CHAPTER 25 Mediterranean Environments

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1 CHAPTER 25 Mediterranean Environments Introduction The regions of the world with a Mediterranean-type climate have very distinctive physical environments. The climate is transitional between tropical arid climates, on the one hand, and humid temperate climates on the other. The stresses imposed by the heat and aridity of the summer months have striking influences on Mediterranean ecosystems vegetation, soils and wildlife. Physical and ecological processes are active in the winter months, when winter precipitation provides the moisture necessary for: weathering; erosion of soils and hill slopes; river action; processes of humification, leaching, clay translocation and clay mineral formation in soils; and the net primary productivity of vegetation. By contrast, the summer is a season of desiccation, drought and relative inactivity in landscape processes. Human activities have had a major impact on regions with Mediterranean environments. The Mediterranean region proper has a long history of human modification of the natural landscape, stretching back into prehistoric times. Impacts from agriculture and nonagricultural industries have continued to increase, so that at the present time they have never been more intense. Although the Mediterranean areas of California, Chile, South Africa and Australia have long-standing impacts from indigenous peoples, modern impacts on a large scale have occurred for only a century and a half. However, in all these regions there are problems of land degradation, fire destruction, desertification and problems of water quantity and quality. Chapter summary Climate: present and past Summer months are dominated by high-pressure cells, whereas in winter depressions bring changeable weather and precipitation. Winter depressions in the Mediterranean region proper come from the south, the west and the north, with their tracks being considerably affected by relief. Rainfall varies from 300 mm to 750 mm, with bimodal distribution in countries along the northern Mediterranean shore and a single maximum in the eastern and southern Mediterranean. Rainfall totals are very variable from season to season, as is shown by values of the coefficient of variation of the order of 30 per cent. Intense storms in autumn are extremely erosive, as they fall on dry, dusty soil surfaces. Soil formation and distribution Hydrolysis of rock-forming minerals in the winter season makes clay minerals and sesquioxides.

2 Dehydration in the summer season leads to some formation of anhydrous iron oxide (haematite) which gives the distinctive red colour to terra rossa soils. Brown soils (terra fusca) have higher organic contents and are moister. During the first rain of autumn clay-sized particles in the A horizon are leached into the Bt horizon. Organic matter is readily oxidized during the hot summer, leading to low contents of nitrogen and phosphorus. Development and adaptation of vegetation During the last glacial maximum the Mediterranean climate was cooler and dryer than at present, with trees on the uplands and dry steppe on the lowlands. The sequence of development during postglacial times has varied from area to area, but the general overall trend has been to produce a climax of Mediterranean evergreen oak forest. The development of vegetation has been much affected by human activities of different cultural ages, the net result being a majority of scrub (maquis) and heath (garrigue) communities. Mediterranean plants, from trees to the smallest herbs, show a variety of ecophysiological and structural adaptations to severe water stress during the hot summer. There are many legumes in the Mediterranean flora, which greatly helps growth in the nitrogen-deficient soils. Distribution of the plant communities Exposure and rain shadow effects lead to many regional differences in rainfall, and hence vegetation, though the two general trends are for aridity to increase from west to east and from north to south. Vegetation zones change rapidly with altitude as precipitation increases and potential evapotranspiration decreases. With increasing altitude the climax vegetation changes from evergreen oak to deciduous forest to a coniferous forest of pine and fir. Aspect is an important control of Mediterranean vegetation, with south-facing slopes having higher temperatures and higher radiation levels, and north-facing slopes having higher soil-moisture levels. The lime (calcium carbonate) content of soils has an important influence on the distribution of plants at the species level. Fire in the landscape Fire is an important ecological factor in Mediterranean ecosystems, being a common occurrence in the landscape.

3 Many plants are adapted to fire, whether by growth adaptations or by rapid regeneration after fire by shoots or seeds. It is difficult to get reliable information on the frequency and causes of fires, as reporting varies in reliability from country to country and with time. Fires are caused by natural means, by deliberate land management, by accident or by arson. Afforestation of Mediterranean lands has greatly increased the fire risk, owing to the widespread planting of pine and eucalyptus. Desertification and soil erosion Desertification refers to the total degradation of land ecosystems, with serious impacts on all aspects of society. Once the vegetation cover falls below 50 per cent, run-off, sediment yield, surface splash and surface sealing all increase dramatically, with a consequent rapid rise in erosion rates. The role of plants in lessening erosion rates is a result of root systems, higher organic matter contents and the improved structure and infiltration beneath plants. Badlands show dramatic effects of gullying, piping and linear erosion slopes, and are frequently found on marls. Surface crusting results from the destruction of soil aggregates by intense rain, and the repacking of the mineral particles and chemicals into a surface skin. Water supply problems Mass tourism, urbanization and agricultural irrigation are making enormous demands on the water supplies of the region. Groundwater supplies from underlying rocks depend upon the specific yield and permeability of the rock. Sedimentary rocks are far more important as aquifers than are igneous and metamorphic rocks, though weathered and fractured igneous and metamorphic rocks can be good aquifers. Along coasts, fresh water is normally in equilibrium with saline ground water, but overpumping of the aquifer causes salt water to penetrate inland. Many Mediterranean countries are reporting problems of sea-water intrusion into coastal aquifers, a problem which either requires expensive reclamation measures or is irreversible. CASE STUDY Wildfires in California during August, September and October 2007 We have seen that two key factors in the spatial structure and ecology of Mediterranean vegetation are the long summer drought and fire. Fires frequently make the headlines

4 when lives and property are threatened, but fire is more common than popularly supposed, and in general vegetation burns every years in the Mediterranean biome, In fact the vegetation structure and the volatile oils in the plants promote burning, and ecologists recognise that both shrublands (maquis, chaparral) and heaths (garrigue) are fire-dependent systems Several species are unable to reproduce without fire, and species diversity and productivity decline after long periods of time without fire. In addition to the flammability of the plants and plant litter, there are factors of climate and weather which promote annual fires. High temperatures and low humidity in summer are clearly important, because fuel flammability is increased by long periods of hot, dry weather. In addition, dry winds provide an important mechanism to increase the fire s intensity and rate of spread. Winds not only drive the fire forward, but bring the flames down into contact with flammable fuel on the ground. For this reason, also, fires spread more quickly and burn with greater intensity when moving upslope. The risk of fire is enormously increased by the incidence of hot, dry winds, and fire-alerts are triggered when such winds are forecast. The sirocco is a hot, dry and dusty South or South-easterly wind blowing from the Sahara in the Mediterranean basin. The Santa Ana is a hot, desiccating wind which blows south-westwards from the Sierra Nevada onto the coastal plain of southern California. It is a föhn-type wind, heating and drying adiabatically as it descends. Forecasts of Santa Ana winds during spring and summer trigger fire-alerts in the State. Human impact on the biome in Europe, Australia and California has radically changed the fire-regime. Urbanisation has brought a greater risk of fire from non-natural causes, both accidental and deliberate. Also, the use of flammable species such as Aleppo pine and eucalyptus in forestry schemes is widespread. The hazard of fire is also greatly increased by housing and commercial development. Towns and rural communities in Mediterranean regions rightly demand protection from fire, but it is now recognised that the policy of preventing and controlling fires has itself changed the nature of the fires. In California, for example, the size of individual fires in the chaparral has greatly increased due to fire-control measures. Before fire-control, natural fires were frequent but small, with even the largest averaging less than 400 hectares in area. This resulted in a patchwork of areas of mature chaparral with large amounts of fuel, scattered amidst areas of more recently burned chaparral with lower fuel loadings, which acted as natural firebreaks controlling the spread of fire. By excluding fires, authorities have produced a landscape with much larger areas of old chaparral with huge amounts of fuel. It is scarcely surprising that large chaparral fires over hectares in extent are now much more common. Warnings about the increased fire-risk and its consequences are dramatically illustrated by the disastrous conflagrations which affected the chaparral-dominated parts of southern California in August, September and October Statistics on wildfires are notoriously imprecise. There are different agencies to whom reports are made, depending partly on land ownership. Preliminary data for 2007 from the California Department of Forestry and Fire Protection (CDF or CAL FIRE) are

5 shown in Table Compared to 2006, the number of fires decreased by 32% but the area burned increased by 36%, with mean fire-size increasing by 100%. Table 1 Number and size of fires reported by CAL FIRE, 2006 and Year Number of fires Total acres burned Total hectares burned Mean fire size, hectares , , , , , , year mean 9, ,264 88, Additional statistics on wildfires in southern California in 2007 are given in Table The large category of unidentified and indeterminate causes will decline as more reports come in. Among human causes are campfires, barbeques, smoking, and equipment malfunction. The small number of fatalities is witness to the efficiency of alarm and warning systems in place. The largest fire was the Witch fire in San Diego county which started on 21 st October 2007 and eventually burned hectares, destroying 1650 homes and 499 outbuildings. The fire was started by human action, but fortunately there were no fatalities. Table 2 Causes and damage reported by CAL FIRE in Causes Lightning 16% Human actions 25% Arson 5% Unidentified/indeterminate 53% Damage to property Homes damaged 574 Homes destroyed 3043 Fatalities Fire department 1 Civilian 1 Population forced to move from their homes >1 million Because fire suppression leads to a dangerous build-up of fuel loads, government authorities in the United States have a programme of prescribed burns under controlled conditions, in order to imitate natural fire regimes and decrease fuel loads. This has been relatively successful, except when these prescribed fires have got out of control with disastrous results. This happened when severe damage was caused to the town of Los Alamos, New Mexico, in 2000.

6 Figure 1 Map of the distribution of fires in Southern California. The map shows the distribution of wildfires in the Mediterranean biome of Southern California which had been contained by the beginning of November Autumn is a common time for wildfires in this biome, as the effects of summer heat and the build-up of flammable organic matter lead to very high fire-risk. Fires can then be easily started by autumn lightning storms or human carelessness. Source: Credit: California Department of Forestry and Fire Protection (CALFIRE) Essay questions 1. Discuss the positive and negative effects of fire in Mediterranean regions. 2. How do Mediterranean plants survive the summer drought? 3. In what ways do Mediterranean processes of soil formation differ from those of cool temperate regions? Discussion topics 1. Discuss the erosional processes operating in badlands. How would you measure the rates of these erosional processes? Discuss why there have been different estimates of erosion rates operating in badlands.

7 2. How will future environmental change influence the processes of desertification seen today in many Mediterranean environments? What conservation measures are likely to be successful? 3. How can Mediterranean countries solve their problems of water quantity and quality? Web resources This 8-year interdisciplinary study of Mediterranean Desertification and Land Use (MEDALUS) was funded by the European Commission (EC). It represents one of the most important recent scientific studies of the Mediterranean environment in Europe, and the pressures to which it is being subjected. Many countries in Mediterranean Europe have active government programmes of research into, and where necessary, rehabilitation programmes for, their Mediterranean ecosystems. The Consejo Superior de Investigaciones Cientificas (CSIC) in Spain is one of the most famous. Follow the web site for two important scientific stations: Centro de Investigaciones sobre Desertificatión (CIDE), Valencia, and Estación Experimental de Zonas Aridas (EEZA), Almería. The website of the California Department of Forestry and Fire Protection.