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.