Vegetation dynamics and restoration trials in limestone quarries: the botticino case study (Brescia, Italy)

Background. All over the world, the naturalistic restoration of abandoned quarry areas represents a real challenge because of the very adverse initial site conditions for plant species colonization. In order to identify the best restoration practices, the present thesis considered, as a case study, the “Botticino extractive basin” (Lombardy, Italy), that is today the second greatest Italian extractive basin and it is famous worldwide for the limestone extraction. In particular, the thesis proposes a multidisciplinary approach based on the study of the local vegetation dynamics, laboratory tests, plant selection for restoration and field experiments to test different restoration techniques. Methods. Spontaneous vegetation dynamics over the whole extractive basin was studied by an ecological approach through 108 plots, that were carried out on surfaces whose “disused time” from quarry abandonment was known; data were analysed by cluster analysis and Canonical Correspondence Analysis (CCA) and compared to the available data on grassland and woodlands related to the study area. We identified successional phases according to the trend of the most common species whose cover significantly increases or decreases with time. To assess the influence of geomorphological heterogeneity on vegetation succession, we studied morphology and ecology of plant species growing on the following three main geomorphological quarry surfaces: artificial cliffs, dump deposits and quarry platforms; data were subjected to statistical analysis (contingency tables) and CCA. Since any restoration project should ensure the soil stabilisation of quarry dump deposits, the contribution of herbaceous root systems to limit superficial movements was studied by calculating the root cohesion of three herbaceous species with different types of root systems (Anthyllis vulneraria, Bromus erectus and Stachys recta). To this aim, we made 421 tensile strength tests by the Stable Micro System TA Hd Plus apparatus and we collected root traits by means of image analysis and the software Winrhizo. In the view of field experiments regarding quarry restoration projects, we carried out some preliminary tests and analysis, such as: a) germination tests on some common species characterizing the local vegetation succession; b) planning the structure and the species composition of the tree layer by using natural woodlands as reference model; c) collection and characterization of the hayseed coming from a “donor grassland” close to the Botticino extractive basin. We performed field experiments in an apposite site of about 600 m2 (consisting in three terraces almost horizontal) selected on a fully exploited quarry inside the Botticino extractive basin. Topsoil showed a clayey texture and some physical (e.g. limited soil depth, high stoniness) and chemical (very alkaline pH, low availability of nutrients) limitations. We tested three different restoration methods: 1) terrace A: hydroseeding of hayseed and plantation of shrub and trees; 2) terrace B: traditional hydroseeding of a commercial seeds mixture and plantation of shrub and trees; 3) terrace C: only plantation of shrub and trees without herb layer. In the tree layer of the three terraces, 98 individuals per terrace were planted (main species: Quercus pubescens, Fraxinus ornus, Cotinus coggygria, Ostrya carpinifolia). One year after the experiment, we collected vegetation data by means of 3 x 3 m plots in which we measured some species traits, e.g. species cover, mean plant height of the herbaceous layers, number of dead individuals; in addition we collected further species traits for each species in four subplot (20 x 20 cm): e.g. number of individuals, cover, maximum plant height. Data were subjected to CCA and compared to reference sites, i.e. the “donor grassland” and a quarry area spontaneously revegetated. Main results. Sinchronic analysis of vegetation allowed to identify 10 plant communities and to assigned them to 5 successional phases: a) pioneer phase (0-2 years from abandonment), b) early phase (3-10 years), c) intermediate phase (11-22 years), d) later phase (23-44 years) and e) advanced phase (>44 years). B. erectus showed the highest value of root cohesion thanks to the high volume of the root system and to the high root tensile strength. Moreover, the selection of herbaceous species should also take into account that root tensile strength strongly decreased with root diameter according to a potential curve and that, as for root volume, it is species-specific. As regards restoration trials, terrace C showed the lowest vegetation cover (15%), plant height and biomass production (16.33 g/m2). Terrace B showed the highest herbaceous plant height (100 cm) and biomass production (355.23 g/m2), although determined only by 6 species (with a clear dominance of Lolium perenne). On the other hand, terrace A showed similar plant height (93.3 cm), but lower biomass production (190.19 g/m2) and a much higher number of species (16), anyway lower than those on the donor grassland (28). As concerns the number of dead tree and shrub in the experimental site, for each single species we recorded the greatest number of dead plants on the terrace B (74.49%), while 18.37% of plants died on terrace A, and only 4.08% on terrace C. Discussion and Conclusion. The succession in the quarry area was partially similar to a primary one and was affected by environmental factors (e.g. stoniness, slope) immediately after the pioneer phase, so that different types of vegetation community grew on different morphological surfaces at the same time. In particular, dump deposits resulted similar to platforms for ecological features and dominant species traits, while artificial cliffs differed significantly, being mostly affected by rockiness and slope. Considering the time needed for the spontaneous vegetation succession and the cost of restoration interventions, human efforts are recommended on dump deposits and platforms. Results also highlighted that the use of species belonging to the Poaceae family (or/and with a fasciculated root system) are recommended for their attitude to stabilize superficial soil layers.The planting of shrub and tree followed by the sowing of hayseed could be a suitable method to ensure a successful restoration. In this way the death of shrub and trees due to the plant competition with the herb layer is avoided and suitable biodiversity levels are ensured. The present thesis provides a procedure for the restoration of calcareous quarries that can be applied on large scale, directing human efforts to reduce the economic costs for restoration.