1. Introduction
It is well known that soil restoration is very difficult in arid and semiarid Mediterranean environments. Along with the climatic aridity, precipitation seasonality and its high inter-annual variability, vegetation establishment is quite uncertain because long annual dry periods and short-lasting torrential rainfalls with a high erosive impact. In more humid areas, when available topsoil is stored before mining/quarrying activities and spread on the surface of mined areas after the cease of mining/quarrying activities, soil restoration and revegetation tend to be rather successful. However, the lack of stockpiled soil due to shallow soils in most arid/semiarid Mediterranean areas, adds negative conditions to the restoration. To cope with this problem, organic amendments have been used to create an artificial soil (technosol) to improve the substrate quality thus enhancing the establishment of autochthonous plants (Clemente et al., 2004; Jorba et al., 2008; Asensio et al., 2013). The application of organic amendments in degraded soils improves physicochemical and biological properties of the degraded systems facilitating the entry of other plant species (Maestre and Cortina, 2004). And fast growing spontaneous cover is efficient at soil erosion control, incorporates carbon into substrates, and accelerates pedogenesis (Moreno de las Heras, et al. 2008).
Nevertheless, during the initial and critical stages of plant development, the beneficial properties of planted species may not be enough to control soil erosion and droughts, for which a mulch cover is recommended, especially in slopes (Morgan and Rickson, 1995; Cook et al., 2011). Surface mulches have been shown to play an important role on soil properties, hydrology, runoff hydraulics, by increasing water infiltration (Tejedor et al., 2003; Adekalu et al., 2007; Luna et al., 2018) and, in the case of organic mulches, improving soil structure and porosity (Walsh et al., 1996; Mulumba and Lal, 2008), and reducing runoff an erosion (Lal, 1976; Prats et al, 2012; Luna et al, 2018). Other mulch functions include reducing evaporation and buffering changes in soil temperature especially in climatically extreme environments (Robichaud et al., 2013). Despite the statement of Bainbridge (2001) that organic mulches can also pose risks during droughts by limiting infiltration into the soil because light rains can easily evaporate from the mulch itself, other authors reported that globally organic mulches have all the former advantages and also provide organic matter to the substrate (Miller and Seastedt 2009, Eldridge et al 2012). Moreover, organic mulches, and specifically woodchips, significantly enhance microbial activities but not plant cover (Espinosa et al 2020, Fehmi et al 2020).
With the final aim to contribute accelerating the process of ecosystem recovery and to ascertain the specific role of organic amendments and woodchip mulches in the most arid part of southern Europe, an experimental restoration was carried out in a degraded soil from an abandoned quarry in semiarid Sierra de Gádor (Almería, SE Spain). As one of the main goals was to achieve a vegetation cover similar to that present in the surrounding natural areas, the three most abundant native species in the area were planted. Besides the planted species, in the first annual survey, we observed annual and perennial herbaceous plants colonizing the experimental plots to different degrees. In addition to ruderal vegetation, pine seedlings were noticed in the plots related to pine cones from the pine woodchips used as mulch. According Daskalakou and Thanos (1996), preserved cones maintain pine seeds from Pinus halepensis much longer than single pinions released much time before when the cones are still on the tree, which might explain any late pine sprouting from the pine mulch.
In the following years of the experimental restoration, pine seedlings increased in size and they could be distinguished from both the planted and the opportunistic species. In limiting environments with dry conditions, the pine seedlings that grow under the canopy of certain shrubs can benefit from the microclimatic protection as well as from a greater availability of resources in the soil, which usually translate into greater survival (Callaway et al. 2002; Armas and Pugnaire, 2005; Thompson et al., 2017).
In the last decades, ecological restoration projects have highlighted the use of wood plant species because they increase the structure and complexity of ecosystems and can accelerate the regeneration of degraded environments (Vallejo et al., 2009). Despite the slow growth of woody species can be a problem in erosion control and soil formation in the first years of plant development, the Aleppo pine (Pinus halepensis Miller) has been extensively used in afforestation in the Western Mediterranean Basin because is a fast-growing conifer (Quezel, 2000). This species grows on all substrates and within most bioclimates of the Mediterranean region. In such locations, the tree is mainly scattered in lower altitudes, though it also grows in mountainous areas. It is a drought-resistant species, growing well in hot areas exposed to frequent forest fires. Within its distribution area, the tree also provides certain ecosystem services, such as improving water infiltration, preventing soil erosion on dry slopes and serving as windbreak. For these reasons, this tree has been key to several afforestation programs. As a result of these massive reforestations, various studies have been carried out to evaluate these actions. In some cases, pine plantations may have positive effects but the opposite in other cases, with negative effects on both the natural vegetation dynamics and in various ecosystem processes (Maestre and Cortina, 2004).
All these facts added new questions to the initial soil restoration experiment which hypothesis was that the combined effect woodchip mulch and organic amendments could effectively enhance the soil properties recovery and consequently could also facilitate the establishment and entry of new species in addition to planted vegetation. Can there be competition between planted vegetation, ruderal species and pines? We hypothesize that an improvement in soil quality due to restoration treatments, as previously stated in similar experiments in the same area by Luna et al (2016a, 2016b, 2017), can promote competition for nutrients, space and light.
Consequently essential characteristics from both restored substrates and vegetation were monitored 6 years after the establishment of experimental plots and statistical procedures on data measured should respond to the raised hypothesis.