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How do size distributions relate to concurrently measured demographic rates? Evidence from over 150 tree species in Panama
- Renato A.F. Lima, Helene C. Muller-Landau, Paulo I. Prado, Richard Condit
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- Journal:
- Journal of Tropical Ecology / Volume 32 / Issue 3 / May 2016
- Published online by Cambridge University Press:
- 11 April 2016, pp. 179-192
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In stable populations with constant demographic rates, size distributions reflect size-dependent patterns of growth and mortality. However, population growth can also affect size distributions, which may not be aligned with current growth and mortality. Using 25 y of demographic data from the 50-ha Barro Colorado Island plot, we examined how interspecific variation in diameter distributions of over 150 tropical trees relates to growth–diameter and mortality–diameter curves and to population growth rates. Diameter distributions were more skewed in species with faster increases/slower decreases in absolute growth and mortality with diameter and higher population growth rates. The strongest predictor of the diameter distribution shape was the exponent governing the scaling of growth with diameter (partial R2 = 0.20–0.34), which differed among growth forms, indicating a role of life history variation. However, interspecific variation in diameter distributions was also significantly related to population growth rates (partial R2 = 0.03–0.23), reinforcing that many populations are not at equilibrium. Consequently, although fitted size distribution parameters were positively related to theoretical predictions based on current size-dependent growth and mortality, there was considerable deviation. These analyses show that temporally variable demographic rates, probably related to cyclic climate variation, are important influences on forest structure.
Chapter Fourteen - Detecting and projecting changes in forest biomass from plot data
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- By Helene C. Muller-Landau, Smithsonian Tropical Research Institute, Matteo Detto, Smithsonian Tropical Research Institute, Ryan A. Chisholm, Smithsonian Tropical Research Institute, Stephen P. Hubbell, Smithsonian Tropical Research Institute, Richard Condit, Smithsonian Tropical Research Institute
- Edited by David A. Coomes, University of Cambridge, David F. R. P. Burslem, University of Aberdeen, William D. Simonson, University of Cambridge
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- Book:
- Forests and Global Change
- Published online:
- 05 June 2014
- Print publication:
- 20 February 2014, pp 381-416
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Summary
Introduction
Increasing atmospheric carbon dioxide, changing climates, nitrogen deposition and other aspects of anthropogenic global change are hypothesised to be changing forest productivity and biomass stocks in tropical forests and elsewhere (Clark 2004; Lewis, Malhi & Phillips 2004; Lewis et al. 2009a; Luo, 2007; Myeni et al. 1997). These hypotheses continue to be much debated, with contrary views on the plausibility of particular mechanisms and on the status of current evidence for or against them (Clark 2007; Friedlingstein et al. 2006; Holtum & Winter 2010; Körner 2009; Wright 2005, 2010). The influence of atmospheric and climate change on forest biomass is of particular interest because of the potential for positive or negative feedbacks. Increases in forest biomass and associated carbon pools would slow the rise in atmospheric carbon dioxide, producing a negative feedback, whereas decreases in forest biomass would have the opposite effect. Uncertainty surrounding these feedbacks is considerable at the global scale, with important implications for global carbon budgets (Luo 2007).
In view of this, it is essential to know whether forests are experiencing changes in productivity and biomass in excess of those typical for their age. Successional forests, those regrowing after disturbances, increase in biomass over time, with the trajectory and duration of this increase varying with forest type (Bormann & Likens 1979; Odum 1969). In the absence of global change, such forests are expected to eventually reach a dynamic equilibrium in which biomass gains from growth and recruitment are balanced by biomass losses from tree death and branchfall, and these old-growth forests thus experience no directional changes in biomass (Odum 1969; Yang, Luo & Finzi 2011). Accordingly, detection of directional changes in biomass in old-growth forests is generally considered evidence of global change influences. When and where such changes are detected, the next critical question concerns prediction of future net carbon fluxes and ultimate carbon stocks of such altered forests.
Demography and biomass change in monodominant and mixed old-growth forest of the Congo
- Jean-Remy Makana, Corneille N. Ewango, Sean M. McMahon, Sean C. Thomas, Terese B. Hart, Richard Condit
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- Journal:
- Journal of Tropical Ecology / Volume 27 / Issue 5 / September 2011
- Published online by Cambridge University Press:
- 02 August 2011, pp. 447-461
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Mbau forest covers much of the Congo, and shifts in its composition could have a large impact on the African tropics. The Ituri forest in east Congo is near a boundary between the monodominant mbau type and non-mbau mixed forest, and two 20-ha censuses of trees ≥ 1 cm diameter were carried out over 12 y to monitor forest change. Based on published diameter allometry, mbau forest had 535 Mg ha−1 biomass above ground and gained 1.1 Mg ha−1 y−1. Mixed forest had 399 Mg ha−1 and gained 3 Mg ha−1 y−1. The mbau tree (Gilbertiodendron dewevrei) increased its share of biomass from 4.1% to 4.4% in mixed forest; other common species also increased. Sapling density declined at both sites, likely because increased biomass meant shadier understorey, but the mbau tree increased in sapling density, suggesting it will become more important in the future. Tree mortality and growth rates were low relative to other tropical forests, especially in the mbau plots. Shifting toward G. dewevrei would represent a large gain in carbon in the mixed forest, but mbau is presently more important as a high-carbon stock: biomass lost during forest harvest could not recuperate for centuries due to slow community dynamics.
Dissecting biomass dynamics in a large Amazonian forest plot
- Renato Valencia, Richard Condit, Helene C. Muller-Landau, Consuelo Hernandez, Hugo Navarrete
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- Journal:
- Journal of Tropical Ecology / Volume 25 / Issue 5 / September 2009
- Published online by Cambridge University Press:
- 01 September 2009, pp. 473-482
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Above-ground biomass (AGB) is increasing in most of the Amazon forests. One hypothesis is that forests are responding to widespread and intense human intervention prior to the European conquest (>500 y ago). In this study we confront this hypothesis with changes in AGB over 6.3 y in a large western Amazonian forest plot (>150 000 shrubs and trees and 1100 species with dbh ≥ 10 mm in 25 ha). We examined AGB flux in different habitats and across diameter classes. The forest lost small stems (4.6%), gained large trees (2.6%), and gained biomass (0.7%). The change in AGB stock was due entirely to this upward shift in size leading to more canopy trees and fewer saplings after just 6 y. Across habitats, the biggest increment in biomass was in the secondary-forest patch (3.4% y−1) which we know was cleared about 27 y ago, whereas mature forest on ridges and valleys had small increases (0.10% and 0.09% y−1, respectively). In both censuses, AGB stocks were >50% higher on the ridge than in the valley while relative growth and mortality were higher in the valley. Mean wood specific gravity (WSG) decreased with increasing diameter class; WSG did not change much between censuses in mature forests and did not contribute to the change in AGB stocks. Our forest increased its standing biomass, but far less than the average reported for other Amazonian forests (i.e. 0.30 vs. 0.98 Mg ha−1 y−1). We find no evidence to support the notion that this forest is recovering from long-past human intervention. Instead of a long-term recovery, we believe the forest changed in response to natural fluctuations of the environment (e.g. changes in precipitation, higher CO2), windstorms or other more recent events. The significant differences in AGB stocks between valley and ridge suggest that the terra firme forests are a mosaic of natural habitats, and that this mosaic is in part responsible for the variation in biomass stocks detected in Amazonian terra firme forests.
Resumen: La biomasa aérea de la mayoría de los bosques amazónicos está incrementando. Una hipótesis es que los bosques están respondiendo a un disturbio humano intenso y ampliamente distribuido, anterior a la llegada de los conquistadores europeos (>500 años atrás). En este estudio se confronta esta hipótesis con los cambios en biomasa encontrados en 6.3 años en una parcela de gran escala de la Amazonia occidental (>150.000 arbustos y árboles con diámetro a la altura del pecho ≥10 mm y 1100 especies en 25 ha). Los resultados se examinan por categorías de diámetro y hábitat. En este período el bosque perdió tallos pequeños (4.6%), ganó árboles grandes (2.6%) y ganó biomasa (0.7%). La ganancia en biomasa fue debida enteramente al incremento de árboles de gran tamaño que significó más árboles de dosel y menos juveniles en apenas 6 años. Entre los hábitats, el mayor incremento en biomasa se encontró en un parche de bosque secundario de colina (3.4%/año), cuya edad es de 27 años, mientras el bosque maduro de las colinas y los valles incrementó escasamente (0.10% y 0.09%/año, respectivamente). Tanto al inicio como al final del estudio, el stock de biomasa fue >50% más grande en la colina que en el valle mientras que el crecimiento y la mortalidad relativa fueron mayores en el valle. La media de la gravedad específica de la madera (GEM) fue menor a mayor clase diamétrica; en el bosque maduro, el cambio en la GEM fue insignificante y no contribuyó al aumento en stocks de biomasa. El bosque incrementó la biomasa aérea pero mucho menos que el promedio reportado para otros bosques amazónicos (i.e. 0.30 vs. 0.98 Mg ha−1/año). No se encontró evidencia que apoye la noción de que el bosque se está recuperando de un disturbio de gran escala ocurrido en el pasado. En su lugar, se cree que el bosque cambió en respuesta a fluctuaciones naturales del ambiente (e.g. cambios en precipitación, mayor concentración de CO2), vendavales u otro tipo de eventos más recientes. La diferencia significativa en los stocks de biomasa encontrada entre el valle y la colina sugiere que la tierra firme es un mosaico de hábitats naturales y que este mosaico podría explicar en parte la variación encontrada en los stocks de biomasa de bosques amazónicos de tierra firme.
Changes in tree species abundance in a Neotropical forest: impact of climate change
- Richard Condit, Stephen P. Hubbell, Robin B. Foster
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- Journal:
- Journal of Tropical Ecology / Volume 12 / Issue 2 / March 1996
- Published online by Cambridge University Press:
- 10 July 2009, pp. 231-256
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The abundance of all tree and shrub species has been monitored for eight years in a 50 ha census plot in tropical moist forest in central Panama. Here we examine population trends of the 219 most numerous species in the plot, assessing the impact of a long-term drying trend. Population change was calculated as the mean rate of increase (or decrease) over eight years, considering either all stems ≥10 mm diameter at breast height (dbh) or just stems ≥100 mm dbh. For stems ≥10 mm, 40% of the species had mean growth rates <1% per year (either increasing or decreasing) and 12% had changes ≥5% per year. For stems ≥100 mm, the figures were 38% and 8%.
Species that specialize on the slopes of the plot, a moist microhabitat relative to the plateau, suffered significantly more declines in abundance than species that did not prefer slopes (stems ≥10 mm dbh). This pattern was due entirely to species of small stature: 91% of treelets and shrubs that were slope-specialists declined in abundance, but just 19% of non-slope treelets and shrubs declined. Among larger trees, slope and non-slope species fared equally. For stems ≥100 mm dbh, the slope effect vanished because there were few shrubs and treelets with stems ≥100 mm dbh. Another edaphic guild of species, those occurring preferentially in a small swamp in the centre of the plot, were no more likely to decline in abundance than non-swamp species, regardless of growth form. Species that preferentially colonize canopy gaps in the plot were slightly more likely to decrease in abundance than non-colonizing species (only for stems ≥10 mm dbh, not ≥100 mm). Despite this overall trend, however, several colonizing species had the most rapidly increasing populations in the plot.
The impact of a 25-year drying trend and an associated increase in the severity of the 4-month dry season is having an obvious impact on the BCI forest. At least 16 species of shrubs and treelets with affinities for moist microhabitats are headed for extinction in the plot. Presumably, these species invaded the forest during a wetter period prior to 1966. A severe drought of 1983 that caused unusually high tree mortality contributed to this trend, and may also have been responsible for sharp increases in abundance of a few gap-colonizers because it temporarily opened the forest canopy. The BCI forest is remarkably sensitive to a subtle climatic shift, yet we do not know whether this is typical for tropical forests because no other large-scale censuses exist for comparison.
Temporal and spatial variability in seedling dynamics: a cross-site comparison in four lowland tropical forests
- Margaret R. Metz, Liza S. Comita, Yu-Yun Chen, Natalia Norden, Richard Condit, Stephen P. Hubbell, I-Fang Sun, Nur Supardi bin Md. Noor, S. Joseph Wright
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- Journal:
- Journal of Tropical Ecology / Volume 24 / Issue 1 / January 2008
- Published online by Cambridge University Press:
- 01 January 2008, pp. 9-18
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Spatial and temporal variation in seedling dynamics was assessed using records of community-wide seedling demography collected with identical monitoring methods at four tropical lowland forests in Panama, Malaysia, Ecuador and French Guiana for periods of between 3 and 10 y. At each site, the fates of between 8617 and 391 777 seedlings were followed through annual censuses of the 370–1008 1-m2 seedling plots. Within-site spatial and inter-annual variation in density, recruitment, growth and mortality was compared with among-site variability using Bayesian hierarchical modelling to determine the generality of each site's patterns and potential for meaningful comparisons among sites. The Malaysian forest, which experiences community-wide masting, was the most variable in both seedling density and recruitment. However, density varied year-to-year at all sites (CVamong years at site = 8–43%), driven largely by high variability in recruitment rates (CV = 40–117%). At all sites, recruitment was more variable than mortality (CV = 5–64%) or growth (CV = 12–51%). Increases in mortality rates lagged 1 y behind large recruitment events. Within-site spatial variation and inter-annual differences were greater than differences among site averages in all rates, emphasizing the value of long-term comparative studies when generalizing how spatial and temporal variation drive patterns of recruitment in tropical forests.
Classification and ecological relationships of seed dormancy in a seasonal moist tropical forest, Panama, Central America
- Adriana Sautu, Jerry M. Baskin, Carol C. Baskin, Jose Deago, Richard Condit
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- Journal:
- Seed Science Research / Volume 17 / Issue 2 / June 2007
- Published online by Cambridge University Press:
- 01 June 2007, pp. 127-140
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This is the first study to determine the class of seed dormancy (or non-dormancy) of a large number of native tree species in a tropical forest, the seasonal moist tropical forest of the Panama Canal Watershed (PCW), or to test the relationships between class of dormancy (or non-dormancy) and various seed and ecological characteristics of the constituent species. Fresh seeds of 49 of 94 tree species were non-dormant (ND), and 45 were dormant (D). Seeds of 23 species had physiological dormancy (PD), 13 physical dormancy (PY), two morphological dormancy (MD), 7 morphophysiological dormancy (MPD) and none combinational dormancy (PY+PD). Seeds with PY were significantly smaller ( < 0.1 g) and drier (moisture content < 16%) at maturity than those that were ND or in the other D classes. Seeds of 62, 42 and 53% of species dispersed in the early rainy, late rainy (LRS) and dry seasons, respectively, were ND. The majority (61%) of species with PD seeds, but only 17% of those with PY seeds, were dispersed in the LRS. The proportion of species with ND seeds was higher in large-size (63%) than in mid-size (35%) and understorey (17%) trees, but differed only slightly between non-pioneers (58%) and pioneers (54%). The proportion of species with D seeds increased only slightly through a precipitation gradient of about 3100 to 1900 mm in the PCW; however, PY increased from 19 to 32% and PD decreased from 63 to 44%.
4 - Neighbourhood effects on sapling growth and survival in a neotropical forest and the ecological-equivalence hypothesis
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- By María Uriarte, Institute of Ecosystem Studies, Millbrook, Stephen P. Hubbell, University of Georgia and Smithsonian Tropical Research Institute, Robert John, University of Georgia and Smithsonian Tropical Research Institute, Richard Condit, Smithsonian Tropical Research Institute, Charles D. Canham, Institute of Ecosystem Studies, Millbrook
- Edited by David Burslem, University of Aberdeen, Michelle Pinard, University of Aberdeen, Sue Hartley, University of Sussex
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- Book:
- Biotic Interactions in the Tropics
- Published online:
- 25 August 2009
- Print publication:
- 08 September 2005, pp 89-106
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Summary
Introduction
In 1980 S. P. Hubbell and R. B. Foster began a long-term, large-scale study of tropical forest dynamics on Barro Colorado Island (BCI), Panama. The objective of the study was to test competing hypotheses about the maintenance of high tree species richness in the BCI forest, and in tropical moist forests more generally. Hubbell and Foster established a 50-ha permanent plot on the summit plateau of BCI, within which all free-standing woody plants with a stem diameter at breast height (DBH) of a centimetre or larger were tagged, measured, mapped and identified by 1982. Subsequent complete censuses of the BCI plot have been conducted from 1985 to 2000 at 5-year intervals. In setting up the BCI plot, Hubbell and Foster (1983) reasoned that whatever diversity-maintaining mechanisms were important, they would have to operate in a spatially dependent manner in communities of sessile plants such as the BCI tree community, which meant that the trees had to be mapped. A decade earlier, Janzen (1970) and Connell (1971) had independently proposed a spatially explicit ‘enemies hypothesis’, now known as the Janzen–Connell hypothesis. They hypothesized that host-specific seed and seedling predators were responsible for maintaining tropical tree diversity by causing dependence on density and frequency (rare species advantage), through an interaction between seed dispersal and density-dependent seed predation.
In 1980, there were essentially just two principal tropical forest diversity theories to test: the enemies hypothesis and its variants, and the ‘intermediate disturbance’ hypothesis (Connell 1977) and its variants that invoked a role for disturbances associated with opening, growth and closure of light gaps (e.g. Ricklefs 1978; Hartshorn 1978; Orians 1982; Denslow 1987).
Reproductive size thresholds in tropical trees: variation among individuals, species and forests
- S. Joseph Wright, M. Alejandra Jaramillo, Javier Pavon, Richard Condit, Stephen P. Hubbell, Robin B. Foster
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- Journal:
- Journal of Tropical Ecology / Volume 21 / Issue 3 / May 2005
- Published online by Cambridge University Press:
- 26 May 2005, pp. 307-315
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Relative size at onset of maturity (RSOM) is defined as size at first reproduction divided by asymptotic maximal size. RSOM is remarkably constant among species within many higher clades of animals, but varies widely among tree species from the Pasoh Forest Reserve, Malaysia according to the work of S. C. Thomas. RSOM was examined for 16 mid-storey and canopy tree species from a second tropical forest at Barro Colorado Island (BCI), Panama. Interspecific variation in RSOM was equally large for BCI and Pasoh and was unrelated to gap dependence or life form for BCI species. The shape of the relationship between size and the proportion of individuals that were reproductive differed between forests, with an abrupt increase over a narrow range of sizes at Pasoh and a more gradual increase over a wider range of sizes at BCI. Both overtopping trees and heavy liana infestation reduced the probability that BCI trees were reproductive. This presumably reflects reduced availability of carbon for reproduction. We speculate that greater liana loads and a greater abundance of large, shade-casting trees at BCI may increase variation among individuals and contribute to the relatively wide range of sizes characterized by a mixture of sterile and fertile individuals observed for most BCI species.
Tropical forest dynamics across a rainfall gradient and the impact of an El Niño dry season
- Richard Condit, Salomon Aguilar, Andres Hernandez, Rolando Perez, Suzanne Lao, George Angehr, Stephen P. Hubbell, Robin B. Foster
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- Journal:
- Journal of Tropical Ecology / Volume 20 / Issue 1 / January 2004
- Published online by Cambridge University Press:
- 05 January 2004, pp. 51-72
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Tropical forest demography and dynamics were examined in three inventory plots across a precipitation gradient in central Panama. The harsh dry season of 1998 that accompanied the 1997–98 El Niño was spanned by censuses at all three sites. The wet and intermediate plots were similar in total species richness, the dry site somewhat lower in diversity; all three sites differed substantially from each other in species composition. Forest-wide growth of large trees was higher at the wet and intermediate sites than at the dry site, but sapling growth was highest at the dry site and lowest at the intermediate site. Forest-wide growth differences were reflected by individual species, for example, saplings of species at the dry site grew faster than saplings of the same species at the intermediate site. Forest-wide mortality was lowest at the dry site and highest at the wet, and this difference was also reflected by individual species. We suggest that low mortality and growth in the drier forest was due to the longer annual dry season and higher deciduousness, and that high sapling growth at the dry site was due to greater light penetration to the forest floor. Growth rates were elevated at all three sites during 1998, possibly due to reduced cloud-cover during the El Niño. Contrary to expectation, mortality during 1998 was not elevated at wet and intermediate sites during the El Niño drought, but was at the dry site. Finally, we found that some species performed poorly at one site and declined in abundance, while having stable or increasing populations at another site, demonstrating that the communities are not at equilibrium.