The dynamics of the carbon storage and fluxes in Scots pine (Pinus sylvestris) chronosequence.

To evaluate the impact of stand age on the ecosystem's C budget, as well as the post-harvest recovery of the C storages and fluxes, a chronosequence of Scots pine stands from the clear-cut stage up to the age of 110 years was studied. An age-related trend of net primary production (NPP) demonstrated effective C accumulation in the young and middle-aged stands and their levelling out thereafter. The understorey vegetation contributed 8-46% to total NPP, being lower in the pole and middle-aged stands, but without a clear age related trend. Annual cumulative soil heterotrophic respiration (Rh) demonstrated stable values along the chronosequence, varying between 3.8 and 5.4 t C ha-1 yr-1. The Rh flux of 2.9 t C ha-1 yr-1 at the clear-cut site did not exceed the corresponding value for stands. The NEP along the chronosequence followed the dynamics of the annual biomass production of the trees, peaking at the middle-aged stage and decreasing in the older stands; the NPP of the trees was the main driver directing the dynamics of NEP. There was no significant correlation between Rh and dynamics of aboveground litter or fine root production, which can partly explain why no relationship was established between annual Rh and stand age. The total ecosystem C stocks followed the same trend as cumulative tree biomass, peaking in the older stands, however, the soil C stocks varied along the chronosequence irrespective of stand age. The post-harvest C compensation point was reached at the age of 7-years and C payback occurred at a stand age of 11-12 years. Stands acted as C accumulating ecosystems and average annual C accumulation was around 2.5 t C ha-1 yr-1, except for the youngest stand and the clear-cut area which acted as C sources. In the oldest stand C budget was almost balanced, with a modest annual accumulation of 0.12 t C ha-1 yr-1.

The initial overreaction of carbon cycle to elevated atmospheric humidity levels off over time - a FAHM study in a young birch forest.

Ecosystem responses to climate change are mainly predicted based on short-term studies. However, the first response can be a temporary overreaction, different from the later response of the more acclimated ecosystem. The current paper is a follow-up study of our previous article, where the effect of elevated atmospheric humidity on forest ecosystem carbon (C) balance was studied in a young silver birch (Betula pendula Roth) forest after two years of humidification. Here, we present the C balance of the same forest measured two years later when humidification treatment had been performed for four years. We revealed that the higher C sequestration capacity of the humidified birch forest ecosystem was an initial overreaction, which levelled off after four years of humidification, when the ecosystem became more acclimated to wetter conditions. Understorey production reacted rapidly and strongly by increasing belowground production more than twofold, but this reaction ceased after four years of humidification treatment. Trees responded to a lesser extent, and the initially decreased aboveground growth was recovered after four years of humidification, when the biomass allocation to tree fine-roots was increased. Our results showed that at early forest age, understorey plant production dominated in the whole ecosystem C sequestration capacity. But in the later stage, the most important C sink was biomass production of birches, and since the tree biomass production no longer differed between the treatments, C sequestration of the whole ecosystem did not differ either. The findings confirm that a preliminary reaction of an ecosystem can be different from the later response, which needs to be taken into account when prognosing the climate change consequences for carbon sequestration.

Predawn leaf conductance depends on previous day irradiance but is not related to growth in aspen saplings grown under artificially manipulated air humidity.

Recent studies have suggested that predawn stomatal opening may enhance early-morning photosynthesis (A) and improve the relative growth rate of trees. However, the causality between night-time stomatal conductance, A, and tree growth is disputable because stomatal opening in darkness can be mediated by previous day photosynthate loads and might be a consequence of growth-related processes like dark respiration (R). To identify linkages between night-time leaf conductance (gl_night), A, R, and tree growth, we conducted an experiment in hybrid aspen saplings grown under different air relative humidity (RH) conditions and previous day irradiance level (IR_pday). Predawn leaf conductance (gl_predawn) depended on RH, IR_pday and R (P < 0.05), whereas early-morning gross A (Agross_PAR500) depended on IR_pday and gl_predawn (P < 0.001). Daytime net A was positively related to Agross_PAR500 and leaf [N] (P < 0.05). Tree diameter and height increment correlated positively with gl at the beginning and middle of the night (P < 0.05) but not before dawn. Although our results demonstrate that gl_night was related to tree growth, the relationship was not determined by R. The linkage between gl_predawn and Agross_PAR500 was modified by IR_pday, indicating that daily CO2 assimilation probably provides feedback for stomatal opening before dawn.

Adaptive root foraging strategies along a boreal-temperate forest gradient.

The tree root-mycorhizosphere plays a key role in resource uptake, but also in the adaptation of forests to changing environments. The adaptive foraging mechanisms of ectomycorrhizal (EcM) and fine roots of Picea abies, Pinus sylvestris and Betula pendula were evaluated along a gradient from temperate to subarctic boreal forest (38 sites between latitudes 48°N and 69°N) in Europe. Variables describing tree resource uptake structures and processes (absorptive fine root biomass and morphology, nitrogen (N) concentration in absorptive roots, extramatrical mycelium (EMM) biomass, community structure of root-associated EcM fungi, soil and rhizosphere bacteria) were used to analyse relationships between root system functional traits and climate, soil and stand characteristics. Absorptive fine root biomass per stand basal area increased significantly from temperate to boreal forests, coinciding with longer and thinner root tips with higher tissue density, smaller EMM biomass per root length and a shift in soil microbial community structure. The soil carbon (C) : N ratio was found to explain most of the variability in absorptive fine root and EMM biomass, root tissue density, N concentration and rhizosphere bacterial community structure. We suggest a concept of absorptive fine root foraging strategies involving both qualitative and quantitative changes in the root-mycorrhiza-bacteria continuum along climate and soil C : N gradients.