Permafrost-wildfire interactions: Active layer thickness estimates for paired burned and unburned sites in northern high-latitudes

Air temperature at northern high latitudes has increased at a higher rate than the global mean, and most general circulation models project that this pattern will continue. Climate warming can increase summer thaw depth and induce permafrost degradation, which may alter the dynamics and functions of northern ecosystems and the lifestyles of northern residents. To address these issues, we developed a process‐based model to simulate permafrost thermal regimes by combining the strength of existing permafrost models and land surface process models. Soil temperature and active layer thickness were simulated by solving the heat conduction equation, with the upper boundary conditions being determined using the surface energy balance and the lower boundary conditions being defined as the geothermal flux. The model integrated the effects of climate, vegetation, ground features, and hydrological conditions on the basis of energy and water transfer in the soil‐vegetation‐atmosphere system. The model was validated against the measurements at four sites in Canada. The simulation results agreed with the measurements of energy fluxes, snow depth, soil temperature, and thaw depth. These results indicate that this physically based model captured the effects of climate, vegetation, and ground conditions on soil temperature and freezing/thawing dynamics, and the model is suitable to investigate the impacts of transient climate change on soil thermal regimes and permafrost degradation and their consequent effects on ecosystem dynamics.

Degradation and local growth of “Xing'an-Baikal” permafrost responding to climate warming and the consequences

Rapid Arctic warming is expected to result in widespread permafrost degradation. However, observations show that site-specific conditions (vegetation and soils) may offset the reaction of permafrost to climate change. This paper summarizes 43 years of interannual seasonal thaw observations from tundra landscapes surrounding the Marre-Sale on the west coast of the Yamal Peninsula, northwest Siberia. This robust dataset includes landscape-specific climate, active layer thickness, soil moisture, and vegetation observations at multiple scales. Long-term trends from these hierarchically scaled observations indicate that drained landscapes exhibit the most pronounced responses to changing climatic conditions, while moist and wet tundra landscapes exhibit decreasing active layer thickness, and river floodplain landscapes do not show changes in the active layer. The slow increase in seasonal thaw depth despite significant warming observed over the last four decades on the Yamal Peninsula can be explained by thickening moss covers and ground surface subsidence as the transient layer (ice-rich upper permafrost soil horizon) thaws and compacts. The uneven proliferation of specific vegetation communities, primarily mosses, is significantly contributing to spatial variability observed in active layer dynamics. Based on these findings, we recommend that regional permafrost assessments employ a mean landscape-scale active layer thickness that weights the proportions of different landscape types.

Long term effects of fire on ectomycorrhizas and soil properties in Nothofagus pumilio forests in Argentina

Introduction. The ongoing climate changes in the permafrost zone associated with global warming significantly impact the temperature regime and the depth of seasonal thawing of permafrost soils. In current design practices, a constant calculated value for the depth of seasonal thawing is adopted; however, in reality, it varies and may exceed the design values. Existing regulatory requirements for design neglect this variation, which underscores the relevance of the study.Aim. To develop a methodology for forecasting changes in the thickness of the seasonal thaw layer based on climate parameters altering with warming.Materials and methods. The methodology of the study involved an analysis of contemporary scientific and technical literature, regulatory documents, methodological literature, and archival data from meteorological stations across Russia; determination of the relationship between changes in the depth of seasonal thawing over time; development of a forecasting methodology for changes in the thickness of the seasonal thaw layer based on climate parameters affected by warming; recommendations for utilizing the results in the development of regulatory and technical documents.Results. The depth of seasonal thawing significantly affects the cost and scope of foundation construction and land planning works. The existing calculation methodology fails to account for climate change trends. The permafrost zone of Russia displays an increase in the temperature of soils and the depth of seasonal thawing. The highest convergence of calculated results with field observations was achieved using the Borey 3D heat engineering program while considering climate change.Conclusions. The depth of thawing is to be determined using a heat engineering program that accounts for climate change. Determination of the foundation bottom depth, as well as strength and deformation calculations, are to consider changes in thaw depth during operation and the impact of global warming.

Effects of fire on spotted owl site occupancy in a late-successional forest

The series of 20-year-long observations of seasonal thawing have been studied on CALM plots in Eurasia and North America. The short-term (2-5 years) fluctuations of the active layer are considered. Similarities and differences in seasonal thawing of tundra soils in circumpolar countries are noted. The dynamics of seasonal thawing is analyzed by the example of two CALM plots in the Chukotka Autonomous District. Time series of changes in the depth of seasonal thawing are described using the mathematical methods. The amplitude, frequency and periods of oscillations are determined for them. The comparative graphical and statistical analysis of climate and seasonally thawed layer variations allowed us to judge on the nature and degree of influence of meteorological factors on the depth of soil thawing. The problems of interpretation of intra-century fluctuations of climatic parameters and seasonal thawing are considered. It is concluded that the duration of the warm season affects significantly the depth of seasonal thawing. It is noted that ignoring the intra-century variations in the parameters of climatic and permafrost landscapes is the cause of inconsistency of judgments about climate change trends and forecasts of permafrost degradation.
 A new concept of “climatic year” was proposed as the conjugation of weather conditions for 2-3 years equally affecting the depth of thawing. The established statistical regularities in changing permafrost-climatic conditions can be used for the preparation of 3-5 and 10-15 summer weather forecasts. The use of the ratio of the average values of the amplitude to the period of short oscillations as an indicator of the stability of the roof of permafrost is justified.
 The hypothesis of the influence of solar and technogenic electromagnetic radiation in the radio-wave range on the degradation of underground ice of the active layer and Arctic ice is proposed.

Global climate change substantially influences vegetation spring phenology, that is, green-up date (GUD), in the northern permafrost region. Changes in GUD regulate ecosystem carbon uptake, further feeding back to local and regional climate systems. Extant studies mainly focused on the direct effects of climate factors, such as temperature, precipitation, and insolation; however, the responses of GUD to permafrost degradation caused by warming (i.e., indirect effects) remain elusive yet. In this study, we examined the impacts of permafrost degradation on GUD by analyzing the long-term trend of satellite-based GUD in relation to permafrost degradation measured by the start of thaw (SOT) and active layer thickness (ALT). We found significant trends of advancing GUD, SOT, and thickening ALT (p<0.05), with a spatially averaged slope of -2.1days decade-1 , -4.1days decade-1 , and +1.1cm decade-1 , respectively. Using partial correlation analyses, we found more than half of the regions with significantly negative correlations between spring temperature and GUD became nonsignificant after considering permafrost degradation. GUD exhibits dominant-positive (37.6% vs. 0.6%) and dominant-negative (1.8% vs. 35.1%) responses to SOT and ALT, respectively. Earlier SOT and thicker ALT would enhance soil water availability, thus alleviating water stress for vegetation green-up. Based on sensitivity analyses, permafrost degradation was the dominant factor controlling GUD variations in 41.7% of the regions, whereas only 19.6% of the regions were dominated by other climatic factors (i.e., temperature, precipitation, and insolation). Our results indicate that GUDs were more sensitive to permafrost degradation than direct climate change in spring among different vegetation types, especially in high latitudes. This study reveals the significant impacts of permafrost degradation on vegetation GUD and highlights the importance of permafrost status in better understanding spring phenological responses to future climate change.

Multi-decadal variations in the oceanic CO2 uptake and biogeochemical parameters over the northern and southern high latitudes

Active-layer thickness, snow depth, minimum soil temperatures, near-surface ground ice, soil heave, and permafrost temperatures have been measured for over 20 years following the 1978 artificial drainage of Lake Illisarvik. Measurements of active-layer thickness and other variables have been made at 25-m intervals along the major and minor axes of the oval-shaped drained-lake bed. Permafrost aggradation commenced in the lake bottom during the first winter following drainage. Before the establishment of vegetation, there was little snow cover, minimum ground temperatures were low, and the active layer was relatively thin. However, both snow depth and minimum ground temperatures have risen where vegetation has grown, the active layer has thickened, and in response, the temperature in permafrost has gradually increased. In the lake bottom, the change in snow depth associated with vegetation growth has been the dominant control on variation in active-layer thickness and not summer weather conditions, which are well correlated with thaw depths along an active-layer course established in the adjacent tundra. Changes in elevation of the surface of the lake bed have been measured with respect to some 40 bench marks anchored in permafrost, and indicate vertical movements of the surface associated with frost heave, thaw subsidence, and the growth of aggradational ice. The ground ice content of near-surface permafrost determined by drilling is in close agreement with the measured uplift of the lake bed. The rate of growth of aggradational ice has been ~0.5 cm a–1 over 20 years.

Differences in the summer insulative value of the zonal vegetation mat affect the depth of thaw along the Arctic bioclimate gradient. Toward the south, taller, denser plant canopies and thicker organic horizons counter the effects of warmer temperatures, so that there is little correspondence between active layer depths and summer air temperature. We examined the interactions between summer warmth, vegetation (biomass, Leaf Area Index, Normalized Difference Vegetation Index), soil (texture and pH), and thaw depths at 17 sites in three bioclimate subzones of the Arctic Slope and Seward Peninsula, Alaska. Total plant biomass in subzones C, D, and E averaged 421 g m−2, 503 g m−2, and 1178 g m−2 respectively. Soil organic horizons averaged 4 cm in subzone C, 8 cm in subzone D, and 14 cm in subzone E. The average late‐August thaw depths in subzones C, D, and E were 44 cm, 55 cm, and 47 cm respectively. Non‐acidic soils in equivalent climates generally have shorter‐stature sedge‐dominated canopies and many frost boils, and consequently have thicker active layers than acidic soils. The trends reported here are useful for palaeo‐ecological reconstructions and predictions of future ecosystem changes in the Low Arctic. Climate change will not lead to uniform thickening of the active layer, and could lead to shallower active layers in some presently dry areas due to paludification. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Low-severity fires in the boreal region: reproductive implications for black spruce stands in between stand-replacing fire events.

Abstract.We investigated the effects of recent fires on the native ant communities in two habitats of north‐west Patagonia that differ in vegetation structural complexity. Using bait traps, we sampled ants in replicated scrub and steppe areas including paired burned and unburned sites. Fires significantly reduced plant cover and ant diversity only in scrub sites. The drop in diversity was due to (a) a reduction in the abundance of rare species associated with woody vegetation, and (b) an increase in the abundance of the dominant species, which thrive in more xeric microclimatic conditions. Consequently, ant assemblage structure of burned scrub approaches that of steppe sites. Our findings suggest that the effects of disturbances on ant assemblages depends both on habitat characteristics, which in turn determine the extent of the changes induced by the disturbance, and on the regional context of the ant fauna, which in turn determines the ability of the ants to deal with the post‐disturbance conditions.

Pollution influences on atmospheric composition and chemistry at high northern latitudes: Boreal and California forest fire emissions

Permafrost, widely distributed in the Northern Hemisphere, plays a vital role in regulating heat and moisture cycles within ecosystems. In the last four decades, due to global warming, permafrost degradation has accelerated significantly in high latitudes and altitudes. However, the impact of permafrost degradation on vegetation remains poorly understood to date. Based on active layer thickness (ALT) monitoring data, meteorological data and normalized difference vegetation index (NDVI) data, we found that most ALT‐monitored sites in the Northern Hemisphere show an increasing trend in NDVI and ALT. This suggests an overall increase in NDVI from 1980 to 2021 while permafrost degradation has been occurring. Permafrost degradation positively influences NDVI growth, with the intensity of the effects varying across land cover types and permafrost regions. Furthermore, based on Mann‐Kendall trend test, we detected abrupt changes in NDVI and environmental factors, further confirming that there is a strong consistency between the abrupt changes of ALT and NDVI, and the consistency between the abrupt change events of ALT and NDVI is stronger than that of air temperature and precipitation. These findings work toward a better comprehending of permafrost effects on vegetation growth in the context of climate change.