Polylepiswoodland dynamics during the last 20,000 years

Abstract

AbstractAimTo determine the palaeoecological influences of climate change and human land use on the spatial distribution patterns ofPolylepiswoodlands in the Andes.LocationTropical Andes above 2,900 m between 2°S and 18°S of latitude.MethodsPollen and charcoal data were gathered from 13 Andean lake sediment records and were rescaled by the maximum value in each site. The rescaled pollen data were used to estimate a mean abundance and coefficient of variation to show woodland expansions/contractions and woodland fragmentation over the last 20,000 years. The rescaled charcoal was displayed as a 200‐year moving median using 500‐year bins to infer the influence of fire on woodland dynamics at landscape scale. Pollen and charcoal were compared with speleothem, clastic flux and archaeological data to assess the influence of moisture balance, glacial activity and human impact on the spatial distribution ofPolylepiswoodlands.ResultsWoodland expansion and fire were correlated with precipitation changes and glacier dynamics fromc. 20 to 6 kcalbp(thousands of calibrated years before present). Charcoal abundances between 20 and 12 kcalbpwere less common than from 12 kcalbpto modern. However, human‐induced fires were unlikely to be the main cause of a woodland decline centred at 11 kcalbp, as woodlands recovered from 10.5 to 9.5 kcalbp(about twofold increase). Charcoal peaks analogous to those that induced the woodland decline at 11 kcalbpwere commonplace post‐9.5 kcalbpbut did not trigger an equivalent woodland contraction. An increase in the coefficient of variation afterc. 5.5 kcalbpsuggests enhanced fragmentation and coincided with the shift from logistic to exponential growth of human populations. Over the last 1,000 years,Polylepisbecame hyper‐fragmented with over half of sites losingPolylepisfrom the record and with coefficients of variation paralleling those of glacial times.Main conclusionsPolylepiswoodlands formed naturally patchy woodlands, rather than a continuous vegetation belt, prior to human occupation in the Andes. The main factors controlling pre‐human woodland dynamics were precipitation and landscape heterogeneity. Human activity led to hyper‐fragmentation during the lastc. 1,000 years.