Built to last? Land-use history, ecological determinants, and land manager perspectives of regeneration of holm oak (Quercus ilex) dehesas

Problem

A successful regeneration of holm oaks is the key to the conservation of the outstanding biodiversity levels in Spanish dehesas. The objective of the present dissertation was to develop strategies for the long-term maintenance of holm oak stands in a typical dehesa region. The amount of regeneration was to be quantified, and the potential impact of land-use and ecological site factors was to be studied. The dissertation consists of six separate studies and is based on a broad methodology, comprising a historical landscape analysis, a regeneration inventory, and a mail survey.

Landscape history

The analysis of landscape history (1700-2000) showed that most dehesas were constructed between the 18th and the beginning 20th century by clearing of dense woodlands and shrublands (so-called monte pardo). As holm oaks can reach an age of 500-700 years, most of the trees in dehesas are relics of the former monte pardo. Practices for the regeneration of holm oaks under conditions of systematic grazing and cultivation have not evolved historically. This led to a regeneration failure.

A GIS analysis of three sets of aerial photographs and orthoimages (1956, 1984, and 1998) showed that stand densities had strongly been reduced in recent decades (from 25.7 to 20.7 trees per ha), probably by clearing, soil tillage, and cultivation.

Regeneration inventory

Alarmingly low densities of holm oak juveniles and saplings (51.2 and 85.0 individuals per ha) were measured, but these figures varied strongly between sites. Regeneration was spatially clustered and bound to specific microhabitats including brush, rock outcrops, and mature trees.

In the analysis of holm oak stand structure, a transition from an inverse J-shaped to a bell-shaped diameter distribution was found whenever monte pardo was cleared to a dehesa. An unbalanced age structure may lead to a gradual dissolution of stands. The threshold for livestock stocking levels supporting regeneration is below all figures presently found in the dehesas.

By this, regeneration failure is an implicit component of the agroforestry system. But it was also found that stands are able to regenerate as soon as agricultural uses are abandoned permanently or temporarily.

Land manager survey

In a last step, managers of private large landholdings were interviewed about their attitudes toward the regeneration of oaks. Unexpectedly, they highly appreciated having holm oaks on their land. Traditions and market incentives were identified as basic determinants of a conservation-awareness.

Outlook

The promotion of natural regeneration through a rotating system of small-scale set-asides over 20-30 years, afforestation and land abandonment is discussed as potential solution for the regeneration failure.