Holm oak

Quercus ilex and Quercus rotundifolia

1. Identification and origin

The holm oak is an evergreen tree species of the Fagaceae family, represented mainly by Quercus ilex L. (Mediterranean holm oak) and Quercus rotundifolia Lam. (Iberian holm oak). Both are native to the Mediterranean region, with a wide distribution in the Iberian Peninsula, North Africa and Southern Europe. They are structuring species of Mediterranean ecosystems, particularly the montados and dehesas.

2. Economic importance

The holm oak has great economic and ecological relevance. It produces acorn, traditionally used in animal feed, especially in the montado system, where it constitutes the basis for fattening the Alentejo pig. The wood is dense and resistant, used in carpentry, firewood and charcoal. Holm oaks also play essential roles in soil conservation, hydrological regulation and maintenance of biodiversity. In Portugal, they are part of agro-silvo-pastoral systems of high ecological and cultural value.

3. Botanical characterisation

An evergreen tree, with a dense and rounded crown, reaching 8–15 m in height. The leaves are coriaceous, persistent, oval to elliptical in shape, with an entire or slightly toothed margin. Q. ilex tends to have narrower and dark green leaves, while Q. rotundifolia has more rounded leaves with a greyish tone. Flowering occurs in spring, with male flowers in pendent catkins and discreet female flowers. The fruit is the acorn, ripening in autumn.

4. Soil and climate requirements

The holm oak is highly adapted to the Mediterranean climate, tolerating hot, dry summers and mild winters. It prefers deep, well-drained soils with a medium to loamy texture, but withstands poor, stony and slightly acidic soils. It is resistant to prolonged drought, thanks to its deep root system. It is sensitive to waterlogging and compacted soils, which favour root diseases.

5. Main pests

• Cork oak and holm oak caterpillar (Lymantria dispar): intense defoliation in years of high pressure.
• Acorn weevils (Curculio spp.): perforation and internal destruction of the fruit.
• Pine processionary moth (Thaumetopoea pityocampa): occasional on young holm oaks.
• Wood-boring beetles (Cerambycidae, Buprestidae): damage to weakened branches and trunks.
• Aphids and scale insects (Aphididae, Coccidae): sap sucking and honeydew production.

6. Main diseases

• Decline associated with Phytophthora cinnamomi: root necrosis, loss of vigour and progressive death.
• Cankers and trunk rots (Botryosphaeriaceae): dry branches and cracks in the bark.
• Powdery mildew (Erysiphe alphitoides): white mycelium on the young leaves.
• Anthracnoses (Colletotrichum spp.): leaf spots and premature leaf drop.

7. General crop management

Management of the holm oak integrates soil conservation practices, grazing control and maintenance of natural regeneration. It is recommended to avoid deep tillage and soil compaction. Grazing must be controlled to allow the development of seedlings. Pruning must be moderate and carried out in dry periods to reduce the risk of infections. Monitoring of decline symptoms is essential, especially in soils susceptible to Phytophthora. Integrated management of the montado includes a balance between trees, pasture and animal load.

Bibliographic references

• EPPO Global Database – Quercus ilex.
• CABI – Forestry Compendium – Quercus ilex.
• FAO (2013). State of Mediterranean Forests. FAO & Plan Bleu.
• Aronson, J., Pereira, J. S., & Pausas, J. G. (Eds.) (2009). Cork Oak Woodlands on the Edge: Ecology, Adaptive Management, and Restoration. Island Press.
• Plieninger, T., & Wilbrand, C. (2001). Land use, biodiversity conservation, and rural development in the dehesas of Spain. Agroforestry Systems, 51, 23–34.