White Mountains Of California Research Articles

Controls on debris-flow avulsions: White Mountains of California and Nevada

Controls on debris-flow avulsions: White Mountains of California and Nevada

Symbiotic interactions above treeline of long‐lived pines: Mycorrhizal advantage of limber pine (Pinus flexilis) over Great Basin bristlecone pine (Pinus longaeva) at the seedling stage

Abstract In response to contemporary changes in climate, many tree species are shifting upslope to find favorable habitat. In the case of obligate ectomycorrhizal species, seedling growth above upper treeline depends on fungal spore availability. In the mountain ranges of the Great Basin, a recent shift in tree species stratification has been recorded, with limber pine (LP, Pinus flexilis) leapfrogging above the ancient bristlecone pine (BCP, Pinus longaeva) forest and establishing above current treeline. We compared the ability of LP and BCP to interact with soil spore banks collected at different microhabitats (next to dead trees, young live trees or in a treeless control) above current treeline in the White Mountains of California. We found an ectomycorrhizal fungal spore bank community composed of 15 species that was dominated by an undescribed and a hitherto unsequenced species of Geopora and Rhizopogon, respectively. This represents a much richer community than was found previously in this system. While both LP and BCP were able to establish ectomycorrhiza, LP was twice as likely to do so, and when comparing only seedlings that were colonized, its root system was colonized to a three‐fold greater extent. BCP seedlings grown on soils collected under young live trees were much more likely to be colonized compared to soils from the other two microhabitats. Synthesis. These differences in ectomycorrhizal receptivity might help to explain why LP is currently establishing at higher rates above the BCP treeline. Furthermore, it is possible that LP saplings above treeline can provide ectomycorrhizal facilitation for BCP seedlings, enabling the subsequent shift of BCP above treeline.

DATING THE METHUSELAH WALK BRISTLECONE PINE FLOATING CHRONOLOGIES

Two floating, ring-width chronologies predate the long bristlecone pine (Pinus longaeva) absolutely-dated, 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.

Spatiotemporal Variability in the Climate Growth Response of High Elevation Bristlecone Pine in the White Mountains of California

AbstractTree‐ring chronologies from bristlecone pine (Pinus longaeva) are a unique proxy used to understand climate variability over the middle to late Holocene. The annual rings from trees growing toward the species' lower elevational range are sensitive to precipitation variability. Interpretation of the ring‐width signal at the upper forest border has been more difficult. We evaluate differences in climate induced by topography (topoclimate) to better understand the dual signals of temperature and moisture. We unmix signals from trees growing at and near the upper forest border based on the seasonal mean temperature (SMT) experienced by each tree. We find that trees growing in exposures with SMT <7.5 ∘C are limited by temperature, while trees with SMT > 7.5 ∘C are limited by moisture. We demonstrate this independently through analysis of growth in the frequency and time domains and using a process model of xylogenesis. Furthermore, we identify increasing moisture sensitivity in trees formerly limited by temperature.

Nitrogenous air pollutants and ozone exposure in the central Sierra Nevada and White Mountains of California – Distribution and evaluation of ecological risks

Ammonia (NH3), nitric oxide (NO), nitrogen dioxide (NO2), nitric acid (HNO3), and ozone (O3) were measured in summers of 2012 and 2013 with passive samplers. Nine monitoring sites were on W-E transect (511 to 3490 m) across central Sierra Nevada Mountains (SNM), and five sites on elevational gradient (1237 to 4346 m) in White Mountains (WM) of California. Levels of pollutants were similar in 2012 and 2013 in all sites. NH3, NO2, and HNO3 were highest near highly polluted Central Valley of California (CVC): maximum summer season means 7.8 μg m−3, 3.0 ppb, and 3.0 μg m−3, respectively. Regional background for NH3, NO2, and HNO3 in SNM occurred >20 km from CVC and >1500 m with seasonal averages: 2.1–4.8 μg m−3; 0.8–1.7 ppb; 1.0–1.8 μg m−3, respectively, during two seasons. Levels of NH3, NO2, and HNO3 in WM remote locations were similar: 1.2–3.3 μg m−3, 0.6–1.1 ppb, and 1.0–1.3 μg m−3, respectively. Seasonal mean O3 (38–60 ppb) in SNM did not change with distance from CVC nor elevation. In WM, O3 and NO mixing ratios were 41–61 ppb and 2.3–4.1 ppb, respectively, increasing with elevation. Even the lowest NH3 concentrations determined in this study were higher than NH3 continental background. This fact, as well as high values of Nreduced/Noxidized near CVC of 1.9 in 2012 and 2.0 in 2013, decreasing with distance to 0.7 in 2012 and 0.8 in 2013, show importance of NH3 emissions from CVC as a contributor to N deposition and ecological impacts in SNM. The phytotoxic O3 indices, AOT40 and W126, for selected sites on SNM and WM transects, showed high potential for negative O3 impacts on vegetation, including forest trees. CapsuleElevated NH3, NO2, and HNO3 on the western slopes of the Sierra Nevada Mountains (SNM) near the Central Valley of California (CVC) decreased with distance from CVC and elevation to regional background levels also recorded at high elevation sites of the White Mountains (WM).

Shrub range expansion alters diversity and distribution of soil fungal communities across an alpine elevation gradient.

Global climate and land use change are altering plant and soil microbial communities worldwide, particularly in arctic and alpine biomes where warming is accelerated. The widespread expansion of woody shrubs into historically herbaceous alpine plant zones is likely to interact with climate to affect soil microbial community structure and function; however, our understanding of alpine soil ecology remains limited. This study aimed to (i) determine whether the diversity and community composition of soil fungi vary across elevation gradients and to (ii) assess the impact of woody shrub expansion on these patterns. In the White Mountains of California, sagebrush (Artemisia rothrockii) shrubs have been expanding upwards into alpine areas since 1960. In this study, we combined observational field data with a manipulative shrub removal experiment along an elevation transect of alpine shrub expansion. We utilized next-generation sequencing of the ITS1 region for fungi and joint distribution modelling to tease apart effects of the environment and intracommunity interactions on soil fungi. We found that soil fungal diversity declines and community composition changes with increasing elevation. Both abiotic factors (primarily soil moisture and soil organic C) and woody sagebrush range expansion had significant effects on these patterns. However, fungal diversity and relative abundance had high spatial variation, overwhelming the predictive power of vegetation type, elevation and abiotic soil conditions at the landscape scale. Finally, we observed positive and negative associations among fungal taxa which may be important in structuring community responses to global change.

A climate‐driven tree line position model in the White Mountains of California over the past six millennia

AbstractAimPosition 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.LocationSheep Mountain, located in the White Mountain Range of California, USA.Time period4750 bce to present.Major taxa studiedGreat Basin bristlecone pine (Pinus longaeva BK Bailey).MethodsWe 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.ResultsThe 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.ConclusionThe 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.

Warming and shrub encroachment decrease decomposition in arid alpine and subalpine ecosystems

ABSTRACT Climate change is shifting species distributions and altering plant community composition worldwide. For instance, with rising temperatures shrubs are encroaching into alpine ecosystems, resulting in important implications for ecosystem functioning. In particular, woody-plant encroachment could slow decomposition in systems traditionally dominated by herbaceous species. To evaluate how litter decomposition responded jointly to warming and shrub presence, we conducted a passive warming chamber experiment in subalpine and alpine plant communities in the White Mountains of California. Passive warming chambers were placed over plots with and without the range-expanding sagebrush Artemisia rothrockii at two elevations. Litter from A. rothrockii and the common perennial herb Trifolium andersonii decomposed for two years under the experimental treatments. Nitrate availability was measured with ion-exchange resins during the same time period. Warming decreased decomposition rates overall, associated with decreased soil moisture, but did not influence soil nitrate availability. Sagebrush presence decreased both decomposition rates and nitrate availability. Hence, future warming in this system will likely reduce decomposition rates, both directly and indirectly, via shrub encroachment. However, impacts on nutrient mineralization are less clear. These findings highlight how shifting species composition, through processes such as range expansions, can influence ecosystem responses to climate change.

The Construction of Sense of Place: Observations from the Field

An introspective look at the illustrative power of field experience, this article makes analogies between field experiences and the larger pursuit of geography research. The setting is the White Mountains of California, but the ideas in this article expand the local experience that informs much larger themes.

Changing climate response in near-treeline bristlecone pine with elevation and aspect

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.

Simultaneous in situ determination of U‐Pb and Sm‐Nd isotopes in monazite by laser ablation ICP‐MS

AbstractResults are presented for in situ simultaneous determination of U‐Pb and Sm‐Nd isotopes in monazite using the Laser Ablation Split‐Stream (LASS) method. This method uses a laser ablation system coupled to a magnetic‐sector inductively coupled plasma mass spectrometer (HR) (ICP‐MS) for measuring U‐Pb isotopes and a multicollector (MC) ICP‐MS for measuring Sm‐Nd isotopes. Ablated material is split using a Y‐connector and transported simultaneously to both mass spectrometers. In addition to Sm and Nd isotopes, the MC‐ICP‐MS is configured to also acquire Ce, Nd, Sm, Eu, and Gd elemental abundances. This approach provides age, tracer isotope, and trace element data in the same ablation volume, reducing sampling problems associated with fine‐scale zoning in accessory minerals and minimizing the material needed for ablation. Precision and accuracy of the U‐Pb method (and the precision of the Sm‐Nd method) is demonstrated with results from well‐characterized monazite reference materials. The LASS results agree within uncertainty with the isotope dilution thermal ionization mass spectrometry (ID‐TIMS) U‐Pb dates. The accuracy of the Sm‐Nd method is assessed by comparing the LA‐MC‐ICP‐MS results with ID‐TIMS determinations on a well‐characterized, in‐house monazite reference material. The LASS method is then applied to monazite from the Birch Creek Pluton in the White Mountains of California as a case study to illustrate the utility of this method for solving geologic problems. The U‐Pb ages and Sm‐Nd isotopic data from the LASS method support the conclusions drawn from previous results that monazite can record timing and information about the source region(s) of hydrothermal fluids.

Probabilistic reconstructions of local temperature and soil moisture from tree-ring data with potentially time-varying climatic response

We explore a probabilistic, hierarchical Bayesian approach to the simultaneous reconstruction of local temperature and soil moisture from tree-ring width observations. The model explicitly allows for differing calibration and reconstruction interval responses of the ring-width series to climate due to slow changes in climatology coupled with the biological climate thresholds underlying tree-ring growth. A numerical experiment performed using synthetically generated data demonstrates that bimodality can occur in posterior estimates of past climate when the data do not contain enough information to determine whether temperature or moisture limitation controlled reconstruction-interval tree-ring variability. This manifestation of nonidentifiability is a result of the many-to-one mapping from bivariate climate to time series of tree-ring widths. The methodology is applied to reconstruct temperature and soil moisture conditions over the 1080–1129 C.E. interval at Methusalah Walk in the White Mountains of California, where co-located isotopic dendrochronologies suggest that observed moisture limitations on tree growth may have been alleviated. Our model allows for assimilation of both data sources, and computation of the probability of a change in the climatic controls on ring-width relative to those observed in the calibration period. While the probability of a change in control is sensitive to the choice of prior distribution, the inference that conditions were moist and cool at Methuselah Walk during the 1080–1129 C.E. interval is robust. Results also illustrate the power of combining multiple proxy data sets to reduce uncertainty in reconstructions of paleoclimate.

Excursions in the 14C record at A.D. 774–775 in tree rings from Russia and America

AbstractThe calibration of radiocarbon dates by means of a master calibration curve has been invaluable to Earth, environmental and archeological sciences, but the fundamental reason for calibration is that atmospheric radiocarbon content varies because of changes in upper atmosphere production and global carbon cycling. Improved instrumentation has contributed to high‐resolution (interannual) radiocarbon activity measurements, which have revealed sudden and anomalous activity shifts previously not observed at the common resolution of 5–10 years of most of the calibration scale. One such spike has been recently reported from tree rings from Japan and then again in Europe at A.D. 774–775, for which we report here our efforts to both replicate its existence and determine its spatial extent using tree rings from larch at high latitude (northern Siberia) and bristlecone pine from lower latitude (the White Mountains of California). Our results confirm an abrupt ~ 15‰ 14C activity increase from A.D. 774 to 776, the size and now the hemispheric extent of which suggest that an extraterrestrial influence on radiocarbon production is most likely responsible.

Fine-Scale Patterns of Soil and Plant Surface Temperatures in an Alpine Fellfield Habitat, White Mountains, California

Within alpine environments the interactions of air temperature, solar irradiance, wind, surface albedo, microtopography, and biotic traits all influence patterns of soil and plant canopy temperatures. The resulting mosaic of surface temperatures has a profound impact on ecosystem processes, plant survival, and ecophysiological performance. Previous studies have documented large and persistent variations in microhabitat temperatures over mesoscale alpine terrains. We have used a novel mobile system to examine changes in soil and plant canopy surface temperatures at spatial scales of centimeters and temporal scales of minutes in an alpine fellfield habitat in the White Mountains of California. In the middle of a summer day, the mean surface temperature differences between points 2, 5, and 10 cm apart were 2.9, 5.4, and 9.0 °C, respectively, and extreme differences of 18 °C or more were found over distances of a few centimeters. These thermal patterns are due not only to substrate material but also to biotic conditions of plant canopy architecture and ecophysiological traits of individual species. The magnitude of temperature variation at these fine scales is greater than the range of warming scenarios in Intergovernmental Panel on Climate Change (IPCC) projections, suggesting that these habitats offer the capacity of significant thermal heterogeneity for plant survival.

Ibuprofen Prevents Altitude Illness: A Randomized Controlled Trial for Prevention of Altitude Illness With Nonsteroidal Anti-inflammatories

Acute mountain sickness occurs in more than 25% of the tens of millions of people who travel to high altitude each year. Previous studies on chemoprophylaxis with nonsteroidal anti-inflammatory drugs are limited in their ability to determine efficacy. We compare ibuprofen versus placebo in the prevention of acute mountain sickness incidence and severity on ascent from low to high altitude. Healthy adult volunteers living at low altitude were randomized to ibuprofen 600 mg or placebo 3 times daily, starting 6 hours before ascent from 1,240 m (4,100 ft) to 3,810 m (12,570 ft) during July and August 2010 in the White Mountains of California. The main outcome measures were acute mountain sickness incidence and severity, measured by the Lake Louise Questionnaire acute mountain sickness score with a diagnosis of ≥ 3 with headache and 1 other symptom. Eighty-six participants completed the study; 44 (51%) received ibuprofen and 42 (49%) placebo. There were no differences in demographic characteristics between the 2 groups. Fewer participants in the ibuprofen group (43%) developed acute mountain sickness compared with those receiving placebo (69%) (odds ratio 0.3, 95% confidence interval 0.1 to 0.8; number needed to treat 3.9, 95% confidence interval 2 to 33). The acute mountain sickness severity was higher in the placebo group (4.4 [SD 2.6]) than individuals receiving ibuprofen (3.2 [SD 2.4]) (mean difference 0.9%; 95% confidence interval 0.3% to 3.0%). Compared with placebo, ibuprofen was effective in reducing the incidence of acute mountain sickness.

An annually resolved bristlecone pine carbon isotope chronology for the last millennium

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 500 yr, 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 δ 13C series, although there are several periods of notable divergence.

Surface ozone in the White Mountains of California

Surface ozone concentrations are presented for four high-elevation sites along a north–south transect along the spine of the White Mountains and a fifth site located at lower elevation approximately 15 km to the west on the floor of the Owens Valley. The ozone data, which were collected from mid-June through mid-October of 2009, include results from two sites, White Mountain Summit (4342 m elevation) and Barcroft Station (3783 m), that are believed to be higher in elevation than any previously investigated sampling locations in North America. Average daily ozone values from the five sampling sites display similar day-to-day and week-to-week temporal fluctuations, which suggest that the sites are experiencing the same regional-scale background patterns in air quality and meteorology. Ozone concentrations increase with increasing elevation, consistent with findings from prior studies in Europe and North America. A linear elevation gradient of +0.0042 ppb m −1 is obtained for July 15–August 15, but analogous gradients for August 15–September 15 and September 15–October 15 show reduced linearity and possibly the onset of a plateau in ozone concentrations for elevations above 2000 m. Average diurnal cycle magnitudes decrease with increasing elevation, falling from ∼25–35 ppb for the Owens Valley site to ∼3–7 ppb at three of the four high-elevation sites. Diurnal cycle magnitudes decrease (or remain roughly constant) at the non-Summit sites during the progression from mid-July to mid-October, but the magnitude of the diurnal cycle at the Summit increases from ∼3 ppb to ∼7 ppb over this same time frame. This latter result is inconsistent with results from previous investigations at other alpine sites, and may indicate the presence of local, topography-influenced mixing dynamics that are unique to the White Mountains. High hourly ozone concentrations at White Mountain Summit are found to correlate with 72-hour HYSPLIT back-trajectories that reflect enhanced levels of ozone transport from polluted regions (such as the Central Valley of California) or meteorological conditions that are favorable for ozone production. Low ozone concentrations at the Summit are found to correlate with HYSPLIT back-trajectories that reflect reduced levels of ozone transport from polluted areas or meteorological conditions that are unfavorable for ozone production. ► We present surface ozone data for high-elevation sites in the White Mountains. ► Our measurements are higher in elevation than any previous North American studies. ► Low O 3 at White Mountain Summit correlates with reduced transport of polluted air from inland areas. ► High O 3 at White Mountain Summit correlates with enhanced transport of polluted air from inland areas.

Temporal stability in bristlecone pine tree-ring stable oxygen isotope chronologies over the last two centuries

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 δ 18O record from two bristlecone pine trees growing at Methuselah Walk in the White Mountains of California showed a dramatic decrease in δ 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 δ18O 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 δ 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.

Monitoring for potential effects of climate change on the vegetation of two alpine meadows in the White Mountains of California, USA

East Shield (3658 m) and Barcroft Gate (3697 m) Meadows in the White Mountains of California on the Inyo National Forest were selected for a pilot baseline monitoring study over five-year intervals to determine potential effects of future climate change on the meadows. Stated as a hypothesis: given the natural range of temporal variability of meadow vegetation, a significant portion of that variability is attributed to the effects of climatic variation. The study is associated with the Global Observation Research Initiative in Alpine Environments Project (GLORIA) hosted by the University of California White Mountain Research Station. Both meadows are sedge–rush–grass wet meadows, or fens, which are sustained by surface water in the form of streams and springs. A comprehensive plant list was compiled and the plant species of each meadow were sampled by a point intercept method along two randomly placed transects. The occurrence of all species at predetermined one-meter interval sample points along the transects were recorded, percent occurrence was calculated, and descriptive statistics were done. The monitoring objective is to detect a 20% biologically meaningful change in the vegetation over time with 90% confidence, and accept a 10% chance of a false change error (Type 1 error). Monitoring is planned to be done in conjunction with tracking change in precipitation and temperature. Three indicators of possible climate change were chosen. One indicator is increase in the abundance of those species whose upper range is at or below meadow elevations. The rationale for this indicator is based the 2007 IPCC report estimation that a doubling of atmospheric CO 2 will increase global temperature with a best estimate of +3.0 °C. Assuming that the present general lapse rate of −6.32 °C per kilometer for the White Mountains will persist, a 3.0 °C increase may result in an equivalent elevation of the meadows of about +475 m. Published elevation ranges indicate that most recorded species are not likely to be affected by the estimated temperature increase. Second and third indicators are a shift in cover from wet meadow to dry meadow species, and the presence and increase of shrub species resulting from postulated reduced precipitation with drying of meadows and encroachment by surrounding shrubs (sagebrush and rabbitbrush). This monitoring study has the potential to detect long-term responses of vegetation to climate change, and reduce uncertainties and confirm or correct the expected consequences of temperature change.

Modeled and observed intra-ring δ 18O cycles within late Holocene Bristlecone Pine tree samples

The oxygen isotope ratio of alpha-cellulose produced by Bristlecone Pine trees from the White Mountains of California was measured for three different 10-year tree-ring sections at near-weekly resolution to investigate how the growing season climate cycle influences the annual to inter-annual isotopic variability of whole-ring samples. We selected sections of wood that grew under periods of generally reduced (155–172 BC) and increased (AD 1106–1122; AD 1827–1834) growth to assess whether or not the environmental parameters that are responsible for ring-width variability, including water availability, also influence the intra-annual cellulosic isotope variability. Each annual ring is characterized by a parabolic function of intraseasonal δ 18O change that reflects humidity and temperature-dependent fractionations of H 2 18O utilized by the tree during the growth period. Subtle shifts in the shape and amplitude of the seasonal δ 18O cycle are attributed to changing growing season length but the differences are not large enough to bias a whole-ring isotopic value. We model the isotope fractionations based on growing season climate variables and tree physiology and find that the measured δ 18O is in close agreement with the model-predicted values. The model retains no information on previous winter conditions and thus confirms that these trees draw upon an isotopically homogenized water source. The intraseasonal δ 18O cycle arises entirely from fractionations due to summer season temperature and humidity. A persistent issue with dendroclimatic reconstructions is addressing seasonal biases in the record, we find that in this environment where growth and precipitation are separated by many months we are able to capture distinct information on both seasons by coupling average annual values with information on sub-seasonal variability.