Fire in the clouds: How changing land use shaped an Andean biodiversity hotspot

Abstract

Past land use, particularly fire, affects modern tropical forests. Charcoal from lake sediments is commonly used to estimate past fire parameters such as burn severity and frequency, but fire intensity also plays a major role in shaping vegetation and vegetation change. Past fire intensity has remained elusive using common paleoecological approaches. We present a new approach to reconstruct past fire (pyrolysis) temperature, a metric of fire intensity, and reveal how human fire use changed and shaped biodiverse Andean montane forests over the last 2100 years. We use spectra obtained from micro-Fourier Transformed Infrared Spectroscopy (FTIR) of individual charcoal particles recovered from the sediments of Lagua de los Condores (Peru) to characterize its chemical composition. We then compare the spectra generated from the sedimentary charcoal fragments with a modern reference dataset to infer the pyrolysis temperature at which they were formed. Reconstructed maximum pyrolysis temperature varied with changes in land use and changes in precipitation. Mid-temperature fires (500–600 °C) dominated the record, and co-occurred with maize cultivation. After 1200 CE the Chachapoya people, referred to as cloud warriors by the Incas, started to use the site for ceremonial purposes as the climate got wetter. We demonstrate a concomitant change in the complete fire regime with fires becoming less severe, less frequent and burning at a lower temperature after this transition. This change in land use resulted in the first forest recovery in 2000 years, which was mainly composed of species with low bark thickness, a trait of fire sensitivity. Our reconstruction of pyrolysis temperature demonstrates that the analysis of fire severity, frequency, and our added metric of intensity, is needed to understand the drivers of past vegetation change.