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Ecological Impacts of Southern Pine Beetle Maria D Ecological Impacts of Southern Pine Beetle Maria D. Tchakerian1 and Robert N. Coulson2 1Associate Research Scientist, Department of Entomology, Knowledge Engineering Laboratory, Texas A&M University, 15 College Station, TX 77843-2475 2Professor, Department of Entomology, Knowledge Engineering Laboratory, Texas A&M University, College Station, TX 77843-2475 Abstract The southern pine beetle (SPB) is the most important biotic disturbance in southern pine forests and causes extensive changes to the forest environment. In this chapter we provide an overview of the ecological impacts of the SPB on forest conditions (the state of the forest) and on forest resources (uses and values associated with the forest). We define ecological impact as the effects—positive or negative—of Keywords SPB activities on the forest ecosystem. The impact on forest conditions is the result of widespread tree mortality, which affects ecological processes such as: biotic disturbance primary production, nutrient cycling, forest succession, and forest composition ecological impact and configuration. We discuss how the SPB affects these ecological processes forest succession through modification of the physical environment and the temporal distribution hydrology of resources. For the ecological impact on forest resources, we emphasize the nutrient cycling impacts of SPB on resources that are affected from an ecological point of view primary production (e.g., hydrology and wildlife). Changes in forest structure resulting from SPB wildlife habitat herbivory can modify key hydrologic processes that control the quantity and quality of water reaching a stream. The ecological impacts of the SPB on wildlife are the result of changes in the distribution and abundance of plant species and insect populations. Increases in SPB densities directly affect the food available for insectivore birds, mainly bark-foraging woodpeckers. The impacts on wildlife have been deduced from changes in wildlife habitat as a result of SPB infestations. An approach to estimate the impacts of SPB herbivory on wildlife habitat in a forest landscape is introduced. 223 15.1.INTRODUCTION The structure, function, and processes of Evaluating the ecological effects of the SPB has forest ecosystems have evolved with natural been a more difficult task than characterizing disturbances such as fire, windthrow, and its social and economic impacts. Understanding pest epidemics (Crow and Perera 2004). In of the natural role of the insect is essential for a the Southeastern United States, southern pine true understanding of the impact (Stark 1987). beetle (Dendroctonus frontalis Zimmermann) Insufficient and inadequate historical data and (SPB) is the most important biotic disturbance lack of ecological theory have limited the efforts and, along with fire, regulates the dynamics of to evaluate the probable role of the SPB in nutrient cycling and succession of pine forests. forest ecosystems. In spite of this, considerable Southern pine beetle outbreaks cause extensive research has been made on the role of the beetle change in forest conditions (the state of the forest as a regulator of southern forest ecosystems at environment) by modifying the processes of multiple scales (Coulson and Stephen 2006, primary production, nutrient cycling, ecological Raffa and others 2008, Schowalter and others succession, and the size, composition, and 1981a). configuration of forest trees. The cumulative effects of SPB outbreaks can impact forest The goal of this chapter is to present a current resources (uses and values associated with the overview of the existing information on the forest environment) such as timber production, impacts of SPB herbivory on forest ecosystems. water quality and quantity, fish and wildlife The specific objectives are: 1. to consider the populations, recreation, grazing capacity, real effects of the SPB on forest conditions (the estate values, biodiversity, endangered species, state of the forest) and 2. to examine the effects and cultural resources (Coulson and Stephen of SPB herbivory on forest resources (uses and 2006). It is obvious that the impact on forest values associated with the forest). Figure 15.1 resources results in economic losses to forest outlines the organization of the topics covered landowners (economic impact) and affects how in this chapter. Although forest resources are humans perceive and use the forest environment considered in this review, we place emphasis (social impact); these topics are discussed in on the impacts of the SPB on resources that are chapters 14 and 16 respectively. affected from an ecological point of view (e.g., water quality and quantity, wildlife, endangered For the purposes of this chapter, ecological species). impact of the SPB is defined as the effects of SPB herbivory on the forest ecosystem. The effects can be perceived as qualitative or quantitative 15.2.SPBimpact on forest changes in conditions and resources associated with the forest ecosystem, and can be positive CONDITIONS or negative (Coulson and Stephen 2006). The Widespread tree mortality by SPB herbivory most evident impact of SPB herbivory on modifies the structure, composition, and pine forests ecosystems is the reduction or function of southern pine ecosystems. This mortality of host tree species. Tree mortality section examines how the SPB brings about by insect herbivory results in increased light change in the forest environment. It considers penetration through the forest canopy, reduced how SPB outbreaks influence the abundance, competition among plants, changes in plant composition, and configuration of forest species composition and biomass, increased vegetation through modifications of the physical rates of water runoff and nutrient leaching, environment and the spatial and temporal higher rates of litter decomposition, and distribution of resources. The ecological redistribution of nutrients (Coulson and Witter processes of primary production, nutrient 1984). One ecological effect of SPB outbreaks cycling, ecological succession, and forest is the shift of forest structure from mature composition and configuration are addressed in and overmature host trees to regenerating further detail. seedlings and competing vegetation species (Land and Rieske 2006). This shift is known 15.2.1.PrimaryProductionand to influence other ecological processes such as NutrientCycling nutrient redistribution, ecosystem succession, Little is known about the effects of SPB and alteration of wildlife habitat (Coulson and outbreaks on ecosystem processes such as Witter 1984). primary production and nutrient cycling. Insect herbivory may stimulate forest productivity by 224 Tchakerian | Coulson Ecological Impact of the Southern Pine Beetle Impacts on Impacts on Forest Conditions Forest Resources Figure 15.1—Diagram outlines the topics and organization of chapter Effects on Primary Effects on Water 15. The probable impacts Quality and of the SPB on forest Production and conditions and forest Nutrient Cycling Quantity resources are examined. Effects on Forest Effects on Wildlife Succession Habitat Effects on Forest Compositions and Configuration selectively killing less productive plants or SPB outbreak have been reported. In contrast, plant parts, therefore enhancing light, water, Romme and others (1986) reported that after and nutrient availability for the survivor a mountain pine beetle (MPB) (Dendroctonus individuals (Mattson and Addy 1975). In ponderosae Hopkins) outbreak, stand level general, nutrients become locked up in living primary productivity declines and leads to a biomass as mature ecosystems tend to have more equitable distribution of biomass and more closed nutrient cycles, with internal resources. With time, individual surviving nutrient cycling exceeding nutrient input and plants respond to the changes produced by the output (Schowalter 1981). Insect herbivory beetle outbreak and plant growth accelerates. accelerates the nutrient cycling by weakening/ The authors described growth increases of 20- killing plants and increasing nutrient transfer 70 percent in canopy lodgepole pines, and by from these nutrient-rich plants to the litter/soil 60-260 percent in understory vegetation for a complex. In addition, plant mortality releases period of 5-20 years after the outbreak. resources such as space, nutrients, and light, increasing the establishment and vigor to other 15.2.2.ForestSuccession vegetation. The SPB is the primary biotic agent affecting yellow pine forest ecosystems, and, in association Successfully SPB-infested trees are usually with fire, determines the successional dynamics dead within weeks of colonization. Resource in these forests (Clarke and others 2000, distribution occurs as a result of leaf fall Coleman and others 2008, Lafon and others episodes lasting a few months and the falling of 2007, Waldron and others 2007). Schowalter dead trees over a period of years (Romme and and others (1981a) suggested the interaction others 2006). The SPB selectively kills suitable of SPB herbivory and fire disturbance as a mature host trees, and subsequent mortality mechanism to maintain early-successional increases nutrient release from pine biomass southeastern coniferous forests, therefore and increases growth of remaining trees preventing ecosystem development toward later- (Schowalter 1981). A review of the literature successional, shade-tolerant hardwood forest. revealed that no direct estimates on primary These authors proposed that herbivory by the production and nutrient
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