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).