16 Aspects of Demography in Post-Fire Mediterranean Plant Communities of Greece M. ARIANOUTSOU 16.1 Introduction Fire is a major ecological factor in many biomes of the world (Rundel 1981). In the Mediterranean ecosystems fire acts as an integral part of their evolutionary history, having shaped their adaptive traits (Naveh 1975). It is beyond any doubt that the specific characteristics of fire regimes have a profound effect on these ecosystems (Mooney and Conrad 1977), the most important of which is related to community and population structure. These are critical issues for any conservation attempt. In this perspective, the biological features that are mostly influenced by fire are reproductive and demographic characteristics which assure a rapid reestablishment by resprouting andlor seed germination (Keeley and Zedler 1978; Arianoutsou and Margaris 1981a). The specific regeneration behaviour of the plants is closely related to their physiological traits and is strongly influenced by fire regime (fire season, intensity, interval). Resprouting occurs from dormant buds or lignotubers effectively pro- tected by the soil. It starts almost immediately after fire in the evergreen sclerophyllous species, while it takes several months, until the onset of the rainy season for the seasonal dimorphic (phryganic) species. This differ- ence has been attributed to the different depths of their root systems. The resprouting ability of the plants is determined by their developmental phase. Many shrubby taxa are successful resprouters, either obligate or facultative. In several Mediterranean ecosystems, fire has been shown to trigger a flush of germination (Purdie 1977; Trabaud 1980; Arianoutsou and Margaris 1981b). Post-fire seed germination is an adaptation of the species which form permanent soil seed banks of hard-coated seeds (Cistaceae, Leguminosae) and of those with a canopy seed bank (Proteaceae, Pinaceae). Seed germination has been attributed to several factors such as mechanical rupture of the hard seed coat induced by the increased tem- peratures developing during fire (Arianoutsou and Margaris 1981b; review Ecological Studies, Vol. 136 Rundel el al. (eds.) Landscape. De~radation- and Biodiversity in Mediterranean-Type Ecosystems O Springer-Verlag Berlin Heidelberg 1998 in Rundel 1981), inactivation of germination inhibitors present in the soil (Christensen and Muller 1975; Keeley and Keeley 1989), effect of charred woody stems (Keeley and Nitzberg 1984) and change in light quality (Roy and Arianoutsou 1985). The temporal destruction of either the whole or the aboveground part of the mature plant by a fire incident triggers the restart of the demographic cycle of the community through the relevant cycle of the plant taxapresent. Many studies have been done in California, Australia, and South Africa on plant community diversity and dynamics in relation to fire, and these are extensively reviewed by Kruger (1983). Not as much work, though, has been done in the Mediterranean Basin, apart from a few demographic studies (Naveh 1974; Papanastasis 1977; Arianoutsou and Margaris 1981a; Arianoutsou 1984; Trabaud and Oustric 1989; Ne'eman et al 1993). This chapter aims to review and synthesize information on the dynamics of . post-fire Mediterranean communities in Greece, as part of a broader study which is evolving under the framework of two research projects sponsored by the European Union - Modelling vegetation dynamics and degrada- tion in Mediterranean ecosystems (MODMED) and Management tech- niques for optimisation of suppression and minimisation of wildfire effects (PROMETHEUS). 16.2 Mediterranean Ecosystems of Greece Mediterranean ecosystems constitute 40% of the terrestrial vegetation of Greece. They can be grouped into three major types according to the prevailing vegetation, and they cover a rainfall regime of very low precipi- tation (close to 100 mm) to adequately higher (700-900 mm). Analytically, the three types are: 1. Communities with seasonal dimorphic dwarf plants, known as phrygana. This community is found mainly in the complex of Cyclades islands, in the islands of Crete, Limnos, Chios, Kefalonia, Leflcada, in West Akarnania and in Southeast Peloponnese. Their vertical structure is restricted to one single layer, that of the dominant shrubs, which are seldom taller than 50 cm. At the mature stage, phrygana is a relatively closed community, leaving some open space between the individuals. Among the most common woody plants are Coridothymus capitatus (thyme), Phlomis fruticosa (Jerusalem sage), Cistus spp. (rockroses), Sarcopoterium spinosum (thorny burnet) and Euphorbia acantho- thamnos (Greek spiny spurge). The majority of the woody species are Aspects of Demography in Post-Fire Mediterranean Plant Communities of Greece 275 seasonal dimorphic, that is they change their leaves and type of branches twice a year, in order to face water stress. Euphorbia acanthothamnos is a summer drought plant. They are all shallow rooted and they grow on calcareous, relatively poor soils. 2. Communities with evergreen sclerophyllous shrubby plants, otherwise referred to as maquis. These cover part of the Halkidiki peninsula in Macedonia, part of continental central Greece and parts of the islands in the Ionian Sea. Deep-rooted plants taller than lm, such as Quercus coccifera (kermes oak), are the predominant species in these communi- ties. Other common taxa are Pistacia lentiscus (lentisc), Arbutus unedo (strawberry tree), Myrtus communis (myrtle), Juniperus phoenicea (phoenician juniper) and Juniperus oxycedrus (prickly juniper). Their leaves are thick, larger than those of the phryganic species and remain the same throughout the year, having a life span of two years. They form very dense stands of one layer, leaving almost no space between indi- viduals at the mature stage. They grow on more fertile soils. 3. Forest communities with pine trees. Pinus halepensis (Aleppo pine) forests are found in Attica, in Euboia, in western Peloponnese and in Halkidiki, while Pinus brutia (brutia pine) forests grow on the islands of the north and southeast Aegean, in Crete and its islets, (see Fig. 16.la). The trees can be up to 15m tall, and generally have a rather poor understory stratum at the middle of their life span (which can be as long as 100 years). However, species diversity is quite high immediately after fire and rich also at the mature stage. The understorey stratum consists of either phryganic species or species of the evergreen sclerophyllous formations. Phrygana exploit the areas with low precipitation and higher summer temperatures, while evergreen sclerophyllous ecosystems occur in more humid and less warm environmental regimes. Mediterranean coniferous forest can be found in situations closer to those prevailing in the latter case (Fig. 16.lb). 16.3 Fire Frequency Fire incidents in the Mediterranean Basin are very common and a high frequency of fires has extended back into the past. Evidence of fire can be identified in the Iron Age, 2600 years B.P., when shepherds and farmers set fires in order to open the forest and the ground for improving pasture and cultivated land (Barry 1960; Dugrand 1964). 6 Fig. 16.la,b. Vegetation map of Greece. Brown areas correspond to phrygana, yellow to Aleppo pine forests and light green to maqnis (Modified c from Mavromatis 1978) b. Precipitation regimes in Greece (Modified from Mariolopoulos and Karapiperis 1955) Aspects of Demography in Post-Fire Mediterranean Plant Communities of Greece 277 In Greece, the first archaeological evidence of fire occurrence in the xerophytic landscapes goes back to the end of the Middle Stone Age: rem- nants of ashed wood in the Kastritsa cave, near Ioannina Lake (Higgs et al. 1967). Liakos (1973) has reviewed the early use of fire and wildfire in ancient Greece. Most fire events occur in evergreen sclerophyllous ecosystems (Fig. 16.2), where they consume approximately 13024ha year-' (Table 16.1), which corresponds to nearly 50% of the total area burned. Following in size are fire events occurring in phrygana and P. halepensis forests (Table 16.1; Fig. 16.2 and 16.3). Most of the fires are caused accidentally or by arson. While there are extensive and detailed studies on fire frequency in Cali- fornia, South Africa and Australia, there are but a few data for the Mediter- ranean Basin. Le Houerou (1973) states that the so-called red belt of southern France is burned regularly every 25-30 years. Forest Service records in Greece are scarce and sometimes not very reliable. Moreover, these data do not reveal any clear pattern of fire frequency, presumably due to multiple reasons encountered as possible fire causes. 16.4 Plant Adaptive Traits Table 16.2 summarizes the adaptive traits that taxa dominant in the Mediterranean ecosystems of Greece have evolved in order to cope with fire. Most of the phanerophytes are resprouters and generally obligate Table 16.1. Vegetation types and five data. (Period of reference 1965-1990; data adapted from Kailidis 1992) Vegetation type Total area Total number Mean area burned (ha) of fires burnedlyear (ha) Abies spp. forest Mixed coniferous forests Pinus nigra forests Pinus halepensis forests Pinus brutia forests Other coniferous forests Forests with deciduous Quercus spp. Forests with other deciduous species Evergreen sclerophyllous shrublands Phrygana M. Arianoutsou J 1 1965 - 1990 TOTAL NUMBER OF FIRES Pinus halepensis forests ther coniferous forests ecidu0~8Quercus forests Other deciduous forests Evergreenu sclerophyllous shrublands