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1. Interannual dynamics of stemwood nonstructural carbohydrates in temperate forest trees surrounding drought

   https://doi.org/10.1007/s11676-022-01566-2  

   Blumstein, Meghan J.; Furze, Morgan E. (February 2023, Journal of Forestry Research)

   NA (Ed.)

   Abstract Interactions between water and carbon dynamics underlie drought-related tree mortality. While whole-tree water relations have been shown to play a key role in the response to and recovery from drought, the role of nonstructural carbohydrates (NSC) and how their storage and allocation changes surrounding drought events deserves further attention and is critical for understanding tree survival. Here, we quantified in situ NSC responses of temperate forest trees to the 2016 drought in the northeastern United States. Sugar and starch concentrations were measured in the stemwood of five tree species from 2014 to 2019, which allowed us to monitor NSCs in relation to climatic conditions before, during, and after the natural drought. We found that immediately following the drought, measured stemwood NSC concentrations decreased. However, NSC concentrations rebounded quickly within three years. Notably, trees allocated proportionally more to starch than to sugars following the 2016 drought. In winter 2017, starch comprised 45% of total stemwood stores, whereas starch made up only 1–2% in other years. Further, we modeled and assessed the climatic drivers of total NSC concentrations in the stem. Variation in total NSC concentrations was significantly predicted by the previous year’s temperature, precipitation, and standardized precipitation-evapotranspiration index (SPEI), with stemwood concentrations decreasing following hotter, drier periods and increasing following cooler, wetter periods. Overall, our work provides insight into the climatic drivers of NSC storage and highlights the important role that a tree’s carbon economy may play in its response and recovery to environmental stress. 

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2. Rapid drought‐recovery of gas exchange in Caragana species adapted to low mean annual precipitation

   https://doi.org/10.1111/pce.14635  

   Bi, Min‐Hui; Jiang, Chao; Yao, Guang‐Qian; Turner, Neil C; Scoffoni, Christine; Fang, Xiang‐Wen (August 2023, Plant, Cell & Environment)

   Abstract While variation in mean annual precipitation (MAP) of the native habitat of a species has been shown to determine the ability of a species to resist a hydraulic decrease during drought, it remains unknown whether these variations in MAP also influence the ability of a species to recover and survive drought. Leaf hydraulic and gas exchange recovery following drought and the underlying mechanisms of these responses in sixCaraganaspecies from habitats along a large precipitation gradient were investigated during rehydration in a common garden. The gas exchange of species from arid habitats recovered more rapidly during rehydration after mild, moderate and severe drought stress treatments than species from humid habitats. The recovery of gas exchange was not associated with foliar abscisic acid concentration, but tightly related to the recovery of leaf hydraulic conductance (K_leaf). The recovery ofK_leafwas associated with the loss ofK_leafduring dehydration under mild and moderate drought stress, and to leaf xylem embolism formation under severe drought stress. Results pointed to the different ability to recover in gas exchange in sixCaraganaspecies post‐drought is associated with the MAP of the species in its native habitat. 

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3. Moisture stress limits radial mixing of non-structural carbohydrates in sapwood of trembling aspen

   https://doi.org/10.1093/treephys/tpad083  

   Peltier, Drew_M P; Nguyen, Phiyen; Ebert, Chris; Koch, George W; Schuur, Edward_A G; Ogle, Kiona (December 2024, Tree Physiology)

   Cernusak, Lucas (Ed.)

   Abstract Dynamics in non-structural carbohydrate (NSC) pools may underlie observed drought legacies in tree growth. We assessed how aridity influences the dynamics of different-aged NSC pools in tree sapwood at two sites with differing climate conditions (‘wet’ vs ‘dry’), which also experienced widespread regional drought 5 years earlier. We used an incubation method to measure the radiocarbon (Δ14C) in CO2 respired from Populus tremuloides Michx. (aspen) tree rings to evaluate NSC storage and mixing patterns, coupled with measurements of NSC (soluble sugars and starch) concentrations and respired δ13C-CO2. At a wet site, CO2 respired from rings formed during 1962–67 was only ~11 years old, suggesting deep sapwood mixing of NSCs as starch. At a dry site, the total NSC was about one-third of wet-site totals, maximum ages in deep rings were lower and ages more rapidly increased in shallow rings and then plateaued. These results suggest historically shallower mixing and/or relatively higher consumption of NSCs under dry conditions. Both sites, however, had similar aged NSC (<1 year) in the most recent six rings, indicative of deep radial mixing following relatively wet conditions during the sampling year. We suggest that the significant differences in NSC mixing among sites are driven by moisture stress, where aridity reduces NSC reserves and restricts the depth of radial mixing. However, dynamic climate conditions in the south-western USA resulted in more complex radial patterns of sapwood NSC age than previously described. We suggest a novel conceptual framework to understand how moisture variability might influence the dynamics of NSC mixing in the sapwood. 

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4. Divergent pathways across vascular plant lineages drive reduced nighttime transpiration during drought

   https://doi.org/10.1093/plphys/kiaf274  

   Pereira, Talitha S; Manandhar, Anju; Cardoso, Amanda A; Martins, Samuel_C V; McAdam, Scott_A M (August 2025, Plant Physiology)

   Abstract Stomatal closure in response to water deficit is crucial for maintaining plant water balance. While the mechanisms driving daytime stomatal closure under drought are well studied, the mechanism driving progressive declines in nighttime transpiration (Enight) during drought remains less understood. To investigate whether either abscisic acid (ABA) or declining leaf water status drives progressive declines in Enight during drought in vascular plants, we conducted experiments using representative fern, gymnosperm, and angiosperm species, including a severe ABA-deficient mutant and tree species. These species span a spectrum of stomatal control by ABA, ranging from insensitive to endogenous ABA in the fern to reliance on ABA for stomatal closure in the herbaceous angiosperm. We found that reductions in Enight during drought are driven by hydropassive stomatal closure in ferns and gymnosperms, transitioning to ABA regulation in gymnosperms under severe stress, and are triggered by ABA in herbaceous angiosperms. In all species, the proportion of total transpiration occurring at night increased as stomata closed during the drought. The reduction of Enight during drought appears to be a convergent stomatal response across vascular land plants but is driven by diverse regulatory mechanisms linked to evolutionary history and ecological strategy. 

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5. Local groundwater decline exacerbates response of dryland riparian woodlands to climatic drought

   https://doi.org/10.1111/gcb.16376  

   Williams, Jared; Stella, John C.; Voelker, Steven L.; Lambert, Adam M.; Pelletier, Lissa M.; Drake, John E.; Friedman, Jonathan M.; Roberts, Dar A.; Singer, Michael Bliss (January 2022, Global Change Biology)

   Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro-isotopic analysis of radial growth and seasonal (semi-annual) carbon isotope discrimination (Δ13C) to investigate the response of riparian cottonwood stands to the unprecedented California-wide drought from 2012 to 2019, along the largest remaining free-flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ13C) during peak drought years. However, patterns of radial growth and Δ13C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ13C for individual trees, and higher inter-correlation of Δ13C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year−1 beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded 3 m. These findings indicate that drought-induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures. 

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