Mean Annual Precipitation Range Research Articles

Trends of Aboveground Net Primary Productivity of Patagonian Meadows, the Omitted Ecosystem in Desertification Studies

The United Nations defines desertification as the loss of productivity in arid and semiarid environments. The extended steppes of Patagonia harbor small meadows whose compounded area is comparatively small, but their aboveground net primary production (ANPP) is up to ten times higher than their surroundings. These meadows then represent a key ecosystem for cattle grazing systems, but there are no descriptions of the trends in their ANPP and, consequently, their carrying capacity, and, as a result, their degradation syndromes. Our objectives were as follows: (1) analyze the trends of mean and spatial heterogeneity of annual ANPP in meadows and neighboring steppes and relate them with precipitation and temperature, (2) evaluate the impact on the livestock carrying capacity of meadows in the region, and (3) evaluate the degradation trends of these meadows, based on a novel description proposed to characterize the trend syndromes of these type of ecosystems. We identified meadow areas across a subcontinental scale in Patagonia, covering a mean annual precipitation range from 129 to 936 mm. We estimated ANPP on a monthly basis from 2000 to 2019 via regional calibrated remote sensing information. In the last two decades, ANPP decreased in 74% of the studied meadow areas, while remaining relatively stable in the nearby steppes. This decrease was relatively higher in the arid end of the analyzed precipitation gradient. Hence, the global carrying capacity for all the studied meadow areas decreased by 8%. Finally, we identified four trend syndromes based on the combination of the ANPP trend and its spatial heterogeneity, calculated as the spatial standard deviation. The predominant trend syndrome, in 55% of the area, was associated with a negative trend of both ANPP and spatial heterogeneity. These results could help prioritize areas where specific management decisions, given the different trend syndromes, could help revert ANPP negative trends.

Host population effects on ectomycorrhizal fungi vary between low and high phosphorus soils of temperate rainforests.

Geographic distinctions in the affinity of tree populations for select ectomycorrhizal fungi (EMF) may occur where strong edaphic pressures act on fungal communities and their hosts. We examine this premise for Pseudotsuga menziesii var. menziesii of southwest British Columbia, using ten native seedlots collected from a range of mean annual precipitation (MAP), as a proxy for podzolization extent and phosphorus (P) deficiencies, and evaluated in contrasting low P and high P soils. After two growing seasons, seedling biomass in the high P soil dwarfed that of the low P soil, and better growth rates under high P were detected for populations from very dry and very wet origins. EMF communities on the high P soil displayed more symmetry among host populations than the low P soil (average community dissimilarity of 0.20% vs. 0.39%, respectively). Seedling foliar P% differed slightly but significantly in relation to MAP of origin. EMF species richness varied significantly among host populations but independently of climatic parameters. There were significant shifts in EMF species abundance related to seedlot MAP, particularly on the low P soil where nonlinear relationships were found for Wilcoxina mikolae, Hyaloscypha finlandica, and Rhizopogon villosulus. Despite efforts to enhance colonization by native fungi, the predominance of ruderal EMF species hindered a realistic evaluation of local adaptation among host-fungi populations. Nevertheless, the shifting affinity in taxa abundance and wider community disparity on low P soil reflected the potential for a consequential host genetic effect related to geographical patterns in P availability across temperate rainforests.

Controls on the presence and storage of soil inorganic carbon in a semi-arid watershed

Soil inorganic carbon (SIC) constitutes ∼40–50% of the terrestrial soil carbon and is an integral part of the global carbon cycle. Rainfall is a primary factor controlling SIC accumulation; however, the distribution and hierarchy of controls on SIC development in arid and semi-arid regions is poorly understood. The Reynolds Creek Experimental Watershed (RCEW) in southwestern Idaho is an ideal location to study factors influencing SIC because it spans a wide mean annual precipitation range (235 mm to 900 mm) along a 1,425 to 2,111 m elevation gradient and has soils derived from a wide variety of parent materials (granite, basalt, dust, and alluvium). We collected soil samples along this elevational gradient to understand local controls on SIC distributions. SIC content was quantified at 71 soil pits and/or augered cores collected between approximately 0–1 m depth or until refusal. Consistent with previous studies, we found variations in precipitation governed the presence or absence of SIC; field measurements of the top 1 m of soils confirm little or no SIC in soils receiving > 500 mm in mean annual precipitation. Below this 500 mm threshold, SIC pools varied substantially and significantly between sites. Results showed that 90% of sites (64 sites) contained less than 10 kg m−2 SIC, 7% (5 sites) contained 10–20 kg m−2, and 3% (2 sites) contain between 24 and 29 kg m−2 SIC. The total SIC within RCEW was estimated at ∼5.17 × 105 Mg. After precipitation, slope consistently ranked as the second most important predictor of SIC accumulation in random forest analysis. Wind-blown dust likely contributed to SIC accumulation; prior work indicates an average dust flux rate in RCEW of about 11 ± 4.9 g m−2 year−1. This study provides an initial model predicting SIC distribution and accumulation in a shrub-dominated dryland watershed.

Evolution of palaeoclimate, palaeoenvironment and vegetation in Central Europe during the Miocene Climate Optimum

The Miocene Climate Optimum was an interval of global climatic warmth characterized by significant global palaeoceanographic changes. In this paper, we describe the parastratotype section of the regional Karpatian stage (late Burdigalian) at Hevlín quarry in the Czech Republic, to improve knowledge of evolving palaeoenvironment and palaeoclimate during the Miocene Climate Optimum. Strontium isotope stratigraphy (87Sr/86Sr) and biostratigraphy allows precise identification of Burdigalian/Langhian boundary, and the studied section is inferred to represent the late Burdigalian time interval (17.2–16 Ma). Analysis of palynological assemblages using the Coexistence Approach and the Plant Functional Type method supported by biomarker study indicates an existence of a warm temperate to subtropical palaeoclimate pattern with a mean annual temperature range of 17–19.5 °C and a high degree of seasonality. The mean annual precipitation range was 1050–1600 mm. The zonal vegetation cover was characterized as transitional between subtropical broad-leaved evergreen forest and warm–temperate to subtropical mixed mesophytic forest. Our palaeoclimatic findings in Central Europe agree with global trends (high temperatures and a high sea level) and indicates the occurrence of rather widespread humid conditions. The proximity to our study site of the Mediterranean-Paratethys sea probably played a critical role in moderating regional palaeoclimate.

Towards species‐level forecasts of drought‐induced tree mortality risk

Summary Predicting species‐level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections.We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South‐Eastern Australia (mean annual precipitation range: 344–1424 mm yr−1). We conducted three experiments: applying CABLE to the 2017–2019 drought; a 20% drier drought; and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO2).The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within‐species’ ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO2 in ameliorating drought stress was consistent across species.Our results represent an important advance in our capacity to forecast the resilience of individual tree species, providing an evidence base for decision‐making around the resilience of restoration plantings or net‐zero emission strategies.

One Stomatal Model to Rule Them All? Toward Improved Representation of Carbon and Water Exchange in Global Models

AbstractStomatal conductance schemes that optimize with respect to photosynthetic and hydraulic functions have been proposed to address biases in land‐surface model (LSM) simulations during drought. However, systematic evaluations of both optimality‐based and alternative empirical formulations for coupling carbon and water fluxes are lacking. Here, we embed 12 empirical and optimization approaches within a LSM framework. We use theoretical model experiments to explore parameter identifiability and understand how model behaviors differ in response to abiotic changes. We also evaluate the models against leaf‐level observations of gas‐exchange and hydraulic variables, from xeric to wet forest/woody species spanning a mean annual precipitation range of 361–3,286 mm yr−1. We find that models differ in how easily parameterized they are, due to: (a) poorly constrained optimality criteria (i.e., resulting in multiple solutions), (b) low influence parameters, (c) sensitivities to environmental drivers. In both the idealized experiments and compared to observations, sensitivities to variability in environmental drivers do not agree among models. Marked differences arise in sensitivities to soil moisture (soil water potential) and vapor pressure deficit. For example, stomatal closure rates at high vapor pressure deficit range between −45% and +70% of those observed. Although over half the new generation of stomatal schemes perform to a similar standard compared to observations of leaf‐gas exchange, two models do so through large biases in simulated leaf water potential (up to 11 MPa). Our results provide guidance for LSM development, by highlighting key areas in need for additional experimentation and theory, and by constraining currently viable stomatal hypotheses.

Impact of Climate Change on the Hydrology of the Upper Awash River Basin, Ethiopia

This study investigated the impacts of climate change on the hydrology of the Upper Awash Basin, Ethiopia. A soil and water assessment tool (SWAT) model was calibrated and validated against observed streamflow using SWAT CUP. The Mann–Kendall trend test (MK) was used to assess climate trends. Meteorological drought (SPEI) and hydrological drought (SDI) were also investigated. Based on the ensemble mean of five global climate models (GCMs), projected increases in mean annual maximum temperature over the period 2015–2100 (compared with a 1983–2014 baseline) range from 1.16 to 1.73 °C, while increases in minimum temperature range between 0.79 and 2.53 °C. Increases in mean annual precipitation range from 1.8% at Addis Ababa to 45.5% over the Hombole area. High streamflow (Q5) declines at all stations except Ginchi. Low flows (Q90) also decline with Q90 equaling 0 m3 s−1 (i.e., 100% reduction) at some gauging stations (Akaki and Hombole) for individual GCMs. The SPEI confirmed a significant drought trend in the past, while the frequency and severity of drought will increase in the future. The basin experienced conditions that varied from modest dry periods to a very severe hydrological drought between 1986 and 2005. The projected SDI ranges from modestly dry to modestly wet conditions. Climate change in the basin would enhance seasonal variations in hydrological conditions. Both precipitation and streamflow will decline in the wet seasons and increase in the dry seasons. These changes are likely to have an impact on agricultural activities and other human demands for water resources throughout the basin and will require the implementation of appropriate mitigation measures.

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.

Quantitative Estimates of Paleoenvironmental Changes in the Late Holocene in the South of the East European Plain as Recorded in the Magnetic Properties of Soils

Information about the past evolutionary changes in the soil cover is essential as a retrospective basis for long-term forecasts of soil development under the changing climatic conditions, and also for understanding the present-day problems of desertification and land degradation in the south of the East European Plain. The principal task of the present study was to estimate quantitatively the shift of the boundary between the steppe and desert-steppe zones in this region during the late Holocene under the impact of global climate variations. The properties of about 100 paleosols of archaeological sites widely represented in the region were used for quantitative reconstruction of paleoprecipitation, paleotemperature, and climate aridity. To obtain regional calibrations, the dependences of the magnetic properties of modern soils on climatic parameters in the south of the East European Plain were analyzed in detail. Samples of 35 soil profiles along the 1500-km-long Voronezh–Volgograd–Astrakhan–Elista–Voronezh transect were used. This transect crosses several natural climatic zones with gradients in the annual precipitation of about 500 mm/yr and in the annual temperatures of about 3°C, which allowed us to obtain reliable relationships between the climatic parameters and the properties of recent soils. Limitations for paleoclimate reconstructions based on the magnetic properties of soils in the range of mean annual precipitation <600 mm/year were identified. Cartographic reconstruction of changes in the De Martonne aridity index (IDM) using GIS modeling methods and paleopedological data indicates that the Volga–Don interfluve has undergone repeated changes in the climatic situation over the past 5000 years resulting in the shifts of the boundaries between soil-geographical zones to distances of up to 200–300 km.

Combined high leaf hydraulic safety and efficiency provides drought tolerance in Caragana species adapted to low mean annual precipitation

Summary Clarifying the coordination of leaf hydraulic traits with gas exchange across closely‐related species adapted to varying rainfall can provide insights into plant habitat distribution and drought adaptation.The leaf hydraulic conductance (K leaf), stomatal conductance (g s), net assimilation (A), vein embolism and abscisic acid (ABA) concentration during dehydration were quantified, as well as pressure–volume curve traits and vein anatomy in 10 Caragana species adapted to a range of mean annual precipitation (MAP) conditions and growing in a common garden.We found a positive correlation between Ψleaf at 50% loss of K leaf (K leaf P 50) and maximum K leaf (K leaf‐max) across species. Species from low‐MAP environments exhibited more negative K leaf P 50 and turgor loss point, and higher K leaf‐max and leaf‐specific capacity at full turgor, along with higher vein density and midrib xylem per leaf area, and a higher ratio of K leaf‐max : maximum g s. Tighter stomatal control mediated by higher ABA accumulation during dehydration in these species resulted in an increase in hydraulic safety and intrinsic water use efficiency (WUEi) during drought.Our results suggest that high hydraulic safety and efficiency combined with greater stomatal sensitivity triggered by ABA production and leading to greater WUEi provides drought tolerance in Caragana species adapted to low‐MAP environments.

Higher resilience to climatic disturbances in tropical vegetation exposed to more variable rainfall

With ongoing global warming, the amount and frequency of precipitation in the tropics is projected to change substantially. While it has been shown that tropical forests and savannahs are sustained within the same intermediate mean annual precipitation range, the mechanisms that lead to the resilience of these ecosystems are still not fully understood. In particular, the long-term impact of rainfall variability on resilience is as yet unclear. Here we present observational evidence that both tropical forest and savannah exposed to a higher rainfall variability—in particular on interannual scales—during their long-term past are overall more resilient against climatic disturbances. Based on precipitation and tree cover data in the Brazilian Amazon basin, we constructed potential landscapes that enable us to systematically measure the resilience of the different ecosystems. Additionally, we infer that shifts from forest to savannah due to decreasing precipitation in the future are more likely to occur in regions with a precursory lower rainfall variability. Long-term rainfall variability thus needs to be taken into account in resilience analyses and projections of vegetation response to climate change. Tropical forests and savannah are more resilient to climate disturbances when they have been exposed to higher rainfall variability in the long-term past, finds an analysis of Brazilian rainfall and tree-cover observations.

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.

PALEOSOLS AND ICHNOFOSSILS OF THE UPPER PENNSYLVANIAN–LOWER PERMIAN MONONGAHELA AND DUNKARD GROUPS (OHIO, USA): A MULTI-PROXY APPROACH TO UNRAVELING COMPLEX VARIABILITY IN ANCIENT TERRESTRIAL LANDSCAPES

The upper Monongahela and lower Dunkard groups of southeastern Ohio include abundant paleosols containing ichnofossils as well as vertebrate, invertebrate, and plant body fossils. These paleosols formed on diverse Late Pennsylvanian to Early Permian landscapes in an alluvial setting. This study investigated small-scale lateral and vertical variability of the paleosols that formed at the distal edge of the Dunkard Basin. Three detailed, 32-m thick stratigraphic sections across a 70 m outcrop were measured in eastern Athens County, Ohio. Seven pedotypes representing soil formation in nine subenvironments were identified through bulk geochemical, clay mineralogical, and petrographic analyses as well as a detailed study of the ichnofossils, body fossils, and macromorphological paleosol properties. Ichnofossils were abundant and included rhizoliths, actively to passively filled burrows, and coprolites. These ichnofossils were produced by various plants, larval and adult arthropods, and micro- to macro-vertebrates. Body fossils within the paleosols included micro- and macro bone fragments and teeth, compression plant fossils, ostracodes, and gastropods. Vertical transitions among the subenvironments were due to both fluvial channel migration and climatic changes. Precipitation was seasonally distributed with a range of mean annual precipitation (MAP) from 300–1000 mm/yr through the section. This study demonstrates that by investigating the small-scale lateral and vertical variations in paleosols, finer resolution paleoclimatic, paleoenvironmental, and paleoecological reconstructions are possible.

Primary Production of Lowland Natural Grasslands and Upland Sown Pastures Across a Narrow Climatic Gradient

Variation in aboveground net primary production (ANPP) is usually studied across wide environmental gradients focusing on spatial averages of zonal natural communities. We studied the spatial and temporal variation of ANPP of upland sown pastures and lowland natural grasslands across a narrow gradient of precipitation and temperature. The Flooding Pampa (Argentina) encompasses an 850–1000 mm range of mean annual precipitation and a 13.8–16.0°C range of mean annual temperature. For 15 100 × 100 km cells, we obtained mean monthly precipitation, temperature, and paddock-level ANPP of upland pastures and lowland grasslands during 8 years. Mean annual ANPP of lowland grasslands and upland sown pastures was positively related to mean annual precipitation. ANPP of upland pastures was 60–80% larger and increased more steeply with mean annual precipitation. ANPP seasonality also changed across the gradient. In lowland grasslands, as mean annual precipitation increased, ANPP monthly maximum increased, minimum decreased, and the duration of the growing season shortened. In contrast, in upland pastures, ANPP monthly maximum was constant, minimum increased, and the growing season lengthened with increasing precipitation. ANPP was more stable across years for lowland grasslands than for upland pastures. The response of annual ANPP to current-year precipitation decreased across the gradient, while the importance of the previous-year precipitation increased. In summary, we found strong spatial and temporal patterns of ANPP across a narrow environmental gradient. In addition, landscape position and species composition heavily influenced those patterns.

Modern pollen assemblages from surface lake sediments and their environmental implications on the southwestern Tibetan Plateau

The relationships amongst modern pollen assemblages, vegetation, climate and human activity are the basis for reconstructing palaeoenvironmental changes using pollen records. It is important to determine these relationships at regional scales due to the development of vegetation under different climatic conditions and human activities. In this paper, we report on an analysis of modern pollen assemblages of 31 surface lake samples from 31 lakes (one sample per lake) on the southwestern Tibetan Plateau where the knowledge of modern pollen and their relationships with vegetation, climate and human activities is insufficient. The region includes five vegetation zones: sub‐alpine shrub steppe, alpine steppe, alpine meadow and steppe ecotone, mountain desert and alpine desert. The lakes span a wide range of mean annual precipitation (50–500 mm) and mean annual temperature (−8 to 6 °C). Modern pollen assemblages from our samples mainly consist of herb taxa (Artemisia, Cyperaceae, Poaceae, Chenopodiaceae, etc.) and some tree taxa (Pinus, Fagaceae, Alnus, etc.). The results indicate that modern pollen assemblages are able to reflect the main vegetation distribution. Redundancy analysis for the main pollen types and environmental variables shows that precipitation is the leading factor that influences the pollen distribution in the study area with the first axis capturing 13.7% of the variance in the pollen data set. The Artemisia/Chenopodiaceae ratio is valid for separating the desert component (&lt;2) from the steppe and other vegetation zones (&gt;2) but is unable to distinguish moisture variations. The Artemisia/Cyperaceae ratio is able to identify meadows (&lt;1) and steppes (&gt;1) and can be used as a moisture index on the southwestern Tibetan Plateau. Our results show that an appropriate range is needed for a modern pollen data set in order to perform pollen‐based quantitative climate reconstructions in one region. It is essential to perform modern studies before using pollen ratios to reconstruct palaeovegetation and palaeoclimate at a regional scale.

Dendrochronology and middle Miocene petrified oak: Modern counterparts and interpretation

This study reports the first successful statistical ‘crossdating’ among many ring width time series from petrified wood, thus providing a replicable continuous annual resolution window into tree growth and environmental influences during the middle Miocene. The petrified samples, of the genus Quercus, originated at the Stinking Water (SW) locality in Oregon, a Miocene-aged exposure associated with the Columbia River Basalts. 40AR/39AR dating on pillow basalt from the locality yielded a weighted Plateau Age of 13.79±0.09Ma placing the death of the trees at the end of the Langhian Stage of the Middle Miocene (15.97±0.05 to 13.65±0.05Ma), during the middle Miocene Climate Transition (MMCT). Ring width time series from 26 radii, 17 different trees, show significant intercorrelation. A Modified Coexistence Approach was applied to determine the likely climate range when the SW trees were growing. The modified approach included regression of site-mean ring width time series statistic values on estimated soil moisture for the site locations, using site-mean data from 126 modern Quercus sites from across the United States. Identification of highly significant linearities indicated strong relationships between ring width intercorrelation and soil moisture and ring width variability and soil moisture. Comparison of individual modern site-mean statistical values with values calculated for the SW locality suggests a mesic growing environment for the SW Quercus, with moderate temperatures. Geographic placement of modern Quercus sites with site-mean statistics similar to the SW values indicates a modern analogue in the eastern United States. Modern distributions of mesic species in the genera present at the SW locality suggest similarities with the central and southern Appalachian Mountains and the Ozark/Ouachita Mountains, indicating a mean annual temperature range of ≅10 °C to ≅15 °C and a mean annual precipitation range of ≅750 mm to ≅1200 mm when the SW Quercus were growing.

Influence of temperature and C4 abundance on n-alkane chain length distributions across the central USA

The distributions of long chain n-alkanes are often utilized as plant-derived paleoenvironmental proxies. However, there is debate regarding the degree to which photosynthetic pathway, temperature and/or hydrology drive the variation in chain length distribution observed among different environments. In order to assess the influence of temperature and photosynthetic pathway on plant n-alkanes, this study examined n-alkane chain length distributions in plants and soils along a transect across mid-continental USA spanning >20°C mean annual temperature with a limited range of mean annual precipitation. We found that (i) longer chain length in plants and soils correlated with higher growing season temperature, with relative humidity also a possible driver, (ii) soils exhibited a stronger correlation with temperature than did individual plants, which may reflect the averaging of plant input to soils and (iii) average chain length values in soils did not correlate with the predominance of C4 plants. The data suggest that the spatial and temporal variation in chain length distributions observed in studies of sediment archives may be driven in large part by growing season temperature and/or aridity rather than photosynthetic pathway (C3 or C4). The findings call for further research into the mechanisms and effects of leaf wax composition on water permeability under different environmental conditions, the relationship between leaf wax n-alkanes and relative humidity, and the incorporation of n-alkanes into soils and sediments.

Relationships among precipitation regime, nutrient availability, and carbon turnover in tropical rain forests

The effect of high precipitation regime in tropical forests is poorly known despite indications of its potentially negative effects on nutrient availability and carbon (C) cycling. Our goal was to determine if there was an effect of high rainfall on nitrogen (N) and phosphorous (P) availability and indexes of C cycling in lowland tropical rain forests exposed to a broad range of mean annual precipitation (MAP). We predicted that C turnover time would increase with MAP while the availability of N and P would decrease. We studied seven Neotropical lowland forests covering a MAP range between 2,700 and 9,500 mm. We used radiocarbon (∆(14)C) from the atmosphere and respired from soil organic matter to estimate residence time of C in plants and soils. We also used C, N, and P concentrations and the stable isotope ratio of N (δ(15)N) in live and dead plant tissues and in soils as proxies for nutrient availability. Negative δ(15)N values indicated that the wettest forests had N cycles that did not exhibit isotope-fractionating losses and were potentially N-limited. Element ratios (N:P and C:P) in senescent leaves, litter, and live roots showed that P resorption increased considerably with MAP, which points towards increasing P-limitation under high MAP regimes. Soil C content increased with MAP but C turnover time only showed a weak relationship with MAP, probably due to variations in soil parent material and age along the MAP gradient. In contrast, comparing C turnover directly to nutrient availability showed strong relationships between C turnover time, N availability (δ(15)N), and P availability (N:P) in senescent leaves and litter. Thus, an effect of MAP on carbon cycling appeared to be indirectly mediated by nutrient availability. Our results suggest that soil nutrient availability plays a central role in the dynamic of C cycling in tropical rain forests.

A modern pollen–climate calibration set based on lake sediments from the Tibetan Plateau and its application to a Late Quaternary pollen record from the Qilian Mountains

AbstractAim Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen–climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data.Location The Tibetan Plateau, between 30 and 40° N and 87 and 103° E.Methods We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (Pann; 31–1022 mm), mean annual temperature (Tann; −6.5 to 1 °C), and mean July temperature (TJuly; 2.6–19.7 °C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for Pann, Tann and TJuly were developed with weighted averaging partial least squares. Model performance was assessed by leave‐one‐out cross‐validation.Results The root mean square errors of prediction (RMSEP) were 104 mm (Pann), 1.18 °C (Tann) and 1.17 °C (TJuly). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (Pann), 15.7% (Tann) and 11.9% (TJuly). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north‐eastern Tibetan Plateau (modern Pann 480 mm; Tann−1 °C). Tann and Pann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 °C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 °C greater than present).Main conclusions The modern pollen–climate calibration set will potentially be useful for quantitative climate reconstructions from lake‐sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance.

Climate and geographic trends in hatch delay of the treehole mosquito, Aedes triseriatus Say (Diptera: Culicidae)

Eggs of Aedes triseriatus mosquitoes are stimulated to hatch when inundated with water, but only a small fraction of eggs from the same batch will hatch for any given stimulus. Similar hatching or germination patterns are observed in desert plants, copepods, rotifers, insects, and many other species. Bet hedging theory suggests that parents stagger offspring emergence into vulnerable life history stages in order to avoid catastrophic reproductive failures. For Ae. triseriatus, a treehole breeding mosquito, immediate hatching of an entire clutch leaves all of the parent's progeny vulnerable to extinction in the event of a severe drought. Natural selection has likely favored parents that pursued a bet hedging strategy where the risk of reproductive failure is distributed over time. Considering treehole mosquitoes, bet hedging theory could be used to predict that hatch delay would be positively correlated with the likelihood of drought. To test this prediction, we collected Ae. triseriatus from habitats that varied widely in mean annual precipitation and exposed them to several hatch stimuli in the laboratory. Here we report that, as predicted, Ae. triseriatus eggs from high precipitation regions showed less hatch delay than areas of low precipitation. This strategy probably allows Ae. triseriatus to cope with the wide variety of climatic conditions that it faces in its extensive geographical range.