Thaw Event Research Articles

Redox-Driven Formation of Mn(III) in Ice.

Redox-driven reactions involving Mn(II) species adsorbed at Mn(IV) oxide surfaces can release Mn(III) in the form of dissolved Mn(III)-ligand species in natural waters. Using pyrophosphate (PP) as a model ligand, we show that freezing accelerates and enhances Mn(III) formation in the form of Mn(III)-PP complexes. This freeze-promoted reaction is explained by the concentration of Mn(IV) oxides and solutes (Mn(II), Na+, and Cl-) into the minute fractions of liquid water locked between ice (micro)crystals - the Liquid Intergrain Boundary (LIB). Time-resolved freezing experiments at -20 °C showed that Mn(III) yields were greatest at low salt (NaCl) content. In contrast, high salt content promoted Mn(III) formation through chloride complexation, although yields became lower as the cryosalt mineral hydrohalite (NaCl·2H2O) dehydrated the LIB by drawing water into its structure. Consecutive freeze-thaw cycles also showed that dissolved Mn(III) concentrations increased within the very first few minutes of each freezing event. Because each thaw event released unreacted PP previously locked in ice, each sequential freeze-thaw cycle increased Mn(III) yields, until ∼80% of the Mn was converted to Mn(III). This was achieved after only seven cycles. Finally, temperature-resolved freezing experiments down to -50 °C showed that the LIB produced the greatest quantities of Mn(III) at -10 °C, where the volumes were greater. Reactivity was however sustained in ice formed below the eutectic (-21.3 °C), down to -50 °C. We suspect that this sustained reactivity was driven by persistent forms of supercooled water, such as Mn(IV) oxide-bound thin water films. By demonstrating the freeze-driven production of Mn(III) by comproportionation of dissolved Mn(II) and Mn(IV) oxide, this study highlights the potentially important roles these reactions could play in the production of pools of Mn(III) in natural water and sediments of mid- and high-latitudes environments exposed to freeze-thaw episodes.

Temperature, moisture and freeze\u2013thaw controls on CO2 production in soil incubations from northern peatlands

Peat accumulation in high latitude wetlands represents a natural long-term carbon sink, resulting from the cumulative excess of growing season net ecosystem production over non-growing season (NGS) net mineralization in soils. With high latitudes experiencing warming at a faster pace than the global average, especially during the NGS, a major concern is that enhanced mineralization of soil organic carbon will steadily increase CO2 emissions from northern peatlands. In this study, we conducted laboratory incubations with soils from boreal and temperate peatlands across Canada. Peat soils were pretreated for different soil moisture levels, and CO2 production rates were measured at 12 sequential temperatures, covering a range from − 10 to + 35 °C including one freeze–thaw event. On average, the CO2 production rates in the boreal peat samples increased more sharply with temperature than in the temperate peat samples. For same temperature, optimum soil moisture levels for CO2 production were higher in the peat samples from more flooded sites. However, standard reaction kinetics (e.g., Q10 temperature coefficient and Arrhenius equation) failed to account for the apparent lack of temperature dependence of CO2 production rates measured below 0 °C, and a sudden increase after a freezing event. Thus, we caution against using the simple kinetic expressions to represent the CO2 emissions from northern peatlands, especially regarding the long NGS period with multiple soil freeze and thaw events.

Effects of Freeze-Thaw Event on Microbial Community Dynamics During Red Clover Ensiling.

Freezing damages in forages represents a major economic loss to agriculture. This study was conducted to investigate the effects of freeze–thaw (FT) event on microbial community dynamics of red clover silage. Results showed that the FT-treated material displayed higher proportions of Weissella and aerobic bacteria, while lower Pantoea and Enterobacter compared with the control material. The FT event promoted the development of Lactobacillus in silage microflora, inducing more intense lactic fermentation after an initial short lag. The aerobic bacteria were suppressed immediately after the onset of ensiling. Microbiomes of the two silages tended to be almost similar after 2 days of ensiling. However, a small number of aerobic bacteria tended to revitalize in the FT silage with prolonged ensiling time, indicated by apparent abundances of Acinetobacter and Pseudomonas at the end of ensiling. The results obtained here suggest that the FT event could promote the development of Lactobacillus during ensiling and the control of aerobe revitalization need to be concerned with silages made from the freeze-damaged forages.

Effect of multiple cycles of freeze-thawing on the RNA quality of lung cancer tissues.

RNA degradation is a major problem in tissue banking. We explored the effect of thawing flash-frozen biospecimens on the quality and integrity of RNA for genetic testing as well as for other cancer research studies. The histological quality of the frozen tumor sections was evaluated by using hematoxylin and eosin staining. RNA extraction from 60 lung cancer tissue samples subjected to various freeze/thaw cycles was performed using the RNeasy Plus isolation kit. RNA integrity was assessed by using an Agilent bioanalyzer to obtain RNA integrity numbers (RIN). Furthermore, RNA from different groups was used for fluorescence Reverse transcription-polymerase chain reaction (RT-PCR) analysis of the echinoderm microtubule-associated protein-like 4 and anaplastic lymphoma kinase (EML4-ALK) fusion gene mutation to verify whether it can be used for research or clinical testing. Highly variable RIN values were observed among the samples, which showed no correlation with the number of freeze/thaw cycles conducted. However, after 3 freeze/thaw cycles (each thaw event lasted for 10min), an increasing number of changes in peak intensity in RINs were observed. After 5 freeze/thaw cycles, RNA integrity decreased to approximately 35%. After 3 freeze/thaw cycles, the RNA could still be used for RT-PCR analysis of EML4-ALK fusion gene mutations; whereas those subjected to 5 freeze/thaw cycles could not. Limited (<3) freeze/thaw cycles did not adversely affect the quality of RNA extracted from tumor tissues and subsequent RT-PCR analysis. Our data could be utilized in the establishment of a standardized procedure for tissue biospecimen collection and storage.

Geomorphological and geochemistry changes in permafrost after the 2002 tundra wildfire in Kougarok, Seward Peninsula, Alaska

AbstractGeomorphological and thermohydrological changes to tundra, caused by a wildfire in 2002 on the central Seward Peninsula of Alaska, were investigated as a case study for understanding the response from ice‐rich permafrost terrain to surface disturbance. Frozen and unfrozen soil samples were collected at burned and unburned areas, and then water isotope geochemistry and cryostratigraphy of the active layer and near‐surface permafrost were analyzed to investigate past hydrological and freeze/thaw conditions and how this information could be recorded within the permafrost. The development of thermokarst subsidence due to ice wedge melting after the fire was clear from analyses of historical submeter‐resolution remote sensing imagery, long‐term monitoring of thermohydrological conditions within the active layer, in situ surveys of microrelief, and geochemical signals recorded in the near‐surface permafrost. The resulting polygonal relief coincided with depression lines along an underground ice wedge network, and cumulative subsidence to 2013 was estimated as at least 10.1 to 12.1 cm (0.9–1.1 cm/year 11 year average). Profiles of water geochemistry in the ground indicated mixing or replenishment of older permafrost water with newer meteoric water, as a consequence of the increase in active layer thickness due to wildfire or past thaw event. Our geocryological analysis of cores suggests that permafrost could be used to reconstruct the permafrost degradation history for the study site. Distinct hydrogen and oxygen isotopic compositions above the Global Meteoric Water Line were found for water from these sites where permafrost degradation with geomorphological change and prolonged surface inundation were suggested.

Differences in field‐scale N2O flux linked to crop residue removal under two tillage systems in cold climates

AbstractResidue removal for biofuel production may have unintended consequences for N2O emissions from soils, and it is not clear how N2O emissions are influenced by crop residue removal from different tillage systems. Thus, we measured field‐scale N2O flux over 5 years (2005–2007, 2010–2011) from an annual crop rotation to evaluate how N2O emissions are influenced by no‐till (NT) compared to conventional tillage (CV), and how crop residue removal (R−) rather than crop residue return to soil (R+) affects emissions from these two tillage systems. Data from all 5 years indicated no differences in N2O flux between tillage practices at the onset of the growing season, but CT had 1.4–6.3 times higher N2O flux than NT overwinter. Nitrous oxide emissions were higher due to R− compared to R+, but the effect was more marked under CT than NT and overwinter than during spring. Our results thus challenge the assumption based on IPCC methodology that crop residue removal will result in reduced N2O emissions. The potential for higher N2O emission with residue removal implies that the benefit of utilizing biomass as biofuels to mitigate greenhouse gas emission may be overestimated. Interestingly, prior to an overwinter thaw event, dissolved organic C (DOC) was negatively correlated to peak N2O flux (r = −0.93). This suggests that lower N2O emissions with R+ vs. R− may reflect more complete stepwise denitrification to N2 during winter and possibly relate to the heterotrophic microbial capacity for processing crop residue into more soluble C compounds and a shift in the preferential C source utilized by the microbial community overwinter.

Hydrologic Alterations from Climate Change Inform Assessment of Ecological Risk to Pacific Salmon in Bristol Bay, Alaska

We developed an integrated hydrologic model of the upper Nushagak and Kvichak watersheds in the Bristol Bay region of southwestern Alaska, a region under substantial development pressure from large-scale copper mining. We incorporated climate change scenarios into this model to evaluate how hydrologic regimes and stream temperatures might change in a future climate, and to summarize indicators of hydrologic alteration that are relevant to salmon habitat ecology and life history. Model simulations project substantial changes in mean winter flow, peak flow dates, and water temperature by 2100. In particular, we find that annual hydrographs will no longer be dominated by a single spring thaw event, but will instead be characterized by numerous high flow events throughout the winter. Stream temperatures increase in all future scenarios, although these temperature increases are moderated relative to air temperatures by cool baseflow inputs during the summer months. Projected changes to flow and stream temperature could influence salmon through alterations in the suitability of spawning gravels, changes in the duration of incubation, increased growth during juvenile stages, and increased exposure to chronic and acute temperature stress. These climate-modulated changes represent a shifting baseline in salmon habitat quality and quantity in the future, and an important consideration to adequately assess the types and magnitude of risks associated with proposed large-scale mining in the region.

Investigations concerning cavitation and frost fatigue in clonal 84K poplar using high-resolution cavitron measurements.

Both drought and freezing-thawing of stems induce a loss of hydraulic conductivity (percentage loss of conductivity [PLC]) in woody plants. Drought-induced PLC is often accompanied by physical damage to pit membranes, causing a shift in vulnerability curves (cavitation fatigue). Hence, if cavitated stems are flushed to remove embolisms, the next vulnerability curve is different (shifted to lower tensions). The 84K poplar (Populus alba × Populus glandulosa) clone has small vessels that should be immune from frost-induced PLC, but results demonstrated that freezing-thawing in combination with tension synergistically increased PLC. Frost fatigue has already been defined, which is similar to cavitation fatigue but induced by freezing. Frost fatigue caused a transition from a single to a dual Weibull curve, but drought-fatigued stems had single Weibull curves shifted to lower tensions. Studying the combined impact of tension plus freezing on fatigue provided evidence that the mechanism of frost fatigue may be the extra water tension induced by freezing or thawing while spinning stems in a centrifuge rather than direct ice damage. A hypothesis is advanced that tension is enhanced as ice crystals grow or melt during the freeze or thaw event, respectively, causing a nearly identical fatigue event to that induced by drought.

Occurrence of heavy metals in rain wastewater on example of urban catchment in Kielce

Rain wastewater flowing from built-up urban areas introduces much pollution into the receiving water environment. The regulation of the Minister of Environment of 24 July 2006 on conditions that have to be met when releasing wastewater into water bodies or soil, and on substances that are particularly hazardous for the aquatic environment only states values of the suspension and those of oil-derived hydrocarbons in rain wastewater delivered to the receiving water. Other hazardous substances, like eg heavy metals, are disregarded. Because of the possibility of bioaccumulation, heavy metals can adversely affect the environment, in particular, that of rain wastewater receiving bodies. The paper presents the results of investigations into the level of rain wastewater pollution with heavy metals flowing into the sewer system from the area of the selected catchment, located in the centre of the city of Kielce, which is typically urban in character. For the sake of analysis, four days of spring thaw event were selected, namely those of 22 Feb. 2010, 23 Feb. 2010, 24 Feb. 2010, and 14 March 2010. During the passage of thaw water, samples of rain wastewater were collected with an automatic device (sampler) and the concentration of the following heavy metals: Ni, Cu, Cr, Zn, Pb, Cd were determined. The conducted statistical analysis concerning the assessment of the strength of statistical relations between individual pairs of heavy metals in spring thaw event indicated the occurrence of a strong correlation relation only between Ni – Cr, Cu – Cr. The results of investigations were presented in a tabular and graphical form to show the range of variation of concentrations of heavy metals for the observed spring thaw event in the examined Si9 rain water sewer.

A 2200-Year Record of Permafrost Dynamics and Carbon Cycling in a Collapse-Scar Bog, Interior Alaska

Recent high-latitude warming is increasing the vulnerability of permafrost to thaw, which is amplified by local disturbances such as fire. However, the long-term ecological effects and carbon dynamics are not well understood. Here we present a 2200-year record of pollen, plant macrofossils, testate amoebae, and apparent carbon (C) accumulation rates from two peat cores in a collapse-scar bog (thermokarst) near Fairbanks, Alaska. A black spruce ecosystem with low apparent C accumulation rates existed on the site during the first ~1500 years of the record. We identify two thaw events, which are linked to local fires. Permafrost aggraded rapidly following the first thaw, which we attribute to local vegetation feedbacks and a cooler climate. The second thaw event at 525 cal y BP is preceded by a stand-replacing fire, as evidenced by a drastic decline in Picea and an initial increase in Epilobium, Salix, and ericaceous shrubs, followed by a sustained increase in Populus. Locally, the forest does not recover for more than 100 years, and the site has remained permafrost-free for the last 500 years. Following thaw, average apparent C accumulation rates (60 to >100 g C m−2 y−1) are 5–6 times higher than average boreal C accumulation rates, indicating that peat C accumulation rates can remain substantially elevated for much more than a century following thaw. The low apparent C accumulation for the formerly forested, permafrost peat (<5 g C m−2 y−1) may suggest that C accumulation increases substantially following thaw, but it remains unknown whether deep peat C loss occurred immediately following thaw. Well-preserved Sphagnum peat dominates during this period of rapid accumulation, except for an interval from ~400 to 275 cal y BP which alternates between Sphagnum and vascular plant-dominated peat and wetter, minerotrophic conditions. A decline in Picea pollen during this interval and again ~100 cal y BP suggests a decrease in suitable substrate for tree growth likely attributable to thermokarst expansion on the collapse-scar margin. These findings suggest that the combined effects of fire and thermokarst will result in a long-term reduction of spruce ecosystems in interior Alaska.

Nitrous oxide fluxes related to soil freeze and thaw periods identified using heat pulse probes

Surface N2O fluxes have not been unequivocally linked to soil profile conditions, in particular the timing of water phase change. The heated needle probe is a sensor that has the potential to monitor in situ apparent volumetric heat capacity (Ca), which considers latent heat transfer, during freezing and thawing. The objective of this study was to relate the timing of N2O flux to the occurrence of soil water phase change between liquid and ice as determined by Ca in no-tillage (NT) and conventional tillage (CT) plots monitored from fall to spring. Half-hourly micrometeorological N2O fluxes were measured using a tunable diode laser trace gas analyzer. Apparent heat capacity was measured at 5-cm depth using three 4-cm-long parallel needles, two equipped with thermistors and one with a heater. Two N2O flux events were observed for CT in January, followed by the main emission event in early March. For NT, only one emission event occurred, with lower magnitude than the CT event, and a later starting and ending date. The apparent heat capacity measured in situ with HPP showed a different temporal pattern between NT and CT, with CT presenting more phase change events. Two out of the three N2O emission events in CT that occurred during winter and early spring occurred immediately after phase change from ice to liquid water at 5-cm depth. The N2O flux associated with the phase change during the main thaw event in CT was an exponential function of the soil surface temperature increasing sharply when T &gt; 0°C, but with smaller fluxes once T was &gt; 5°C. The temperature response observed is consistent with the suggestion of a breakdown in the N2O reduction process in the 0 to 5°C range, while the N2O production enzymes are less affected by low temperature.Key words: Nitrous oxide flux, freeze-thaw cycles, heat pulse probes, no-tillage, conventional tillage

Spring Thaw and Its Effect on Terrestrial Vegetation Productivity in the Western Arctic Observed from Satellite Microwave and Optical Remote Sensing

Abstract Global satellite remote sensing records show evidence of recent vegetation greening and an advancing growing season at high latitudes. Satellite remote sensing–derived measures of photosynthetic leaf area index (LAI) and vegetation gross and net primary productivity (GPP and NPP) from the NOAA Advanced Very High Resolution Radiometer (AVHRR) Pathfinder record are utilized to assess annual variability in vegetation productivity for Alaska and northwest Canada in association with the Western Arctic Linkage Experiment (WALE). These results are compared with satellite microwave remote sensing measurements of springtime thaw from the Special Sensor Microwave Imager (SSM/I). The SSM/I-derived timing of the primary springtime thaw event was well correlated with annual anomalies in maximum LAI in spring and summer (P ≤ 0.009; n = 13), and GPP and NPP (P ≤ 0.0002) for the region. Mean annual variability in springtime thaw was on the order of ±7 days, with corresponding impacts to annual productivity of approximately 1% day−1. Years with relatively early seasonal thawing showed generally greater LAI and annual productivity, while years with delayed seasonal thawing showed corresponding reductions in canopy cover and productivity. The apparent sensitivity of LAI and vegetation productivity to springtime thaw indicates that a recent advance in the seasonal thaw cycle and associated lengthening of the potential period of photosynthesis in spring is sufficient to account for the sign and magnitude of an estimated positive vegetation productivity trend for the western Arctic from 1982 to 2000.

Evidence for freeze–thaw events and their implications for rock weathering in northern Canada: II. The temperature at which water freezes in rock

AbstractMany undertakings have used either a single value or a narrow window of temperatures as a threshold for the freezing of water within rock. These temperatures vary from 0 to −5 C, with most windows being in the range −1 to −4 C. Based on thermal data, these thresholds are commonly used to ‘count’ the number of freeze–thaw events as a basis for determining freeze–thaw weathering. Data collected from northern Canada indicate that the temperature at which freezing occurs can vary substantially, even for the same site. Using exotherm and zero curtain observations from bricks, at angles of 90 and 45, aligned to the four cardinal aspects, the various temperatures at which water froze are shown. Bricks on the north and east commonly did exhibit freezing, based on exotherms, within the window −1 to −5 C, while data for the south and west aspects showed substantial variation, with freezes sometimes between −6·4 and −8·9 C. The data were evaluated for evidence of zero curtain effects (indicative of water freezing), but no unequivocal events could be found, and it is suggested that, at the scale of observation used here, they are unlikely. It would therefore appear that the use of thermal thresholds may not be meaningful for evaluation of freeze–thaw events. The available data also indicate many instances when temperatures went substantially sub‐zero (e.g. −20 C) and yet no indication of water freezing occurred – most likely because there was no water available to freeze. This indicates that any form of freeze–thaw event counting, in the absence of some indicator of the presence of water and that it actually froze, is flawed, as thermal conditions alone are not adequate to indicate the occurrence or not of actual freeze–thaw weathering events. These data suggest that evaluations of freeze–thaw occurrence based simply on thermal thresholds may be substantially in error. Copyright © 2006 John Wiley &amp; Sons, Ltd.

An analysis of mechanisms of ice-wedge casting based on geotechnical centrifuge simulations

Here we interpret the outcomes of scaled geotechnical centrifuge simulation of ice-wedge casting in terms of the likely significance of Quaternary ice-wedge pseudomorphs observed within different host sediments. Six experiments were completed in which 1/30th scale models of an ice-wedge embedded within frozen host sediments beneath a 25 mm thick unfrozen active layer were thawed from the surface downwards in the geotechnical centrifuge under a 30 times gravity (30× g) acceleration. Host sediment granulometry and/or ice contents were varied in each model, with host materials comprising medium sands, fine sandy silts and silty clays. The model ice-wedge was 50 mm at the top, 150 mm deep, and extended across the full width of the 450 mm wide test box. Centrifuge scaling laws indicate that under an acceleration of 30× g, stress distribution was equivalent to a 13.5 m long section of a 4.5 m high and 1.5 m wide full-scale prototype ice-wedge, covered in an active layer of thickness equivalent to 0.75 m. Thermal regimes, measured pore pressures during thaw, observed thaw consolidation and measured host sediment geotechnical properties are utilised in the interpretation of casting mechanisms. During a single uniform thaw event it is shown that arching of infilling sediment and the formation of a void is likely if negative pore pressures are developed in the host sediment. In fine silt and clays high ice contents are more likely than in sands, thaw consolidation is greater, positive pore pressures encourage complete filling of the ice-wedge void, and soft sediment deformation is likely to cause deformation of the cast and reduce its width and depth. Though natural casting mechanisms are likely to be more complex than those simulated here, modelling experiments highlight the need for care when inferring original ice-wedge geometry from observed shape and size of Quaternary ice-wedge casts.

Assessing snowmelt dynamics with NASA scatterometer (NSCAT) data and a hydrologic process model

The presence of snow strongly impacts the exchange of moisture and energy between the land surface and atmosphere. In the interior of the northern hemisphere continents, snowmelt on frozen soils can cause or exacerbate major floods. Microwave remote sensing from satellite platforms has the potential to monitor the freeze–thaw status of soil and overlying snow packs over large areas. We evaluate the backscatter response of the NSCAT scatterometer to changing snow surface conditions, especially freeze and thaw status, using a macroscale hydrology model and the NSCAT backscatter data for the upper Mississippi River basin of the north central U.S. and the Boreal Ecosystem Atmosphere Study (BOREAS) region in central Canada. We compared the snowmelt conditions simulated by the Variable Infiltration Capacity (VIC) macroscale hydrology model driven with surface meteorological observations with NSCAT measurements for 1996–1997 snow season. A mid-winter thaw event (in February) and late season melt (April–May) are evaluated for both regions. Comparison of backscatter images with daily and hourly-modeled snow surface wetness and temperature showed that the model agreed with the backscatter for snow surface wetness on some days but not on others. Factors such as NSCAT overpass times, vegetation on the ground and their freeze–thaw state, and liquid moisture content appear to contribute to these discrepancies.

Changes in oxidation‐reduction state and antioxidant enzymes in the roots of jack pine seedlings during cold acclimation

The role of glutathione and the response of components of the ascorbic acid‐glutathione cycle in cold acclimation and the acquired freezing tolerance of jack pine (Pinus banksiana Lamb.) seedlings were investigated. An increase in the reduced to oxidized glutathione mole ratio was correlated with the increase in root soluble and membrane‐bound protein thiol concentrations during cold acclimation and after a freeze and thaw event. All the enzymes involved in the ascorbic acid‐glutathione cycle were regulated by low temperatures and increased activities of ascorbic peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, and glutathione reductase were observed after the conditioning of the seedlings to low temperatures. Our results suggest that these enzymes play a protective role following the exposure of the seedlings to freezing temperatures.

Xylem dysfunction caused by water stress and freezing in two species of co‐occurring chaparral shrubs

ABSTRACTWater transport from the roots to leaves in chaparral shrubs of California occurs through xylem vessels and tracheids. The formation of gas bubbles in xylem can block water transport (gas embolism), leading to shoot dieback. Two environmental factors that cause gas embolism formation in xylem conduits are drought and freezing air temperatures. We compared the differential vulnerabilities of Rhus laurina and Ceanothus megacarpus, co‐dominant shrub species in the coastal regions of the Santa Monica Mountains of southern California, to both water stress‐induced and freezing‐induced embolism of their xylem. Rhus laurina has relatively large xylem vessel diameters, a deep root system, and a large basal burl from which it vigorously resprouts after wildfire or freezing injury. In contrast, Ceanothus megacarpus has small‐diameter vessels, a shallow root system, no basal burl and is a non‐sprouter after shoot removal by wildfire. We found that R. laurina became 50% embolized at a water stress of –3 MPa and 100% embolized by a freeze–thaw cycle at all hydration levels. In contrast, C. megacarpus became 50% embolized at a water stress of –9 MPa and 100% embolized by freeze–thaw events only at water potentials lower than –3 MPa. Reducing thaw rates from 0·8 °C min−1 to 0·08 °C min−1 (the normal thaw rate measured in situ) had no effect on embolism formation in R. laurina but significantly reduced embolism occurrence in well‐hydrated C. megacarpus (embolism reduced from 74 to 35%). These results were consistent with the theory of gas bubble formation and dissolution in xylem sap. They also agree with field observations of differential shoot dieback in these two species after a natural freeze–thaw event in the Santa Monica Mountains.

Xylem Embolism in Ring‐Porous, Diffuse‐Porous, and Coniferous Trees of Northern Utah and Interior Alaska

Xylem embolism was measured in nine tree species for one or more years. Species were ring—porous (Quercus sp.), diffuse—porous (Alnus, Betula, Populus spp.) or coniferous (Picea, Larix, Abies spp.). Intraspecific (Populus tremuloides) and intrageneric (Betula, Alnus) comparisons were made between sites in northern Utah and interior Alaska. Most embolism, &gt;90% in some dicot species, occurred in winter. Within sites, dicot trees embolized more than conifers. Between sites, Alaskan dicot trees embolized less than their Utah counterparts. Differences were explained by vulnerability to embolism caused by freeze—thaw cycles. Most conifers were entirely resistant, whereas dicot trees were vulnerable. Less embolism in Alaskan dicot trees was associated with fewer freeze—thaw events in Alaska vs. Utah. Vulnerability was positively correlated with conduit volume and hydraulic conductance per unit xylem area (ks). Tracheids were superior to vessels in avoiding freeze—thaw—induced embolism, and had lower ks. At the other extreme, ring—porous xylem had the highest ks but lost &gt;90% of hydraulic conductance after a single freeze—thaw event. Vulnerability to water—stress—induced cavitation was not correlated with conduit volume or ks. Dicot species either reversed winter embolism by refilling vessels with positive root pressures during spring (Betula, Alnus spp.), or tolerated it and relied on new xylem production to restore hydraulic conductance (Quercus sp.). Conifers reversed embolism by refilling tracheids in the absence of positive pressure. Populus species behaved inconsistently, showing some reversal one year but none the next. Even without embolism reversal, Populus species had hydraulic conductances per unit leaf area equal to other diffuse—porous species.

Effect of freeze–thaw cycles on the freezing characteristics of a clayey silt at various overconsolidation ratios

Repeated freezing and thawing affect the structure of clayey silts over a wide range of consolidation ratios. While the overall void ratio of the thawed soil either decreased, as in lightly overconsolidated soils, or increased, as in heavily overconsolidated samples, freezing and thawing caused a reduction in segregation potential after each freeze–thaw event. All the changes occurred during the first three cycles. The maximum reduction in segregation potential was about 50% in all samples. The implications for frost heave predictions in the field are discussed. Key words: freeze–thaw, clayey silt, segregation potential, frost heave.