Aim: Few paleoenvironmental studies have been performed in Araucaria-Nothofagus forests, which are highly vulnerable to ongoing threats from climate change and anthropogenic activities. The primary goal of this work is to reconstruct past environmental changes related to fire disturbances over the last 1800 years in Tolhuaca National Park, Chile.

Location: Tolhuaca National Park, Araucanian region (38.2°S; 71.8°W), Northwestern Patagonia, Chile.

Taxa: Araucaria araucana (Araucariaceae), Nothofagus spp. (Nothofagaceae).

Methods: We completed charcoal and pollen analyses to create two new paleoecological records that span 1800 years. We compared the lake-based reconstruction with the available local tree-ring fire scar chronologies from the last 430 years. Using these data, we compute forest index changes, biomass burning trends, and compare with estimates of archaeological radiocarbon density. We place our inferences with context of published regional paleoclimatic proxies from the Patagonian-Andean region.

Results: Our results showed that fire activity was higher than present between 200 and 1500 CE, with peaks around 200-400 CE and 1100-1500 CE. Periods with high fire activity are associated with reduced forest cover, as Araucaria declined when mixed-severity fire regime occurred for extended periods. Pollen assemblages suggested a shift from dry to wet climate conditions at 1500 CE, and from 1750 CE onward, the arrival of exotic species reflected the land-use changes related to forest clearance and transhumance practices.

Main conclusions: The paleoenvironmental reconstructions showed changes in vegetation, fire, and climate over the past 1800 years in TNP. Wildfires have been the main disturbance process modifying the vegetation structure in the Araucaria and Nothofagus forests. Since 1750 CE intensive post-Hispanic land-use changes (forest clearances by fire and logging) took place in the study area, reducing the native vegetation cover. Climate variability, modulated by SAM-like and ENSO-like conditions, influenced the fire activity (availability and flammability of fuels), concomitantly with high archaeological density. The recent (after 2000 CE) increase of catastrophic wildfires may negatively affect the conservation strategies of Araucaria-Nothofagus forests.

Chronology: Two sediment cores were collected in Verde (112 cm) and Malleco (120 cm) lakes using an UWITEC gravity corer. Radionuclide measurements using a gamma spectrometer were performed to estimate the ²¹⁰Pb, ²²⁶Ra (to support ²¹⁰Pb activity), and ¹³⁷Cs activities on the first 20 cm). In addition, AMS radiocarbon dating was performed in both lakes. Radiocarbon ages were calibrated using the SHCal20 curve (Hogg et al., 2020). The age-depth models were computed using an integrated chronology based on ²¹⁰Pb, ¹³⁷Cs, and ¹⁴C dates through Bayesian analyses in “rPlum” package (Aquino-López et al., 2018; Blaauw et al., 2020) in R platform.

Charcoal analyses: To infer fire regime, macroscopic (>125 µm) charcoal fractions were analyzed in Verde and Malleco cores. 2 cm³ of sediment were taken at 1 cm intervals for the length of the core, sieved through 125 μm and 250 μm screens, following the methodology outlined by Whitlock and Larsen (2001). 

Pollen analyses: Verde and Malleco cores were subsampled for 1 cm³ (Verde) and 2 cm³ of sediment (Malleco) at discrete 2 to 5 cm intervals. The samples were analyzed using conventional procedures (Faegri & Iversen, 1989). To calculate pollen concentration and accumulation rate, two tablets of Lycopodium clavatum were added to each sample (Stockmarr, 1971). Pollen identification was carried out using a modern reference collection (Palynology Lab-UACh) and palynological keys (Heusser, 1971; Markgraf & D'Antoni, 1978).

References:

Aquino-López, M. A., Blaauw, M., Christen, J. A., & Sanderson, N. K. (2018). Bayesian Analysis of 210Pb Dating. Journal of Agricultural, Biological and Environmental Statistics, 23(3), 317-333. doi:10.1007/s13253-018-0328-7

Blaauw, M., Christen, J. A., & Aquino-López, M. A. (2020). rplum: Bayesian Age-Depth Modelling of '210Pb'-Dated Cores. R package version 0.1.4.  https://CRAN.R-project.org/package=rplum

Faegri, K., & Iversen, J. (1989). Textbook of pollen analysis. Londres: John Wiley & Sons Ld.

Heusser, C. J. (1971). Pollen and spores of Chile: Modern types of the Pteridophyta, Gymnospermae, and Angiospermae. Tucson: University of Arizona Press.

Hogg, A. G., Heaton, T. J., Hua, Q., Palmer, J. G., Turney, C. S. M., Southon, J., . . . Wacker, L. (2020). SHCal20 Southern Hemisphere Calibration, 0–55,000 Years cal BP. Radiocarbon, 62(4), 759-778. doi:10.1017/RDC.2020.59

Markgraf, V., & D'Antoni, H. L. (1978). Pollen Flora of Argentina. Modern Spores and Pollen Types of Pteridophyta, Gymnospermae and Angiospermae. Tucson: The University of Arizona Press.

Stockmarr, J. (1971). Tablets with spores used in absolute pollen analysis. Pollen spores, 13, 615-621. 

Whitlock, C., & Larsen, C. (2001). Charcoal as a proxy fire. In J. P. Smol, H. J. B. Birks, & W. M. Last (Eds.), Tracking Environmental Change Using Lake Sediments: Terrestrial, Algal, and Siliceous Indicators (Vol. 3, pp. 75-97). Dordrecht, The Netherlands: Kluwer Academic Publishers.