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5 - Economic Impacts and Drivers of Deforestation
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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- Global Deforestation
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Summary
Background
The classic economic issue surrounding deforestation is whether and how forest resources are allocated optimally in order to maximize net benefits to people (Sills and Pattanayak, 2006). Net benefits are the value of goods and services (people's willingness to pay in terms of money or some other valuable resource) minus their opportunity costs (what people have to pay or what resources they have available to invest to obtain the goods and services) (Sills and Pattanayak, 2006). The opportunity costs of forested land can vary considerably (Chomitz et al., 2006; Table 5.1). For instance, in Brazil's cerrado region, converting native woodlands to soy results in land worth more than $3,000 per hectare, whereas land in the Atlantic forest of Bahia, Brazil, is worth just $400 per hectare. As discussed by Sills and Pattanayak (2006), net benefits can be calculated from two perspectives. Private benefits and costs directly affect the families or companies making decisions about how resources are allocated. The private net benefits of deforestation are equal to the value of returns from agriculture minus the value of forgone future forest production. Future forest production includes timber that could be sold, as well as goods consumed directly from forest products (i.e., nontimber forest products). However, these private benefits and costs do not include adverse environmental effects, termed “environmental externalities,” that result from deforestation (e.g., see Table 5.2). The other perspective on net benefits takes into account the cost of externalities to calculate social or public benefits and costs. Many socially valuable goods and services do not have prices because they are public goods and are not traded in markets. Public goods and services refer to those whose benefits cannot be denied to anyone and cannot be divided up and sold. For example, biodiversity and carbon sequestration are public goods provided by forests.
When forest is initially cleared, there are both immediate and future costs and benefits of the harvested timber. The future stream of net benefits from land uses such as agriculture and ranching is equal to the value of outputs minus the cost of inputs. To compare costs and benefits at their equivalent present value, future values are adjusted by a discount rate. The discount rate can vary considerably in time and with location (e.g., Weitzman, 1994). For instance, interest rates in Madagascar have varied between 5% and 22.8% (Kremen et al., 2000).
Frontmatter
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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2 - Hydrological and Climatic Impacts
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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- Global Deforestation
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Summary
Introduction
Forest ecosystems strongly affect the water cycle through their impact on evapotranspiration, precipitation, infiltration, runoff, and, consequently, soil erosion and stream-flow (see following sections). The removal of forest vegetation leads to an increase in water yields (e.g., Bosch and Hewlett, 1982; Section 2.4) and a shift in the predominant mechanism of runoff generation (Dunne and Black, 1970a; Dunne, 1978; Section 2.3). It also enhances snowpack accumulation and shortens the snowmelt season (Section 2.4). Moreover, in deforested watersheds, evapotranspiration is strongly reduced (Section 2.7). The impact on precipitation is more complex: Large-scale (i.e., >105 km2) deforestation is expected to reduce regional precipitation (e.g., Bonan, 2008a), though the effect of forest removal also depends on synoptic patterns of atmospheric circulation and geographic setting (e.g., latitude, location with respect to mountain ranges and oceans). The deforestation of small watersheds (<10 km2) is not expected to have a substantial impact on precipitation, whereas the clearing of intermediate sized areas (15,000–50,000 km2 [Lawrence and Vandecar, 2015]) could increase local precipitation (see also Chapter 4). Deforestation can also affect the hydrologic conditions and microclimate of nearby (downwind) ecosystems (Ray et al., 2006).
Landmasses receive water as precipitation and lose it either as water vapor fluxes into the atmosphere (evapotranspiration) or as surface and subsurface flows in the liquid phase (runoff). In recent years, these two fluxes have been named green water and blue water flows, respectively, to stress the fact that evapotranspiration receives a strong contribution from vegetation (transpiration) (Falkenmark and Rockstrom, 2004; Figure 2.1). As explained in the following sections, the overall effect of deforestation on the hydrologic cycle is a decrease in water vapor fluxes and increase in runoff. Thus, green water flows decrease and blue water flows increase. This means that more water is likely to become available for societal withdrawals (but also for environmental uses) in areas located downstream from the cleared watershed. In turn, forest management can strongly impact the water resources of a watershed, and sometimes the thinning of woody vegetation has been proposed as an option to increase water availability in semiarid areas (e.g., Ingebo, 1971; Griffin and McCarl, 1989). Such an approach, however, has only limited applicability because forest removal and the consequent increase in overland flow have the effect of increasing sediment yields and soil erosion rates, thereby damaging the landscape, often irreversibly within human timescales.
Index
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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Contents
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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6 - Synthesis and Future Impacts of Deforestation
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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Summary
Benefits of Preserving Forests
Forests provide an expansive range of environmental benefits (Table 5.2) that have local to regional (e.g., flood control) and global (e.g., carbon sequestration) relevance (Foley et al., 2007). Hydrological benefits of forests include regulating water supply and river discharge (i.e., moderating high and low flows) by increased transpiration, water storage beneath the forest, and increased travel time for water to reach streams/rivers. Climate benefits of forests include maintaining available precipitation via precipitation recycling (in areas where this feedback exists) and regulating local and global temperature both directly – by reducing diurnal sensible heat fluxes and nocturnal radiative cooling – and indirectly − by taking up atmospheric CO2 during photosynthesis. For instance, a review by Lawrence and Vandecar (2015) highlighted that complete deforestation of the tropics would lead to a 0.1–1.3°C increase in temperature across the tropics and drying of approximately –270 mm yr–1 (or up to 10%–15% decrease of annual rainfall). The presence of forests can also affect edaphic processes by reducing soil erosion and enhancing soil formation. Biogeochemical benefits of forests include enhancing nutrient availability and reducing nutrient losses, thereby increasing the amount of nutrients available for plant uptake and aiding in sustaining forest growth. Forests also provide many ecological services such as maintaining biodiversity and regulating a range of dynamical trophic relationships. As discussed in Chapter 4, forests can be important for maintaining their own habitat, possibly reducing the occurrence of disturbances such as fire or landsliding, and, in turn, maintaining the wide span of benefits described previously. Forests provide many economic benefits to societies from nontimber forest products (NTFPs) that are harvested from them. These NTFPs provide food (e.g., nuts, honey, bush meat, and fruits), medicine, construction materials (e.g., rubber), bioprospecting (i.e., value for new pharmaceutical products), and agricultural products such as fodder for livestock. Forests also have recreation, cultural, intellectual, aesthetic, and spiritual values that are important to society. Thus, deforestation strongly affects the environment and society. In the following sections we review and summarize some of its major impacts.
Ecohydrological and Climate Impacts of Deforestation
Most of the past research on the hydrological impacts of deforestation has focused on changes in water yields and flow regulation. The removal of forest vegetation causes a reduction in soil infiltration and evapotranspiration and, consequently, an increase in infiltration-excess runoff and soil erosion.
Preface
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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Summary
Deforestation disrupts hydrological processes, climate, biogeochemical cycling, and socioenvironmental dynamics. It can lead to irreversible losses of biodiversity, natural capital, and rural livelihoods, while favoring an unsustainable use of natural resources and enhancing unbalanced relationships between private benefits and public losses associated with land clearance. Deforestation is a disturbance because it leads to biomass losses over timescales much shorter than those needed for forest regeneration. In some cases recovery is not possible because the disturbance induces a shift in forest ecosystems to a permanently deforested state by impacting the availability of resources and environmental conditions that are necessary for forest regeneration.
According to the 2010 Food and Agriculture Organization (FAO) Forest Resource Assessment, forests cover 41 billion hectares, or 31% of the global land surface, yet used to cover nearly 50% of the global land surface 8,000 years ago. While the current rate of deforestation has decreased since the 1990s from 16 million ha yr−1 to 13 million ha yr−1, it remains relatively high. Deforestation alters the coupled natural and human systems with important impacts on the potential for forests to regenerate. Understanding these impacts is also important in light of international programs that seek to provide financial incentives for reduced deforestation and have an estimated market potential of U.S. $10 billion.
This book is motivated by the need for a comprehensive cross-disciplinary analysis of the existing literature on global deforestation. We review the geography of deforestation, analyze the major drivers and effects of forest loss, and examine theories as well as empirical evidence on how forests affect their natural environment. We stress how forest removal may cause the loss of important ecosystem functions, leading to a permanent and nearly irreversible shift to a treeless state. We investigate the biotic-abiotic feedbacks that determine the stability and resilience of forest ecosystems and analyze the socioeconomic processes underlying current patterns of deforestation. While doing so, we review a large number of recent studies on this body of literature and synthesize information across disciplines, thereby bridging the physical and biological sciences with the social sciences.
This analysis addresses a broad readership of ecologists, hydrologists, economists, biogeochemists, geographers, resource analysts, and policy makers whose work is related to deforestation. As such, it was written with the goals of readability and accessibility by both social and natural scientists.
Plate section
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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4 - Irreversibility and Ecosystem Impacts
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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Summary
Background on Irreversibility and Bistability in Deforested Ecosystems
In this chapter, we examine the potential situations in which deforestation induces a change in the physical and/or the chemical environment that leads to a loss of environmental conditions necessary to sustain forest vegetation. The reversibility or irreversibility of deforestation is often determined by the absence or presence of positive feedbacks of adequate strength.
Sudden and often irreversible changes in the structure and functioning of ecosystems are typically associated with the existence of multiple stable ecosystem states (e.g., May, 1977; Holling, 1973). We focus on the case of a system that can be stable both with and without forest vegetation (e.g., Box 4.1), although bistable forest dynamics can also emerge in systems with two forested states but with different species compositions (e.g., Pastor and Post, 1988; Ridolfi et al., 2008). The presence of alternative states or “attractors” is commonly associated with positive feedbacks (i.e., a sustained sequence of processes) between forest vegetation and its physical environment, though bistability may emerge in nonlinear dynamics even in the absence of such feedbacks (e.g., Ridolfi et al., 2011; Petraitis, 2013). A change among attractors may be an effect of changes in environmental conditions or disturbance regime that are sustained by changes in forest vegetation (e.g., Wilson and Agnew, 1992). The magnitude of the perturbation required to push the system into the basin of attraction of the stable “deforested state” depends on the resilience of the “forest state,” a property defined as the ability of the system to recover that state after a disturbance (Holling, 1973). The occurrence of shifts between ecosystem states depends both on the magnitude of the external disturbance and on the resilience of the initial state of the system (e.g., Folke et al., 2004). As the resilience of an ecosystem's state declines, it becomes increasingly vulnerable to state shifts such that progressively smaller external events can cause regime shifts (Holling, 1973). When a disturbance imposed on a system causes some critical bifurcation point to be passed, this can produce a shift to an alternative stable state (e.g., a state of low vegetation) (Kuznetsov, 1995; Figures 4.1 and 4.2).
3 - Biogeochemical Impacts
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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Summary
Terrestrial vegetation is often limited by nutrients, particularly nitrogen and/or phosphorus, and in some cases potassium, calcium, sulfur, magnesium, silica, and other micronutrients or trace minerals (Vitousek and Howarth, 1991). On a global scale, Fisher et al. (2012) found the average reduction in terrestrial plant productivity due to nutrient limitation to be between 16% and 28% (Figure 3.1). One major factor altering patterns of nutrient cycling is land use change due to deforestation. Deforestation alters nitrogen and phosphorus cycling, both of which can feed back to affect the carbon cycle and atmospheric CO2 concentrations. In this chapter, we examine carbon, nitrogen, and phosphorus cycling beneath forests and the effect of deforestation on these cycles.
Carbon Cycle
In this section, we briefly describe the carbon cycle in undisturbed forests to provide the basis for understanding how deforestation alters this cycle. Next, we examine global pools of carbon stored in forests and the soils beneath these forests as well as the rate at which forests across different latitudinal belts take up carbon. Finally, we consider how deforestation might alter the carbon balance of forests and forest soils.
Carbon Cycle in Undisturbed Forests
Gross primary production (GPP) of an ecosystem represents the gross uptake of atmospheric CO2 that is used for photosynthesis. Plants use energy in the synthesis of new plant tissue and the maintenance of living tissues (Luyssaert et al., 2007). Because of the costs associated with growth and maintenance of leaves, wood, and roots, some photoassimilated compounds are lost from the ecosystem as autotrophic respiration (Ra) (Figure 3.2; Luyssaert et al., 2007). The fraction that is used for maintenance respiration can vary widely (i.e., 0.23–0.83 for different forest types as determined from a literature review of 60 different studies), yet in many forest studies it is generally assumed to be a constant value at 0.5 (DeLucia et al., 2007). The energy that is not used for respiration is the net primary production (NPP) and is equal to GPP – Ra. A large fraction of NPP is used in the production of leaves, wood, and roots and a portion of the standing biomass is transferred annually to litter (Luyssaert et al., 2007). This carbon enters the soil when C contained in leaf and woody litter, dead roots, mycorrhizal turnover, and carbon exudates from roots are transferred to the forest soil (Figure 3.2).
References
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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1 - Introduction: Patterns and Drivers
- Christiane Runyan, The Johns Hopkins University, Paolo D'Odorico, University of Virginia
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Summary
Definitions and Classifications of Forest Ecosystems
Forests, which can be defined as woody plant communities in areas that are large enough to modify the local environment and microclimate (Chang, 2002), currently cover 3.8 billion hectares, roughly 30% of the Earth's land surface (FAO, 2010). Approximately 25% of the world's forested area is located in Europe, followed by South America (21%), North and Central America (17%), Africa (17%), Asia (15%), and Oceania (5%) (Table 1.1). The most extensive forest biomes are tropical, boreal, and temperate (Figure 1.1). Tropical forests cover approximately 1.76 billion hectares (or 42% of the world's forested area), followed by boreal forests with their roughly 1.37 billion hectares (or ~33% of the world's forests), and temperate forests (1.04 billion hectares, or 25% of the world's forested areas) (IPCC, 2000).
Forest ecosystems play a fundamental role in the dynamics of the Earth system and provide services of great environmental, societal, and economic value. They are a major determinant of the regional and global climate (Chapter 2), modulate water and nutrient cycling (Chapter 3), and provide invaluable resources and services (Chapter 5) that have played a crucial role for the social, economic, and cultural development of several civilizations. Depending on how resources derived from forests are used, they can be either renewable (i.e., not depleted) or nonrenewable (Chang, 2002). Moreover, the rise and fall of several civilizations in human history have been determined by their use and overuse of forests (Box 1.1).
This book is concerned with the ongoing phenomenon of global deforestation (Box 1.2). This Introduction and the following chapters will discuss the major drivers along with the environmental and societal implications of deforestation. It will also analyze social-environmental processes that affect the stability and resilience of forest ecosystems and their ability to recover after deforestation.
Box 1.1 Deforestation and the Collapse of Past Civilizations
Geographers and anthropologists have often related the decline of past civilizations to environmental degradation resulting from deforestation. Deforestation might have triggered the collapse of the Viking, Maya, Anasazi, and Rapa Nui civilizations (Diamond, 2005; Turner and Sabloff, 2012). In most of these cases, deforestation enhanced soil erosion, thereby leading to the permanent loss of soil resources. In Chapter 4, we will discuss some positive feedbacks that could prevent forest regeneration.
Global Deforestation
- Christiane Runyan, Paolo D'Odorico
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- 05 April 2016
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- 18 April 2016
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Global Deforestation provides a concise but comprehensive examination of the variety of ways in which deforestation modifies environmental processes, as well as the societal implications of these changes. The book stresses how forest ecosystems may be prone to nearly irreversible degradation. To prevent the loss of important biophysical and socioeconomic functions, forests need to be adequately managed and protected against the increasing demand for agricultural land and forest resources. The book describes the spatial extent of forests, and provides an understanding of the past and present drivers of deforestation. It presents a theoretical background to understand the impacts of deforestation on biodiversity, hydrological functioning, biogeochemical cycling, and climate. It bridges the physical and biological sciences with the social sciences by examining economic impacts and socioeconomic drivers of deforestation. This book will appeal to advanced students, researchers and policymakers in environmental science, ecology, forestry, hydrology, plant science, ecohydrology, and environmental economics.