Incidence of Large Wood Borers in the Conservation of dehesa Islands Forests in Southwestern Spain

Abstract

The dehesa is a traditional agrosilvopastoral ecosystem characterized by a savanna-like structure. It has been in decline since the middle of the last century, in particular in agricultural environments where it has been relegated to isolated forests. This paper presents a study into the reduction over the past 50 years. One of the main causes of this decline has been identified as xylophagous insects, specifically the wood borer Cerambyx welensii Kuster (Coleoptera: Cerambycidae). Based on estimations in both the population size, using Jolly Seber models, and the level of tree injury, using a single-index model, the study investigates the role the insect has played in the decline of the dehesa. Surface area loss was found to be greater than 50%, while canopy cover has diminished linearly over the period studied, with the risk of disappearance of the woodland increasing in the same magnitude as borer population density. The highest wood borer population densities observed corresponded to stands with an average canopy cover loss of 1.68% a year, indicating a risk of completely losing the forest cover in the short term (10–30 years). The proposed tree injury index was successful in categorizing the size of the insect population, and seems to be a useful indicator for describing the relationship between the two variables. According to this indicator, woodlands with more than 10% of trees with severe or very severe injuries are at risk. Follow-up work should be undertaken to investigate long-term demographic changes in wood borer populations, and management practices should be developed to reduce the population size, minimize forest loss and preserve these relict oak trees in unconnected landscapes.