Effects of undergrowth clearing on the bird communities of the Northwestern Mediterranean Coppice Holm oak forests
Introduction
Mediterranean forests have traditionally been subject to strong human pressure through activities such as forestry, coal production or vegetation clearing (Blondel and Aronson, 1999). However, recent socioeconomic changes in some Mediterranean regions have induced a reduction of most of these activities typically leading to an accumulation of vegetation, especially in the shrub and lianas layers of the undergrowth. In this context, undergrowth clearings are currently the main management activity effecting forests in many regions particularly in the north-western Mediterranean basin. Mechanical undergrowth clearing aims to reduce the densely developed shrub and liana vegetation layers and are mainly used to eliminate the combustible load but also to control tree competition and favour sapling regeneration or access to the forest (Garolera, 1991, CPF, 1992, Mesón and Montoya, 1993). The increase in the number and impact of forest fires during recent decades (Terradas, 1996) has boosted the use of undergrowth clearing, although, the effectiveness of this forestry technique to fight the spread of fire has been questioned because of its low cost effectiveness (Camprodon, 2001).
Although the influence of forestry, fire or grass management on biological diversity in the Mediterranean scrubs, garrigues and maquis has been studied (Blondel, 1981, Prodon, 1988, Pons, 1998), the effects of undergrowth clearing have been largely neglected and are not currently used in the criterion which guides forest management in the region. Clearing is likely to have a strong impact on biodiversity, since it entails a significant elimination of forest biomass. In Mediterranean forests, more than 40% of the total floristic richness, as quantified by forest plants, is located in the shrub layer. Furthermore, most of the arthropod biomass inhabiting Holm oak forest, essential for the undergrowth feeding of diverse faunal groups, also occurs in the shrub undergrowth (pers. obs.). Vegetable biomass is often a good indicator of the density and specific bird composition of forest habitats (MacArthur, 1964, Wilson, 1974, Levin, 1976, Prodon and Lebreton, 1981, James and Wamer, 1982). In particular, a recurrent positive relation between bird richness, density and extension of the undergrowth layer has been found (for example, Prodon and Lebreton, 1981, Canterbury and Blockstein, 1997, Donald et al., 1997, Díaz et al., 1998). Crawford et al. (1981) concluded that the crown cover and the height of the undergrowth layer were the most important factors in explaining the distribution of forest passerines. These studies linking undergrowth vegetation and bird communities suggest that an intense modification of the shrub layer either by mechanical means, grass or prescribed burning, may have a significant impact on bird fauna. In fact, the situation after a total clearing of the undergrowth can be compared to one after a forest fire in which trophic availability and nest site availability are limited, forcing territorial birds to breed in unusual locations, consequently increasing their exposure to predation or reducing their food supply (Renwald, 1977, Wright, 1981, Winter and Best, 1985, Petersen and Best, 1987). In spite of these predicted effects, coppice regeneration may favour rapid nest establishment even before the development of a more dense vegetation layer by growth of new seeds (Brooker and Rowley, 1995, Pons, 1996). However, unlike burning, clearing of the undergrowth does not affect tree canopies and therefore, undergrowth recovery tends to be slower unless clearing is accompanied by tree thinning.
The objective of this study is to determine the association between forest birds and the structure of the shrub layer in Mediterranean forests and test the hypothesis that undergrowth clearing has a strong impact on the occurrence of forest birds, especially those known to rely on low vegetation layers. Furthermore, we explored the combined impact of undergrowth clearing with other forestry practices (i.e. tree thinning) on the bird community. We conclude by suggesting some recommended guidelines aimed at making forest management compatible with the conservation of species associated with Mediterranean shrubby vegetation.
Section snippets
Study area
The study area, of about 2000 km2, is located in the Catalan Pre-littoral System (north-east Iberian Peninsula). It has a relatively rainy coastal climate with a short cool winter (de Bolòs, 2001). Coppice Holm oak forests (Viburno-Quercetum ilicis) are dominant in most mountain areas up to 1400–1500 m. The arboreal layer of coppice Holm oak forests is rather short and the trees never reach a great size: 8.5 m average maximum height, 12.4 cm average diameter at breast height (dbh), with few trees
Structural differences between forest types
The cover of the understory was significantly related to forest type. In dense, reference forests, shrub cover reached, below 2 m in height, values above 50%. This cover decreased to 26–42% in partially cleared forests and to 10–27% in completely cleared forests (Table 1). Total tree density showed similar figures between different cleared forests, with abundance of sprouts, ranging between 1.750 and 2.400 trees/ha, largely within the 5–15 cm class and rarely surpassing 30 cm. On the other hand,
Discussion
Change in perceived habitat quality is the key factor determining birds’ response to disturbances (Sousa, 1984, Wiens, 1989, Bazzaz, 1991). Prodon and Lebreton (1981) showed that as small changes in vegetal cover occur, significant impacts on the bird community may be detected when overall vegetal cover in the undergrowth is low. However, if ligneous vegetation is dense enough, clearing of the vegetation may have negligible effects on birds. In line with these predictions, the abundance of
Applied conclusions
The reduction of the vegetal biomass of the undergrowth is a convenient forestry practice in particular forest management circumstances. However, maintaining large extensions of forests with a low combustible load is a titanic and economically unrealizable task with obvious ecological impacts (Mesón and Montoya, 1993). Consequently, to improve biodiversity conservation it is important to ensure a certain number of patches of the different shrubs and lianas species and to apply clearing to
Acknowledgements
This study has been carried out thanks to the financing of the Environmental Department of the Generalitat de Catalunya (Catalan Government) and the Forest Technology Centre of Catalonia. At the same time, to carry out the project we have been able to count on the collaboration and the support of the technical services of the Environmental Department, particularly Joan Montserrat and Emili Bassols technicians of the Natural Park of the Zona Volcànica de la Garrotxa (Catalonia). Also, Enric
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