Effects of undergrowth clearing on the bird communities of the Northwestern Mediterranean Coppice Holm oak forests

https://doi.org/10.1016/j.foreco.2005.10.044Get rights and content

Abstract

Undergrowth clearing is a widespread forest management technique used in many Mediterranean regions to reduce dense vegetation in order to prevent fire or to facilitate other forest exploitation activities. Here, we analyze the effects of undergrowth clearing on biodiversity by focusing on the variations in bird diversity in Holm oak forests in Catalonia (north-east Iberian Peninsula) under different forest management regimes: (1) coppice Holm oak forests where the undergrowth layer has been completely cleared, (2) partially cleared forests and (3) cleared and tree thinned forests and finally, (4) undisturbed forests. The synchronic comparison approach was used in conjunction with a before–after control impact (BACI) experiment in which the effects of undergrowth clearing were explicitly measured. Complete undergrowth clearing resulted in the almost complete disappearance of three Warbler species (Subalpine Warbler, Sardinian Warbler and Garden Warbler). Partial clearing also led to a marked reduction in the numbers of these three species, but the presence of Sardinian and Garden Warblers was maintained in the treated forests. Complete undergrowth clearing accompanied by tree thinning also led to a decrease in amount of undergrowth species and involved additional negative effects for species such as Wren, Robin, Blackbird and Blackcap. Only one species, the Nightjar, appeared to benefit from undergrowth clearing, while many others increased their numbers only when clearing was applied together with tree thinning: Turtle dove, Mistle thrush and Cirl bunting. Undergrowth clearing brought about a significant simplification in the vertical structure of the forest, which probably reduced foraging opportunities and breeding resources for most undergrowth species. These effects became more pronounced when tree thinning was applied together with undergrowth clearing. To reconcile forest management and bird diversity, undergrowth clearing should be applied selectively to ensure that a certain number of undergrowth patches, preferably corresponding to different shrub species, are not affected by management treatments.

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

References (46)

  • F.A. Bazzaz

    Characteristics of populations in relation to disturbance in natural and man modified ecosystems

  • C.J. Bibby et al.

    Bird Census Techniques

    (2000)
  • J. Blondel

    Structure and dynamics of bird communities in Mediterranean habitats

  • J. Blondel et al.

    Biology and Wildlife of the Mediterrenean Region

    (1999)
  • M. Brooker et al.

    The significance of territory size and quality in the mating strategy of the splendid fairy-wren

    J. Anim. Ecol.

    (1995)
  • J. Camprodon

    Tratamientos forestales y gestión de la fauna vertebrada

  • G.E. Canterbury et al.

    Local changes in a breeding bird community following forest disturbance

    J. Field Ornithol.

    (1997)
  • CPF, 1992. Documentació Tècnica 1. El bosc mediterrani humit. Centre de la Propietat Forestal....
  • H.S. Crawford et al.

    Songbird population response to silvicultural practices in central Appalachian hardwoods

    J. Wildl. Manage.

    (1981)
  • O. de Bolòs
    (2001)
  • M. Díaz et al.

    Breeding bird communities in pine plantations of the Spanish plateau: biogeography, landscape and vegetation effects

    J. Appl. Ecol.

    (1998)
  • P.F. Donald et al.

    Winter bird communities in forest plantation in western England and their response to vegetation, growth stage and grazing

    Bird Study

    (1997)
  • J.T. Emlen

    Habitat selection by birds following a forest fire

    Ecology

    (1970)
  • R.J. Fuller

    Bird Life of Woodland and Forest

    (1995)
  • E. Garolera

    El bosc mediterrani humit. Explotació forestal de Mollfulleda

    Apunts de Silvicultura

    (1991)
  • M.J. Greenacre

    Theory and Applications of Correspondence Analysis

    (1984)
  • S.H. Hurlbert

    Pseudoreplication and the design of ecological field experiments

    Ecol. Monogr.

    (1984)
  • Recommendations for an international standard for a mapping method in bird census work

    Bird Study

    (1969)
  • F.C. James et al.

    Relationships between temperate forest bird communities and vegetation structure

    Ecology

    (1982)
  • D.H. Janzen

    The eternal external threat

  • S.A. Levin

    Population dynamics models in heterogeneous environments

    Ann. Rev. Ecol. Syst.

    (1976)
  • D.H. Lovejoy

    Edge and other effects of isolation on Amazon fragments

  • R.H. MacArthur

    Environmental factors affecting bird species diversity

    Ann. Nat.

    (1964)
  • Cited by (0)

    View full text