2_Gomez.indd 13Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.DOI: 10.15201/hungeobull.66.1.2 Hungarian Geographical Bulletin 66 2017 (1) 13–28. Mediterranean tree crops Tree crops are a key element of the Euro- pean agricultural landscape with more than 13 million hectares of permanent tree crops in the EU-28. The majority of them, approxi- mately 80 per cent of the surface, are concen- trated in areas with Mediterranean type of climate (Table 1). This is because the majority of these crops in the EU (such as olives, citrus or almonds) are best grown under a Mediter- ranean type of climate. The only exception among the dominant tree crops are vines. The 3.2 million hectares of vines in the EU- 28 are distributed across the continent among 21 countries, from Sweden to Malta, albeit the majority of them are also concentrated in Mediterranean areas. The major reason for that distribution is the favourable conditions in terms of tem- perature and radiation. Other reasons are the rusticity of some of these tree crops, particu- larly olives and almonds, which allows cul- tivation in areas not suitable for other crops or grazing and their double role as a food and cash crop. However, the Mediterranean type of climate is characterized by a limited, and highly variable, precipitation in relation Sustainability using cover crops in Mediterranean tree crops, olives and vines – Challenges and current knowledge José A. GÓMEZ1 Abstract Tree crops cover a large area of European landscape, 13.3 million hectares, with olive, grapes, nuts and almonds been the most extended and mostly concentrated in Mediterranean areas. The cultivation of tree crops in rain limited Mediterranean areas depend on an adequate management of water balance that, been historically mostly based on bare soil, has created severe erosion and offsite contamination problems. Temporary cover crops can be an alternative to control these problems with a larger effect on erosion control than on reducing runoff, and a moderate impact on soil properties. This impact depend strongly on the ability to implement temporary cover crops that achieve a significant development during the rainy season while simultaneously minimizing the competition for soil water with the major crop, which is not always easy in commercial farms. This balance between soil protection and yield has been achieved in some conditions but not in others, and a significant reduction in yield has been reported for some situations. This potential risk of yield decrease, combine with the difficulty to see a collapse in yield due to soil degradation by water erosion in the short/ medium term can explain, partially, the reluctance of farmers for an extensive use of temporary cover crops. The development of improved strategies for using temporary cover crops which could include the use of water balance models, new varieties better adapted to the region, and strategies for restoring ground cover in severely degraded orchards seems to be necessary, coupled with regulations and incentive to their use by farmers. Future research should focus in the less understood elements of this system, among them root development, biomass production, phenology under different microclimate of the cover crops and the main tree crops, use of cover crops mixes, which are hampering the tuning of the system for specific conditions. It is also necessary a better definition and measurement of the impacts of cover crops on biodiversity that should be related to the landscape conditions. Keywords: olive, vines, sustainability, water balance, erosion, Mediterranean 1 Institute for Sustainable Agriculture (IAS) CSIC. Avenida Menéndez Pidal S/N 14004. Cordoba, Spain. E-mail: joseagomez@ias.csic.es Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.14 Ta bl e 1 . S um m ar y of tr ee cr op s e xt en sio n in th e E ur op ea n U ni on in 1 ,0 00 h a EU m em be r co un tr ie s To ta l O liv es G ra pe s C itr us A lm on ds N ut s A pp le s Pe ar s Pe ac he s an d ne ct ar in es C he rr ie s EU 2 8 co un tr ie s Be lg iu m Bu lg ar ia C ze ch R ep ub lic D en m ar k G er m an y Es to ni a Ir el an d G re ec e* Sp ai n* Fr an ce ** C ro at ia * Ita ly * C yp ru s* La tv ia Li th ua ni a Lu xe m bo ur g H un ga ry M al ta * N et he rl an ds A us tr ia Po la nd Po rt ug al * Ro m an ia Sl ov en ia Sl ov ak ia Fi nl an d Sw ed en U ni te d K in gd om 13 ,3 33 41 96 46 7 19 5 3 13 1, 35 7 5, 49 1 1, 03 8 84 2, 77 5 30 7 34 7 13 1 1 37 65 55 9 84 4 38 8 19 14 3 3 46 4, 99 2 0 0 0 0 0 0 0 93 8. 3 2, 52 6. 5 17 .1 17 .5 1, 13 0. 4 11 .0 0 0 0 .. 0 0 0 0 35 1. 3 0 .. 0 0 0 0 3, 17 8 0 38 .7 15 .8 0 10 0. 10 0 0 10 9. 8 94 1. 1 75 3. 9 25 .6 68 3. 8 5. 8 0 0 1. 3 73 .1 0. 7 0. 2 44 .8 0. 6 17 8. 9 17 7. 7 15 .7 8. 8 0 0. 1 1. 8 52 1 0 0 0 0 0 0 0 49 .1 0 30 2. 46 4. 16 2. 17 14 0. 16 2. 69 0 0 0 0 0 0 0 0 19 .8 0 0 0 0 0 0 0 65 4 0 0. 57 0 0 0 0 0 12 .5 7 54 8. 60 1. 12 0. 31 57 .4 3 2. 76 0 0 0 0. 20 0 0 0 0 30 .1 5 0 0 0 0 0 0 1, 24 0 5. 00 6. 76 0 0 1. 00 0 12 .0 0 54 .9 5 69 7. 90 52 .4 1 10 .5 2 19 8. 39 3. 08 0 0 5. 00 0. 60 0 0 3. 00 13 .0 0 17 3. 08 3 0 0 0 0 0 53 9 7. 06 4. 81 8. 98 1. 38 31 .6 5 0. 90 0. 62 12 .9 3 30 .7 9 52 .5 0 5. 80 53 .0 1 0. 63 2. 80 11 .7 0 0. 24 33 .3 6 0 7. 91 6. 97 16 2. 40 13 .6 6 60 .2 8 2. 64 3. 65 0. 60 1. 30 20 .0 0 11 7 9. 08 0. 34 0. 88 0. 36 1. 92 0 0 4. 97 23 .6 4 5. 36 1. 04 30 .1 5 0. 08 0. 20 0. 90 0. 02 2. 89 0 8. 60 0. 44 9. 20 12 .0 1 3. 46 0. 21 0. 13 0 0. 10 1. 00 22 6 0 3. 71 0. 48 0 0 0 0 48 .1 0 86 .5 1 9. 89 1. 06 67 .5 1 0. 45 0 0 0 .. 0 0 0. 17 2. 40 3. 75 1. 65 .. 0. 40 0 0 0 17 3 1. 3 9. 3 2. 3 1. 1 7. 2 0 0 13 .8 26 .5 8. 1 3. 1 29 .4 0. 2 0. 1 0. 8 0 16 .1 0 0. 8 0. 2 39 .1 6. 4 5. 7 0. 2 0. 2 0 0. 1 0. 7 *C ou nt ri es w ith p re do m in an t M ed ite rr an ea n cl im at e. * *S om e ar ea s w ith M ed ite rr an ea n cl im at e. .. = D at a no n av ai la bl e. S ou rc e: O w n el ab or at io n fr om Eu ro st at (2 01 6) a va ila bl e da ta . 15Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28. resources in areas where irrigation, which is almost exclusively deficit irrigation, has expanded in recent decades. In an effort to mitigate some of these prob- lems it has been an continuous attempt for in introducing the use of cover crops in tree crops on Mediterranean areas, at least since 1969 (Ruíz de Castroviejo, J. 1969). It is worth clarifying that when talking about cover crops in the context of rainfed (or deficit irrigation) tree crops in Mediterranean conditions we al- ways refer to temporary cover crops. Photo 2 summarized the concept of temporary cover crops which is based on seeding, or allowing growing, of herbaceous vegetation in the lanes during the rainfall season (autumn/fall and winter) controlling chemically or mechanically the cover crop in early spring to prevent losses of soil water by transpiration, and maintain- ing its residues over the surface until next fall when, ideally, it will regrow from seeds pro- duced during the previous year. This communication revises some of the issues regarding sustainable cultivation of tree crops in Mediterranean conditions with the use of cover crops, focusing particularly in olives and vines. Modification of soil properties, erosion and runoff losses at plot scale Most of the available information to evaluate the impact of the use of temporary cover crop as an alternative to bare soil comes from ex- periments at plot scale. Figure 2 summarizes results from experiments carried out under natural rainfall conditions in experiments lasting 2 or more years in plots at least 12 m long. This criterion was followed to limit the bias induced by short term experiments, sim- ulated rainfall, or those performed at very small scale not including relevant processes. Figure 2 (top side) shows how the use of cover crops has a clear and significant effect on reducing soil losses in olive orchards and vineyards at plot scale. In all the experiments this reduction was found, with an average reduction close to 60 per cent. to the potential evapotranspiration (ETo) and by a dry season during the period of maxi- mum temperature and ETo (Figure 1). Agronomical practices in orchards in Mediterranean areas have evolved in the direction of prioritizing the improvement of soil water balance for the tree, to insure productivity and survival of trees and crops under limiting water conditions. Historically this has been achieved com- bining three major elements. One is a low tree plant density, which allows a large soil volume for the roots to explore for soil water, with the other two been a limitation of the canopy size by pruning and elimination of weeds to prevent competition for soil water with the tree. This, agronomically sounded, strategy has been successful for allowing tree cultivation over centuries in Mediterranean areas, but it has also created landscapes, like the one shown in Photo 1 characterized by a simplified landscape with limited ground cover on sloping areas. This has resulted in some environmental problems, particularly severe in some areas of the Mediterranean. Several studies have noted these prob- lems, particularly in olives growing areas (e.g. Beauffoy, G. 2001; Scheidel, A. and Krausmann, F. 2011). They can be sum- marized in: soil degradation by accelerated water erosion, decrease of water quality by offsite contamination, decrease of biodiver- sity and an increasing pressure on water Fig. 1. Average monthly precipitation and potential evapotranspiration (ETo) for Cordoba, Southern Spain, from 2001 to 2015. Error bars indicates stand- ard deviation. Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.16 Photo 1. View of olive cultivation in a mountainous area in Southern Spain (Montefrío). Photo 2. Evolution of a temporary cover crop in an olive orchard during the four seasons of the year. 17Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28. The effect on average annual runoff is shown in Figure 2 (down side). In this case the effect of the use of cover crops is not as clear and although there is an overall reduc- tion in average annual runoff of approxi- mately 25 per cent, this reduction is site spe- cific with some orchards and vineyard pre- senting very small reductions in cover crops (CC) compared to bare soil by conventional tillage (CT) or no tillage with bare soil with herbicide (NT) or even slight increase in runoff, with others showing a large reduc- tions. The reasons for that different answer in runoff and soil losses have been discussed in detail elsewhere (e.g. Gómez, J.A. et al. 2011). They can be summarized in that while the reduction in soil losses is primarily the re- sult of physical protection by the cover crop and its residues, the mechanism controlling infiltration is more complex and varied with sites. In situations where infiltration is lim- ited by surface sealing or reduced porosity of the top soil the over crop has a clear effect, however in situations while the infiltration rate is controlled by saturation of the soil profile or by subsurface layers the effect of the cover crops is very small or negligible. In Mediterranean areas it is frequent to have orchards and vineyards on shallow soils and also periods of high precipitation in which the soil profile is close to satura- tion. It reasonable to expect that this differ- ent answer in runoff and soil losses when using cover crops can be a widespread phe- nomenon in Mediterranean tree crops. It is worth noting that Maetens, W. et al. (2012) in a metanalysis of plot experiments in Europe also detected a higher effect of conservation tillage in reducing soil losses compared run- off losses when compared to conventional systems. Figure 3 shows for two long term ex- periments in vineyards and olives the annual variability of the reduction in runoff and soil losses. It is apparent the same overall trend commented before and also that this variabil- ity must be related to the interaction between rainfall, soil conditions and soil management within each year, since the overall correlation with annual rainfall is weak. The spatial distribution of soil properties within an orchards or vineyard is different to those in a field crop, since it has a mosaic pattern in which the influence of the tree and the cover crop induces differences in some of them, like infiltration rate or bulk density. When interpreting and modelling hydrologi- cal processes, such as runoff generation, water balance or water erosion, this heterogeneity depicted in photos needs to be considered (Photo 3a and 3b). For instance, Castro, G. et al. (2006) showed the relevance of run-on in the under canopy and cover crop area with some of the runoff generated in the area of the lane with bare soil These effects have been, sometimes, incorporated into the efforts for modelling runoff and water erosion in olives and vineyards at hillslope scale. For instance, Romero, P. et al. (2007) developed and validat- ed values for the CN method for different soil Fig. 2. Comparison of average annual runoff losses (top) and soil losses (down) between cover crops (CC) and bare soil management by tillage (CT) or herbicide (NT) in olives and vineyards. Source: Own elaboration from data in Biddoccu, M. et al. 2016, and Gómez, J.A. et al. 2009a, 2011. Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.18 management in olive orchards, and these CN values have been used successfully in water balance models in olives (Abazi, U. et al. 2012). The CN method has also been used for determining runoff losses in water balance modes in vines in Mediterranean conditions (e.g. Celette, F. et al. 2010) although in these case the CN values were apparently taken from the values developed for orchards in USA by the USDA. The effect of soil manage- ment in water erosion in olives and vines has been incorporated in RUSLE through cali- bration of C values for specific conditions. Gómez, J.A. et al. (2003) proposed several C values for different olive plant density and soil management in orchards considering the influence of the variation of soil moisture content during the year. These C values seem to provide reliable pre- dictions of soil losses when compared to long term erosion rates estimations (Vanwallegem, T. et al. 2011) or plot data (Marin, V.J. 2013). Auerswald, K. and Schwab, A. (1999) pro- posed C values for USLE for different soil management and vine plant density in Germany, although to our knowledge, these values have not been validated. When com- paring C values for vines proposed by differ- ent authors in Europe (Gómez, J.A. et al. 2016) it is noticeable that they show large differenc- es even for apparently similar managements. This is probably for a combination of differ- ences in the conditions for which they have been determined and the lack of a standard approach for its calibration and validation. Overall, all the C values proposed for olives and vines capture the trend towards reduced erosion with the use of cover crops, albeit there is the need for extensive validation to evaluate the uncertainty existing on the pre- dicted values of soil loss. The modification of soil properties induced by the cover crop in an orchard and vine tend to be limited to the area where the cover crop is implanted, usually only a fraction of the or- chard (see Photo 3–4), and tend to be concen- trated in the top 0–20 cm of the soil (see Gómez, J.A. et al. 2009a). For this reason their overall impact on nutrient and carbon content in the orchards and vines, albeit significant, tend to be limited and related to the spatial extension of the cover crop strip. An element of major concern when extrapolating the benefits of the cover crops, in term of runoff and soil loss reduction, from experimental areas to com- mercial farms should be the large variability in the “quality” of the cover crop found in dif- ferent farms (Photo 5–6). This “quality” should be understood as the ability to provide enough ground cover and biomass during the rainy season in a significant area of the orchard. In transects within a relatively small areas Gómez, J.A. et al. (unpublished data) measured in spring (before killing the cover crop) values of aboveground biomass for the cover crop area from 0.1 t/ha (almost bare soil) to 1.8 t/ha (which provided a good ground cover). There are several reasons for this large dis- parity in cover crops development, among them differences in soil quality, seed bank and soil management among different orchards. Fig. 3. Annual ratio of soil (top) and runoff losses (down) between cover crops (CC) and bare soil man- agement by tillage (CT) or herbicide (NT) in olives and vineyards. Source: Own elaboration from data in Biddoccu, M. et al. (2016), and Gómez, J.A. et al. (2011) and unpublished data. 19Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28. Photo 3–4. View of orchards showing the area of influence of the olive canopy (top) and the cover crop (down). Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.20 Photo 5–6. Comparison of two olive orchards declaring use of cover crops, Note narrow over crop strips in the upper picture compared to the one below. 21Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28. Similar differences in cover crop biomass production in the lanes of olive orchards have also been noted by other authors (e.g. Vicente-Vicente, J.L. 2017). These results highlight the need of more focused efforts in developing innovative strategies for achiev- ing successful implementation of temporary cover crops in these situations which in many cases are associated to severely de- graded soils. Gómez, J.A. et al. (2009b) noted this heterogeneity of cover crop conditions as one of the reasons for the large variability found in organic olive orchards with cover crop management. Gómez, J.A. et al. (2014a) discussed the implications of these large differences between experimental results and field situations when trying to estimate regional erosion rates for olive growing ar- eas in Andalusia. He noted a variation of approximately 30 per cent in the predicted average erosion rate and severely degraded area estimation under current common ag- ricultural policy (CAP) regulations regard- ing the compulsory use of cover crops when introducing a decrease in the efficiency of these cover crops based on calibrating the C factor of RUSLE based on observations of cover crops status from field visits to several orchard in the region. Water balance and yield Water is the major limiting factor for agri- cultural production in semiarid environment with soil management playing a major role in controlling that water balance (Hender- son, D.W. 1979). A modification of soil man- agement such as the use of temporary cover crops in Mediterranean tree crop cannot be successful without understanding the im- plications for yield due to the modification of the water actually available to the crop. Figure 4 depicts the results of some experi- ments comparing the impact on olive fruit and wine yield of temporary cover crops in olives and wines. It is apparent that in some situations the system of temporary cover crops has been adjusted to provide soil pro- tection while achieving yields that are similar to those under bare soil management (e.g. CC controlled in early spring in Figure 4), although in other situations, (e.g. those con- trolled in mid-late spring in Figure 4) there is a significant decrease in yield. This decrease when comparing those ap- proaches (CC vs. CT) has been noted by oth- er researchers in long-term experiments (e.g. Ferreira, I.Q. et al. 2013). This potential risk of a yield decrease remains a major obstacle for expanding the use of temporary cover crops in Mediterranean tree crops particu- larly under rainfed conditions. Another tool to fine tune the management of cover crops under a broad range of conditions is the use of simulation models to study its impact on water balance. The literature describes several models developed for vines or olives. For instance, Celette, F. et al. (2011) presented WALIS as a simple model to simulate water partition- ing in a crop association and use it to study the case of an intercropped vineyard, while Abazi, U. et al. (2013) presented WABOL, other conceptual model for the case of inter- cropped olives. These studies concluded that the models provided realistic simulations, and they could be useful tools in providing a better understanding of cover crops in ol- ives and vines. However, in both studies the authors mentioned the need for an extensive validation of the model results, which to date Fig. 4. Comparison of vine and olive yield in conven- tional tillage (CT) and temporary cover crop (CC). Source: Own elaboration from data in Gómez, J.A. 2005, and Ruíz-Colmenero, M. et al. 2011. Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.22 still lacking. Parameterization of these models is of paramount importance and some of their key parameters still remain relatively poorly understood. Among those less understood are the phenology and root development of the tree crops and cover crops species under different conditions, the effect of capillary rise of subsurface layers during the dry season, and improved determination of the transpi- ration of the tree and cover crops in complex situation such as only partial ground cover or vertic soils are among the processes on which future research could be focused. Even with the caveats mentioned by the authors, these conceptual models have pro- vided insight into the feasibility of cover crop use under different conditions. Figure 5 sum- marizes the results of a study made by Abazi, U. et al. (2012) in which the variations in olive transpiration under different conditions in cover crop and conventional tillage condi- tions were evaluated for Andalusia (Southern Spain). The model results predicted for some situations no significant differences in olive transpiration while it also predicted in oth- er locations that CT seems to have slightly higher transpiration compared to CC, which agree with the agronomical experiments pre- viously commented. These conceptual models incorporate the effect of soil depth into soil water storage ca- pacity, and so they have the potential to be used in the evaluation on the decrease of vine or olives potential productivity due to the reduction of soil water availability accompa- nying the decrease of available soil depth by accelerated erosion. Gómez, J.A. et al. (2014a) evaluate the effect of decreasing soil depth on olive potential productivity under two con- trasting situations both characteristic of large areas in the Mediterranean: soils with rela- tively good water holding capacity and stony soils with worse water holding capacity. Figure 6 summarizes some of the major re- sults of this study. One is that for soils with relatively deep rooting zones and good soil water holding capacity the decrease in poten- tial yield appears clearly only at very shallow soil depths (see lines for Cordoba situation in Figure 6). The other is that the slope of the de- crease in potential yield with decreasing soil depth is not very steep, so the year to year decrease in potential year can be masked by other factors such as climate variability, pest and effect of agronomical practices. Both facts combined can help to under- stand, at least partially, the low priority giv- en by farmers to the implementation of soil erosion control practices in olives. Basically, because the effects of soil degradation in the reduction of potential yield are difficult to be observed in the short or medium term, and its worst effects will be suffered in the future. Vanwalleghem, T. et al. (2011) noted this situ- ation in an mountainous olive growing area in Southern Spain in which the loss of ap- Fig. 5. Predicted olive transpiration for the aver- age conditions rainfed olives in eight locations in Andalusia under conventional tillage (CT) and tem- porary cover crop (CC) for period 2006–2010. Error bars are standard deviation. Source: Adapted from Abazi, U. et al. 2012. Fig. 6. Potential olive tree yield for different average annual rainfall and rooting depth for two contrast- ing situations: Obejo, sandy soils with coarse mate- rial and moderate water holding capacity; Cordoba, fine textured soils with high water holding capacity. Source: Adapted from data in Gómez, J.A. et al. 2014a. 23Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28. proximately 40 cm of rooting depth (from 120 to 80 cm approximately) in olive orchards in the area in the time span of two centuries was accompanied by an increase in yield, attrib- uted to improved agronomical practices. This situation, soil degradation due to soil erosion which is not currently decreas- ing yields dramatically and it will not do it in the medium term, can be a recurrent pat- tern in some of the tree crops growing areas in Mediterranean regions. All these facts con- sidered suggest the need for regulations and incentives for erosion control on tree crops growing areas in the Mediterranean regions, particularly when most of the cost of erosion from these areas has been played downstream. Costs of soil erosion from agricultural areas in Europe has been estimated by Montanarella, L. (2007) as an average of 48 EUR/ha per year (within the range from 4.8 to 93.0 EUR/ha per year) with off-site damages representing more than 90 per cent of this costs. A review of possible strategies for implementation cover crops Table 2 summarizes the major kind of cover crops alternatives and some of the main is- sues regarding the choice of the option best suited for a given objective, as well as some of the major features and decisions to be considered regarding their implantation and management. In the context of limited water availability the decision for temporary cover crops aimed mostly to soil management has oriented many of the experiences in olives and vines towards the use of grasses. Several research projects has pursued the selection of grasses from local species which present a shorter growing cycle and could emerge with the first rains in fall and com- plete the seed development by late winter or early spring. This is the situation depicted in Photo 7 in which a difference in phenology of several weeks can be appreciated among several grasses. A shorter, best adapted, cycle will results in a lower risk for water compe- tition but also in a better persistence of the Ta bl e 2 . S um m ar y of a lte rn at iv es o f c ov er cr op s b as ed o n ob jec tiv es a nd m aj or q ue st io ns re ga rd in g m an ag em en t p ra ct ic es Pu rp os e K in d of c ov er c ro ps M ai n fe at ur es M an ag em en t A lte rn at iv es D ec is io ns Bi od iv er si ty M ix es , i nc lu di ng s ev er al sp ec ie s w ith fl ow er s C om po si tio n, p er si st en ce o f t he d iff e- re nc es s pe ci es , p he no lo gy C om po si tio n of m ix C on tr ol m et ho ds : h er bi ci de , m ow in g, g ra zi ng , t ill ag e? Ex te ns io n of c ov er c ro p W hi ch u s? C os t C on tr ol m et ho d: W he n? F re qu en cy ? La yo ut in th e sl op e, w id th o f co ve r c ro p? Fe rt ili ty Le gu m es /L eg um es a nd gr as se s A nn ua ls o r p er en ni al s? Ph en ol og y? Re si lie nc e? Si ze ? Pr ec oc ity ? Bi om as s pr od uc tio n an d gr ou nd c ov er ? Er os io n G ra ss es G ra zi ng Le gu m es /L eg um es a nd gr as se s Tr affi ca bi lit y G ra ss es Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.24 introduced cover crop in the plot, since it will have greater chances of producing seed before been controlled. In the search of better adapter species of grasses, precocity in emergence and a shorter size (an eventually lower biomass production) are also characters favoured. In vineyards, and lately although sporadically in olives, it is relatively frequent the use of mixes combining many species designed to increase biodiversity providing a large period with flowers in the orchard (e.g. Sweet, R.M, et al. 2010; Gómez, J.A. et al. 2014b). There is a limited understanding of the dy- namic of these mixes composed by a large number of different species. Gómez, J.A. et al. (2017) noted how a large number of them were not found in surveys in the seeded plots one and two years after their seeding, indicat- ing how a lower number of species composed the majority of the flora in the plots. A bet- ter understanding the dynamic of mixes, in terms not only of composition and long term evolution but also in terms or air and root biomass production of the different compo- nents are necessary if we want to evaluate these promising new alternatives using water balance models. The use of less diverse mixes can be useful in this objectives, as well as in optimizing expenditure in seed of species that could actually been viable in a mix for a given condition. Figure 7 shows preliminary results of a study comparing the evaluation of a simple mix with three species chosen from local flora for their potential. Despite all these efforts, statistics indi- cates that in many situations farmers still choose not to seed but to develop a cover crop from the flora naturally present in the Photo 7. View of a cover crops experiment in Cordoba (Southern Spain) in early May. It is apparent the dif- ferent in phenology between raygrass (front of picture still green) with Bromus (mid position in the picture, already eared and dried). 25Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28. orchard or vineyard. In Spain, for instance of the 30 per cent of the olive orchards us- ing some kind of cover crops, 97 per cent of them opted for natural weeds and only 3 per cent were seeded (MAGRAMA 2013). Cost is probably the major reason for this situation, although other reasons, such as the loose coupling between severe erosion and yield losses discussed above can also play a role. Within this context it might be appropriate to consider strategies for introducing cover crops that will require a very limited cost for farmers, for instance species that could be easily propagated by them. Also concen- trating more studies in situations where the naturally present weeds cannot be an alter- native, such as in extremely degraded soils with poor fertility and exhausted seed bank. Effects on biodiversity An improvement in biodiversity is one of the benefits frequently mentioned when recom- mending the use of cover crops in tree crops under Mediterranean conditions. However, for an issue which is extremely complex in- volving different orders of plants and animals and different scales the experimental data are relative limited and indicate less conclusive results than when compared to other of the questions commented in this article. For instance, Beaufoy, G. (2008) evaluating the results of a project evaluating the future of olive production in sloping land in sev- eral EU countries noted how the evaluation of the impact on biodiversity was extremely superficial, indicating the need for a more focused research. In the last years more pub- lications have been published on the subject indicating the need for establishing a clear link between the biodiversity indicator meas- ured and the landscape conditions where the study was performed. Paredes, D. et al. (2015) presented the results of a metanalysis evaluating the effect of cover crops in olive orchards in reducing the effect of several pests in Andalusia (Southern Spain), expect- ed due to the increase of natural predators for these pests when using cover crops. Their results show that the presence or not of cover crops explained a very small part of the pest response, with local, landscape and regional variability explaining a large proportion of the variability in pest response variables. This study points to perennial vegetation close to the focal crop as a promising alter- native strategy for conservation biological control that should receive more attention. Focusing in a different indicator of biodiver- sity, songbirds, Castro-Caro, J.C. et al. (2015) predicted that the presence of ground cover and landscape heterogeneity would have a positive effect on songbird communities, although the effect would be greatest in ho- mogeneous environments. The same team, however, in another study (Castro-Caro, J.C. et al. 2014) measured a different response in the abundance and rich- ness of omnivorous vs insectivorous birds to the use of cover crops depending on the pres- ence or not of hedgerows. In their study, they indicated how the richness of insectivorous birds increased with the presence of cover crops, or hedgerow, in the olive orchards, with a maximum increase in richness when both elements (cover crops and hedgerows were present simultaneously). However, in the case of omnivorous birds they did not found a significant increase with any the presence of a cover crop, hedgerows, or both elements in the olive orchards compared to an orchard managed with a bare soil and not hedgerows. Fig 7. Distribution of root biomass with depth for dif- ferent cover crops alternatives. Source: Adapted from Soriano, M.A. et al. (2016). Gómez, J.A. Hungarian Geographical Bulletin 66 (2017) (1) 13–28.26 These examples illustrate the complexity of the relationship between use of cover crops and biodiversity. In this context it is not sur- prising that metanalysis evaluating the impact of cover crops on biodiversity in vineyards have found a moderate impact (Winter, S. et al. 2016). However, despite this complexity many of the studies on biodiversity indicate that for a proper understanding of the effect of cover crops in Mediterranean tree crops they need to be linked to the landscape structure and, particularly, to the role of other vegeta- tion in that landscape. The need for this link has been noted also in erosion studies. For instance, Gómez, J.A. et al. (2014c) in study in a small catchment on a vertic soil note the rel- evance of gully erosion which could explain the high erosion rates in very rainy years which had high runoff coefficients. It is clear that much benefit could be achieved if some of the future studies evalu- ating the impact of cover crops could incor- porate this across-scale effects and interac- tion with other vegetation for hydrological and biodiversity studies. Also for innovative approaches in the design of environmental regulations that link the benefits of the use of vegetation on landscape, biodiversity and erosion control on solid technical knowledge. Conclusion Soil protection, enhancement of biodiversity and water quality are three major ecosystem services that should be delivered by agricul- tural areas in addition to crop production. Tree crops cover a large area of the European landscape, particularly in the Mediterranean areas. Although research have demonstrated the potential of temporary cover crops to deliver those services in Mediterranean tree crops this potential is not fully exploited. The need to balance two conflicting objectives: an appropriate ground cover vs. an adequate management of the cover crop to limit its wa- ter consumption by transpiration to prevent yield reductions, results in many farm situa- tions in a reduced ground cover and biomass production, which it is not enough to deliver those ecosystem services. The conservative approach of many farm- ers to cover crops reflects also the limited understanding of key elements that are hampering the fine tuning of the system for specific farm conditions, which is a critical element for success. Future research should focus in the less understood elements of the tree and cover crops system such as: cover crops and tree root distribution and develop- ment; biomass production; phenology under different microclimate of the cover crops and the main tree crops; or performance of cover crops mixes. It is also necessary a better defi- nition and measurement of the impacts of cover crops on biodiversity that should be related to the landscape conditions. This research should lead to the develop- ment of improved strategies for using tempo- rary cover crops which could include the use of water balance models, new varieties better adapted to the region, and strategies for re- storing ground cover in severely degraded orchards. 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