4.3. Interactions between plant communities
Biodiversity of plant species was greater in the experimental plots with
topsoil because they had a previous seed bank (Golos et al., 2016), and
in treatments with organic amendments. However, biodiversity was not
proportional to total plant cover. This means that large increases in
nutrient inputs do not ensure high plant diversity. Other authors
(Moreno de las Heras et al., 2008; Pallavicini et al., 2015) also found
a confrontation between high cover of fast-growing spontaneous species
and species diversity. This fact may due to competitions or space
problems.
The native species planted did not show the colonizing character of the
spontaneous vegetation. The establishment of native shrubs depends on
various factors in addition to fertile soil, such as the quality of
organic matter, salinity, space (Hess et al., 2019; Clemente et al.,
2004, Vallejo et al, 2012). That is why they have not achieved the
highest cover where there were more nutrients. As seen in the DCA (Fig
8), each of the three planted native species preferred different niches.Anthyllis sp. established better over topsoil, butAnthyllis terniflora was more tolerant to both higher salinity
and coexistence with spontaneous species. In contrast, Macrochloa
tenacissima preferred the marl substrate with less presence of
spontaneous species.
Spontaneous species were also influenced by soil restoration treatments
and by total vegetation cover. The number of pines was similar in most
of the treatments, except in compost, where there is a large difference
between the two types of substrate. High vegetation cover can limit the
germination of pines that need a lot of light in the early stages of
their development (Skordilis and Thanos, 1997; Calama et al., 2017).
This is consistent with our results, where the spontaneous species
covered almost twice the surface in the topsoil substrate (65%) than in
the marl (36.6%).
Despite it has been mentioned that spontaneous colonizers can prevent
native vegetation development due to competition for nutrients, space,
light (Holl, 2002) and delay successional advance (Moreno de las Heras
et al., 2008), the improvement showed by our amended substrates with
regards un-amended ones (Luna et al, 2016a, 2016b, 2017, 2018) indicates
that spontaneous vegetation is an essential starting point for a suite
of successional mechanisms favoring soil restoration.
The three native planted species are better adapted to local habitats
and this fact is key over all in a semi-arid area where conditions are
more extreme (Asensio et al., 2013b; Sheoran et al., 2010). However,
these three species require a site quality that is not normally achieved
in the short term. Many works have reported that vegetation cover is a
key parameter for soil stabilization and erosion control in these
disturbed areas (Franklin et al., 2012; Bochet et al., 2010). For this
reason, earlier colonists that are mostly herbaceous can aid in limiting
erosion, as well as favoring successional changes.
In summary, the use of woodchips from pine silvicultural treatments
could be used in soil restoration projects as a low-cost mulching
technique. Besides protecting against soil erosion and improving the
soil water content, they contain cones inducing the establishment of
pine seedlings. However, proper pine management will be necessary to
avoid the negative impacts reported for dense stands of this
Mediterranean pine species if biodiversity maintenance and sustainable
ecosystem services are sought (Bellot et al., 2004; Maestre and Cortina,
2004; Moreno-GutiƩrrez et al. 2012). An initial restoration with native
shrubs on organically amended substrates might be compatible with sparse
pines as they both will benefit from two organic matter sources (added
organic amendments and organic mulch), and their litter will contribute
to enhance soil fertility. The reorientation of woody trees, pines in
this case, towards species compositional states that are more similar to
native habitats, could be faster using adaptive forest management
practices (Onaindia et al. 2013).
The balance between these positive and negative plant interactions will
condition the rate and direction of its handling and can therefore be
used to influence the adequate development of a degraded system towards
the target community as stated by Siles et al. (2008).