For decades, conservation and research initiatives in tropical forests have focused almost exclusively on old-growth forests because scientists believed that these “pristine” ecosystems housed superior levels of biodiversity. With Second Growth, Robin L. Chazdon reveals those assumptions to be largely false, bringing to the fore the previously overlooked counterpart to old-growth forest: second growth.
Even as human activities result in extensive fragmentation and deforestation, tropical forests demonstrate a great capacity for natural and human-aided regeneration. Although these damaged landscapes can take centuries to regain the characteristics of old growth, Chazdon shows here that regenerating—or second-growth—forests are vital, dynamic reservoirs of biodiversity and environmental services. What is more, they always have been.
With chapters on the roles these forests play in carbon and nutrient cycling, sustaining biodiversity, providing timber and non-timber products, and integrated agriculture, Second Growth not only offers a thorough and wide-ranging overview of successional and restoration pathways, but also underscores the need to conserve, and further study, regenerating tropical forests in an attempt to inspire a new age of local and global stewardship.
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The Promise of Tropical Forest Regeneration in an Age of Deforestation
By Robin L. Chazdon
The University of Chicago PressCopyright © 2014 The University of Chicago
All rights reserved.
PERCEPTIONS OF TROPICAL FORESTS AND NATURAL REGENERATION
Indigenous knowledge does indeed hold valuable information on the role that species play in ecologically sustainable systems. Such knowledge is of great value for an improved use of natural resources and ecological services, and could provide invaluable insights and clues for how to redirect the behavior of the industrial world towards a path in synergy with the life-support environment on which it depends.—Gadgil et al. (2003, p. 156)
1.1 Viewing Forests as a Cycle
In the holistic worldview of indigenous resource managers, the forest has no end and no beginning—the forest is a cycle that is managed to provide for their needs. The Dayak are forest-dwelling tribes in Borneo with a deep knowledge of forest regeneration and management. For over 4,000 years, their lives have revolved around a system of shifting cultivation based on the regenerative capacities of the tropical forest ecosystem. Their life cycle is completely interwoven with the forest's own regeneration cycle. The Benuaq Dayaks of Datarban in East Kalimantan recognize that many factors affect the rate of forest regeneration, including soil conditions, rainfall, temperature, slope, and aspect. They define five phases of forest regeneration following a short (1–2 yr.) period of cultivation in swidden fields (ladang) that are carved out of the primary forest. Within 1–3 years after cultivation ceases, dense young scrub (kurat uraq) covers the former swidden field (Poffenberger and McGean 1993). This phase can take 3–5 years if soils are compacted, eroded, or heavily leached, and is dominated by light-demanding grasses, perennial shrubs, herbs, and fast-growing tree species that reach heights of 3–4 meters.
The second phase (kurat tuha) follows 2–5 years after fields are fallowed. Trees reach 5 centimeters or more in diameter and heights of about 5–6 meters. The undergrowth is filled with dense shrubs, lianas (woody vines), and herbaceous species that grow rapidly at high light levels. After 3–10 years on good quality soil, the third phase begins (kurat batang muda). Pioneer trees now reach 10–15 centimeters in diameter. The upper canopy begins to close, reducing light in the understory. Understory grasses and herbs then start to die out. After 9–16 years, depending on soil quality, the fourth (and longest) phase begins (kurat batang tuha ). Shade-tolerant tree saplings fill in the spaces in the understory vacated by shrubs, herbs, and grasses. The forest canopy is now virtually closed by trees with diameters well over 10 centimeters. Over the next 100–180 years, the forest changes gradually in structure, composition, and spatial complexity until it returns to the status of primary (old-growth) forest (hutan bengkar;fig. 1.1).
The Dayaks' understanding of natural regeneration strikingly resembles that of forest ecologists and foresters. This knowledge has been part of their cultural tradition for as long as there have been Dayaks (Sardjono and Samsoedin 2001; Setyawan 2010). Dayak farmers have careful procedures for determining whether a site is suitable for opening a new swidden field. One test involves cutting a length of stem from a wild ginger plant and burying it in the forest soil. If the stem resprouts within 3–5 days, the conditions of soil moisture and nutrient availability are considered suitable for cultivation. Certain tree species in the Dipterocarpaceae family and certain understory herbs are additional indicators of desirable soil conditions. When the field is cleared, large valuable trees are spared to provide timber and honey in future years. Stumps are left to resprout as coppice regeneration, accelerating regrowth and reducing weed growth.
The Dayaks' sophisticated understanding of forest regeneration is shared by other forest-dwelling peoples who have practiced shifting cultivation and harvested forest products for millennia (Wiersum 1997). Over 16,000 kilometers away, the Yucatec Maya have practiced shifting cultivation for over 3,000 years in the northern Maya lowlands of Mesoamerica. Their way of life also depends on understanding and managing forest regeneration. Their vocabulary includes 6 terms for successional stages of the forest and more than 80 terms for soil characteristics (fig 1.2; Gómez-Pompa 1987; Barrera-Bassols and Toledo 2005). The Soligas of the Western Ghats in India have developed similar traditional knowledge to support their shifting-cultivation lifestyle (Madegowda 2009). Indigenous peoples have learned how forests respond to different types of disturbance and which species of plants and animals appear and proliferate in different phases of forest regeneration. They recognize particular species as indicators of soil conditions. Traditional ecological knowledge enables them to practice adaptive management, adjusting their impact on the forests to sustain regeneration, even over multiple cycles of cultivation.
1.2 The Resilience of Tropical Forests
Traditional shifting cultivators have developed adaptive management practices that sustain the forest regeneration cycle. Their lives and the future of their forest areas depend on this partnership. When the cycle is broken, forests lose their intrinsic capacity to regenerate. There is a limit to the resilience of tropical forests, to their ability to recover from disturbances and reassemble all of their parts. The traditional practices of the Dayaks of Datarban are now no longer possible, because the area of forest available for their use is insufficient to allow long-term rotations (Poffenberger and McGean 1993). In their traditional practice, forests would not be cleared until they reached the fourth stage (kurat batang tuha). But increasing population pressures on limited forest lands have forced many farmers to reduce their fallow periods to 5–10 years. Young fallows are being cleared for fields before they have fully recovered soil organic matter and nutrients and before weeds have been controlled by regrowth. Fires are frequent in young scrub, suppressing woody regrowth and favoring weeds and invasive grasses, such as Imperata cylindrica. This new regimen reduces the forest's regenerative capacity. The shifting cultivation system that was sustainable for over 4,000 years has now become unsustainable (Coomes et al. 2000; Lawrence et al. 2010).
Tropical forests are often considered to be highly fragile and vulnerable ecosystems because of their complex vertical structure, high species diversity, and intricate networks of species interactions (Chazdon and Arroyo 2013). Immediately following a major disturbance, devastation appears irreversible. However, what makes a forest ecosystem fragile is not its immediate destruction but rather interference with its intrinsic resilience (see figs. 1.1 and 1.2). Tropical forests, like all ecosystems, are naturally subjected to disturbances of variable intensity, frequency, and duration. Forest ecosystems are always in flux. Forces of nature, such as hurricanes, floods, or volcanic eruptions, cause major disturbances. Human activities—such as shifting cultivation, permanent cultivation, grazing, and logging—also cause major disturbances by removing and fragmenting vegetation and degrading soils. Human and natural disturbances often act together to influence forest dynamics. For example, selective logging increases the susceptibility of forests to burning.
Resilience is a feature of a complex adaptive system that is capable of self-reorganization in the aftermath of disturbance—the capacity to return, over time, to a state similar to the predisturbance state (Holling 1973; Levin 1998). This reorganization, in the case of ecosystems, is embodied in the concept of succession and in the forest regeneration cycle (see fig. 1.1; Messier et al. 2013). The gradual reassembly process can take more than a century, especially for ecosystems composed of hundreds of species of long-lived trees. Moreover, each phase in the process can be influenced by factors internal and external to the system (Bengtsson et al. 2003; Chazdon and Arroyo 2013). Tropical forest regeneration is influenced by prior land use, initial colonization, climate, soils, and seed dispersal from forests in the surrounding landscape. When obstacles to regeneration prevail, succession is arrested and a new type of ecosystem develops, such as a grass- or fern-dominated ecosystem (see box 7.1). In these cases, careful human intervention is needed to get succession back on track. Evaluating the resilience of tropical forests involves more than understanding the ecological process of forest regeneration. Humans are active participants in both the disturbance and the recovery of natural systems; the delineation between social and ecological systems is artificial and arbitrary (Berkes and Folke 1998). Resilience is a feature of the combined socioecological system that includes both ecological and human components.
1.3 Forest Regeneration, Succession, and Forest Degradation
Following a small, localized disturbance, spontaneous forest regeneration occurs in disturbed patches within the forest matrix. Gap-phase regeneration is a normal part of forest dynamics (see box 4.1). During forest succession, the spatial unit undergoing regeneration is the entire forest stand (Chokkalingam and de Jong 2001). Regeneration is a commonly used term to describe regrowth following forest disturbance at a range of spatial scales, analogous with the regeneration, regrowth, or reconstitution of tissues or organs following damage or loss. Natural regeneration can apply to an individual tree or population, a single tree species, a small forest patch, an entire stand, an assemblage, or an ecosystem, and refers to the regrowth or reestablishment of these units.
Succession is a process linked to the assemblage of species composing a particular ecosystem. Natural regeneration of populations, species, and assemblages occurs throughout all stages of succession. Trees that regenerate during the early phases of succession constitute a different set of species from trees that regenerate during later phases of succession (see box 5.1). Following selective logging disturbances, many trees remain standing in the forest, but they may sustain considerable damage. Successional processes in formerly selectively logged forests drive stand-level changes in structure and composition, but from a different starting point than in former agricultural sites.
The designation naturally regenerating forest used in the Forest Resource Assessment (FRA) of the United Nations Food and Agriculture Organization (FAO) can be applied to forests regenerating following selective logging of trees or to forests regrowing on former agricultural land that was completely cleared (FAO 2010). A "primary forest" is relatively stable in structure and composition, whereas a "secondary forest" develops after the original forest has been cleared and regenerates spontaneously. The term secondary forest also widely refers to selectively logged forests, creating much ambiguity (Chokkalingam and de Jong 2001). Throughout this book, I distinguish between selectively logged forests and second-growth forests on cleared land.
In 2011, the FRA published a working paper entitled "Assessing forest degradation: Towards the development of globally applicable guidelines" (FAO 2011). More than 50 definitions of forest degradation have been formulated for different applications (Lund 2009). Forest degradation is caused by humans and is often ascribed to poor or inadequate management or misuse. In 2002, the International Tropical Timber Organization estimated that up to 850 million hectares of tropical forest and forest land were in a degraded state (ITTO 2002). The International Union for the Conservation of Nature (IUCN) and the Global Partnership for Forest Landscape Restoration (GPFLR) have initiated a global effort to restore 150 million hectares of degraded and deforested land by 2020, a fraction of the 2 billion hectares worldwide that offer opportunities for landscape-level restoration (GPFLR 2012). According to ITTO, a degraded forest supplies fewer goods and services and maintains limited biological diversity (ITTO 2002, 2005). The Convention on Biological Diversity (CBD) stated that "a degraded forest is a secondary forest that has lost, through human activities, the structure, function, species composition or productivity normally associated with a natural forest type expected on that site" (Secretariat of the Convention on Biological Diversity 2002, p. 154). If we were grading a forest's condition on a US university scale, a degraded forest would receive a grade less than an A (94%–100%). But it could be anywhere from an A–to an F (failing).
These definitions imply that any forest regenerating on former pastures or cultivated fields is a degraded forest, because the structure, function, composition, productivity, and services are reduced compared to the original, or "natural," forest. Indeed, this inclusive approach is implicit in the CBD's definition. But this definition reflects underlying confusion over the definition of secondary forest. Putz and Redford (2010) proposed that secondary forests that develop after complete deforestation should be distinguished from degraded forests, which are derived from old-growth forests and retain some of the original forest structure and composition. Secondary forests are young, second-growth forests that take time to develop the features of a "mature," or old-growth, forest in the same region and climate zone. Yet young forests are labeled degraded simply because they are young, regardless of their potential for recovery. They are often viewed as "damaged goods." In fact, the term secondary implies that these forests are secondary in quality and in value. For this reason, I prefer to use the term second-growth, or regenerating, forests in this book.
How do we escape this quagmire of terms? The way forward is to view forests as dynamic entities. The current application of the label degraded forest to any forest that has been impacted by human activities (hunting, logging, fragmentation, grazing, or cultivation) reflects a static view of the forest that fails to consider the potential for spontaneous natural regeneration to regain "lost" properties. Rather, forests should be viewed as resilient systems, with intrinsic capacities to reorganize and recover. If all forests affected by humans are lumped into a single category of degraded forest by forestry, biofuel industry, and climate mitigation policies, we are overlooking one of the greatest conservation opportunities in human history. In India, ecosystems that experience moderate to heavy disturbance are classified as wastelands (Ravindranath et al. 1996). The way forward is to move beyond the labels of degradation, deforestation, and devastation to promote forest regeneration, regrowth, and restoration on a massive scale. We can work toward re-creating sustainable forest regeneration cycles and rebuilding resilient ecosystems. We should not simply condemn all forests affected by human activities and auction them off to the highest bidder. If forests can regrow following complete sterilization of the island of Rakata in 1883, our hope can be restored too (see box 6.1).
1.4 The Geographic Extent of Deforestation and Forest Regeneration across the Tropics
Deforestation in the tropics is a major environmental issue of our time, with far-reaching consequences for earth's biodiversity, climate, and life-support systems (MEA 2005). During the last 100 years, old-growth tropical forests have been cleared and replaced with agriculture, pastures, plantations, and young regenerating forests at unprecedented rates. From 1990 to 2000, 8.6 million hectares were deforested across the tropics, including wet and dry zones (see plate 1 for "hot spots" of deforestation; Mayaux et al. 2005). Approximately 42% of the world's tropical forests are seasonally dry forests, which have experienced even higher rates of deforestation than humid tropical forests. Only 16% of the dry forests of South and Southeast Asia remained in 2001, compared to 40% in Latin America (Miles et al. 2006).
Excerpted from Second Growth by Robin L. Chazdon. Copyright © 2014 The University of Chicago. Excerpted by permission of The University of Chicago Press.
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Table of Contents
List of Illustrations
Perceptions of Tropical Forests and Natural Regeneration
1.1 Viewing Forests as a Cycle
1.2 The Resilience of Tropical Forests
1.3 Forest Regeneration, Succession, and Forest Degradation
1.4 The Geographic Extent of Deforestation and Forest Regeneration across the Tropics
1.5 The Tropical Forests of the Future
Ancient Human Legacies in Tropical Forest Landscapes
2.2 The Peopling of the Tropics
2.3 Impacts of Early Hunter-Gatherer Societies
2.4 The Development of Agriculture
2.5 Holocene Climate Variability, Forest Change, and Agricultural Expansion
Landscape Transformation and Tropical Forest Regeneration through Prehistory
3.2 Earthworks and Landscape Transformations
3.3 Prehistoric Fires: Synergies between Natural and Human Causes
3.4 Ancient Soil Modifications
3.5 The Scale of Prehistoric Human Impacts in the Neotropics
3.6 Paleoecological Reconstruction of Tropical Forest Regeneration
Tropical Forest Dynamics and Disturbance Regimes
4.2 Disturbance Regimes in Tropical Forest Regions
4.3 Gap Dynamics and the Forest Growth Cycle
4.4 Detection of Tropical Forest Disturbance
4.5 Are Old-Growth Tropical Forests Stable?
Successional Pathways and Forest Transformations
5.2 Variability in Successional Pathways
5.3 Successional Stages and Species Classification
5.4 Forest Definitions and Concepts
5.5 Approaches to Studying Tropical Forest Succession
Tropical Forest Succession on Newly Created Substrates
6.2 Biological Legacies and Local Resource Availability
6.3 Colonization and Succession on Landslides
6.4 Succession following Volcanic Eruptions
6.5 Riverbank Succession
Forest Regeneration following Agricultural Land Uses
7.2 Effects of Land Use and Biological Legacies on Propagule Availability and Modes of Regeneration
7.3 Effects of Land Use on Site Quality and Resource Availability
Forest Regeneration following Hurricanes and Fires
8.2 Hurricane Damage and Regeneration
8.3 Tropical Forest Regeneration after Single and Recurrent Fires
Forest Regeneration following Selective Logging and Land-Use Synergisms
9.2 Harvesting Intensity, Forest Disturbance, and Postlogging Forest Regeneration
9.3 Effects of Logging on Animal Abundance and Diversity
9.4 Consequences of Land-Use Synergisms for Forest Regeneration
Functional Traits and Community Assembly during Secondary Succession
10.2 Environmental Gradients during Succession
10.3 Successional Changes in Life-Form Composition
10.4 Functional Traits of Early and Late Successional Species
10.5 Environmental Filtering, Functional Diversity, and Community Assembly during Succession
10.6 A General Scheme for Community Assembly during Secondary Succession
Recovery of Ecosystem Functions during Forest Regeneration
11.2 Loss of Nutrients and Carbon during Conversion of Forest to Agriculture
11.3 Accumulation of Carbon and Nutrients during Forest Regeneration
11.4 Nutrient Cycling and Nutrient Limitation
11.5 Hydrology and Water Balance
Animal Diversity and Plant-Animal Interactions in Regenerating Forests
12.2 Animal Diversity in Regenerating Forests
12.3 Plant-Herbivore Interactions during Forest Regeneration
12.4 Seed Dispersal and Predation during Forest Regeneration
12.5 Pollination in Regenerating Forests
Tropical Reforestation Pathways
13.2 Reforestation Goals and Decisions
13.3 Reforestation through Management of Forest Fallows
13.4 Ecological Forest Restoration in the Tropics
13.5 Recovery of Biodiversity during Reforestation
13.6 Recovery of Ecosystem Properties during Reforestation
Regenerating Forests in Tropical Landscapes
14.2 Land-Use Transitions and Forest Transitions
14.3 The Landscape Context of Forest Regeneration
14.4 Socioecological Drivers of Tropical Reforestation
14.5 Enhancing Forest Regeneration and Human Livelihoods in the Landscape Matrix
Synthesis: The Promise of Tropical Forest Regeneration in an Age of Deforestation
15.1 The Power of Forest Regeneration
15.2 Tropical Forest Change and Resilience
15.3 The Current and Future Value of Regenerating Tropical Forests
15.4 New Approaches to Promoting Forest Regeneration