What is a tree in the Mediterranean Basin hotspot? A critical analysis

Abstract

Background: Tree species represent 20% of the vascular plant species worldwide and they play a crucial role in the global functioning of the biosphere. The Mediterranean Basin is one of the 36 world biodiversity hotspots, and it is estimated that forests covered 82% of the landscape before the first human impacts, thousands of years ago. However, the spatial distribution of the Mediterranean biodiversity is still imperfectly known, and a focus on tree species constitutes a key issue for understanding forest functioning and develop conservation strategies. Methods: We provide the first comprehensive checklist of all native tree taxa (species and subspecies) present in the Mediterranean-European region (from Portugal to Cyprus). We identified some cases of woody species difficult to categorize as trees that we further called "cryptic trees". We collected the occurrences of tree taxa by "administrative regions", i.e. country or large island, and by biogeographical provinces. We studied the species-area relationship, and evaluated the conservation issues for threatened taxa following IUCN criteria. Results: We identified 245 tree taxa that included 210 species and 35 subspecies, belonging to 33 families and 64 genera. It included 46 endemic tree taxa (30 species and 16 subspecies), mainly distributed within a single biogeographical unit. The countries with the highest tree richness are Greece (146 taxa), Italy (133), Albania (122), Spain (155), Macedonia (116), and Croatia (110). The species-area relationship clearly discriminated the richest central-eastern (Balkans) and northern (Alpine and Cevenno-Pyrenean) biogeographical provinces, against the five western provinces in the Iberian Peninsula. We identified 44 unrecognized "cryptic trees", representing 21% of the total trees. Among the 245 taxa identified, 19 are considered to be threatened (15 CR + EN + VU) or near threatened (4 NT) by IUCN. Conclusions: The Mediterranean-European region includes an unsuspectedly high number of tree taxa, almost 200 tree taxa more than in the central European region. This tree diversity is not distributed evenly and culminates in the central-eastern part of the Mediterranean region, whereas some large Tyrrhenian islands shelter several narrow endemic tree taxa. Few taxa are recognized as threatened in the IUCN Red list, and the vulnerability of these species is probably underestimated.

A Random Forest Modelling Procedure for a Multi-Sensor Assessment of Tree Species Diversity

“…Yet, direct and indirect factors, such as habitat loss, landscape degradation, fire, soil erosion, anthropogenic activities, demographics, and climate change threaten and lead to a decline in the forest biodiversity levels [6]. In particular, Mediterranean forest habitats, characterized as one of the richest biodiversity hotspots worldwide are among the most threatened on Earth [7,8], identified as priority area for conservation [9]. In such areas, biodiversity monitoring systems are needed for identifying the location, the magnitude, and the rate of changes in order to prevent or mitigate losses and enhance the resilience capacity of forest areas through appropriate management plans and conservation strategies.…”

A comprehensive, genus-level time-calibrated phylogeny of the tree flora of Mediterranean Europe and an assessment of its vulnerability

“…The 64 study genera (Appendix S1) are reported in the most recent checklist of tree taxa native to Mediterranean Europe (Médail et al 2019). This checklist reports 210 tree species in the north of the Mediterranean area spanning the zone comprised between Portugal and Greece.…”

“…This checklist reports 210 tree species in the north of the Mediterranean area spanning the zone comprised between Portugal and Greece. Médail et al (2019) define trees as plants with four main features: "(i) secondary growth; (ii) perennials (they live many years, mostly for decades or centuries); (iii) typically (at least in some environmental conditions or locations) having a single stem or trunk, growing orthotropic with monopodial or more often sympodial branching systems, and bearing lateral branches at some distance from the ground; and (iv) the height of the mature individual is at least three meters". Four of these genera (Chamaerops, Phillyrea, Spartium and Tetraclinis) are strict Mediterranean Basin endemics, while three others (Myrtus, Nerium and Ceratonia) are Tethys endemics that are also found in the Irano-Turanian and Saharan biogeographic regions (Zohary and Feinbrun-Dothan 1966).…”

“…We determined the vulnerability to extinction for every woody plant species included in the checklist of Médail et al (2019), assigning it to one of four following IUCN Red List categories: Endangered (EN), Vulnerable (VU), Near Threatened (NT) and Data Deficient (DD) (Appendix S6). This was done by consulting the IUCN Red List of Threatened Species (https://www.iucnredlist.org/) in February 2019.…”

“…The tree flora of the Mediterranean Basin is particularly remarkable and far more species-rich than that of temperate Europe (Quézel and Médail 2003;Montoya et al 2007). Mediterranean Europe shelters no less than 45 endemic tree species (Médail et al 2019), for which scarcely any phylogenetic information is available. Trees are likely to be excellent witnesses of deep history events and the tree flora is known to be a good proxy of biodiversity in general and in the Mediterranean region (Kati et al 2004;Thompson 2005;Petit and Hampe 2006;Conord et al 2012).…”