Barcelona Field Studies Centre

Garraf Natural Park: Human Activity and Vegetation Adaptation

Most of the Garraf has a typical southern Mediterranean landscape. The vegetation is a dense scrub of one to three metres in height, some of African origin. Garraf is the most northerly area of Europe where this vegetation grows.

The vegetation on the coastal slopes tends to be MAQUIS - a dense tangle of undergrowth usually found in impermeable areas or where soils are damper. Maquis is from the French word meaning 'resistance' relating to the armoured protection of the species.

Where the underlying rock is permeable, such as limestone, GARRIGUE vegetation is found. This is less dense and lower lying scrub which includes many aromatic plants such as rosemary and lavender.

In the drier and hotter interior of the Garraf, the holm (or evergreen) oak and laurestine dominate in the damper valleys. Vegetation has adapted to the dry, hot conditions and poor soils where water drains away rapidly. Where the shrubs have leaves, these are evergreen, which enables the plants to make full use of the relatively short periods favourable to growth in spring and autumn. Adaptations such as leathery leaves, long roots and the ability to close the leaf around the stomata helps survival through the summer drought.

Some species have all their leaves reduced to spines to reduce evapo-transpiration, such as the Kermes Oak, Gorse, Juniper and Heather.

Broom has no leaves, transpiring through its green stem. Plants with flat, hard, waxy and leathery leaves that provide insulation include Mock-Privet, Laurestine and Rosemary.

The stomata of the Evergreen Oak is able to close to preserve moisture. Other plants, such as the Rock Rose shrivel their leaves in times of drought to reduce evapo-transpiration. Some plants accumulate large quantities of water within their structures, such as the Prickly Pear, Cat's Claw and the Agave.

Strong aromas and resins repel animals. Such plants include Rosemary, Lavender and Thyme.

Some of the plant species are particularly resistant to fire. When their stems above ground are completely destroyed they are able to send up new shoots.

Carritx, normally only found on the North African coast, probably arrived in the Garraf from seeds dropped by migratory birds.

The lack of vegetation cover is due to past human activity on the Garraf - the grazing of goats and growing of crops. There is much evidence of the terraces or 'Feixes'. Overgrazing and over-cultivation led to soil erosion that removed the soil essential for tree regeneration.

Trees not only protected the soil from heavy rain and erosion. They also provided shade and the shelter they offered from the wind helped increase humidity levels, especially because of the water vapour received by evaporation and transpiration from the tree foliage, the undergrowth and the soil surface. The blanket of moister air would, in turn, prevent rapid evaporation from the ground. Beneath the tree canopy therefore, the soil would remain relatively cool and moist during the hot summer days even after weeks of drought.

With the felling or grazing of the forest, the soil would have been exposed to the wind and sun and dried to a considerable depth in only a few days. Also the leaf-fall which had persistently supplied new humus to the surface layers was now cut off; no longer were mineral nutrients being drawn by the plant roots from deep within the soil and then returned to the soil through leaf decay. The nutrient cycle was therefore altered as well as the water cycle. Without humus, the soil lost its 'stickiness' or adhesive qualities. The winter rains were able to run off the surface carrying the soil particles away. In a very short time, bare rock was exposed, and where pockets of soil remain, these are poor in humus and prone to drought.

The 'terra rossa' or red soils are often said to be characteristic of the Mediterranean region. They are widespread on the Garraf. The redness is due to the presence of iron (ferrous) oxide (Fe0), one of the products of weathering of the limestone through the action of naturally-acidic rain water. Were there more humus in the soil, the redness would be obscured and the whole aspect of the Garraf (and much of the Mediterranean landscape) would be very different.

Today, fire is a main controlling influence. In the past, it has been common practice to clear the land for farming by burning the vegetation. Fires are also started today by careless visitors, arson and lightning strikes. Burning favours shrubs like the lentisk and the cistus which can shoot freely from the base of the stems no matter how often they are burned. The saplings of trees, on the other hand, cannot survive.

 

Vegetation and Human Value

 

LENTISK

The red fruit gives an oil that can be consumed. It has been considered an aphrodisiac. The fruit also provides an indelible ink. An incision in the stem produces a green, very aromatic liquid, with an odour similar to turpentine. This is called the Turpentine of Kios. Its taste is slightly bitter but not unpleasant. Chewing the fruit strengthens the gums and whitens the teeth.

BLACKTHORN

The fruit ripens at the end of August, turning from green to black. It is poisonous. A short time after eating it, you would experience violent stomach pains, vomiting, kidney and respiratory failure and death. In very small doses, the plant is used as a laxative.

WILD OLIVE

The green fruit is very bitter. The ripe fruit is black and the oil is used for cooking. It is a popular medicine. The leaves are used to relieve high blood pressure and also for hypoglycaemia.

CAROB TREE

The fruit is very nutritious and is used in cattle feed. It has been used for human consumption in the past. The pods are rich in calcium and sugars and can be eaten fresh, dried or crushed. It can be made into an omelette (tortilla). The fruit can be substituted for cocoa in the manufacture of chocolate and the seeds have sometimes been used as a substitute for coffee. The seeds (quilates or carats) were used in ancient times to weigh gold. The quality of gold plating today is still measured in carats (in fractions of 24).

JUNIPER TREE

The berries are poisonous, but in low doses are used to treat kidney diseases and as a stimulant. Oil obtained from the wood (oil of Miera) is used as an antiseptic. The reddish timber is highly valued by cabinet-makers.

KERMES OAK

The acorns are used as animal feed. They are bitter but have been eaten by people in times of food shortage. Roasted, they lose their bitterness and can be used as a coffee substitute. Cochineal colouring (for materials) is also obtained from them.

DWARF PALM

Its location in The Garraf is unique in Europe - it marks its most northerly site in the continent. The palm dates are edible, but not popular. The leaves have been used for hat and basket weaving and making brooms and rope. It is protected by law.

HOLM (or EVERGREEN) OAK

The acorns are very bitter, but toasted, they lose this taste. They are used as animal feed. The bark is used as a dye for wool, giving a maroon colour.

LAURESTINE

The fruit is poisonous, producing inflammation of the mouth, vomiting and diarrhoea. It is used as a medicine but its rapid toxicity in higher doses causes it to be rarely used today.

ALEPPO PINE

It is an important forestry tree, planted to help prevent soil erosion in arid zones. The pine cone can be eaten, but is very oily. The bark is rich in tannin dye.

STRAWBERRY TREE

The fruit can be eaten, but too many produces headaches as the ripe ones contain alcohol! It is used to make alcoholic drinks, sweets and marmalade.

PRICKLY PEAR

The fruit is edible, but remove the spines first! Alcohol can be made from the fruit pulp. It can be used to make a deliciously refreshing cold drink.