Seasonal Frost Research Articles

Characterizing Seasonal and Residual Ices at the South Pole of Mars Using a Universal Set of CRISM Spectral Endmembers

AbstractCurrent maps of compositional variation across south polar ice exposures on Mars do not resolve the meter‐scales at which erosional processes are most active, ultimately limiting our understanding of how the deposits form and evolve and how they can be used to interpret long‐term climate records. In this study, we use k‐means clustering and random forest classification to identify and map a set of universal spectral endmembers across 167 high‐resolution observations acquired during southern summer by the Compact Reconnaissance Imaging Spectrometer for Mars. The 21 endmembers show distinct combinations and strengths of key infrared absorption features reflecting diverse mixtures of CO2 ice, H2O ice, and dust. The resulting compositional framework can be used to characterize the nature of both seasonal CO2 frost and the residual ices it overlies across a variety of terrains. Following the large dust event of Mars Year 28, the residual CO2‐ice deposits (RCD) were covered by an unusually thick or long‐lived deposit of seasonal frost. Within the RCD, low‐albedo material around erosional features display H2O ice absorptions consistent with exposures around the outer margins of the RCD. These peripheral water‐ice deposits show unexpected variation in CO2 ice and dust content. Most notably, regions within several km of the edge of the RCD display spectral contributions from CO2 ice even after seasonal frost has been removed. These results can inform investigations focused on the dynamics of seasonal CO2 deposition, the development of erosional morphologies, and the creation of climate records in south polar stratigraphy.

Seasons in Ancient Indian Medicine

Most notions of the seasons in works of ancient Indian medicine list frost, spring, summer, rainy season, autumn and winter as the particular times of the year. However, in some contexts, the hibernal season frost (śiśira) is left out and replaced by a second rainy season, called beginning of the rain (prāvṛṣ) and placed between summer and the actual rainy season. In this paper, I firstly introduce the concept of the seasons and the division of the year into two halves. Secondly, I examine the dichotomy of two seasonal schemes inside one scientific corpus, which varies regarding the included seasons. Concerning this matter, I follow the investigations of Francis Zimmermann in order to prove that the two schemes are utilized both intentionally and systematically in specific contexts. The two seasonal schemes serve diverse cases, being of use for the physician in different aspects of his work.

Durability Analysis of Small Assembled Buildings in Irrigation Canal System

With the rapid development of agricultural economy, people are paying more and more attention to how to apply high-efficiency technologies that save resources to improve agricultural production efficiency, so water-saving irrigation technology has gradually developed. China’s agricultural irrigation technology is relatively backward. In addition to the inappropriate irrigation methods and irrigation systems, problems such as siltation, seepage, and frost heave damage in irrigation canals have seriously affected the durability and service life of the canals. The backward irrigation technology seriously restricts the development of agricultural water-saving irrigation. Compared with large irrigation channels, the fabricated reinforced concrete small irrigation channels studied in this paper are less prone to frost heave damage and infiltration problems, and have the advantages of standardized production, simple transportation and installation, and convenient maintenance. In order to study the durability issues such as the basic characteristics, frost heave damage, and service life of fabricated irrigation channels, this paper takes the channel concrete and the formed channel as the research objects, and discusses the research on the properties of the channel concrete through theoretical research, numerical analysis, experiments, and other methods. Strength properties, water penetration resistance, cyanide ion penetration resistance and frost resistance; simulate the seasonal frost heave failure process of the channel; finally, on the basis of the test data, the service life aims to explore the safety and applicability of the fabricated reinforced concrete irrigation channel during the design use period.

Association genetics of early season cold and late season frost tolerance in Sorghum bicolor

AbstractSorghum [Sorghum bicolor (L.) Moench] is a multipurpose crop that is highly susceptible to low temperature stress. Early season cold (ESC) greatly affects seed germination, seedling vigor, and root architecture, whereas late‐season frost (LSF) reduces fertility, yield, and nutrition quality and content in sorghum. Here, an association mapping study was conducted to delineate the genetics of ESC and LSF tolerance in sorghum. A total of 10 single nucleotide polymorphisms (SNPs) and 17 quantitative trait loci (QTL) were identified for ESC, and 40 SNPs were identified for LSF. The diversity panel exhibited extensive variation for plant traits measured for their association with ESC and LSF tolerance. Two QTL colocalized with the classical tannin genes Tan1 (S4_61580203) and Tan2 (S2_7777693) were identified for ESC tolerance. We also identified additional putative candidate genes such as Sobic.001G157100 (Nonphototropic hypocotyl 3, NPH3), Sobic.001G156600 (Lectin receptor‐like serine/threonine kinase), and Sobic.006G061100 (Sucrose nonfermenting 1 related protein kinase 1, SnRK1) for ESC tolerance and Sobic.006G139900 (UDP‐glucoronosyl and UDP‐glucosyl transferase), Sobic.002G187400 (Serine/threonine protein kinase), and Sobic.004G333700 (Ca2+/calmodulin‐dependent protein phosphatase) for LSF tolerance. Tan1 and Tan2 play a major role in seed dormancy and induce resistance against soil‐borne pathogens under cold and wet soil conditions. Seed tannin content may be used as an indirect selection marker for identifying cold tolerant germplasm for breeding. NPH3 and SnRK1 regulate seed germinability under cold stress in various crops. Candidates identified in this study could be further exploited to develop low temperature tolerant cultivars as proxies in marker‐assisted breeding, genomic selection, and genetic engineering.

Long‐Term Simulation of Snow Cover and Its Potential Impacts on Seasonal Frost Dynamics in Croplands Across Southern Canada

AbstractIn northern climates, accurate simulation of thermal and hydrological budgets for farmlands overwinter is crucial for both an accurate prediction of spring flooding and the successful management of nutrient losses. As snow cover influences soil freezing dynamics, it has been hypothesized that reduced snow cover due to warmer winters might intensify soil freezing. The present study was designed to test this hypothesis. Drawing upon observed snow depth and soil temperature data collected from six research farms across Southern Canada over various time spans from 1989 to 2020, the root zone water quality model, integrated with the simultaneous heat and water model (RZ‐SHAW), was calibrated and validated. The potential influence of warmer winter on shifts in soil frost dynamics was evaluated by estimating soil freezing dynamics for each farmland site under various air temperature scenarios using the RZ‐SHAW model. Soil frozen depth in the eastern Canada sites increased with increasing air temperature in some years but decreased under the highest air temperature increases of 3.5°C. The monthly relationship between snow depth and soil frozen depth was determined through partial correlation analysis. Snow was most effective in alleviating soil freezing in the months of January and February, a period when snow cover depth was least affected by warming air temperatures. This study suggests that the hypothesis of increasing soil frozen depth under global warming‐induced snow cover reduction holds true in conditions where soil energy lost through reduced snow cover outweighed the soil energy gained through warmer air temperature.

Climate change projections for UK viticulture to 2040: a focus on improving suitability for Pinot Noir

Between 1981–2000 and 1999–2018, growing season average temperatures (GST) in the main UK viticulture regions have warmed ~1.0 °C and are now more reliably >14.0 °C GST. This warming has underpinned the rapid expansion of the UK viticulture sector and its current focus on growing grape varieties for sparkling wine. Near-term (2021–2040) climate change may condition opportunities for further variety and/or wine style changes. Using the latest high-resolution (5 km) ensemble (×12) of downscaled climate change models for the UK (UK Climate Projections; UKCP18) under Representative Concentration Pathway (RCP) 8.5, we calculate near-term trends and variability in bioclimatic indices (BCIs). We simulate the projected repetition of the UK’s highest yielding season—2018—and use an analogue approach to model the 1999–2018 mean growing season temperatures from Pinot Noir producing areas of Champagne (France), Burgundy (France) and Baden (Germany) over the UK during 2021–2040. We also project, across the UK for the 2021–2040 period, BCI values of recent high-quality vintage years from Champagne and Burgundy. GST are projected to increase from a 1999–2018 spatial range of 13.0 (minimum threshold) –15.7 °C to a future (2021–2040) range of 13.0–17.0 °C, and Growing Degree Days (GDD) from 850 (minimum threshold) –1267 to 850–1515. Growing season precipitation (GSP) is projected to decline in some UK viticulture areas but is not modelled as a limiting viticulture factor. High inter-annual weather variability is simulated to remain a feature of the UK viticulture climate and early season frost risk is likely to occur earlier. Large areas of the UK are projected to have >50 % of years within the bioclimatic ranges experienced during the 2018 growing season, indicating potential higher yields in the future. The 1999–2018 mean Champagne, Burgundy and Baden GST and GDD are projected for much of England and some areas in the far south and south-east of Wales during 2021–2040, with significant areas projected to have >25 % of years within the BCI ranges of top Champagne vintages. These results indicate greater potential for Pinot Noir for sparkling wines and shifting suitability to still red wine production. Accounting for changes in variety suitability and wine styles will be essential to maximise opportunities and build resilience within this rapidly expanding wine region.

Integrated hydrogeological and hydrogeochemical dataset of an alpine catchment in the northern Qinghai–Tibet Plateau

Abstract. Climate warming has significantly changed the hydrological cycle in cold regions, especially in areas with permafrost or seasonal frost. Groundwater flow and its interactions with surface water are essential components of the hydrological process. However, few studies or modeling works have been based on long-term field observations of groundwater level, temperature, hydrogeochemistry, or isotopic tracers from boreholes due to obstacles such as remote locations, limited infrastructure, and harsh work conditions. In the Hulugou catchment, an alpine catchment in the headwater region of the Heihe River on the northern Qinghai–Tibet Plateau (QTP), we drilled four sets of depth-specific wells and monitored the groundwater levels and temperatures at different depths. Surface water (including river water, glacier meltwater, and snow meltwater), precipitation, groundwater from boreholes, spring water, and soil water were sampled to measure the abundances of major and minor elements, dissolved organic carbon (DOC), and stable and radioactive isotopes at 64 sites. This study provides a dataset of these groundwater parameters spanning 6 consecutive years of monitoring/measurements. These data can be used to investigate groundwater flow processes and groundwater–surface water interactions on the QTP under global climate change. The dataset provided in this paper can be obtained at https://doi.org/10.5281/zenodo.6296057 (Ma et al., 2021b) and will be subject to further updates.

Influence of permafrost and hydrogeology on seasonal and spatial variations in water chemistry of an alpine river in the northeastern Qinghai-Tibet Plateau, China

Known as the third pole of the world, the Qinghai-Tibet Plateau has been experiencing rapid permafrost warming and thawing over the last few decades. However, the impact of permafrost distribution and hydrogeology on river hydrochemistry in alpine areas remains unclear. This study conducted four sampling campaigns to reveal the temporal and spatial variations in and factors driving river hydrochemistry in the upper reaches of the Heihe River, the northeastern Qinghai-Tibet Plateau. We found that the concentrations of major ions and total dissolved solids (TDS) in river water showed substantial seasonal variations; the concentrations were generally lower during the initial thawing and thawed periods than during the initial freezing period. However, solute fluxes during the thawed period were much higher than those during the frozen period. The concentrations of major ions and TDS gradually decreased to a minimum from the permafrost meander (PM) section to the seasonal frost meander (SFM) section and then increased the seasonal frost canyon section. Using the revised forward model, we found that river solutes were contributed by carbonate weathering (mean 38.9%) > sulfide oxidation (22.9%) > evaporite dissolution (20.2%) > atmospheric precipitation (8.7%) > silicate weathering (5.0%) > glacial meltwater (4.3%). The higher TDS, Na+, Cl−, Ca2+, Mg2+, and SO42− concentrations in the PM section reflected the influence of freeze-out fractionation. The concentrations of major ions and TDS were lowest in the SFM section, indicating that the riparian porous aquifer was essential in regulating river hydrochemistry, thus reducing its spatiotemporal variations in the alpine area. In the mountain glacier–hillslope–riparian porous aquifer–river system, the river was mainly recharged by groundwater with insufficient water–rock interactions due to the rapid flow owing to the high elevation difference and high permeability of the riparian quaternary porous aquifers. Our findings provide insights into the construction of hydrogeochemical models in alpine areas and are practically important for the scientific management of water resources in the Qinghai-Tibet Plateau.

Simulations of Soil Water and Heat Processes for No Tillage and Conventional Tillage Systems in Mollisols of China

Soil water and temperature are important factors to reflect variations in soil heat and water flows especially for tillage systems. The objective of this study was to evaluate the performance of the CoupModel in predicting the effect of tillage practices on soil water and heat processes for conventional tillage (CT) and no-tillage (NT) systems with straw mulching on semi-arid and high-latitude Mollisols of northeast China. This model was calibrated and evaluated in a three-year tillage experiment from 2009 to 2011 in a field experiment station, using field measurements of daily soil temperature and water storage in profiles for CT and NT separately. The results showed that under the model, soil temperatures were well simulated at 0–90 cm soil depths for CT, as indicated by R2 ≥ 0.97, the nRMSE = 27.5–38.7% and −1.02 °C ≤ ME ≤ −0.31 °C, and soil water storage at 0–130 cm soil depth (R2 = 0.01–0.06, the nRMSE = 19.6–37.1%, 13.3 mm ≤ ME ≤ 28.2 mm) was simulated with more uncertainty. “Moderate to good agreements” were achieved for NT. In general, the temporal and spatial variations of soil temperature and water for NT were well simulated by CoupModel. Although NT decreased soil evaporation—thus improving soil water content, especially in the root zone soil—and lowered the soil frozen depths, it reduced the soil temperatures, which could influence crop growth. It was concluded that the CoupModel proved to be a functional tool to predict soil heat and water processes for CT and NT systems in high-latitude seasonal frost conditions of Mollisols in China to estimate the soil temperature, water, energy balance, and frost depth dynamics in relatively complex systems that combined plant dynamics with tillage and/or no tillage covered with straw mulching in the soil surface.

Long-term simulation of snow cover and its potential impacts on seasonal frost dynamics in croplands across southern Canada

Long-term simulation of snow cover and its potential impacts on seasonal frost dynamics in croplands across southern Canada

Stabilization of sulfide soil with by-product originated hydraulic binder in a region with seasonal frost – A field investigation

Fine-grained soils often show a low bearing capacity as well as a high frost susceptibility. These aspects are a challenge for the needs of infrastructure. The addition of hydraulic binder to fine-grained soils is common worldwide to improve the soil properties for engineering purposes. The classical hydraulic binders are lime and cement, but nowadays more and more by-producs are used as well like e.g. fly ash, slag or filter dust. The binders are called “hydraulic” because they react with water. By this reaction new minerals are formed, connecting the soil particles together. This improves the properties of the soil: A strength increase is even visible when the curing takes place in cold environment or after freezing and thawing cycles. Another challenge that occurs in fine-grained sulfidic soils is their possible acidification, when aerated due to e.g. excavation or drainage. Sulfide minerals in contact with oxygen produce sulfuric acid. The low pH caused by this oxidation can mobilize metals from the soil minerals, with harmful consequences for the environment. The addition of lime or calcite is one possible action to improve acid sulphate soils for agricultural and aqua-cultural purposes. The high pH of the lime and calcite increases the buffer capacity of the soil. However, the addition of hydraulic binder to a fine-grained sulfide soil in order to improve both the engineering properties and to buffer the potential acidification is sparsely investigated. In the present publication a field investigation is described where a cement mixture with cement kiln dust (CKD) is used alone and in combination with a calcite-rich by-product from the paper industry to improve a fine-grained sulfide soil for possible usage in earthworks. Samples taken from the surface after one year show a buffering of the potential acidification. Additionally, a strength increase can be seen in the stabilized soil when compressed and stored in a tube in field conditions.

Collapse wedges in periglacial eolian sands evidence Late Pleistocene paleoseismic activity of the Vienna Basin Transfer Fault (western Slovakia)

Studies of seismically triggered deformations in the depositional record contribute significantly to our knowledge of earthquake recurrence over geological time. This paper describes soft sediment deformation structures preserved in a periglacial Upper Pleistocene succession of eolian sand in the eastern Vienna Basin, Central Europe. The strata were formed dominantly by wind ripples migrating across a sand sheet, less frequently by deposition of fine-grained sand and silt from suspension during decreased wind speed and by small-scale eolian dunes. Most prominent deformations observed are collapse wedges, which are interpreted as being formed along dilatational fractures. Further detected deformations include chaotically disturbed strata, folded strata, and a pop-up structure caused by sliding. The tensile fractures are oriented systematically in a prevailing N–S direction, perpendicular to the W–E extension suggested for the transtensional Vienna Basin Transfer Fault, which underlies the study area. Repeated seismic shock is identified as the trigger of the deformations. The mechanism of deformation implies some degree of cohesion within the deformed strata, and this may be attributed to seasonal frost and the presence of vadose water within the upper levels of the sediment. These deformations appear periodically, in 25 distinct horizons of strata internally deformed by collapse wedges, and in 40 horizons if deformation both by collapse wedges and/or chaotically disturbed beds is considered. The vertical distribution of deformations allowed the calculation of the recurrence periods of earthquakes with the use of a Bayesian age-depth model; this, in turn, was based on seven OSL ages. The calculation yielded mean recurrence periods of ca. 100 years, though this figure is biased by a relatively high degree of uncertainty in the dating and in the age-depth model. Even with this caveat, the present study reveals the underexplored potential of periglacial eolian deposits to preserve paleoseismological signals.

Impact of Early Season Temperatures in a Climate-Changed Atmosphere for Michigan: A Cool-Climate Viticultural Region

This study assesses the impacts of observed (2001–2012) and projected climate change on early season heat accumulation for grape production (viticulture) in a cool-climate region of the world (Michigan, USA). Observational data were generated from a weather station located in the center of one of the most important appellations located in the SW part of the state. Climate change projections retrieved from a high spatiotemporal weather model using atmospheric conditions simulated for the end of the 21st Century. All the temperature variables considered demonstrated significant warming trends especially during the months of March, April and May. Temperature differences, increases in heat accumulation, and changes to potential frost events would necessitate new approaches to vineyard management. In fact, it is likely that vine budburst will occur earlier and early season frost develop as a new challenge. However, our study results are comparable with other viticulture regions of the world, with a warming trend of at least 3 to 5 °C in the months leading up to the growing season by the end of the 21st Century. Therefore, effective climate change adaptations will be important to the grape and wine industry in this region. Management strategies are needed to minimize climate risks while taking advantage of new opportunities related to improved climatic conditions for growing more late-ripening European Vitis vinifera grape varieties, relevant for producing higher quality wines.

Integrated environmental records in Late Pleistocene Poland: The paleofluvial regime and paleoclimate inferred from Krosinko site

Multiproxy analyses performed at the Krosinko site in the Warsaw–Berlin ice marginal valley have provided data that characterize the depositional conditions, the paleofluvial regime, and the paleoclimate and help reconstruct the paleo-environment at a time of significant sediment (alluvium) accumulation (starting in central Poland ca. 45 ka BP). The sedimentological study, including sand-grain morphology and micromorphology analyses, reveals the presence at Krosinko of a meandering river and of a braided river during the Eemian (MIS 5e) and Upper Pleniweichselian (MIS 2), respectively. The fluvial regime changed over time, from low-energy, through high-energy, to low-energy, as documented by the differently formed sedimentary units A–D (vertical sedimentary succession from base to top: sandy silt, sand, gravelly sand, and sand). Sedimentary unit A and the lower part of unit B were accumulated in the Eemian interglacial (MIS 5e) by the low-energy meandering river. The high-energy fluvial environment during the Weichselian is associated with long, intense sediment transport, organic remains, and flints, as confirmed by taphonomic studies on conifer cones (Picea cf. abies) and wood remains (Juniperus). The river operated under distinct environmental conditions. First, during the accumulation of the upper part of unit B and unit C, there was a strong influence of freezing and aeolian weathering, which weakened significantly over time (unit D). This in turn indicates a change in climate from periglacial (upper part of unit B and unit C), when we also document the presence of permafrost in the subsurface or relatively deep seasonal frost, to temperate (unit D). With warming in the Holocene (MIS 1), an oxbow lake developed in Krosinko, the presence of which is recorded by peat.

Freeze-Thaw Behavior of Stabilized Clayey Soil with Red Mud and Cement

The clayey soils in areas with seasonal frost are exposed to at least one freeze-thaw cycle every year and worsen their engineering properties. To prevent the engineering properties of clayey soils, it is necessary to improve the freeze-thaw resistance of them. In this study, the clayey soil was stabilized by using red mud and cement additive materials. Prepared samples of clayey soil and stabilized clayey soil were subjected to the unconfined compressive test. To investigate the effects of red mud and cement additive materials on the freeze-thaw resistance of clayey soil, the natural and stabilized expansive soil samples were exposed to the freeze-thaw cycles under laboratory conditions. The obtained results showed that the red mud and cement additive materials increased the freeze-thaw resistance of clayey soil. Consequently, it was concluded that red mud and cement additive materials can be successfully used to improve the freeze-thaw resistance of clayey soils.

Seasonal Performance Evaluation of Pavement Base Using Recycled Materials

Using recycled pavement materials to construct new pavement base is currently an important construction strategy bringing improved sustainability. This study investigates the long-term performance of pavement bases constructed with recycled concrete aggregate (RCA), reclaimed asphalt pavement (RAP), and blends with natural aggregates in a seasonal frost region. The stabilization effect of fly ash on RAP was studied as well. In situ falling weight deflectometer (FWD) tests were routinely conducted to provide seasonal deflection data, which were used to back-calculate the layer modulus. Seasonal changes in the base layer modulus along with the pavement ride quality were monitored. One of the two lanes at the test sections was consistently subjected to traffic loading, whereas the other one was not. Findings from this field research indicated that after undergoing over 8 years of naturally seasonal freeze-thaw conditions, 100% RCA, 50% RCA, plus 50% natural aggregates, and 100% RAP, presented improved performance over 100% natural aggregates. However, 50% RAP blended with 50% natural aggregates performed comparably to natural aggregates only, and fly ash did not provide considerable improvement on the long-term performance of 50% RAP plus 50% natural aggregate base. Seasonal climatic variations turned out to affect pavement performance more critically than traffic loading.

Песчаные почвы лесных ландшафтов Амуро-Зейской равнины

Historically, soils of the Cambisol group have been recognized as zonal soils of the Upper Amur Basin Region. However, the wide distribution in the territory of the Amur-Zeya Plain of forest soils of light granulometric composition that were formed on loose sedimentary deposits makes it possible to identify an area of Arenosols and Podzols, for the first time described within the study area. The common features in the researched soils are a well-structured thin humus horizon and quartzfeldsparic mineralogical composition causing low intensity of the processes of weathering and metamorphism of mineral substance. The initial stages of biochemical weathering of mineral matter are diagnosed in soils of the Humic Arenosol subgroup. The most intensive processes of weathering and metamorphism of mineral matter are diagnosed in the middle horizons of the Rubic Arenosol group, which is associated with the formation of seasonal frost and the associated cycles of freezing-thawing of the soil profile. In the soils of the Entic Podzol subgroup, the process of iron illuviation is diagnosed, with formation of the maximum accumulation in the lower part of the soil profile at the boundary with the soil-forming rock. Keywords: FOREST SOILS, NEOGENIC SANDS, SOIL MORPHOLOGY, THE UPPER AMUR BASIN REGION

Integrating observations and models to determine the effect of seasonally frozen ground on hydrologic partitioning in alpine hillslopes in the Colorado Rocky Mountains, USA

AbstractThis study integrated spatially distributed field observations and soil thermal models to constrain the impact of frozen ground on snowmelt partitioning and streamflow generation in an alpine catchment within the Niwot Ridge Long‐Term Ecological Research site, Colorado, USA. The study area was comprised of two contrasting hillslopes with notable differences in topography, snow depth and plant community composition. Time‐lapse electrical resistivity surveys and soil thermal models enabled extension of discrete soil moisture and temperature measurements to incorporate landscape variability at scales and depths not possible with point measurements alone. Specifically, heterogenous snowpack thickness (~0–4 m) and soil volumetric water content between hillslopes (~0.1–0.45) strongly influenced the depths of seasonal frost, and the antecedent soil moisture available to form pore ice prior to freezing. Variable frost depths and antecedent soil moisture conditions were expected to create a patchwork of differing snowmelt infiltration rates and flowpaths. However, spikes in soil temperature and volumetric water content, as well as decreases in subsurface electrical resistivity revealed snowmelt infiltration across both hillslopes that coincided with initial decreases in snow water equivalent and early increases in streamflow. Soil temperature, soil moisture and electrical resistivity data from both wet and dry hillslopes showed that initial increases in streamflow occurred prior to deep soil water flux. Temporal lags between snowmelt infiltration and deeper percolation suggested that the lateral movement of water through the unsaturated zone was an important driver of early streamflow generation. These findings provide the type of process‐based information needed to bridge gaps in scale and populate physically based cryohydrologic models to investigate subsurface hydrology and biogeochemical transport in soils that freeze seasonally.

Frost‐driven lower treelines in Angola and their implications for tropical forest–grassland mosaics

AbstractQuestionsForest–grassland boundaries in the tropics are primarily interpreted as driven by fire and herbivory. So far, frost has received little attention as driver of tropical vegetation boundaries. Here, we study mid‐altitudes in south‐central Africa and ask the following questions: (a) is there a lower treeline between grasslands and forests on the Angolan Plateau; (b) if so, do topoclimatic differences play a role in it; and (c) what do our results imply for tropical highlands globally?LocationThe Angolan Plateau (10°–16°S and 13°–21° E).MethodsWe mapped vegetation by applying a supervised support vector machine‐based classification on a Landsat satellite image. We used a 1,000‐m spaced point grid to extract the mean vertical distribution of the vegetation units from a digital elevation model. The topographic position of each point was calculated using a channel network approach. Microclimatic data were collected across topographic gradients at two sites along the Plateau. Analyses focussed on the number of nights where temperature dropped below 0℃, median and minimum temperatures, and diurnal temperature ranges.ResultsLower treelines separate Miombo forests from geoxyle grasslands in all major valley systems. Treelines are located 150–200 m below the headwater divides. Nocturnal outgoing net long‐wave radiation and resulting cold‐air pooling cause frequent frosts in the valleys during the dry season. Topography controls frost patterns, with minima of down to −7.5℃ and diurnal temperature amplitudes up to 40℃.ConclusionsThe results show that vegetation patterns in central Angola are shaped by frost. The processes causing frost depend on air humidity. Drier atmosphere and lower global temperatures indicate a longer frost season, lower minima and frost extending to lower elevations and latitudes for the Pleistocene. Pleistocene forest fragmentations and the rise of endemism‐rich grasslands in comparable tropical highlands worldwide may be explained by crossing thermic thresholds via increasing seasonal aridity.

Assessing temporal response to biomass removal: A framework for investigating evolving constraints on boreal stand development

Impacts of harvest intensity and soil disturbance on site productivity are likely to vary with stand development stage and interactive processes related to ecosystem function. On upland boreal sites, stand productivity may initially be constrained by microclimatic conditions. However, increasing nutrient demands and overstory light interception associated with canopy development may place progressively greater emphasis on nutrient availability, profile water storage capacity and asymmetric competition. We illustrate these effects using 20-yr post-harvest data from fourteen jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) BSP) Long-term Soil Productivity (LTSP) installations in northern Ontario.Initial growth and ten-year survival were as great or greater without forest floors (NFF), but by year 15 dominant height increments were usually greater with intact forest floors (FF). Growing season frost occurrence and high vapor pressure deficits occurred more frequently with FF than NFF treatments during the initial stand establishment phase. Similarly, mean growing season soil temperatures were 2-5˚C warmer with NFF during this phase, with stand Leaf Area Index (LAI) values < 0.5. During the following Accelerated Growth phase, treatment differences in microclimate dissipated as LAI increased. Foliar N concentrations and N:P ratios, and soil net N mineralization were usually greater from an early age with FF, and often declined as canopy closure approached. Likewise, modelled site-level constraints related to soil water availability increased with stand development, peaking at LAI values associated with the stem exclusion stage. Placing post-harvest response (i.e., impacts of harvest intensity) in the context of relationships among stand structural development, changes in microclimate, and resource supply and demand should provide greater insights into longer-term effects of establishment practices.