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1. Linking Post-fire Tree Density to Carbon Storage in High-Latitude Cajander Larch (Larix cajanderi) Forests of Far Northeastern Siberia

   https://doi.org/10.1007/s10021-024-00913-0  

   Alexander, H D; Paulson, A K; Loranty, M M; Mack, M C; Natali, S M; Pena, H; Davydov, S; Spektor, V; Zimov, N (August 2024, Ecosystems)

   Abstract With climate warming and drying, fire activity is increasing in Cajander larch (Larix cajanderiMayr.) forests underlain by continuous permafrost in northeastern Siberia, and initial post-fire tree demographic processes could unfold to determine long-term forest carbon (C) dynamics through impacts on tree density. Here, we evaluated above- and belowground C pools across 25 even-aged larch stands of varying tree densities that established following a wildfire in ~ 1940 near Cherskiy, Russia. Total C pools increased with increased larch tree density, from ~ 9,000 g C m⁻²in low-density stands to ~ 11,000 g C m⁻²in high and very high-density stands, with increases most pronounced at tree densities < 1 stem m⁻²and driven by increased above- and belowground (that is, coarse roots) and live and dead (that is, woody debris and snags) larch biomass. Total understory vegetation and non-larch coarse root C pools declined with increased tree density due to decreased shrub C pools, but these pools were relatively small compared to larch biomass. Fine root, soil organic matter (OM), and near surface (0–30 cm) mineral soil (MS) C pools varied little with tree density, although soil C pools held most (18–28% in OM and 44–51% in MS) C stored in these stands. Thus, if changing fire regimes promote denser stands, C storage will likely increase, but whether this increase offsets C lost during fires remains unknown. Our findings highlight how post-fire tree demographic processes impact C pool distribution and stability in larch forests of Siberian permafrost regions. 

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2. Deep roots of Carex aquatilis have greater ammonium uptake capacity than shallow roots in peatlands following permafrost thaw

   https://doi.org/10.1007/s11104-021-04978-x  

   Albano, Lucas J.; Turetsky, Merritt R.; Mack, Michelle C.; Kane, Evan S. (May 2021, Plant and Soil)

   null (Ed.)

   Aims Climate warming in northern ecosystems is triggering widespread permafrost thaw, during which deep soil nutrients, such as nitrogen, could become available for biological uptake. Permafrost thaw shift frozen organic matter to a saturated state, which could impede nutrient uptake. We assessed whether soil nitrogen can be accessed by the deep roots of vascular plants in thermokarst bogs, potentially allowing for increases in primary productivity. Methods We conducted an ammonium uptake experiment on Carex aquatilis Wahlenb. roots excavated from thermokarst bogs in interior Alaska. Ammonium uptake capacity was compared between deep and shallow roots. We also quantified differences in root ammonium uptake capacity and plant size characteristics (plant aboveground and belowground biomass, maximum shoot height, and maximum root length) between the actively-thawing margin and the centre of each thermokarst bog as a proxy for time-following-thaw. Results Deep roots had greater ammonium uptake capacity than shallow roots, while rooting depth, but not belowground biomass, was positively correlated with aboveground biomass. Although there were no differences in aboveground biomass between the margin and centre, our findings suggest that plants can benefit from investing in the acquisition of resources near the vertical thaw front. Conclusions Our results suggest that deep roots of C. aquatilis can contribute to plant nitrogen uptake and are therefore able to tolerate anoxic conditions in saturated thermokarst bogs. This work furthers our understanding of how subarctic and wetland plants respond to warming and how enhanced plant biomass production might help offset ecosystem carbon release with future permafrost thaw. 

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3. Carbon Emissions From Fires in Eastern Siberian Larch Forests

   https://doi.org/10.1111/gcb.70247  

   Delcourt, Clement_J F; Rogers, Brendan M; Akhmetzyanov, Linar; Izbicki, Brian; Scholten, Rebecca C; Shestakova, Tatiana A; van_Wees, Dave; Mack, Michelle C; Sass‐Klaassen, Ute; Veraverbeke, Sander (May 2025, Global Change Biology)

   ABSTRACT Siberian boreal forests have experienced increases in fire extent and intensity in recent years, which may threaten their role as carbon (C) sinks. Larch forests (Larixspp.) cover approximately 2.6 million km²across Siberia, yet little is known about the magnitude and drivers of carbon combustion in these ecosystems. To address the paucity of field‐based estimates of fuel load and consumption in Siberian larch forests, we sampled 41 burned plots, one to two years after fire, in Cajander larch (Larix cajanderi) forests in the Republic of Sakha (Yakutia), Russia. We estimated pre‐fire carbon stocks and combustion with the objective of identifying the main drivers of carbon emissions. Pre‐fire aboveground (trees and woody debris) and belowground carbon stocks at our study plots were 3.12 ± 1.26 kg C m⁻²(mean ± standard deviation) and 3.50 ± 0.93 kg C m⁻². We found that combustion averaged 3.20 ± 0.75 kg C m⁻², of which 78% (2.49 ± 0.56 kg C m⁻²) stemmed from organic soil layers. These results suggest that severe fires in Cajander larch forests can result in combustion rates comparable to those observed in North American boreal forests and exceeding those previously reported for other forest types and burning conditions in Siberia. Carbon combustion was driven by both fire weather conditions and landscape variables, with pre‐fire organic soil depth being the strongest predictor across our plots. Our study highlights the need to better account for Siberian larch forest fires and their impact on the carbon balance, especially given the expected climate‐induced increase in fire extent and severity in this region. 

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4. Woody debris and snag carbon pools across a post-fire tree density gradient in far northeastern Siberia, 2010 - 2017

   https://doi.org/10.18739/A2833N091  

   Alexander, Heather; Paulson, Alison; Loranty, Michael; Natali, Susan; Mack, Michelle (January 2024, NSF Arctic Data Center)

   Fire severity is increasing in larch forests of the Siberian Arctic as climate warms, and initial fire impacts on tree demographic processes could be an especially important determinant of long-term forest structure and carbon (C) dynamics. We hypothesized that changes in post-fire larch recruitment impact C accumulation through tree density impacts on understory microclimate and permafrost thaw. We tested these hypotheses by quantifying C pools across a Cajander larch (Larix cajanderi Mayr.) tree density gradient within a fire perimeter near Cherskiy, Russia that burned in ~1940. Across the density gradient, from 2010 - 2017 we inventoried larch trees and harvested ground-layer vegetation to estimate above ground contribution to C pools. We also quantified snag and woody debris C pools and sampled below ground C pools (soil, fine roots, and coarse roots) in the organic + upper mineral soils. Our findings should highlight the potential for a climate-driven increase in fire severity to alter tree recruitment, successional dynamics, and C cycling in Siberian larch forests. 

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5. Understory vegetation carbon pools across a post-fire tree density gradient in far northeastern Siberia, 2015 - 2017

   https://doi.org/10.18739/A24B2X66C  

   Alexander, Heather; Paulson, Alison; Loranty, Michael; Natali, Susan; Mack, Michelle (January 2024, NSF Arctic Data Center)

   Fire severity is increasing in larch forests of the Siberian Arctic as climate warms, and initial fire impacts on tree demographic processes could be an especially important determinant of long-term forest structure and carbon (C) dynamics. We hypothesized that changes in post-fire larch recruitment impact C accumulation through tree density impacts on understory microclimate and permafrost thaw. We tested these hypotheses by quantifying C pools across a Cajander larch (Larix cajanderi Mayr.) tree density gradient within a fire perimeter near Cherskiy, Russia that burned in ~1940. Across the density gradient, from 2010 - 2017 we inventoried larch trees and harvested ground-layer vegetation to estimate above ground contribution to C pools. We also quantified woody debris C pools and sampled below ground C pools (soil, fine roots, and coarse roots) in the organic + upper mineral soils. Our findings should highlight the potential for a climate-driven increase in fire severity to alter tree recruitment, successional dynamics, and C cycling in Siberian larch forests. 

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