3.2. Vegetation
Total plant cover (Table 2) positively and significantly increased from non-amended plots to compost-amended plots, being the plots with sewage-sludge in the middle. In all plots (treatments) the topsoil showed plant cover values significantly higher than the marl substrate. However, planted vegetation cover had a different trend, reaching a percentage much higher in the treatment of sewage sludge compared with compost and no amendment.
The cover of spontaneous species (table 2) shows values in the topsoil treatments (6.6%, 16.6% and 36.6%) that doubles those of the marl treatments (11%, 33.3% and 65%) and like the total cover, the spontaneous species show a significantly higher percentage with organic amendments, especially with compost.
In the variable number of pines (table 2) we can observe considerable differences depending on the treatment. There are no significant differences between marl substrate and topsoil on plots without organic amendment and sewage sludge. However, compost provoked an opposite behaviour according the initial substrate where applied: on marl substrate, the number of pines is the maximum in all treatments but on the topsoil, the lowest.
The pine cover is related to the number of pines, reaching the highest cover in the treatment of marl with compost amendment and the lowest cover in the topsoil-compost treatment.
In summary, the number of pines, as well as their coverage, depended on the interaction between the amendment and the substrate: a) only one amendment favoured the pines and b) the way the amendment works depended on the substrate.
However, pine height (Fig. 4) is quite similar with the two organic amendments and somewhat different on the two substrates, with larger values in the marl substrate. As expected, height values decrease from compost-amended plots to non-amended plots. Only the organic amendment factor had a significant influence on the height of pines (P <0.03), but there were not many differences in tree height within type of substrate, and only on non-amended plots, pines were slightly higher with topsoil than on the marl substrate.
In general, the slenderness index in the different plots and under the two substrates (Fig. 5) followed a somewhat similar pattern than pine height. The largest indices for both substrates were found in compost-amended plots. Except on the sewage-sludge treatment, the slenderness index values are significantly higher on topsoil than on marl substrate. The lowest indices were found on the marl substrate and non-amended plots.
The relationships between number of pines and TOC per substrate type (compound samples) (Fig. 6) indicate that on the marl substrate the regression is positive (R2 = 0.82; P = 0.06), though not significant. However, on the topsoil, the regression is negative and significant (R2 = 0.99; P < 0.02). Total cover showed somewhat positive and negative relationships with biodiversity (Shannon index) on the two substrates (Fig. 7). This relationship is negative and significant (R2 = 0.99,P < 0.03) over topsoil and it is positive and no significant (R2 = 0.41; P = 0.07) over the marl substrate.
The DCA results in Fig. 8 show that the first axis separates the plots over topsoil and those over marl. Likewise, the two different organic amendments and their control are explained by axis 2: at the lower part, plots without amendments, at the intermediate part, the plots with sewage sludge, and at the upper part, the plots with compost. Pine coverage (Ph) is more associated to the marl than to the topsoil where most of the spontaneous species are found. The spontaneous species are located in the upper left quadrant, associated with amendments and topsoil. The planted species (Mt , Ac , At ) do not follow the same pattern of preference for soil treatments.Anthyllis species are situated of the left side of ordination axis 1 and are associated with topsoil; however, Macrochloa tenacissima is associated with the marl substrate.