Abstract
Permafrost forests play an important role in the global carbon budget due to the huge amounts of carbon stored below ground in these ecosystems. Although fine roots are considered to be a major pathway of belowground carbon flux, separate contributions of overstory trees and understory shrubs to fine root dynamics in these forests have not been specifically characterized in relation to permafrost conditions, such as active layer thickness. In this study, we investigated fine root growth and morphology of trees and understory shrubs using ingrowth cores with two types of moss substrates (feather- and Sphagnum mosses) in permafrost black spruce (Picea mariana) stands along a north-facing slope in Interior Alaska, where active layer thickness varied substantially. Aboveground biomass, litterfall production rate, and fine root mass were also examined. Results showed that aboveground biomass, fine root mass, and fine root growth of black spruce trees tended to decrease downslope, whereas those of understory Ericaceae shrubs increased. Belowground allocation (e.g., ratio of fine root growth/leaf litter production) increased downslope in both of black spruce and understory plants. These results suggested that, at a lower slope, belowground resource availability was lower than at upper slope, but higher light availability under open canopy seemed to benefit the growth of the understory shrubs. On the other hand, understory shrubs were more responsive to the moss substrates than black spruce, in which Sphagnum moss substrates increased fine root growth of the shrubs as compared with feather moss substrates, whereas the effect was unclear for black spruce. This is probably due to higher moisture contents in Sphagnum moss substrates, which benefited the growth of small diameter (high specific root length) fine roots of understory shrubs. Hence, the contribution of understory shrubs to fine root growth was greater at lower slope than at upper slope, or in Sphagnum than in feather-moss substrates in our study site. Taken together, our data show that fine roots of Ericaceae shrubs are a key component in belowground carbon flux at permafrost black spruce forests with shallow active layer and/or with Sphagnum dominated forest floor.
Highlights
Permafrost ecosystems store huge amounts of carbon below ground, which accounts for 50% of global belowground organic carbon pool (Tamocai et al, 2009; Mishra et al, 2021)
Temperature and volumetric water contents (VWC) were monitored by only one sensor per each ground cover type in each plot, mean organic layer temperature during the growing seasons (May–September of 2018–2019, 10 cm depth) appeared lower at NE260 (7.0– 7.9◦C) than at NE360 and NE330 (8.8–9.4◦C) (Table 1 and Supplementary Figure 4), while VWC were greater under Sphagnum mosses than under feather mosses (Supplementary Figure 4)
Litterfall from black spruce trees was greater in the order of NE360 > NE330 > NE260, whereas leaf litter production of understory plants was significantly greater at NE330 than in two other plots (Figure 1)
Summary
Permafrost ecosystems store huge amounts of carbon below ground (about 1,000 Pg), which accounts for 50% of global belowground organic carbon pool (Tamocai et al, 2009; Mishra et al, 2021). Frozen soil can be a harsh environment for vascular plants, coniferous forests dominated with black spruce (Picea mariana) (Interior Alaska, Northwestern Canada) and larch (Larix gmelinii and L. cajanderi) (central and eastern Siberia) are widely distributed in permafrost regions (Viereck et al, 1983; Abaimov, 2010; Fujii et al, 2020), covering more than 20% of the boreal forest globally (Osawa and Zyryanova, 2010) Considering this large area of distribution, it is important to improve our knowledge of carbon dynamics in permafrost forests to better understand terrestrial carbon cycling. Since understory plants were reported to make a significant contribution to aboveground productivity in permafrost or poorly drained black spruce stands (O’Connell et al, 2003; Ruess et al, 2003; Bond-Lamberty et al, 2004), it would be valuable to evaluate fine root growth of understory plants separately from black spruce trees to better understand how the belowground carbon flux is controlled in permafrost forests
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