Dynamics of understorey herbaceous plant diversity following shrub clearing of cork oak forests: A five-year study

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Abstract

Cork oak forest management has been traditionally oriented towards optimization of cork production. Shrub clearing is a traditional silvicultural practice aimed to: (1) facilitate cork extraction (conducted every nine years); (2) increase cork yield by reducing competition from neighbouring shrubs; and (3) reduce fire risk by decreasing fuel load. These silvicultural practices, however, may interfere with current objectives of conservation and sustainable use of biodiversity. The effects of shrub clearing on different diversity components were assessed in three structurally contrasting forests sites, located in Southern Spain. A fenced experimental plot (1 ha) was established at each forest site. Half of each plot was shrub-cleared and the other half was left unmanaged (control stand). Afterwards, the presence of herbaceous species was monitored during five consecutive years in a total of 120 permanent quadrats (1 m2). Species richness (α- and γ-diversity) increased with the clearing, mainly by expansion of open grassland species, while β-diversity declined due to the homogenizing process associated with this expansion. Thus, the herbaceous species composition was modified by these silvicultural practices, especially the second year following treatment application. Effects differed across forest stands, being more marked in the Open Woodland, while there was no significant effect observed in the closed Forest. Understorey herbaceous communities were resilient to shrub clearing and initial diversity values were approximately restored after five years. The high resprouting potential of shrubs contributed to this resilience. Based on the results of this study, we propose several low costs strategies to be incorporated in forest management plans to reconcile cork oak extraction with the maintenance of biodiversity.

Introduction

Mediterranean plant communities exhibit a remarkable biodiversity, which is in part maintained by a certain level of anthropogenic exploitation derived from ancient land use practices (Pons and Quézel, 1985, Blondel and Aronson, 1999, Grove and Rackham, 2001). Current forest policy is concerned with the development of sustainable forest management practices that allow simultaneous exploitation of goods and services, and preservation of structural and functional attributes of forest ecosystems (Riley, 1995, Brown et al., 2001, Thomas et al., 2006). Understanding how silvicultural practices impact plant community structure (e.g., biodiversity) and function is thus a critical goal of current applied ecology (Franklin, 1993, Roberts and Gilliam, 1995, Pimentel et al., 1997, Decocq et al., 2004, Young et al., 2005).

The management of cork oak forests in the Mediterranean Basin has been mainly oriented towards maximization of annual yield in cork production. For this purpose, the forest is typically divided into various management units which are periodically managed (roughly every nine years, depending on the locality) for cork extraction. Shrub clearing is a common silvicultural practice associated with cork extraction, which consists of cutting of shrubby cover and damaged trees every nine years, with the following objectives: (1) to facilitate the extraction of oak tree bark to obtain cork; (2) to increase cork production by reducing competition from neighbouring shrubs; and (3) to reduce the risk of fires by decreasing the amount of fuel. Traditionally, shrub clearing was carried out manually and only around productive cork oaks, saving money and energy, and indirectly minimizing the damaged area. Today, due to both public subsidies to forestry and availability of motor powered machinery, shrub clearing is far more intensive and extensive, producing a more homogeneous and intense perturbation in managed forests. Shrub clearing could negatively impact herbaceous plant species diversity in the forest understorey.

In general, forest management has been identified as one of the main causes of plant diversity loss (Gilliam and Roberts, 1995); however, an increasing number of studies show that silvicultural practices can have a positive or neutral effect on understorey plant species richness (e.g., North et al., 1996, Gale et al., 1998, Jenkins and Parkers, 1999, Battles et al., 2001). Nevertheless, the number of plant species (species richness) is only one component of biological diversity; the understorey species composition, the spatial scale (β- and γ-diversity), as well as other components, such as number of endemic species and taxonomic singularity of the elements (Ojeda et al., 1995), must be also taken into consideration (Zavala and Oria, 1995). In addition, the effects of silvicultural practices should be evaluated over a temporal sequence. This long-term perspective is critical for the design of sustainable and conservation-oriented management practices (Scarascia-Mugnozza et al., 2000, Rees et al., 2001).

In this study, we investigated the effects of shrub clearing on various components of understorey herbaceous biodiversity, and their temporal trends along a period of five years. Using cross-comparisons between shrub-cleared and paired unmanaged stands, we examined whether initial biodiversity values (previous to treatment application) were restored before completion of the cork extraction cycle. The study was conducted in three stands located within one of the largest cork oak forests in Europe, “Los Alcornocales” Natural Park (South Spain). The selected stands varied structurally, representing a wide range of cork oak forests in this region: an open woodland with nearby grasslands, a woodland with small trees and intermediate tree density, and a closed forest.

Specifically, we sought to answer the following questions: (i) What are the effects of shrub clearing on the different diversity components (α-, β- and γ- diversity) of the herbaceous understorey?; (ii) Is there any temporal change in herbaceous composition after shrub clearing?; (iii) What is the balance between local colonization and extinction rates?; (iv) Are these changes, if any, consistent across forest stands?; and (v) How persistent are these effects over a five-year period?

Section snippets

Study area and forest sites

The study area is located in the oak forests of Aljibe Mountains, near the Strait of Gibraltar, in Southern Spain. Bedrock is dominated by Oligo-Miocenic sandstone, with a rough relief and a highest peak of 1092 m a.s.l. Climate is subhumid mediterranean-type, with cool and wet winters, alternating with warm and dry summers. Mean annual temperature ranges from 14.6 to 18.4 °C, with a mean monthly maximum of 36 °C (July) and mean monthly minimum of 2 °C (January). Mean annual rainfall varies from

Results

In general, shrub-clearing treatment altered the different diversity components of the herbaceous community, though the effects were not persistent through time and varied as function of the forest site. In fact, effects associated with shrub clearing were most marked in the Open Woodland, while almost all were non-significant in the closed Forest site (Table 2 and Fig. 2). As expected, the percentage of woody plant cover in the shrub layer just after treatment application (2000) was much lower

Diversity components and turnover rates

The different diversity components were modified by the effect of shrub clearing practices, probably as a consequence of the new abiotic conditions resulting from structural changes in the shrub layer. In a parallel study (Quilchano et al., 2008), we demonstrated that shrub clearing reduced effective cover (LAI) of cork oak forests and, consequently, resulted in a higher light availability at ground level. In addition, the elimination of shrubs, which are piled and burned during these

Acknowledgements

We thank the “Consejería de Medio Ambiente” (Andalusian Government), and the then Director of Los Alcornocales Natural Park, Felipe Oliveros, for the facilities and support to carry out field work. We thank to Juan Arroyo for his help with the sampling design, and to Chelo Quilchano, Laura Noejovich, Carmen Navarro, Maite Domínguez and M. Antonio Santiago for their assistance in the field work. This study was supported by grant PFPU-MEC to IMPR, by the Spanish MEC projects Heteromed

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