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Plant succession and soil degradation in desertified areas (Fuerteventura, Canary Islands, Spain)
Catena 59 (2005) 117 – 131 www.elsevier.com/locate/catena
Plant succession and soil degradation in desertified areas (Fuerteventura, Canary Islands, Spain) Antonio Rodrı´guez Rodrı´guez*, Juan Luis Mora, Carmen Arbelo, Juan Bordon Departamento de Edafologı´a y Geologı´a, Facultad de Biologı´a, Universidad de La Laguna, La Laguna, Tenerife, Canary Islands, Spain Received 13 February 2003; received in revised form 24 June 2004; accepted 21 July 2004
Abstract The eastern Canary Islands constitute a region that is vulnerable to desertification processes, mainly due to its intensely arid climate and the action of man. Vegetation in the island of Fuerteventura has been profoundly transformed over the last few hundred years, and the greater part of the insular territory is presently covered by substitution brush. In this work, a study of soil degradation processes relating to plant cover transformation is presented. To this end, soils that characterize the present-day countryside of the island and those associated with enclaves of original vegetation are studied, and the sequential variation of soil properties along the plant succession is established by means of multivariant analysis of environmental gradients. The results indicate that the original vegetation was established on soils with a low natural quality, severely limited by natural aridity, salinity and sodicity as well as the water and wind erosion processes dominant in the island. These same ecological factors condition the quality of the island’s present soils, although the degradation of the plant cover has increased the
* Corresponding author. Fax: +34 922 318311. E-mail address: [email protected] (A. Rodrı´guez Rodrı´guez). 0341-8162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2004.07.002
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severity of several soil degradation processes, particularly those of a physical and biological nature. D 2004 Elsevier B.V. All rights reserved. Keywords: Multivariant analysis; Soil quality; Canary Islands; Plant succession; Soil degradation
1. Introduction Soils sustaining mature ecosystems are in sustained and stable equilibrium with their environmental conditions. They are thus attributed a high environmental quality, even using them as references to evaluate soil quality in certain ecological settings (Doran and Jones, 1996, Rodrı´guez Rodrı´guez et al., 2002a). The processes leading ecosystems towards maturity give place to active soils in equilibrium, with minimum free energy and a high degree of self-organization (Margalef, 1974). Degradation of an ecosystem usually causes a decrease in soil quality together with a regression of the ecological succession (Arbelo et al., 2002, Rodrı´guez Rodrı´guez et al., 2002a,b). The eastern Canaries and, in particular, the island of Fuerteventura, are subjected to a clear desertification, enhanced by their proximity to the Sahara Desert, extreme aridity and periods of persistent drought, in addition to the flat relief subject to the influence of the dominant northwest winds, frequent winds from the Sahara and torrential rains (Rodrı´guez Rodrı´guez, 2001). Under the present climatic conditions, alteration of geological material, accumulation of organic material and migration of materials in the soil are very limited, leading soil degradation processes to predominate over those of soil formation (Torres Cabrera, 1995). This has been accompanied during the last several hundred years by an intense land degradation process caused by human activity (desertification). The conquest of Fuerteventura by Europeans took place in the early XVth century, while the greater part of the island was still covered by a succulent xerophytic scrub locally known as bTabaiba scrubQ (Webb and Berthelot, 1835–1850; Pitard and Proust, 1908; Abreu Galindo, 1955; Herna´ndez-Rubio, 1983; Serra Ra´fols, 1986; Cabrera, 1996; Santos, 1988, 2000; Rodrı´guez Delgado et al., 2000, 2004, etc.). After the conquest, degradation of tabaiba shrubs started to accelerate as shrubs were used for fuel or animal fodder. Soil tillage was carried out in the more favorable topographical areas and grazing of numerous goat herds are still present today. Therefore, tabaiba scrub was reduced to areas largely unfavorable for human use and inaccessible both to man and their livestock (Gonza´lez Henrı´quez et al., 1986; Gonza´lez Anto´n, 1998; Santos, 2000; Cabrera, 2001). These formations that covered the island at the time of this human intervention had clear associations with western North African vegetation (Fuerteventura is only 100 km away from this continent), from southern Morocco to Senegal and the Cape Verde coasts along the whole oceanic Sahara (Santos, 2000).
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The aim of this study is to evaluate the effect of the transformation of the original tabaiba scrub into serial substitution communities (shrub steppe and halophile scrub) on present soil quality in a particular setting, Fuerteventura, where soil quality is limited by natural geographic factors.
2. Material and methods 2.1. Study area The island of Fuerteventura is situated in the Canary Archipelago between 28845V04U and 28802V16U north latitude, and 13849V12U and 14830V24U west longitude. It is the second largest of the Canaries (1655 km2) and the closest to the continental margin of Africa (115 km to the nearest point). Its orography is relatively flat and the highest altitude is Pico de La Zarza on the Jandia Peninsula at 807 m. Annual rainfall is less than 200 mm (169 mmF100) and is concentrated between November and February. Potential evapotranspiration exceeds 900 mm/year (918 mm/year—Thornthwaite method and 1800 mm/year—Evaporation measured by tank method) and the average temperature is around 20 8C (19.6 8CF0.8) (Torres Cabrera, 1995). The dominant soil climate is Aridic and Thermic. The predominant soils are Aridisols: Petrocalcids, Haplocalcids, Petroargids and Haplocambids (Torres Cabrera, 1995). The dominant bioclimate in the island is desertic hyperarid infra-thermomediterranean, and the main potential vegetation is Tabaiba scrub, which is succulent in appearance and dominated by species of the genus Euphorbia, such as E. balsamifera Aiton, E. regisjubae Webb and Berth, E. handiensis Burchard and E. canariensis L. Tabaiba scrub presently occupies a very small area, restricted to isolated sites of difficult access such as ravines and steep slopes. The present vegetation of the island mainly features scrub and disperse grassland, enriched by plant species spurned by livestock. Short-lived therophytes are characteristic (Stipa capensis Thunb, Mesembryanthemum nodiflorum L. and Aizoon canariense L., among others), spiny xerophytes such as Launaea arborescens (Batt.) Murb. and Lycium intricatum Boiss, bushy halophylous Chenopodiaceae belonging to the genera Salsola, Suaeda and Chenoleoides (Rodrı´guez Delgado et al., 2000). 2.2. Field and laboratory analysis With the aim of characterizing the original vegetation and associated soils, the pockets of Tabaibal vegetation still subsisting in Fuerteventura were located, and 10 plots (2020 m) were selected for study. In order to study the soils and flora currently dominant in the island, systematic sampling was carried out in areas measuring 55 km, disregarding those points situated over aeolian sand substrates, to a total of 69 plots (Fig. 1). The field work was carried out in March and April, 2001. A soil sample resulting from the mixture of three subsamples of the first 25–30 cm of soil was collected in each plot. This depth was chosen because the soil holds the greatest portion of developed plant roots and because this soil is the most sensitive to degradation processes.
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Fig. 1. Location of the 69 study sites separating plant communities according to TWINSPAN results.
The general characteristics of the soil and the land in each plot were described according to the Soil Survey Manual (Soil Survey Division Staff, 1993) (Table 1), and a plant inventory was performed by means of line-intersection method. This method evaluates the relative coverage of each species from the projection of the canopy of individuals above lines situated at regular intervals on the ground (Ferna´ndez-Palacios and De los Santos, 1996). The nomenclature of the plant species follows Acebes Ginove´s et al. (2001). In the laboratory, the physical and chemical properties related to soil degradation processes were analyzed: water retention, bulk density, particle-size composition, structural stability, soil reaction, electrical conductivity, soluble cations and anions, exchangeable cations, calcium carbonate, available phosphorus and micronutrients, organic matter and total nitrogen. The analytical methods employed are given in Table 1. Vegetation cover data were analyzed by means of a divisive hierarchical classification—Two-Way Indicator Species Analysis (TWINSPAN) (Hill, 1979) that delimited like groups of plots characterized by the presence of certain indicator plant species. By way of a Detrended Correspondence Analysis (DCA) (Hill and Gauch, 1980), a graphic ordination of the plots was obtained on the basis of its floristic composition, allowing the relationship between the vegetation and soil types to be studied as well as the relationship among plot groups defined by TWINSPAN. In order to study the relationship between the vegetation and soil variables, a Canonical Correspondence Analysis (CCA) (Ter Braak, 1986) was used. The most suitable variables for this analysis were selected by means of a Montecarlo test ( p=0.05,
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Table 1 Methods Analysis
Methodology
Oxidizable carbon Total nitrogen Calcium carbonate Soil reaction Soil solution and soluble ions
Walkley and Black, 1934 Kjedahl method Bernard calcimeter (MAPA, 1974) Soil/water suspension 1:2.5 ratio Soil/water extract 1:1 ratio Emission photometry (Na+, K+) Atomic absorption spectrometry (Ca2+, Mg2+) HCl potentiometry (HCO3 ) AgNO3 potentiometry (Cl ) Bower and Wilcox, 1965 (SO24 ) Bower et al., 1952 Emission photometry (Na+, K+) Atomic absorption photometry (Ca2+, Mg2+) Olsen et al., 1954 DTPA extraction (Lindsay and Norwell, 1978) Hot-water soluble boron (Gupta, 1979) Gravimetric method Richards, 1980 Undisturbed samples, tin cylinders of known volume (240 ml) Dispersion with sodium hexametaphosphate, densimeter method, sieving for the sandy fraction Bartoli et al., 1991 Soil Survey Division Staff, 1993
Exchangeable cations
Available P Available Fe, Cu, Mn and Zn Available B Water content Water retention Bulk density Particle size Aggregate stability Rocky outcrops, soil depth, aggregate size, hardness, porosity, pore size, root abundance , root size
5000 iterations). The same test was employed to determine the significance of the canonical axes obtained. A comparison was made of the values of all the properties studied in the groups of plots defined by the Kruskal–Wallis and Mann–Whitney U-tests. The statistical analyses were performed using the programs SPSS (Anon, 1990) and CANOCO (Ter Braak and Sˇmilauer, 1998).
3. Results A total of 46 plant species was identified in the floristic inventories. Therophytes predominate (50% of the species), followed by Chamaephytes (24%), Nanophanerophytes (20%) and other life-forms (Hemycryptophytes and Geophytes). The classification of the plots by means of TWINSPAN (Fig. 2) discriminates firstly plots with Tabaiba scrub vegetation, and then delimits a shrub-steppe and halophile scrub with species of a coastal or ruderal nature. The territorial distribution of these plant formations (Fig. 1) situates the shrub-steppe in areas of the inland sites and on the leeward flanks of the island of Fuerteventura, while the halophile scrub is situated in coastal areas exposed to the predominant northwest winds.
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spp.
Fig. 2. Vegetation analysis by means of a Two-Way Indicator Species Analysis (TWINSPAN).
The original Tabaiba scrub vegetation is located in both coastal areas and in the inland sites. The structure of the three plant communities differs in the abundance of the different plant life-forms (Table 2). The Tabaiba scrub possesses a greater perennial shrub cover than the substitution vegetation in comparison with annual herbaceous species, and may afford more effective protection to the soil surface. Ordination by way of DCA (Fig. 3) reveals the successional sequence of the plant communities. The plots of Tabaiba scrub make up a highly homogeneous and differentiated unit, with only slight differences between coastal and inland areas. The two substitution brush types are clearly distinct from the Tabaiba scrub, although the shrub steppe presents a floristic composition that is closer to that of the Tabaiba scrub plots of the inland sites, and the halophile scrub is more similar to the Tabaiba scrub of the coastal areas. In regard to the soil type, the soils of the plots studied are Aridisols and Entisols, with Calcids (65% of the plots) and Argids (19%), followed by Fluvents (7%) and Orthents (6%) predominating. As shown by the DCA (Fig. 3), Tabaiba scrub frequently develops over Argids in inland areas and over Calcids in coastal areas. The corresponding substitution brush Table 2 Vegetation life forms (%; mean and standard deviation) Nanophanerophytes Chamaephytes Hemicryptophytes Geophytes Therophytes Total coverage
Tabaiba scrub
Shrub steppe
Halophile scrub
27.4F13.1 [a] 2.4F3.7 [a] 0.0F0.0 [ab] 0.0F0.1 [a] 0.5F1.3 [b] 30.3F15.6 [a]
5.6F6.7 [b] 7.4F5.5 [b] 0.2F0.5 [a] 0.1F0.2 [a] 8.4F14.8 [b] 21.6F15.9 [a]
6.2F4.0 0.8F1.8 0.0F0.0 0.0F0.1 4.5F6.2 11.5F6.3
[b] [a] [b] [a] [b] [b]
Values followed by the same character do not exhibit significant differences (Kruskal–Wallis and Mann–Whitney tests, pV0.05).
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Fig. 3. Detrended Correspondence Analysis of vegetation.
displays an analogous behaviour: the shrub steppe is frequently situated on Argids, while the halophytic scrub occupies, mainly Calcids. In general, the soils studied are poor in organic matter, clay–loam in texture, slightly alkaline, with a high degree of carbonation, very saline and extremely sodic. The mean values for each plant community are indicated in Table 3. Despite the limited size of the samples, which makes it difficult to find significant differences, the soils of the Tabaiba scrub exhibit significantly higher values of organic matter (oxidizable carbon), porosity and abundant roots, especially evident for tabaiba scrub in coastal sites (Fig. 4), while they present very disperse values of other relevant properties such as salinity or particle size. The CCA (Fig. 4) shows the relationship between the vegetation and the most significant soil variables (Montecarlo test, pV0.05).
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Table 3 Soil properties (mean and standard deviation) Tabaiba scrub Water content (%) Water retention (g kg 1) (dry) Particle size (g kg 1)
Shrub steppe
3.76F0.52 [a] 4.03F1.16 [a] 33 kPa 26.0F4.1 [a] 27.5F5.0 [a] 1500 kPa 15.9F2.2 [a] 15.3F2.1 [a] Clay 303F121 [ab] 348F93 [a] Silt 446F157 [a] 372F93 [a] Sand 251F126 [a] 280F133 [a] Aggregate stability (%) 17.5F6.3 [a] 14.5F6.8 [a] Bulk density (g cm 3) 1.1F0.2 [a] 1.2F0.2 [a] pH 8.6F0.2 [a] 8.8F0.3 [a] E.C. (1:1 soil/water 6.0F7.5 [a] 2.6F7.2 [b] ratio, dS m 1) Soluble ions 0.9F1.0 [a] 0.7F1.7 [a] Ca2+ (cmolc kg 1) Mg2+ 0.7F0.7 [a] 0.4F1.1 [b] 0.2F0.2 [ab] 0.1F0.1 [a] K+ Na+ 3.7F5.1 [a] 1.6F5.3 [b] SO24 0.5F0.8 [ab] 0.2F0.3 [a] Cl 4.7F6.9 [a] 2.6F8.9 [b] 0.3F0.3 [a] 0.4F0.3 [a] HCO3 Calcium carbonate 63F60 [a] 103F100 [ab] (g kg 1) 19.5F3.3 [ab] 22.7F7.3 [a] Exchangeable Ca2+ Mg2+ cations 5.7F2.2 [a] 4.7F2.0 [a] (cmolc kg 1) K+ 3.0F1.5 [a] 2.9F1.2 [a] Na+ 6.9F5.3 [a] 6.5F9.7 [a] Oxidizable C 10.3F5.3 [a] 6.2F2.5 [b] (g kg 1) Total nitrogen 1.24F0.47 [a] 0.77F0.24 [ab] (g kg 1) 23.8F16.1 [a] 15.1F9.1 [a] P-Olsen (g kg 1) Micronutrients Fe 4.7F1.9 [a] 3.6F1.7 [a] (g kg 1) Cu 1.2F0.4 [a] 1.0F0.5 [a] Mn 5.0F1.2 [a] 4.4F1.5 [a] Zn 0.3F0.2 [a] 0.3F0.1 [a] B 3.5F2.7 [a] 2.1F1.8 [a] Surface pebbles Very many Very many (2–75 mm) (15–90%) [a] (15–90%) [a] Surface stones Very many Very many (250–600 mm) (15–90%) [a] (15–90%) [a] Rocky outcrops Common (2–10%) [a] Very few (b2%) [b] Soil depth Very shallow (0–25 cm) [a] Shallow (25–50 cm) [a] Aggregate size Coarse (5–10 mm) [a] Coarse (5–10 mm) [a] Hardness Soft [a] Soft [a] Porosity Common (1–5/cm2) [a] Common(1–5/cm2) [a] Pore size Very fine (b0.5 mm) [a] Very fine (b0.5 mm) [a] Root abundance Few (b1/cm2) [a] Few (b1/cm2) [a] Root size Fine (1–2 mm) [a] Very fine (b1 mm) [a] Soil macrofauna Present [a] Absent [a]
Halophile scrub 3.25F1.31 [a] 26.2F5.3 [a] 15.7F3.6 [a] 250F67 [b] 352F123 [a] 393F145 [b] 12.7F6.2 [a] 1.2F0.2 [a] 8.8F0.5 [a] 10.0F11.0 [a] 1.4F1.8 [a] 0.9F1.1 [a] 0.7F2.4 [b] 5.8F8.0 [a] 1.1F1.8 [b] 10.9F15.5 [a] 0.4F0.3 [a] 163F137 [b] 16.5F4.1 [b] 4.4F1.8 [a] 3.2F2.7 [a] 9.0F10.9 [a] 5.5F2.9 [b] 0.63F0.28 [b] 19.8F10.0 [a] 2.7F1.8 [b] 0.7F0.4 [b] 2.6F1.6 [b] 0.3F0.3 [a] 4.2F5.3 [a] Very many (15–90%) [a] Very many (15–90%) [a] Very few (b2%) [b] Shallow (25–50 cm) [a] Medium (2–5 mm)[a] Soft [a] Few (b1/cm2) [b] Fine (1–2 mm) [a] Very few(b0.2/cm2) [b] Very fine (b1 mm) [a] Absent [a]
Values followed by the same character do not exhibit significant differences (Kruskal–Wallis and Mann–Whitney tests, pV0.05).
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Fig. 4. Canonical Correspondence Analysis of soil properties and vegetation.
The first ordination axis obtained is highly significant ( F-ratio=6.22, p=0.0002), and seems to represent the variations undergone in the succession process, with the Tabaiba scrub plots around the positive semiaxis and the plots with substitution vegetation on the negative semiaxis. This axis shows a highly positive correlation with total nitrogen, oxidizable carbon, aggregate stability and root abundance, and negative correlations with the sand content and the pH. Thus, the Tabaiba scrub appears to be associated with healthy, more dynamic soils, with a greater environmental quality, while the regressive succession is associated with processes of erosion and of chemical and biological degradation. However, the depth of the soil and the presence of outcrops of the material of origin present their most unfavorable values in the Tabaiba scrub soils. This must be attributed to
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the fact that the Tabaiba scrub subsists at present only on land rejected for other uses because of steep slopes and high stoniness, where soils are generally shallow and with numerous rocky outcrops. On the other hand, the second axis discriminates the plots according to their geographical setting. Thus, Tabaibal plots along coastal zones and especially the halophile shrub plots are associated with the positive semiaxis, which correlates with salinity-related parameters, such as electrical conductivity, soluble sodium content, and bioavailable boron, or to carbonation processes prompted by organogenic marine sands. The shrub steppe and the Tabaibal plots away from the seashore are placed closer to the negative semiaxis, which correlates with the bioavailable Mn content and a relative Casaturation in the exchange complex. This environmental gradient is also evidenced by the soil type in each plot type: the Calcids are grouped around the positive semiaxis, whereas the Argids are placed close to the negative semiaxis. The whole of the CCA axis is statistically very significant ( Fratio=1.79, p=0.0002).
4. Discussion and conclusions The dominant vegetation in the past in Fuerteventura seems to have had a high ecological value. The initial distribution of tabaiba scrub over the whole island and the salinity analysis of soils in the current sites of this community show that this was initially found in soils of variable salinity (from E.C.=1.3 to 490 dS/m) and that these enclaves can be spatially grouped into Tabaiba scrub plots of inland sites (E.C.=4.4F3.9 dS/m) and Tabaiba scrub plots of the coastal areas (E.C.=26.5F19.3 dS/m). At the present time, Tabaiba scrub occupies both protected sites inland and exposed steep slopes in the lower areas. This diversity of biotopes is reflected in a wide range of variations in the properties of the associated soils (particularly those related to their saline– sodic state) and their typology (Calcids, Argids, Orthents). Degradation of tabaiba scrub due to human intervention and the appearance of scrubland, steppe or grassland in a regressive ecological succession, is also well documented (Rivas Martı´nez, 1987; Rivas Martı´nez et al., 1993). For the case of tabaiba scrub dominated by species of the Euphorbia genus, the serial substitution communities are: "Arid pasture" with indicator species such as Launaea, Atractylis, Cenchrus, similar to the Shrub steppe we described, "Ruderal fruticose community", featuring Frankenia, Suaeda, Chenoloides as indicator species also similar to the Halophile scrub that we described, accompanied by "Ruderal grasses" (Mesembryanthemum, Aizoon, Stipa, Medicago, etc.) (Gonza´lez Henrı´quez et al., 1986, Rivas Martı´nez et al., 1993, Arco-Aguilar et al., 1997, Rodrı´guez-Delgado et al., 1997, 2000). It, therefore, seems clear that the situation we describe here is a regressive succession that transforms the mature stage or series head (Tabaiba scrub) into subserial communities as a result of human causes. Here, we try to study the effects these changes in plant cover have on the soil properties and soil quality.
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An intense and continued human alteration has led to the substitution of the Tabaiba scrub by brush which has a floristic composition highly influenced by the soil properties. In the lower areas, under greater influence by the sea, with a predominance of Calcids, the high salinity has favored the establishment of halophile plants. In the higher inland areas, protected from the sea spray, where Argids are frequent, succession has given place to a vegetation of gramineae and small shrubs that are less salinity tolerant. Although there is a large amount of literature that has studied the distribution of patches of vegetation in desertic and arid zones in relation to edaphic properties that condition an unequal distribution of the soil moisture regime (Yair and Danin, 1980; Yair, 1983; Kadmon et al., 1989; Casanave and Valentin, 1992; Hairsine et al., 1992; Holzapfel et al., 1995; Schreiber et al., 1995; Puigdefa´bregas and Sa´nchez, 1996; Bergkamp, 1998; Puigdefa´bregas et al., 1999, Domingo et al., 2001, etc.), we consider that our situation is not comparable. As mentioned before, the present distribution of tabaiba scrub is due to the existence of enclaves that have remained undegraded in areas of difficult access: steep slopes, shallow soils and areas with rocky outcrops. The natural drainage of these slopes at times of occasional rainstorms is by natural drainage channels that take the runoff to the sea or to small reservoirs in the ravines, typical for the hydrological dynamics of the eastern islands. There are no significant differences either in the soil moisture content of the different plant communities or their water retention capacity (Table 3). They would not, therefore, be expected to have different soil moisture regimes nor should there be more growth of the vegetation after the rains in one area than in another. A short-lived cover of annual grasses appears for 2–3 months in the winter after the rains and covers the whole island, without apparent differences in the intensity of growth of the perennial plant forms commonly observed in other arid areas (Noy-Meir and Seligman, 1979). Substitution of the Tabaiba scrub has given place to a more disperse vegetation, richer in annual plants and with less radicular development, that offers the soil little protection and a scanty contribution of organic matter. As a result, the regressive plant succession seems to have produced an increase in the severity of several soil degradation processes. Thus, the substitution of the original vegetation is associated in Fuerteventura with skeletization, which is evident in the relative enrichment in sand, and is a product of erosion. The incidence of erosive processes, particularly wind erosion, seems to be greater on halophile scrub, which affords scant plant cover and is located on the lower, more exposed areas. Vegetation regression is also associated with a deterioration of the biological health of the soil, with diminished organic matter and nitrogen contents. Coinciding with the degradation of the plant cover, a certain tendency is also detected towards a greater alkalinization of the soils, although they were already of an alkaline nature. A similar phenomenon has been described in other areas of the Canary Islands (Rodrı´guez Rodrı´guez et al., 2002a), also associated with a regressive plant succession. The natural conditions of the island caused by aridity result in generally poor quality soils (Torres Cabrera, 1995). However, these characteristics are not an obstacle to tabaiba scrub establishment over the whole island, even in coastal sites most affected by salinity and sodicity.
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As a consequence of the degraded plant cover of the island, tabaiba scrub has been reduced to steep slopes, with shallow soils and numerous rocky outcrops, while in the rest of Fuerteventura the degradation of the original vegetation has been accompanied by small changes in the physical and chemical properties of the soils, such that the acquired quality of the soils of Fuerteventura is close to their natural quality, strongly limited in the recent past and presently by the aridity, salinity and sodicity and erosive processes. In earlier studies of fragile areas, showing a low resilience against human transformations that cause significant environmental impact (Arbelo et al., 2002, Rodrı´guez Rodrı´guez et al., 2002a,b), it has been established that the degradation of the ecosystem always implies a regression of the ecological succession accompanied by processes of soil degradation that cause a decrease in soil quality. In turn, in these areas, natural or ecosystem-induced regeneration, implying progression in the ecological succession, invariably involves an improvement in soil properties. However, like the scenario studied here, in arid zones with greater resilience and where, under original conditions, the vegetation is already established on low-quality saline–sodic soils affected by wind erosion processes, the relationships between plant cover and soil degradation are unclear. Here, plant succession regression only implies changes in soil properties as a consequence of the intensified physical (erosion) and biological degradation processes, without apparently causing a loss of quality in soils already presenting a low soil quality. The rehabilitation of these ecosystems should tend to enhance the natural ecological succession, delaying the incidence of physical and biological soil degradation processes, without having a determining influence on the improvement of chemical quality.
Acknowledgements This work has been supported by the Research Project REN2000 1178-GLO "Methodological design for soil degradation assessment on a detailed scale (1:50.000)" (Spanish Ministry of Science and Technology). We are indebted to Dr. A. Yair and an unknown reviewer for their valuable comments and suggestions on an earlier version of the manuscript.
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Plant succession and soil degradation in desertified areas (Fuerteventura, Canary Islands, Spain) Antonio Rodrı´guez Rodrı´guez*, Juan Luis Mora, Carmen Arbelo, Juan Bordon Departamento de Edafologı´a y Geologı´a, Facultad de Biologı´a, Universidad de La Laguna, La Laguna, Tenerife, Canary Islands, Spain Received 13 February 2003; received in revised form 24 June 2004; accepted 21 July 2004
Abstract The eastern Canary Islands constitute a region that is vulnerable to desertification processes, mainly due to its intensely arid climate and the action of man. Vegetation in the island of Fuerteventura has been profoundly transformed over the last few hundred years, and the greater part of the insular territory is presently covered by substitution brush. In this work, a study of soil degradation processes relating to plant cover transformation is presented. To this end, soils that characterize the present-day countryside of the island and those associated with enclaves of original vegetation are studied, and the sequential variation of soil properties along the plant succession is established by means of multivariant analysis of environmental gradients. The results indicate that the original vegetation was established on soils with a low natural quality, severely limited by natural aridity, salinity and sodicity as well as the water and wind erosion processes dominant in the island. These same ecological factors condition the quality of the island’s present soils, although the degradation of the plant cover has increased the
* Corresponding author. Fax: +34 922 318311. E-mail address: [email protected] (A. Rodrı´guez Rodrı´guez). 0341-8162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2004.07.002
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severity of several soil degradation processes, particularly those of a physical and biological nature. D 2004 Elsevier B.V. All rights reserved. Keywords: Multivariant analysis; Soil quality; Canary Islands; Plant succession; Soil degradation
1. Introduction Soils sustaining mature ecosystems are in sustained and stable equilibrium with their environmental conditions. They are thus attributed a high environmental quality, even using them as references to evaluate soil quality in certain ecological settings (Doran and Jones, 1996, Rodrı´guez Rodrı´guez et al., 2002a). The processes leading ecosystems towards maturity give place to active soils in equilibrium, with minimum free energy and a high degree of self-organization (Margalef, 1974). Degradation of an ecosystem usually causes a decrease in soil quality together with a regression of the ecological succession (Arbelo et al., 2002, Rodrı´guez Rodrı´guez et al., 2002a,b). The eastern Canaries and, in particular, the island of Fuerteventura, are subjected to a clear desertification, enhanced by their proximity to the Sahara Desert, extreme aridity and periods of persistent drought, in addition to the flat relief subject to the influence of the dominant northwest winds, frequent winds from the Sahara and torrential rains (Rodrı´guez Rodrı´guez, 2001). Under the present climatic conditions, alteration of geological material, accumulation of organic material and migration of materials in the soil are very limited, leading soil degradation processes to predominate over those of soil formation (Torres Cabrera, 1995). This has been accompanied during the last several hundred years by an intense land degradation process caused by human activity (desertification). The conquest of Fuerteventura by Europeans took place in the early XVth century, while the greater part of the island was still covered by a succulent xerophytic scrub locally known as bTabaiba scrubQ (Webb and Berthelot, 1835–1850; Pitard and Proust, 1908; Abreu Galindo, 1955; Herna´ndez-Rubio, 1983; Serra Ra´fols, 1986; Cabrera, 1996; Santos, 1988, 2000; Rodrı´guez Delgado et al., 2000, 2004, etc.). After the conquest, degradation of tabaiba shrubs started to accelerate as shrubs were used for fuel or animal fodder. Soil tillage was carried out in the more favorable topographical areas and grazing of numerous goat herds are still present today. Therefore, tabaiba scrub was reduced to areas largely unfavorable for human use and inaccessible both to man and their livestock (Gonza´lez Henrı´quez et al., 1986; Gonza´lez Anto´n, 1998; Santos, 2000; Cabrera, 2001). These formations that covered the island at the time of this human intervention had clear associations with western North African vegetation (Fuerteventura is only 100 km away from this continent), from southern Morocco to Senegal and the Cape Verde coasts along the whole oceanic Sahara (Santos, 2000).
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The aim of this study is to evaluate the effect of the transformation of the original tabaiba scrub into serial substitution communities (shrub steppe and halophile scrub) on present soil quality in a particular setting, Fuerteventura, where soil quality is limited by natural geographic factors.
2. Material and methods 2.1. Study area The island of Fuerteventura is situated in the Canary Archipelago between 28845V04U and 28802V16U north latitude, and 13849V12U and 14830V24U west longitude. It is the second largest of the Canaries (1655 km2) and the closest to the continental margin of Africa (115 km to the nearest point). Its orography is relatively flat and the highest altitude is Pico de La Zarza on the Jandia Peninsula at 807 m. Annual rainfall is less than 200 mm (169 mmF100) and is concentrated between November and February. Potential evapotranspiration exceeds 900 mm/year (918 mm/year—Thornthwaite method and 1800 mm/year—Evaporation measured by tank method) and the average temperature is around 20 8C (19.6 8CF0.8) (Torres Cabrera, 1995). The dominant soil climate is Aridic and Thermic. The predominant soils are Aridisols: Petrocalcids, Haplocalcids, Petroargids and Haplocambids (Torres Cabrera, 1995). The dominant bioclimate in the island is desertic hyperarid infra-thermomediterranean, and the main potential vegetation is Tabaiba scrub, which is succulent in appearance and dominated by species of the genus Euphorbia, such as E. balsamifera Aiton, E. regisjubae Webb and Berth, E. handiensis Burchard and E. canariensis L. Tabaiba scrub presently occupies a very small area, restricted to isolated sites of difficult access such as ravines and steep slopes. The present vegetation of the island mainly features scrub and disperse grassland, enriched by plant species spurned by livestock. Short-lived therophytes are characteristic (Stipa capensis Thunb, Mesembryanthemum nodiflorum L. and Aizoon canariense L., among others), spiny xerophytes such as Launaea arborescens (Batt.) Murb. and Lycium intricatum Boiss, bushy halophylous Chenopodiaceae belonging to the genera Salsola, Suaeda and Chenoleoides (Rodrı´guez Delgado et al., 2000). 2.2. Field and laboratory analysis With the aim of characterizing the original vegetation and associated soils, the pockets of Tabaibal vegetation still subsisting in Fuerteventura were located, and 10 plots (2020 m) were selected for study. In order to study the soils and flora currently dominant in the island, systematic sampling was carried out in areas measuring 55 km, disregarding those points situated over aeolian sand substrates, to a total of 69 plots (Fig. 1). The field work was carried out in March and April, 2001. A soil sample resulting from the mixture of three subsamples of the first 25–30 cm of soil was collected in each plot. This depth was chosen because the soil holds the greatest portion of developed plant roots and because this soil is the most sensitive to degradation processes.
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Fig. 1. Location of the 69 study sites separating plant communities according to TWINSPAN results.
The general characteristics of the soil and the land in each plot were described according to the Soil Survey Manual (Soil Survey Division Staff, 1993) (Table 1), and a plant inventory was performed by means of line-intersection method. This method evaluates the relative coverage of each species from the projection of the canopy of individuals above lines situated at regular intervals on the ground (Ferna´ndez-Palacios and De los Santos, 1996). The nomenclature of the plant species follows Acebes Ginove´s et al. (2001). In the laboratory, the physical and chemical properties related to soil degradation processes were analyzed: water retention, bulk density, particle-size composition, structural stability, soil reaction, electrical conductivity, soluble cations and anions, exchangeable cations, calcium carbonate, available phosphorus and micronutrients, organic matter and total nitrogen. The analytical methods employed are given in Table 1. Vegetation cover data were analyzed by means of a divisive hierarchical classification—Two-Way Indicator Species Analysis (TWINSPAN) (Hill, 1979) that delimited like groups of plots characterized by the presence of certain indicator plant species. By way of a Detrended Correspondence Analysis (DCA) (Hill and Gauch, 1980), a graphic ordination of the plots was obtained on the basis of its floristic composition, allowing the relationship between the vegetation and soil types to be studied as well as the relationship among plot groups defined by TWINSPAN. In order to study the relationship between the vegetation and soil variables, a Canonical Correspondence Analysis (CCA) (Ter Braak, 1986) was used. The most suitable variables for this analysis were selected by means of a Montecarlo test ( p=0.05,
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Table 1 Methods Analysis
Methodology
Oxidizable carbon Total nitrogen Calcium carbonate Soil reaction Soil solution and soluble ions
Walkley and Black, 1934 Kjedahl method Bernard calcimeter (MAPA, 1974) Soil/water suspension 1:2.5 ratio Soil/water extract 1:1 ratio Emission photometry (Na+, K+) Atomic absorption spectrometry (Ca2+, Mg2+) HCl potentiometry (HCO3 ) AgNO3 potentiometry (Cl ) Bower and Wilcox, 1965 (SO24 ) Bower et al., 1952 Emission photometry (Na+, K+) Atomic absorption photometry (Ca2+, Mg2+) Olsen et al., 1954 DTPA extraction (Lindsay and Norwell, 1978) Hot-water soluble boron (Gupta, 1979) Gravimetric method Richards, 1980 Undisturbed samples, tin cylinders of known volume (240 ml) Dispersion with sodium hexametaphosphate, densimeter method, sieving for the sandy fraction Bartoli et al., 1991 Soil Survey Division Staff, 1993
Exchangeable cations
Available P Available Fe, Cu, Mn and Zn Available B Water content Water retention Bulk density Particle size Aggregate stability Rocky outcrops, soil depth, aggregate size, hardness, porosity, pore size, root abundance , root size
5000 iterations). The same test was employed to determine the significance of the canonical axes obtained. A comparison was made of the values of all the properties studied in the groups of plots defined by the Kruskal–Wallis and Mann–Whitney U-tests. The statistical analyses were performed using the programs SPSS (Anon, 1990) and CANOCO (Ter Braak and Sˇmilauer, 1998).
3. Results A total of 46 plant species was identified in the floristic inventories. Therophytes predominate (50% of the species), followed by Chamaephytes (24%), Nanophanerophytes (20%) and other life-forms (Hemycryptophytes and Geophytes). The classification of the plots by means of TWINSPAN (Fig. 2) discriminates firstly plots with Tabaiba scrub vegetation, and then delimits a shrub-steppe and halophile scrub with species of a coastal or ruderal nature. The territorial distribution of these plant formations (Fig. 1) situates the shrub-steppe in areas of the inland sites and on the leeward flanks of the island of Fuerteventura, while the halophile scrub is situated in coastal areas exposed to the predominant northwest winds.
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spp.
Fig. 2. Vegetation analysis by means of a Two-Way Indicator Species Analysis (TWINSPAN).
The original Tabaiba scrub vegetation is located in both coastal areas and in the inland sites. The structure of the three plant communities differs in the abundance of the different plant life-forms (Table 2). The Tabaiba scrub possesses a greater perennial shrub cover than the substitution vegetation in comparison with annual herbaceous species, and may afford more effective protection to the soil surface. Ordination by way of DCA (Fig. 3) reveals the successional sequence of the plant communities. The plots of Tabaiba scrub make up a highly homogeneous and differentiated unit, with only slight differences between coastal and inland areas. The two substitution brush types are clearly distinct from the Tabaiba scrub, although the shrub steppe presents a floristic composition that is closer to that of the Tabaiba scrub plots of the inland sites, and the halophile scrub is more similar to the Tabaiba scrub of the coastal areas. In regard to the soil type, the soils of the plots studied are Aridisols and Entisols, with Calcids (65% of the plots) and Argids (19%), followed by Fluvents (7%) and Orthents (6%) predominating. As shown by the DCA (Fig. 3), Tabaiba scrub frequently develops over Argids in inland areas and over Calcids in coastal areas. The corresponding substitution brush Table 2 Vegetation life forms (%; mean and standard deviation) Nanophanerophytes Chamaephytes Hemicryptophytes Geophytes Therophytes Total coverage
Tabaiba scrub
Shrub steppe
Halophile scrub
27.4F13.1 [a] 2.4F3.7 [a] 0.0F0.0 [ab] 0.0F0.1 [a] 0.5F1.3 [b] 30.3F15.6 [a]
5.6F6.7 [b] 7.4F5.5 [b] 0.2F0.5 [a] 0.1F0.2 [a] 8.4F14.8 [b] 21.6F15.9 [a]
6.2F4.0 0.8F1.8 0.0F0.0 0.0F0.1 4.5F6.2 11.5F6.3
[b] [a] [b] [a] [b] [b]
Values followed by the same character do not exhibit significant differences (Kruskal–Wallis and Mann–Whitney tests, pV0.05).
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Fig. 3. Detrended Correspondence Analysis of vegetation.
displays an analogous behaviour: the shrub steppe is frequently situated on Argids, while the halophytic scrub occupies, mainly Calcids. In general, the soils studied are poor in organic matter, clay–loam in texture, slightly alkaline, with a high degree of carbonation, very saline and extremely sodic. The mean values for each plant community are indicated in Table 3. Despite the limited size of the samples, which makes it difficult to find significant differences, the soils of the Tabaiba scrub exhibit significantly higher values of organic matter (oxidizable carbon), porosity and abundant roots, especially evident for tabaiba scrub in coastal sites (Fig. 4), while they present very disperse values of other relevant properties such as salinity or particle size. The CCA (Fig. 4) shows the relationship between the vegetation and the most significant soil variables (Montecarlo test, pV0.05).
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Table 3 Soil properties (mean and standard deviation) Tabaiba scrub Water content (%) Water retention (g kg 1) (dry) Particle size (g kg 1)
Shrub steppe
3.76F0.52 [a] 4.03F1.16 [a] 33 kPa 26.0F4.1 [a] 27.5F5.0 [a] 1500 kPa 15.9F2.2 [a] 15.3F2.1 [a] Clay 303F121 [ab] 348F93 [a] Silt 446F157 [a] 372F93 [a] Sand 251F126 [a] 280F133 [a] Aggregate stability (%) 17.5F6.3 [a] 14.5F6.8 [a] Bulk density (g cm 3) 1.1F0.2 [a] 1.2F0.2 [a] pH 8.6F0.2 [a] 8.8F0.3 [a] E.C. (1:1 soil/water 6.0F7.5 [a] 2.6F7.2 [b] ratio, dS m 1) Soluble ions 0.9F1.0 [a] 0.7F1.7 [a] Ca2+ (cmolc kg 1) Mg2+ 0.7F0.7 [a] 0.4F1.1 [b] 0.2F0.2 [ab] 0.1F0.1 [a] K+ Na+ 3.7F5.1 [a] 1.6F5.3 [b] SO24 0.5F0.8 [ab] 0.2F0.3 [a] Cl 4.7F6.9 [a] 2.6F8.9 [b] 0.3F0.3 [a] 0.4F0.3 [a] HCO3 Calcium carbonate 63F60 [a] 103F100 [ab] (g kg 1) 19.5F3.3 [ab] 22.7F7.3 [a] Exchangeable Ca2+ Mg2+ cations 5.7F2.2 [a] 4.7F2.0 [a] (cmolc kg 1) K+ 3.0F1.5 [a] 2.9F1.2 [a] Na+ 6.9F5.3 [a] 6.5F9.7 [a] Oxidizable C 10.3F5.3 [a] 6.2F2.5 [b] (g kg 1) Total nitrogen 1.24F0.47 [a] 0.77F0.24 [ab] (g kg 1) 23.8F16.1 [a] 15.1F9.1 [a] P-Olsen (g kg 1) Micronutrients Fe 4.7F1.9 [a] 3.6F1.7 [a] (g kg 1) Cu 1.2F0.4 [a] 1.0F0.5 [a] Mn 5.0F1.2 [a] 4.4F1.5 [a] Zn 0.3F0.2 [a] 0.3F0.1 [a] B 3.5F2.7 [a] 2.1F1.8 [a] Surface pebbles Very many Very many (2–75 mm) (15–90%) [a] (15–90%) [a] Surface stones Very many Very many (250–600 mm) (15–90%) [a] (15–90%) [a] Rocky outcrops Common (2–10%) [a] Very few (b2%) [b] Soil depth Very shallow (0–25 cm) [a] Shallow (25–50 cm) [a] Aggregate size Coarse (5–10 mm) [a] Coarse (5–10 mm) [a] Hardness Soft [a] Soft [a] Porosity Common (1–5/cm2) [a] Common(1–5/cm2) [a] Pore size Very fine (b0.5 mm) [a] Very fine (b0.5 mm) [a] Root abundance Few (b1/cm2) [a] Few (b1/cm2) [a] Root size Fine (1–2 mm) [a] Very fine (b1 mm) [a] Soil macrofauna Present [a] Absent [a]
Halophile scrub 3.25F1.31 [a] 26.2F5.3 [a] 15.7F3.6 [a] 250F67 [b] 352F123 [a] 393F145 [b] 12.7F6.2 [a] 1.2F0.2 [a] 8.8F0.5 [a] 10.0F11.0 [a] 1.4F1.8 [a] 0.9F1.1 [a] 0.7F2.4 [b] 5.8F8.0 [a] 1.1F1.8 [b] 10.9F15.5 [a] 0.4F0.3 [a] 163F137 [b] 16.5F4.1 [b] 4.4F1.8 [a] 3.2F2.7 [a] 9.0F10.9 [a] 5.5F2.9 [b] 0.63F0.28 [b] 19.8F10.0 [a] 2.7F1.8 [b] 0.7F0.4 [b] 2.6F1.6 [b] 0.3F0.3 [a] 4.2F5.3 [a] Very many (15–90%) [a] Very many (15–90%) [a] Very few (b2%) [b] Shallow (25–50 cm) [a] Medium (2–5 mm)[a] Soft [a] Few (b1/cm2) [b] Fine (1–2 mm) [a] Very few(b0.2/cm2) [b] Very fine (b1 mm) [a] Absent [a]
Values followed by the same character do not exhibit significant differences (Kruskal–Wallis and Mann–Whitney tests, pV0.05).
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Fig. 4. Canonical Correspondence Analysis of soil properties and vegetation.
The first ordination axis obtained is highly significant ( F-ratio=6.22, p=0.0002), and seems to represent the variations undergone in the succession process, with the Tabaiba scrub plots around the positive semiaxis and the plots with substitution vegetation on the negative semiaxis. This axis shows a highly positive correlation with total nitrogen, oxidizable carbon, aggregate stability and root abundance, and negative correlations with the sand content and the pH. Thus, the Tabaiba scrub appears to be associated with healthy, more dynamic soils, with a greater environmental quality, while the regressive succession is associated with processes of erosion and of chemical and biological degradation. However, the depth of the soil and the presence of outcrops of the material of origin present their most unfavorable values in the Tabaiba scrub soils. This must be attributed to
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the fact that the Tabaiba scrub subsists at present only on land rejected for other uses because of steep slopes and high stoniness, where soils are generally shallow and with numerous rocky outcrops. On the other hand, the second axis discriminates the plots according to their geographical setting. Thus, Tabaibal plots along coastal zones and especially the halophile shrub plots are associated with the positive semiaxis, which correlates with salinity-related parameters, such as electrical conductivity, soluble sodium content, and bioavailable boron, or to carbonation processes prompted by organogenic marine sands. The shrub steppe and the Tabaibal plots away from the seashore are placed closer to the negative semiaxis, which correlates with the bioavailable Mn content and a relative Casaturation in the exchange complex. This environmental gradient is also evidenced by the soil type in each plot type: the Calcids are grouped around the positive semiaxis, whereas the Argids are placed close to the negative semiaxis. The whole of the CCA axis is statistically very significant ( Fratio=1.79, p=0.0002).
4. Discussion and conclusions The dominant vegetation in the past in Fuerteventura seems to have had a high ecological value. The initial distribution of tabaiba scrub over the whole island and the salinity analysis of soils in the current sites of this community show that this was initially found in soils of variable salinity (from E.C.=1.3 to 490 dS/m) and that these enclaves can be spatially grouped into Tabaiba scrub plots of inland sites (E.C.=4.4F3.9 dS/m) and Tabaiba scrub plots of the coastal areas (E.C.=26.5F19.3 dS/m). At the present time, Tabaiba scrub occupies both protected sites inland and exposed steep slopes in the lower areas. This diversity of biotopes is reflected in a wide range of variations in the properties of the associated soils (particularly those related to their saline– sodic state) and their typology (Calcids, Argids, Orthents). Degradation of tabaiba scrub due to human intervention and the appearance of scrubland, steppe or grassland in a regressive ecological succession, is also well documented (Rivas Martı´nez, 1987; Rivas Martı´nez et al., 1993). For the case of tabaiba scrub dominated by species of the Euphorbia genus, the serial substitution communities are: "Arid pasture" with indicator species such as Launaea, Atractylis, Cenchrus, similar to the Shrub steppe we described, "Ruderal fruticose community", featuring Frankenia, Suaeda, Chenoloides as indicator species also similar to the Halophile scrub that we described, accompanied by "Ruderal grasses" (Mesembryanthemum, Aizoon, Stipa, Medicago, etc.) (Gonza´lez Henrı´quez et al., 1986, Rivas Martı´nez et al., 1993, Arco-Aguilar et al., 1997, Rodrı´guez-Delgado et al., 1997, 2000). It, therefore, seems clear that the situation we describe here is a regressive succession that transforms the mature stage or series head (Tabaiba scrub) into subserial communities as a result of human causes. Here, we try to study the effects these changes in plant cover have on the soil properties and soil quality.
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An intense and continued human alteration has led to the substitution of the Tabaiba scrub by brush which has a floristic composition highly influenced by the soil properties. In the lower areas, under greater influence by the sea, with a predominance of Calcids, the high salinity has favored the establishment of halophile plants. In the higher inland areas, protected from the sea spray, where Argids are frequent, succession has given place to a vegetation of gramineae and small shrubs that are less salinity tolerant. Although there is a large amount of literature that has studied the distribution of patches of vegetation in desertic and arid zones in relation to edaphic properties that condition an unequal distribution of the soil moisture regime (Yair and Danin, 1980; Yair, 1983; Kadmon et al., 1989; Casanave and Valentin, 1992; Hairsine et al., 1992; Holzapfel et al., 1995; Schreiber et al., 1995; Puigdefa´bregas and Sa´nchez, 1996; Bergkamp, 1998; Puigdefa´bregas et al., 1999, Domingo et al., 2001, etc.), we consider that our situation is not comparable. As mentioned before, the present distribution of tabaiba scrub is due to the existence of enclaves that have remained undegraded in areas of difficult access: steep slopes, shallow soils and areas with rocky outcrops. The natural drainage of these slopes at times of occasional rainstorms is by natural drainage channels that take the runoff to the sea or to small reservoirs in the ravines, typical for the hydrological dynamics of the eastern islands. There are no significant differences either in the soil moisture content of the different plant communities or their water retention capacity (Table 3). They would not, therefore, be expected to have different soil moisture regimes nor should there be more growth of the vegetation after the rains in one area than in another. A short-lived cover of annual grasses appears for 2–3 months in the winter after the rains and covers the whole island, without apparent differences in the intensity of growth of the perennial plant forms commonly observed in other arid areas (Noy-Meir and Seligman, 1979). Substitution of the Tabaiba scrub has given place to a more disperse vegetation, richer in annual plants and with less radicular development, that offers the soil little protection and a scanty contribution of organic matter. As a result, the regressive plant succession seems to have produced an increase in the severity of several soil degradation processes. Thus, the substitution of the original vegetation is associated in Fuerteventura with skeletization, which is evident in the relative enrichment in sand, and is a product of erosion. The incidence of erosive processes, particularly wind erosion, seems to be greater on halophile scrub, which affords scant plant cover and is located on the lower, more exposed areas. Vegetation regression is also associated with a deterioration of the biological health of the soil, with diminished organic matter and nitrogen contents. Coinciding with the degradation of the plant cover, a certain tendency is also detected towards a greater alkalinization of the soils, although they were already of an alkaline nature. A similar phenomenon has been described in other areas of the Canary Islands (Rodrı´guez Rodrı´guez et al., 2002a), also associated with a regressive plant succession. The natural conditions of the island caused by aridity result in generally poor quality soils (Torres Cabrera, 1995). However, these characteristics are not an obstacle to tabaiba scrub establishment over the whole island, even in coastal sites most affected by salinity and sodicity.
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As a consequence of the degraded plant cover of the island, tabaiba scrub has been reduced to steep slopes, with shallow soils and numerous rocky outcrops, while in the rest of Fuerteventura the degradation of the original vegetation has been accompanied by small changes in the physical and chemical properties of the soils, such that the acquired quality of the soils of Fuerteventura is close to their natural quality, strongly limited in the recent past and presently by the aridity, salinity and sodicity and erosive processes. In earlier studies of fragile areas, showing a low resilience against human transformations that cause significant environmental impact (Arbelo et al., 2002, Rodrı´guez Rodrı´guez et al., 2002a,b), it has been established that the degradation of the ecosystem always implies a regression of the ecological succession accompanied by processes of soil degradation that cause a decrease in soil quality. In turn, in these areas, natural or ecosystem-induced regeneration, implying progression in the ecological succession, invariably involves an improvement in soil properties. However, like the scenario studied here, in arid zones with greater resilience and where, under original conditions, the vegetation is already established on low-quality saline–sodic soils affected by wind erosion processes, the relationships between plant cover and soil degradation are unclear. Here, plant succession regression only implies changes in soil properties as a consequence of the intensified physical (erosion) and biological degradation processes, without apparently causing a loss of quality in soils already presenting a low soil quality. The rehabilitation of these ecosystems should tend to enhance the natural ecological succession, delaying the incidence of physical and biological soil degradation processes, without having a determining influence on the improvement of chemical quality.
Acknowledgements This work has been supported by the Research Project REN2000 1178-GLO "Methodological design for soil degradation assessment on a detailed scale (1:50.000)" (Spanish Ministry of Science and Technology). We are indebted to Dr. A. Yair and an unknown reviewer for their valuable comments and suggestions on an earlier version of the manuscript.
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