Polylepis woodland dynamics during the last 20,000 years

Abstract

Aim: To determine the palaeoecological influences of climate change and humanland use on the spatial distribution patterns ofPolylepiswoodlands in the Andes.Location:Tropical Andes above 2,900 m between 2°S and 18°S of latitude.Methods:Pollen and charcoal data were gathered from 13 Andean lake sedimentrecords and were rescaled by the maximum value in each site. The rescaled pollendata were used to estimate a mean abundance and coefficient of variation to showwoodland expansions/contractions and woodland fragmentation over the last20,000 years. The rescaled charcoal was displayed as a 200-year moving medianusing 500-year bins to infer the influence of fire on woodland dynamics at land-scape scale. Pollen and charcoal were compared with speleothem, clastic flux andarchaeological data to assess the influence of moisture balance, glacial activity andhuman impact on the spatial distribution ofPolylepiswoodlands.Results:Woodland expansion and fire were correlated with precipitation changesand glacier dynamics fromc. 20 to 6 kcalBP(thousands of calibrated years beforepresent). Charcoal abundances between 20 and 12 kcalBPwere less common thanfrom 12 kcalBPto modern. However, human-induced fires were unlikely to be themain cause of a woodland decline centred at 11 kcalBP, as woodlands recoveredfrom 10.5 to 9.5 kcalBP(about twofold increase). Charcoal peaks analogous tothose that induced the woodland decline at 11 kcalBPwere commonplace post-9.5 kcalBPbut did not trigger an equivalent woodland contraction. An increase inthe coefficient of variation afterc. 5.5 kcalBPsuggests enhanced fragmentation andcoincided with the shift from logistic to exponential growth of human populations.Over the last 1,000 years,Polylepisbecame hyper-fragmented with over half of siteslosingPolylepisfrom the record and with coefficients of variation paralleling thoseof glacial times.