Responses of fine-root biomass and production to drying depend on wetness and site nutrient regime in boreal forested peatland

Abstract

IntroductionPeatlands are terrestrial-carbon hotspots, where changes in carbon pools and fluxes potentially caused by drying or warming may have significant feedbacks to climate change. In forested peatlands, fine-root biomass (FRB), and production (FRP) are important carbon pools and fluxes, but they and their depth distribution and plant functional type (PFT) composition are poorly known.MethodsWe studied the effects of persistent water-table level (WTL) drawdown on these characteristics in four forested boreal peatland site types that varied in soil nutrient and WTL regimes, ground vegetation and tree stand characteristics. Each site type was represented by a pair of one undrained and one drained site. Two pairs were nutrient-poor, Scots pine dominated sites, one very wet and one relatively dry in their undrained condition. The other two pairs were nutrient-rich, Norway spruce dominated sites, again one wetter and one drier in the undrained condition. FRB was estimated by separating and visually identifying roots from soil cores extending down to 50 cm depth. FRP was estimated using ingrowth cores covering the same depth, and the separated roots were identified using infrared spectroscopy.Results and discussionBoth FRB and FRP varied widely both within and among the different types of boreal forested peatland. In FRB, the clearest differences were seen in the two originally wettest sites, nutrient-poor tall-sedge pine fen and nutrient-rich herb-rich spruce swamp: FRB was smaller in the drained site compared to the undrained site in the pine fen, but the opposite was true in the spruce swamp. FRP was generally higher in the nutrient-poor, pine-dominated sites than the nutrient-rich, spruce-dominates sites. The depth distribution of FRB was more superficial than that of FRP, except for the most nutrient-rich spruce swamp. Tree and shrub roots dominated both FRB and FRP, except for the undrained pine fen, where graminoids and forbs dominated. Even there, these PFTs were replaced by trees and shrubs at the drained site. Site wetness and nutrient regime both thus clearly regulated FRB and FRP of the forested peatland site types studied, and both need to be considered when making any generalizations.

The effect of drainage on fine‐root biomass, production, and turnover in hemiboreal old‐growth forests on organic soils

“…However, the observed high variability (stand and soil characteristics) among the drained sites suggests that the drainage effect was likely confused with other biotic and abiotic factors. It is highly likely that, besides the differences related to the ditch systems (Finér & Laine, 1998;Rezapour et al, 2022), the fundamental soil heterogeneity (Janssens et al, 2002) and tree-species-specific effects (Lampela et al, 2023) played…”

“…In forestry, ditch network maintenance is carried out approximately once or twice during a stand rotation (~100 years) (Hökkä et al, 2000), which suggests that the ditch systems at our sites might have gradually degraded over the last 20-30 years. The difficulty in precisely evaluating the GWL effect on the FR characteristics in different boreal peatlands has also been reported in a short-term (2-year) study by Lampela et al (2023), the effects overlapping with other biotic and abiotic factors, and also being shaped by tree-species-specific effects. In our study, the long-term effect of drainage on the ecosystem was suggested by the significantly higher mean diameter, basal area, and standing volume of Norway spruce at the drained Ugale and Auce sites (Table 1).…”

“…In this study, we aimed to provide a comparison between the FRB and FRP in old-growth, fertile, drained, and undrained forests on organic soils dominated by Scots pine in the hemiboreal forest zone. Because it was known that the FRB can be higher in undrained Scots-pine-dominated peatland sites in the boreal region (Lampela et al, 2023), we hypothesized that we would find higher Scots pine FRB and FRP in the undrained sites than in the drained sites in the hemiboreal forest zone. We also hypothesized that Scots pine, as the dominant tree species, would have higher estimates of FRB and FRP than the other functional groups.…”

“…However, as only one site had an exceptionally high Scots pine FRP, this was likely caused by site-specific effects coupled with the meteorological conditions. In the study published by Lampela et al (2023), the possible reasons for the extremely low/high FRP and FRB estimates are likely linked to nutrient availability and the sampling method.…”

“…Due to the laborious sampling procedure, previous studies have often used only a small number of sites per study, but because forests on organic soils vary greatly in terms of fertility gradient, more studies are required for a better understanding of FRP and C fluxes in these ecosystems (Bhuiyan et al, 2017), especially in the hemiboreal forest zone, where the available information on forests on organic soils is fragmented and scarce. Some novel aspects of the short‐term effects of drainage (2 years) on FR characteristics in boreal peatlands have been recently published (Bhuiyan et al, 2023; Lampela et al, 2023) that highlight the multilevel influence of abiotic and biotic factors.…”