Ancient human occupation alters modern forest traits, structure and biomass in neotropical forests.

Christopher Doughty , Ben Wiebe, Megan Seeley, Jesus Aguirre-Gutierrez, Francisco Estrada-Belli, Jason Nesbitt, Marcello Canuto, Luke Auld-Thomas

There is considerable debate as to how extensively neotropical forests were shaped by ancient people. This uncertainty has implications for the global carbon cycle because forest plots may be disproportionately located in formerly occupied areas, potentially biasing estimates of tropical forest growth and carbon uptake. Ground measurements within the formerly occupied Maya city of Xmakabatun (400BCE to 800CE), now covered by forest, revealed significantly higher forest biomass, modified leaf traits, and distinct species composition on archaeological structures (i.e. ancient buildings) relative to control and former agricultural areas. We then used GEDI and trait-based remote sensing across the broader region to show that forest regions with greater quantities of known archaeological settlement also had significantly higher biomass and modified leaf traits. We next extend this approach to the Amazon basin, where ancient land-use histories are less certain. Using similar remote sensing techniques, we found that forests near Amazonian Dark Earth (terra preta) sites (N=503) were shorter but had slightly more foliage (~6%), whereas forests near geoglyphs (N=961) were taller but had much less bulk (20%) than the control forest plots. From these relationships, we estimate that ancient human occupation has increased modern Amazonian biomass by, on average, 2.8±1.6%, an amounts equivalent to ~1% of global above-ground biomass. Together, these results demonstrate that ancient land use has left detectable legacies in the traits and structure of modern forests and suggest that remotely sensed biomass and/or leaf trait datasets may also provide a tool for regional-scale archaeological mapping.