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Chapter 10
Post-Fire Management of Exotic Forests Joaquim S. Silva and Hélia Marchante
10.1 Introduction The expression “exotic forests” may be associated to a wide range of forest types. One first differentiation should be made between the plantations of exotic trees established with a specific economic purpose, and the self-regenerated stands of naturalized species. According to the report on European Forest Types (EEA 2007) this wide range of forest formations is classified as Forest Type 14.2 – Plantations of not-site-native species and self-sown exotic forest. This forest type is included in the broader Forest Category 14 – Plantations and self-sown exotic forest. Plantations of not-site-native species include the forest stands aimed at providing raw material for industrial purposes. In many cases these are managed using intensive short-rotation silviculture, falling into what is frequently known as industrial forestry. Between the species most commonly used in commercial plantations in Europe we may find: Eucalyptus spp., Populus spp., Picea sitchensis, Pinus radiata, P. contorta, Pseudotsuga menziesii and Tsuga heterophylla. The second sub-type, self-sown exotic forest, includes species which have been naturalized in many European countries, like: Robinia pseudoacacia, Ailanthus altissima, Prunus serotina and different species of Acacia. However, the assignment of a species to one of
J.S. Silva (*) Centre of Applied Ecology, Institute of Agronomy, Technical University of Lisbon, Lisbon, Portugal H. Marchante College of Agriculture, Polytechnic Institute of Coimbra, Coimbra, Portugal Department of Life Sciences, Centre for Functional Ecology, University of Coimbra, Coimbra, Portugal
F. Moreira et al. (eds.), Post-Fire Management and Restoration of Southern European Forests, Managing Forest Ecosystems 24, DOI 10.1007/978-94-007-2208-8_10, © Springer Science+Business Media B.V. 2012
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these two sub-types of forest is not straightforward, as some of them may be represented in both groups. The cases of Robinia pseudoacacia and Eucalyptus spp. are good examples since both are used in plantations and both are referred as invasive species, as explained later in this chapter. Invasive species are able to regenerate and spread naturally, competing successfully with local species. In many situations the natural regeneration of these species intermingles with native species, originating mixed stands which are increasingly altering the forest composition of natural communities (Richardson 1998; Silva et al. 2011). In ecological terms, forest plantations are characterized by a highly simplified forest structure and composition and low richness of associated fauna, when compared to natural and semi-natural forests (EEA 2007), whereas self-sown exotic forests may present different composition and structure, depending on the characteristics of the species involved. The post-fire management of exotic woody species poses a series of different questions which basically depend: on fire severity, on the stand characteristics (plantation, self-sown forest), on the species composition previous to fire and on the objectives pretended for the burned area (which are very much associated with the stand characteristics). The post-fire management of a burned plantation is frequently directed to replant the same tree species, whereas the post-fire management of burned stands of self-regenerated exotic trees, either it is absent or, if existing, will most likely be directed to the conversion to a different forest type. This aspect is crucial because in many cases, self-sown exotic forests are the result of poor or absent management (Silva et al. 2011). Therefore, it is expectable that abandoned land colonized by invasive species, will stay abandoned after a wildfire, which will in many cases aggravate the invasion problem (Keeley et al. 2011). For either case we may have species which are relatively well adapted to fire occurrence, such as the case of many Eucalyptus spp. (Gill 1977, 1997; Gouveia et al. 2010; Marques et al. 2011) and others which have no specific traits known to be related with fire occurrence, like some North American conifers introduced in Europe. Considering this diversity of situations, it becomes clear that there is no typical procedure to be adopted in the post-fire management of exotic forests, contrarily to what may be the case in other forest types presented in this book. In fact, given the different variables to consider, there are different measures that can be taken. Because of the scarcity of information on the post-fire conversion of burned exotic plantations, we assume that in most situations, land managers will basically decide on replanting the burned stand or simply converting it to another productive type of plantation. However, these direct replacements of one plantation by another do not always follow good management practices, frequently presenting negative consequences at different levels, particularly in the conservation of soil and water regime (e.g. Shakesby et al. 2002; Smith et al. 2011). On the other hand there is a big challenge in managing the self-regenerated stands of exotic species because of the associated difficulties. These stands may even result from former or from existing burned plantations. In fact, as we will see further in the present chapter, forest plantations of exotic species can be one of the pathways leading to plant invasions, and fire may have a positive effect in this context. Therefore the paradigm of ecosystem restoration, as it is presented in this book for
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most forest types, is difficult to apply to the specific case of exotic forests, even if there are common aspects to consider such as the case of soil and water conservation, for example. This chapter aims at presenting a broad perspective on the post-fire management of exotic forests taking into account a European context and, given its increasing importance, clearly emphasizing the problem of invasive tree species. It is divided in two main sections. The first includes a characterization of the specific issue of exotic forests and fire in the European context. The second presents post-fire management alternatives, and includes detailed information for common types of exotic forests, taking into account their ecological and economic importance in southern Europe and their relevance in terms of post-fire management. An additional section presents two case studies concerning two important exotic species and their invasive behavior.
10.2 Exotic Forests and Fire 10.2.1 Basic Definitions and Concepts Exotic species can be classified in terms of their regeneration level and introduction pathway, among other criteria. According to the definitions by Lambdon et al. (2008) we apply the term exotic (the same as alien, introduced, non-native, or non-indigenous) to those species in a given area whose presence there is due to intentional or unintentional human involvement, or which have arrived there without the help of people from an area in which they are alien. From these we should distinguish different regeneration levels. Casual species are those that may reproduce occasionally outside cultivation in an area, but that eventually die out because they do not form self-replacing populations, and rely on repeated introductions for their persistence. Naturalized species are those that sustain self-replacing populations for a period of time long enough to experience extreme climatic events in the area, and reproduce without direct intervention by people (or in spite of human intervention) by recruitment from seed or vegetative parts capable of independent growth. Invasive species are a subset of naturalized plants that produce reproductive offspring, often in very large numbers, at considerable distances from the parent plants and thus have the potential to spread over a large area. The original definition from Richardson et al. (2000) presents additional criteria in order to quantify “considerable distances from the parent plants”: >100 m over 6 m over 3 years for species spreading by roots, rhizomes, stolons or creeping stems. Exotic species can also be classified according to their introduction pathways (Lambdon et al. 2008). Released species have been released deliberately into the wild (e.g., for the enrichment of the native flora, landscaping, etc.), whereas escaped species have escaped into the wild from cultivation. Escapes can be associated with different activities, including forestry (e.g. Eucalyptus spp.), ornamental purposes (e.g. Ailanthus altissima) or soil stabilization (e.g. Acacia spp.) for example.
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10.2.2 Distribution and Importance of Exotic Forests in Europe There is a clear correlation between the establishment of plantations and the area of forests dominated by exotic species (MCPFE 2007). In total, about 8.1 million ha, (5.2%) of the total forest area of Europe, are dominated by exotic tree species in 32 reporting MCPFE countries (Ministerial Conference for the Protection of Forests in Europe, excluding the Russian Federation), including 10% dominated by invasive species (MCPFE 2007). According to EEA (2007) European countries have a total of 7% of forests classified in forest category 14 – Plantations and self-sown exotic forest and there are 5 countries with more than 50% of forest area assigned to this forest category (Ireland, Belgium, The Netherlands, United Kingdom and Denmark, in decreasing order) (Table 10.1). Combining the information from EEA (2007) and from MCPFE (2007), France and Spain are the countries presenting the largest surface occupied by plantations and self-sown exotic woody species with more than two million ha each, followed by the United Kingdom and Germany, with more than 1.5 million ha each. If we retain only the countries from southern Europe, corresponding to higher fire proneness (JRC 2010), the country presenting the highest percentage of forest Category 14 is Portugal (18%), followed by France (15%) and Spain (12%). These figures may be underestimated at least for Portugal where, according to the last National Forest Inventory, there are 23% of Eucalyptus globulus stands (AFN 2010), all of them obviously corresponding to forest category 14. All other southern European countries are referred as having less than 10% of plantations and self-sown exotics. The MCPFE report (MCPFE 2007) presents an incomplete list of countries with data on the forest surface dominated by exotic species. The countries presenting the highest surfaces are the United Kingdom (1.4 million ha), France (1.1 million ha), Hungary (0.8 million ha) Sweden (0.6 million ha) and Italy (0.4 million ha). When looking at the forest surface of exotics which are considered invasive, Hungary leads the list (426,000 ha), followed by Italy (282,000 ha), the Russian Federation (55,000 ha), Slovakia (26,000 ha) and Austria (22,000 ha). However, this information is far from being complete and excludes some southern countries which are prone to invasive species, like Portugal and Spain. On the other hand, the criteria to consider a species as invasive may result in misleading information because it was not, up to our knowledge, based on quantitative studies. For example Eucalyptus globulus is referred by some authors as having invasive behavior in some areas in Portugal (Marchante et al. 2008a) and Spain (Dana et al. 2004) but was not apparently considered as so in the MCPFE report (MCPFE 2007). This calls for common procedures and common objective criteria among European countries in order to have a reliable assessment on the real situation in terms of exotic forests and invasive woody species (Hulme et al. 2009). According to estimations by Köble and Seufert (2001) the main tree species exotic to Europe in terms of occupied surface are (in decreasing order): Eucalyptus spp., Picea sitchensis, Robinia pseudoacacia, Pseudotsuga menziesii and Pinus contorta. According to the European database on exotic species (DAISIE 2008) there are 10 different species of Eucalyptus essentially distributed in the southern western Europe.
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In particular, Eucalyptus globulus is a very important species, occupying around one million ha in the Iberian Peninsula (Schelhaas et al. 2006). Robinia pseudoacacia is the most widely distributed exotic tree species in Europe being naturalized in 32 regions (Pyšek et al. 2009). Robinia pseudoacacia has been cultivated in several countries due to its excellent regeneration capacity and strong competitiveness, but now it accounts for half of the area dominated by invasive tree species in Europe. Within the southern European countries we should also consider Pinus radiata which has been widely planted in northern Spain particularly in the Basque Country, where it occupies around 150,000 ha (Mena-Petite et al. 2004; EEA 2008). Besides the referred species, we should also add some problematic invasive tree species even if they still do not occupy the top rank in terms of forest surface. The team of the European project DAISIE proposed a list of 100 of the worst invasive species in Europe (DAISIE 2009). This list is composed of 18 plant species including 4 tree species: Acacia dealbata, Ailanthus altissima, Prunus serotina and Robinia pseudoacacia.
10.2.3 When Exotics Become Invasive 10.2.3.1 Social and Ecological Aspects The problem of invasive species is becoming more evident as it is quantified in terms of economic costs. In Europe, most expenses generated by invaders are in the form of management costs (eradication, control, monitoring, and environmental education programs) in natural areas (Vilà et al. 2010). The crudest estimate of total known monetary impact of alien species (including animals and plants) in Europe is close to 10 billion Euros annually (Hulme et al. 2009). Andreu et al. (2009) have evaluated the economic costs of management initiatives associated with invasive species only in natural protected areas of Spain as more than 50 million Euros in 10 years. From this amount, around 60% was allocated to the management of Eucalyptus spp. Nonetheless and despite the enormous amounts of funds which are allocated to solve or at least to mitigate the problem of plant invasions, these are perceived by stakeholders as not so important, in comparison to other environmental concerns, according to a survey performed in Spain by Andreu et al. (2009). At the European level only 2% of the public feel biological invasions as important threats to biodiversity (Hulme et al. 2009). Therefore, in what concerns the particular issue of post-fire management, it is not surprising the lack of concern even amongst specialists. Within the European COST FP0701 Project (Post-fire forest management in southern Europe) a survey was distributed among country representatives about the importance of invasive exotic species in burned areas. The results of the questionnaire showed that among the 14 respondents only 4 (Israel, Lithuania, Portugal and Switzerland) considered invasive species in burned areas a relevant problem. The tree species referred by the respondents were: Acacia spp. (Israel, Portugal), Pinus halepensis (Israel), Robinia pseudoacacia (Lithuania, Switzerland) and Ailanthus
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Table 10.1 Estimated surface occupied by plantations and self-sown exotic forests (Category 14) according to EEA (2007); estimated surface occupied by introduced forest species (Type 14.2) according to MCPFE (2007); and estimated surface occupied by invasive tree species according to MCPFE (2007) Forest surface Category 14 Type 14.2 Invasive Country 1000 ha 1000 ha % 1000 ha % 1000 ha % Andorra 16 0 0 − − − − Austria 3,862 − − 53 1 22
