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Bristlecone pine (Pinus longaeva) (PILO) trees exhibit exceptional longevity. Their tree‐ring width (TRW) series offer valuable insights into climatic variability. Maximum latewood density (MXD) typically correlates better with temperature variations than TRW, yet PILO MXD records are non‐existent due to methodological challenges related to tree‐ring structure. Here, we used an X‐ray Computed Tomography (X‐ray CT) toolchain on 51 PILO cores from the California White Mountains to build a chronology that correlates significantly (r = 0.66, p < 0.01) with warm‐season (March‐September) temperature over a large spatial extent. This led to the first X‐ray CT‐based temperature reconstruction (1625–2005 CE). Good reconstruction skill (RE = 0.51, CE = 0.32) shows that extending MXD records across the full length of the PILO archive could yield a robust warm‐season temperature proxy for the American Southwest over millennia. This breakthrough opens avenues for measuring MXD in other challenging conifers, increasing our understanding of past climate further, particularly in lower latitudes. Plain Language Summary: Ancient Bristlecone pine trees can live for several millennia and hold invaluable climate information. Their annual rings were used to develop millennium‐length records of the Holocene climate. Maximum latewood density (MXD), which is the highest wood density value in the latewood of a tree ring, has been shown to closely follow summer temperature in different conifer species, but not yet in Bristlecone pine. The gnarly and twisted growth of these ancient trees has presented significant hurdles for MXD analysis. Here we apply an X‐ray Computed Tomography toolchain that allows us to 3D scan through the tissue of a tree ring and to map MXD variations. Using this new technique, we were able to reconstruct warm‐season temperature for the American Southwest back to 1625 CE. With these findings, we are confident that a full‐length reconstruction (back to 2575 BCE) can yield the longest annually resolved temperature reconstruction for this continent. Key Points: We present the first X‐ray Computed Tomography‐derived maximum latewood density‐based temperature reconstruction using Bristlecone pine tree coresBristlecone pine maximum latewood density is a reliable proxy for warm‐season temperature over a large part of the American SouthwestOur reconstruction (1625–2005 CE) contains low‐frequency variability and can be prolonged over a large part of the Holocene [ABSTRACT FROM AUTHOR]

Abstract Bristlecone pine (Pinus longaeva) (PILO) trees exhibit exceptional longevity. Their tree‐ring width (TRW) series offer valuable insights into climatic variability. Maximum latewood density (MXD) typically correlates better with temperature variations than TRW, yet PILO MXD records are non‐existent due to methodological challenges related to tree‐ring structure. Here, we used an X‐ray Computed Tomography (X‐ray CT) toolchain on 51 PILO cores from the California White Mountains to build a chronology that correlates significantly (r = 0.66, p

Water-use efficiency (WUE), weighing the balance between plant transpiration and growth, is a key characteristic of ecosystem functioning and a component of tree drought resistance. Seasonal dynamics of tree-level WUE and its connections with drought variability have not been previously explored in sky-island montane forests. We investigated whole-tree transpiration and stem growth of bristlecone (Pinus longaeva) and limber pine (Pinus flexilis) within a high-elevation stand in central-eastern Nevada, United States, using sub-hourly measurements over 5 years (2013–2017). A moderate drought was generally observed early in the growing season, whereas interannual variability of summer rains determined drought levels between years, i.e., reducing drought stress in 2013–2014 while enhancing it in 2015–2017. Transpiration and basal area increment (BAI) of both pines were coupled throughout June–July, resulting in a high but relatively constant early season WUE. In contrast, both pines showed high interannual plasticity in late-season WUE, with a predominant role of stem growth in driving WUE. Overall, bristlecone pine was characterized by a lower WUE compared to limber pine. Dry or wet episodes in the late growing season overrode species differences. Our results suggested thresholds of vapor pressure deficit and soil moisture that would lead to opposite responses of WUE to late-season dry or wet conditions. These findings provide novel insights and clarify potential mechanisms modulating tree-level WUE in sky-island ecosystems of semi-arid regions, thereby helping land managers to design appropriate science-based strategies and reduce uncertainties associated with the impact of future climatic changes. [ABSTRACT FROM AUTHOR]

Water-use efficiency (WUE), weighing the balance between plant transpiration and growth, is a key characteristic of ecosystem functioning and a component of tree drought resistance. Seasonal dynamics of tree-level WUE and its connections with drought variability have not been previously explored in sky-island montane forests. We investigated whole-tree transpiration and stem growth of bristlecone (Pinus longaeva) and limber pine (Pinus flexilis) within a high-elevation stand in central-eastern Nevada, United States, using sub-hourly measurements over 5 years (2013–2017). A moderate drought was generally observed early in the growing season, whereas interannual variability of summer rains determined drought levels between years, i.e., reducing drought stress in 2013–2014 while enhancing it in 2015–2017. Transpiration and basal area increment (BAI) of both pines were coupled throughout June–July, resulting in a high but relatively constant early season WUE. In contrast, both pines showed high interannual plasticity in late-season WUE, with a predominant role of stem growth in driving WUE. Overall, bristlecone pine was characterized by a lower WUE compared to limber pine. Dry or wet episodes in the late growing season overrode species differences. Our results suggested thresholds of vapor pressure deficit and soil moisture that would lead to opposite responses of WUE to late-season dry or wet conditions. These findings provide novel insights and clarify potential mechanisms modulating tree-level WUE in sky-island ecosystems of semi-arid regions, thereby helping land managers to design appropriate science-based strategies and reduce uncertainties associated with the impact of future climatic changes.

Edmund Schulman is rightly honored for quantifying the age of bristlecone pines and discovering individuals significantly older than giant sequoias (Sequoiadendron gigantea), previously thought to be the oldest living things. However, George Engelmann inferred the potential for great age in his description of bristlecone pine (Pinus aristata) almost a century before, in 1863. Staff from Inyo National Forest re-made Engelmann's inference, and publically asserted that White Mountain bristlecones might outlive giant sequoias before Schulman had published any results of his bristlecone research. Schulman sampledWhiteMountains pines after seeing a photograph and caption associated with an article by founders of the University of California White Mountain Research Station. Although Schulman's correspondence and publications make this clear, incorrect theories regarding his decision to come to theWhite Mountains have been published and are promulgated at the Schulman Grove Visitor Center in the White Mountains. This paper places Schulman's work in its historic context by recovering forgotten information about attempts by Inyo National Forest staff and White Mountain Research Station to call attention to the trees. It also recovers details of Schulman's and C. Wesley Ferguson's activities in the White Mountains range as documented in field notes and Thomas Harlan's Bristlecone Pine Project database. [ABSTRACT FROM AUTHOR]

Two floating, ring-width chronologies predate the long bristlecone pine (Pinus longaeva) absolutelydated, ring-width chronology from the Methuselah Walk (MWK) site in the White Mountains of California. The two non-overlapping floating chronologies were derived from samples that crossdate internally but are temporally unconnected to each other and to the nearly 9000-year, ring-width sequence that is crossdated to the calendar year. We used radiocarbon wiggle-matching and crossdating to place the two floating sequences more accurately in time and to better understand the temporal relationships between the three time series. The trees from the oldest floating sequence were alive near the beginning of the Pleistocene/Holocene boundary and they do not overlap with the other chronologies because of a gap of two-to-three centuries between the two floating series. However, the trees from the younger floating sequence likely do overlap with the long, calendar-dated MWK chronology. We find a possible 57-year overlap that connects these two. If confirmed with additional work, the resulting tree-ring dated annual record from this single location will span 10,359 years, a unique accomplishment in dendrochronology. [ABSTRACT FROM AUTHOR]

Xylem structure optimizes water conductivity while preventing hydraulic failure via embolism resistance, but how this process is modulated by climate variability and how it affects secondary growth in mature trees is still not fully understood, particularly in water-limited environments. Using quantitative wood anatomy techniques, we estimated xylem anatomical proxies for hydraulic efficiency (xylem specific conductivity, Ks) and safety (cell wall reinforcement, wrein) in two western US conifers, Pinus flexilis and Pinus longaeva , at a montane and subalpine location respectively. We built two large datasets (570 rings for P. flexilis , 635 rings for P. longaeva) to investigate 1) the variability of anatomical parameters (i.e lumen diameter, cell wall thickness) and hydraulic proxies along the stem in the five outermost rings (2009–2013); 2) the response of hydraulic proxies to daily climate over a period of 24 years (1990–2013); and 3) the relationship between xylem hydraulic architecture and basal area increment (BAI). Lumen diameter scaled along the stem following a power function, but the scaling patterns of cell wall thickness and hydraulic proxies differed significantly between species. From 1990–2013, Ks decreased in both species, whereas wrein increased only in P. longaeva , while no trends were observed in BAI. Climate sensitivity of Ks peaked over a longer period (84–102 days) compared to wrein (20–55 days), responding to increasing minimum temperature. In both species, Ks was a better predictor of BAI than wrein , indicating that, even under severely water-limited conditions, radial growth is linked to hydraulic efficiency rather than safety. Based on the variability of cell density along the stem, the trade-off between hydraulic efficiency and safety in P. longaeva appeared to be controlled by a strategy of space occupation. Characterizing the mechanistic relationship between xylem anatomy, plant hydraulic functioning, and stem growth is necessary to better understand climate-growth relationships in the western US and species' growth plasticity under future climate change scenarios. • Tracheid size scales along the stem following a power function. • Xylem hydraulic traits scale differently along the stem between species. • Hydraulic efficiency and safety respond to evaporative demands in different periods. • Hydraulic efficiency prevails on safety as a predictor of basal area growth. • The relationship xylem hydraulics – plant growth is not affected by drought stress. [ABSTRACT FROM AUTHOR]

Abstract: Aim: Position of the alpine tree line ecotone around the globe corresponds to a growing season isotherm of approximately 6°C. Accordingly, tree lines are important indicators of Holocene global and regional environmental change. A central line of inquiry in tree line ecology is to better understand the mechanisms that dictate tree line position on the landscape, as well as the environmental conditions that cause upslope and downslope shifts in its position at centennial to millennial time‐scales. Here, we present a climate‐driven model to predict tree line position over the past 6,000 years. Location: Sheep Mountain, located in the White Mountain Range of California, USA. Time period: 4750 bce to present. Major taxa studied: Great Basin bristlecone pine (Pinus longaeva BK Bailey). Methods: We use a climate‐driven tree line position model that utilizes a topoclimate raster surface of growing season average temperature to predict the spatial position and area of the alpine tree line ecotone across the mountain range. We then produce a time series of tree line position predictions at 500‐year intervals from 4750 bce to present, and compare the predictions to the growth dates and spatial locations of 61 remnant bristlecone pine samples from above modern tree line. Results: The model indicates that tree line position in the White Mountains, CA migrated downslope throughout the Holocene until approximately 750 ce, rebounded slightly upslope by 1250 ce, and has since likely remained stationary. Applying the model under present‐day climatic conditions suggests the current tree line at Sheep Mountain may be out of climatic equilibrium by up to 250 vertical metres in some places. Conclusion: The results support independent conclusions from global tree line analyses, underscore the temperature sensitivity of the tree line ecotone, and further develop our understanding of climate‐driven tree line dynamics. [ABSTRACT FROM AUTHOR]

Mountain conifers in the Great Basin of North America have provided some of the longest, continuous, and annually resolved paleoclimate records. Climate-growth relationships at the cellular level, which help understand wood formation processes that underlie dendroclimatic reconstructions, are at present largely unexplored in the Great Basin. We analyzed 42 trees located in the Snake Range (eastern Nevada, USA) at three sites along an elevation gradient. Sampled species included white fir ( Abies concolor ), Douglas fir ( Pseudotsuga menziesii ), limber pine ( Pinus flexilis ), bristlecone pine ( Pinus longaeva ), and Engelmann spruce ( Picea engelmannii ). Wood anatomical features were quantified for two consecutive years, 2011 and 2012. Lumen area, cell wall thickness, lumen diameter, and wall-to-cell ratio were measured for the total ring as well as for earlywood and latewood. Mean standardized tracheidograms highlighted differences between 2011 and 2012, in particular concerning lumen area and wall-to-cell ratio. Most annual variation was due to earlywood, rather than latewood. Anatomical parameters of limber pine, the only species that could be tested at both the montane and subalpine sites, varied with elevation. Principal component analysis showed that the main axis of variability was related to dimensional parameters (e.g. lumen area), which reflected differences in water availability. [ABSTRACT FROM AUTHOR]

Six hundred and eleven Great Basin bristlecone pines ( Pinus longaeva) were surveyed in two separate groves in California's White Mountains. The presence and direction of spiral grain were recorded for each tree as well as elevation above sea level, horizon angles, latitude and longitude, trunk diameter, whether the tree was dead, and whether the trunk was broken. The proportions of left-handed, right-handed and straight trees were similar in every part of both groves, although the groves lie at different elevations. No significant correlation was found between the direction of spiral grain and any environmental factor. The hypothesis that spiral grain is an adaptation to distribute sap evenly between the roots and the crown in Pinus longaeva is not strongly supported, since spiral grain is not correlated with asymmetric environments and most trees exhibit <90° rotation through the main stem. The data also do not support the idea that spiral grain makes the tree more resistant to breaking in strong winds. Right-handed spiral grain is predicted by this hypothesis, but most bristlecone pines are either left-handed or exhibit no spiral grain. Bristlecone pines are often uprooted from thin soils by strong winds, but rarely are the main stems broken by this mechanism. Spiral grain in Pinus longaeva growing in California's White Mountains does not appear to be under environmental control. [ABSTRACT FROM AUTHOR]

The instrumental temperature record is of insufficient length to fully express the natural variability of past temperature. High elevation tree-ring widths from Great Basin bristlecone pine ( Pinus longaeva) are a particularly useful proxy to infer temperatures prior to the instrumental record in that the tree-rings are annually dated and extend for millennia. From ring-width measurements integrated with past treeline elevation data we infer decadal- to millennial-scale temperature variability over the past 4,500 years for the Great Basin, USA. We find that twentieth century treeline advances are greater than in at least 4,000 years. There is also evidence for substantial volcanic forcing of climate in the preindustrial record and considerable covariation between high elevation tree-ring widths and temperature estimates from an atmosphere-ocean general circulation model over much of the last millennium. A long-term temperature decline of ~−1.1 °C since the mid-Holocene underlies substantial volcanic forcing of climate in the preindustrial record. [ABSTRACT FROM AUTHOR]

Abstract: AMS-based 14C measurements have been obtained on 53 dendrochronologically-dated Bristlecone pine (Pinus longaeva) wood samples in decadal increments spanning 2300–2750cal BP with particular interest focused on the decades centered on 2405cal BP and 2625cal BP. In general, there is overall agreement with the current IntCal04/09 consensus calibration curve for this period. For the 2400–2410cal BP interval, our Bristlecone-based 14C value is consistent with the 14C value obtained by Belfast on Irish oak for that decade but not on German oak values obtained by Seattle. Our 14C value for the 2620–2630cal BP interval is consistent with the German oak (Heidelberg)-based 14C decadal value. The 10Be-based reconstruction of 14C variations over the 2620–2630cal BP interval also supports the Heidelberg data although clearly additional clarifications are required before the current tree ring-based 14C and ice core-based 10Be data for this interval can be accurately interpreted and valid inferences obtained. [Copyright &y& Elsevier]

Abstract: We present the first near millennium-length, annually resolved stable isotope record from bristlecone pines (Pinus longaeva, D.K Bailey). The carbon isotope ratios from the cellulose of seven trees from the White Mountains of California, corrected for anthropogenic changes in atmospheric chemistry, are used to reconstruct growing season (June through August) precipitation back to AD 1085. Extremely negative isotope results are strongly correlated with proposed severest El Niño events over the last 500yr, and similar values in the first half of the millennium are used to reconstruct a further 13 strong El Niño events, concentrated in the 12th Century and the mid 13th and 14th Centuries. Ring-width chronologies from adjacent sites in the White Mountains demonstrate a high degree of decadal covariance with the δ13C series, although there are several periods of notable divergence. [Copyright &y& Elsevier]

Abstract: Bristlecone pine trees are exceptionally long-lived, and with the incorporation of remnant material have been used to construct multi-millennial length ring–width chronologies. These chronologies can provide valuable information about past temperature and moisture variability. In this study, we outline a method to build a moisture-sensitive bristlecone chronology and assess the robustness and consistency of this sensitivity over the past 1200yr using new reconstructions of Arkansas River flow (AD 1275–2002 and 1577–2002) and the summer Palmer Drought Sensitivity Index. The chronology, a composite built from parts of three collections in the central Rocky Mountains, is a proxy for decadal-scale moisture variability for the past 18 centuries. Since the sample size is small in some portions of the time series, the chronology should be considered preliminary; the timing and duration of drought events are likely the most robust characteristics. This chronology suggests that the region experienced increased aridity during the medieval period, as did much of western North America, but that the timing and duration of drought episodes within this period were somewhat different from those in other western locations, such as the upper Colorado River basin. [Copyright &y& Elsevier]

The absolutely dated bristlecone pine (Pinus longaeva) tree-ring chronology spans almost 9000 years, offering great potential for inferring past environmental change. Existing ring width chronologies have been widely used to produce some of the most influential millennial length temperature reconstructions for the Northern Hemisphere. A recently published ™18O record from two bristlecone pine trees growing at Methuselah Walk in the White Mountains of California showed a dramatic decrease in ™18O between AD 1850 and 1920 (c. 13‰), interpreted as indicating a major shift in Pacific storm tracks over the past 300 years. Here we present new bristlecone pine ™18O time series from 15 trees at three White Mountains sites, including two series from Methuselah Walk. Whilst occasional high interannual variability is observed in our ™18O series, none of our chronologies exhibit an equivalent pronounced or sustained twentieth-century decrease, suggesting the earlier results are anomalous and may require palaeoclimatic re-interpretation. [ABSTRACT FROM AUTHOR]

Abstract: Many years of low growth identified in a western USA regional chronology of upper forest border bristlecone pine (Pinus longaeva and Pinus aristata) over the last 5000 yr coincide with known large explosive volcanic eruptions and/or ice core signals of past eruptions. Over the last millennium the agreement between the tree-ring data and volcano/ice-core data is high: years of ring-width minima can be matched with known volcanic eruptions or ice-core volcanic signals in 86% of cases. In previous millennia, while there is substantial concurrence, the agreement decreases with increasing antiquity. Many of the bristlecone pine ring-width minima occurred at the same time as ring-width minima in high latitude trees from northwestern Siberia and/or northern Finland over the past 4000–5000 yr, suggesting climatically-effective events of at least hemispheric scale. In contrast with the ice-core records, the agreement between widely separated tree-ring records does not decrease with increasing antiquity. These data suggest specific intervals when the climate system was or was not particularly sensitive enough to volcanic forcing to affect the trees, and they augment the ice core record in a number of ways: by providing confirmation from an alternative proxy record for volcanic signals, by suggesting alternative dates for eruptions, and by adding to the list of years when volcanic events of global significance were likely, including the mid-2nd-millennium BC eruption of Thera. [Copyright &y& Elsevier]

We examined Pinus aristata Engelm. stands in four locations in Colorado: Almagre Mountain, Black Mountain, Goliath Peak and Quartzville. All stands are located at 3200–3700 m and face south-southeast. We measured maximum mass-based assimilation rates (Amax) and nitrogen (N) and phosphorus (P) foliar concentrations on six foliar age classes, from which instantaneous photosynthetic N- and P-use efficiencies (PNUE and PPUE, respectively) and P:N ratios were estimated. Leaf mass per area (LMA) was also determined for each foliar age class from each site. Foliar age, P and N concentrations, and the P:N ratio explained the most variation in Amax when data from all sites were combined. Leaf mass per area did not vary with foliar age class. Both P and N limit Amax, although P appears to be more limiting. The critical P:N ratio is approximately 0.12. Results for Black Mountain differed from the other sites, as Amax was not correlated with age and was negatively correlated with LMA and P. Current findings showed no evidence of N saturation at the Front Range sites (Almagre Mountain and Goliath Peak); however, because P is a limiting nutrient, increased anthropogenic N availability at sites in the Front Range may cause adverse effects on photosynthesis, and perhaps growth, in the future. [ABSTRACT FROM AUTHOR]

In the White Mountains of California, eight bristlecone pine ( Pinus longaeva ) tree-ring width chronologies were developed from trees at upper treeline and just below upper treeline along North- and South-facing elevational transects from treeline to ~90 m below. There is evidence for a climate-response threshold between approximately 60–80 vertical m below treeline, above which trees have shown a positive growth-response to temperature and below which they do not. Chronologies from 80 m or more below treeline show a change in climate response and do not correlate strongly with temperature-sensitive chronologies developed from trees growing at upper treeline. Rather, they more closely resemble lower elevation precipitation-sensitive chronologies. At the highest sites, trees on South-facing slopes grow faster than trees on North-facing slopes. High growth rates in the treeline South-facing trees have declined since the mid-1990s. This suggests the possibility that the climate-response of the highest South-facing trees may have changed and that temperature may no longer be the main limiting factor for growth on the South aspect. These results indicate that increasing warmth may lead to a divergence between tree growth and temperature at previously temperature-limited sites.

Seed and pollen were collected from Great Basin bristlecone pine trees (Pinus longaeva D. K. Bailey) at Mammoth Creek (Utah) and the Methuselah Grove (California). Analyses determined whether seed and pollen viability, seed weight and germinability, and seedling biomass decreased with increasing age of the parent tree and whether putative mutations increased with increasing parent tree age. There were signifcant differences in seed weight and seedling biomass between the two locations. Seed from the Methuselah Grove was lighter and Methuselah seedling biomass was lower at all four harvest dates. However, regression analyses did not identify any relationship between tree age and any variable.

We investigated the interaction between soil water supply and atmospheric evaporative demand for driving the seasonal pattern of transpiration in sky-island high-elevation forest ecosystems. Sap flow measurements were collected at 10-minute intervals for five consecutive years (2013–2017) on two co-occurring subalpine conifers, i.e. limber pine (Pinus flexilis) and bristlecone pine (Pinus longaeva). Our study site is part of the Nevada Climate-ecohydrological Assessment Network (NevCAN), and is located at 3355 m a.s.l. within an undisturbed mixed-conifer stand. We found that seasonal changes in soil moisture regulated transpiration sensitivity to atmospheric conditions. Sap flow density was mainly limited by evaporative demands under non-water limiting conditions, but was influenced only by soil moisture when water availability decreased. Daily sap flow density increased with radiation and soil moisture in June and July when soil moisture was generally above 10%, but correlated only with soil moisture in August and September when soil drought occurred. Sap flow sensitivity to vapor pressure deficit and solar radiation was therefore reduced under decreasing soil moisture conditions. Transpiration peaked in mid-to-late June during both dry and wet years, with a lower peak in late summer during wet years. Normalized mean daily canopy conductance of both species declined with decreasing soil moisture (i.e., increasing soil drought). Severe soil drying (i.e., soil moisture <7% at 20 cm depth), which was rarely detected in wet summers (2013–2014) but occurred more frequently in dry summers (2015–2017), induced a minimum in crown conductance with unchanged low-level sap flow, which might potentially trigger hydraulic failure. The minimum sap flow level under severe soil drought was higher for limber pine than bristlecone pine, possibly because of wider tracheids in limber compared to bristlecone pine. Our findings provide insights into physiological mechanisms of drought-induced stress for iconic sky-island five-needle pines located at high elevation in xeric environments. Unlabelled Image • Sap flow of five-needle pines in sky-island ecosystems is rarely studied. • Sap flow response to evaporative demand was reduced with decreasing soil moisture. • Pinus flexilis sap flow responded to prior wet winters more than Pinus longaeva. • Greater late-season soil water availability enhanced sap flow for both pines. • Severe soil drying induced minimum crown conductance with unchanged stem sap flow. [ABSTRACT FROM AUTHOR]