Coupled climate-forest growth shifts in the Chilean Patagonia are decoupled from trends in water-use efficiency

Ecological regime shifts may lead to a loss of resilience if the affected ecosystem experiences rapid and irreversible changes affecting its structure and function. Coupled regime shifts in climate variability and forest growth have been rarely described, albeit they should play a prominent role on forest dynamics. Patagonia hosts the largest forested area across the mid-latitudes of the southern Hemisphere where some unique and long-lived conifer species as Pilgerodendron uviferum (Cupressaceae) are found. In this region, the prominent 1970s climate shift was characterized by a very wet period leading to drier climate conditions afterwards, which caused an unprecedented drop in productivity in most forest types and tree species. However, it is unknown how such climate shift affected P. uviferum radial growth and intrinsic water-use efficiency (iWUE) since this species endemic to the Patagonia inhabits wet and often water-logged sites. Here we studied the response of P. uviferum in growth and iWUE across different sites in the Chilean Patagonia. Unexpectedly, in two out of four sites we found a positive shift in P. uviferum growth, which started in the 1970s. Such growth enhancement, however, was not related to trends in iWUE. Trees in sites showing growth increase presented a loss of sensitivity to the growing-season temperatures after the 1970s. Site and climate were the main predictors of growth trends, whereas the observed iWUE increase was attributed as a response to atmospheric CO2 concentration increases. We conclude that the sustained growth increase observed in P. uviferum was not due to a CO2-fertilization effect because iWUE improvements were similar in sites with or without growth enhancement. Thus, growth and iWUE are decoupled in Patagonian P. uviferum forests.