Climate and physiological signals in tree-ring stable isotopes of Polylepis tarapacana from the South American Altiplano
Polylepis tarapacana is the longest paleoclimatic tree-ring archive in the South American southern tropics. It grows up to 5200 m a.s.l. in the South American Altiplano, a semiarid-high elevation Andean Plateau. P. tarapacana ring-widths (RW) have provided centuries of past hydroclimate information, but the potential use of tree-ring stable isotopes for paleoclimatic or ecophysiological studies remained understudied for this species. Here, we developed a network of four RW, oxygen (δ18O) and carbon (δ13C) stable isotope chronologies from P. tarapacana tree rings for 1950-present along a latitudinal-aridity gradient (18ºS-22ºS). Among these three parameters, δ18O recorded the strongest current growing season January to March precipitation signal (r=0.6-0.8), coincident with the peak of the South American Summer Monsoon in this region. This finding allowed for the development of the first centennial precipitation reconstruction (1700-2013) based on P. tarapacana tree-ring δ18O, which explains more than 55% of the variance in precipitation. From the physiological perspective, the development of this network was crucial to identify that current-growing season temperature regulated RW at the northern-wetter sites, while prior-growing season precipitation influenced RW at the southern-drier sites. Warm and dry current growing seasons resulted in more enriched tree-ring δ13C and δ18O at all study sites. The synchronous (asynchronous) climate conditions influencing RW and stable isotopes, as well as significant (non-significant) correlations between RW and δ13C in the northern (southern) sites, indicate a coupling (decoupling) between wood formation and leaf gas exchange for wetter (drier) conditions along the aridity gradient.