Paludification reduces black spruce growth rate but does not alter tree water use efficiency in Canadian boreal forested peatlands

Background:Black spruce(Picea mariana(Mill.)BSP)-forested peatlands are widespread ecosystems in boreal North America in which peat accumulation,known as the paludification process,has been shown to induce forest growth decline.The continuously evolving environmental conditions(e.g.,water table rise,increasing peat thickness)in paludified forests may require tree growth mechanism adjustments over time.In this study,we investigate tree ecophysiological mechanisms along a paludification gradient in a boreal forested peatland of eastern Canada by combining peat-based and tree-ring analyses.Carbon and oxygen stable isotopes in tree rings are used to document changes in carbon assimilation rates,stomatal conductance,and water use efficiency.In addition,paleohydrological analyses are performed to evaluate the dynamical ecophysiological adjustments of black spruce trees to site-specific water table variations.Results:Increasing peat accumulation considerably impacts forest growth,but no significant differences in tree water use efficiency(iWUE)are found between the study sites.Tree-ring isotopic analysis indicates no iWUE decrease over the last 100 years,but rather an important increase at each site up to the 1980 s,before iWUE stabilized.Surprisingly,inferred basal area increments do not reflect such trends.Therefore,iWUE variations do not reflect tree ecophysiological adjustments required by changes in growing conditions.Local water table variations induce no changes in ecophysiological mechanisms,but a synchronous shift in iWUE is observed at all sites in the mid-1980 s.Conclusions:Our study shows that paludification induces black spruce growth decline without altering tree water use efficiency in boreal forested peatlands.These findings highlight that failing to account for paludification-related carbon use and allocation could result in the overestimation of aboveground biomass production in paludified sites.Further research on carbon allocation strategies is of utmost importance to understand the carbon sink capacity of these widespread ecosystems in the context of climate change,and to make appropriate forest management decisions in the boreal biome.

Forecasting the development of boreal paludified forests in response to climate change: a case study using Ontario ecosite classification

Background：Successional paludification,a dynamic process that leads to the formation of peatlands,is influenced by climatic factors and site features such as surficial deposits and soil texture.In boreal regions,projected climate change and corresponding modifications in natural fire regimes are expected to influence the paludification process and forest development.The objective of this study was to forecast the development of boreal paludified forests in northeastern North America in relation to climate change and modifications in the natural fire regime for the period 2011–2100.Methods：A paludification index was built using static（e.g.surficial deposits and soil texture）and dynamic（e.g.moisture regime and soil organic layer thickness）stand scale factors available from forest maps.The index considered the effects of three temperature increase scenarios（i.e.＋1°C,＋3°C and＋6°C）and progressively decreasing fire cycle（from 300 years for 2011–2041,to 200 years for 2071–2100）on peat accumulation rate and soil organic layer（SOL）thickness at the stand level,and paludification at the landscape level.Results：Our index show that in the context where in the absence of fire the landscape continues to paludify,the negative effect of climate change on peat accumulation resulted in little modification to SOL thickness at the stand level,and no change in the paludification level of the study area between 2011 and 2100.However,including decreasing fire cycle to the index resulted in declines in paludified area.Overall,the index predicts a slight to moderate decrease in the area covered by paludified forests in 2100,with slower rates of paludification.Conclusions：Slower paludification rates imply greater forest productivity and a greater potential for forest harvest,but also a gradual loss of open paludified stands,which could impact the carbon balance in paludified landscapes.Nonetheless,as the thick Sphagnum layer typical of paludified forests may protect soil organic layer from drought and deep burns,a significant proportion of the territory has high potential to remain a carbon sink.

The development process of a temperate montane peatland and its controlling factors since the middle Holocene

Peatlands are some of the largest carbon reservoirs in terrestrial ecosystems and play a key role in the global carbon cycle.Understanding peatland development,carbon accumulation processes,and the peatland response to varying forcing factors over different temporal and spatial scales helps reveal the underlying processes and general patterns of these ecosystems.To assess the role of climate and local conditions in peatland development,the basal samples from 23 peat cores and three well dated long peat cores were used to explore peatland initiation,lateral expansion,and carbon accumulation rate in the Baijianghe peatland located in the Changbai Mountains,Northeast China.Our results reveal that the Baijianghe peatland was initiated from forest conditions at 7.9 cal.kyr BP and then expanded laterally by paludification.The rapid expansion between 5 and4 cal.kyr BP likely resulted from high precipitation and gentle topography.The mean carbon accumulation rates of the three long peat cores were 36.3,39.1 and 48.4 g C m^(-2)yr^(-1),respectively,which are higher than rates from the northern peatlands.Both climate and local conditions have exerted an important influence on carbon accumulation rates in the Baijianghe peatland since the middle Holocene.The carbon accumulation patterns between 5 and 1.5 cal.kyr BP were probably linked to local conditions rather than climatic settings,including topography,hydrological conditions,and plant composition.The consistently decreasing carbon accumulation rate values at all locations within the BJH peatland over the last 1.5 cal.kyr BP suggests that climate is the primary control.This study highlights the varying primary controls on the process of peatland development and reveals the important role of local conditions in carbon accumulation.