Fernando J. Pulido Diaz is a professor at the University of Extremadura (Plasencia) who has been doing research for years on the Dehesa. This is a savannah landscape, typical of the South West of Spain and the South East of Portugal – where it is referred to as Montado. Quite a surprising landscape for most northern Europeans. It is one of the largest agroforestry systems in the world, covering 2,3 millions of hectares in Spain and 0,7 millions in Portugal, dominated by Quercus ilex/rotundifolia (Holm oak) and Q. suber (Cork Oak), depending on soil characteristics. An amazing habitat for wildlife, it also involves a complex cycle of agricultural practices. Fernando Diaz has accepted to answer a few questions.
A. G. Fernando, what do we know about the origins of this landscape? Did it evolve out of dense forests where Quercus were the only climacic species? Why did local communities not mix dense forests with fully cleared arable land instead of moving extensively to the more intermediary agricultural Dehesa landscape? And where does the word “Dehesa” come from?
F. P. Iberian oak forest have been cleared by human populations since the Neolithic, but dehesas as a systematic type of land use dates back to the early Middle Age, when the word “dehesa” (from the latin “deffesa”) also implies a legal status for grazed lands which were defended from uncontrolled use. Oak trees were retained because they increase forage, acorn and firewood production.
A. G. Are the origins similar for the Portuguese Montado and for the equivalent landscapes that exist in California?
F. P. California oak woodlands have been managed by natives for millennia but with a much less intensive use because they did not practice livestock husbandry nor tree thinning or pruning. The origins of the montado apparently differ from the dehesa because of the greater importance of cork in Portugal.
A. G. In the Dehesa, the holm oaks are pruned, a bit like fruit trees in orchards. What are the rhythm and functions of such pruning?
F. P. Quite surprisingly, there are not conclusive data of the effect of pruning on acorn production. Therefore, the main benefit is firewood. Pruning was performed every four year (matching the cycle of cereal crops) in the traditional dehesa system (before 1960). Currently, pruning does not follow a specified time schedule and it is performed according to firewood demands.
A. G. Nowadays, one often hears that this landscape is under threat. Is this true? And if so, what are its causes? Too deep ploughing? Too intense grazing? New mushrooms? New Insects? Climate change?
F. P. Yes, it is certainly true mainly because of tree regeneration failure and tree diseases. In general, the system is now used much more intensively due to a three-fold increase in stocking rates as compared to traditional levels.
A. G. One interesting finding seems to be that when Holm Oaks are grown in less dense formation, they can stand the drought better than in denser formations, which is crucial in the long summers of the south of Spain and Portugal.
F. P. Yes, our research group and others has provided evidence of increase tree vigour in low-density stands, and this affects growth and acorn production, being the result of decreased competition for water resources.
A. G. Can you tell us a bit more about your idea of conceiving a sustainable Dehesa in terms of a mosaic managed in a dynamic way - what Blondel would perhaps refer to as a metaclimacic system?
F. P. As Dehesa is a kind of wood pasture, tree regeneration is inherent to its definition and it can only be achieved by reducing or eliminating livestock impact in some parts of the farm in which natural vegetation is allowed to increase. This is the basis for a mosaic of plots with different grazing intensities.
A. G. Among the possible principles for a more sustainable agriculture are the following two. First, we should try to raise less livestock and go more vegetarian, which would lead to less energy input for the same amount of calories, less methane emissions, etc. Second, we should move towards cultivating perennial plants – including cereal crops – rather than annual ones, that require much more input. Could Dehesas be preserved if we were to follow these two principles? Pasture surfaces would probably have to be reduced and we would probably need to focus more on the shrub stage, experimenting mixes of large oaks and smaller fruit shrubs and bushes such as feijoa, pomegranate, or even small almond or carob trees that are especially drought resistant. Have there been interesting experiments in that direction, transforming Dehesas into a new type of forest gardens somehow?
F. P. Fruit trees have not generally been introduced in dehesas because of soil/water limitations. Among the potential candidates, pistachio trees is probably the best and some parcels have been planted in the last few years. However, our group is leading a project promoting the use of oaks as fruit trees. In fact, oaks have been pruned in a way similar to fruit trees. Certain acorn varieties can be marketed as snack or processed food (including beverage) and in the future this will provide additional income to landowners.
A. G. Have there also been experiments either to use holm oaks for other functions (e.g. edges) or to mix holm oaks in a Dehesa Savannah with other large tree species? I am thinking about Californian oaks, Q. canariensis or even non-oak species such as Carob or Fig trees for instance. One feature of the Dehesa is that it looks like a vulnerable monoculture of oaks that could perhaps gain from some diversification while preserving its general structure. With other large tree species that could stand the same pruning methods, that would have deep roots as well, and fruits that are more edible for humans than acorns, etc.
F. P. Yes, dehesa is a vulnerable open forest, but there it is also the result of soil traits limiting cultivation. The problems here are (1) the high investment needed in the plantations you suggest at a large scale, (2) the chance for farmers to get benefits from traditional practices (though with moderate to low profitability), and (3) the cultural inertia of historically grazing systems. In addition, any landscape change derived from cultivation should be assessed for its negative effects on biodiversity.
A. G. What would your main advice be to someone in charge of a few hectares of Dehesa who would want to contribute to its preservation through methods that are neither costly, nor heavy to put in place? And is there one pilot project that you would recommend every Dehesa and Montado amateur to visit?
F. P. Dehesas, by definition, are large farms over 50-100 hectares. You cannot manage a few hectares as dehesa in a profitable way unless you introduce a number of complementary uses such as agrotourism and cropping in case the soil is appropriate. Two examples of interesting pilot projects are the Herdade do Freixo do Meio (on the portuguese side) and Casablanca (on the Spanish one).
A. G. Many thanks, Fernando
And if you want some extra references, here are a few: Fernando Pulido's google scholar page, a small film on the Monfrague dehesa with Fernando Pulido, a great link on the history of Montado (in Portuguese), and two portuguese products made of acorn.