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1. Consequences of climatic thresholds for projecting fire activity and ecological change

   https://doi.org/10.1111/geb.12872  

   Young, Adam M. ; Higuera, Philip E. ; Abatzoglou, John T. ; Duffy, Paul A. ; Hu, Feng Sheng ; Gillespie, ed., Thomas ( February 2019 , Global Ecology and Biogeography)

   Ecological properties governed by threshold relationships can exhibit heightened sensitivity to climate, creating an inherent source of uncertainty when anticipating future change. We investigated the impact of threshold relationships on our ability to project ecological change outside the observational record (e.g., the 21st century), using the challenge of predicting late‐Holocene fire regimes in boreal forest and tundra ecosystems.

   Boreal forest and tundra ecosystems of Alaska.

   850–2100 CE.

   Not applicable.

   We informed a set of published statistical models, designed to predict the 30‐year probability of fire occurrence based on climatological normals, with downscaled global climate model data for 850–1850 CE. To evaluate model performance outside the observational record and the implications of threshold relationships, we compared modelled estimates with mean fire return intervals estimated from 29 published lake‐sediment palaeofire reconstructions. To place our results in the context of future change, we evaluate changes in the location of threshold to burning under 21st‐century climate projections.

   Model–palaeodata comparisons highlight spatially varying accuracy across boreal forest and tundra regions, with variability strongly related to the summer temperature threshold to burning: sites closer to this threshold exhibited larger prediction errors than sites further away from this threshold. Modifying the modern (i.e., 1950–2009) fire–climate relationship also resulted in significant changes in modelled estimates. Under 21st‐century climate projections, increasing proportions of Alaskan tundra and boreal forest will approach and surpass the temperature threshold to burning, with > 50% exceeding this threshold by > 2 °C by 2070–2099.

   Our results highlight a high sensitivity of statistical projections to changing threshold relationships and data uncertainty, implying that projections of future ecosystem change in threshold‐governed ecosystems will be accompanied by notable uncertainty. This work also suggests that ecological responses to climate change will exhibit high spatio‐temporal variability as different regions approach and surpass climatic thresholds over the 21st century.

    

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2. South American Summer Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models

   https://doi.org/10.5194/cp-18-2045-2022  

   Orrison, Rebecca ; Vuille, Mathias ; Smerdon, Jason E. ; Apaéstegui, James ; Azevedo, Vitor ; Campos, Jose Leandro ; Cruz, Francisco W. ; Della Libera, Marcela Eduarda ; Stríkis, Nicolás M. ( January 2022 , Climate of the Past)

   Abstract. The South American Summer Monsoon (SASM) is the maindriver of regional hydroclimate variability across tropical and subtropicalSouth America. It is best recorded on paleoclimatic timescales by stableoxygen isotope proxies, which are more spatially representative of regionalhydroclimate than proxies for local precipitation alone. Network studies ofproxies that can isolate regional influences lend particular insight intovarious environmental characteristics that modulate hydroclimate, such asatmospheric circulation variability and changes in the regional energybudget as well as understanding the climate system sensitivity to externalforcings. We extract the coherent modes of variability of the SASM over thelast millennium (LM) using a Monte Carlo empirical orthogonal function(MCEOF) decomposition of 14 δ18O proxy records and compare themwith modes decomposed from isotope-enabled climate model data. The twoleading modes reflect the isotopic variability associated with (1) thermodynamic changes driving the upper-tropospheric monsoon circulation(Bolivian High–Nordeste Low waveguide) and (2) the latitudinaldisplacement of the South Atlantic Convergence Zone (SACZ). The spatialcharacteristics of these modes appear to be robust features of the LMhydroclimate over South America and are reproduced both in the proxy dataand in isotope-enabled climate models, regardless of the nature of themodel-imposed external forcing. The proxy data document that the SASM wascharacterized by considerable temporal variability throughout the LM, withsignificant departures from the mean state during both the Medieval ClimateAnomaly (MCA) and the Little Ice Age (LIA). Model analyses during theseperiods suggest that the local isotopic composition of precipitation isprimarily a reflection of upstream rainout processes associated with monsoonconvection. Model and proxy data both point to an intensification of themonsoon during the LIA over the central and western parts of tropical SouthAmerica and indicate a displacement of the South Atlantic Convergence Zone(SACZ) to the southwest. These centennial-scale changes in monsoon intensityover the LM are underestimated in climate models, complicating theattribution of changes on these timescales to specific forcings and pointingtoward areas of important model development. 

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3. 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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4. Flood regime alters the abiotic correlates of riparian vegetation

   https://doi.org/10.1111/avsc.12572  

   Steinberg, Kelly A. ; Eichhorst, Kim D. ; Rudgers, Jennifer A. ; Hölzel, ed., Norbert ( April 2021 , Applied Vegetation Science)

   Predicting the influence of climate change on riparian plant communities improves management strategies. The sensitivity of riparian vegetation to climate and other abiotic factors depends on interactions between properties of the ecosystem, like flood regime, and plant characteristics. To explore these interactions, we addressed three questions: (a) does the composition and diversity of riparian vegetation vary with the flood regime; (b) do abiotic correlates of vegetation, including climate and groundwater, differ between sites that flood compared to locations that did not experience floods; and (c) which plant functional groups account for differential plant community sensitivity to abiotic factors between flood regimes?

   Middle Rio Grande Valley, New Mexico.

   We used long‐term observations of plant community composition, groundwater depth, precipitation and interpolated temperature from 24 sites spanning 210 km of the Rio Grande riparian cottonwood–willow forest to explore the relative importance of climate and hydrologic correlates of riparian vegetation diversity and composition.

   Riparian plant diversity was higher at sites flooding compared to non‐flooding sites. Plant diversity positively tracked shallower groundwater depth at flooding sites, but was best predicted by intra‐annual groundwater variability at non‐flooding sites. Plant community composition correlated with groundwater depth and air temperature at all sites, but at non‐flooding sites, also with intra‐annual groundwater variability and precipitation. Relationships between native plant cover and potential environmental drivers diverged strongly between the two flood regimes; non‐native plant cover had only weak relationships with most environmental predictors.

   The current flood regime of a site determined the climate and hydrologic factors that best predicted riparian plant community composition and diversity. Relationships between plant diversity or total cover and groundwater, temperature, precipitation, or groundwater variability can change in strength or direction depending on a site's flood history, highlighting the importance of flood regime to predicting the sensitivity of riparian woodlands to future environmental change.

    

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5. Constraining long‐term model predictions for woody growth using tropical tree rings

   https://doi.org/10.1111/gcb.17075  

   Xu, Xiangtao ; van der Sleen, Peter ; Groenendijk, Peter ; Vlam, Mart ; Medvigy, David ; Moorcroft, Paul ; Petticord, Daniel ; Ma, Yixin ; Zuidema, Pieter A. ( December 2023 , Global Change Biology)

   The strength and persistence of the tropical carbon sink hinges on the long‐term responses of woody growth to climatic variations and increasing CO₂. However, the sensitivity of tropical woody growth to these environmental changes is poorly understood, leading to large uncertainties in growth predictions. Here, we used tree ring records from a Southeast Asian tropical forest to constrain ED2.2‐hydro, a terrestrial biosphere model with explicit vegetation demography. Specifically, we assessed individual‐level woody growth responses to historical climate variability and increases in atmospheric CO₂(C_a). When forced with historical C_a, ED2.2‐hydro reproduced the magnitude of increases in intercellular CO₂concentration (a major determinant of photosynthesis) estimated from tree ring carbon isotope records. In contrast, simulated growth trends were considerably larger than those obtained from tree rings, suggesting that woody biomass production efficiency (WBPE = woody biomass production:gross primary productivity) was overestimated by the model. The estimated WBPE decline under increasing C_abased on model‐data discrepancy was comparable to or stronger than (depending on tree species and size) the observed WBPE changes from a multi‐year mature‐forest CO₂fertilization experiment. In addition, we found that ED2.2‐hydro generally overestimated climatic sensitivity of woody growth, especially for late‐successional plant functional types. The model‐data discrepancy in growth sensitivity to climate was likely caused by underestimating WBPE in hot and dry years due to commonly used model assumptions on carbon use efficiency and allocation. To our knowledge, this is the first study to constrain model predictions of individual tree‐level growth sensitivity to C_aand climate against tropical tree‐ring data. Our results suggest that improving model processes related to WBPE is crucial to obtain better predictions of tropical forest responses to droughts and increasing C_a. More accurate parameterization of WBPE will likely reduce the stimulation of woody growth by C_arise predicted by biosphere models.

    

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