Written by: Devin Hollister
Quercus suber, or cork oak, is a tree species native to the Mediterranean basin possessing both economic and ecological importance and whose evolutionary adaptations have led to its success as a species and its cultivation by man. Today, the projected warming of the Mediterranean and the uncertain future of the cork industry on which Quercus suber is dependent for survival in much of its range, threaten the continued success of the tree.
Whether or not Quercus suber remains a prominent species of the Mediterranean flora is contingent upon its ability to adapt to rising global temperatures and the human management of cork forests. Due to the positive effects they have on the environment, are important targets for future research and conservation efforts.
The Evolution and Climatic Adaptions of Quercus suber
The evolutionary history of Quercus suber has been the focus of many recent studies, and as a result, much has been determined about its origins. Cork oak is in Spain, Portugal, Northern Africa, Italy, and many islands throughout the Mediterranean. Genetic mapping of these populations suggests the species originated during the early Cenozoic era in the Western Mediterranean and reached its current distribution due to the separation of the landmasses surrounding the Iberian continent starting in the late Oligocene and into the Miocene (Magri et al. 2007); this makes Quercus suber a Mediterranean endemic, and as such it has become well adapted to the characteristics of the Mediterranean climate zone.
A Mediterranean climate is having warm, dry summers and relatively mild winters during which most precipitation occurs. Though Mediterranean climate zones are often associated with poor soil quality and varying topography (Moreno and Pulido 2007), water-shortage, especially in the summer months, is the primary environmental restriction for plant growth in these regions. One way that Quercus suber has adapted to overcome this obstacle is by having evergreen, sclerophyllous leaves covered by a thick, waxy cuticle that prevents water loss through evapotranspiration.
At the cellular level, cork oak leaves efficiently control their stomata to maintain efficient water uptake to loss levels even during the dry summer, in effect allowing the tree to avoid drought conditions (Nardini et al. 1999). Quercus suber also exhibits drought avoidance by increasing its year-round access to water. Like many Mediterranean plant species, it often has an extensive root system with a deep taproot that can continue to take water after the topsoil has dried (Ja et al. 2011).
While adaption to drought stress has defined many features of Quercus suber, the fire has also played an essential role in shaping the evolutionary history of cork oak and other Mediterranean flora.
Due to its seasonal lack of precipitation, Mediterranean biomes have frequent fire regimes caused by both natural and later, anthropogenic fires. As a result, many endemic plants have developed mechanisms for dealing with periodic burning. The thick bark and cork cambium from which cork oak gets its name, sometimes reaching a thickness of up to thirty centimeters (Catry et al. 2012), most likely arose as an evolutionary response to fire; this is because the insulating properties of cork that is one of the reasons it is so commercially viable also serve to protect the inner vascular tissue from the heat generated by fires (Catry et al. 2012).
Cork also serves other valuable plant functions such as aiding in the transport of nutrients, and protection from desiccation, damage from insect pests, and their associated plant pathogens (Catry et al. 2012). In addition to being fire-tolerant, Quercus suber also shows properties of being fire resilient. For example, cork oak is one of the only trees found in Europe capable of resprouting epicormically after burning, enabling it to regenerate more quickly after being severely burnt (Catry et al. 2012).
Despite the many adaptations of Quercus suber to its environment, its ultimate fitness may experience challenges by rapid changes occurring throughout its range.
The response of Quercus suber to Rising Mediterranean Temperatures
As average global temperatures continue to rise as a result of anthropogenic climate change, many ecosystems are rapidly changing, affecting species compositions and distributions around the world. The Mediterranean basin looks to be one of the regions most affected by climate change, with projected increases between two and four degrees Celsius over the next one hundred years (Ramirez-Valiente et al. 2009). There are many reports of a decline in the area of Quercus suber forest across Central Europe and Northern Africa. (Ja et al. 2011).
To deal with such climatic changes, cork oak and other plants can adopt many different strategies, one of these being seasonal shifts in range. As temperatures warm and the southern habitats become uninhabitable, Quercus suber may be able to expand its range northward or to higher elevations where the development of milder and shorter winters can allow it to grow it historically was unable to (Ramirez-Valiente et al. 2009). Rather than avoiding warmer temperatures, Quercus suber might also be able to adapt to its environment’s increased stresses.
Cork oak can potentially adapt physiologically to rising temperatures and increased drought in two ways: a population through natural selection and on the individual level through phenotypic variation. The first strategy involves the adaptation of the species through the range of beneficial traits in subsequent generations. It is dependent on the heritability of these traits, that is, the extent to which these traits successfully pass down genetically.
Genetic analyses have shown that some characteristics such as stem diameter and carbon isotope discrimination (an expression of the relative abundance of different carbon isotopes) are highly heritable and that there are no known genetic linkages between traits that would possibly act as an evolutionary constraint, suggesting that natural selection is likely to promote drought-tolerance in future generations of cork oak (Ramirez-Valiente et al. 2011).
Additionally, an individual tree can respond to water stress by adjusting its phenotype to minimize water loss while facing drought conditions. For example, a tree can concentrate its biomass in its roots rather than its leaves and stem, decrease the surface area of its leaves by decreasing leaf size and leaf curling, and increase leaf thickness, serving to maximize water uptake and to prevent the loss of water (Ja et al. 2011). The prominence of these variations correlates with increasing temperatures.
Many researchers can see these changes at the cellular level; this is primarily in the form of stomatal mutations that limit evapotranspiration. Quercus suber afflicted with water stress often exhibit higher stomatal density, decreased stomata size, and relatively small guard cells (Ja et al. 2011). Also, on the level of individual adaptation, the cork oak has been shown to alter its life history strategy to increase the likelihood its offspring will survive establishment in drier habitat.
The size of the acorn of Quercus suber, which bears a positive correlation to the survival of the seed, has been observed to increase in lineages of cork oak from drought-stressed environments (Ramirez-Valiente et al. 2009). Despite these adaptations, Quercus suber forests are still in decline, and this is likely to continue as long as global temperatures continue to climb. Much of the future of the cork oak lies in its cultivation and the cork industry, the fate of which is currently being challenged by new industry trends.
The Cork Industry
Much of the world’s cork oak is grown in vast agrisilvipastoral woodlands for the harvesting of cork. As these systems depend on human upkeep to persist, the success of Quercus suber links to the cork industry’s economic viability. Called dehesas in Spain and montados in Portugal, these agroecosystems, spaced evenly, are stands of cork oak with wheat and grains growing in the understory for part of the year and the grazing of native grasses by cattle, goats, and pigs, which primarily eat the acorn of Quercus suber, after the grains have been harvested (Moreno and Pulido 2007).
The trees themselves, which are usually harvested every nine to ten years (Jove et al. 2011), are essential parts of the ecosystem, as the cork and firewood they provide comprise a significant proportion of the economic value of dehesas and montados. The shade and nutrient cycling they provide allows for the growth of the crops and grasses in the semi-arid Mediterranean climate. Recently, changes in the management of cork oak forests and competition from synthetic materials have threatened to decrease the monetary value of these systems, bringing into question the status of Quercus suber and its current habitat.
The discontinuation of more traditional dehesa/montado management practices has led to the degradation of the cork oak habitat. Such changes include increased mechanization, the abandonment of low-input specialized techniques in favor of simplified system management using agricultural chemicals and fertilizer, and overgrazing due to the reduction of shepherding and cattle subsidization (Moreno and Pulido 2007).
Changes in the wine industry, which accounts for the majority of the economic value of cork, have also led to the decline of Quercus suber forests. Many wine producers have pushed toward synthetic stoppers in place of traditional cork ones, as they are cheaper to produce and eliminate cork taint (Normandin 1980). In an attempt to persuade consumers to support this transition, the environmental impact of cork harvesting is now questionable.
There may be some truth in claims suggesting the cork industry is harmful to trees, as the debarking of Quercus suber results in increased fire susceptibility, water loss, interruption of nutrient transport, and decreased protection from pathogens (Catry et al. 2012). However, most scientists and agriculturalists agree the harvesting of cork is a renewable and sustainable practice due to its role in preserving biodiversity and other positive impacts it has on the environment.
The habitat complexity created by cork oak forests serves as a refuge for many endangered species. For instance, the Iberian lynx, the Spanish imperial eagle, and the black stork prevents the encroachment of invasive grasses and supports higher levels of biodiversity than nearby natural ecosystems.
Furthermore, dehesas and montados occupy land with soil quality that is often unsuitable for other types of commercial agriculture. Their removal has had such consequences as increased soil erosion and the accumulation of over-aged tree stands (Moreno and Pulido 2007). Overall, the positive effects of the cultivation of Quercus suber in the manner of agrisilvicultures make it an excellent example of integrated land use and merit its conservation.
Future Research and Goals
The decline of Quercus suber in Mediterranean ecosystems is a complex problem, and as such, its conservation demands a multi-faceted approach. Additional research will help further assess the response of cork oak to climate change and new management practices and provide possible solutions. Some potential areas of research include the effect of climate change on other Mediterranean plants and how their interactions with Quercus suber may affect its adaption, and genetic analysis for drought-resistant strains of cork oak to be used in future cultivation.
Research can indirectly raise the fitness of cork oak by mitigating climate change effects, especially in the Mediterranean basin. And as Quercus suber is dependent on the cork industry for much of its range, it can also be protected by augmenting the value of the tree; this can be done by increasing the use of cork for purposes other than wine bottling, providing tourism opportunities in Dehesa and montados, and raising public awareness of the benefits of choosing natural corked wine over wine with synthetic stoppers (Moreno and Pulido 2007).
For millions of years, Quercus suber adapted to its environment, and for hundreds of years, humans have cultivated it while maintaining a healthy relationship with the Mediterranean habitat. Given the benefits of cork oak forests on biodiversity and the environment, it is necessary to note that these processes need to continue.
Works Cited
Catry, F.X., F. Moreira, J.G. Pausas, P.M. Fernandes, F. Rego, E. Cardillo, and T. Curt. 2012. Cork oak vulnerability to fire: the role of bark harvesting, tree characteristics, and abiotic factors. PLoS ONE 6: 1-9.
Ja, K.M., L.S. Han, and W.S. Young. 2011. Cork oak (Quercus suber L.) forest decline in Tunisia: a linkage between physiological adaptation and stress. Scientific Research and Essays 6: 1143-1146.
Jove, P., M.A., Olivella, and L. Cano. 2011. Study of the variability in the chemical composition of bark layers of Quercus suber L. from different production areas. BioResources 6: 1806-1815.
Magri, D., S. Fineschi, R. Bellarosa, A. Buonamici, F. Sebastiani, B. Schirone, M.C. Simeone, and G.G. Vendramin. 2007. The distribution of Quercus suber chloroplast haplotypes matches the palaeogeographical history of the western Mediterranean. Molecular Ecology 16: 5259-5266.
Moreno, G., and F.J. Pulido. 2009. The functioning, management, and persistence of dehesas. Pp. 127-160. In: A.R. Rigueiro, J. McAdam, and M. R. Mosquera-Losada (eds). Agroforestry in Europe: Current Status and Future Prospects. Springer, Dordrecht, Netherlands.
Nardini, A., M.A. Lo Gullo, and S. Saleo. 1999. Competitive strategies for water availability in two Mediterranean Quercus species. Plant, Cell, and Environment 22: 109-116.
Normandin, D. 1980. The econmics of cork in France. Revue Forestiere Francaise 32: 79-90.
Ramirez-Valiente, J.A., F. Valladares, A.D. Huertas, S. Granados, I. Aranda. 2011. Factors affecting cork oak growth under dry conditions: local adaptation and contrasting additive genetic variance within populations. Tree Genetics and Genomes 7: 285-295.
Ramirez-Valiente, J.A., F. Valladares, L. Gil, and I. Aranda. 2007. Population differences in juvenile survival under increasing drought are mediated by seed size in cork oak (Quercus suber L.). Forest Ecology and Management 257: 1676-1683.
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