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Austral Ecology -
"The book deals with a topic of major conservation importance, is wide-ranging in scope, and includes many genuinely insightful chapters written by leading researchers."
"The book deals with a topic of major conservation importance, is wide-ranging in scope, and includes many genuinely insightful chapters written by leading researchers."
"A thought provoking and current overview of the sources of risk to the biodiversity of tropical forests. It would make for a useful volume for a graduate or advanced undergraduate seminar class. Its summaries of emerging threats constitute a report card on how well scentists and consertvationists are doing in grasping the magnitude and complexity of human-caused changes in the tropics."
"In his foreword, Thomas E. Lovejoy writes that 'this book is unquestionably the best and most up-to-date effort to document the chilling panoply of threats to tropical forests.'--I do agree!"
"With 23 essays by 49 contributors, Emerging Threats to Tropical Forests offers a pan-tropical overview of the formidable challenges we continue to face in the field of tropical forest conservation....An excellent book. . . . All of the contributing authors bring original insights and critical information to our ongoing consideration of the problem of tropical forest conservation. This book is especially recommended for tropical ecologists, resource managers, and conservation educators, but it is relevant for all readers interested in the future of this fundamentally important global resource."
— John Kanowski
— Kenneth R. Young
— Francois M. Catzeflis
— Karin Rita Gastreich
Simon L. Lewis, Oliver L. Phillips, and Timothy R. Baker
With contributions by M. Alexiades, S. Almeida, L. Arroyo, S. Brown, J. Chave, J. A. Comiskey, C. I. Czimczik, A. Di Fiore, T. Erwin, N. Higuchi, T. Killeen, C. Kuebler, S. G. Laurance, W. F. Laurance, J. Lloyd, Y. Malhi, A. Monteagudo, H. E. M. Nascimento, D. A. Neill, P. Núñez Vargas, J. Olivier, W. Palacios, S. Patiño, N. C. A. Pitman, C. A. Quesada, M. Saldias, J. N. M. Silva, J. Terborgh, A. Torres Lezama, R. Vásquez Martínez, and B. Vinceti
Ecosystems worldwide are changing as a result of numerous anthropogenic processes. Some important processes, such as deforestation, are physically obvious, whereas others, such as defaunation and surface fires, may be subtler but affect biodiversity in insidious ways (cf. Lewis et al. 2004b; W. F. Laurance, chap. 5 in this volume; Terborgh and Nuñez-Iturri, chap. 13 in this volume). Atmospheric changes, such as increased rates of nitrogen deposition and especially increases in air temperatures and increasing carbon dioxide concentrations, are altering the environment of even the largest and most well-protected areas (e.g., Prentice et al. 2001; Galloway and Cowling 2002; Malhi and Wright 2004). Anthropogenic atmospheric change will certainly become more significant during this century; atmospheric carbon dioxide concentrations are likely to reach levels unprecedented for the past 20 million or even 60 million years (Retallack 2001; Royer et al. 2001). Nitrogen-deposition rates and climates are predicted to move far beyond that of Quaternary envelopes (Prentice et al. 2001; Galloway and Cowling 2002). Moreover, the rate of change in all these basic ecological drivers is likely to be without precedent in the evolutionary span of most species on Earth today (Lewis et al. 2004a). This, then, is the Anthropocene: we are living through truly epoch-making times (Crutzen 2002).
Given the scale of these changes, it is clear that all ecosystems on Earth are already very likely to have been altered by human activities. Recent research suggests that seemingly undisturbed tropical forests that are far from areas of deforestation are indeed undergoing profound shifts in structure, dynamics, productivity, and function. In this chapter, we draw together, review, and synthesize the recent results from a network of long-term monitoring plots across tropical South America that indicate how these forests are changing.
Changes in tropical forest structure, dynamics, productivity, and function are of great societal importance for three reasons: First, tropical forests play an important role in the global carbon cycle and hence the rate of climate change; about 40% of terrestrial carbon stocks lie within tropical forests (Malhi and Grace 2000). Second, because tropical forests house at least half of all Earth's species, changes in these high-biodiversity forests will have a large impact on global biodiversity (Groombridge and Jenkins 2003). Finally, because different plant species vary in their ability to store and process carbon, changes in both climate and biodiversity are potentially linked by feedback mechanisms (e.g., P. M. Cox et al. 2000; Lewis 2005). Overall, small yet consistent changes across the tropical forest biome are likely to have critical impacts on the global carbon cycle, global biodiversity conservation, and the rate of climate change-and hence human welfare.
Recent evidence suggests that remaining South American (and perhaps other tropical) rainforest is currently an important global carbon sink (Malhi and Grace 2000). The evidence is from (1) long-term monitoring plots, which show that forest stands are increasing in aboveground biomass (Phillips et al. 1998, 2002b; Baker et al. 2004a); (2) micrometeorological techniques, which indicate that mature Amazon forests may be a carbon sink (Grace et al. 1995; Araujo et al. 2002; Malhi et al. 2002b), albeit with substantial seasonal and interannual variability; and (3) inverse modeling of atmospheric carbon dioxide concentrations, which shows that tropical ecosystems may contribute a carbon sink of between 1 and 3 gigatons (1 gigaton = 1 billion metric tons) per year (e.g., Rayner and Law 1999; Rodenbeck et al. 2003).
These findings of a substantial tropical carbon sink are consistent with modeling and laboratory studies that imply changes in the productivity of tropical forests in response to increasing carbon dioxide (e.g., Lloyd and Farquhar 1996; Norby et al. 1999; Lewis et al. 2004a). However, the reader should note that all three approaches contain unresolved and currently debated possible areas of error:
1. The possibility exists that some of the biomass increase from forests may reflect forest recovery from episodic disturbance events, which may cause rapid and large biomass losses, whereas recovery is slow and steady; therefore, in this case, if by chance a limited number of monitoring sites fail to include rare, large-disturbance events, they may lead to erroneous extrapolations (Körner 2003). 2. Micrometeorological techniques contain errors in night-time flux measurements, which are not yet adequately addressed (e.g., Baldocchi 2003). 3. The very small number of air-sampling locations and poorly understood atmospheric transport across the tropics leave findings from inverse modeling studies open to debate (Houghton 2003).
In addition, the idea that atmospheric carbon dioxide increases are causing the tropical carbon sink is also controversial (for a recent discussion, see Lewis et al. 2004a). However, because all three lines of evidence suggest the existence of a sink-and efforts to overcome limitations in each line of research have generally confirmed the presence of a sink-it is reasonable to suggest that tropical forests likely provide a substantial buffer against global climate change. Indeed, the results from long-term forest-monitoring plots suggest that intact Amazonian forests have increased in biomass by about 0.3% to 0.5% per year and, hence, if all tropical forests are similarly increasing, they sequester carbon at approximately the same rate that the European Union (in January 2004) emits it by burning fossil fuels (Phillips et al. 1998; Malhi and Grace 2000; Baker et al. 2004a).
Large-scale environmental changes, such as increasing atmospheric carbon dioxide concentrations and rising air temperatures, will alter fundamental ecological processes and in turn will likely effect important changes in tropical biodiversity. In fact, this has already occurred in better-studied temperate areas (e.g., Parmesan and Yohe 2003) and in a well-studied old-growth tropical forest landscape (W. F. Laurance et al. 2004b). The interactive "balance" among tens of thousands of tropical plant species and millions of tropical animal species is certain to shift, even within the largest and best-protected forest ecosystems, which are traditionally thought of as "pristine" wilderness. These areas are vital refugia-where global biodiversity may most easily escape the current extinction crisis-because they are large enough to allow some shifts in the geographic ranges of species in response to global changes and are afforded some protection from industrial development such as logging and agriculture. However, how most tropical forest taxa will respond to rising temperatures and carbon dioxide concentrations, among other global changes, is currently unknown (Thomas et al. 2004).
Biodiversity change has inevitable consequences for climate change because different plant species vary in their ability to store and process carbon. For example, shifts in the proportion of faster-growing light-demanding species may alter the carbon balance of tropical forests. Long-term forest-plot data show that mature humid neotropical forests are a net carbon sink of ca. 0.6 gigatons per year (Phillips et al. 1998; Baker et al. 2004a). However, tree mortality rates have increased by as much as about 3% per year in recent decades, causing an increase in the frequency of tree-fall gaps (Phillips and Gentry 1994; Phillips et al. 2004). A shift in the composition of forests toward gap-favoring, light-demanding species with high growth rates, at the expense of more shade-tolerant species, is plausible (Körner 2004). Such fast-growing species generally have lower wood specific gravity, and hence lower carbon content for a given size (West et al. 1999), than do shade-tolerant trees. An Amazon-wide decrease in mean wood specific gravity of just 0.4% would cancel out the current carbon sink effect that is apparently caused by accelerated plant productivity. Whether such changes are occurring is currently poorly understood, but it is clear that the biodiversity and climate-change issues are closely linked and merit further study.
In this chapter, we present a summary of the latest findings from permanent plots monitored by a large network of Amazon-forest researchers, known as RAINFOR (Red Amazónica de Inventarios Forestales, or Amazon Forest-Inventory Network). The studies associated with RAINFOR have the following goals (Malhi et al. 2002c):
1. Quantify long-term changes in forest biomass and stem turnover. 2. Relate current forest structure, biomass, and dynamics to local climate and soil properties. 3. Attempt to understand the extent to which climate and soils will constrain future changes in forest dynamics and structure. 4. Attempt to understand the relationships among productivity, mortality, and biomass. 5. Use these results to predict how changes in climate may affect the biomass and productivity of the Amazon forest as a whole and to inform basin-scale carbon-balance models. 6. Search for evidence of change in forest composition and biodiversity (tree and liana species) over time. 7. Create a forum for discussion and standardize sampling methods among different research groups.
Here we summarize findings from old-growth forests in terms of (1) structural, (2) dynamic-process, and (3) functional change over the past two decades. Details of the exact plot locations, inventory and monitoring methods, and issues relating to collating and analyzing plot data are largely omitted from this chapter for reasons of space but are discussed in detail elsewhere (Phillips et al. 2002a, 2002b, 2004; Baker et al. 2004a, 2004b; Lewis et al. 2004b; Malhi et al. 2004). In addition, the evolving debates following the discovery that stem turnover had increased across the tropics since the 1950s (Phillips and Gentry 1994; Sheil 1995; Phillips 1995, 1996; Condit 1997; Phillips and Sheil 1997) and that long-term-monitoring plots in Amazonia increased in biomass during the 1980s and 1990s (Phillips et al. 1998; D. A. Clark 2002; Phillips et al. 2002b) are also relevant.
THE PLOT NETWORK
A RAINFOR plot is an area of forest where all trees above 10 cm diameter at breast height (dbh, measured at 1.3 m height or above any buttress or other deformity) are tracked individually over time. All trees are marked with a unique number, measured, mapped, and identified. Periodically (generally, approximately every 5 years) the plot is revisited; all surviving trees are remeasured, dead trees are noted, and trees recruited to 10 cm dbh are uniquely numbered, measured, mapped, and identified. This allows the calculation of (1) the cross-sectional area that trees occupy (termed "basal area"), which can be used with allometric equations to estimate tree biomass (Baker et al. 2004a); (2) tree growth (the sum of all basal-area increments for surviving and newly recruited stems over a census interval); (3) the total number of stems present; (4) stem recruitment (the number of stems added to a plot over time); and (5) mortality (either the number or basal area of stems lost from a plot over time). We present data from 50 to 91 plots, depending on selection criteria for different analyses (most critically, the number of census intervals from a plot and whether only stem-count data or the full tree-by-tree data set is available). The plots span South America (fig. 1.1), including Bolivia, Brazil, Ecuador, French Guiana, Peru, and Venezuela. Most are 1 ha in size and comprise about 600 trees of greater than or equal to 10 cm dbh. The smallest are 0.4 ha and the largest is 9 ha, all large enough to avoid undue influence by the behavior of an individual tree (Chave et al. 2003). Many plots have been monitored for more than a decade, although they range in age from 2 to 25 years. The earliest plot inventory was taken in 1967, the latest in 2002.
Among 59 plots monitored in old-growth Amazon forests where we have access to the full tree-by-tree data, there has been a significant increase in aboveground biomass between the first and last time they were measured. Over approximately the past 20 years, the increase has been 0.61 ± 0.22 tons of carbon per ha per year, or a relative increase of 0.50 ± 0.17% per year (mean ± 95% confidence interval; Baker et al. 2004b). Across all plots, the aboveground biomass change is normally distributed and shifted to the right of zero (fig. 1.2). This estimate is slightly higher than that documented by Phillips et al. (1998), who used a smaller and earlier data set, and similar to that documented by Lewis et al. (2004b), who used a smaller data set, where each plot was monitored over much longer periods of time.
We can crudely estimate the magnitude of the South American carbon sink by multiplying 0.61 tons per ha per year by the estimated area of mature neotropical humid-forest cover (ca. 8,705,100 [km.sup.2]; FAO 1993), which yields a value of about 0.5 gigatons of carbon per year. If we further assume that the ratio of aboveground to belowground biomass is 3:1 (cf. Phillips et al. 1998), and that belowground biomass is increasing in proportion to aboveground biomass, then the sink increases to 0.7 gigatons of carbon per year. If other biomass components, such as small trees, lianas, and coarse woody debris, are also increasing in biomass, then the sink may be fractionally larger still. However, these estimates depend critically on (1) how representative of South American forests are the 59 plots studied by Baker et al. (2004a) and (2) assumptions about the extent of mature, intact forest remaining in South America. Both areas of uncertainty suggest that we treat these extrapolations with caution.
D. A. Clark (2002) raised two concerns about the original findings of Phillips et al. (1998) that Amazon biomass was increasing, suggesting that (1) some floodplain plots that Phillips et al. considered mature may still be affected by primary succession, and that (2) large buttress trees in some plots may have been measured not above the buttress, as protocols dictate, but around it. However, neither potential error leads to a significant overestimation of aboveground-biomass increases because the carbon sink remains when either all plots on old floodplain substrates or those that may have buttress problems are removed from the analysis (Baker et al. 2004a).
Among 91 plots monitored across South America, there was a significant increase in stem density between the first and last time they were measured. The increase has been 0.84 ± 0.77 stems per ha per year (fig. 1.3; paired t-test, t = 2.12, P = 0.037), or a 0.15 ± 0.13% per-year increase (Phillips et al. 2004). Across all plots, stem-change rates are approximately normally distributed and slightly shifted to the right of zero (fig. 1.3). (The number of plots used here is more than that used in the biomass study, largely because complete tree-by-tree data are required to calculate biomass using Baker et al.'s [2004a] methods, whereas stem-change data can often be obtained from published studies.) The same test using 59 plots (from the study by Baker et al. 2004a) shows a similar increase in stem density (0.16 ± 0.15% per year), whereas a smaller but much longer-term data set (50 plots from Lewis et al. 2004b) shows a slightly larger increase (0.18 ± 0.12% per year).
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