Growing olive trees

Olive systems may be categorised into 'intensive', with over 150 trees per hectare; 'extensive', with 100-150 trees per hectare; 'marginal', with less than 100 trees per hectare; and 'abandoned'.

A particular feature of olive groves is the many small niches found in the old trees themselves and in the stone walls which support the traditional terraces. These niches harbour a variety of reptiles and invertebrates, and provide nesting sites for several bird species which thrive in the open £parkland' habitats of old, widely-spaced olive groves. The olives themselves provide an important food resource for overwintering birds, whilst the ground, undisturbed by man for most of the year, supports orchids, ground-nesting birds, and many butterflies which enjoy these open, sunny habitats. Alongside these species live a variety of predators, such as the scops owl, foxes and stoats, together with other mammal species, including badgers, hedgehogs, porcupines and dormice.

Biodiversity generally increases as the olive groves become more extensive, until finally scrubbing over of abandoned groves fundamentally changes their open 'parkland' habitat. Particularly important for biodiversity are old trees, wide spacings, limited tillage and human activity, as well as an absence of the pesticides used in the more intensive plantations.

The traditional production systems, which harbour the richest wildlife communities, use occasional cultivation or grazing to control weeds between the trees and, in upland areas, use terracing with supporting walls to reduce soil erosion on slopes. Under more intensive systems, terracing is less usual and the combination of frequent mechanical weed control, thin soils and steep slopes often leads to very serious problems of soil erosion, with figures quoted for losses of up to 80 tonnes of topsoil per hectare per year in parts of Spain. The most modern systems, using newer dwarf varieties at over 200 trees per hectare, tend to be sited on flatter land and to use herbicides for weed control; this reduces the problem of soil erosion but establishes a virtual monoculture with minimal biodiversity. The final stage in intensification is to use irrigation, allowing higher tree densities and increasing yields by almost 50% compared with intensive dryland systems. Irrigation brings with it a new environmental impact, through the exploitation of scarce water resources and the construction of reservoirs to store irrigation water.

Olive groves are Europe's oldest farmed habitat, with many trees in Italy dating back five or six centuries and some being a thousand years old. Olive trees thrive in conditions of low rainfall, with long dry summers and mild winters, where the natural vegetation comprises Mediterranean scrubland with holm and cork oaks.