Stable isotopes of pedogenic carbonates as indicators of paleoecology in the Plio-Pleistocene (upper Bed I), western margin of the Olduvai Basin, Tanzania

Paleosol carbonates from trenches excavated as part of a landscape-scale project in Bed I of Olduvai Gorge, Tanzania, were analyzed for stable carbon and oxygen isotopic composition. The approximately 60,000-year interval ( approximately 1.845-1.785 Ma) above Tuff IB records evidence for lake and fluvial sequences, volcanic eruptions, eolian and pedogenic processes, and the development of a fluvial plain in the western margin of the basin. Significant temporal variation in the carbonate delta(18)O values records variation of local precipitation and supports the shifts in climatic conditions interpreted from the lithologic record. During this period, carbonate delta(13)C values varied between depositional facies indicating that the paleolandscape supported a local biomass of about 40-60% C(4) plants within a mosaic of grassy woodlands and wooded grasslands. The lithologic and stable isotope record in this small lake basin indicates the area was much wetter, with more woody C(3) plants, during this interval than is the semi-arid area today. The record also reflects the variation in climatic conditions (wet/dry) documented by other global climate proxies for this time.     

Natural and Anthropogenic Changes in Chesapeake Bay During the Last 1000 Years

Sediment cores from tributaries, marshes and the main stem of Chesapeake Bay were analyzed for paleoecological indicators of climate change and land use. Indicators include pollen and seeds of terrestrial and aquatic plants, diatoms, charcoal, nutrients, and trace metals. Two major events, one climatic and the other anthropogenic, occurred within the last millennium. The Medieval Climatic Anomaly and the Little Ice Age are recorded in Chesapeake sediments by terrestrial indicators of dry conditions for 200 years, beginning about 1000 years ago, followed by increases in wet indicators from about 800 to 400 years ago. There were no corresponding shifts in estuarine diatoms and seeds of submerged macrophytes. During the last few centuries following European settlement, deforestation and agriculture have resulted in the transport of large sediment and nutrient loads to estuarine waters. The terrestrial flora shifted from arboreal to herbaceous, and much of the estuarine benthic biota was replaced by pelagic species. These changes had a profound effect on the Chesapeake fishery. In assessing risks associated with climate change, it must be recognized that changes wrought by human activity are likely to influence effects of future climate change, in ways not evident from the fossil record.     

The salt lakes of western Canada: A paleolimnological overview

The northern Great Plains of western Canada contain many saline and hypersaline lakes. Deadmoose and Waldsea Lakes in south-central Saskatchewan are meromictic, with saline Mg-Na-SO₄-Cl waters overlying denser brines of similar composition. Mineralogical, chemical, palynological, and stable isotope analyses of the sediments in the Waldsea basin indicate the lake was much shallower about 4 000 years ago in response to a warmer and drier climate. Since that time water levels have generally increased in the basin giving rise to higher organic productivity and greater inorganic carbonate precipitation. Within this overall trend there is also evidence of several lower water stages during the last 3 000 years. The stratigraphy preserved in the Deadmoose basin suggests considerably lower water levels about 1 000 years ago.Ceylon Lake, located about 350 km south of the Waldsea-Deadmoose area, is presently a shallow, saline playa. The basin originated about 15 000 years ago as a glacial meltwater spillway. Stratigraphic variation in evaporite and carbonate mineralogy shows that the basin evolved from a relatively low salinity, riverine lake to one in which initially Na-rich and then Mg-rich hypersaline brines dominated.Lake Manitoba is a large, hyposaline lake located in the eastern Great Plains about 700 km from the Deadmoose-Waldsea area. Stable oxygen and carbon isotope analyses of the endogenic carbonates in the basin indicate gradually increasing levels of organic productivity but decreasing temperatures between 9 000 and 5 000 years B.P. Between about 4 000 and 2 000 years ago the isotope ratios suggest relatively stable temperatures followed by a strong decrease during the most recent 2 000 year period.     

Tracking the rise of eukaryotes to ecological dominance with zinc isotopes

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote‐rich ecosystem could have provided a means of more efficiently sequestering organic‐derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.     

A late pleistocene and holocene vegetation and environmental record from Lake Wangoom, Western Plains of Victoria, Australia

Pollen, microfaunal and sedimentological evidence from the top 20 m of sediment in a closed volcanic crater lake is used to construct a detailed record of vegetation and environmental conditions through the Holocene and a substantial part of the Late Pleistocene. Radiocarbon dating suggests that the sequence covers tha last 51,000 yr or so. High lake-levels and the presence of forest or woodland vegetation indicate that the Holocene and the basal few thousand years of the record experienced wet and warm conditions. The earliest period was succeeded by a long phase of lower but variable moisture levels before more arid conditions resulted in the replacement of forest and woodland by herbaceous vegetation and frequent lake drying. Periods of slight amelioration occurred between about 27,000 and 19,000 and before 15,000 radiocarbon yr ago. Maximum aridity occurred between about 19,000 and 10,000 yr B.P., a period incorporating the height of the last glacial. During the Pleistocene, the lake became gradually more saline but has been fresh during the Holocene, even under low lake-levels.

An increase in charcoal values is recorded, probably around 20,000 years ago, and this could have resulted from increased burning as a result of the activities of Aboriginal people. This may have also caused a change in understorey vegetation from one composed largely of Asteraceae, to grassland.

The record is compared with others from southeastern Australia and there is good correspondence through the last 20,000 or 30,000 yr but, before this, problems of dating and the condensed nature of most sequences prevent detailed correlation.     

Carbon isotopic record of terrestrial ecosystems spanning the Late Miocene extinction of Oreopithecus bambolii, Baccinello Basin (Tuscany, Italy)

Oreopithecus bambolii is a Late Miocene hominoid with an extensive fossil record in the Baccinello Basin (Tuscany, Italy), and was the only western European hominoid to survive a major extinction event ca. 9.6 Ma (millions of years ago). Oreopithecus lived in the insular Tusco-Sardinian paleobioprovince, where it evolved many unique anatomical specializations that make it important for understanding the mechanisms and history of Late Miocene hominoid evolution. The eventual extinction of Oreopithecus and its associated fauna ca. 6.5 Ma has generally been attributed to interaction with species that arrived from continental Europe following tectonic collision of the Tusco-Sardinian province with mainland Italy, but palynological, paleontological, and sedimentological records indicate an environmental shift toward more variable climate across the extinction event.To explore the possibility of environmental change as a contributing factor in the extinction of Oreopithecus, we developed a stable carbon and oxygen isotope record from organic matter in paleosols from the Baccinello Basin. These data show very low temporal and spatial variability (indicating plant ecosystem stability through time and space) and provide no evidence for ecologically significant changes in floral composition spanning the extinction event, suggesting that environmental change was not an underlying cause for the extinction of Oreopithecus and its associated fauna. The carbon isotope values fall entirely within the range of isotopic variability for modern plants following the C₃ photosynthetic pathway (trees, shrubs, cool-season grasses), indicating that C₄ vegetation (warm-season grasses) was not an important component of biomass. When corrected for temporal variation in the carbon isotopic composition of atmospheric carbon dioxide, the paleosol carbon isotope values are consistent with predicted values based on modern plants and the Baccinello palynoflora, supporting the reliability of paleosol isotopic records as paleoecological proxies.     

Pythons metabolize prey to fuel the response to feeding

We investigated the energy source fuelling the post-feeding metabolic upregulation (specific dynamic action, SDA) in pythons (Python regius). Our goal was to distinguish between two alternatives: (i) snakes fuel SDA by metabolizing energy depots from their tissues; or (ii) snakes fuel SDA by metabolizing their prey. To characterize the postprandial response of pythons we used transcutaneous ultrasonography to measure organ-size changes and respirometry to record oxygen consumption. To discriminate unequivocally between the two hypotheses, we enriched mice (= prey) with the stable isotope of carbon (13C). For two weeks after feeding we quantified the CO2 exhaled by pythons and determined its isotopic 13C/12C signature. Ultrasonography and respirometry showed typical postprandial responses in pythons. After feeding, the isotope ratio of the exhaled breath changed rapidly to values that characterized enriched mouse tissue, followed by a very slow change towards less enriched values over a period of two weeks after feeding. We conclude that pythons metabolize their prey to fuel SDA. The slowly declining delta13C values indicate that less enriched tissues (bone, cartilage and collagen) from the mouse become available after several days of digestion.     

Pleistocene climate, phylogeny, and climate envelope models: an integrative approach to better understand species' response to climate change

Mean annual temperature reported by the Intergovernmental Panel on Climate Change increases at least 1.1°C to 6.4°C over the next 90 years. In context, a change in climate of 6°C is approximately the difference between the mean annual temperature of the Last Glacial Maximum (LGM) and our current warm interglacial. Species have been responding to changing climate throughout Earth's history and their previous biological responses can inform our expectations for future climate change. Here we synthesize geological evidence in the form of stable oxygen isotopes, general circulation paleoclimate models, species' evolutionary relatedness, and species' geographic distributions. We use the stable oxygen isotope record to develop a series of temporally high-resolution paleoclimate reconstructions spanning the Middle Pleistocene to Recent, which we use to map ancestral climatic envelope reconstructions for North American rattlesnakes. A simple linear interpolation between current climate and a general circulation paleoclimate model of the LGM using stable oxygen isotope ratios provides good estimates of paleoclimate at other time periods. We use geologically informed rates of change derived from these reconstructions to predict magnitudes and rates of change in species' suitable habitat over the next century. Our approach to modeling the past suitable habitat of species is general and can be adopted by others. We use multiple lines of evidence of past climate (isotopes and climate models), phylogenetic topology (to correct the models for long-term changes in the suitable habitat of a species), and the fossil record, however sparse, to cross check the models. Our models indicate the annual rate of displacement in a clade of rattlesnakes over the next century will be 2 to 3 orders of magnitude greater (430-2,420 m/yr) than it has been on average for the past 320 ky (2.3 m/yr).     

Paoay Lake,northern Luzon,the Philippines: a record of Holocene environmental change

The last 7000 years of environmental history for Paoay Lake and its surrounding landscape is examined through the analysis of pollen, diatoms, charcoal, mineral magnetics and AMS dating. Basal sediments contain shells of Cerithiidae and the saline‐tolerant diatom Diploneis indicating that this was an estuarine environment before becoming a freshwater lake after 6000 bp . Pollen analysis shows that submontane forests, characterized by Pinus pollen, underwent a major disturbance around 5000 years ago, recovering to previous levels by 1000 years ago. Charcoal as an indicator of fire is abundant throughout record, although the highest levels occur in the earlier part of the record, between 6500 and 5000 years ago. An aspect of the project was to examine whether there is evidence of land clearance and agricultural development in the region during the late Holocene. While a clear signal of human impact in the record remains equivocal, there appears to be a correspondence between submontane forest decline and mid‐Holocene ocean data that depict warmer and possibly drier conditions for the region. The study highlights the vulnerability of these montane forests to forecasts of a warmer and drier climate in the near future.     

Speciation, diversity, and Mode 1 technologies: the impact of variability selection

Over geological timescales, organisms encounter periodic shifts in selective conditions driven by environmental change. The variability selection hypothesis suggests that increases in environmental fluctuation have led to the evolution of complex, flexible behaviours able to respond to novel and unpredictable adaptive settings. This hypothesis is tested via the framework of a single locus genetic model in which an invading ‘versatilist’ allele competes with two opposed specialists in a selection regime driven by a fluctuating environment, modelled initially as a sine wave and subsequently as an empirical climate curve covering the past 5 million years. Results demonstrate that generalist alleles achieve fixation in the sine wave environment, whilst versatilist alleles do so in the empirical environment, even at a range of very low fitness advantages over the basic generalist template. Variability selection is found to be a particularly strong force between approximately 2.5 and 1.2 Ma (millions of years ago). These results are discussed in relation to the spread of Oldowan lithics and the patterns of speciation and extinction documented in the hominin fossil record. It is suggested that the flexibility required for survival in a variable climatic regime may have been a stimulus to the development of the first stone tool technologies, whilst the ecological opportunities provided by heightened variability may have been a factor in prompting the hominin adaptive radiation evidenced during this period.     

Climate and weather impact timing of emergence of bats

Interest in forecasting impacts of climate change have heightened attention in recent decades to how animals respond to variation in climate and weather patterns. One difficulty in determining animal response to climate variation is lack of long-term datasets that record animal behaviors over decadal scales. We used radar observations from the national NEXRAD network of Doppler weather radars to measure how group behavior in a colonially-roosting bat species responded to annual variation in climate and daily variation in weather over the past 11 years. Brazilian free-tailed bats (Tadarida brasiliensis) form dense aggregations in cave roosts in Texas. These bats emerge from caves daily to forage at high altitudes, which makes them detectable with Doppler weather radars. Timing of emergence in bats is often viewed as an adaptive trade-off between emerging early and risking predation or increased competition and emerging late which restricts foraging opportunities. We used timing of emergence from five maternity colonies of Brazilian free-tailed bats in south-central Texas during the peak lactation period (15 June-15 July) to determine whether emergence behavior was associated with summer drought conditions and daily temperatures. Bats emerged significantly earlier during years with extreme drought conditions than during moist years. Bats emerged later on days with high surface temperatures in both dry and moist years, but there was no relationship between surface temperatures and timing of emergence in summers with normal moisture levels. We conclude that emergence behavior is a flexible animal response to climate and weather conditions and may be a useful indicator for monitoring animal response to long-term shifts in climate.     

Man, vegetation and climate during the Holocene in the territory of Sagalassos, Western Taurus Mountains, SW Turkey

Past vegetation change and the influence of climate change and anthropogenic pressure during the Holocene is constructed from a series of palynological records sampled from three locations within the territory of the antique site of Sagalassos. The disappearance of the original deciduous oak woodlands and increases in anthropogenic indicator species around 5300 and 4300 b.c. correspond with an increase in settlements in the region. A period of drought following the deforestation may have hampered the recovery of deciduous oak. The timing of the onset of the Bey?ehir Occupation Phase (BO-Phase) in the territory differs between locations, estimates ranging from ca. 1000–800 b.c. to the start of the Hellenistic period (334 b.c.). The most intense period of arboriculture coincides with the Roman and late-Roman periods. Increases in human pressure on the landscape as reflected in the pollen record correspond with an increased rate of sedimentation and fire activity. The timing of the end of the BO-Phase again differs between locations. Estimates range from the 4th century a.d. to the mid 7th century a.d., when a region-wide shift to dry environmental conditions is observed. Numerical analyses show that post BO-Phase vegetation change is largely driven by climate and displays a succession of dry and wet periods that coincided with well-defined European climate shifts, including the Medieval Climate Anomaly and the Little Ice Age. Current agricultural activities in the region are of a very recent (20th century) origin.     

Holocene sand beaches of southern California: ENSO forcing and coastal processes on millennial scales

A proxy record of sand beach accretion for the past 10,000 years has been assembled from radiocarbon dates on the Pismo clam, Tivela stultorum, in archaeological sites along the southern California coast. When this record is compared with numerous climate proxies, it appears that El Niño-Southern Oscillation (ENSO) controls on wave climate and sediment flux have acted upon regional geomorphology at different sea levels to either accrete or erode the Holocene beaches of southern California. Tivela dates from the Santa Maria coast indicate that perennial sand beaches built by 9000 years ago in response to abundant riverine sediment contained by the natural groin at Point Sal, wave sheltering by the massive headland of Point Buchon, and Early Holocene El Niño events. On the western Santa Barbara coast, sand beaches were forming by 7000 years ago in response to high sand fluxes from the Santa Ynez Mountains to the many small littoral catchments, possibly aided by high local rates of uplift. Decline of these sand beach habitats 5000-4000 years ago coincides with increased El Niño-driven wave energy. In accord with slowing in sea-level rise ca. 6000 years ago, sand beaches were most widespread in the period 6000-5000 years ago on Estero Bay, the western Santa Barbara coast, and west of Point Dume. However, Tivela dates first appear 5000 years ago in the Oceanside and Silver Strand littoral cells of the San Diego region. This lag coincides with the Middle Holocene shift to a more variable climate and modern periodicity in El Niño events that increased sediment supply to the southern coast. The ontogeny of the littoral cells provides timelines for modeling coastal evolution with implications for sand beach ecology, prehistoric human coastal adaptations, and coastal planning for future climate change.     

Water consumption in Iron Age,Roman, and Early Medieval Croatia

Patterns of water consumption by past human populations are rarely considered, yet drinking behavior is socially mediated and access to water sources is often socially controlled. Oxygen isotope analysis of archeological human remains is commonly used to identify migrants in the archeological record, but it can also be used to consider water itself, as this technique documents water consumption rather than migration directly. Here, we report an oxygen isotope study of humans and animals from coastal regions of Croatia in the Iron Age, Roman, and Early Medieval periods. The results show that while faunal values have little diachronic variation, the human data vary through time, and there are wide ranges of values within each period. Our interpretation is that this is not solely a result of mobility, but that human behavior can and did lead to human oxygen isotope ratios that are different from that expected from consumption of local precipitation. Am J Phys Anthropol 154:535–543, 2014. © 2014 Wiley Periodicals, Inc.     

Stable oxygen isotopes (δ 18O) in Austrocedrus chilensis tree rings reflect climate variability in northwestern Patagonia, Argentina

The stable oxygen isotope (δ ¹⁸O) composition of Austrocedrus chilensis (D. Don) Endl. (Cupressaceae) tree rings potentially provide retrospective views of changes in environment and climate in the semi-arid lands of Patagonia. We report the development of the first annually resolved δ ¹⁸O tree-ring chronology obtained from natural forests of the foothills of the northwestern Patagonian Andes. The isotope record spans between 1890 and 1994 AD. We explore the probable links between this record and the climate of the region. Air temperatures during summer conditions are significantly, but not strongly, inversely correlated with annual δ ¹⁸O values from Austrocedrus tree rings. The strongest correlations are between the southern oscillation index (SOI) and the tree rings. The existence of millennial-age Austrocedrus trees in northern Patagonia provides interesting possibilities for examining these climate-related isotopic signals over most of the last 1,000 years.     

Cesare Emiliani (1922–1995), pioneer of Ice Age studies and oxygen isotope stratigraphy

Cesare Emiliani established that the ice ages of the last half million years or so are cyclic phenomena, which gave strong support to the hypothesis of Milankovitch. He discovered the cycles when analyzing foraminifers from long deep-sea cores for their oxygen isotope composition (cores were from the Swedish Deep-Sea Expedition, from the Lamont collection and later from collections made at Miami). Emiliani’s method has become the standard procedure for interpreting the deep-sea record in terms of ocean and climate history. Emiliani introduced a time scale suggesting that the cycles are typically 40 000 years in duration, and he defended this scale for almost 20 years. He also thought that temperature was a more important influence on oxygen isotope variations of the ocean than the buildup and decay of northern hemisphere ice sheets. Both these notions proved incorrect. However, his insistence that the Milankovitch mechanism, in conjunction with ice dynamics and crustal response to loading, is the driving force behind the climate cycles of the Quaternary proved well founded.     

Ecological stability in a changing world? Reassessment of the palaeoenvironmental history of Cuatrociénegas,Mexico

Previous work on the palaeoenvironmental history of Cuatrociénegas, Coahuila, Mexico, suggests that the local environments of the basin have been relatively stable since the last glacial period. Reassessment of the palaeoenvironmental record from Cuatrociénegas, combined with the analysis of a glacial-age packrat midden (∼ 16,900 cal yr bp ), shows that this region has experienced substantial climatic and ecological changes during the Late Quaternary. Woodlands occurred near the valley floor until ca. 11,000 years ago, migrating upslope as climate warmed. The apparent lack of change in the flora of the basin floor may be attributable to high soil salinity. This edaphic control led to earlier interpretations that the region was environmentally stable.     

Paleoceanographic shifts and global events recorded in late Pliocene shallow marine deposits (2.80–2.55 Ma) of the Sea of Japan

Fossil ostracod assemblages from the upper Pliocene Kuwae Formation of the Kurokawa area, Niigata Prefecture, central Japan were investigated to discern the high resolution paleoceanographic changes in the Sea of Japan during the transitional interval from a warm to a cold climate in the period from 2.80 to 2.55 Ma, dated previously from diatom and nannofossil datum horizons and magnetostratigraphy. The studied ostracod assemblages did not contain Tsushima Warm Core Current taxa known in the modern Sea of Japan, and most of them are cryophilic and circumpolar. The combinations of recognized fossil ostracod assemblages differ from the modern ones of the region, suggesting that the shallow water area was probably colder than that of today. The Kuwae Formation was deposited during several bathymetric fluctuations between upper bathyal and lower sublittoral zone. A large-scale shift from upper bathyal to lower sublittoral condition, which dominated the depositional setting, occurred at 2.70 Ma in the Tainai area, and occurred rapidly during 40,000 years (2.70–2.66 Ma). This shallowing mirrored the shifts induced by global cooling recorded in various evidence such as oxygen isotope data from deep-sea core. Detailed paleodepth fluctuations show four shallowing events that occurred in this area between 2.80 and 2.55 Ma. The third and fourth shallowings at 2.70 and 2.60 Ma were both responses to global climatic cooling corresponding to the oxygen isotope stages G6 and/or G4, and to stage 104, respectively; deduced from the contemporary abundance of cold water species in the study section, observations of shallowing events in the northwestern Pacific Ocean, and the IRD event recorded in high latitude seas.     

Climatic influences on fire regimes in the northern Sierra Nevada mountains, Lake Tahoe Basin, Nevada, USA

Aim The goal of this study was to understand better the role of interannual and interdecadal climatic variation on local pre‐EuroAmerican settlement fire regimes in fire‐prone Jeffrey pine (Pinus jeffreyi Grev. & Balf.) dominated forests in the northern Sierra Nevada Mountains. Location Our study was conducted in a 6000‐ha area of contiguous mixed Jeffrey pine‐white fir (Abies concolor Gordon & Glend.) forest on the western slope of the Carson Range on the eastern shore of Lake Tahoe, Nevada. Methods Pre‐EuroAmerican settlement fire regimes (i.e. frequency, return interval, extent, season) were reconstructed in eight contiguous watersheds for a 200‐year period (1650–1850) from fire scars preserved in the annual growth rings of nineteenth century cut stumps and recently dead pre‐settlement Jeffrey pine trees. Superposed epoch analysis (SEA) and correlation analysis were used to examine relationships between tree ring‐based reconstructions of the Palmer Drought Severity Index (PDSI), Southern Oscillation Index (SOI), Pacific Decadal Oscillation (PDO) and pre‐EuroAmerican fire regimes in order to assess the influence of drought and equatorial and north Pacific teleconnections on fire occurrence and fire extent. Results For the entire period of record (1650–1850), wet conditions were characteristic of years without fires. In contrast, fire years were associated with drought. Drought intensity also influenced fire extent and the most widespread fires occurred in the driest years. Years with widespread fires were also preceded by wet conditions 3 years before the fire. Widespread fires were also associated with phase changes of the PDO, with the most widespread burns occurring when the phase changed from warm (positive) to cold (negative) conditions. Annual SOI and fire frequency or extent were not associated in our study. At decadal time scales, burning was more widespread during decades that were dryer and characterized by La Niña and negative PDO conditions. Interannual and interdecadal fire–climate relationships were not stable over time. From 1700 to 1775 there was no interannual relationship between drought, PDO, and fire frequency or extent. However, from 1775 to 1850, widespread fires were associated with dry years preceded by wet years. This period also had the strongest association between fire extent and the PDO. In contrast, fire–climate associations at interdecadal time scales were stronger in the earlier period than in the later period. The change from strong interdecadal to strong interannual climate influence was associated with a breakdown in decadal scale constructive relationships between PDO and SOI. Main conclusions Climate strongly influenced pre‐settlement pine forest fire regimes in northern Sierra Nevada. Both interannual and interdecadal climatic variation regulated conditions conducive to fire activity, and longer term changes in fire frequency and extent correspond with climate‐mediated changes observed in both the northern and southern hemispheres. The sensitivity of fire regimes to shifts in modes of climatic variability suggests that climate was a key regulator of pine forest ecosystem structure and dynamics before EuroAmerican settlement. An understanding of pre‐EuroAmerican fire–climate relationships may provide useful insights into how fire activity in contemporary forests may respond to future climatic variation.     

Tree‐ring stable isotopes record the impact of a foliar fungal pathogen on CO2 assimilation and growth in Douglas‐fir

Swiss needle cast (SNC) is a fungal disease of Douglas‐fir (Pseudotsuga menziesii) that has recently become prevalent in coastal areas of the Pacific Northwest. We used growth measurements and stable isotopes of carbon and oxygen in tree‐rings of Douglas‐fir and a non‐susceptible reference species (western hemlock, Tsuga heterophylla) to evaluate their use as proxies for variation in past SNC infection, particularly in relation to potential explanatory climate factors. We sampled trees from an Oregon site where a fungicide trial took place from 1996 to 2000, which enabled the comparison of stable isotope values between trees with and without disease. Carbon stable isotope discrimination (Δ¹³C) of treated Douglas‐fir tree‐rings was greater than that of untreated Douglas‐fir tree‐rings during the fungicide treatment period. Both annual growth and tree‐ring Δ¹³C increased with treatment such that treated Douglas‐fir had values similar to co‐occurring western hemlock during the treatment period. There was no difference in the tree‐ring oxygen stable isotope ratio between treated and untreated Douglas‐fir. Tree‐ring Δ¹³C of diseased Douglas‐fir was negatively correlated with relative humidity during the two previous summers, consistent with increased leaf colonization by SNC under high humidity conditions that leads to greater disease severity in following years.