In 677 North American Neotoma pollen records, vegetation change was not faster after 3 ka than at 3–6 ka without post-settlement samples; in 986 European records it was 12% faster

Question: Mottl et al. (2021, Science) report that vegetation change accelerated on all continents from 4.6–2.9 ka, and they give 2.9 ka for North America. Does this acceleration occur before Euro-American settlement? Approach: We downloaded all 5,738 Neotoma pollen datasets for North America and Europe. We used Neotoma chronologies; we converted radiocarbon-year chronologies with IntCal20. We calculated the chord-distance rate of change (RoC) between adjacent 500-year bins (one random sample per bin, 150-grain rarefaction, 100 replicates). We compared each site with itself: 0–3 ka against 3–6 ka. We repeated this without samples younger than AD 1800, AD 1600, or the Ambrosia-rise settlement horizon. Europe is the positive control. Results: With all samples, the North American rate increased only in the youngest 500-year step, by 15.6% (95% CI 11.3–19.7). Ambrosia, Poaceae and Rumex supplied 60% of this excess. Without post-1800 or post-settlement samples, the median North American late/mid ratio was 0.985 (0.972–0.997) and 0.986 (0.975–1.001) at 677 sites. In 986 European sites, the rate increased gradually from about 3 ka. Without post-1800 samples, the late/mid ratio was 1.12 (1.10–1.14) and the youngest step was +29%. The contrast stayed the same for the Mottl et al. site set, unscaled distance, 250-year bins, 300-grain rarefaction, chronology type, deposit type and sub-regions. North American youngest-step rates were below deglacial rates at the same sites (ratio 0.86–0.88). Limitations: We made no new age models and did not propagate age uncertainty. The settlement horizon is approximate. A small youngest-step residual (0–4% unscaled) in some subsets can be late-Holocene climate or age-model leakage. We did not identify why Mottl et al.'s outputs show a North American rise at 0.5–1.5 ka; smoothing, moving windows, pooled bin means and age models are candidates.

fossil pollen · Neotoma · rate of change · Holocene · vegetation change · human impact · European settlement · Ambrosia rise · palaeoecology · North America · Europe · replication