Root‐associated fungi not tree density influences stand nitrogen dynamics at the larch forest–tundra ecotone

Abstract

Greater tree density and forest productivity at the tundra–taiga ecotone (TTE) are expected with climate warming, with potential feedbacks to the climate system. Yet, competition for nitrogen (N) may impact TTE dynamics. Greater tree density will likely increase N demand, while reducing N supply through soil shading and slower decomposition rates. We explored whether characteristics of roots and root‐associated fungi important to N acquisition responded to changes in density at the TTE and were related to above‐ground stand productivity and N cycling. We measured rooting depth, uptake of N forms among soil layers and ectomycorrhizal (EcM) colonization and composition along a natural tree density gradient of monodominant larch Larix cajanderi in northeastern Siberia. We tested relationships between larch root and fungal characteristics, above‐ground productivity and stand‐level N cycling parameters. Overall, there was preferential uptake of ammonium compared to glycine or nitrate. Nitrogen uptake was greatest in shallow soils of the organic horizon and related to root chemistry, root‐associated fungi and above‐ground N cycling parameters, but the direction of these relationships depended on N form. Uptake of different N forms, rooting depth and EcM colonization and composition were not related to tree density, but fungal composition was correlated with root N chemistry and above‐ground N cycling parameters. In addition to EcM, the abundance of dark septate endophytes and other ascomycetous taxa was positively related to N uptake and above‐ground N cycling parameters. Synthesis. There was little impact of tree density on root and fungal parameters related to N acquisition suggesting intraspecific larch competition for N was not amplified with increased density. There was, however, a strong impact of root‐associated fungi on N uptake and stand N dynamics regardless of tree density. Together, this suggests an important role of root‐associated fungi on broadscale patterns of N cycling in TTE larch forests independent of changes in tree density expected with climate warming.

Increasing tree density accelerates stand‐level nitrogen cycling at the taiga–tundra ecotone in northeastern Siberia

“…This contrasts, however, with our observations of greater resin‐sorbed NO 3 − ‐N, a relatively depleted N source, in mineral but not organic soils. The greater NO 3 − ‐N in the mineral soils may reflect lower demand by larch from this soil layer and thus a greater amount in the soil, given that larch and other species were not observed to have appreciable amounts of fine roots in mineral soils and N uptake from mineral soils is lower than that from organic soils (Hewitt, Miller, et al, 2022a).…”

Fire‐Induced Carbon Loss and Tree Mortality in Siberian Larch Forests

“…Our finding of highly lethal, stand replacing fires in Siberian larch forests is consistent with some previous observations that have not been widely cited in the international literature (Tsvetkov, 2006a(Tsvetkov, , 2006b. Siberian larch trees growing on continuous permafrost have shallow root systems, which often do not exceed the depth of the SOL (Hewitt et al, 2022;Kajimoto, 2010;Kropp et al, 2019). Surface fires can therefore cause extensive root damage that ultimately results in tree mortality (Fang et al, 2018;Tsvetkov, 2006a).…”

Peat loss collocates with a threshold in plant–mycorrhizal associations in drained peatlands encroached by trees

“…Temperature increases c. 1°C are also expected to induce encroachment by trees in northern peatlands (Heijmans et al, 2013), suggesting that long-term drainage and climate warming have similar effects on vegetation in these ecosystems. Some studies on climate-induced tree encroachment into carbonrich soils have reported a concomitant shift in the fungal community composition from root-associated Ascomycota, including the functional guild ErM fungi and the class Archaeorhizomycetes, to Basidiomycota and Mucoromycota, including the functional guilds EcM fungi and saprotrophs (Tonjer et al, 2021;Hewitt et al, 2022). Clemmensen et al (2021), on the contrary, showed a change in the assemblage of EcM fungal functional trait and explained losses of organic matter at the transition from birch forest to heath tundra by the activity of rhizomorph-forming EcM genera with class II peroxidases, especially Cortinarius.…”

“…1°C are also expected to induce encroachment by trees in northern peatlands (Heijmans et al ., 2013), suggesting that long‐term drainage and climate warming have similar effects on vegetation in these ecosystems. Some studies on climate‐induced tree encroachment into carbon‐rich soils have reported a concomitant shift in the fungal community composition from root‐associated Ascomycota, including the functional guild ErM fungi and the class Archaeorhizomycetes, to Basidiomycota and Mucoromycota, including the functional guilds EcM fungi and saprotrophs (Tonjer et al ., 2021; Hewitt et al ., 2022). Clemmensen et al .…”