2Universita degli Studi della Basilicata, Dipartimento di Produzione Vegetale
3University of the Aegean, Department of Human Geography
4Unuversity of Amsterdam, Department of Physical Geography and Soil Science
1. Definition of ESAs
The different types of ESAs to desertification can be analysed in relation to various
parameters such as landforms, soil, geology, vegetation, climate, and human action. Each of
these parameters is grouped into various uniform classes with respect the its behaviour on
desertification and weighting factors are assigned in each class. Then the following four
qualities are evaluated (a) soil quality, (b) climate quality, (c ) vegetation quality, and (d)
management quality. After the computation of four indices for each quality, the ESAs to
desertification are defined by combining them (Fig. 14). All the data defining the four
qualities are introduced in a regional geographical information system (GIS), and overlaid in
accordance with the developed algorithm and maps of ESAs to desertification are compiled.
This approach includes parameters which can be easily found in existing soil, vegetation, and
climate reports of an area.
Three general types of Environmentally Sensitive Areas (ESAs) to desertification can
be distinguished based on the stage of land degradation:
Type A: Areas already highly degraded through past misuse, presenting a threat to the
environment of the surrounding areas. For example, badly eroded areas subject to high runoff
and sediment loss. This may cause appreciable flooding downstream and reservoir
sedimentation. These are critical ESAs.
Type B: Areas in which any change in the delicate balance of natural and human activity is
likely to bring about desertification. For example, the impact of predicted climate change due
to greenhouse warming is likely to enchance reduction in the biological potential due to
drought causing areas to lose their vegetation cover, subject to greater erosion, and finally
shift to the Type A category. A land use change, as for example, a shift towards cereals
cultivation, on sensitive soils might produce immediate increase in runoff and erosion, and
perhaps pesticide and fertiliser pollution downstream. These are fragile ESAs.
Type C: Areas threatened by desertification under significant climate change, if a particular
combination of land use is implemented or where offsite impacts will produce severe
problems elsewhere, for example pesticide transfer to downslope or downstream areas under
variable land use or socio-economic conditions. This would also include abandoned land
which is not properly managed. This is a less severe form of Type B, for which nevertheless
planning is necessary. These are potential ESAs.
Areas with deep to very deep, nearly flat, well drained, coarse-textured or finer soils, under
semi-arid or wetter climatic conditions, independently of vegetation are considered as non-
threatened by desertification.
1. Definizione delle ESAs
I diversi tipi d’ESAs alla desertificazione possono essere analizzati in relazione ai vari
parametri come morfologia del suolo, profondità del suolo, composizione geologica,
vegetazione, clima o azioni umane. Ognuno di questi parametri è raggruppato in vari classi
uniformi in relazione alla sua influenza sulla desertificazione, e di pesi assegnati ad ogni
classe. In seguito sono valutati quattro parametri: (a) la qualità del terreno, (b) la qualità
del clima, (c) la qualità della vegetazione e (d) la qualità della gestione. Dopo aver calcolato
quattro indici per ciascun qualità del suolo, si procede alla definizione delle ESAs alla
desertificazione combinando i quattro indici (Fig. 14). Tutti i dati che definiscono i quattro
parametri del suolo sono introdotti in un GIS regionale, e sovrapposti secondo gli algoritmi
sviluppati, quindi sono prodotte le mappe delle ESAs alla desertificazione.
Si possono distinguere tre tipi generali d’ESAs alla desertificazione in base al grado
di degradazione del suolo:
Tipo A: Aree già altamente degradate tramite il cattivo uso del terreno, che presenta una
minaccia all’ambiente delle aree circostante. Per esempio, aree molto erose soggette ad
un’alto deflusso e perdita di sedimenti. Queste aree sono denominate ESAs critiche.
Tipo B: Aree dove qualsiasi cambiamento del delicato equilibrio delle attività naturali o
umane molto probabilmente porterà alla desertificazione. Per esempio, l’impatto del previsto
cambiamento climatico causato dall’effetto serra probabilmente determinerà una riduzione
del potenziale biologico causata dalla siccità provocando la perdita della copertura vegetale
in molte aree, che saranno soggette ad una maggiore erosione, e diventeranno di Tipo A. Un
cambiamento nell’uso del suolo, per esempio uno spostamento verso la coltivazione di cereali
su suoli sensibili potrebbe produrre un immediato aumento del deflusso e dell’erosione, e
forse l’inquinamento a valle da parte di pesticidi e fertilizzanti. Queste aree sono denominate
ESAs fragili.
Tipo C: Aree minacciate dalla desertificazione sono soggette ad un significativo
cambiamento climatico; se una particolare utilizzazione del suolo è praticata con criteri
gestionali non corretti si potranno creare seri problemi, per esempio il scorrimento di
pesticidi lungo la pendice e deposito a valle dei principi attivi nocivi alla vegetazione. Questo
tipo è meno severo del precedente, ma ciò nonostante è necessaria attuare una pianificazione
delle aree. Queste aree sono denominate ESAs potenziali.
Aree profonde o molto profonde, pianeggiati, ben drenati, e con tessitura grossolana o
suoli con particelle più fini, soggette a condizioni semi-aride o con condizioni climatiche più
umide, indipendentemente della loro copertura vegetale sono considerate come aree non
soggette a desertificazione o comunque soggetti al lento processo di degradazione e
comunque stabili.
2. Ορισµός των ΠΕΠ
Οι διαφορετικές κατηγορίες των ΠΕΠ για την απερήµωση µπορούν να αναλυθούν σε σχέση µε
διάφορες παραµέτρους όπως η γεωµορφολογία, το έδαφος, η γεωλογία, η βλάστηση, το κλίµα
και οι ανθρώπινες δραστηριότητες. Κάθε µια από αυτές τις παραµέτρους οµαδοποιείται σε
διάφορες οµοιογενείς κατηγορίες σε σχέση µε τη συµπεριφορά της προς την απερήµωση και σε
κάθε κατηγορία δίνονται συντελεστές βαρύτητας. Μετά οι ακόλουθες τέσσερεις ποιότητες
(α) ποιότητα εδάφους, (β) ποιότητα κλίµατος, (γ) ποιότητα βλάστησης και (δ)
ποιότητα διαχείρισης. Μετά από τον υπολογισµό των τεσσάρων δεικτών (ενός για κάθε
κατηγορία) οι ΠΕΠ για την απερήµωση προσδιορίζονται από το συνδυασµό τους. (Εικ. 14). Όλα
τα δεδοµένα που προσδιορίζουν τις τέσσερεις ποιότητες εισάγονται σε ένα γεωγραφικό
πληροφοριακό σύστηµα και γίνονται διαδοχικές επιθέσεις µε βάση τον αλγόριθµο που
αναπτύχθηκε οπότε και φτιάχνονται οι χάρτες των ΠΕΠ. Αυτή η προσέγγιση περιλαµβάνει
παραµέτρους που µπορούν να βρεθούν εύκολα σε διαθέσιµες εκθέσεις για το έδαφος, το κλίµα
και τη βλάστηση µιας περιοχής.
Τρεις γενικές κατηγορίες Περιβαλλοντικά Ευαίσθητων Περιοχών (ΠΕΠ) για την απερήµωση
µπορούν να διακριθούν µε βάση το στάδιο της υποβάθµισης
Κατηγορία Α
Περιοχές ήδη υποβαθµισµένες λόγω κακής χρήσης στο παρελθόν που
παρουσιάζουν κινδύνους για το περιβάλλον γειτονικών περιοχών. Για παράδειγµα έντονα
διαβρωµένες περιοχές που υπόκεινται σε υψηλή επιφανειακή απορροή και απώλεια ιζήµατος.
Αυτό µπορεί να προκαλέσει σηµαντικά πληµµυρικά φαινόµενα στα κατάντη και πρόσχωση των
φραγµάτων. Αυτές είναι οι κρίσιµες ΠΕΠ.
Κατηγορία Β
Περιοχές στις οποίες κάθε αλλαγή στην λεπτή ισορροπία φυσικής και ανθρώπινης
δραστηριότητας είναι πιθανόν να προκαλέσει απερήµωση. Για παράδειγµα η επίδραση της
προβλεφθείσας κλιµατικής αλλαγής λόγω του φαινοµένου του θερµοκηπίου είναι πιθανόν να
προκαλέσει µείωση του βιολογικού δυναµικού λόγω της ξηρασίας που θα οδηγήσει στην
απώλεια της φυτοκάλυψης από ορισµένες περιοχές, θα τις εκθέσει έτσι σε µεγαλύτερη διάβρωση
και τελικά θα τις µετατοπίσει στην κατηγορία Α. Μια αλλαγή της χρήσης γης, όπως για
παράδειγµα µια µετατόπιση προς την καλλιέργεια σιτηρών σε ευαίσθητα εδάφη µπορεί να
προκαλέσει άµεση αύξηση στην απορροή και τη διάβρωση και ίσως µόλυνση από φυτοφάρµακα
και λιπάσµατα στα κατάντη. Αυτές είναι ευαίσθητες ΠΕΠ.
Κατηγορία Γ
Περιοχές που απειλούνται από την απερήµωση κάτω από σηµαντική κλιµατική
αλλαγή, αν εφαρµοστεί κάποιος ειδικός συνδυασµός χρήσεων γης ή εκεί όπου δηµιουργούνται
έντονα προβλήµατα από επιδράσεις που ξεκινούν από αλλού, για παράδειγµα η µεταφορά σε
χαµηλότερες περιοχές φυτοφαρµάκων που χρησιµοποιήθηκαν ψηλότερα κάτω από διαφορετικές
χρήσεις γης ή κοινωνικοοικονοµικές συνθήκες. Αυτό θα περιλάµβανε ακόµα εγκαταλελειµµένη
γη η οποία δε διαχειρίζεται σωστά. Αυτή είναι µια λιγότερο σοβαρή περίπτωση από την
κατηγορία Β, για την οποία όµως είναι απαραίτητος ο σχεδιασµός. Αυτές είναι οι δυνητικές
Περιοχές µε βαθιά ως πολύ βαθιά, σχεδόν επίπεδα, καλά αποστραγγιζόµενα, χονδρόκοκκα ή και
πιο λεπτόκκοκα εδάφη κάτω από ηµίξηρες ή και πιο υγρές κλιµατικές συνθήκες ανεξάρτητα από
τη βλάστηση θεωρούνται σαν µη απειλούµενες από την απερήµωση.
2. Definição de ESAs
Os diferentes tipos de ESAs no contexto da desertificação podem ser analisados relativamente
a vários parâmetros, tais como morfologia, solos, geologia, coberto vegetal, clima, e acção
antrópica. Cada um destes parâmetros é agrupado em diferentes classes uniformes que
reflectem o seu comportamento relativamente à desertificação, sendo atribuídos factores de
ponderação para cada classe. Seguidamente, as quatro qualidades dos recursos da terra são
avaliadas: (a) qualidade de solo. (b) qualidade de clima. (c) qualidade de vegetação e (d)
qualidade de gestão. Depois do tratamento informático dos quatro índices para cada
qualidade, as ESAs são definidas pelo cruzamento dessas qualidades (Fig. 14). Toda a
informação relativa às diferentes qualidades dos recursos da terra é introduzida num Sistema
de Informação Geográfica (SIG) de base regional, sendo sobrepostos os diferentes níveis de
informação de acordo com um algoritmo matemático, de modo a produzir mapas de Áreas
Ambientalmente Sensíveis (ESAs) no contexto da desertificação. Esta abordagem inclui
parâmetros que podem ser facilmente acessíveis em estatísticas e fontes de informação sobre
solos, clima e vegetação.
Podem distinguir-se, deste modo, três tipos principais de ESAs no contexto da
desertificação, baseadas no seu estado de degradação:
Tipo A: Áreas já bastante degradadas devido a uma incorrecta utilização no passado,
constituindo uma ameaça para o ambiente das áreas envolventes. Por exemplo, áreas
severamente erodidas, sujeitas a elevados índices de escorrência superficial e de perda de
solo. Neste caso podem ocorrer a jusante inundações com alguma gravidade e a
sedimentação das albufeiras. Estas são as ESAs Críticas.
Tipo B: Áreas onde qualquer alteração no delicado equilíbrio entre o meio natural e as
actividades humanas pode conduzir o ecossistema no sentido da desertificação. Por exemplo,
o impacto da previsível alteração climática devido ao aquecimento global (efeito de estufa)
pode potenciar uma redução do potencial biológico devido à ocorrência de secas, causando
uma redução do coberto vegetal, um aumento da erosão do solo e por fim uma mudança para
a categoria do Tipo A. Uma mudança do uso do solo, por exemplo no sentido do cultivo de
cereais, em áreas de solos com elevada sensibilidade, pode produzir um aumento imediato
nos fenómenos de escorrência superficial e na erosão hídrica, e talvez ainda problemas de
poluição a jusante devido ao arrastamento de pesticidas e fertilizantes. Estas são ESAs
Tipo C: Áreas ameaçadas pela desertificação em face de uma significativa alteração
climática, se uma particular combinação de usos do solo for implementada e onde impactos
externos podem produzir graves problemas, como por exemplo a transferência de pesticidas
ao longo das vertentes e cursos de água para áreas a jusante, sujeitas a uma variedade de
usos de solo e condições socioeconómicas. Esta situação inclui também as terras que são
abandonadas e não devidamente geridas posteriormente. Trata-se de uma forma menos
severa que o Tipo B, para a qual, no entanto, é necessário ordenamento e gestão. Estas são
as ESAs Potenciais.
As áreas com solos profundos ou muito profundos, quase planos, bem drenados, com textura
grosseira ou mais fina, sob condições climáticas semi-áridas ou mais húmidas,
independentemente do coberto vegetal, são consideradas como não ameaçadas ou não
afectadas pela desertificação.
1. Definición de AMS
Los diferentes tipos de AMS a la desertificación pueden ser analizados en relación a varios
parámetros como geoformas, suelo, geología, vegetación, clima y acción humana. Cada uno
de estos parámetros se agrupa dentro de varias clases uniformes con respecto a su
comportamiento sobre el proceso de desertificación y se les asignan factores de ponderación
dentro de cada clase. Se pueden evaluar las siguientes cuatro calidades de la tierra: (a)
calidad del suelo, (b) calidad del clima, (c) calidad de la vegetación y (d) calidad de la
gestión. Tras la computación de los cuatro índices para cada calidad de la tierra, las ASM a
la desertificación son definidas por la combinación de estos índices (Fig. 14). Todos los datos
que definen las cuatro calidades de la tierra se introducen en un sistema de información
geográfica (SIG) regional, se superponen de acuerdo con el algoritmo desarrollado, y los
mapas de las AMS a la desertificación son compilados. Esta aproximación incluye
parámetros que se pueden encontrar fácilmente en los informes existentes sobre suelo,
vegetación y clima de un área.
Se pueden distinguir tres tipos diferentes de Areas Medioambientalmente Sensibles
(AMS) a la desertificación, sobre la base del estado de degradación de la tierra:
Tipo A: Areas que ya están altamente degradadas por el mal uso pasado, que presentan una
amenaza para el medio ambiente de las áreas adyacentes. Por ejemplo, areas muy
erosionadas sujetas a alta escorrentía y alta producción de sedimentos, que pueden causar
inundaciones aguas abajo y colmatación de embalses. Son las AMS críticas.
Tipo B: Areas en las cuales cualquier cambio en el delicado balance entre la naturaleza y la
actividad humana puede, verosílmente, causar desertificación. Por ejemplo, el impacto del
cambio climático que se predice debido al efecto invernadero verosílmente intensificará la
reducción del potencial biológico debido a que la sequía causará la pérdida de la cobertura
vegetal en ciertas áreas, que sufrirán mayor erosión y finalmente cambiarán a la categoría de
Tipo A. Un cambio en el uso del suelo, como por ejemplo un cambio hacia el cultivo del
cereal, sobre suelos sensibles pudiera producir un incremento inmediato en la ecorrentía y la
erosión, y quizá contaminación por pesticidas y fertilizantes aguas abajo. Estas áreas son las
AMS frágiles.
Tipo C: Areas amenazadas por desertificación ante un cambio climático significativo, si se