Introduction
The dehesas of southern Spain and Portugal (where they are called montados) are man-made ecosystems characterized by a savannah-like structure and very high biodiversity, where the trees (mainly holm oak, Quercus ilex) are viewed as an integrated part of the system, and as a result are planted, managed and regularly pruned. They are traditionally grazed by mixed livestock at low densities, and are probably the oldest agroforestry systems still in existence in Europe, having been developed some two thousand years ago and changed very little until the last 30-40 years.
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The dehesa region of the Iberian peninsula (from Joffre 1999).
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The dehesa system
The dehesa system is characterized by a savannah-like open tree layer, mainly dominated by Mediterranean evergreen oaks - holm oak (Quercus ilex) and cork oak (Q. suber) and to a lesser extent by the deciduous Q. pyrenaica and Q. faginea. Other trees which may be included are wild olive (Olea europaea silvestris), stone pine (Pinus pinea) and wild pear (Pyrus bourgaeana). Shrubs include the strawberry tree (Arbutus unedo) and Quercus coccifera. The holm oak trees have been selected over centuries for the production of sweet acorns as a high quality stock feed. These trees will have been planted deliberately or may have self-seeded. Below the tree layer is a grass layer used for grazing, or occasionally cultivated cereals. These systems occupy over 6 million ha (60,000 square kilometers) in the west and south west of Spain and southern Portugal.
The high biodiversity is due to a variety of grazed, shrubby and cultivated types of dehesas and from differences in stand composition, density and structure. The mosaic of habitats supports a rich diversity of plants and animals.
The traditional system was highly diversified in terms of livestock types (sheep, goats, pigs and cattle). Hardy local breeds were used - merino sheep (for wool), retinta, avilena and morucha cattle and iberico (iberian) pigs. The pigs and sheep were tended by herders. The herders and the sheep migrated in summer to northern mountain ranges due to the scarcity of forage during drought. The average stocking levels were equivalent to 2 suckling ewes/ha in winter and 0.7-1 suckling ewe/ha in summer. The pigs graze the seasonal acorn production between October and February, gaining about 60 kg live weight in 75 days (9 kg of Q. ilex acorns corresponds to the production of 1 kg of pork meat). Today the grazing of the high value Iberian pig is the most profitable component of the dehesa system.
The herbaceous layer is usually composed of native annual vegetation (grasses - Lolium, Bromus, Hordeum etc., legumes - clovers, medicagos, serradela [Ornitophus sp] etc., crucifers) which are used for grazing. Sometimes cultivated cereals (oats, barley, wheat, rye) are grown (occupying 9% of the total land area), traditionally in long rotations and closed nutrient cycles without external inputs of fodder, fertilizers and chemicals (often using fallows of 3 years or more between crops). Control of invasion of the dehesa by shrubby species, "matorral" (mainly Cistus ladaniferus, C. salviaefolius and C. monspeliensis) , has traditionally been by (1) manually uprooting and (2) clearing and horse ploughing by landless peasants. The peasants were granted rights to grow cereals in exchange. Pasture productivity was traditionally managed by directing livestock manure to selected places by nocturnal penning (for example, penning sheep in an area of 2 m2 per ewe for 2-3 successive nights).
The oak trees are pruned several times during their lives. Cork oak is pruned quite differently to holm oak; in the former the main product is cork, while in the latter it is acorn production. Holm oak is formation pruned in the early years to achieve a crown with three main vertical branches; this favors the horizontal spread of the fruiting branches. Maintenance pruning every 7 to 9 years allows opening of the crown and sustainability of acorn production. Production pruning provides firewood and browse during severe droughts, reducing the leaf area hence aiding tree survival. With appropriate pruning, tree cover can extend to 40% of the land area, but still allow 96% of the soil surface to grow grasses. Trees reduce the ambient and ground temperature beneath them, act as nutrient pumps, increase soil organic matter and reduce wind speed.
Several other secondary products such as firewood, charcoal and tannin production, hunting (partridges, wood pigeons, rabbits, wild boar), fish farming and raising of poultry are still important in the dehesa economy.
A typical dehesa farm with the average of about 45 trees per ha produces annually (per hectare):
- 1500-3000 kg grasses
- 450-500 kg acorns from holm and cork oaks
- 150-300 kg tree leaves for fodder
- 30 kg cork from cork oaks
- 37 kg charcoal from pruned branches
- 40-70 kg livestock
The 9% area of cultivated land produces about 100 kg grains (fed to animals) and 115 kg cereals (used as bedding or sold).
The animal component produces some 570 kg manure.
Any land use system in this region has to cope with the high variability of the Mediterranean / Continental climate, and the density of trees is a critical factor in the system being able to survive droughts. The density varies depending on the average annual rainfall, rising with the annual rainfall until annual rainfall levels reached 650-700 mm, when the tree density reached 40-50 per ha. Where annual rainfall is below 600 mm, the tree density is low, under 20 per ha. Rainfall under 500 mm can only support very scattered trees which don't really form an integrated system.
The soils in dehesa regions are generally poor, especially in phosphorus and calcium, and low in organic matter. Soils under the tree canopies are richer in nutrients and organic matter, due partly to the trees acting as nutrient pumps (mineral accumulators), and partly due to the attraction of shade for grazing animals who leave their dung there. Moisture retention beneath trees is better, and in droughts the soils under trees are slower to dry than in the open.
Changes since the 1950's
Since the mid 1950's there have been big changes in the dehesa areas, more than in the previous 1700 years. These have been, a) a decrease in agro-pastoral management
b) an increase in deforestation and clearing in order to extend cropping lands,
c) reforestation, and
d) a huge decrease in human population, not least because of inequitable land ownership, with much of the land still owned by relatively few people.
When grazing pressure is removed, dehesas are invaded by Mediterranean matorral species, which in turn increase the fire risk. Shrubby species are now controlled entirely by mechanical means - ploughing etc.
Modern trends are a specialization towards lamb and beef production and use of techniques such as free range grazing at high stocking levels or cross-breeding with high-performance breeds (rather than using hardy regional breeds.) A wider range of livestock is usually found on public owned lands (about 18% of the total). Fodder concentrates are used and increasingly chemical fertilizers, pesticides etc. are being used, with the result that biodiversity is decreasing, soil degradation is increasing.
One major problem for the continuation of the dehesas is the gradual decay of the tree canopy. With the decline of pig raising, farmers lost the fundamental interest in the acorn crops from the holm oaks, hence the trees are neglected and ageing; new trees are rarely planted, and the intensification of grazing makes natural regeneration unlikely.
Climate models for southern Europe show that a doubling of carbon dioxide would increase the probability of extreme events such as severe drought; and that in the dehesa regions, a decrease in total annual rainfall can be expected. A doubling of CO2 in 70 years might lead to a reduction in rainfall of 80 mm - enough to cause problems in many of the dehesas with annual rainfall currently under 700 mm (about 88% of the entire region).
Overall, the dehesas are under severe threat over the next 100 years. Although various regional, national and EU programs are being targeted at helping farmers retain the systems, all this may come to little if climate changes make their sustainability impossible.
References
Joffre, R et al: The dehesa system of southern Spain and Portugal as a natural ecosystem
mimic. Agroforestry Systems 45: 57-79, 1999.
Plieninger, T & Wilbrand, C: Land use, biodiversity conservation, and rural development in the
dehesas of Cuatro Lugares, Spain. Agroforestry Systems 51: 23-34, 2001
Garcia Trujillo, R & Mata, C: The Dehesa: an extensive livestock system in the Iberian
Peninsula. Proceedings of the 2nd NAHWOA Workshop, Cordoba, 8-11 January 2000.
(Reprinted by permission from Agroforestry News, a publication of the Agroforestry Research Trust, Devon, UK)
By Martin Crawford, Agroforestry Research Trust