Browsing by Author "Christie, Duncan A."

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    Radiocarbon bomb-peak signal in tree-rings from the tropical Andes register low latitude atmospheric dynamics in the Southern Hemisphere
    (2021) Ancapichun, Santiago; De Pol-Holz, Ricardo; Christie, Duncan A.; Santos, Guaciara M.; Collado-Fabbri, Silvana; Garreaud, Rene; Lambert, Fabrice; Orfanoz-Cheuquelaf, Andrea; Rojas, Maisa; Southon, John; Turnbull, Jocelyn C.; Creasman, Pearce Paul
    South American tropical climate is strongly related to the tropical low-pressure belt associated with the South American monsoon system. Despite its central societal role as a modulating agent of rainfall in tropical South America, its long-term dynamical variability is still poorly understood. Here we combine a new (and world's highest) tree-ring C-14 record from the Altiplano plateau in the central Andes with other C-14 records from the Southern Hemisphere during the second half of the 20th century in order to elucidate the latitudinal gradients associated with the dissemination of the bomb C-14 signal. Our tree-ring C-14 record faithfully captured the bomb signal of the 1960's with an excellent match to atmospheric C-14 measured in NewZealand but with significant differences with a recent record from Southeast Brazil located at almost equal latitude. These results imply that the spreading of the bomb signal throughout the Southern Hemisphere was a complex process that depended on atmospheric dynamics and surface topography generating reversals on the expected north-south gradient in certain years. We applied air-parcelmodeling based on climate data to disentangle their different geographical provenances and their preformed (reservoir affected) radiocarbon content. We found that air parcel trajectories arriving at the Altiplano during the bomb period were sourced i) from the boundary layer in contact with the Pacific Ocean (41%), ii) fromthe upper troposphere (air above the boundary layer, with no contact with oceanic or continental carbon reservoirs) (38%) and iii) fromthe Amazon basin (21%). Based on these results we estimated the Delta C-14 endmember values for the different carbon reservoirs affecting our recordwhich suggest that the Amazon basin biospheric C-14 isoflux could have been reversed fromnegative to positive as early as the beginning of the 1970's. Thiswould imply amuch faster carbon turnover rate in the Amazon than previouslymodelled. (c) 2021 Elsevier B.V. All rights reserved.
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    Regeneration patterns and persistence of the fog-dependent Fray Jorge forest in semiarid Chile during the past two centuries
    (2008) Gutierrez, Alvaro G.; Barbosa, Olga; Christie, Duncan A.; Del-Val, Ek; Ewing, Holly A.; Jones, Clive G.; Marquet, Pablo A.; Weathers, Kathleen C.; Armesto, Juan J.
    The persistence of rainforest patches at Fray Jorge National Park (FJNP) in semiarid Chile (30 degrees 40'S), a region receiving approximately 147 mm of annual rainfall, has been a source of concern among forest managers. These forests are likely dependent on water inputs from oceanic fog and their persistence seems uncertain in the face of climate change. Here, we assessed tree radial growth and establishment during the last two centuries and their relation to trends in climate and canopy disturbance. Such evaluation is critical to understanding the dynamics of these semiarid ecosystems in response to climate change. We analyzed forest structure of six forest patches (0.2-22 ha) in FJNP based on sampling within 0.1 ha permanent plots. For the main canopy species, the endemic Aextoxicon punctatum (Aextoxicaceae), we used tree-ring analysis to assess establishment periods, tree ages, growing trends and their relation to El Nino Southern Oscillation (ENSO), rainfall, and disturbance. The population dynamics of A. punctatum can be described by a continuous regeneration mode. Regeneration of A. punctatum was sensitive to different canopy structures. Growth release patterns suggest the absence of large scale human impact. Radial growth and establishment of A. punctatum were weakly correlated with rainfall and ENSO. If water limits forests patch persistence, patches are likely dependent on the combination of fog and rain water inputs. Forest patches have regenerated continuously for at least 250 years, despite large fluctuations in rainfall driven by ENSO and a regional decline in rainfall during the last century. Because of the positive influence on fog interception, forest structure should be preserved under any future climate scenario. Future research in FJNP should prioritize quantifying the long-term trends of fog water deposition on forests patches. Fog modeling is crucial for understanding the interplay among physical drivers of water inputs under climate change.