Low Mean Annual Temperature Research Articles

Relative weathering intensity of calcite versus dolomite in carbonate‐bearing temperate zone watersheds: Carbonate geochemistry and fluxes from catchments within the St. Lawrence and Danube river basins

Calcite and dolomite solubilities in open weathering environments are proportional to pCO2 and inversely proportional to temperature, and dolomite solubility is progressively greater than calcite below 25°C. The continent‐scale weathering budget reveals the significance of the Northern Hemisphere (NH) to globally integrated riverine fluxes of Ca2+, Mg2+, and HCO3−. The NH contributes 70% of the global HCO3− flux while only 54% of the riverine discharge. We present results of a comparative hydrogeochemical study of carbonate mineral equilibria and weathering fluxes in two NH carbonate‐rich river basins. Surface water geochemistry and discharge were determined for headwater streams in Michigan and Slovenia within the St. Lawrence and Danube river basins. Michigan watersheds are established atop carbonate‐bearing glacial drift deposits derived from erosion of Paleozoic strata with thick soil horizons (100–300 cm). Slovenia watersheds drain Mesozoic bedrock carbonates in alpine and dinaric karst environments with thin soil horizons (0–70 cm). Carbonate weathering intensity is a parameter that normalizes river runoff and HCO3− concentration to catchment area (meq HCO3− km−2 s−1), summing calcite and dolomite contributions, and is used to gauge the effects of climate, land use, and soil thickness on organic‐inorganic carbon processing rates. Importantly, Michigan riverine discharge is one‐tenth of Slovenian rivers, providing the opportunity to evaluate the kinetics of carbonate mineral equilibration. The study rivers are HCO3− − Ca2+ − Mg2+ waters, supersaturated for calcite at pCO2 values in excess of the atmosphere. As discharge varies, HCO3− concentrations differ by less than 20% for any location, and Mg2+/Ca2+ remains relatively fixed for Michigan (0.5) and Slovenia streams (0.4), requiring that dolomite dissolution exceed calcite on a mole basis. The ability of calcite and dolomite dissolution to keep pace with increased discharge indicates carbonate weathering is limited only by water flux and temperature‐dependent solubility in these watersheds. Carbonate weathering intensity in Michigan and Slovenia exceeds the world average by factors between 2 and 20, and dolomite weathering intensity, estimated from riverine Mg2+ fluxes, exceeds the world average by factors between 2 and 15. Thus global fluxes of carbonate‐related weathering products appear heavily skewed toward carbonate‐bearing environments at higher latitudes with relatively low mean annual temperatures and high discharge.

The influence of climatic features on distribution and infection of root-knot nematodes <i>Meloidogyne</I> spp, n cowpea growing areas in Nigeria

The effect of climatic features on distribution and infection of root knot nematodes was investigated in 248 cowpea growing areas in 31 states and the Federal Capital Territory of Nigeria. The studies revealed pronounced incidence of Meloidogyne javanica (65.5%) at high altitude of 500-2000 m, high mean annual maximum temperature of 32-40 °C, low mean annual minimum temperature of 11-18 °C, low mean annual precipitation of 0.1-1530 mm and in hot Sudan savannah vegetation of six months or less of wet and dry season, prevailing in the Core North (North east and North west) of the northern region. In contrast, M. incognita (83.1%) was overwhelmingly dominant at low attitude of 15-650 m, mean annual maximum temperature of 24-27 °C, mean annual minimum temperature of 21-23 °C, mean annual precipitation of 1170-4700 mm and in warm humid forest vegetation of 8 months or more of wet season and 4 months or less of dry season, operating in the southern region (Southeast, Southwest and Southsouth). The two species (M. incognita) (44.2%) and M. javanica (51.7%) were, however, common at intermediate altitude of 305-1859 m, moderate mean annual maximum temperature of 29-32 °C, moderate mean annual minimum temperature of 18-21 °C, moderate mean annual precipitation of 800-2000 mm and in moderate Guinea savannah vegetation of 7 months of wet season and 5 months of dry season, prevailing in the Middle belt (North central). The distribution of M. arenaria though sparse (4.1%), followed that of M. incognita in relation to altitude, temperature, precipitation and vegetation. In overall distribution, M. incognita (51.8%) was more widespread than M. javanica (44.1%). The occurrence of pure population of M incognita (31.9%) and M. javanica (37.1%) was encountered. Meloidogyne arenaria was never found as a pure population or as a mixture with M. javanica but with M. incognita (2.0%) or with M. incognita and M. javanica together (4.0%). The mean galling infection by the two species was significantly more severe in the humid forest southern region (11-50% of the root system galled) than in the dry Sudan savannah Core North (1-30% of the root system galled). Generally, Meloidogyne incognita caused more severe mean galling infection (11-30% of the root system galled) than M. javanica (1-10% of the root system galled). The polarized ecological distribution and infection of the M. incognita and M. javanica in the present studies show the climatic preference of the nematodes, and gives a guide in targeting a workable control strategy. Keywords: Climatic features, cowpea, distribution, infection, root-knot nematodes. Ife Journal of Science Vol. 8 (2) 2006: pp. 185-192

Tectonic driving of Neoproterozoic glaciations: Evidence from extreme oxygen isotope signature of meteoric water in granite

The global context of glaciation in the Neoproterozoic has been hypothesized to result from the massive reorganization of the Earth's land and ocean systems due to breakup and dispersal of the supercontinent Rodinia at about 750 Ma. This hypothesis gains support from unusually light O isotope records of hydrothermally altered rocks in a rift tectonic zone at that time, which is indicative of interaction with meteoric water of low mean annual temperature, and thus tectonic driving of cold paleoclimate at the time of hydrothermal alteration. Very negative δ 18O values of − 14.4 to − 10.0‰ are found for garnet from Neoproterozoic granite in South China, which are the lightest O isotope record so far reported for minerals from igneous rocks. Negative δD values of − 129 to − 109‰ are obtained for garnet, magnetite and zircon. Thus high-T meteoric-hydrothermal alteration occurred during magma emplacement. SHRIMP U–Pb dating for magmatic and hydrothermal zircons yields two groups of ages at 782 ± 3 Ma and 748 ± 3 Ma, respectively, responsible for granite crystallization and hydrothermal alteration. The garnet is in O isotope disequilibrium with zircon, indicating differential effects of subsolidus hydrothermal alteration on the minerals of different O diffusion rates. Occurrence of the unusually negative δ 18O granite at mid-low paleolatitudes provides a geochemical proxy for a cold paleoclimate at 748 ± 3 Ma, possibly a continental glaciation corresponding to the Kaigas iceage. It suggests a tectonic link to the climatic effect of uplifted rift flanks due to the Rodinia breakup at about 750 Ma, and thus the ice–fire interaction by syn-rift magmatism of low δ 18O imprints in association with the mantle event of asthenospheric upwelling. Hence the tectonic driving is evident for regional glaciations in supercontinental rift basins.

Echinococcus multilocularis in the red fox Vulpes vulpes from the East Carpathian region of Poland and the Slovak Republic.

The occurrence of Echinococcus multilocularis in the Poland-Slovak frontier zone of the East Carpathian region was assessed, for comparison with that in adjacent regions in both countries. A total of 392 red foxes from Poland and 427 red foxes from the Slovak Republic were examined from 2001 to 2004. Significant differences in prevalences were observed in foxes captured from the borderland and adjacent zones in both countries. The mean prevalence of E. multilocularis in the Polish borderland reached 45.7+/-18.6% and in the Slovak border 35.0+/-10.7%. In both countries, the prevalence of E. multilocularis in red foxes from adjacent districts, outside the frontier Carpathian region, was considerably lower (18.9+/-9.2% in Poland and 20.8+/-9.0% in Slovakia). These differences are probably due to geomorphological and ecological factors, which contribute to the survival of the tapeworm eggs and the subsequent spread of infection. The Carpathian regions of northeast Slovakia and southeast Poland are characterized by specific climatic conditions such as low mean annual air temperatures, low temperatures in active soil surfaces, high soil humidities and a high mean annual rainfall.

Glacier mass balance reconstruction by sublimation induced enrichment of chemical species on Cerro Tapado (Chilean Andes)

Abstract. A 36 m long ice core down to bedrock from the Cerro Tapado glacier (5536 m a.s.l, 30°08' S, 69°55' W) was analyzed to reconstruct past climatic conditions for Northern Chile. Because of the marked seasonality in the precipitation (short wet winter and extended dry summer periods) in this region, major snow ablation and related post-depositional processes occur on the glacier surface during summer periods. They include predominantly sublimation and dry deposition. Assuming that, like measured during the field campaign, the enrichment of chloride was always related to sublimation, the chemical record along the ice core may be applied to reconstruct the history of such secondary processes linked to the past climatic conditions over northern Chile. For the time period 1962–1999, a mean annual net accumulation of 316 mm water equivalent (weq) and 327 mm weq loss by sublimation was deduced by this method. This corresponds to an initial total annual accumulation of 539 mm weq. The annual variability of the accumulation and sublimation is related with the Southern Oscillation Index (SOI): higher net-accumulation during El-Niño years and more sublimation during La Niña years. The deepest part of the ice record shows a time discontinuity; with an ice body deposited under different climatic conditions: 290 mm higher precipitation but with reduced seasonal distribution (+470 mm in winter and –180 mm in summer) and –3°C lower mean annual temperature. Unfortunately, its age is unknown. The comparison with regional proxy data however let us conclude that the glacier buildup did most likely occur after the dry mid-Holocene.

Younger Dryas glacial landsystems in North West Scotland: an assessment of modern analogues and palaeoclimatic implications

By explicitly linking process to form, landsystem models developed in modern environments provide powerful tools for the reconstruction of past environments from the sedimentological and geomorphological record. Some landform characteristics, however, may originate by more than one process (equifinality), so although a particular depositional model may “fit” the data, it is not necessarily the correct one. Here, we critically examine the use of modern analogues to interpret glacial landforms, focusing on “hummocky moraine”, a distinctive landform association consisting of numerous sharp-crested moraine mounds and ridge fragments, which occurs on many valley floors in upland Britain. Detailed studies of examples of Younger Dryas age in the North West Highlands of Scotland show that they represent ice-contact fans which have been affected to varying degrees by ice push and subglacial deformation, and that they formed at active, oscillating ice margins. The sedimentological data are inconsistent with an origin by englacial thrusting, as has been proposed for similar moraines elsewhere in upland Britain on the basis of morphological criteria. Comparison with modern glacial landsystems shows that, although aspects of “hummocky moraines” in the study area have close modern counterparts, none provides an exact analogue in all respects. This may reflect the distinctive climate of western Scotland during the Younger Dryas. Estimates of mean July temperatures in the Younger Dryas in Scotland, based on chironomid assemblages, are c. 8 °C at sea-level. When combined with reconstructed equilibrium line altitudes for contemporaneous glaciers, these estimates indicate mean annual precipitation totals in the range of 2358±337 mm yr −1 at sea-level in NW Scotland, c. 26% higher than present-day values. The implied heavy snowfall suggests that glaciers in western Scotland during the Younger Dryas were of high activity type, unlike modern glaciers in Svalbard, for example. High glacier velocities and high winter precipitation would suppress permafrost development, despite low mean annual temperatures. The climatic environment also favoured rapid oscillations of glacier margins, during both glacier build-up and retreat phases. Climatic factors, combined with the availability of large quantities of subglacial debris, led to the development of distinctive glacial landsystems, which may have no direct modern analogue.

Stoichiometry and large-scale patterns of leaf carbon and nitrogen in the grassland biomes of China

Nitrogen (N) and carbon-nitrogen (C:N) ratio are key foliar traits with great ecological importance, but their patterns across biomes have only recently been explored. We conducted a systematic census of foliar C, N and C:N ratio for 213 species, from 41 families over 199 research sites across the grassland biomes of China following the same protocol, to explore how different environmental conditions and species composition affect leaf N and C:N stoichiometry. Leaf C:N stoichiometry is stable in three distinct climatic regions in Inner Mongolia, the Tibetan Plateau, and Xinjiang Autonomous Region, despite considerable variations among co-existing species and among different vegetation types. Our results also show that life form and genus identity explain more than 70% of total variations of foliar N and C:N ratio, while mean growing season temperature and growing season precipitation explained only less than 3%. This suggests that, at the biome scale, temperature affects leaf N mainly through a change in plant species composition rather than via temperature itself. When our data were pooled with a global dataset, the previously observed positive correlation between leaf N and mean annual temperature (MAT) at very low MATs, disappeared. Thus, our data do not support the previously proposed biogeochemical hypothesis that low temperature limitations on mineralization of organic matter and N availability in soils lead to low leaf N in cold environments.

Soil organic matter dynamics in lands continuously cultivated with maize and soybean in Heilongjiang Province, Northeast China: Estimation from natural 13C abundance

Northeast China is the main production area of maize and soybean in China. In the present study, the rates of decomposition and replacement of soil organic carbon (SOC) were estimated using the soil inventory collected since 1991 from long-term maize and soybean cultivation plots in Heilongjiang Province, Northeast China, to evaluate the sustainability of the present cultivation system. The total carbon (C) content in soil was stable without any significant changes in the plots (approximately 28.5 g C kg−1). The δ13C value of soil organic matter under continuous maize cultivation increased linearly with an annual increment of 0.07 from −23.9 in 1991, which indicated that approximately 13% of the initial SOC was decomposed during the 13-year period of maize cultivation, with a half-life of 65 years. Slow decomposition of SOC was considered to result from the low annual mean temperature (1.5°C) and long freezing period (170–180 days year−1) in the study area. In contrast, the amount of organic C derived from maize increased in the soil with a very slow annual increment of 0.17 g C kg−1, probably because of the removal of all the plant residues from the plots. Based on the soil organic matter dynamics observed in the study plots, intentional recycling/maintenance of plant residues was proposed as a way of increasing soil fertility in maize or soybean cultivation.

The influence of seawater temperature on zooid size and growth rate in Pentapora fascialis (Bryozoa: Cheilostomata)

Zooid size variation and growth banding in the bryozoan Pentapora fascialis, collected from the South of Great Britain to the South of Italy, were investigated in order to test their relationship with the variation of seawater temperature. Zooid length appeared to be more sensitive than zooid area and width to temperature variation. The longest zooids were budded in localities that typically experience low seasonal variation in temperature (3.2–7.5°C) and low mean annual temperature (about 11°C). Zooid length demonstrated the strongest relationship with the range of seasonal variation in temperature rather than the mean annual temperature. Growth bands produced during colder months were less high than those produced during warmer months. Measurements of these alternating bands were used to calculate growth rate of the laminae, ranging from 20.7 at Plymouth to 29.3 mm year−1 at Tino Island. Significant differences in zooid length were found between the two bands, with longer zooids within the band developed in colder months and shorter zooids in bands produced in the warmer period. We conclude that zooid size can be used as an environmental proxy of the seasonal temperature regimes experienced by the species.

Soil δ15N patterns in old-growth forests of southern Chile as integrator for N-cycling

Old-growth forests of southern Chile represent an important reserve of temperate (rain) forests in the world. Wetter and colder forest ecosystems appear to be more efficient in conserving and recycling N such that mostly non-plant available N species are lost, which could be indicated by more depleted δ15N values of the soil and plants. Hydrological N loss from the old-growth forests in southern Chile occurs mainly via dissolved organic nitrogen and not via dissolved inorganic N. Forest disturbances (e.g. fire, clear-cutting or enhanced N deposition) cause (abrupt) changes in ecosystem N-cycling processes. In this study, we hypothesized that δ15N signatures of soil profiles under old-growth forests could be used as an integrator for ecosystem N-cycling, and changes of these δ15N profiles could be valuable to assess ecosystem resilience towards disturbances. Six old-growth forests were selected in the phytogeographical region of the Valdivian rain forest in southern Chile. One of the sites has been partly burned in February 2002. First, we observed that ecosystems with higher mean annual precipitation and lower mean annual temperature were relatively more depleted in 15N. Secondly, we found that a forest fire caused a 100-fold increase of the nitrate export and induced an enrichment of the soil δ15N signal in the upper 20 cm.

Groundwater recharge, circulation and geochemical evolution in the source region of the Blue Nile River, Ethiopia

Geochemical and environmental isotope data were used to gain the first regional picture of groundwater recharge, circulation and its hydrochemical evolution in the upper Blue Nile River basin of Ethiopia. Q-mode statistical cluster analysis (HCA) was used to classify water into objective groups and to conduct inverse geochemical modeling among the groups. Two major structurally deformed regions with distinct groundwater circulation and evolution history were identified. These are the Lake Tana Graben (LTG) and the Yerer Tullu Wellel Volcanic Lineament Zone (YTVL). Silicate hydrolysis accompanied by CO 2 influx from deeper sources plays a major role in groundwater chemical evolution of the high TDS Na–HCO 3 type thermal groundwaters of these two regions. In the basaltic plateau outside these two zones, groundwater recharge takes place rapidly through fractured basalts, groundwater flow paths are short and they are characterized by low TDS and are Ca–Mg–HCO 3 type waters. Despite the high altitude (mean altitude ∼2500 masl) and the relatively low mean annual air temperature (18 °C) of the region compared to Sahelian Africa, there is no commensurate depletion in δ 18O compositions of groundwaters of the Ethiopian Plateau. Generally the highland areas north and east of the basin are characterized by relatively depleted δ 18O groundwaters. Altitudinal depletion of δ 18O is 0.1‰/100 m. The meteoric waters of the Blue Nile River basin have higher d-excess compared to the meteoric waters of the Ethiopian Rift and that of its White Nile sister basin which emerges from the equatorial lakes region. The geochemically evolved groundwaters of the YTVL and LTG are relatively isotopically depleted when compared to the present day meteoric waters reflecting recharge under colder climate and their high altitude.

Patterns of community change among ammonia oxidizers in meadow soils upon long-term incubation at different temperatures.

The effect of temperature on the community structure of ammonia-oxidizing bacteria was investigated in three different meadow soils. Two of the soils (OMS and GMS) were acidic (pH 5.0 to 5.8) and from sites in Germany with low annual mean temperature (about 10 degrees C), while KMS soil was slightly alkaline (pH 7.9) and from a site in Israel with a high annual mean temperature (about 22 degrees C). The soils were fertilized and incubated for up to 20 weeks in a moist state and as a buffered (pH 7) slurry amended with urea at different incubation temperatures (4 to 37 degrees C). OMS soil was also incubated with less fertilizer than the other soils. The community structure of ammonia oxidizers was analyzed before and after incubation by denaturing gradient gel electrophoresis (DGGE) of the amoA gene, which codes for the alpha subunit of ammonia monooxygenase. All amoA gene sequences found belonged to the genus Nitrosospira. The analysis showed community change due to temperature both in moist soil and in the soil slurry. Two patterns of community change were observed. One pattern was a change between the different Nitrosospira clusters, which was observed in moist soil and slurry incubations of GMS and OMS. Nitrosospira AmoA cluster 1 was mainly detected below 30 degrees C, while Nitrosospira cluster 4 was predominant at 25 degrees C. Nitrosospira clusters 3a, 3b, and 9 dominated at 30 degrees C. The second pattern, observed in KMS, showed a community shift predominantly within a single Nitrosospira cluster. The sequences of the individual DGGE bands that exhibited different trends with temperature belonged almost exclusively to Nitrosospira cluster 3a. We conclude that ammonia oxidizer populations are influenced by temperature. In addition, we confirmed previous observations that N fertilizer also influences the community structure of ammonia oxidizers. Thus, Nitrosospira cluster 1 was absent in OMS soil treated with less fertilizer, while Nitrosospira cluster 9 was only found in the sample given less fertilizer.

Yield and its stability, crop diversity, adaptability and response to climate change, weather and fertilisation over 75 years in the Czech Republic in comparison to some European countries

Yield and its stability, crop diversity, adaptability and response to climate change, weather and fertilisation over 75 years in the Czech Republic in comparison to some European countries

Chilling-dependent photoinhibition, nutrition and growth analysis of Eucalyptus nitens seedlings during establishment.

Effects of chilling-dependent photoinhibition on gas exchange, chlorophyll fluorescence, growth and nutrition of Eucalyptus nitens (Deane and Maiden) Maiden seedlings were assessed for 70 weeks after transplanting 9-month-old seedlings in early winter. One month before transplanting, the seedlings were assigned to fertilized or nutrient-deprived treatments. Immediately after transplanting, half the seedlings in each nutrient treatment were placed in shadecloth tree shelters. The experimental site was at an altitude of 700 m, which is considered marginal for the establishment of E. nitens plantations in Tasmania because of low mean annual minimum temperatures. Overnight frosts followed by sunny morning conditions in the first 20 weeks after transplanting (early June to early October) caused severe photoinhibition. Predawn maximal photochemical efficiency (Fv/Fm) and maximum net photosynthesis (Amax) were depressed in nutrient-deprived seedlings compared with fertilized seedlings, although shading partially alleviated this difference. Neither Fv/Fm nor Amax recovered to values observed before transplanting until > 20 weeks after transplanting. During this period, non-photochemical quenching (NPQ) was high in seedlings in all treatments, although NPQ was lower in shaded, fertilized seedlings than in seedlings in the other treatments. Total foliar nitrogen (N) concentration increased up to 42 weeks after transplanting in the nutrient-deprived seedlings in parallel with increasing relative growth rate (RGR). Fractionation of N- and phosphorus (P)-containing compounds indicated that differences in protein N accounted for the treatment differences in total seedling N. Nucleic acid P increased and inorganic P decreased during growth periods, although total seedling P remained constant. Among treated seedlings, height growth was greatest in shaded seedlings: this was probably a result of apical dominance effects because RGR was higher in unshaded seedlings than in shaded seedlings. Thus, the shade treatment alleviated chilling-dependent photoinhibition and maximized growth during winter, but limited growth during warmer periods and therefore overall growth.

The maritime Subantarctic: a distinct periglacial environment

Recent research on slope processes and weathering on Marion Island has highlighted distinctive attributes of the maritime Subantarctic periglacial environment, compared to other periglacial environments. This is reflected morphologically by sorted stripe characteristics and stratified solifluction forms, but the wind factor and low diurnal temperature range also results in differences with other diurnal frost environments. Its hyper-maritime setting at low mean annual temperatures results in very high frost cycle frequencies, with associated effectiveness in surface sediment transport and patterned ground development. In addition, a high frequency of wetting and drying cycles is observed in rocks. Small seasonal temperature ranges and steep temperature profiles indicate a high sensitivity to climate change in the Subantarctic, as reflected in a rich relict periglacial record on most islands. This sensitivity appears absent in high tropical environments of low annual temperature range.

Molecular and physiological diversity in the bipolar lichen-forming fungus Xanthoria elegans

Genetic and physiological diversity was assessed in isolates of the lichen-forming fungus Xanthoria elegans originating from a range of geographic localities and climatic regimes including polar and temperate regions. In vitro cultures were established and isolates examined by production of multi-locus RAPD markers and by sequence comparison of the ITS region of the rDNA tandem repeat unit. Both molecular techniques revealed significant variability. Phylogenetic analysis of RAPD profiles (226 markers) clustered isolates from particular locations together and Mantel's test showed a correlation between genetic divergence and extent of geographic separation. Phylogenies based on ITS sequence divergence were not well supported and did not correlate with the geographic origin of samples, suggesting a single locus approach to be less informative than RAPD data in a study of this nature. Physiological investigations also revealed significant variability between isolates with different geographic origins. Temperature had a significant effect on relative growth rate (RGR) such that X. elegans originating from sites with lower mean annual temperatures had significantly higher RGRs at all test temperatures between 2° and 18°C. Enhanced metabolic activity might be an adaptation for growth in colder climates. The results demonstrate high genetic diversity within this morphologically variable species in response to geographic and environmental factors, and are discussed in relation to data obtained for non-lichenised fungal species and the possibility of cryptic speciation.

Neoproterozoic sand wedges: crack formation in frozen soils under diurnal forcing during a snowball Earth

Thermal contraction cracking of permafrost produced sand-wedge polygons at sea level on the paleo-equator during late Neoproterozoic glacial episodes. These sand wedges have been used as evidence for high (≥54°) paleo-obliquity of the Earth’s ecliptic, because cracks that form wedges are hypothesized to require deep seasonal cooling so the depth of the stressed layer in the ground reaches ≥1 m, similar to the measured depths of cracks that form wedges. To test the counter hypothesis that equatorial cracks opened under a climate characterized by a strong diurnal cycle and low mean annual temperature (snowball Earth conditions), we examine crack formation in frozen ground subject to periodic temperature variations. We derive analytical expressions relating the Newtonian viscosity to the potential crack depth, concluding that cracks will form only in frozen soils with viscosities greater than ∼10 14 Pa s. We also show numerical calculations of crack growth in frozen soils with stress- and temperature-dependent rheologies and find that fractures may propagate to depths 3–25 times the depth of the thermally stressed layer in equatorial permafrost during a snowball Earth because the mean annual temperature is low enough to keep the ground cold and brittle to relatively great depths.

Temperature data for phenological models.

In an arid environment, the effect of evaporation on energy balance can affect air temperature recordings and greatly impact on degree-day calculations. This is an important consideration when choosing a site or climate data for phenological models. To our knowledge, there is no literature showing the effect of the underlying surface and its fetch around a weather station on degree-day accumulations. In this paper, we present data to show that this is a serious consideration, and it can lead to dubious models. Microscale measurements of temperature and energy balance are presented to explain why the differences occur. For example, the effect of fetch of irrigated grass and wetting of bare soil around a weather station on diurnal temperature are reported. A 43-day experiment showed that temperature measured on the upwind edge of an irrigated grass area averaged 4% higher than temperatures recorded 200 m inside the grass field. When the single-triangle method was used with a 10 degrees C threshold and starting on May 19, the station on the upwind edge recorded 900 degree-days on June 28, whereas the interior station recorded 900 degree-days on July 1. Clearly, a difference in fetch can lead to big errors for large degree-day accumulations. Immediately after wetting, the temperature over a wet soil surface was similar to that measured over grass. However, the temperature over the soil increased more than that over the grass as the soil surface dried. Therefore, the observed difference between temperatures measured over bare soil and those over grass increases with longer periods between wettings. In most arid locations, measuring temperature over irrigated grass gives a lower mean annual temperature, resulting in lower annual cumulative degree-day values. This was verified by comparing measurements over grass with those over bare soil at several weather stations in a range of climates. To eliminate the effect of rainfall frequency, using temperature data collected only over irrigated grass, is recommended for long-term assessment of climate change effects on degree-day accumulation. In high evaporative conditions, a fetch of at least 100 m of grass is recommended. Our results clearly indicate that weather stations sited over bare soil have consistently higher degree-day accumulations. Therefore, especially in arid environments, phenology models based on temperature collected over bare soil are not transferable to those based on temperature recorded over irrigated grass. At a minimum, all degree-day-based phenology models reported in the literature should clearly describe the weather station site.

Neutral and Acidic Sugars in Particle‐Size Fractions as Influenced by Climate

Changes in the composition and amount of saccharides have been shown to reflect soil organic matter (SOM) dynamics. The effect of climate on soil monosaccharide pools was investigated in this study. Particle‐size fractions were obtained from composite samples taken from the top 10 cm of soil at 18 native grassland sites along mean annual temperature (MAT) and precipitation (MAP) transects from Saskatoon, Canada to southern Texas, USA. Neutral and acidic sugars were determined in the bulk soil, <2‐μm (clay), 2‐ to 20‐μm (silt), 20‐ to 250‐μm (fine sand), and 250‐ to 2000‐μm (coarse sand) size separates. As particle size decreased, the concentration of monosaccharides decreased significantly from 297 g kg−1 soil organic C (SOC) in coarse sand to 174 g kg−1 SOC in the silt fractions, but increased to 239 g kg−1 SOC in clay. Ratios of hexoses to pentoses increased with decreasing particle size, indicating that SOM of the finer fractions contained more microbe‐derived saccharides, this effect being more pronounced at lower MAT. The concentrations of neutral saccharides decreased in silt and fine‐sand fractions as MAT decreased, but increased in all fractions <250 μm as MAP increased. The concentration of acidic sugars in clay and silt was related only to MAP. The results suggest that the moisture regime primarily affected the saccharide concentrations of the finer particle‐size fractions, whereas the temperature regime affected primarily the saccharide concentrations of the coarser fractions. Particle‐size fractionation was thus a useful tool in decoding the differing effects of MAT and MAP on saccharide dynamics.

Near-Surface Thermal Profiles in Alpine Bedrock: Implications for the Frost Weathering of Rock

The rates of many processes that control rock breakdown into transportable particles are dependent upon rock temperature. In particular, frost-cracking depends largely upon the time spent within a range of subzero temperatures I call the frost cracking window. I present simple analytic solutions, detailed time series of thermal data from a field site, and a numerical model of subsurface temperatures that constrain the expected depth dependence of frost cracking. Analytic solutions using sinusoidal surface temperature histories result in predicted vertical profiles of frost-cracking intensity that depend upon the location of the frost-cracking window within the range of surface temperatures. In some cases, the expected frost cracking intensity decreases monotonically with depth, while in others it displays a distinct maximum at depth. I use hourly temperatures measured over the 1-yr interval April 1995 through March 1996, at 8 depths up to 42 cm into a granitic bedrock surface in the Laramie Range, Wyoming, to constrain an in situ thermal diffusivity of 1.7 mm2 s-1. These temperature histories suggest that frost cracking at this site should decrease monotonically with depth into the rock. I also use this time series of temperatures to calibrate a numerical thermal model in which the top boundary condition is set by a surface radiation balance. The data require both a low albedo of the rock surface (0.1), and a high atmospheric transmissivity (0.9). I then explore the expected near-surface temperatures at other sites by running the model with different annual mean temperatures, latitudes, and slopes. The resulting simulations suggest that at lower mean annual temperatures, as are found in higher latitude and altitude sites, the frost cracking maximum should both amplify and deepen into the subsurface.