Mean Annual Temperature Range Research Articles

Denudation rates of granitic regolith along climatic gradient in Eastern China

In order to assess the impact of the monsoonal climate on denudation, physical erosion, and chemical weathering processes, we measured cosmogenic nuclides 10Be, 26Al and immobile element Zr in five granitic regolith profiles across Northeast to South China. The 10Be and 26Al concentrations are 2.98 × 104–25.20 × 104 atoms g−1 and 2.18 × 105–16.62 × 105 atoms g−1, respectively. Compiling previous depth profile data from Leymon and Beacon Heights, we find that the 26Al/10Be ratios systematically increase or decrease with the depth. Our numerical model shows that muons contributions can produce higher 26Al/10Be ratios (greater than 6.75) in both steady-state and non-steady state scenarios, which can result in underestimated burial age (initial 26Al/10Be ratio) and exaggerated denudation rates (due to non-negligible production from muons) for the application of dual-nuclide in Earth Science. The denudation rates, chemical weathering rates and physical erosion rates derived from cosmogenic nuclides and Zr are 28–45, 9–31 and 6–22 m Ma−1, respectively. Chemical weathering rates, excluding the JLN profile, comprise a large proportion of the denudation rates (greater than 50%). Despite the wide range of mean annual precipitation (MAP) and mean annual temperature (MAT), the denudation rates vary without an apparent gradient from northeast to south. However, the physical erosion and chemical weathering results show good relationships with climate, suggesting the limitation type changed from kinetic-limited in the north, to supply-limited in the south. Chemical weathering indices (chemical index of alteration, CIA and weathering intensity factor, WIF) also indicate that the extent of chemical weathering strengthens from northeast to south and increases with chemical weathering rates, but decreases with physical erosion rates.

Genotype-Environment mismatch of kelp forests under climate change.

Climate change is increasingly impacting ecosystems globally. Understanding adaptive genetic diversity and whether it will keep pace with projected climatic change is necessary to assess species' vulnerability and design efficient mitigation strategies such as assisted adaptation. Kelp forests are the foundations of temperate reefs globally but are declining in many regions due to climate stress. A lack of knowledge of kelp's adaptive genetic diversity hinders assessment of vulnerability under extant and future climates. Using 4245 single nucleotide polymorphisms (SNPs), we characterized patterns of neutral and putative adaptive genetic diversity for the dominant kelp in the southern hemisphere (Ecklonia radiata) from ~1000km of coastline off Western Australia. Strong population structure and isolation-by-distance was underpinned by significant signatures of selection related to temperature and light. Gradient forest analysis of temperature-linked SNPs under selection revealed a strong association with mean annual temperature range, suggesting adaptation to local thermal environments. Critically, modelling revealed that predicted climate-mediated temperature changes will probably result in high genomic vulnerability via a mismatch between current and future predicted genotype-environment relationships such that kelp forests off Western Australia will need to significantly adapt to keep pace with projected climate change. Proactive management techniques such as assisted adaptation to boost resilience may be required to secure the future of these kelp forests and the immense ecological and economic values they support.

Temperature patterns of soil carbon: nitrogen: phosphorus stoichiometry along the 400 mm isohyet in China

Responses of terrestrial ecosystems to global warming have gained widespread attention. Soil carbon: nitrogen: phosphorus (C:N:P) stoichiometry regulate various functions and services in terrestrial ecosystems. Investigating the stoichiometric changes of soil C:N:P along the temperature gradient provides valuable insights into the long-term impacts of global warming. However, the inconsistent results were found in previous studies. The collinearity between temperature and precipitation might account for it since the variation in precipitation was rarely limited in previous studies. To make the precipitation effect become minimized, we conducted a 6000-km soil investigation along the 400 mm isohyet in China with a broad mean annual temperature (MAT) range of 14.2 °C. Soil C, N and P concentrations and soil C:N, C:P and N:P ratios all decreased with increasing MAT. Most of these temperature patterns were not affected by vegetation type and soil type, which indicated that the decreasing trends of soil C:N:P stoichiometry with increasing temperature might be common phenomena. MAT is the most important influential factor of soil C and N concentrations and soil C:P and N:P ratios along the temperature gradient; and soil properties, vegetation and soil types also exerted some effects on the changes of soil C and N concentrations and soil C:P and N:P ratios. For soil P, its variation was jointly influenced by MAT, soil properties, vegetation and soil types. The change of soil C:N ratio was mainly influenced by MAT and vegetation type. Furthermore, the decreased soil C:P and N:P ratios with increasing MAT indicated that soil C and N lowered faster than soil P as temperature increased, leading to the imbalanced soil C-N-P.

Novel biogeographic patterns across latitudinal and elevational gradients: A case study with tropical montane epiphytes lends insights to conservation

AbstractAimWhile latitudinal and elevational gradients of range size have been thoroughly studied, the ways these gradients might interact with each other to shape species distributions in complex montane environments are not well understood. We examined how elevational and latitudinal gradients interact to structure individual species’ distributions and larger geographic range size patterns.LocationNeotropics.TaxaTwo epiphytic plant genera: Peperomia and Elaphoglossum.MethodsWe compiled 35,382 GBIF records for 505 species, calculated species’ latitudinal and elevational range extents, and examined gradients in richness and range size. For individual species, we analysed the relationship between elevational and latitudinal occurrences.ResultsApproximately 50% of species demonstrate a significant, negative relationship between their elevational and latitudinal occurrences; most of these species occupy a non‐random, relatively narrow range of mean annual temperatures across their latitudinal distribution. Across species there is a positive relationship between latitudinal and elevational extent, a phenomenon we hereafter refer to as ‘Stevens’ pattern’. Average latitudinal extent of species’ ranges increased at higher latitudes, in support of Rapoport's rule. Average elevational extent increased with elevation in the global dataset (consistent with Rapoport's elevational rule), but most subsets of the data demonstrated a peak in average extent size at mid‐elevations.Main conclusionsThe prevalence of species with negative elevation‐by‐latitude relationships, along with their non‐random tracking of temperature, suggests that many tropical species with broad geographic distributions are more temperature sensitive than their broad ranges might otherwise suggest. Consequently, even tropical species that occur across a wide range of latitudes and elevations might be threatened by climate change. These wide‐ranging species drive the occurrence of two biogeographic patterns: Rapoport's elevational rule and Stevens’ pattern. Finally, while Rapoport's rule and its elevational corollary were supported in part, the unexpected occurrence of many species restricted to high elevations near the equator suggests a possible focus for conservation effort.

Climate change assessment using spatial climate datasets: Theodore Roosevelt National Park (South Unit), 1895-2019

Climate change poses a major challenge to park managers of the United States National Park Service. Several studies have surveyed system-wide climate change, but limited funds prevent the development of local-scale climate change studies. We examined climate change at Theodore Roosevelt National Park (South Unit) in Medora, North Dakota (USA), using spatial climate datasets. We surveyed the period 1895-2019 to provide a long-term perspective, and more closely analyzed the period 1960-2019, when the quality assurance of the database was highest, concerns over anthropogenic climate change effects were greatest, and relevance to current management was most critical. Our results show moderate changes over the past 60 yr. Interannual and decadal-scale climatic variation dominated long-term climate and weakened the statistical significance of most long-term climate trends. The most pronounced changes were an increase in mean annual minimum air temperature and a decrease in mean annual diurnal temperature range. Mean annual precipitation showed a small increase, while an increase in fall precipitation was the most pronounced seasonal precipitation change. Estimated snow water equivalent and snow fraction increased slightly. Climatic water balance results indicate an increase in mean annual soil moisture storage and annual moisture index. Vegetation growing season changes include an increase in the frost-free season length but a decrease in the length of the thermal growing season. The sign and magnitude of change of individual climate variables is sensitive to the period of record examined. Study results provide a baseline for park managers to assess climate change impacts upon grassland productivity, wildfire hazard, and invasive species, and reveal an opportunity to lengthen the tourism season in this summer peak tourism climate.

Statistical Forecast of Daily Maximum Air Temperature in Arid Areas at Summertime

Based on historical observations of summers for the period from 2004 to 2018 with a focus on daily maximum and minimum air temperatures and wind speed recorded at 0600 GMT, a non-linear regression hypothesis is developed for forecasting daily maximum air temperature (Tmax) in arid areas such as Baghdad International airport station, which has a hot climate with no cloud cover or rain. Observations with dust storm events were excluded, thus this hypothesis could be used to predict daily Tmax on any day during summers characterized by fair weather. Using mean annual daily temperature range, daily minimum temperature, and the trend of maximum temperature with wind speed, Tmax was forecasted and then compared to those recorded by meteorological instruments. To improve the accuracy of the hypothesis, daily forecast errors, bias, and mean absolute error were analyzed to detect their characteristics through calculating relative frequencies of occurrence. At the end of this analysis, a value of (-0.45ºC) was added to the hypothesis as a bias term.

Temperature adaptation of soil microbial respiration in alpine, boreal and tropical soils: An application of the square root (Ratkowsky) model.

Warming is expected to stimulate soil microbial respiration triggering a positive soil carbon-climate feedback loop while a consensus remains elusive regarding the magnitude of this feedback. This is partly due to our limited understanding of the temperature-adaptive response of soil microbial respiration, especially over broad climatic scales. We used the square root (Ratkowsky) model to calculate the minimum temperature for soil microbial respiration (Tmin , which describes the temperature adaptation of soil microbial respiration) of 298 soil samples from alpine grasslands on the Tibetan Plateau and forest ecosystems across China with a mean annual temperature (MAT) range from -6°C to +25°C. The instantaneous soil microbial respiration was determined between 4°C and 28°C. The square root model could well fit the temperature effect on soil microbial respiration for each individual soil, with R2 higher than 0.98 for all soils. Tmin ranged from -8.1°C to -0.1°C and increased linearly with increasing MAT (R2 =0.68). MAT dominantly regulated Tmin variation when accounting simultaneously for multiple other drivers (mean annual precipitation, soil pH and carbon quality); an independent experiment showed that carbon availability had no significant effect on Tmin . Using the relationship between Tmin and MAT, soil microbial respiration after an increased MAT could be estimated, resulting in a relative increase in respiration with decreasing MAT. Thus, soil microbial respiration responses are adapted to long-term temperature differences in MAT. We suggest that Tmin = -5 + 0.2×MAT, that is, every 1°C rise in MAT is estimated to increase Tmin of respiration by approximately 0.2°C, could be used as a first approximation to incorporate temperature adaptation of soil microbial respiration in model predictions. Our results can be used to predict future changes in the response of soil microbial respiration to temperature over different levels of warming and across broad geographic scales with different MAT.

Regional coordination between riparian dependence and atmospheric demand in willows (Salix L.) of western North America

AbstractAimPlants vary in their hydrological and climatic niches. How these niche dimensions covary among closely related species can help identify co‐adaptations to hydrological and climatic factors, as well as predict biodiversity responses to environmental change.LocationWestern United States.MethodsRelationships between riparian dependence and climate niches of willows (Salix L.) were assessed, incorporating phylogenetics and functional traits to understand the adaptive nature of those relationships. The riparian dependence niche was estimated as the mean distance between georeferenced occurrence records and the nearest stream based on the National Hydrography Database. Results were compared to oaks (Quercus L.), a less riparian‐dependent clade, with the expectation of different niche relationships.ResultsWillows generally occurred closer to streams than expected by chance, but riparian dependence varied substantially among species. Riparian dependence was positively correlated with mean annual temperature and diurnal temperature range niche, both indicators of atmospheric demand on evapotranspiration. Phylogenetic independent contrast correlations for these relationships were significant as well, and the high degree of niche convergence among species indicated evolutionarily labile co‐adaptations to riparian dependence and atmospheric demand. Plant height increased with mean annual temperature niche, and specific leaf area increased with residual variation in height, indicating underlying morphological correlates of niche variation. Oaks, on the other hand, exhibited no relationship between atmospheric demand and riparian dependence, and weaker niche relationships with riparian dependence overall.Main conclusionsThese results support the assertion that hydric‐adapted, woody riparian plants compensate for increased atmospheric demand on transpiration with a reliable supply of water provided by riparian habitats and that this trade‐off may be unique from mesic–xeric woody plants. Conservation of warm‐adapted riparian trees and shrubs under increasing temperatures and atmospheric demand may necessitate reversal of groundwater depletion. Cool‐adapted species may be best conserved through maintenance or expansion of riparian buffers as they become more riparian obligate with warming.

Calibrating branched GDGTs in bones to temperature and precipitation: Application to Alaska chronological sequences

Our recent study suggests that branched glycerol dialkyl glycerol tetraethers (brGDGTs) in ancient bones may have great potential for reconstructing in-situ paleoclimate conditions. However, one major problem with reconstructing past climate and environmental conditions using brGDGTs in ancient bones is the lack of calibrations specific to bones. Here we investigate the distribution of brGDGTs in 191 modern weathered bone samples in China with a broad range of mean annual air temperature (MAAT: −3.3-25.6 °C) and mean annual precipitation (MAP: 163-2062 mm), and present the first bone-specific brGDGT temperature and precipitation calibrations. We find that the best correlation with temperature is found in bones with a higher ratio of 6-methyl over 5-methyl brGDGT isomers, suggesting a preferential production of 6-methyl brGDGTs by bacteria inside bones. The bone-specific temperature relationship is distinct from published mineral soil calibrations (e.g., with virtually no influence from soil pH changes). We hypothesize that the lower calibration residual values result from more homogenous and semi-closed environment for bacterial growth inside the bones than soil environments. We also demonstrate that, in semi-arid and semi-humid regions (MAP: 200-710 mm), brGDGT distributions in cancellous bones with relatively high percentages of 6-methyl isomers are strongly correlated with mean annual precipitation. Application of our bone-specific brGDGT calibrations to Alaskan chronological sequences yields new insights to high latitude temperature and precipitation variations for the past 30,000 years.

Climate Factors and Wolbachia Infection Frequencies in Natural Populations of Drosophila melanogaster

The endosymbiotic bacteria Wolbachia are widespread in natural populations of Drosophila melanogaster, which is a cosmopolitan and synanthropic species. Various aspects of Wolbachia infection are studied in many arthropod species, including fruit flies. However, the influence of climatic factors on the level of Wolbachia infection in natural populations of fruit flies has not been studied in detail. The influence of mean temperature, precipitation, and potential evaporation was investigated on Wolbachia infection rates in ten D. melanogaster populations (collected in Europe) and newly obtained estimates were combined with the data from other studies for different continents and for different climatic zones (280 populations, 10 806 isofemale lines). The effect of climatic factors on the infection frequency was revealed. The highest infection rates are observed in the mean annual temperature range of 20–25°C, which corresponds to fly rearing conditions in the laboratory. The clinal distribution of Wolbachia infection rates in the Eurasian fruit fly populations was described. Logistic regression models showed that climatic factors have a stronger impact on bacterial infection rates within climatic zones rather than continents.

Not so robust: Robusta coffee production is highly sensitive to temperature.

Coffea canephora (robusta coffee) is the most heat-tolerant and 'robust' coffee species and therefore considered more resistant to climate change than other types of coffee production. However, the optimum production range of robusta has never been quantified, with current estimates of its optimal mean annual temperature range (22-30°C) based solely on the climatic conditions of its native range in the Congo basin, Central Africa. Using 10years of yield observations from 798 farms across South East Asia coupled with high-resolution precipitation and temperature data, we used hierarchical Bayesian modeling to quantify robusta's optimal temperature range for production. Our climate-based models explained yield variation well across the study area with a cross-validated mean R2 =.51. We demonstrate that robusta has an optimal temperature below 20.5°C (or a mean minimum/maximum of ≤16.2/24.1°C), which is markedly lower, by 1.5-9°C than current estimates. In the middle of robusta's currently assumed optimal range (mean annual temperatures over 25.1°C), coffee yields are 50% lower compared to the optimal mean of ≤20.5°C found here. During the growing season, every 1°C increase in mean minimum/maximum temperatures above 16.2/24.1°C corresponded to yield declines of ~14% or 350-460kg/ha (95% credible interval). Our results suggest that robusta coffee is far more sensitive to temperature than previously thought. Current assessments, based on robusta having an optimal temperature range over 22°C, are likely overestimating its suitable production range and its ability to contribute to coffee production as temperatures increase under climate change. Robusta supplies 40% of the world's coffee, but its production potential could decline considerably as temperatures increase under climate change, jeopardizing a multi-billion dollar coffee industry and the livelihoods of millions of farmers.

Calibrating bacterial tetraether distributions towards in situ soil temperature and application to a loess-paleosol sequence

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) produced by soil-dwelling bacteria offer a promising tool for reconstructing terrestrial temperatures. However, in most previous studies, due to lack of soil temperature data, brGDGTs are calibrated to the air rather than the soil temperature. This may impede our understanding of the accurate response of brGDGTs to temperature, and thus affect the quantitative paleotemperature reconstruction using these lipids. Here, we investigated modern soil brGDGTs and the corresponding soil temperature across a large climatic gradient in China (mean annual soil temperature (MAST) range: −2.7 to 26.2 °C). The results show that the MAST is higher than the mean annual air temperature (MAAT) by 0–6 °C, and the difference is related to vegetation coverage. This supports the idea that vegetation can modulate MAST and points to the necessity of exploring the direct response of brGDGTs to soil temperature. Employing stepwise regression (sr), we developed MASTsr and MAATsr calibrations, which improve accuracy and reduce the error compared with previous global MAAT calibrations. In Lantian loess-paleosols, the MASTsr calibration resulted in ∼4 °C lower glacial temperatures and a ∼10 °C deglacial warming comparable with other terrestrial records and climate models. However, the global MATmr and MBT’/CBT calibrations produced abnormally higher glacial temperatures, while the empirical MAATsr calibration overestimated MAAT during the deglacial period with low vegetation coverage. This demonstrates that the calibration with soil temperature is preferred for quantitative paleotemperature reconstruction. Nevertheless, soil brGDGTs might be useful for inferring MAAT if underlying surface conditions are sufficiently constrained.

Air temperature and precipitation analyses on a small Mediterranean island: the case of the remote island of Lastovo (Adriatic Sea, Croatia)

The paper analyses a series of annual, monthly, and daily air temperatures, and annual and monthly precipitation at the Lastovo meteorological station on the island of Lastovo (Croatia) during the 1948-2018 period. The small carbonate Adriatic island of Lastovo is the most remote inhabited island in the Croatian part of the Adriatic Sea. The absolute minimum annual air temperatures range between -6.8 °C (recorded in 1963) and 4.0 °C (recorded in 1974), with an average value of -1.2 °C. The Mean annual air temperatures range between 14.7 °C (recorded in 1980) and 17.4 °C (recorded in 2018), with an average value of 15.8 °C. The absolute maximum annual air temperatures range between 31.7 °C (recorded in 1959) and 38.3 °C (recorded in 1998), with an average value of 34.8 °C. A strong jump in the minimum annual temperatures started in 1972. The mean annual temperature jump began nine years later in 1981, while the maximum annual temperature jump occurred in 1992. The values of the t-test for all three analysed annual temperature indices (ATI) substantiate the conclusion that the average values in two subperiods defined by the RAPS method are statistically significant at the level p<<0.01. The increasing trend of air temperatures is significantly higher for the time series of the average maximum temperature Tmax than for the time series of the average minimum temperature Tmin. The most prominent increasing trend occurs in June and July. The number of warm and hot days in Lastovo has continuously increased over the analysed period. The increasing trends for both indices are statistically significant at the level p<0.01. The day-to-day (DTD) method established a decrease in the night-time to night-time variability, and an increase in the day-time to day-time temperature variability. The number of frost days is steadily decreasing. The average annual precipitation in the 1948-2018 period was 666 mm, while the minimum and the maximum precipitation was 368 mm and 1089 mm, respectively. No trends have been established for the annual and the monthly precipitation time-series during the 1948-2018 period, though a statistically insignificant drop in annual precipitation is found after 1982.

Impacts of climate change on the distribution of Cymbidium kanran and the simulation of distribution pattern

In this study, data of 19 climatic factors were downloaded from the World Climate website. A total of 233 Cymbidium kanran distribution data were obtained through online review and field visits. Using MaxEnt model and combined with ArcGIS spatial analysis technology, the potential distribution area and distribution pattern of C. kanran in different periods were simulated, as well as its distribution during the last glacial period and 2070. The results showed that the curve indexes (AUC) value of the model training set was 0.957, and the AUC value of the verification set was 0.953, indicating that the prediction accuracy of the model was very high. The current distribution of C. kanran was mostly affected by the driest quarter precipitation, mean annual precipitation, wettest quarter precipitation, and mean annual temperature range. The contribution rates were 50.3%, 15.9%, 8.4% and 4.4%, respectively, with the total contribution rate being 79.0%. In the last glacial period, C. kanran mainly distributed in Wuyi Mountain, Luojing Mountain, Nanling, Taiwan's five major mountains and some hills in the northern part of Guangxi. From now to 2070, the distribution of C. kanran area will decrease by 22.4%. The southwestern part of Guangxi, the central part of Yunnan, and the junctions of Jiangxi, Fujian and Guangdong provinces will expand, while that in eastern Jiangxi, western Fujian, and the border between these two provinces will shrink.

The Fate of Endangered Rock Sedge (Carex rupestris) in the Western Carpathians—The Future Perspective of an Arctic-Alpine Species under Climate Change

Carex rupestris is an endangered and rare arctic-alpine element of the Western Carpathian flora. Given the geographically isolated and spatially restricted peripheral ranges of arctic-alpine species, there is a good chance that many species of conservation concern irreversibly disappear from the regional flora under the ongoing climate change. In this study, we gathered all existing data on the presence of C. rupestris and focused on its current and future distribution in the Western Carpathians. We found that although the distribution of the species is fragmented and scarce, C. rupestris occurs in several mountain ranges, in four distinct plant community types, which differ considerably in altitude, geological bedrock, and other habitat characteristics. In contrast to the relatively broad range of occupied habitats, C. rupestris shows a narrow temperature niche (mean annual temperature range 0.4–4.0 °C). Ensembles of small models based on climatic characteristics and local topography show that regardless of the climate change scenario (rcp2.6, rcp8.5), many current occurrence sites, mainly in the peripheral zones of the range, will face the excessive loss of suitable environmental conditions. It is expected that the Tatra Mountains will be the only mountain range retaining potentially suitable habitats and providing possible refugia for this cold-adapted species in the future. Such severe shrinkage of distribution ranges and associated geographic isolation raises serious concerns for the fate of the arctic-alpine species in the Western Carpathians.

Effect of climate on provincial-level banana yield in the Philippines

This study assessed the effect of climate on banana yield in the Philippines using provincial level yield data and different climatic variables (i.e. annual rainfall, frequency of wet days, precipitation seasonality, annual mean temperature, temperature seasonality, and annual mean diurnal temperature range) derived from Climate Research Unit – Time Series (CRU-TS) dataset from year 1991–2016. First, trend analysis was applied to the time series of banana yield data for each province to determine presence of linear trend. Then, based on the result of linear trend analysis, multiple regression analysis was used to assess the effect on climate on provincial-level banana yield. Trend analysis showed that 71% of banana producing area in the country experience significant yield trend while multiple regression analysis showed that only 10% of the banana producing areas in the country is significantly affected by climate. Compared to rainfall, temperature variables, specifically temperature seasonality has the greater influence on provincial-level banana yield in the Philippines.

Baikal glacier system, new findings

Three glaciers and other small glacial formations, which are not mentioned in the USSR Catalog of glaciers as well as in any published article, were found in the area of the Verkhneangarsky (upper Angara) Range on satellite images. Earlier, researchers from the Institute of Geography of Siberian Branch of RAS described a group of glaciers on the Baikal Mountains and the Barguzin Range, which together with the above group of the Verkhneangarsky Range are related to the Baikal basin by the climate conditions. Ogdynda-Maskit Glacier has been described on the spot in 2017, while other glaciers and snow patches are to be investigated in situ in 2018. Together with the above Baikal and Barguzin glaciers, new data allow identification of the Baikal Glacial system. The article presents analysis of climate changes in the region. Trends in mean annual temperature range from 1 to 4 °C/50 years, while the same of summer temperatures change from 1 to 3 °C/50 years, but without any evident spatial distribution between them. Another situation takes place in the spatial distribution of precipitation: trends in total precipitation decrease towards the Baikal Lake, while precipitation of the cold period, on the contrary, slightly increases towards the Lake, but with rather small trend – 10–40 mm per 50 years. The last fact confirms the influence of the Baikal basin on the alimentation of glaciers and snow patches on the Verkhneangarsky Range. Currently, glaciers of the Verkhneangarsky Range undergo a slow regression.

Isotopic evidence for oligotrophication of terrestrial ecosystems.

Human societies depend on an Earth system that operates within a constrained range of nutrient availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. Examining patterns of foliar N concentrations and isotope ratios (δ15N) from more than 43,000 samples acquired over 37 years, here we show that foliar N concentration declined by 9% and foliar δ15N declined by 0.6-1.6‰. Examining patterns across different climate spaces, foliar δ15N declined across the entire range of mean annual temperature and mean annual precipitation tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ15N of tree rings and leaves from herbarium samples over the past 75-150 years. These patterns are consistent with the proposed consequences of elevated atmospheric carbon dioxide and longer growing seasons. These declines will limit future terrestrial carbon uptake and increase nutritional stress for herbivores.

Widespread occurrence of distinct alkenones from Group I haptophytes in freshwater lakes: Implications for paleotemperature and paleoenvironmental reconstructions

Alkenones are C35–C42 polyunsaturated ketone lipids that are commonly employed to reconstruct changes in sea surface temperature. However, their use in coastal seas and saline lakes can be hindered by species-mixing effects. We recently hypothesized that freshwater lakes are immune to species-mixing effects because they appear to exclusively host Group I haptophyte algae, which produce a distinct distribution of alkenones with a relatively consistent response of alkenone unsaturation to temperature. To evaluate this hypothesis and explore the geographic extent of Group I haptophytes, we analyzed alkenones in sediment and suspended particulate matter samples from lakes distributed throughout the mid- and high latitudes of the Northern Hemisphere (n=30). Our results indicate that Group I-type alkenone distributions are widespread in freshwater lakes from a range of different climates (mean annual air temperature range: −17.3–10.9 °C; mean annual precipitation range: 125–1657 mm yr−1; latitude range: 40–81°N), and are commonly found in neutral to basic lakes (pH > 7.0), including volcanic lakes and lakes with mafic bedrock. We show that these freshwater lakes do not feature alkenone distributions characteristic of Group II lacustrine haptophytes, providing support for the hypothesis that freshwater lakes are immune to species-mixing effects. In lakes that underwent temporal shifts in salinity, we observed mixed Group I/II alkenone distributions and the alkenone contributions from each group could be quantified with the RIK37 index. Additionally, we observed significant correlations of alkenone unsaturation (U37K) with seasonal and mean annual air temperature with this expanded freshwater lakes dataset, with the strongest correlation occurring during the spring transitional season (U37K=0.029⁎T−0.49; r2=0.60; p<0.0001). We present new sediment trap data from two lakes in northern Alaska (Toolik Lake, 68.632°N, 149.602°W; Lake E5, 68.643°N, 149.458°W) that demonstrate the highest sedimentary fluxes of alkenones in the spring transitional season, concurrent with the period of lake ice melt and isothermal mixing. Together, these data provide a framework for evaluating lacustrine alkenone distributions and utilizing alkenone unsaturation as a lake temperature proxy.

The relation between global palm distribution and climate

Fossil palms provide qualitative evidence of (sub-) tropical conditions and frost-free winters in the geological past, including modern cold climate regions (e.g., boreal, or polar climates). The freeze intolerance of palms varies across different organs and life stages, with seedlings in particular less tolerant of sub-zero temperatures than adult plants, limiting successful establishment of populations while permitting adult palms to survive in cultivation outside their natural ranges. Quantitatively, palms indicate minimum cold month mean temperature (CMMT) at 2–8 °C in palaeoclimate reconstructions. These data have accentuated model-proxy mismatches for high latitudes during Paleogene hyperthermals when palms expanded poleward in both hemispheres. We constructed a manually filtered dataset of >20,000 georeferenced Arecaceae records, by eliminating cultivars. Statistically derived mean annual temperature, mean annual temperature range, and CMMT thresholds for the Arecaceae and lower rank subfamilies and tribes reveal large differences in temperature sensitivity depending on lower taxonomic classification. Cold tolerant tribes such as the Trachycarpeae produce thresholds as low as CMMT ≥ 2.2 °C. However, within the palm family, CMMT < 5 °C is anomalous. Moreover, palm expansion into temperate biomes is likely a post-Palaeogene event. We recognize a CMMT ≥ 5.2 °C threshold for the palm family, unless a lower taxonomic rank can be assigned.