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1. A global perspective on the climate‐driven growth synchrony of neighbouring trees

   https://doi.org/10.1111/geb.13090  

   Tejedor, Ernesto ; Serrano‐Notivoli, Roberto ; de Luis, Martin ; Saz, Miguel Angel ; Hartl, Claudia ; St. George, Scott ; Büntgen, Ulf ; Liebhold, Andrew M. ; Vuille, Mathias ; Esper, Jan ; et al ( March 2020 , Global Ecology and Biogeography)

   Previous work demonstrated the global variability of synchrony in tree growth within populations, that is, the covariance of the year‐to‐year variability in growth of individual neighbouring trees. However, there is a lack of knowledge about the causes of this variability and its trajectories through time. Here, we examine whether climate can explain variation in within‐population synchrony (WPS) across space but also through time and we develop models capable of explaining this variation. These models can be applied to the global tree cover under current and future climate change scenarios.

   Global.

   1901–2012.

   Trees.

   We estimated WPS values from a global tree‐ring width database consisting of annual growth increment measurements from multiple trees at 3,579 sites. We used generalized linear mixed effects models to infer the drivers of WPS variability and temporal trends of global WPS. We then predicted WPS values across the global extent of tree cover. Finally, we applied our model to predict future WPS based on the RCP 8.5 (2045–2065 period) emission scenario.

   Areas with the highest WPS are characterized by a combination of environments with both high mean annual temperature (>10°C) and low precipitation (<300 mm). Average WPS across all temperate forests has decreased historically and will continue to decrease. Potential implications of these patterns include changes in forest dynamics, such as higher tree growth and productivity and an increase in carbon sequestration. In contrast, the WPS of tropical forests of Central and South America will increase in the near future owing to reduced annual precipitation.

   Climate explains WPS variability in space and time. We suggest that WPS might have value as an integrative ecological measure of the level of environmental stress to which forests are subjected and therefore holds potential for diagnosing effects of global climate change on tree growth.

    

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2. Different climate sensitivity for radial growth, but uniform for tree-ring stable isotopes along an aridity gradient in Polylepis tarapacana , the world’s highest elevation tree species

   https://doi.org/10.1093/treephys/tpab021  

   Rodriguez-Caton, Milagros ; Andreu-Hayles, Laia ; Morales, Mariano S ; Daux, Valérie ; Christie, Duncan A ; Coopman, Rafael E ; Alvarez, Claudio ; Rao, Mukund Palat ; Aliste, Diego ; Flores, Felipe ; et al ( February 2021 , Tree Physiology)

   Cernusak, Lucas (Ed.)

   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. 

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3. Radial Growth Patterns Associated with Tree Mortality in Nothofagus pumilio Forest

   https://doi.org/10.3390/f10060489  

   Rodríguez-Catón, Milagros ; Villalba, Ricardo ; Srur, Ana ; Williams, A. Park ( June 2019 , Forests)

   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. 

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4. Continental‐scale tree‐ring‐based projection of Douglas‐fir growth: Testing the limits of space‐for‐time substitution

   https://doi.org/10.1111/gcb.15170  

   Klesse, Stefan ; DeRose, Robert Justin ; Babst, Flurin ; Black, Bryan A. ; Anderegg, Leander D. L. ; Axelson, Jodi ; Ettinger, Ailene ; Griesbauer, Hardy ; Guiterman, Christopher H. ; Harley, Grant ; et al ( June 2020 , Global Change Biology)

   A central challenge in global change research is the projection of the future behavior of a system based upon past observations. Tree‐ring data have been used increasingly over the last decade to project tree growth and forest ecosystem vulnerability under future climate conditions. But how can the response of tree growth to past climate variation predict the future, when the future does not look like the past? Space‐for‐time substitution (SFTS) is one way to overcome the problem of extrapolation: the response at a given location in a warmer future is assumed to follow the response at a warmer location today. Here we evaluated an SFTS approach to projecting future growth of Douglas‐fir (Pseudotsuga menziesii), a species that occupies an exceptionally large environmental space in North America. We fit a hierarchical mixed‐effects model to capture ring‐width variability in response to spatial and temporal variation in climate. We found opposing gradients for productivity and climate sensitivity with highest growth rates and weakest response to interannual climate variation in the mesic coastal part of Douglas‐fir's range; narrower rings and stronger climate sensitivity occurred across the semi‐arid interior. Ring‐width response to spatial versus temporal temperature variation was opposite in sign, suggesting that spatial variation in productivity, caused by local adaptation and other slow processes, cannot be used to anticipate changes in productivity caused by rapid climate change. We thus substituted only climate sensitivities when projecting future tree growth. Growth declines were projected across much of Douglas‐fir's distribution, with largest relative decreases in the semiarid U.S. Interior West and smallest in the mesic Pacific Northwest. We further highlight the strengths of mixed‐effects modeling for reviving a conceptual cornerstone of dendroecology, Cook's 1987 aggregate growth model, and the great potential to use tree‐ring networks and results as a calibration target for next‐generation vegetation models.

    

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5. Delineating Environmental Stresses to Primary Production of U.S. Forests From Tree Rings: Effects of Climate Seasonality, Soil, and Topography

   https://doi.org/10.1029/2019JG005499  

   Dannenberg, Matthew P. ; Song, Conghe ; Wise, Erika K. ; Pederson, Neil ; Bishop, Daniel A. ( February 2020 , Journal of Geophysical Research: Biogeosciences)

   Primary production is the entry point of energy and carbon into ecosystems, but modeling responses of primary production to “environmental stress” (i.e., reductions of primary production from nonoptimal environmental conditions) remains a key challenge and source of uncertainty in our understanding of Earth's carbon cycle. Here we develop an approach for estimating annual “environmental stress” from tree rings based on the proportion of the optimal diameter growth rate (from species‐specific allometric equations) that is realized in a given year. We assessed climatic, topographic, and soil drivers of environmental stress, as well as their interactions, using both empirical model experiments and linear mixed effect models. Climate gradients and interannual climate variability dominated spatial and temporal variability of environmental stress in much of the western United States, where the tree‐ring environmental stress index was positively correlated with antecedent climatic water balance (precipitation minus potential evapotranspiration) and negatively correlated with temperature and vapor pressure deficit. Excluding topographic and soil information from empirical models reduced their ability to capture spatial gradients in environmental stress, particularly in the eastern United States, where growth was not as strongly limited by climate. Mean climate conditions and topographic characteristics had significant interaction effects with the climatic water balance, indicating an increasing importance of winter moisture for warmer and drier sites and as elevation and topographic wetness index increased. These results suggest that including effects of antecedent climate (particularly in dry regions) and site topographic and soil characteristics could improve parameterization of environmental stress effects in primary production models.

    

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