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.