Maritime pine forests at their Italian edge reveal surprising biogeographic value

Deep in the hills south of Siena, where the Farma stream cuts through siliceous bedrock and the summer air smells of resin and sun-baked heather, maritime pine forests rise in monospecific stands that botanists have argued about for half a century. Were these trees planted by nineteenth-century foresters intent on stabilizing eroding soils, or do they represent the natural southeastern outpost of a species whose mainland European range otherwise peters out in Liguria? A new study published in the journal Vegetation Classification and Survey offers the most rigorous answer yet, and it leans decisively toward the second possibility. By classifying hundreds of vegetation plots from across the Italian peninsula, researchers have shown that the maritime pine forests of central-southern Tuscany form a distinct plant community with a formal phytosociological identity, and that their understory harbors species of genuine biogeographic and conservation significance.

Maritime pine, Pinus pinaster, is a medium-sized conifer with a west-Mediterranean and Atlantic distribution stretching from southwestern Europe and the Iberian Peninsula through France and Italy to northwestern Africa. In mainland Italy it reaches its eastern native limit, occurring naturally in Liguria, Tuscany, Sicily, and Sardinia. The species is ecologically versatile, growing from sea level to 2000 meters on schists, serpentines, sandstones, granites, and volcanic soils, in climates ranging from humid to sub-humid. It often forms pure stands but also mixes with oaks and other pines. Across the Mediterranean Basin, thermophilous pine forests dominated by species such as Pinus halepensis, Pinus brutia, Pinus pinea, and Pinus pinaster typically colonize extreme sites: exposed, warm, dry rocky slopes, ultramafic bedrocks, marls, dolomites, and limestones. These forests correspond to the EUNIS habitat type T3A, Mediterranean lowland to submontane Pinus forest, and they have been studied syntaxonomically in Italy since the 1970s.

The research team, led by scientists working in the Colline Metallifere hilly-mountain complex southwest of Siena, focused on an area between 100 and 800 meters in elevation, within the catchment basin of the Farma stream, a tributary of the Merse river. The lithologies there are dominated by siliceous rocks of the Verrucano Group, silty schists with sandstone and quartzites that give rise to acidic soils, with limited outcrops of karstic limestone and ophiolites. The macroclimate is temperate with a strong submediterraneity index: mean temperatures range from 5 degrees Celsius in January to 23.1 degrees in July, and precipitation is seasonal, with autumn and winter months averaging 40 millimeters per month and summer months dropping to between 20 and 40 millimeters. Significant microclimatic differences exist across the area, with rainfall decreasing from west to east across the mountain chain, greater temperature variation on ridges, and thermal inversions in valley floors. The study sites fall within three Special Areas of Conservation of the Natura 2000 network and four Nature Reserves, in a well-preserved landscape with low human density and no industrial activity.

To capture the full floristic variation of these forests, the researchers sampled 50 original vegetation relevés of 10 by 10 meters during May and June 2022, placed subjectively across the major environmental gradients of the study area. They recorded the presence and percentage cover of every taxon. To place their data in a peninsular context, they retrieved 244 additional published and unpublished relevés from Liguria and Tuscany held in the CircumMed Forest Database. The combined dataset of 294 plots was then filtered to retain only relevés where maritime pine had a cover value of at least 15 percent and where its cover exceeded the combined cover of broadleaved tree species, a compromise threshold that excluded open vegetation with scattered pines while including true forests and open pine woodlands. The final dataset comprised 251 relevés, each transformed so that cover values of the same species in different layers were combined into a single value accounting for overlap.

The classification was performed using Two-way Indicator Species Analysis, or TWINSPAN, with five pseudospecies cut levels of species cover at 0, 2, 5, 10, and 20 percent. The first division of the analysis separated two main pine forest communities, suggesting a distinction based primarily on substrate. Cutting the classification at the second level yielded four clusters. To visualize patterns, the team ran a Principal Coordinate Analysis based on a square-root transformed Bray-Curtis dissimilarity matrix calculated on square-root transformed cover values. The clusters separated cleanly along the two main axes: the first distinguished communities along a substrate gradient, while the second represented a latitudinal gradient. Species showing a correlation coefficient greater than 0.5 with the first two axes were superimposed on the ordination diagram, and the geographic coordinates of each cluster were mapped to aid interpretation.

The results were floristically revealing. Cluster 1, concentrated in eastern Liguria, includes mainly acidophilous species such as Castanea sativa, Pteridium aquilinum, and Teucrium scorodonia. Cluster 2, distributed across central-southern Tuscany and marginally in western Liguria, encompasses evergreen acidophilous species including Arbutus unedo, Calluna vulgaris, and Erica scoparia. Cluster 3, confined to a small serpentine area in southern Tuscany, reports a mixture of heavy-metal tolerant species such as Plantago holosteum and Thymus striatus, with a particularly high share of species endemic to Italy. Cluster 4, located across eastern and western Liguria, shows a mixture of grassland and garrigue species on serpentine-derived soils, including Brachypodium phoenicoides, Euphorbia spinosa subsp. ligustica, Plantago maritima subsp. serpentina, and Thymus serpyllum. Diagnostic species were defined as those with a phi coefficient of at least 0.3, and constant species as those with a percentage frequency above 20 percent.

To characterize the ecology of each cluster, the researchers applied Ecological Indicator Values, ordinal scales that denote the position at which each plant species reaches peak abundance along environmental gradients of temperature, light, soil moisture, soil nitrogen status, and soil reaction. Differences in unweighted community medians among clusters were tested with a Kruskal-Wallis H omnibus test followed by post-hoc Dunn’s multiple comparison test with Holm correction. The analysis revealed significant differences among clusters in soil reaction, nitrogen, moisture, and light conditions, but not in temperature. Cluster 1 had the highest mean soil moisture and the lowest mean light availability, while Cluster 2 had the lowest mean soil reaction, confirming its acidophilous character. Cluster 3 showed the lowest soil moisture and nitrogen values and the highest light values, indicating a markedly xerothermic community, whereas Cluster 4 had the lowest temperature and the highest soil reaction. Life form spectra further differentiated the clusters, with Cluster 2 showing the highest percentage of phanerophytes and the lowest of hemicryptophytes.

Based on these analyses, the team classified the forests of central-southern Tuscany within the association Erico scopariae-Pinetum pinastri, belonging to the alliance Genisto pilosae-Pinion pinastri of the order Pinetalia halepensis in the class Pinetea halepensis. This community includes thermophilous and mesophilous species primarily distributed in the Atlantic and Western Mediterranean regions. The occurrence of taxa such as Erica arborea, Erica scoparia, Calluna vulgaris, Cistus salviifolius, Fraxinus ornus, and Genista pilosa confirms the alliance placement. The forests on acidic substrates elsewhere were assigned to the association Erico arboreae-Pinetum pinastri, while those on ultramafic substrates in Liguria were placed in the Euphorbio ligusticae-Pinetum pinastri, for which the authors propose conservation of the association name under the International Code of Phytosociological Nomenclature. Pine plantations on serpentine in southern Tuscany were treated as an informal group, in line with previous decisions not to describe formal syntaxa for clearly planted stands.

The biogeographic story emerging from the understory is perhaps the most striking finding. The central-southern Tuscan forests host a significant number of Stenomediterranean species alongside a Eurimediterranean group, but they also carry a substantial Euro-Siberian influence, with European, Eurasian, and Boreal taxa such as Calluna vulgaris, Danthonia decumbens, Molinia arundinacea, and Potentilla erecta. The occurrence of Calluna vulgaris holds particular phytogeographic interest, as it marks the southern boundary of its Italian distribution. The unique combination of a hyperoceanic climate and dystric cambisols and ferric podzols with highly acidic raw humus in this part of Tuscany supports Atlantic and Mediterranean-Atlantic species such as Erica scoparia, Genista pilosa, and Teucrium scorodonia. Recent phytogeographical and archival studies suggest these pine forests may be natural rather than planted, and genetic work indicates the Tuscan pines are most similar to those in nearby Corsica, and to a lesser extent to those in Liguria and southern France.

From a conservation perspective, the studied communities are not currently listed in Annex I of the European Habitats Directive, partly because extensive historical reforestation with maritime pine makes it difficult to distinguish native sites from artificially established ones. Yet old-established plantations of native pine species can develop an understory of natural species composition that renders them valuable for nature conservation. The authors suggest attributing these communities to the habitat type 9540, Mediterranean pine forests with endemic Mesogean pines, specifically the Franco-Italian Mesogean pine forests subtype, and to the EUNIS type T3A. They support extending the recognized natural distribution of maritime pine into central-southern Tuscany, arguing that the combination of favourable climatic and edaphic conditions allows the species to thrive there, and that its pioneering nature has driven expansion and recovery of natural conditions over recent decades in southern Europe. The study underscores the importance of conserving these forests, both because the dominant pine sits at the southeasternmost boundary of its peninsular distribution and because of the phytogeographically significant species sheltering beneath its canopy.

Subject of Research: Vegetation diversity of Pinus pinaster forests in the Italian Peninsula

Article Title: Vegetation diversity of Pinus pinaster forests in the Italian Peninsula

Article References: Ciaramella, D., Viti, M. M., Landi, M., Chytrý, K., Angiolini, C., & Bonari, G. (2024). Vegetation diversity of Pinus pinaster forests in the Italian Peninsula. Vegetation Classification and Survey, 5, 289-300. https://doi.org/10.3897/vcs.118023

Image Credits: AI Generated

DOI: 10.3897/VCS.118023

Keywords: Vegetation, diversity, Pinus, pinaster, forests, Italian, Peninsula, scientific research