Thawed Soil Layers Research Articles

Subsurface mixing of different soil water pools in a permafrost-underlain headwater catchment, Qilian Mountains, Northeastern Tibetan Plateau

Recently studies postulate that there are two distinct soil water pools, i.e., mobile and immobile soil water (MSW and IMSW), and different soil waters have different mobility to generate runoff. Evaluating the influences of soil thawing on the subsurface mixing between different soil waters is important for understanding subsurface hydrological processes. In this study, the isotopic differences between different soil waters were evaluated in a permafrost-underlain headwater catchment by comparing the variations of stable isotopes and seasonal origin of MSW and bulk soil water (BSW), including both IMSW and MSW. BSW was sampled using soil augering and cryogenic vacuum distillation in thawed and frozen soil layers, and MSW was collected using a zero-tension lysimeter in thawed soil layers. The results show the following: (1) The average δ2H values of MSW, thawed BSW, and frozen BSW were similar during the spring snowmelt period (SSP), this can be attributed to weak evaporation and their similar origin, which is supported by the high line-conditioned excess values and their similar seasonal origin index values. (2) The enriched MSW collected during the SRP compared to the spring MSW and was associated with enriched summer precipitation, and the IMSW experienced strong evaporation during the period from the SSP to the SRP, both leading to enriched summer BSW compared to the spring BSW. (3) The frozen and thawed BSW had very similar isotopic compositions and seasonal origin, indicating the close hydrological connection between them and the important recharge of the melting frozen soil to soil water or runoff. The different freeze–thaw stages and soil moisture conditions controlled the stable isotope variations and mixing of different soil waters, this results in the different mobility of the mobile/immobile soil waters involved in the runoff generation processes during the SRP and the SSP, and therefore these features are useful to verify the isotopes-aided precipitation–runoff models applied in such catchments.

Ideas and perspectives: Alleviation of functional limitations by soil organisms is key to climate feedbacks from arctic soils

Abstract. Arctic soils play an important role in Earth's climate system, as they store large amounts of carbon that, if released, could strongly increase greenhouse gas levels in our atmosphere. Most research to date has focused on how the turnover of organic matter in these soils is regulated by abiotic factors, and few studies have considered the potential role of biotic regulation. However, arctic soils are currently missing important groups of soil organisms, and here, we highlight recent empirical evidence that soil organisms' presence or absence is key to understanding and predicting future climate feedbacks from arctic soils. We propose that the arrival of soil organisms into arctic soils may introduce “novel functions”, resulting in increased rates of, for example, nitrification, methanogenesis, litter fragmentation, or bioturbation, and thereby alleviate functional limitations of the current community. This alleviation can greatly enhance decomposition rates, in parity with effects predicted due to increasing temperatures. We base this argument on a series of emerging experimental evidence suggesting that the dispersal of until-then absent micro-, meso-, and macroorganisms (i.e. from bacteria to earthworms) into new regions and newly thawed soil layers can drastically affect soil functioning. These new observations make us question the current view that neglects organism-driven “alleviation effects” when predicting future feedbacks between arctic ecosystems and our planet's climate. We therefore advocate for an updated framework in which soil biota and the functions by which they influence ecosystem processes become essential when predicting the fate of soil functions in warming arctic ecosystems.

Applicability analysis of thermosyphon for thermally stabilizing pipeline foundation permafrost and its layout optimization

Rapid permafrost thawing triggered by heat release from the buried warm-oil pipeline would result in the instability of the pipeline, posing a potential risk of an oil spill in permafrost regions. Therefore, how to precisely control the ground temperature to mitigate pipeline foundation permafrost from thawing is of significant importance. Thermosyphon is a widely-accepted and extensively-used countermeasure against permafrost thawing in permafrost engineering. This paper presents 3-year (2015–2018) monitored data of ground temperature and geophysical survey results conducted by electrical resistivity tomography in April 2018, to investigate the thermal stabilizing effect of thermosyphons installed nearby the China-Russia crude oil pipeline (CRCOP) and the influencing factors. Furthermore, numerical simulation tests were conducted to evaluate their long-term cooling applicability and to optimize their layout parameters. Field observations show that the thermosyphons can cool down and even freeze the thawed soil layers around the CRCOP, prohibiting the infiltration of water into the thawing front to some extent and effectively mitigating the rapid thawing of permafrost beneath the pipeline. The performance of the thermosyphons is greatly influenced by the method and time of thermosyphon installation, as well as layout parameters including the number and longitudinal spacing. Two pairs of thermosyphons with a shorter spacing are unable to control the thawing of the underlying permafrost due to the non-anticipated higher oil temperature, the thermal erosion of water ponding, and climate warming. The simulated results indicate that thermosyphons perform well in the first 5 years after installation. Thermosyphon lowers the mean annual temperature of soils surrounding it by about 2 and 1.4 times when the evaporator section length is increased by 50% and the longitudinal spacing is decreased by 0.5 m, respectively. The application of thermosyphons has proven its strength for thermally stabilizing the pipeline foundation permafrost along the CRCOP.

Progressively thawed soil layers differed in microbial properties and CO2 emission rates

Freeze–thaw events can profoundly reorganise water, salts, and nutrients within the soil matrix, which consequently disturbs soil microbes and perturbs carbon metabolism. However, it remains unclear how soil microbial properties differentiate as they progressively thaw and detach from the still frozen layer as often occurs in a sloped field. In this study, a Mollisol (< 0.5 mm) was refilled into columns and frozen at −15 °C for 12 h. To simulate detachment while thawing, the frozen soil columns were progressively peeled off, from the outer to the inner core, into six layers (T1 ∼ T6). Our results show that: (1) The soil water content significantly declined from 95.3% in the outermost layer T1 to the inner core T6 of 24.7%, whilst the soil electrical conductivity evidently increased from 45.8 μS·cm−1 in the T1 to 73.7 μS·cm−1 in the T6. (2) The six frozen-thawed layers had more observed microbial species and distinct β diversity compared to the unfrozen CK, and the OTU-based co-occurrence network of the soil bacteria in the T1 was apparently better interconnected than that in the T6. However, the outer layers released significantly less CO2 on average than the CK and inner layers over the seven days of incubation. Although cautions must be taken in extrapolating our results to field scale processes, our study was the first (to our knowledge) to distinguish the heterogeneity of soil inter-layer microbial properties during progressive thawing. Our findings collectively suggest progressive thawing may introduce a non-negligible but long-neglected heterogeneity in carbon biogeochemistry along slopes.

Temporal Variation of Suspended Sediment and Solute Fluxes in a Permafrost-Underlain Headwater Catchment on the Tibetan Plateau

Under global warming, the permafrost-underlain headwater catchments of the Tibetan Plateau have undergone extensive permafrost degradation and changes in precipitation characteristics, which may substantially alter the riverine suspended sediment and riverine solute fluxes. However, these fluxes and their influencing factors in such catchments are poorly understood. We studied the suspended sediment and solute fluxes in a permafrost-underlain headwater catchment on the northeastern Tibetan Plateau, based on comprehensive measurements of various water types in spring and summer in 2017. The daily flux of suspended sediment in spring was close to that in summer, but heavy rainfall events following a relatively long dry period made the largest contribution to the suspended sediment fluxes in summer. The riverine solute flux (in tons) was 12.6% and 27.8% of the suspended sediment flux (in tons) in spring and summer, indicating the dominating role of physical weathering in total material exportation. The snowmelt mobilized more suspended sediment fluxes and fewer solutes fluxes than summer rain, which may be due to the meltwater erosion and freeze–thaw processes in spring and the thicker thawed soil layer and better vegetation coverage in summer, and the longer contact time between the soil pore water and the soil and rock minerals after the thawing of frozen soil. The input of snowmelt driven by higher air temperatures in spring and the direct input of rainfall in summer would both act to dilute the stream water; however, the supra-permafrost water, with high solute contents, recharged the adjacent streamflow as frozen soil seeps and thus moderated the decrease in the riverine solute content during heavy snowmelt or rainfall events. With the permafrost degradation under future global warming, the solute fluxes in permafrost-underlain headwater catchments may increase, but the suspended sediment flux in spring may decrease due to the expansion of discontinuous permafrost areas and active layer thickness.

Dynamic Responses of a Pile with a Cap under the Freezing and Thawing Processes of a Saturated Porous Media Considering Slippage between Pile and Soil

The freezing/thawing stratification effect of seasonal factors or artificial disturbances in frozen soil regions has an important influence on the vertical vibration of the pile–soil–cap system. Taking into account the slippage between the pile and soil, a simplified layered analytical model of the vertical vibration of the pile–soil–cap system in a double-layered stratum under the freezing and thawing processes of a saturated porous medium was established, and the analytical solution of the dynamic response on the top of the pile cap was obtained. In this model, frozen saturated porous media and Biot’s porous media theory were used to simulate frozen soil and unfrozen soil, respectively. The validation of the slippage model was first verified by comparison with the results of the existing model tests. This was followed by a dynamic model test of the pile–soil–cap system in a self-made, ground-freezing system. In comparison with the analytical results and the experimental results of model tests under the freezing/thawing processes, the validation of the present model is further verified. A comprehensive parametric study reveals that the parameters of the frozen or thawed soil layer have significant effects on the amplitude–frequency curve of the vertical vibration of the pile foundation.

Changes in plot-scale runoff generation processes from the spring–summer transition period to the summer months in a permafrost-dominated catchment

The hydrological regimes in permafrost-dominated catchments have unique characteristics. However, studies based on plot-scale experiments of runoff generation processes and the factors influencing these processes are limited on the Tibetan Plateau, which is experiencing rapid warming and permafrost degradation. Runoff generation processes were studied on a standard runoff plot (5 × 20 m) in a permafrost-dominated catchment on the Tibetan Plateau with an alpine meadow cover to analyze these processes during the spring–summer transition period (from mid-May to early June) and in the summer months of June and July. The discharge, including surface and subsurface runoff, the soil hydrothermal and moisture conditions, and the meteorological conditions were monitored from May 2018 to May 2019. Partial Least-Squares Path Modeling was used to identify the influencing factors in the two time periods. The lateral subsurface flow at 30–60 cm depth accounted for the majority of the discharge from the runoff plot (>99%) and showed a sharp increase when the soil water contents of the thawed soil layers exceeded a threshold value that varied from 0.41 to 0.52. During the spring–summer transition period, the lateral subsurface flow at 30–60 cm depth was promoted by both event precipitation and the antecedent precipitation with an average runoff ratio of 1.7, indicating that the antecedent moisture condition maintained by the underlying permafrost drives runoff generation via meltwater from snow and frozen soil. In the summer months, the lateral subsurface flow at 30–60 cm depth was directly promoted by event precipitation, especially the moderate precipitation that occurred in this period, with a reduced runoff ratio of 0.5 as a result of deepening of the thawed soil layer from 30 to >60 cm, enhanced evapotranspiration and an increased soil water storage capacity caused by an increase of air temperature and the soil temperature. Prediction models obtained via Multiple Linear Regressions of the identified influencing factors were able to accurately estimate the discharge. This study shows the important role of lateral subsurface flow in runoff generation processes in this permafrost area and the most important influencing factors in different seasons.

Using stable isotopes to identify major flow pathways in a permafrost influenced alpine meadow hillslope during summer rainfall period

AbstractGlobal warming has leaded to permafrost degradation, with potential impacts on the runoff generation processes of permafrost influenced alpine meadow hillslope. Stable isotopes have the potential to trace the complex runoff generation processes. In this study, precipitation, hillslope surface and subsurface runoff, stream water, and mobile soil water (MSW) at different hillslope positions and depths were collected during the summer rainfall period to analyse the major flow pathway based on stable isotopic signatures. The results indicated that (a) compared with precipitation, the δ2H values of MSW showed little temporal variation but strong heterogeneity with enriched isotopic ratios at lower hillslope positions and in deeper soil layers. (b) The δ2H values of middle‐slope surface runoff and shallow subsurface flow were similar to those of precipitation and MSW of the same soil layer, respectively. (c) Middle‐slope shallow subsurface flow was the major flow pathway of the permafrost influenced alpine meadow hillslope, which turned into surface runoff at the riparian zone before contributing to the streamflow. (d) The slight variation of δ2H values in stream water was shown to be related to mixing processes of new water (precipitation, 2%) and old water (middle‐slope shallow subsurface flow, 98%) in the highly transmissive shallow thawed soil layers. It was inferred that supra‐permafrost water levels would be lowered to a less conductive, deeper soil layer under further warming and thawing permafrost, which would result in a declined streamflow and delayed runoff peak. This study explained the “rapid mobilization of old water” paradox in permafrost influenced alpine meadow hillslope and improved our understanding of permafrost hillslope hydrology in alpine regions.

Seismic profile solution under conditions of thawing permafrost with technogenic depression

The article given presents the results of geophysical research of the certain types of geodynamical phenomenon, which is generally located on areas with permafrost soil layers. The detailed description to the research methods of refracting interface of the technogenic thawing ground area under the conditions of flooded pressure by refraction and reflection methods is given. Setting of the problem and detection of the nature of the changes in the properties and configuration of the permafrost and thawed soil layers in the grounds of embankment is practically solved by the integrated application of seismic methods that in theory have different kinematics due to propagation of elastic waves and methods of interpretation the seismic profile.

Impact of unusually wet permafrost soil on understory vegetation and CO2 exchange in a larch forest in eastern Siberia

This study investigated the CO2 exchange over a 10-year period (2005–2014) inside and above a larch-dominant forest in the central Lena river basin, eastern Siberia. A wet-soil condition, such as that found in the active layer (seasonally thawed soil layer of upper permafrost), containing unusually high soil water close to saturation and partial surface waterlogging, was prolonged during the warm season of 2005–2009. In later years, the soil layer closer to the ground surface became dry (∼10% volumetric water content), although the deeper part remained relatively wet (∼30%). We quantitatively compared the whole forest and the understory CO2 exchanges to detect the separate effects of excessive soil waters on the overstory and understory vegetation. The conventional light and temperature response functions for half-hourly CO2 fluxes, that is, the net ecosystem exchange of daytime and night-time, respectively, were applicable to the understory observations. Comparison of the fitting parameters of the light response function at two levels revealed a smaller maximum net ecosystem exchange (NEE) under light saturation with a steep response under weak light conditions for the understory. The CO2 exchanges at the understory increased from the wet-soil period to the drying soil period by 46% (1.3 g C m−2 d−1) of gross primary production (GPP) and 29% (1.2 g C m−2 d−1) of ecosystem respiration (ER), while no trend was found in the ecosystem scale fluxes. These increases were due to an increasing understory biomass, changes in plentiful light and soil water in the inside-canopy environments, and enhanced turbulent mixing. The decline in the larch contribution could be compensated for by the understory growth and the remaining wetness of the active layer, which indicated that the interactions between the larch and the understory supported the stability of carbon cycles in this forest ecosystem.

Glucose addition increases the magnitude and decreases the age of soil respired carbon in a long-term permafrost incubation study

Higher temperatures in northern latitudes will increase permafrost thaw and stimulate above- and belowground plant biomass growth in tundra ecosystems. Higher plant productivity increases the input of easily decomposable carbon (C) to soil, which can stimulate microbial activity and increase soil organic matter decomposition rates. This phenomenon, known as the priming effect, is particularly interesting in permafrost because an increase in C supply to deep, previously frozen soil may accelerate decomposition of C stored for hundreds to thousands of years. The sensitivity of old permafrost C to priming is not well known; most incubation studies last less than one year, and so focus on fast-cycling C pools. Furthermore, the age of respired soil C is rarely measured, even though old C may be vulnerable to labile C inputs. We incubated soil from a moist acidic tundra site in Eight Mile Lake, Alaska for 409 days at 15 °C. Soil from surface (0–25 cm), transition (45–55 cm), and permafrost (65–85 cm) layers were amended with three pulses of uniformly 13C-labeled glucose or cellulose every 152 days. Glucose addition resulted in positive priming in the permafrost layer 7 days after each substrate addition, eliciting a two-fold increase in cumulative soil C loss relative to unamended soils with consistent effects across all three pulses. In the transition and permafrost layers, glucose addition significantly decreased the age of soil-respired CO2-C with Δ14C values that were 115‰ higher. Previous field studies that measured the age of respired C in permafrost regions have attributed younger Δ14C ecosystem respiration values to higher plant contributions. However, the results from this study suggest that positive priming, due to an increase in fresh C supply to deeply thawed soil layers, can also explain the respiration of younger C observed at the ecosystem scale. We must consider priming effects to fully understand permafrost C dynamics, or we risk underestimating the contribution of soil C to ecosystem respiration.

Wildfire as a major driver of recent permafrost thaw in boreal peatlands

Permafrost vulnerability to climate change may be underestimated unless effects of wildfire are considered. Here we assess impacts of wildfire on soil thermal regime and rate of thermokarst bog expansion resulting from complete permafrost thaw in western Canadian permafrost peatlands. Effects of wildfire on permafrost peatlands last for 30 years and include a warmer and deeper active layer, and spatial expansion of continuously thawed soil layers (taliks). These impacts on the soil thermal regime are associated with a tripled rate of thermokarst bog expansion along permafrost edges. Our results suggest that wildfire is directly responsible for 2200 ± 1500 km2 (95% CI) of thermokarst bog development in the study region over the last 30 years, representing ~25% of all thermokarst bog expansion during this period. With increasing fire frequency under a warming climate, this study emphasizes the need to consider wildfires when projecting future circumpolar permafrost thaw.

Mechanism of the influence of soil freezing depth on winter runoff

The depth of soil freezing in river watersheds is a factor governing winter runoff formation. The freezing depth regulates the redistribution of stored soil moisture between thawed and frozen soil layers. The moisture stored in the thawed soil layer is spent for winter runoff, while that in the frozen zone forms snowmelt runoff. The depth of soil freezing has considerably decreased over the period of climate changes, resulting in an increase in winter runoff and greater losses of snowmelt runoff.

NifH-Harboring Bacterial Community Composition across an Alaskan Permafrost Thaw Gradient.

Since nitrogen (N) is often limiting in permafrost soils, we investigated the N2-fixing genetic potential and the inferred taxa harboring those genes by sequencing nifH gene fragments in samples taken along a permafrost thaw gradient in an Alaskan boreal soil. Samples from minimally, moderately and extensively thawed sites were taken to a depth of 79 cm to encompass zones above and below the depth of the water table. NifH reads were translated with frameshift correction and 112,476 sequences were clustered at 5% amino acid dissimilarity resulting in 1,631 OTUs. Sample depth in relation to water table depth was correlated to differences in the NifH sequence classes with those most closely related to group I nifH-harboring Alpha- and Beta-Proteobacteria in higher abundance above water table depth while those related to group III nifH-harboring Delta Proteobacteria more abundant below. The most dominant below water table depth NifH sequences, comprising 1/3 of the total, were distantly related to Verrucomicrobia-Opitutaceae. Overall, these results suggest that permafrost thaw alters the class-level composition of N2-fixing communities in the thawed soil layers and that this distinction corresponds to the depth of the water table. These nifH data were also compared to nifH sequences obtained from a study at an Alaskan taiga site, and to those of other geographically distant, non-permafrost sites. The two Alaska sites were differentiated largely by changes in relative abundances of the same OTUs, whereas the non-Alaska sites were differentiated by the lack of many Alaskan OTUs, and the presence of unique halophilic, sulfate- and iron-reducing taxa in the Alaska sites.

Seasonal changes and vertical distribution of root standing biomass of graminoids and shrubs at a Siberian tundra site

Shrub expansion is common in the tundra biome and has been linked to climate warming. However, the underlying mechanisms are still not fully understood. This study aimed to investigate the seasonal and vertical rooting patterns of different plant functional types, which is important for predicting tundra vegetation dynamics. We harvested root samples by soil coring and investigated seasonal changes in root biomass and vertical root distribution across a vegetation gradient, focusing on the differences between graminoids and dwarf shrubs, at a northeastern Siberian tundra. Graminoid fine root biomass increased significantly during the growing season, whereas that of shrubs was already high at the beginning and did not change later on. Shrubs had a much shallower rooting pattern than graminoids. Also, shrub roots did not respond to increases in permafrost thawing depth over the growing season, whereas graminoids grew fine roots in deeper, recently thawed soil layers during the growing season. Our results show that shrubs are predominantly shallow-rooted and grow roots earlier than graminoids, which allows shrubs to take advantage of the nutrient pulse after snowmelt in the early growing season. In contrast, the deep-rooted graminoids can access the nutrients in deeper soil and may profit from increasing permafrost thawing depth. The outcome of the competitive interactions between graminoids and shrubs in tundra may depend on the balance between the benefits associated with earlier root growth and deeper root distribution, respectively. The shrub expansion with climate warming observed in recent decades suggests that earlier root growth in the upper soil layer may be more important than increased rooting depth later in the growing season.

Influence of moss and lichen spatial mosaics on radial growth of Gmelin larch (Central Evenkia)

The results of research studies on the influence of high moss-and-lichen mosaicity typical for the northern taiga in Central Evenkia on the dynamics of radial growth of Gmelin larch trees (Larix gmelinii (Rupr.) Rupr.) are reported. Three sample plots were laid out in the larch ledum–cowberry–green moss forest on the northwest facing slope (8°–10°) in close proximity to each other; they differed in the proportions of area occupied by the principal components of a moss–lichen cover (Aulacomnium turgidum, Pleurozium schreberi, Cladonia rangiferina, and Hylocomium splendens), spatial distribution, and thermal properties of seasonally thawed soil layers. The worst thermal soil properties under the current climatic conditions have determined the comparatively low radial growth of trees in the sampling area, reduced response to air-temperature variations in the period of preseason cambium activity and at the beginning of the radial growth period, and heightened the response to this factor at the end of a season.

Thermal regimes and degradation modes of permafrost along the Qinghai-Tibet Highway

Permafrost on the Qinghai-Tibet Plateau (QTP) is widespread, thin, and thermally unstable. Under a warming climate during the past few decades, it has been degrading extensively with generally rising ground temperatures, the deepening of the maximum summer thaw, and with lessening of the winter frost penetration. The permafrost has degraded downward, upward and laterally. Permafrost has thinned or, in some areas, has totally disappeared. The modes of permafrost degradation have great significance in geocryology, in cold regions engineering and in cold regions environmental management. Permafrost in the interior of the QTP is well represented along the Qinghai-Tibet Highway (QTH), which crosses the Plateau through north to south and traverses 560 km of permafrost-impacted ground. Horizontally, the degradation of permafrost occurs more visibly in the sporadic permafrost zone in the vicinity of the lower limit of permafrost (LLP), along the margins of taliks, and around permafrost islands. Downward degradation develops when the maximum depth of seasonal thaw exceeds the maximum depth of seasonal frost, and it generally results in the formation of a layered talik disconnecting the permafrost from the seasonal frost layer. The downward degradation is divided into four stages: 1) initial degradation, 2) accelerated degradation, 3) layered talik and 4) finally the conversion of permafrost to seasonally frozen ground (SFG). The upward degradation occurs when the geothermal gradient in permafrost drops to less than the geothermal gradients in the underlying thawed soil layers. Three types of permafrost temperature curves (stable, degrading, and phase-changing transitory permafrost) illustrate these modes. Although strong differentiations in local conditions and permafrost types exist, the various combinations of the three degradation modes will ultimately transform permafrost into SFG. Along the QTH, the downward degradation has been proceeding at annual rates of 6 to 25 cm, upward degradation at 12 to 30 cm, and lateral degradation in the sporadic permafrost zone at 62 to 94 cm during the last quarter century. These rates exceed the 4 cm per year for the past 20 years reported for the discontinuous permafrost zone in subarctic Alaska, the 3 to 7 cm per year reported in Mongolia, and that of the thaw-stable permafrost in subarctic Yakutia and Arctic Alaska.

06/00176 Grassland bird response to harvesting switchgrass as a biomass energy crop: Roth, A. M. et al. Biomass and Bioenergy, 2005, 28, (5), 490–498

This study investigated the CO2 exchange over a 10-year period (2005–2014) inside and above a larch-dominant forest in the central Lena river basin, eastern Siberia. A wet-soil condition, such as that found in the active layer (seasonally thawed soil layer of upper permafrost), containing unusually high soil water close to saturation and partial surface waterlogging, was prolonged during the warm season of 2005–2009. In later years, the soil layer closer to the ground surface became dry (∼10% volumetric water content), although the deeper part remained relatively wet (∼30%). We quantitatively compared the whole forest and the understory CO2 exchanges to detect the separate effects of excessive soil waters on the overstory and understory vegetation. The conventional light and temperature response functions for half-hourly CO2 fluxes, that is, the net ecosystem exchange of daytime and night-time, respectively, were applicable to the understory observations. Comparison of the fitting parameters of the light response function at two levels revealed a smaller maximum net ecosystem exchange (NEE) under light saturation with a steep response under weak light conditions for the understory. The CO2 exchanges at the understory increased from the wet-soil period to the drying soil period by 46% (1.3 g C m−2 d−1) of gross primary production (GPP) and 29% (1.2 g C m−2 d−1) of ecosystem respiration (ER), while no trend was found in the ecosystem scale fluxes. These increases were due to an increasing understory biomass, changes in plentiful light and soil water in the inside-canopy environments, and enhanced turbulent mixing. The decline in the larch contribution could be compensated for by the understory growth and the remaining wetness of the active layer, which indicated that the interactions between the larch and the understory supported the stability of carbon cycles in this forest ecosystem.

Effects of daylength and temperature on root elongation in tundra graminoids.

Effects of soil temperature and daylength on root elongation of Carex aquatilis, Dupontia fischeri, and Eriophorum angustifolium were studied under both field and phytotron conditions. Late season decrease in root elongation rate and cessation of root elongation in Dupontia and Eriophorum are shown to be controlled by decreasing daylength. During the growing season, low temperature is not a direct factor in limiting root growth in any of the three species despite the presence of permafrost and low soil temperatures in the shallow thawed soil layer. In the phytotron, temperature-dependence of root elongation is related to experimental conditions characterized by continuous light. Plants of all three species are capable of root growth at near-freezing temperatures.