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

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.

Transition along gradient from warm to mesic temperate forests evaluated by GAMM

Aims The aim of the study was to discover what set of variables best explains the transition from warm to mesic forest vegetation.Based on various variables grouped into sets(geomorphological,ecological,structural,soil characteristics and chorological),six models were built and tested by generalized additive mixed models(GAMMs).We assumed that each set of variables has different explanatory power.Our aim was to compare the six different models(sets of variables),to test which model best explains the species turnover in forest communities along the transition between warm and mesic temperate forests and to try to find reasons for the different explanatory power of the models.Methods The research took place in the southern part of the Balkan Peninsula.Field sampling was done according to standard methods.The gradient from warm to mesic forests was defined as the turnover of species and evaluated by projection of samples on the first unconstrained DCA axis.Geomorphological,ecological,structural and soil characteristics,together with chorological sets of variables,were regressed on the turnover of species composition.Based on the five sets of variables,six models were constructed and tested by generalized additive mixed models.Important Findings Ecological conditions best explain the change of forest communities along the gradient;evolution and the development of vegetation reflected in chorotypes are also of high importance;geomorphology and structure seem not to change so dramatically and soil shows the least significant differences of all.Ecological variables are the most important set of variables in the transition between warm and mesic temperate forests but eco-evolutionary dynamics after the Pleistocene should also be taken into consideration.