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1. Positive response of tree productivity to warming is reversed by increased tree density at the Arctic tundra-taiga ecotone

   https://doi.org/10.1139/cjfr-2020-0466  

   Walker, Xanthe; Alexander, Heather D.; Berner, Logan; Boyd, Melissa A; Loranty, Michael M.; Natali, Sue; Mack, Michelle C. (January 2021, Canadian Journal of Forest Research)

   null (Ed.)

   The transition zone between the northern boreal forest and the arctic tundra, known as the tundra-taiga ecotone (TTE) has undergone rapid warming in recent decades. In response to this warming, tree density, growth, and stand productivity are expected to increase. Increases in tree density have the potential to negate the positive impacts of warming on tree growth through a reduction in the active layer and an increase in competitive interactions. We assessed the effects of tree density on tree growth and climate-growth responses of Cajander larch (Larix cajanderi) and on trends in the normalized difference vegetation index (NDVI) in the TTE of Northeast Siberia. We examined 19 mature forest stands that all established after a fire in 1940 and ranged in tree density from 300 to 37,000 stems ha-1. High density stands with shallow active layers had lower tree growth, higher stand productivity, and more negative growth responses to growing season temperatures compared to low density stands with deep active layers. Variation in stand productivity across the density gradient was not captured by Landsat derived NDVI, but NDVI did capture annual variations in stand productivity. Our results suggest that the expected increases in tree density following fires at the TTE may effectively limit tree growth and that NDVI is unlikely to capture increasing productivity associated with changes in tree density. 

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2. Lower‐intensity restoration interventions drive greater seedling establishment for later‐successional tree species

   https://doi.org/10.1111/1365-2664.70116  

   Joyce, Francis H; Zahawi, Rakan A; Holl, Karen D (September 2025, Journal of Applied Ecology)

   Abstract Recovery of tree community composition in restored tropical forests relies on successful recruitment of later‐successional species. However, the long‐term effects of different restoration interventions on establishment success of arriving seeds are poorly understood.We evaluated the effects of three restoration treatments on the seed‐to‐seedling transition for later‐successional tree species in a fragmented agricultural landscape in southern Costa Rica. Restoration plots (0.25 ha) were established in a block design nearly two decades prior and spanned a gradient of intervention intensity: natural regeneration (not planted), applied nucleation (planted tree clusters) and plantation (fully planted). We conducted seed addition experiments from 2021 to 2023 using eight species at seven replicate restoration sites and in four nearby remnant forests. We defined the seed‐to‐seedling transition as two stages: seedling emergence and post‐emergence survival.Seedling emergence was highest in natural regeneration (48%), intermediate in applied nucleation (37%) and lowest in plantation (25%) and remnant forests (27%), with high variability across species. First‐year survival of emerged seedlings was greater in applied nucleation than in plantation and was lower in remnant forest than in all restoration treatments. On average, seedlings grew slightly taller in natural regeneration compared to the other treatments.Despite seed mass spanning four orders of magnitude among the eight species, this trait did not explain variation in seedling emergence or survival.Synthesis and applications: Taken together, results suggest that after ~15 years, applied nucleation and natural regeneration plots may provide more favourable conditions for establishment of later‐successional trees than do plantation plots. When the seed‐to‐seedling transition is not a major barrier for the recruitment of later‐successional tree species in restoration plots, practitioners should focus on increasing seed arrival for these species. Seed addition could be a viable strategy to introduce later‐successional species that fail to colonize restoration sites, particularly in restoration sites with low intensity interventions (i.e. natural regeneration or applied nucleation). 

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3. Variable retention harvesting influences belowground plant-fungal interactions of Nothofagus pumilio seedlings in forests of southern Patagonia

   https://doi.org/10.7717/peerj.5008  

   Hewitt, Rebecca E.; Taylor, Donald Lee; Hollingsworth, Teresa N.; Anderson, Christopher B.; Martínez Pastur, Guillermo (January 2018, PeerJ)

   Background The post-harvest recovery and sustained productivity of Nothofagus pumilio forests in Tierra del Fuego may be affected by the abundance and composition of ectomycorrhizal fungi (EMF). Timber harvesting alters EMF community structure in many managed forests, but the impacts of harvesting can vary with the management strategy. The implementation of variable retention (VR) management can maintain, increase, or decrease the diversity of many species, but the effects of VR on EMF in the forests of southern Patagonia have not been studied, nor has the role of EMF in the regeneration process of these forests. Methods We evaluated the effects of VR management on the EMF community associated with N. pumilio seedlings. We quantified the abundance, composition, and diversity of EMF across aggregate (AR) and dispersed (DR) retention sites within VR managed areas, and compared them to primary forest (PF) unmanaged stands. EMF assemblage and taxonomic identities were determined by ITS-rDNA sequencing of individual root tips sampled from 280 seedlings across three landscape replicates. To better understand seedling performance, we tested the relationships between EMF colonization, EMF taxonomic composition, seedling biomass, and VR treatment. Results The majority of EMF taxa were Basidiomycota belonging to the families Cortinariaceae ( n  = 29), Inocybaceae ( n  = 16), and Thelephoraceae ( n  = 8), which was in agreement with other studies of EMF diversity in Nothofagus forests. EMF richness and colonization was reduced in DR compared to AR and PF. Furthermore, EMF community composition was similar between AR and PF, but differed from the composition in DR. EMF community composition was correlated with seedling biomass and soil moisture. The presence of Peziza depressa was associated with higher seedling biomass and greater soil moisture, while Inocybe fibrillosibrunnea and Cortinarius amoenus were associated with reduced seedling biomass and lower soil moisture. Seedling biomass was more strongly related to retention type than EMF colonization, richness, or composition. Discussion Our results demonstrate reduced EMF attributes and altered composition in VR treatments relative to PF stands, with stronger impacts in DR compared to AR. This suggests that VR has the potential to improve the conservation status of managed stands by supporting native EMF in AR. Our results also demonstrate the complex linkages between retention treatments, fungal community composition, and tree growth at individual and stand scales. 

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4. Understory plant diversity and composition across a postfire tree density gradient in a Siberian Arctic boreal forest

   https://doi.org/10.1139/cjfr-2020-0483  

   Paulson, Alison K.; Peña, Homero; Alexander, Heather D.; Davydov, Sergei P.; Loranty, Michael M.; Mack, Michelle C.; Natali, Susan M. (May 2021, Canadian Journal of Forest Research)

   null (Ed.)

   Cajander larch (Larix cajanderi Mayr.) forests of the Siberian Arctic are experiencing increased wildfire activity in conjunction with climate warming. These shifts could affect postfire variation in the density and arrangement of trees and understory plant communities. To better understand how understory plant composition, abundance, and diversity vary with tree density, we surveyed understory plant communities and stand characteristics (e.g., canopy cover, active layer depth, and soil organic layer depth) within 25 stands representing a density gradient of similarly-aged larch trees that established following a 1940 fire near Cherskiy, Russia. Understory plant diversity and mean total plant abundance decreased with increased canopy cover. Canopy cover was also the most important variable affecting individual species’ abundances. In general, tall shrubs (e.g., Betula nana subsp. exilis) were more abundant in low-density stands with high light availability, and mosses (e.g., Sanionia spp.) were more abundant in high-density stands with low light availability. These results provide evidence that postfire variation in tree recruitment affects understory plant community composition and diversity as stands mature. Therefore, projected increases in wildfire activity in the Siberian Arctic could have cascading impacts on forest structure and composition in both overstory and understory plant communities. 

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5. 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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