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Abstract. Given the short span of instrumental precipitationrecords in the South American Altiplano, longer-term hydroclimatic recordsare needed to understand the nature of climate variability and to improvethe predictability of precipitation, a key natural resource for thesocioeconomic development in the Altiplano and adjacent arid lowlands. Inthis region grows Polylepis tarapacana, a long-lived tree species that is very sensitive tohydroclimatic changes and has been widely used for tree-ring studies in thecentral and southern Altiplano. However, in the northern sector of thePeruvian and Chilean Altiplano (16–19∘ S)still exists a gap of high-resolution hydroclimatic data based on tree-ringrecords. Our study provides an overview of the temporal evolution of thelate-spring–mid-summer precipitation for the period 1625–2013 CE at thenorthern South American Altiplano, allowing for the identification of wet ordry periods based on a regional reconstruction from three P. tarapacana chronologies. Anincrease in the occurrence of extreme dry events, together with a decreasingtrend in the reconstructed precipitation, has been recorded since the 1970sin the northern Altiplano within the context of the last ∼4 centuries. The average precipitation over the last 17 years stands outas the driest in our 389-year reconstruction. We reveal a temporal andspatial synchrony across the Altiplano region of dry conditions since themid-1970s. Independent tree-ring-based hydroclimate reconstructions andseveral paleoclimatic records based on other proxies available for thetropical Andes record this synchrony. The influence of El Niño–SouthernOscillation (ENSO) on the northern Altiplano precipitation was detected byour rainfall reconstruction that showed past drier conditions in our studyregion associated with ENSO warm events. The spectral properties of therainfall reconstruction showed strong imprints of ENSO variability atdecadal, sub-decadal, and inter-annual timescales, in particular from thePacific NIÑO 3 sector. Overall, the recent reduction in precipitation incomparison with previous centuries, the increase in extreme dry events andthe coupling between precipitation and ENSO variability reported by thiswork is essential information in the context of the growing demand for waterresources in the Altiplano. This study will contribute to a betterunderstanding of the vulnerability and resilience of the region to theprojected evapotranspiration increase for the 21st century associated withglobal warming. 

Abstract Tree growth is generally considered to be temperature limited at upper elevation treelines, yet climate factors controlling tree growth at semiarid treelines are poorly understood. We explored the influence of climate on stem growth and stable isotopes for Polylepis tarapacana Philipi, the world’s highest elevation tree species, which is found only in the South American Altiplano. We developed tree-ring width index (RWI), oxygen (δ18O) and carbon (δ13C) chronologies for the last 60 years at four P. tarapacana stands located above 4400 m in elevation, along a 500 km latitude aridity gradient. Total annual precipitation decreased from 300 to 200 mm from the northern to the southern sites. We used RWI as a proxy of wood formation (carbon sink) and isotopic tree-ring signatures as proxies of leaf-level gas exchange processes (carbon source). We found distinct climatic conditions regulating carbon sink processes along the gradient. Current growing-season temperature regulated RWI at northern-wetter sites, while prior growing-season precipitation determined RWI at arid southern sites. This suggests that the relative importance of temperature to precipitation in regulating tree growth is driven by site water availability. By contrast, warm and dry growing seasons resulted in enriched tree-ring δ13C and δ18O at all study sites, suggesting that similar climate conditions control carbon-source processes along the gradient. Site-level δ13C and δ18O chronologies were significantly and positively related at all sites, with the strongest relationships among the southern drier stands. This indicates an overall regulation of intercellular carbon dioxide via stomatal conductance for the entire P. tarapacana network, with greater stomatal control when aridity increases. This manuscript also highlights a coupling (decoupling) between physiological processes at leaf level and wood formation as a function of similarities (differences) in their climatic sensitivity. This study contributes to a better understanding and prediction of the response of high-elevation Polylepis woodlands to rapid climate changes and projected drying in the Altiplano. 

Tree mortality is a key process in forest dynamics. Despite decades of effort to understand this process, many uncertainties remain. South American broadleaf species are particularly under-represented in global studies on mortality and forest dynamics. We sampled monospecific broadleaf Nothofagus pumilio forests in northern Patagonia to predict tree mortality based on stem growth. Live or dead conditions in N. pumilio trees can be predicted with high accuracy using growth rate as an explanatory variable in logistic models. In Paso Córdova (CO), Argentina, where the models were calibrated, the probability of death was a strong negative function of radial growth, particularly during the six years prior to death. In addition, negative growth trends during 30 to 45 years prior to death increased the accuracy of the models. The CO site was affected by an extreme drought during the summer 1978–1979, triggering negative trends in radial growth of many trees. Individuals showing below-average and persistent negative trends in radial growth are more likely to die than those showing high growth rates and positive growth trends in recent decades, indicating the key role of droughts in inducing mortality. The models calibrated at the CO site showed high verification skill by accurately predicting tree mortality at two independent sites 76 and 141 km away. Models based on relative growth rates showed the highest and most balanced accuracy for both live and dead individuals. Thus, the death of individuals across different N. pumilio sites was largely determined by the growth rate relative to the total size of the individuals. Our findings highlight episodic severe drought as a triggering mechanism for growth decline and eventual death for N. pumilio, similar to results found previously for several other species around the globe. In the coming decades, many forests globally will be exposed to more frequent and/or severe episodes of reduced warm-season soil moisture. Tree-ring studies such as this one can aid prediction of future changes in forest productivity, mortality, and composition. 

South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions. 

Fire scars on trees are created by excessive heat from a fire that kills the vascular cambium. Although, fires are one of the most important forest disturbances in Patagonia, the effects of fire on tree physiology and wood anatomy are still unknown. In this study, we hypothesized that abnormal functioning of the cambium after a fire will induce anatomical changes in the wood. We also assumed that these anatomical changes would affect xylem safety transport.

We quantified wood anatomical traits inNothofagus pumilio, the dominant subalpine tree species of Patagonia, using two approaches: time and distance. In the first, anatomical changes in tree rings were compared before, during, and after fire occurrence. In the second, the spatial extent of these changes was evaluated with respect to the wound by measuring anatomical traits in sampling bands in two directions (0° and 45°) with respect to the onset of healing.

Reductions in lumen diameter and vessel number were the most conspicuous changes associated with fire damage and observed in the fire ring and subsequent post‐fire rings. In addition, the fire ring had more rays than in control rings. In terms of distance, anatomical changes were only restricted to short distances from the wound.

Post‐fire changes in wood anatomical traits were confined close to the wound margins. These changes might be associated with a defense strategy related to the compartmentalization of the wound and safety of water transport.