The Tree

JULI G. PAUSAS, JOÃO S. PEREIRA, AND JAMES ARONSON

To understand and appreciate it properly, we should first recognize that cork oak is in many ways a typical Mediterranean tree. It can survive adverse conditions of both human and nonhuman origin. It resists cutting, grazing, prolonged drought, and fire but not extreme cold. On suitable, deep soils and with adequate rainfall, the tree may reach up to 20 meters tall and live for several centuries. However, it has one feature that is extremely rare throughout the plant kingdom: an outer coat of insulation consisting of corky bark of continuous layers of suberized cells (see Chapter 5), up to 20 centimeters thick, that may have evolved as an adaptation to fire (see Color Plates la, 1b). What is more, the tree survives and grows new bark when the original bark on its trunk has been removed.

Like other evergreen Mediterranean oaks, cork oaks survive drought, thanks in part to their extensive and deep root systems. During a drought, the tree may protect crucial organs and tissues from dehydration by closing stomata on leaves, restricting water loss, and the tree's deep roots may tap water from the deeper soil or subsoil (Pereira et al. 2006). The deep root system of cork oak helps the tree maintain water status and xylem conductance above lethal levels throughout the summer drought period (see Chapter 6). In some cases, under severe drought, the tree may shed its leaves and resprout when the drought is over (in spring). During the early stages of plant life, there is a clear priority for root growth (Maroco et al. 2002). This early investment in roots, rather than in stems and foliage, may contribute to survival in the first years in drought-prone environments because seedling survival cannot be guaranteed before roots reach a soil depth that holds available water in summer. Symbiosis with mycorrhizae that live in or on the roots is also an important aid to cork oak seedlings in resisting drought, as described in Chapter 7.

Once aboveground parts begin to develop, cork oak has a unique leafing phrenology. For an evergreen tree, it has short-lived foliage and a late flushing pattern (Pereira et al. 1987; Escudero et al. 1992). In fact, the average leaf life expectancy is only about 1 year, much shorter than in other evergreen oaks, such as Iberian holm oak (Quercus rotundifolia = Q. ilex subsp. ballota), whose leaves last 1-3 years, or the kermes oak (Q. coccifera), whose leaves can last 5-6 years. Leaf phenology is under strong genetic control, and the beginning of shoot flushing of populations belonging to different provenances but cultivated together can vary by as much as 4 weeks, from late March to late April.

Cork oak leaves themselves are also well designed to cope with an unpredictable climate. They are sclerophyllous, which means they are stiff, thick, and waxy (see Color Plate 1d). This is typical of many trees and shrubs that grow in regions with strong seasonal water deficits, such as the Mediterranean. They are also small, which allows efficient heat dissipation, thus partly avoiding overheating in the hot summer. In cork oak, as in many kinds of trees whose roots tap water from deep in the soil, supplementary cooling is often achieved through transpiration, as stomata open for some time on long summer days. Harmful leaf tissue dehydration is prevented by the highly efficient gradual closure of stomata (see Chapter 6, where other adaptations to drought are discussed in more detail).

Sclerophylly is often considered an adaptive trait of woody plants in seasonally dry climates, but it does not automatically confer greater tolerance to drought, and it may have evolved because it provides protection from many different types of stress (Read and Stokes 2006), such as poor mineral nutrition or attacks by defoliators (Salleo and Nardini 2000). In fact, sclerophylly implies a long leaf development time and fairly high ratio of carbon to nitrogen, both of which traits make the foliage undesirable to herbivores. Thus, most defoliators attacking cork oak feed on the young, tender leaves, before sclerophylly fully develops. This, in turn, conditions the nature of the web of organisms dependent on cork oak leaves.

Biogeography

Cork oak occurs in regions with average annual precipitation above 600 millimeters and average temperature near 15°C (Blanco et al. 1997). In Europe, it is low winter temperatures that appear to set the geographic distribution limits and most cork oak stands are located in areas below 800 meters in altitude. Cork oak leaves are less tolerant to frost (Larcher 2000; Garcia-Mozo et al. 2001) and to drought than those of the more widespread holm oak. In addition, whereas holm oak is indifferent to soil types, cork oak usually grows in acidic soils on granite, schist, or sandy substrates or, more rarely, in limestone-derived soils or in neutral soils overlying dolomitic bedrocks (Chapter 8).

Today cork oak occurs only in the western Mediterranean (Figure 1.1), from Morocco and the Iberian Peninsula to the western rim of the Italian peninsula. It also flourishes on all the large islands between the Iberian and Italian peninsulas, and in scattered parts of southern France and some coastal plains and hilly regions of Morocco, Algeria, and Tunisia. Total...