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Top1. Introduction
The Mediterranean basin region is a global hotspot of biodiversity and presents a wide diversity of habitats and ecosystems. Biodiversity is a widely used term in ecology and natural resource management, and it is a key item in nature conservation (Do et al., 2015; Feroz et al., 2016).
Greece is distinguished for its rich flora. The country is geographically located in SE Europe and occupies the southern tip of the Balkan Peninsula (the farthest east of the three peninsulas of Europe), which, unlike the other two, doesn’t have the very high mountains separating it from the rest of Europe. It is an inland country with a large number of islands and is close to Asia and Africa. Also, it contains a variety of climatic conditions (29 climatic classes according to the Thornwaite classification scheme), a fact attributable to its geographic location, terrain ruggedness and the presence of the sea. The vascular plants of Greece include 5758 species and 1970 subspecies, representing 6620 taxa, belonging to 1073 genera and 185 families. Many of these plant species are endemic, as a result to the location and the particular climatic, geological and topographical conditions (Dimopoulos et al., 2013).
Land abandonment constitutes a characteristic element of people's dynamic relationship with natural ecosystems. In the Mediterranean region, which has undergone anthropogenic intervention for thousands of years (Kosmas et al., 2000), there has been dramatic abandonment of traditional land uses, agriculture and livestock farming over the past decades. Land abandonment may be due to natural causes or socio-economic factors (Lambin & Meyfroidt, 2010). Due to the dramatic decline in land productivity over the last decades, areas that have been introduced into cultivation in the past century are increasingly being abandoned (Karakosta, 2012; Kosmas et al., 2000).
According to Lambin & Meyfroidt (2010), socio-economic causes seem to be the most important cause of land abandonment. Naveh & Kutiel (1989), Lepart & Debussche (1992) and Cramer et al. (2007) refer that the environmental and socio-economic changes of recent decades have led to dramatic abandonment of the countryside resulting in the abandonment of cultivated land throughout the world. The abandonment of land is due to socio-economic factors that force the rural population to move from the countryside to large urban centers (Macdonald et al., 2000; Bonet & Pausas, 2004; Papanastasis, 2008; Karakosta, 2012).
MacDonald et al. (2000) report that the abandonment of agricultural land is due to a lack of profit in agricultural activity, the inability to adapt to new farming practices due to the small agricultural pieces of land and the natural aging of the active agricultural population. Several studies (Lavorel et al., 1998; Lavorel et al., 1999; McIntyre et al., 1999) point out that the estimation of the effects of external factors on vegetation, such as abandonment, can be investigated with the assistance of plant functional groups.
Woody plant species are among the most important components of terrestrial ecosystems (e.g. maquis and abandoned olive groves) and affect the overall composition of their communities and environment (Paganová & Jureková, 2012; Solomou & Sfougaris, 2015). Especially, they offer food such as leaves, flowers, pollen, nectar, seeds, and fruit which are important for the wildlife diets (Solomou & Sfougaris, 2015). In addition, woody plant species diversity is fundamental to the overall natural ecosystems biodiversity, because woody plants provide habitats for almost all other species (Feroz et al., 2016). Also, structural diversity measured as variation across a vertical stand profile appears to be a good ecological indicator of the conservation of woody species diversity (Feroz et al., 2016; Neumann & Starlinger, 2001).