Did Indigenous Depopulation of the Americas Lead to Reforestation, CO2 Drawdown, and the LIA? Empirical Evidence from Log Buildings

Several causes have been proposed for the Little Ice Age (LIA), including reduced solar forcing, heightened volcanic activity, and internal climate variability. Recently, it was hypothesized that the global CO₂ drawdown, centered on ~1610 CE and the coldest part of the LIA, may have been caused by reforestation following depopulation of Indigenous Peoples of the Americas. Here, we propose that if forests experienced massive recruitment into abandoned fields, tree rings from historic log buildings, representing fertile, upland sites, should capture those land-use changes. We hypothesize that compared to less habitable old-growth forests, historic logs will have 1) synchronous recruitment, 2) rapid early growth, and 3) little evidence of disturbance. We used tree-ring evidence from 18 historic buildings (n = 361 logs), 8 old-growth sites (n = 197 trees), and 1 reference stand from an abandoned field in eastern North America to test our hypotheses. All sites document a period of elevated recruitment beginning in the 1670s. Most historic sites (61- 83%) had fast early growth, similar to the reference site, indicating high-light conditions (Fig 1). The rate of disturbance was lower at historic sites with fast growth (5.3 events/century, 95% CI [3.6, 7.0) than at old-growth sites with slow growth (23.5 events/century, 95% CI [17.5, 29.5], consistent with the idea that most historic logs were harvested from fast-growing, second-growth forests. Our results support widespread forest establishment peaking at ~1690, around the time of indigenous depopulation in the study area. This is broadly coincident with the timing of modeled depopulation, reforestation, and measured carbon drawdown. However, the recruitment event occurred across forest sites and coincides with a major drought event in eastern North America. The timing of a regional drought, depopulation, and subsequent recruitment of trees , all within the period of ~1650-1690 CE, warrants further research into interactions between indigenous land use, climate, and CO₂.