Radial growth decline in a tropical Andean treeline in Bolivia

The impact of rising temperatures on tropical treeline ecosystems remains understudied. Here we report on the growth history of Polylepis pepei trees growing in Madidi National Park, a tropical forest setting at elevational treeline in the Andes-Amazon ecotone of Bolivia. Using dendrochronological methods, we developed an annually-resolved tree-ring width chronology spanning from 1850 to 2018 CE. To our knowledge this is the longest tree-ring record for this species (168 years). The standardized ring-width chronology revealed a significant radial growth decline after the 1962–1963 growth ring (i.e. 1963–2018). From 1960–2015, smaller ring-widths were associated with drier and warmer conditions during the previous wet season (∼ November–March), while wetter and cooler conditions led to enhanced growth in the following year. P. pepei radial growth decline coincided with a significant increase in minimum temperature and decrease in precipitation and diurnal temperature range that was observed both locally and regionally across tropical South America between 1960–2015. These climate trends at treeline may indicate a reduction in moisture convergence and transport to such higher elevation settings in the Andes. If temperatures continue to rise at current rates, P. pepei growth may continue to decline, threatening the survival of tropical treeline ecosystems in Bolivia.