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. 2006 Aug;98(2):439–447. doi: 10.1093/aob/mcl122

Effect of Root System Morphology on Root-sprouting and Shoot-rooting Abilities in 123 Plant Species from Eroded Lands in North-east Spain

JOAQUÍN GUERRERO-CAMPO 1,*, SARA PALACIO 1, CARMEN PÉREZ-RONTOMÉ 2, GABRIEL MONTSERRAT-MARTÍ 1
PMCID: PMC2803465  PMID: 16790468

Abstract

• Background and Aims The objective of this study was to test whether the mean values of several root morphological variables were related to the ability to develop root-borne shoots and/or shoot-borne roots in a wide range of vascular plants.

• Methods A comparative study was carried out on the 123 most common plant species from eroded lands in north-east Spain. After careful excavations in the field, measurements were taken of the maximum root depth, absolute and relative basal root diameter, specific root length (SRL), and the root depth/root lateral spread ratio on at least three individuals per species. Shoot-rooting and root-sprouting were observed in a large number of individuals in many eroded and sedimentary environments. The effect of life history and phylogeny on shoot-rooting and root-sprouting abilities was also analysed.

• Key Results The species with coarse and deep tap-roots tended to be root-sprouting and those with fine, fasciculate and long main roots (which generally spread laterally), tended to be shoot-rooting. Phylogeny had an important influence on root system morphology and shoot-rooting and root-sprouting capacities. However, the above relations stood after applying analyses based on phylogenetically independent contrasts (PICs).

• Conclusions The main morphological features of the root system of the study species are related to their ability to sprout from their roots and form roots from their shoots. According to the results, such abilities might only be functionally viable in restricted root system morphologies and ecological strategies.

Keywords: Root architecture, root morphology, root-borne shoots, shoot-borne roots, sprouting, soil erosion, functional strategies

INTRODUCTION

One key morphological feature of plants is their sprouting ability, which is fundamental with regard to understanding their functional and evolutionary strategies (Bond and Midgley, 2001, 2003; Del Tredici, 2001). Sprouting promotes a protraction of the life span of mature individuals following disturbance, while the development of shoot-borne roots or root-borne shoots facilitates vegetative reproduction (Jeník, 1994; Del Tredici, 2001). Sprouting requires the presence of surviving meristems and stored reserves. The allocation of resources to storage has an important cost on growth and reproduction, so sprouters generally show less seed production, less seedling survival and slower growth rates than non-sprouters (Hansen et al., 1991; Iwasa and Kubo, 1997; Verdú, 2000; Bond and Midgley, 2001, 2003). Sprouting capacity depends on several factors, especially plant age and size, and the type and severity of disturbances (Bellingham and Sparrow, 2000; Bond and Midgley, 2001). Bellingham and Sparrow (2000) proposed a model based on the trade-off between allocation to re-sprouting vs. seeding. As the frequency of disturbance increases, re-sprouting rates in plant communities also increase (Midgley, 1996). When disturbance exceeds the tolerance threshold of sprouters, seeders have an advantage because of their higher growth rates and their greater colonization capacity. Following the predictions of Bellingham and Sparow's model, regeneration responses must follow a continuum. However, in most studies only a few plant-response types are distinguished. For example, only two types (seeders and sprouters) are generally used to describe plant responses to intense fire events and other severe disturbances (Vesk and Westoby, 2004). In contrast, Groff and Kaplan (1988) differentiated four structural classes of rooted plants: class 1, bipolar plants, lacking shoot-borne roots and root-borne shoots; class 2, plants with only shoot-borne roots, i.e. plants able to root from layered branches, stems or stem-derived structures, such as rhizomes, stolons, lignotubers, etc.; class 3, plants with only root-borne shoots, i.e. plants with root systems able to produce shoots, commonly known as root suckers; and class 4, plants with the ability to produce both shoot-borne roots and root-borne shoots. This approach has the advantage that it takes into account both the ability of plants to sprout from its roots and the ability to form roots from its shoots.

The functionality of a root system is partially determined by its size and its morphological features (Fitter, 1991). It is generally assumed that deep roots help to avoid water stress in semi-arid and Mediterranean-type climates by reaching deep soil horizons that store water even in the driest periods (Levitt, 1980; Fitter and Hay, 1987; Canadell and Zedler, 1994; Canadell et al., 1996). However, Schenk and Jackson (2002) found that plants of sub-humid climates had deeper maximum rooting depths than plants of dry climates, and that the below-ground/above-ground mass quotient increased with aridity for herbaceous plants only. Lateral spread enables roots to increase the explored soil surface (Rundel and Nobel, 1991). Extended lateral roots are advantageous for water and nutrient uptake after small rainfall events, or to survive in areas with shallow soils, where the highest nutrient concentration is found in the superficial soil horizons (Canadell and Zedler, 1994).

Plants of arid and semi-arid environments normally produce two types of roots: fine and coarse roots (Wilcox et al., 2004). The former are more active in the absorption of water and nutrients and have higher rates of growth and turnover (Caldwell and Richards, 1986; Kummerow, 1989; Wilcox et al., 2004). Root absorption capacity is related to total and specific root length (SRL, total root length/root dry weight) which in turn is enhanced by a higher fine-root density (Fitter, 1991). Plants with highly competitive root systems have high fine-root densities (root length per soil volume) that explore the soil intensively, while plants with less competitive root systems have low fine-root densities that can explore large volumes of soil but do not exploit it intensively (Caldwell and Richards, 1986). Less competitive root systems can be advantageous in infertile soils as they are better adapted to exploit mobile or spatially uneven resources (Fitter and Hay, 1987; Fitter et al., 1988; Fitter, 1991). Coarse roots can penetrate hard soils and reach deep horizons and water sources (Goss, 1977; Walker and Noy-Meir, 1982), allowing better anchorage than fine roots and the storage of carbon, nutrients and water (James, 1984; Boot and Mensink, 1990). Water storage in coarse roots can be advantageous for species that undergo long periods of drought (Kummerow, 1982).

In this study the approach of Groff and Kaplan (1988) was followed, considering both shoot-rooting and root-sprouting abilities of vascular plant species. Few studies have considered the morphological implications of root systems with regard to these characters. Several authors have reported that species with thick and/or very deep roots tend to sprout after fire (Hellmers et al., 1955; James, 1984; Canadell and Zedler, 1994), but most of these studies have been based on general field observations without detailed analyses and measurements (but see Bell et al., 1996). It is generally accepted that sprouters in fire-prone ecosystems usually allocate more reserves to roots, as compared to shoots, than non-sprouters (Pate et al., 1990; Hansen et al., 1991; Bell et al., 1996). However, studies on sprouting after fire have not discriminated between sprouts produced from roots, stems or stem-derived structures (e.g. stolons or rhizomes). Furthermore, literature dealing with sprouting responses under edaphic disturbances is very scarce (but see Sakai et al., 1995) as most studies on plant sprouting responses have dealt with responses to fire (López-Soria and Castell, 1992; Trabaud, 1992), hurricanes or wind-throws (Bellingham et al., 1994; Zimmerman et al., 1994) and tree cuttings (Flinn and Wein, 1977; Mazzoleni and Esposito, 1993).

Several studies have demonstrated the functional relevance of morphological variables such as the maximum root depth (Canadell and Zedler, 1994; Canadell et al., 1996), the lateral root spread (Rundel and Nobel, 1991), the specific root length (SRL), which is related to fine-root density (Fitter, 1991), and the main-root diameter (James, 1984; Boot and Mensink, 1990). In the present study, we formulate the hypothesis that plants with different abilities to develop root-borne shoots and shoot-borne roots might show different values for these four variables. To test this hypothesis a comparative analysis was performed including the aforementioned root morphological features and the capacity to develop shoot-borne roots and/or root-borne shoots for the most common 123 species of vascular plants growing in eroded lands from the north-eastern Iberian Peninsula. The effect of life history and phylogeny on root-sprouting and shoot-rooting abilities was also explored.

MATERIAL AND METHODS

Species and study area

A survey of 734 vegetation relevés was performed on scrub pastures and badlands affected by different degrees of soil erosion. The 123 most frequent phanerogams, i.e. those that appeared in at least 5 % of the relevés, were selected. This large species set included most of the growth forms found in the area, from shrubs to annual plants. The area surveyed was located between the middle Ebro Valley and the Pre-Pyrenees (north-east Spain), and comprised the following substrates: almost pure gypsum (350 mm average annual rainfall), gypsum mixtures with clays or marls (400–500 mm), Miocene clays from the Somontanos (500 mm), Eocene Pre-Pyrenean marls (800 mm) and Eocene Pyrenean flysch (1000 mm). For more details about the study area and field survey see Guerrero-Campo (1998) and Guerrero-Campo and Montserrat-Martí (2000, 2004).

Root morphological features

To estimate the different types of rooting or sprouting characteristics, several eroded sites were surveyed in detail, particularly highly eroded surfaces, talus slopes and sedimentary sites. Data were pooled with previous observations to build a large data base. For each species the cases of observed root-borne shoots and shoot-borne roots were recorded. There was continuous variation between the species that produced large amounts of shoot-borne roots and those that produced no shoot-borne roots. A similar variation pattern was observed with regard to root-borne shoots. However, in order to simplify the interpretation of results, and due to the great number of species analysed, only the presence/absence of each ability was considered in the study-species set.

At least three well-developed specimens of each species were carefully excavated within natural populations growing at sites with few stones and rocks, in order to ensure a free development of root systems. To facilitate excavation, plants were selected growing near the faces of road through-cuts or gullies. When large differences were found between the root systems of the three specimens, additional individuals were excavated. Selected specimens were well-developed adults except for the few large shrubs studied, which were young individuals. In each root system measurements were made of the maximum depth, maximum lateral spread, absolute diameter (basal diameter of the main root axis at 10 cm depth, or at 5 cm in the shallow-rooted annual plants) and the relative basal root diameter. The relative basal root diameter is an index that relates absolute basal root diameter (mm) with the average general dimensions of the root system, i.e. root depth (cm) and root lateral spread (cm). It was calculated according to the following formula:

graphic file with name M1.gif

If the root system had more than one root axis the thickest one was measured. The specific root length (SRL, total root length/root weight) was considered as a variable related to the absorption capacity and the intensity of exploration of the soil volume (Caldwell and Richards, 1986; Fitter, 1991). The large species set compromised the measurement of the SRL of all sampled species in detail. Accordingly, SRL was measured in the whole root system of 14 representative species, which were ranked according to their SRL values. Four categories of SRL were then recognized within the gradient of variation displayed by the 14 reference species. Finally, unmeasured species were assigned to one of the four SRL categories by visual comparison with measured plants, which had been pressed and preserved in a herbarium. However, the SRL of those species that were difficult to ascribe to one of the four categories by visual assessment were also measured. To enable comparison between SRL data and the rest of variables, the remaining root variables were grouped in categories. Accordingly, species maximum root depth, species depth/lateral spread ratio, and average relative basal root diameter were included in four categories, while average absolute basal root diameters were included in three categories. Continuous data for these variables for the study species can be found online in the Supplementary Information.

Statistical analyses

Log-likelihood ratio analyses (G) were used to assess the relationships between shoot-rooting and root-sprouting abilities and root morphological features, to compare those abilities and plant life history, and to explore the differences between these relationships in each of the main families studied. A taxonomic analysis was performed to test whether the results were affected by relations above the species level (Felsenstein, 1985; Harvey and Pagel, 1991). Provided that we had discrete variables and little phylogenetic information on the taxa used, we chose the paired comparison method combined with a sign test (Harvey and Pagel, 1991; Ackerly, 2000). This method offers a very robust and conservative approach to significance testing for independent contrasts, maintaining very low Type I error rates, but greatly reducing statistical power for detecting non-zero correlations (Ackerly, 2000). For each set of two subtaxa belonging to the same taxa (e.g. species within genera) and differing in shoot-rooting or root-sprouting capacity, the trend of the relationship was positive, negative or neutral. If more than two taxa were available (for example if there were three genera in a family), we chose the taxon to be included in the pair randomly. Different taxa were then used for a similar comparison at the next taxonomic level (e.g. families within orders), always following parsimony rules and using the angiosperm phylogeny of Soltis et al. (2000). After combining the results at different taxonomic levels, several one-tailed binomial tests were performed to verify whether there were more positive or negative relationships than expected or if there were no underlying relationships (relationships were significant when P ≤ 0·05). All statistical analyses were conducted with SPSS 11·0 (SPSS Inc, Chicago, USA).

RESULTS

Root systems and root/shoot relationships

Shoot-rooting species

Shoot-rooting species were quite abundant among the studied species. There was not a significant relationship between shoot-rooting capacity and maximum root depth, relative root depth or absolute basal root diameter (Fig. 1). Shoot-rooting capacity was significantly associated with high or intermediate SRL (Fig. 1C). In addition, shoot-rooting was related to the relative basal root diameter, being significantly more frequent in species with fine relative root diameters than in species with coarse (Fig. 1E).

Fig. 1.

Fig. 1.

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Number of shoot-rooting and non shoot-rooting species as related to (A) maximum depth, (B) root depth/root lateral spread ratio, (C) fine-root density (SRL), (D) absolute basal root diameter, and (E) relative basal root diameter. G-statistic and P-values are provided after a log-likelihood ratio analysis (α = 0.05).

When annuals and grass-like species were excluded from the analyses, the above relations stood with the exception of relative basal root diameter. The above relationships also stood after applying analyses based on phylogenetically independent contrasts (PICs; (Table 1). Accordingly, the SRL and the relative basal root diameter were related to the shoot-rooting capacity, but not the relative root depth, the maximum root depth or the absolute basal root diameter (Table 1).

Table 1.

Taxonomic relatedness analysis of the relationship between shoot-rooting capacity and the studied morphological features of roots

Taxonomic level Maximum depth Relative depth SRL Absolute diameter Relative diameter
Species within genera
    Centaurea − − + − −
    Convolvulus − − = − −
    Ononis = = + = =
    Hippocrepis + = − + +
    Linum + − = + =
Genera within families
    Rosaceae − − + = −
    Rubiaceae + − + − −
    Apiaceae + + + + −
Families within orders
    Lamiales + − + − −
    Euphorbiales + − + − −
Orders within subclasses
    Cariophyllidae + + + − −
    Rosiflorae − − + + −
Total 6+, 5− 2+, 8− 9+, 1− 4+, 6− 1+, 9−
P (one-tailed binomial test) 0·500 0·054 0·011 0·377 0·011
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+, indicates that the relationship for the set of subtaxa belonging to the same taxon was positive, i.e. shoot-rooting species had greater values of the measure variables than non shoot-rooting species; –, indicates that the relationship was negative; =, indicates no significant differences. P-values smaller than 0·05 are in bold.

Root-sprouting species

Root-sprouting species had significantly deeper root systems than non root-sprouting species (Fig. 2A). There were no significant differences in the relative root depth of root-sprouting and non root-sprouting species, although there were no root-sprouting species with low values of this ratio (Fig. 2B). Root-sprouters had significantly lower SRL than non root-sprouters, root-sprouting ability being absent in species with high SRL (Fig. 2C). Root-sprouting was significantly associated with coarse or medium relative basal root diameters, but never with very low values (Fig. 2D). This tendency was also evident in the case of absolute diameter. Accordingly, plants with root-borne shoots never had fine main roots (Fig. 2E). Coris monspeliensis and Euphorbia minuta were species that could form root-borne shoots and had the finest main roots (around 2 mm at 10 cm depth, see Supplementary Information).

Fig. 2.

Fig. 2.

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Number of root-sprouting and non root-sprouting species as related to (A) maximum depth, (B) root depth/root lateral spread ratio, (C) fine-root density (SRL), (D) absolute basal root diameter, and (E) relative basal root diameter. G-statistic and P-values are provided after a log-likelihood ratio analysis (α = 0.05).

When analyses based on phylogenetically independent contrast (PICs) were applied, the above relationships stood. All root system features analysed, except the root depth/root lateral spread ratio, were significantly related with root-sprouting (Table 2). However, when annuals and grass-like species were excluded from the analyses, all significant relationships disappeared.

Table 2.

Taxonomic relatedness analysis of the relationship between root-sprouting capacity and the studied morphological features of roots

Taxonomic level Maximum depth Relative depth SRL Absolute diameter Relative diameter
Species within genera
    Centaurea + − − − −
    Euphorbia + = + + +
    Teucrium + = = + +
    Linum + − = + +
Genera within families
    Boraginaceae + + − + +
    Cariophyllaceae = − − + +
    Fabaceae + − − + −
    Primulaceae + + − + +
    Rosaceae + − − = +
    Apiaceae − + – + +
Families within orders
    Dipsacales + − − + +
Orders within subclasses
    Asteriflorae + + − + +
    Cariophyllidae − + − − =
Total 10+, 2− 5+, 6− 1+, 10− 10+, 2− 10+, 2−
P (one-tailed binomial test) 0·019 0·500 0·005 0·019 0·019
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+, indicates that the relationship for the set of subtaxa belonging to the same taxon was positive, i.e. root-sprouting species had greater values of the measure variables than non root-sprouting species; –, indicates that the relationship was negative; =, indicates no significant differences. P-values smaller than 0·05 are in bold.

Life history and root–shoot relationships

Life history had a strong effect on the ability to form roots from shoots; shoot-rooting species being more frequent among perennial plants (Fig. 3). Life history had also a significant effect on root sprouting ability. Thus, annual plants were unable to form shoots from their roots (Fig. 3).

Fig. 3.

Fig. 3.

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Number of species with different root/shoot relationships within the main life history types considered. G-statistic and P are provided after a log-likelihood ratio analysis (α = 0.05).

Phylogeny and root–shoot relationships

Phylogeny had a significant effect on the shoot-rooting (G = 47·5, P < 0·001) and root-sprouting (G = 13·1, P = 0·004) abilities of the study species. The Cistaceae studied were unable both to root-sprout and to form roots from their shoots; Poaceae were only able to form roots from their shoots but not to sprout from their roots; while Asteraceae and Fabaceae had all kinds of combinations of both abilities. The percentage of variation of root–shoot relationships in the main taxonomic families is summarized in Fig. 4.

Fig. 4.

Fig. 4.

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Number of species with different root/shoot relationships within the main families considered. G-statistic and P are provided after a log-likelihood ratio analysis (α = 0.05).

DISCUSSION

In general, our results agree with the hypothesis that the ability to produce shoot-borne roots and root-borne shoots is related to basic morphological characteristics of the root system. Nevertheless, other linked aspects like phylogeny and life history had some effect on these relationships, although they did not change the main tendencies found in the study.

Well-developed tap roots were generally associated with deep root systems and with species developing root-borne shoots, while fasciculate roots with numerous fine roots were usually associated with species forming shoot-borne roots, but never root-borne shoots. In the studied species, shoot-rooting was related to laterally spreading root systems reaching moderate depths, high fine-root densities (SRL) and fine main root axes. These features are clearly adaptive for those species, which tend to spread horizontally. Decumbent shoots or shoot-derived structures, like stolons and rhizomes, allow plants to explore the horizons close to the soil surface. Such features favour wide-spread clonal growth, which in turn is favoured in fertile sites (Shumway, 1995; Jónsdóttir and Watson, 1997). In shoot-rooting species, shoots and shoot-derived structures can work as storage organs (de Kroon and van Groenendael, 1990), while roots mainly function as absorbing organs. To maximize the absorbing capacity, roots allocate most of their resources to develop numerous fine roots, achieving a high fine-root density and a high SRL (Kummerow, 1981; Caldwell and Richards, 1986). Grass-like species might be an extreme within shoot-rooting species, as they present a very high fine-root density (SRL) and a massive production of shoot-borne roots. However, some shoot-rooting species, such as Gypsophila hispanica, Santolina chamaecyparissus, Rosmarinus officinalis and Thymus vulgaris, lacked several of the characteristic features of shoot-rooting plants, like a high fine-root density (SRL), significant lateral root spread or shallow rooting depths. These species might use their ability to develop shoot-borne roots to maintain a dense fine root layer near the soil surface, which might enable them to take advantage of small and moderate rainfall events and to exploit the high fertility of the upper soil layer. In addition, some of these species have deep main roots that can reach water stored in deep soil horizons. This architecture is called a mixed or dual root system (Dawson and Pate, 1996) and seems to be favoured by the seasonal Mediterranean climate (Cannon, 1949; Canadell and Zedler, 1994; Dawson and Pate, 1996). We found clear examples of this root type in Salsola vermiculata, Gypsophila hispanica, Atriplex halimus and Rosmarinus officinalis. All of these species are woody plants, and hence they fit the predictions of the two-layer model of soil partitioning between woody and herbaceous plants (Walker and Noy-Meir, 1982). According to this model, only woody species can obtain resources from the deepest part of the soil, while both woody and herbaceous species compete for resources in the upper soil layers (Walker and Noy-Meir, 1982; but see Schenk and Jackson, 2002).

A different trend is followed by species bearing root-borne shoots, which might be a highly adaptive feature when roots perform important functions other than absorption. Roots from root-sprouting species tend to grow deeper with low lateral spread and tend to be thicker in order to penetrate tough soil horizons (Goss, 1977; Fitter, 1991). Accordingly, in this study the basal diameter of the main root was always greater than 2 mm in species with the ability to root-sprout. The need to store water and nutrient reserves to supply root sprouting might also lead to greater main root diameters (Kummerow, 1982; Bell et al., 1996). In Mediterranean plants, root swelling is frequently used to provide space for resource storage to survive fire, defoliation and drought (James, 1984; Kummerow, 1989; Canadell and Zedler, 1994). Non-structural carbohydrates, particularly starch, are necessary to start sprouting, so the amount of stored starch in roots, stumps or stems is a good indicator of the species' sprouting ability (Rundel and Parsons, 1980; Baker et al., 1982; Bell et al., 1996). According to Bellingham and Sparrow (2000) and Iwasa and Kubo (1997), the ability to sprout depends on the presence of storage organs. For example, in graminoid and shrub species of the Australian areas where fire is an important factor, sprouters present at least four times higher root starch concentrations and root : shoot ratios than seeders (Pate et al., 1990; Bell and Pate, 1996). In addition, these root systems can explore deep horizons and store water and nutrients. The high amount of resources invested in the construction of such large and expensive roots might be compensated for by the species' ability to survive disturbances, producing root-borne shoots when the above-ground biomass is destroyed (Iwasa and Kubo, 1997).

Life history might affect root system features and root/shoot relationships, as it is a very important attribute of plant functional strategy (Grime, 2001). Annual species were unable to produce shoot-borne roots or root-borne shoots, with the exception of annual grass-like species that showed a certain ability to form roots from their shoots. This is an inherent characteristic of the ecological strategy of annual plants, which are commonly ruderal species with short-lived organs and high reproductive and growth rates (Grime, 2001). This strategy is incompatible with sprouting ability, as it requires allocation to storage organs with detriment to resource investment to growth and reproduction (Bellingham and Sparrow, 2000; Bond and Midgley, 2001). Consequently, annual species share many root system characteristics, including shallow maximum depths, fine main roots and intermediate or high SRL. The species that can produce shoot-borne roots and/or root-borne shoots might have a very different functional strategy, since their goal is long-term survival. Accordingly, such species comprised perennial herbs and woody plants in our study.

Functional strategies and life histories were not the only factors that explained the observed relationships between root system morphology and the capacity of plants to root-sprout and form roots from shoots. Phylogeny also had an important influence as it shapes many root system features (Klepper, 1987; de Kroon and van Groenendael, 1990) such as the main-root diameter (Fitter, 1991). However, other root attributes such as root depth and lateral root spread are less influenced by phylogeny (Guerrero-Campo, 1998). Phylogeny explained some differences in the observed shoot–root relations of the study species at a family level: all the studied Cistaceae were unable both to root sprout and to produce shoots from their roots; and Poaceae were all shoot-rooting plants. However, Fabaceae, Asteraceae, Lamiaceae and a single genus such as Centaurea all included different classes of root/shoot relationships. Furthermore, most relationships found in this study between the features of root systems and the types of root/shoot relationship stood after applying phylogenetically independent contrast. Such results agree with the hypothesis that sprouting and clonality are not conservative, but quite labile traits (Klimes et al., 1997; Bond and Midgley 2003). Accordingly, clonality has appeared several times during evolution, so clonal species might constitute a phylogenetically heterogeneous group (Klimes et al., 1997). Nevertheless, a recent study has shown that sprouters of the Mediterranean flora correspond to older lineages than non-resprouters or seeders (Pausas and Verdú, 2005).

In conclusion, our results demonstrate that the main morphological features of the root system of the 123 study species are clearly related to their faculty to produce root-borne shoots and/or shoot-borne roots. These relations are not only mediated by phylogeny, as the life history and the ecological strategy of each species are also crucial. According to the results, the ability to develop shoot-borne roots and root-borne shoots might only be possible and functionally viable in restricted root morphologies.

SUPPLEMENTARY INFORMATION

Supplementary Information available online at http://aob.oxfordjournals.org/ contains information on the morphological features of the root system of the 123 most frequent vascular species from the eroded lands that were studied. Both the categorized data used in the analyses and the original continuous data are provided.

Acknowledgments

We are grateful to Pilar Castro Díez, Patricio García Fayos, Mónica Bardají Mir, Professor Mike Cramer and an anonymous referee for their valuable comments on the manuscript, and to Miguel Verdú for his assistance. This study was supported by the Spanish Ministry of Science and Technology, projects AMB93-0806, REN2002-02635/GLO and RTA 2005-00100. J. G. C. and S. P. were supported by a grant from the Government of Aragon (BMA 15/93) and a FPU scholarship (AP-2001-2311, MEC, Spain), respectively.

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