Fine root distribution in Dehesas of Central-Western ... - Springer Link

Springer 2005

Plant and Soil (2005) 277:153–162 DOI: 10.1007/s11104-005-6805-0

Fine root distribution in Dehesas of Central-Western Spain G. Moreno1,3, J.J. Obrador1, E. Cubera1 & C. Dupraz2 1

I.T.Forestal, Centro Universitario, UEX, Plasencia 10600, Ca´ceres, Spain. 2INRA, UMR-SYSTEM, MONTPELLIER, 34060, Cedex, France. 3Corresponding author*

Received 15 January 2005. Accepted in revised form 28 April 2005

Key words: Agroforestry, core-break method, grasses, open woodland, Quercus ilex, root length density

Abstract A Dehesa is a structurally complex agro-silvo-pastoral system where at least two strata of vegetation, trees and herbaceous plants coexist. We studied the root distribution of trees (Quercus ilex L.) and herbaceous plants, in order to evaluate tree and crops competition and complementarity in Dehesas of Central Western Spain. 72 soil cores of 10 cm diameter (one to two metre deep) were taken out around 13 trees. Seven trees were intercropped with Avena sativa L. and six trees were in a grazed pasture dominated by native grasses. Soil coring was performed at four distances from the tree trunks, from 2.5 (beneath canopy) till 20 m (out of the canopy). Root length density (RLD) of herbaceous plants and trees was measured using the soil core-break method. Additionally, we mapped tree roots in 51 profiles of 7 recently opened road cuts, located between 4 and 26 m of distance from the nearest tree. The depth of the road cuts varied between 2.5 and 5.5 m. Herbaceous plant roots were located mostly in the upper 30 cm, above a clayey, dense soil layer. RLD of herbaceous plants decreased exponentially with depth until 100 cm depth. Holm-oak showed a much lower RLD than herbs (on average, 2.4 versus 23.7 km m)3, respectively, in the first 10 cm of the soil depth). Tree RLD was surprisingly almost uniform with depth and distance to trees. We estimated a 5.2 m maximum depth and a 33 m maximum horizontal extension for tree roots. The huge surface of soil explored by tree roots (even 7 times the projection of the canopy) could allow trees to meet their water needs during the dry Mediterranean summers. The limited vertical overlap of the two root profiles suggests that competition for soil resources between trees and the herbaceous understorey in the Dehesa is probably not as strong as usually assumed. Abbreviations: RLD – Root length density; Nroot – Number of root; d50 and d95 – Depth of 50% and 95% cumulative root density, respectively; CL and ST – Farm names, Cerro Lobato and Sotillo, respectively

Introduction Dehesas are multi-purpose open woodlands where at least two strata of vegetation coexist. They have been common in the Iberian Peninsula, at least since the middle ages (Montero et al., 1998). At present, Dehesas cover 3.1 million hectares in western Spain and Portugal, and they are considered as habitats to be preserved * FAX No: +34-927425209. E-mail: [email protected]

because of the high biological diversity they support (Dı´ az et al., 1997). However, in the last decades, a significant decrease in extension and tree density has been occurring as a consequence of increased mechanisation, changes in land use and death of trees in over-aged stands (Plieninger et al., 2003). A better knowledge of the role of trees on dehesa functioning and sustainability could contribute to improve its management and conservation. Some pioneer studies on the effect of trees in dehesa functioning have shown the positive

154 effects of trees on soil nutrient contents (Escudero, 1985), soil water storage capacity (Joffre and Rambal, 1988), water stress for the underlying herbaceous stratum (Joffre and Rambal, 1993), and pasture production (Puerto et al., 1987). Several authors have also shown the positive effect of tree clearance on the physiological status of remaining trees (e.g., Infante et al., 1999 and Montero et al., 2004) and productivity (e.g., Diaz et al., 1997). The improved physiological status of dehesa trees could be due to an increase in the available soil volume, and thus water and nutrients, for each individual tree. Joffre et al. (1999) stressed the need for better knowledge of the extension of the tree root system to understand the implications of soil water balance on the stability of the dehesa (tree-tree and tree-understorey interactions), and therefore to predict the consequences of long-term climatic and land use changes. Much of the competition among plants takes place underground (Casper and Jackson, 1997), and below-ground competition knowledge is a major difficulty for understanding simultaneous agroforestry systems (van Noordwijk et al., 1996). The use of a higher proportion of below ground resources can be achieved if deep networks of tree roots are able to capture water or nutrients draining or leaching below the rooting zone of the crops (van Noordwijk et al., 1996). Understanding and predicting ecosystem functioning (e.g. nutrient cycling, carbon and water fluxes) requires an accurate assessment of plant root distribution (Jackson et al., 1996). A realistic map of root length density, both horizontal and vertical, is needed to model possible interactions (facilitation, competition and complementarity) between plants. However, many models (e.g. HyPAR: Mobbs et al., 1999; WaNuLCAS: van Noordwijk and Lusiana, 2000) assume a simple shape for tree root system: an exponential decrease with soil depth and distance from the tree trunk is often considered. This simplicity is explained by the difficulty and complexity of root studies, which have resulted in a lack of quantitative information on root systems (Jose et al., 2001; Smith et al., 1999). In the last decades several new methods have been proposed to describe the root system: soil windows, minirhizotrons, soil core washing, (Smit et al., 2000). However, all them have certain

limitation in term of collecting representative samples and of cost or time required (Smit et al., 2000). To address the issue of representativeness, some authors applied a combination of methods, e.g. trenches plus soil cores (e.g. Jones et al., 1998, Silva and Rego, 2003), in order to characterise the horizontal and/or vertical root extension and to determine the root density. Additionally, to reduce the labour needs and speed up the process, van Noordwijk et al. (2000) proposed an indirect method to estimate the root length density by counting roots emerging from the horizontal planes of broken soil cores. To our knowledge, only two studies have been carried out to characterise the root systems in dehesas (Barrera et al., 1987; Joffre et al., 1987), and both were limited to natural grasses roots, in the first 30 and 60 cm of soil, respectively. None of these two works dealt with the tree root distribution. The present study focus on the root distribution (fine root length density) of both tree (holm-oak) and herbaceous vegetation (cereal crop or natural grasses) considering both vertical and horizontal dimensions. Additionally, we have documented the effect of soil tillage on the root density of the trees. To achieve that we used two different methods, based on the study of soil cores and on the record of root maps in recently opened road cuts.

Material and methods Study area The study has been carried out in two dehesas (Cerro Lobato CL and Sotillo ST) of C-W Spain (3941¢ N–613¢ W; altitude: 380 m.a.s.l.), with an average tree density of 35 tree ha)1 (Quercus ilex L. in both grazed and cropped plots). Average tree dimensions were 44.9 cm DBH (diameter at breast height), 10.4 m canopy width and 7.8 m tree height. In grazed plots the main grasses were Lolium rigidum Gaudin, Plantago lanceolata L., Erodium sp. L., Taraxacum obovatum (Willd.) DC. and Echium plantagineum L. These species were also abundant as weeds in the intercropped (oats) plots given that herbicides are not applied in dehesas crops. The aboveground biomass of the intercrop was low: 5.2 and 2.9 Mg of dry weight per ha in CL and ST, respectively. The difference

155 reflected the difference of the fertilisation schemes: 200 and 50 kg of NPK 7/12/7 ha)1 in CL and ST farms, respectively. Cropped plots were cultivated only one year every four years, and grazed plots were never cropped during the last 20 years. The climate is semi-arid Mediterranean, with an annual rainfall of 579 mm, mean annual temperature of 16.2 C, and mean annual potential evapotranspiration of 864 mm. Climate is classified as subtropical Mediterranean, following the Papadakis classification (1966), with dry, warm and cold (with frost) periods of 4, 3 and 5 months, respectively. Soils are chromic Luvisols (FAO, 1998) in both farms, developed over tertiary sediments with abundant gravels and stones of quartzite, one or several very red argic horizons, with slight brown and silty-sand texture in the surface horizon and a very sandy layer in depth (below 100 cm). Soils also showed poor internal drainage, resulting in variegated colours and/or pseudo-gleic, low soil chemical fertility, and occasional CaCO3 accumulation between 1.5 and 2 m depth. Soil cores: Root Length Density Soil cores were taken with a stainless steel soil column cylinder with a cutting shoe and a removable cover (diameter 10 cm, length 1000 mm), inserted into the soil with a heavy electrical powered percussion hammer (Makita HM 1800, provided by Eijkelkamp, Giesbeek, The Netherlands). Between 23rd–28th April 2002 thirty-six soil cores of 1 m-depth were extracted at 2, 5, 8 and 12 m distance from the tree trunk of 4 intercropped holm-oaks, at two orientations (three orientations in one tree). Between 10th and 20th May 2003, thirty-six soil cores of 2 m-depth (at maximum) were taken at 2, 5, 10 and 20 m of distance from the tree trunk of 3 intercropped holm-oaks and 6 ‘intergrazed’ holm-oaks. Soil cores were covered by a gutter (two halves of a PVC tube) and transparent plastic to avoid damage during transport to the laboratory, where they were stored at 6 C in a cold chamber to keep the roots fresh until analysis. Cylindrical soil cores were divided into 10 cm-length samples. Each sample was then broken by hand in two parts, and the number of tree and herbaceous plants fine roots (using a 2 mm diameter threshold for tree fine roots) were recorded in

both sides of the sample parts. Decayed tree roots were excluded. Holm-oak roots were identified by their black cork, while grass roots were white. The youngest tree roots (growing tips) are also white, but much thicker than crop/grass roots. Differentiating crop and weed roots was not possible. The method of soil core-break allowed us to estimate the Root Length Density (RLD) from the number of roots sticking out of two soil surfaces of a horizontally broken soil core (van Noordwijk et al., 2000). A total of 65 randomly selected samples were washed each year to provide a direct calibration of root length versus counts (van Noordwijk et al., 2000). To wash samples, different filters between 2 and 0.125 mm mesh size were used. This was done to avoid losing fine roots. All samples had tree fine roots, and only 46 had herbaceous roots. Roots obtained from the washing activity were laid on plastic paper and then photocopied and the length of the fine roots was measured manually for each soil core. Data are expressed as Root Length Density (km m)3 of soil) because root length is a better indicator of root system functions in terms of uptake of water and nutrients than root weight and root number (Jones et al., 1998). Road cuts: maximum tree rooting depth and horizontal spread We took advantage of current works in the road that cross the study area, to check the maximum distance and depth of the holm oak root system. Road cuts had been recently (3–4 months) opened. We counted the number of emerging tree roots (fine and coarse). Roots were counted in 51 profiles, located every 5 m in 7 different road cuts. To identify the position of the root (depth and distance from the nearest tree), we used a metallic square of 50 cm size, divided into small squares of 100 cm2. The maximum depth of the profiles varied between 250 and 550 cm. The slope of the road cuts was measured to calculate the actual depth of roots. We were able to confirm (by means of recent aerial photographs) in four of the road cuts that no trees had been removed during the works. In these cases we measured the distance to the nearest tree. For the other three road cuts we confirmed the previous existence of some trees where the road cuts had been created, and data were

156 only used to describe the root distribution in depth. We therefore have 51 profiles for the description of vertical root distribution, and only 34 for horizontal root distribution. Results are expressed as number of roots per m2. Schenk and Jackson (2002a) have shown that patterns of root profiles based on root length and root count do not differ. Thus, results of both methods (soil cores and road cuts) could be compared. Data analysis A simple linear regression was found to be fit for calibrating the soil core-break method (relationship between RLD and the number of counted roots, as suggested by van Noordwijk et al., 2000). Root density has been regressed with depth (linear and non-linear regressions) or with both depth and distance (multiple regression) in order to describe rooting patterns of both trees and herbaceous plants. Differences between treatments and position in RLD were assessed by analysis of variance (ANOVA). Two-way ANOVAs were applied to detect differences in mean values of RLD (dependent variable) between distance and depth (as independent variables) for both herbaceous plants and trees. Two-way ANOVAS were also applied to contrast the effect of soil management (cropped versus grazed) on RLD at different distances or depths. Results are expressed as F values (and degree of freedom) and significance level (P). Depth of 50% and 95% cumulative root density (d50 and d95, respectively) were calculated according to the Gale and Grigal (1987) model. Following these authors, a non-linear regression was used to fit the function fc = 1 – bd to the profile of cumulative root fraction (fc), from the soil surface to depth d (cm). b is the fitted ‘‘extinction coefficient’’. Values of d50 and d95 were then calculated from d50 = Ln (0.5) / Ln (b) and from d95 = Ln(0.05) / Ln (b), respectively.

tree fine roots (Figure 1). The relationships was better for herbaceous roots than for tree roots (R2 = 0.85 vs 0.42, respectively). This difference is partly explained because the range of Nroot and RLD was much lower for tree (0–3800 roots m)2 and 0–8 km m)3) than for herbs (0–28000 roots m)2 and 0–44 km m)3). A further explanation is probably linked to the patchy pattern of tree roots: tree roots are less evenly distributed in the soil core sample volume, resulting in a less accurate prediction from the core-break count. Both regressions were however highly significant, with p < 0.001 (n = 46 and 65 for herbs and trees, respectively). Vertical profiles of root length density Herbaceous RLD was very high in the first 10 cm of the soil (Figure 2), decreasing very sharply, exponentially with depth: RLDkm.m)3 = 122.1*Depthcm)0.999 (R2 = 97.0; F1,18 = 714; p < 0.00001). At 40 cm depth, RLD was ten times lower than in the first 10 cm. The depth of 50% cumulative root length (d50) was found at 10.7 cm (Figure 2 and Table 1). Results of a two-way ANOVA (depth and distance as independent variables) showed significant differences between consecutive depths till 60 cm (F9,442 = 94.46; p < 0.001); then, differences were not significant. Below 90 cm, herbaceous plant roots were only found very occasionally, reaching a maximum rooting depth of 100 cm. By contrast, a linear but non-significant decrease in tree RLD was observed from 0 till 200 cm of soil (F9,442 = 0.36; p < 0.952). At 2 m depth the holm-oak RLD was still about half with respect to the uppermost soil layer (Figure 2). From the regression between tree RLD and depth (RLDkm.m)3 = 2.24–0.0056 · Depthcm; R2 = 0.56; F1,18 = 22.9; P < 0.00015), the expected maximum rooting depth for holm-oak in this area was estimated at 400 cm, and the d50 value at 96.4 cm (Figure 2 and Table 1). Lateral root distribution

Results Linear relationships for root length density estimation The calibration curves between Nroot and RLD were determined separately for herbaceous and

RLD varied significantly with distance to the tree trunk, for both trees and herbaceous plants (Figure 3a). Herbaceous plants RLD was significantly higher at 10 and 20 m of distance than at 2.5 and 5 m of distance (F3,442 = 10.26; p < 0.0004). Tree RLD decreased smoothly

157 with the distance to the tree. Significant differences were detected between 2.5 and 5 m and between 5 and 10 m (F3,442 = 7.64; p < 0.001). In spite of the differences with distance, the RLD profile shape did not vary with distance, neither for herbaceous plants nor for holm-oak (Figure 3b). In fact, we did not find any significant interaction between both factors (Distance · Depth) either for herbaceous plants (F27,442 = 1.16; P < 0.289) or for trees (F27,442 = 0.80; p < 0.702). Only a slight increase in d50 value with the distance for herbaceous plant roots has been estimated (9.6, 9.3, 10.6 and 14.4 at 2, 5 10 and 20 m of distance); the opposite was observed for tree roots (67, 69, 57 and 55 cm at 2, 5 10 and 20 m of distance, considering only the roots of the first 200 cm of soil). Effect of soil management on root distribution

Figure 1. Linear regressions between the number of roots crossing a horizontal plane (Nroot) and root length density (RLD) for (a) herbaceous plants (oats and native grasses), and (b) Holm-oak (only fine-roots).

Two main differences have been observed in the root distribution when two different soil management types (cropped or grazed) are compared (Figure 4a). Root length density of herbaceous plants was much lower in grazed plots (native grasses) than in intercropped plots (oats + weeds) (F1,249 = 5.55; p = 0.004), at any distance (not-significant interaction: F6,249 = 0.062; p = 0.991). d50 value was also clearly deeper in intercropped plots than in grazed plots (13.9 and 7.4 cm, respectively; Table 1). The profile of RLD was similar for both cropped and grazed plots (Figure 4b). Tree RLD was however very similar in both types of management, cropped and grazed plots (Figure 4a). The only difference was observed in the top soil, where intercropped trees had a lower RLD than grazed trees (Figure 4b), although the differences were not statistically significant (F1,104 = 1.18; p = 0.31). d50 value was only slightly deeper in intercropped plots than in grazed plots (69.1 and 64.6 cm, respectively, considering only the two first metres of depth). Tree roots in road cuts

Figure 2. Variation of root length densities with soil depth for holm-oak and herbaceous plants in dehesas developed over chromic Luvisols in Central-Western Spain. The inset shows the cumulative fractional root distribution plotted against the soil depth.

This study confirmed results cores. The maximum depth found was 450 cm, with a d50 a d50 value of 96.4 found in (Table 1).

obtained with soil where roots were value of 81.2 cm vs the soil core study

158 Table 1. Comparison of the root profiles of holm-oak and herbaceous vegetation in dehesas of Central-Western Spain with average values from recent comprehensive reviews Vegetation type

b

R2

d50d (m)

d95d (m)

Maximum rooting depth (m)

Herbaceous plants Oats + weeds Native grasses (mostly annual) Holm-oak (soil cores)e Holm-oak (road cuts) Mediterranean woody plants Temperate grassland Crops

0.937 0.951 0.911 0.993 0.992

96.0 97.4 98.8 98.9 99.5

0.943b 0.961b

82.0 94.3

0.11 0.14 0.07 0.96 0.81 0.19a 0.12b 0.17b

0.46 0.60 0.32 4.16 3.51 1.71a 0.51b 0.75b

1 1 0.8 4 4.5 5.2c 2.6c 2.1c

a

Schenk and Jackson (2002a) averaged from 475 root studies. Jackson et al. (1996) averaged data from many different species from all over the world. c Canadell et al. (1996) averaged data from 82 species of temperate grassland. d d50d and d95d indicate the depth corresponding to 50 and 95%, respectively, of the cumulative root fraction. Both values are estimate from the Gage and Grigal (1987) model: Y = 1-bd, where Y is the cumulative root fraction from the surface to depth d (cm), and b is the fitted ‘‘extinction coefficient’’. e The equation RLDkm.m)3 = 2.24–0.0056 · Depthcm (see text) was applied to get values of RLD from 200 to 400 cm depth. b

A significant decrease in the number of tree roots with distance was also found (F3,27 = 96.81; p = 3.0E)05). Significant differences were

found between 0–10 m and 10–15 m, between 10–15 m and 15–20 m, but not between 15–20 m and >20 m. The maximum measured distance

Figure 3. (a) Mean values of RLD of holm-oak and herbaceous plants (oats and native grasses) measured at different distances to the tree trunk in dehesas. (b) Distribution of RLD plotted against the distance to the tree trunk and the depth for both herbaceous plants and tree.

Figure 4. Distribution of the tree (holm-oak) and herbaceous plants (oats and native grasses) RLD at differences distances (a) and depth (b) under two different types of dehesa management: cropped and grazed.

159 was 26 m, where roots were found even at 3 m of depth (data not shown). Considering the relationship between the number of roots and both distance and depth, the following equation was found: Nroot (m2) = 76.2–0.14*Depth(cm)–2.23*Distance(m); with R2 = 0.56 (F2,155 = 36.24; p < 0.0001). According to the stepwise analysis, the amount of variability explained by both parameters (their contribution to R2) was 39% and 18%, for distance and depth, respectively. From this equation, 33 m was estimated as the maximum distance, and 520 cm as the maximum depth.

Discussion Herbaceous plants root system Most of the herbaceous plant roots were located in the first centimetres of the soil, a common pattern for most of the herbaceous plants in the world (Jackson et al., 1996). Native grasses showed a very shallow root system, with d50 at 7.4 cm, and with 94% of the root length in the first 30 cm of soil. The maximum rooting depth was evidenced at about 80 cm. The oat crop had a deeper rooting pattern than native grasses, with ‘only’ 78% of the root length in the first 30 cm, and with a maximum rooting depth of 100 cm. Other authors have reported deeper root systems for temperate grassland and crops than those found in this study (e.g., Canadell et al., 1996, Jackson et al., 1996; see Table 1). The apparent low capacity of oats and native grasses to go deep in our study area could be explained by the presence of a very clayey soil layer between 40–80 cm depth. Nevertheless, a very shallow root system for native grasses of dehesas of Quercus ilex has also been reported by Barrera et al. (1987) and Joffre et al. (1987). The higher RLD of cropped plants as compared to natural pasture grasses was a surprise. This result does not coincide with the reported values by Jackson et al. (1996), who found that crops showed very low root density when compared to most other biomes (even a tenth part respect to temperate grassland). The denser and deeper root system in oat crops with respect to native grasses could induce an additional

competition for soil resources (mainly water) with trees when compared to the pasture. Holm-oak root profiles Surprisingly, the root profiles of mature holmoaks were almost uniform with depth. Most reported tree root profiles feature a significant decrease with depth, and this decrease often follows an exponential negative pattern (Jackson et al., 1996). Nevertheless, a linear or quasi-linear decrease of root density has been also reported by few authors (e.g., Kummerov and Mangan, 1981, Schulze et al., 1996). As a consequence, we put in evidence an unusual deep root system for Quercus ilex in this dehesa study, with a d50 value between 96.4 and 81.3 cm, and with a d95 value between 417 and 351 cm (with soil cores and road cuts methods, respectively). Schenk and Jackson (2002a), after an exhaustive review of 475 root studies, concluded that most of the plants have at least 50% of the roots in the first 30 cm of the soil, even in the desert. For sclerophyllous Mediterranean plants, these authors reported mean values of 19 and 171 cm for d50 and d95 (Table 2). Rather shallow root systems, with most of the roots in the first 50 cm of the soil depth, have also been reported in the Iberian Peninsula for forests of Quercus ilex (Canadell and Roda´, 1991, Lo´pez et al., 2001), for Quercus coccifera (Can˜ellas and San Miguel, 2000), for a sand dune shrub community (Martı´ nez et al., 1998), and for Erica and Ulex species (Silva and Rego, 2003). A deep-root pattern is often found in waterlimited situations, mainly for species with taproots in desert, savannah, tropical evergreen forest and sclerophyllous shrubland and forest (Canadell et al., 1996). We have found a maximum rooting depth for holm-oak at around 5 m, meanwhile Canadell et al. (1996) and Canadell and Roda´ (1991) reported maximum rooting depth of only 3.7 and 1 m, respectively, for close forests of Quercus ilex. Regarding other evergreen Quercus species there are several references with maximum rooting depth between 5 and 10 m (see Canadell et al., 1996). These authors reported a mean value of 5.2 m for sclerophyllous shrubland and forest of the world.

160 Lateral root distribution In semiarid conditions, the survival of trees facing severe drought conditions is only possible if the tree root system can extend beyond the influence of the tree canopy (Joffre and Rambal, 1999). Lateral root spread influences how many neighbours compete for resources available to plants in an ecosystem (Schenk and Jackson, 2002b). In the present study, the maximum lateral rooting (estimated at 33 m) was slightly larger than the average distance between trees (26 m). This pattern may be common in semiarid open woodland. Schenk and Jackson (2002b) pointed out that larger lateral root spreads were found in plants growing at low density in dry environments, where plants can explore the soil in interspaces between plants. These authors reported several cases of trees with maximum lateral root spread above 20 m. An outstanding consequence of this result is that lateral roots can explore the whole inter-tree space, allowing full use of the soil volume by mature trees in dehesas. The surface of explored soil by roots could spread up to 7 times the projected area of the canopy. The dynamics of soil water content in the same plots (Cubera et al., 2004) showed that soil water beyond the tree canopy was depleted throughout the summer, while no grasses were active, confirming that water was extracted by trees. Our results support thus the hypothesis that mature tree density in dehesas could be water-availability dependent (Joffre and Rambal, 1999). Combined root system: implication on competition for soil resources To reduce competition with crops/grasses for below-ground resources, a tree should have a deep root system and little root proliferation near the top of the profile, thereby enabling the herbaceous plants to utilise resources from near the soil surface, while trees have sole access to deeper layers (Schroth, 1995). We have shown such a pattern of spatial separation between herbaceous plants and tree root systems. Trees had a much deeper root system, with a rather low RLD in the upper layers of the soil, and herbaceous vegetation did not reach deep layers, where tree roots were still abundant.

This rooting pattern contributes to reducing below-ground competition, thereby probably falling into the general category of ’niche separation’ (Casper and Jackson, 1997). Thus, although water limitation is an important feature in most dehesas (including our study area), this does not necessarily mean that competition for water is high. Many authors have shown that woody plants took up more water from deeper layers than herbaceous ones (e.g. Ehleringer et al., 1991, Sala et al., 1989) avoiding thus a direct competition. In fact, this two-layer model appears to be most appropriate in drier regimes and in systems with substantial winter precipitation (Schenk and Jackson, 2002b), as it is the case of Iberian dehesas. The possible existence of this water partitioning in dehesas should be addressed in future research, and is only relevant when the rainfall pattern allows deep soil layer to be systematically refilled during the cold season. The very high herbaceous RLD in the first 10 cm of soil could induce a strong competition for nutrients with trees, as a result of the fact that nutrients (mainly N) may be available only in the upper soil layers (Jackson et al., 1996). This extensive and deep root system of the trees may indicate a very good capability to avoid any nitrate leaching from the cropped area. Most leached nitrates can be captured by the tree roots, as the evergreen oak trees are active throughout the year. If we follow the assumption that roots grow only as deeply and as far as needed to fulfil the plant resource requirements (Schenk and Jackson, 2002b), it is obvious that mature holm-oaks need a huge volume of soil to capture below-ground resources in oligotrophic soils, under a semi-arid climate with a long summer drought. As Joffre et al. (1999) have pointed out, dehesas have to cope with the high variability of the Mediterranean climate; the tree extensive root system undoubtedly must contribute both in adapting to natural conditions and in overcoming unpredictability. Holm-oak RLD decreased very slowly with distance and depth. This rooting pattern should have important consequences in modelling the coexistence of tree and grass/crop. It is at odds with the commonly assumed pattern of tree fine roots distributions, that is often described with an exponential decrease with soil depth and distance from the tree trunk. We have

161 found a limited vertical overlap of tree and herbaceous understorey root systems, and this feature is probably a key to the stability and productivity of this agro-silvo-pastoral system. However, we documented the rooting pattern only in spring, and more information is still required on temporal dynamics of the fine roots of both holm-oak and herbaceous plants in the dehesa.

Acknowledgements This study was supported by the European Union (SAFE project: Silvoarable Agroforestry For Europe QLK5-CT)2001–0560), by the Spanish Ministerio de Ciencia y Tecnologı´ a (MICASA project) and by the Consejerı´ a de Educacio´n de Extremadura (CASA project). Elena Cubera has been awarded a grant by the Consejerı´ a de Educacio´n de la Junta de Extremadura (Spain) and Jesu´s Obrador has been awarded a grant by ANUIES (Me´xico).

References Barrera I, Galindo P and Go´mez J M 1987 Modelo de distribucio´n de la biomasa radical en funcio´n de la profundidad. Anuario del CEBA de Salamanca 12, 313–323. Canadell J, Jackson R B, Ehleringer J R, Mooney H A, Sala O E and Schulze E-D 1996 Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583–595. Canadell J and Roda´ F 1991 Root biomass of Quercus ilex in a montane Mediterranean forest. Can. J. For. Res. 21, 1771– 1778. Can˜ellas I and San Miguel A 2000 Biomass of root and shoot systems of Quercus coccifera shrublands in Eastern Spain. Ann. For. Sci. 57, 803–810. Casper B B and Jackson B J 1997 Plant competition underground. Annu. Rev. Ecol. Syst. 28, 545–570. Cubera E, Montero M J and Moreno G 2004 Effect of land use on soil water dynamics in dehesas of Central-Western Spain. In Advances in GeoEcology 37: Sustainability of Agrosilvopastoral systems –Dehesas, Montados Eds. S Schnabel and A Ferreira pp. 109–123. Catena Verlag, Reiskirchen. Dı´ az M, Campos P and Pulido F J 1997 The Spanish dehesas: a diversity in land-use and wildlife. In Farming and Birds in Europe: The common Agricultural Policy and its Implications for Bird Conservation Eds. D J Pain and M W Pienkowski pp. 178–209. Academic press, Cambridge, UK. Ehleringer J R, Phillips S L, Schuste W F S and Sandquist D R 1991 Differential utilization of summer rains by desert plants: implications for competition and climate change. Oecologia 88, 430–434.

Escudero A 1985 Efectos de a´rboles aislados sobre las propiedades quı´ micas del suelo. Rev. Ecol. Biol. Sol. 22(2), 149–159. FAO 1998 World reference base for soil resources. FAO, ISRIC and ISCC, Rome. 109 pp. Gale M R and Grigal D F 1987 Vertical root distributions of northern tree species in relation to successional status. Can. J. For. Res. 17, 829–834. Infante J M, Damesin C, Rambal S and Ferna´ndez-Ale´s R 1999 Modelling leaf gas exchange in holm-oak trees in southern Spain. Agr. For. Meteorol. 95, 203–223. Jackson R B, Canadell J, Ehleringer J R, Mooney H A, Sala O E and Schulze E.-D 1996 A global analysis of root distributions for terrestrial biomes. Oecologia 108, 389–411. Joffre R, Leiva Morales M J, Rambal S and Ferna´ndez Ale´s R 1987 Dynamique racinaire et extraction de l’eau du sol par des gramine´es pe´rennes et annuelles me´diterrane´ennes. Acta Oecol. Oecol. Plant 8(22), 181–194. Joffre R and Rambal S 1988 Soil water improvement by trees in the rangelands of southern Spain. Oecol. Plant 9, 405–422. Joffre R and Rambal S 1993 How tree cover influences the water balance of Mediterranean rangelands. Ecology 74, 570–582. Joffre R, Rambal S and Ratte J P 1999 The dehesa system of southern Spain and Portugal as a natural ecosystem mimic. Agroforest. Syst. 45, 57–79. Jose S, Gillespie A R, Seifert J R and Pope P E 2001 Comparison of minirhizotron and soil core methods for quantifying root biomass in a temperate alley cropping system. Agroforest. Syst. 52, 161–168. Jones M, Sinclair F L and Grime V L 1998 Effect of tree species and crown pruning on root length and soil water content in semi-arid agroforestry. Plant Soil 201, 197–207. Kummerov J and Mangan R 1981 Root system in Quercus dumosa Nutt. dominated chaparral in southern California. Acta Oecol. 2, 177–188. Lo´pez B, Sabate´ S and Gracia C 2001 Vertical distribution of fine root density, length density, area index and mean diameter in a Quercus ilex forest. Tree Physiol. 21, 555–560. Martı´ nez F, Merino O, Martı´ n A, Garcı´ a Martı´ n D and Merino J 1998 Belowground structure and production in a Mediterranean sand dune community. Plant Soil 201, 209–216. Mobbs D C, Lawson G J, Friend A D, Crout N M J, Arah J R M, Hodnet M G 1999 Hypar Model for Agroforestry System. Technical Manual. DFID Forestry Research Programme. Montero G, San Miguel A and Can˜ellas I 1998 System of Mediterranean Silviculture ‘La Dehesa’. In Agricultura Sostenible Eds. R M Jime´nez Dı´ az. and J de Lamo Espinos pp. 519–554. Mundi Prensa, Madrid. Montero M J, Obrador J J, Cubera E and Moreno G 2004 The role of dehesa land use on tree water status in CentralWestern Spain. In Advances in GeoEcology 37: Sustainability of Agrosilvopastoral systems – Dehesas, Montados. Eds. S Schnabel and A Ferreira. pp. 125–136. Catena Verlag, Reiskirchen. Papadakis J 1966 Climates of the world and their agricultural potentialities. DAPCO, Rome. 174 pp. Plieninger T, Pulido F J and Konold W 2003 Effects of land-history on size structure of holm oak stands in Spanish dehesas: implications for conservation and restoration. Environ. Conserv. 30, 61–70. Puerto A, Garcı´ a J A and Garcı´ a A 1987 El sistema de ladera como elemento esclarecedor de algunos efectos del arbolado

162 sobre el pasto. Anuario del CEBA de Salamanca 12, 297–312. Sala O E, Golluscio R A, Laurenroth W K and Soriano A 1989 Resource partitioning between shrubs and grasses in the Patagonian steppe. Oecolgia 81, 501–505. Schenk H J and Jackson R B 2002a The global biogeography of roots. Ecol. Monogr. 72(3), 311–328. Schenk H J and Jackson R B 2002b Rooting depths, lateral root spreads and below-ground/above-ground allometries of plants in water-limited ecosystems. J. Ecol. 90, 480–494. Schroth G 1995 Tree root characteristics as criteria for species selection and systems design in agroforestry. Agrofor. Syst. 30, 125–143. Schulze E-D, Mooney H A, Sala O E, Jobbagy E, Buchmann N, Bauer G, Canadell J, Jackson R B, Loreti J, Oesterheld M and Ehleringer J R 1996 Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia. Oecologia 108, 503–5111. Silva J S and Rego F C 2003 Root distribution of a Mediterranean shrubland in Portugal. Plant Soil 255, 529– 540.

Smith D M, Jackson N A, Roberts J M and Ong C K 1999 Root distribution in a Grevillea robusta-maize agroforestry system in semi-arid Kenya. Plant Soil 211, 191–205. Smit A L, Bengough A G, Engels C, van Noordwijk M, Pellerin S and van de Geijn M, Brouwer G, Meijboom F, Do Rosario G, Oliveira M and Bengough A G 2000 Trench Profile Techniques and Core Break Methods. In Root Methods: A Handbook Eds. L Smit, A G Bengough, C Engels, M van Noordwijk, S Pellerin and van de Geijn pp. 211–233. Springer-Verlag, Berlin Heidelberg. van Noordwijk M, Lawson G, Soumare A, Groot J J R and Hairiah A 1996 Root distribution of trees and crops: competition and/or complementarity. In Tree-crop interactions A Physiological approach Eds. C K Ong and P Huxley pp. 319–364. CAB International, Wallingford. van Noordwijk M and Lusiana B 2000 WaNuLCAS 2.0, Background on a model of water nutrient and light capture in agroforestry systems. International Centre for Research in Agroforestry (ICRAF), Bogor, Indonesia. 186 pp. Section editor: T. Kalapos