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ABSTRACT: In tropical forests, pioneer tree species regenerate from seeds dispersed directly into canopy gaps and from seeds that persisted in soil seed banks before gap formation. Life-history models have suggested that selection for the long-term persistence of tree seeds in the soil should be weak because persistence potentially reduces population growth rate by extending generation time and because adult life spans may exceed the return interval of favorable recruitment sites. Here we use accelerator mass spectrometry to carbon-date seeds of three pioneer tree species extracted from undisturbed seed banks in seasonally moist lowland Neotropical forest. We show that seeds of Croton billbergianus, Trema micrantha, and Zanthoxylum ekmannii germinate successfully from surface soil microsites after 38, 31, and 18 years, respectively. Decades-long persistence may be common in large-seeded tropical pioneers and appears to be unrelated to specific regeneration requirements.
The American Naturalist 03/2009; 173(4):531-5. · 4.72 Impact Factor
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ABSTRACT: We examined direct and indirect impacts of millennial-scale climate change on fire regimes in the south-central Brooks Range, Alaska, USA, using four lake sediment records and existing paleoclimate interpretations. New techniques were introduced to identify charcoal peaks semi-objectively and to detect statistical differences between fire regimes. Peaks in charcoal accumulation rates provided estimates of fire return intervals (FRIs), which were compared among vegetation zones identified by fossil pollen and stomata. Climatic warming between ca. 15 000–9000 yr BP (calendar years before Common Era [CE] 1950) coincided with shifts in vegetation from herb tundra to shrub tundra to deciduous woodlands, all novel species assemblages relative to modern vegetation. Two sites cover this period and show decreased FRIs with the transition from herb to Betula-dominated shrub tundra ca. 13 300– 14 300 yr BP (FRI mean ¼ 144 yr; 95% CI ¼ 120–169 yr), when climate warmed but remained cooler than present. Although warming would have favored shorter FRIs in the shrub tundra, the shift to more continuous, flammable fuels relative to herb tundra was probably a more important cause of increased burning. Similarly, a vegetation shift to Populus-dominated deciduous woodlands overrode the influence of warmer-and drier-than-present summers, resulting in lower fire activity from ca. 10 300–8250 yr BP (FRI mean ¼ 251 yr; 95% CI ¼ 156– 347 yr). Three sites record the mid-to-late Holocene, when climatic cooling and moistening allowed Picea glauca forest–tundra and P. mariana boreal forests to establish ca. 8000 and 5500 yr BP, respectively. FRIs in forest–tundra were either similar to or shorter than those in the deciduous woodlands (FRI mean range ¼ 131–238 yr). The addition of P. mariana ca. 5500 yr BP increased landscape flammability, overrode the effects of climatic cooling and moistening and resulted in lower FRIs (FRI mean ¼ 145 yr; 95% CI ¼ 130–163). Overall, shifts in fire regimes were strongly linked to changes in vegetation, which were responding to millennial-scale climate change. We conclude that shifts in vegetation can amplify or override the direct influence of climate change on fire regimes, when vegetation shifts significantly modify landscape flammability. Our findings emphasize the importance of biophysical feedbacks between climate, fire, and vegetation in determining the response of ecosystems to past, and by inference, future climate change.
Ecological Monographs 01/2009; 79:201-219. · 7.43 Impact Factor
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ABSTRACT: Understanding feedbacks between terrestrial and atmospheric systems is vital for predicting the consequences of global change, particularly in the rapidly changing Arctic. Fire is a key process in this context, but the consequences of altered fire regimes in tundra ecosystems are rarely considered, largely because tundra fires occur infrequently on the modern landscape. We present paleoecological data that indicate frequent tundra fires in northcentral Alaska between 14,000 and 10,000 years ago. Charcoal and pollen from lake sediments reveal that ancient birch-dominated shrub tundra burned as often as modern boreal forests in the region, every 144 years on average (+/- 90 s.d.; n = 44). Although paleoclimate interpretations and data from modern tundra fires suggest that increased burning was aided by low effective moisture, vegetation cover clearly played a critical role in facilitating the paleofires by creating an abundance of fine fuels. These records suggest that greater fire activity will likely accompany temperature-related increases in shrub-dominated tundra predicted for the 21(st) century and beyond. Increased tundra burning will have broad impacts on physical and biological systems as well as on land-atmosphere interactions in the Arctic, including the potential to release stored organic carbon to the atmosphere.
PLoS ONE 02/2008; 3(3):e0001744. · 4.09 Impact Factor
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ABSTRACT: Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition (13C and 14C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.
Nature 08/2005; 436(7050):538-41. · 36.28 Impact Factor
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ABSTRACT: Paleorecords of the middle Holocene (MH) from the North American mid-continent can offer insights into ecological responses to pervasive drought that may accompany future climatic warming. We analyzed MH sediments from West Olaf Lake (WOL) and Steel Lake (SL) in Minnesota to examine the effects of warm/dry climatic conditions on prairie-woodland ecosystems. Mineral composition and carbonate delta(18)O were used to determine climatic variations, whereas pollen assemblages, charcoal delta(13)C, and charcoal accumulation rates were used to reconstruct vegetation composition, C(3) and C(4) plant abundance, and fire. The ratio of aragonite/calcite at WOL and delta(18)O at SL suggest that pronounced droughts occurred during the MH but that drought severity decreased with time. From charcoal delta(13)C data we estimated that the MH abundance of C(4) plants averaged 50% at WOL and 43% at SL. At WOL C(4) abundance was negatively correlated with aragonite/calcite, suggesting that severe moisture deficits suppressed C(4) plants in favor of weedy C(3) plants (e.g., Ambrosia). As climate ameliorated C(4) abundance increased (from approximately 33 to 66%) at the expense of weedy species, enhancing fuel availability and fire occurrence. In contrast, farther east at SL where climate was cooler and wetter, C(4) abundance showed no correlation with delta(18)O-inferred aridity. Woody C(3) plants (e.g., Quercus) were more abundant, biomass flammability was lower, and fires were less important at SL than at WOL. Our results suggest that C(4) plants are adapted to warm/dry climatic conditions, but not to extreme droughts, and that the fire regime is controlled by biomass-climate interactions.
Proceedings of the National Academy of Sciences 02/2004; 101(2):562-7. · 9.68 Impact Factor
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ABSTRACT: Annually laminated sediments (varves) offer an effective means of acquiring high-quality palaeoenvironmental records. However, the strength of a varve chronology can be compromised by a number of factors, such as missing varves, ambiguous laminations and human counting error. We assess the quality of a varve chronology for the last three millennia from Steel Lake, Minnesota, through comparisons with nine AMS 14 C dates on terrestrial plant macrofossils from the same core. These comparisons revealed an overall 8.4% discrepancy, primarily because of missing/uncountable varves within two stratigraphic intervals characterized by low carbonate concentrations and obscure laminations. Application of appropriate correction factors to these two intervals results in excellent agreement between the varve and 14 C chronologies. These results, together with other varve studies, demonstrate that an independent age-determination method, such as 14 C dating, is usually necessary to verify, and potentially correct, varve chronologies.
