cjabradshaw / savannacorridors

Analysis of palaeoecological records across South-East Asia to determine the evidence for regime shifts between open savannas and dense tropical forests occurred since the Last Glacial Maximum

Testing evidence for savanna corridors in South-East Asia since the Last Glacial Maximum

Analysis of palaeoecological records across South-East Asia to determine the evidence for regime shifts between open savannas and dense tropical forests occurred since the Last Glacial Maximum

Code contributors: Rebecca Hamilton, Corey J. A. Bradshaw, Frédérik Saltré, Jesse Wolfhagen

Accompanies paper:

Hamilton, R, N Amano, CJA Bradshaw, F Saltré, R Patalano, D Penny, J Stevenson, J Wolfhagen, P Roberts. 2023. Seasonal forests, not long-term savanna corridors, dominated in South-East Asia during the Last Glacial Maximum. Proceedings of the National Academy of Sciences of the USA doi:10.1073/pnas.2311280120

Abstract

The dominant paradigm is that large tracts of Southeast Asia’s lowland rainforests were replaced with a ‘savanna corridor’ during the cooler, more seasonal climates of the Last Glacial Maximum (23000 to 19000 years ago). This interpretation has implications for understanding the resilience of Asia’s tropical forests to projected climate change, implying a vulnerability to ‘savannization’. A savanna corridor is also an important foundation for archaeological interpretations of how humans moved through and settled insular Southeast Asia and Australia. Yet an up-to-date, multi-proxy, and empirical examination of the palaeoecological evidence for this corridor is lacking. We did qualitative and statistical analyses of 59 palaeoecological records across Southeast Asia to test the evidence for Last Glacial Maximum (LGM) savannization and clarify the relationships between methods, biogeography, and ecological change in the region from the start of Late Glacial Period (119000 years ago) to the present. The pollen records typically show montane forest persistence during the Last Glacial Maximum, while δ¹³C biomarker proxies indicate the expansion of C₄-rich grasslands. We reconcile this discrepancy by hypothesizing the expansion of montane forest in the uplands, and replacement of rainforest with seasonally dry tropical forest mosaics in the lowlands. We also find that smooth forest transitions between 34000 and 2000 years ago point to the capacity of Southeast Asia’s ecosystems both to resist and recover from climate stressors, suggesting resilience to savannization. Finally, the timing of ecological change observed in our combined datasets indicates an ‘early’ onset of the LGM in Southeast Asia from ~ 30000 years ago.

Scripts

• SavannaCorridorGithub.R

Data

Site-specific palaeoecological proxy series (raw data)

• G6-4.csv, SH19014_grass.csv, G5_6_149P2.csv, NPK2.csv, hordorli.csv, 18300.csv, 18323.csv, 18302.csv, CB19.csv, MD063075.csv, NS-0725.csv, 17964.csv, DDA.csv, G4_K12P1.csv, PB-A.csv, GEOB100693.csv, PSS.csv, GeoB100537.csv, G5_2_056P.csv, LL2.csv, RD-3.csv, KUM3.csv, G4_K4P3.csv, BYK2.csv, TOW9.csv, BJ8_03_91GGC.csv, MAT10_2B.csv, SO189_144KL.csv, MC1.csv, mbelen.csv

Compiled, age-resampled, standardised compilation of time series (34 ka to 2 ka), with site coordinates (lon/lat)

• SC_all_SACor.csv

Spatially constrained bivariate correlations to reproduce site-by-site correlation heatmap

• cor_matrix_allmean.csv

Supplementary data

• database descriptors (Excel file)

R packages

Scripts require following R libraries

• spatstat, gstat, maps, sp, ape, permute, ggplot2, dplyr, boot, tmvnsim, wCorr, hrbrthemes