Radial growth decline in a tropical Andean treeline in Bolivia

Abstract. Relative to research efforts in higher latitiudes, the impact of climate shifts in the tropical treeline remains understudied. Little is known about the tree growth dynamics and climate response at this treeline over the past few centuries, and at present under a rapidly changing environment. Here we provide information on recent changes in tree-ring patterns of Polylepis pepei BB.Simpson, a tropical tree species that grows in a monospecific forest at the elevational treeline in the Andes-Amazon ecotone of Bolivia and identify factors that limit its radial growth. We first developed a ring width (RW) chronology spanning 1867–2018 C.E. using dendrochronological methods and independently verified annual periodicity with radiocarbon dating. The RW chronology indicates a significant (p < 0.01) radial growth decline in P. pepei since 1997, a trend that mirrors a decrease reported in other Polylepis species from the drier central Andes of South America. P. pepei tree-ring width (RW) was mostly limited by mean, minimum, and maximum temperature and precipitation during austral summer (November–January). Over the instrumental period (1981–2019) prior-year temperatures negatively affected current-year tree growth (p < 0.05), while prior-year wet conditions were associated with higher growth (p < 0.05). Gridded temperature records (1901–2019) showed a significant increase in minimum temperatures and a decline in the diurnal temperature range since 1967, which may reduce orographic convection and water availability at higher elevations where our forest is located. In situ daily measurements from dataloggers in the forest recorded higher temperatures and lower relative humidity values when data was available. Our results suggest less moisture availability associated with warming conditions was related to the observed tree-growth decline. If temperature continues to rise at current rates, one of the highest-elevation tree species on the globe, P. pepei, could face severe consequences. This work provides insights into the past and historical trends of a tropical Andean treeline, which shows a recent decline also observed in other high-elevation forests (4657–4800 m.a.s.l.) of tropical South America (>17° S).