Do eucalypt plantations provide habitat for native forest biodiversity?
Highlights
► Understory diversity and composition in plantations and other communities was compared. ► Diversity was lower in plantations than in native communities at local scales. ► Species composition of eucalypt plantations was similar to shrublands when young. ► Native forests were the most distinctive community, and harboured the rarest species. ► Eucalypt plantations provide low habitat value to support native biodiversity.
Introduction
In many countries, plantation forestry depends on a few fast-growing tree species, often exotic. These species contribute significantly to the economic growth of many regions, but may also produce substantial changes in natural ecosystems, with impacts on biodiversity and ecosystem services, which can be nevertheless reduced with proper management, thus increasing the sustainability of this important economic sector (Richardson, 1998, Hartley, 2002).
Tree plantations are expanding worldwide, while natural forests are in decline and increasingly fragmented (FAO, 2010). Temperate forests have been highly influenced by human activities through history and their extension has been dramatically reduced throughout the world, mainly due to overexploitation and conversion to farmland and tree plantations (Hannah et al., 1995, Teixido et al., 2010). Among them, oak (e.g. Quercus robur) forests harbor a rich biodiversity, which is endangered as a consequence of their decline (Castro et al., 2001). In fragmented landscapes, species persistence depends on their ability to use different habitats, so that less suitable habitats may still favor the connectivity of the most suitable habitats in the landscape (Tischendorf and Fahrig, 2000, Lindenmayer, 2009). Remaining fragments of native habitats such as forests are often surrounded by a matrix of modified semi-natural habitats, such as tree plantations and crops, which can still provide habitat for species associated to natural forests (Lindenmayer and Hobbs, 2004, Brockerhoff et al., 2008). Given the relevance of tree plantations in many regions and the potential similarity between them and natural forests, at least in terms of abiotic conditions, it is important to determine their ability to harbor native biodiversity which would favor the connectivity of the landscape and, with it, the persistence of native populations at a regional scale (McIntyre and Hobbs, 1999, Bender and Fahrig, 2005).
The potential role of tree plantations for biodiversity conservation is controversial. Some point to lower biodiversity in plantations as compared to natural forests while others highlight their capacity to harbor native biodiversity (reviewed in Stephens and Wagner, 2007, Brockerhoff et al., 2008, Bremer and Farley, 2010). However, as pointed out by Stephens and Wagner (2007), many studies on the contribution of plantations to biodiversity are based on inappropriate comparisons. An appropriate comparison depends largely on the aims of the plantations, but, in a general sense, plantations should be at least compared with the land uses they usually replace. This should include comparisons to those replaced both directly and indirectly, i.e. considering also the potential vegetation in the replaced areas after natural succession. In addition, plantations are usually even-aged and subject to management. Management intensity and the age and structure of the stands determine the ability of plantations to harbor biodiversity (Lindenmayer and Hobbs, 2004). For example, plantations experience changes in environmental conditions related to age (e.g. canopy closure). These changes affect species differently, according to their habitat preferences, and may result in differences in diversity, structure and species composition between ages (Bunn et al., 2010). Management practices such as periodic clearing of understory vegetation might have more drastic effects than any competitive or allopathic effects of the planted trees (Atauri et al., 2004). For this reason, it is important to specify the management context of the study sites. Moreover, comparisons with other communities are often based on species diversity but do not consider differences/similarities in species composition, which is essential to assess the possible role of plantations in harboring native biodiversity.
Eucalypts (chiefly E. globulus, E. grandis, E. nitens and E. camaldulensis) have become the most widely planted hardwood species in the world, mostly for paper production. The success of eucalypts can be attributed to their fast growth, coppicing ability, wide adaptability to soils and climate, and the unpalatability of its leaves (Turnbull, 1999). However, the expansion of eucalypt plantations has raised concerns regarding ecosystem degradation (e.g. loss of biodiversity) and eucalypts have become a major source of conflicts between foresters and conservationists in many countries. Studies on the effects of eucalypt plantations on biodiversity are surprisingly scarce considering the spread of these plantations around the world, but some point to lower biodiversity in eucalypt plantations as compared to other communities (e.g. Bará-Temes et al., 1985, Pina, 1989, Basanta et al., 1989, Proença et al., 2010a). In contrast, other studies show their positive effect in regeneration of native understory plants (e.g. in Ethiopia, Michelsen et al., 1996, Yirdaw and Luukkanen, 2003).
In this study, we analyzed the diversity and composition of understory plants in the most common plant communities in the natural and semi-natural landscape in the study region (NW Spain): low-management eucalypt plantations of three different age-stages (young, intermediate and mature) as compared to natural forests, shrublands and pine plantations, which are the communities most commonly replaced by eucalypt plantations, directly or indirectly, and thus are the most appropriate for comparison (Stephens and Wagner, 2007). Understory plants are used here as appropriate biodiversity indicators as their diversity is often correlated with that of other taxonomic groups (Kati et al., 2004, Rodrigues and Brooks, 2007). In addition, understory plants are especially significant in temperate forests in terms of total diversity, and favor the development and diversity of other groups (Gilliam, 2007). They are also important ecosystem service providers (Quijas et al., 2010). Our aim in this study is to assess the value of eucalypt plantations to favor habitat connectivity at a landscape scale by determining their ability to provide habitat for species associated with the natural vegetation.
Section snippets
Study area context
The NW of Spain (Galicia) is the most important forestry region in the country (Manuel and Gil, 2002). E. globulus has been intensively cultivated in this region, mostly for the paper industry (Riesco, 2004), and has multiplied its cover by 6 over the last 25 years, both by intentional planting and natural spread (Manuel and Gil, 2002). Most plantations are privately owned in small, scattered holdings (83% of the area covered by eucalypts in pure stands) which leads to excessive land
Biovolume
Plant biovolume in the understory (of herbs, shrubs, trees, and total) differed among habitats (χ25df > 6.6, P < 0.001, Fig. 1). Shrub biovolume clearly determined the trend in total biovolume, which decreased from shrublands to native forests, with intermediate values in eucalypt and pine plantations (Fig. 1). Native forests had the highest tree species biovolume in the understory (seedlings, small saplings), followed by mature eucalypt and pine plantations (Fig. 1). Herbaceous biovolume was
Diversity patterns
At local scales (plot and site), diversity was generally higher in native communities (native forests and shrublands) and lower in plantations. Total diversity, i.e. accumulated across all study sites, was lowest in eucalypt plantations in intermediate age, but was relatively high in other plantations, and similar to that in native forests. This was mainly due to a higher dissimilarity among sites (species turnover) in young and mature eucalypt plantations. In the case of young plantations,
Conclusions
Low-management plantations can harbor high amounts of plant biovolume in the understory although with relatively low local diversity. Although successional changes were found within the understory of eucalypt plantations with increasing age, key components of native forests were not found in mature eucalypt forests. Pine plantations supported an understory that was more similar to that in native forests, but failed also to provide a suitable habitat for the most characteristic species of native
Role of the funding source
The sponsors had no role on the collection, analysis and interpretation of data; in the writing of the manuscripts; nor in the decision to submit the article for publication.
Acknowledgements
We thank Cristina E. Ramalho for helpful discussions on the study, Cristina Vences and Paola Acuña for assistance in the field, and Adolfo Cordero for comments on the manuscript. This research was funded by Project 08MRU024371PR (Consellería de Innovación e Industria, Plan INCITE, Xunta de Galicia) to MCC. MCC was supported by an Isidro Parga Pondal Contract (Xunta de Galicia). Comments of two anonymous referees and the editor contributed to increase the quality of the manuscript.
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