David W. Stahle

Tree-ring chronologies were used to reconstruct growing-season precipitation for the North American Corn Belt from 1182–2023 CE. The reconstruction explains close to 60% of the variance in instrumental May–July precipitation totals (1901–2014) and reproduces the teleconnections with circulation and surface climate variables detected in the instrumental precipitation data, substantiating the skill of the reconstruction and indicating episodic co-occurrence of drought between major crop production regions of the central United States and Eurasia. The episode with five consecutive years of deficit May through July precipitation totals in the Corn Belt from 1930–1934 during the Dust Bowl drought was exceeded by several uninterrupted droughts reconstructed during the pre-instrumental period. The early 17 ^th century drought (1627–1639) was the most severe, sustained, and unbroken drought in the reconstruction for at least 600-years and has no analog in the modern instrumental observations. This drought coincided with decadal dryness in China and eastern Europe and provides a paleoclimatic example of simultaneous megadrought in major breadbasket regions of Eurasia and North America. The decade of wetness during the early 21 ^st century in the instrumental observations appears to have been unmatched for 700-years and may have been enhanced by increased humidity and radiation changes associated with the dramatic intensification of agriculture in the Corn Belt.

Assessing long-term hydroclimatic variability in central Mexico is essential to understand regional water availability and groundwater recharge for urban centers such as Querétaro. This study developed a multi-century winter–spring precipitation reconstruction for the Sierra Gorda Biosphere Reserve (SGBR) using ring width chronologies of Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco. Standard dendrochronological techniques were applied to develop a 284-year master chronology (1731–2015). Following the accepted Subsample Signal Strength criterion (SSS ≥ 0.85) for chronology reliability, the reconstruction was restricted to the 1744–2015 period, yielding a statistically robust 271-year December–April precipitation record. A bootstrapped ordinary least-squares regression model relating tree-ring indices to instrumental December–April precipitation was calibrated and validated using split-sample cross-validation, explaining 46% of the instrumental precipitation variance (R² = 0.46) and yielding positive verification statistics (RE = 0.38–0.58; CE = 0.37–0.57). Spatial field correlations against gridded climate data (CRU TS4.08) confirmed a broad regional hydroclimatic signal centered over the Sierra Madre Oriental. Continuous wavelet transform (CWT), spectral analysis, superposed epoch analysis (SEA), and wavelet coherence (WTC) revealed significant interannual (2–8 years) and decadal (10–20 years) variability associated with large-scale ocean–atmosphere climate modes, including the El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO), and Tropical North Atlantic (TNA) index. The pronounced sensitivity of these conifer forests to pre-monsoonal moisture deficits highlights their vulnerability to projected warming and increasing spring evapotranspiration stress. Although the reconstruction is limited to pre-monsoonal (December–April) precipitation, it provides a robust centuries-long baseline for contextualizing regional hydroclimatic variability and supports water-resource management, groundwater conservation, and climate-adaptation strategies in central Mexico.

The magnitude of the terrestrial carbon sink remains a key uncertainty in future climate projections, in part due to poorly understood links between carbon uptake and its allocation to woody biomass in vegetation. Here, in this study, we show that photosynthesis and aboveground growth occur asynchronously across diel to seasonal scales in eight North American oak species. Across 137 tree ring sites, current-year annual growth was insensitive to climate variability after midsummer despite 26 to 36% of annual gross primary productivity (GPP) occurring during this period. Hourly GPP flux and growth measurements at four sites spanning seven site years further demonstrate that wood formation ceases earlier than photosynthesis and is restricted to periods of low atmospheric aridity and temperature. This photosynthesis-growth decoupling intensifies with interannual variability in vapor pressure deficit ( r = 0.86, P < 0.05), suggesting that by assuming tight coupling between photosynthesis and woody biomass, current earth system models may overestimate long-term carbon sequestration in forests.

Recent years have seen strong droughts and floods in the Amazon basin, the largest center of atmospheric convection on land. To assess to what degree these events are extreme in a historical perspective requires accurate and long‐term climate data, which are generally lacking for this part of the world. Here, we developed a 131‐year oxygen isotope chronology from exactly dated tree rings of Cedrela odorata from the eastern Amazon Basin. The chronology (1885–2016) correlates strongly with observed wet‐season rainfall totals ( r = −0.71, 1951–2016) and stream discharge over the eastern equatorial Amazon. In contrast to oxygen isotope chronologies further inland that record basin‐wide rainfall, our new record provides a good rainfall proxy for the eastern Amazon basin alone and shows that extreme precipitation events are also driven by ENSO.

Our understanding of pre‐modern El Niño Southern Oscillation (ENSO) variability is reliant on proxy records, often distant from the center of ENSO activity in the equatorial Pacific Ocean. Here, we assess the relationship between reconstructed soil moisture in four distant ENSO‐influenced regions over the past 400 years. A major breakdown in the teleconnection of regional drought conditions in Asia, Eastern Australia, and North America is identified around 1700 CE. The statistically significant decline in inter‐series correlations ( p < 0.01) represents a previously unknown aspect of global hydroclimate dynamics. We hypothesize that the disruption was driven by ENSO weakening and/or by a large‐scale multi‐decadal reconfiguration of ocean‐atmosphere circulation. Data assimilation estimates of soil moisture from the same regions fail to produce results of the same magnitude, potentially due to an overreliance of ENSO influence on the boundaries of spatial covariance in the underlying climate models.

The term ‘Miracle March’ is colloquially used in California to describe an anomalously wet spring that alleviates drought conditions caused by a winter precipitation deficit. This study centers on the influential water year 1991, which is the standard reference point for this phenomenon. The spring precipitation impact on water year totals and the geographic extent of the spring precipitation anomaly during the 1991 ‘Miracle March’ (MM91) is without equal in the observational record. This exceptional precipitation episode was initiated by a week-long sequence of extreme atmospheric rivers that started landfalling on February 27th. To this day, MM91 is routinely referenced as a basis for hope of drought alleviation in California water management. MM91 demonstrates the role that an exceptional spring precipitation season can play in determining the cessation of drought, but a historical analysis also shows just how unusual such conditions are.

Photorespiration can limit gross primary productivity in terrestrial plants. The rate of photorespiration relative to carbon fixation increases with temperature and decreases with atmospheric [CO ₂ ]. However, the extent to which this rate varies in the environment is unclear. Here, we introduce a proxy for relative photorespiration rate based on the clumped isotopic composition of methoxyl groups ( R –O–CH ₃ ) in wood. Most methoxyl C–H bonds are formed either during photorespiration or the Calvin cycle and thus their isotopic composition may be sensitive to the mixing ratio of these pathways. In water-replete growing conditions, we find that the abundance of the clumped isotopologue ¹³ CH ₂ D correlates with temperature (18–28 °C) and atmospheric [CO ₂ ] (280–1000 ppm), consistent with a common dependence on relative photorespiration rate. When applied to a global dataset of wood, we observe global trends of isotopic clumping with climate and water availability. Clumped isotopic compositions are similar across environments with temperatures below ~18 °C. Above ~18 °C, clumped isotopic compositions in water-limited and water-replete trees increasingly diverge. We propose that trees from hotter climates photorespire substantially more than trees from cooler climates. How increased photorespiration is managed depends on water availability: water-replete trees export more photorespiratory metabolites to lignin whereas water-limited trees either export fewer overall or direct more to other sinks that mitigate water stress. These disparate trends indicate contrasting responses of photorespiration rate (and thus gross primary productivity) to a future high-[CO ₂ ] world. This work enables reconstructing photorespiration rates in the geologic past using fossil wood.

Recent severe droughts, extreme floods, and increasing differences between seasonal high and low flows on the Amazon River may represent a twenty-first-century increase in the amplitude of the hydrologic cycle over the Amazon Basin. These precipitation and streamflow changes may have arisen from natural ocean–atmospheric variability, deforestation within the drainage basin of the Amazon River, or anthropogenic climate change. Tree-ring reconstructions of wet-season precipitation extremes, substantiated with historical accounts of climate and river levels on the Amazon River and in northeast Brazil found in the Brazilian Digital Library, indicate that the recent river-level extremes on the Amazon may have been equaled or possibly exceeded during the preinstrumental nineteenth century. The “Forgotten Drought” of 1865 was the lowest wet-season rainfall total reconstructed with tree-rings in the eastern Amazon from 1790 to 2016 and appears to have been one of the lowest stream levels observed on the Amazon River during the historical era according to first-hand descriptions by Louis Agassiz, his Brazilian colleague João Martins da Silva Coutinho, and others. Heavy rains and flooding are described during most of the tree-ring-reconstructed wet extremes, including the complete inundation of “First Street” in Santarem, Brazil, in 1859 and the overtopping of the Bittencourt Bridge in Manaus, Brazil, in 1892. These extremes in the tree-ring estimates and historical observations indicate that recent high and low flow anomalies on the Amazon River may not have exceeded the natural variability of precipitation and streamflow during the nineteenth century. Significance Statement: Proxy tree-ring and historical evidence for precipitation extremes during the preinstrumental nineteenth century indicate that recent floods and droughts on the Amazon River may have not yet exceeded the range of natural hydroclimatic variability.

Subsets of annual and sub-annual tree-ring chronologies are used to reconstruct seasonal precipitation totals in northern California. The specific seasons selected for reconstruction are based on the strongest monthly precipitation signals recorded in the tree-ring data. Earlywood width of gray pine is best correlated with Oct-Dec precipitation at the onset of the wet season. Latewood width of ponderosa pine is correlated with Mar–Apr totals at the end of the wet season. These earlywood and latewood width chronologies are used to develop separate reconstructions of precipitation for the “autumn” (Oct–Dec) and “spring” (Mar–Apr) seasons. Total ring-width chronologies of blue oak are highly correlated with October–April precipitation totals and are used to reconstruct precipitation for the “wet season.” We then computed one additional skillful reconstruction by subtracting the reconstructed spring totals from the wet season precipitation estimates (i.e., “winter” [Oct–Feb]). We compare the winter and spring reconstructions because they are well calibrated and provide an interesting long-term perspective on the interaction of winter–spring precipitation amounts near March 1, when important reservoir management decisions are often made. Consecutive wet winter and very wet spring precipitation anomalies increased after 1950 in the instrumental and reconstructed time-series, often coinciding with the largest spring streamflow and flood events recorded on the American River at Folsom. Once the sub-annual tree-ring data can be improved, it may be possible to develop discrete reconstructions of early-, middle-, and late-season precipitation for the past 250 to 500 years, to help define natural variability and anthropogenic forcing of seasonal precipitation totals in California.

Reverence for old trees sparked early forest conservation efforts, even as ancient woodlands were cut with impunity