Jukka Pumpanen

Precipitation and net ecosystem exchange are the most important drivers of DOC flux in upland boreal catchments

According to recent studies, dissolved organic carbon (DOC) concentrations in rivers throughout the boreal zone are increasing. However, the mechanistic explanation of this phenomenon is not yet well known. We studied how the short and long-term changes in precipitation, soil temperature, soil water content, and net ecosystem exchange (NEE) are reflected to DOC concentrations and runoff DOC fluxes in two small forested upland catchments in Southern Finland. We used continuous eddy covariance measurements above the forest and runoff flow measurements from the catchment areas conducted over a 15 year long time period to study the correlation between NEE, gross photosynthetic production, total ecosystem respiration, litter production, and runoff DOC. In addition, we looked for the most important environmental variables in explaining the interannual changes in runoff DOC by using multiple linear regression. Finally, we studied the temporal connection between runoff DOC concentrations, precipitation, soil water content, and NEE by using wavelet coherence analysis technique. Our results indicate that the DOC concentrations have increased over the last 15u2009years. The DOC flux was to a large extent determined by the amount of precipitation, but the previous years NEE and litter production had also a small but significant effect on runoff DOC fluxes.

Effects of prolonged drought stress on Scots pine seedling carbon allocation

As the number of drought occurrences has been predicted to increase with increasing temperatures, it is believed that boreal forests will become particularly vulnerable to decreased growth and increased tree mortality caused by the hydraulic failure, carbon starvation and vulnerability to pests following these. Although drought-affected trees are known to have stunted growth, as well as increased allocation of carbon to roots, still not enough is known about the ways in which trees can acclimate to drought. We studied how drought stress affects belowground and aboveground carbon dynamics, as well as nitrogen uptake, in Scots pine (Pinus sylvestris L.) seedlings exposed to prolonged drought. Overall 40 Scots pine seedlings were divided into control and drought treatments over two growing seasons. Seedlings were pulse-labelled with 13CO2 and litter bags containing 15N-labelled root biomass, and these were used to follow nutrient uptake of trees. We determined photosynthesis, biomass distribution, root and rhizosphere respiration, water potential, leaf osmolalities and carbon and nitrogen assimilation patterns in both treatments. The photosynthetic rate of the drought-induced seedlings did not decrease compared to the control group, the maximum leaf specific photosynthetic rate being 0.058 and 0.045 µmol g-1 s-1 for the drought and control treatments, respectively. The effects of drought were, however, observed as lower water potentials, increased osmolalities as well as decreased growth and greater fine root-to-shoot ratio in the drought-treated seedlings. We also observed improved uptake of labelled nitrogen from soil to needles in the drought-treated seedlings. The results indicate acclimation of seedlings to long-term drought by aiming to retain sufficient water uptake with adequate allocation to roots and root-associated mycorrhizal fungi. The plants seem to control water potential with osmolysis, for which sufficient photosynthetic capability is needed.

Studying the impact of living roots on the decomposition of soil organic matter in two different forestry-drained peatlands

Background and aimsForestry drainage is the main management practice of peatlands in Finland. The influence of drainage and management on carbon (C) fluxes may vary, e.g., depending on the original peatland type. We have studied C fluxes in two forestry-drained peatlands with different nutrient status.MethodsOur hypothesis that the differences in the C balance between these two sites can be attributed to differences in soil respiration rates, and in particular to the priming effect, was tested with laboratory microcosm flux measurements and 14C isotopic partitioning method. A two-pool mixing-model based on the natural difference in the respired 14CO2 between the peat and plants was employed.ResultsWe found no statistically significant priming effect in either nutrient-poor or nutrient-rich soil, respectively.ConclusionsAs no differences in priming effect were found, we can conclude that the nutrient status of the sites does not affect the priming effect in the peat soils studied here, thus our results suggest that organic soils do not support priming to the same extent as mineral soils.

137Cs distributions in soil and trees in forest ecosystems after the radioactive fallout – Comparison study between southern Finland and Fukushima, Japan

The nuclear accidents at Chernobyl and Fukushima released large amounts of (137)Cs radionuclides into the atmosphere which spread over large forest areas. We compared the (137)Cs concentration distribution in different parts of two coniferous forest ecosystems (needle litter, stems and at different depths in the soil) over short and long term periods in Finland and Japan. We also estimated the change in (137)Cs activity concentrations in needle and soil between 1995 and 2013 in Southern Finland based on the back-calculated (137)Cs activity concentrations. We hypothesized that if the (137)Cs activity concentrations measured in 1995 and 2013 showed a similar decline in concentration, the (137)Cs activity concentration in the ecosystem was already stable in 1995. But if not, the (137)Cs activity concentrations were still changing in 2013. Our results showed that the vertical distribution of the (137)Cs fallout in the soil was similar in Hyytiälä and Fukushima. The highest (137)Cs concentrations were observed in the uppermost surface layers of the soil, and they decreased exponentially deeper in the soil. We also observed that (137)Cs activity concentrations estimated from the samples in 1995 and 2013 in Finland showed different behavior in the surface soil layers compared to the deep soil layer. These results suggested that the (137)Cs nuclei were still mobile in the surface soil layers 27 years after the accident. Our results further indicated that, in the aboveground parts of the trees, the (137)Cs concentrations were much closer to steady-state when compared to those of the surface soil layers based on the estimated declining rates of (137)Cs concentration activity in needles which were similar in 1995 and 2013. Despite its mobility and active role in the metabolism of trees, the (137)Cs remains in the structure of the trees for decades, and there is not much exchange of (137)Cs between the heartwood and surface layers of the stem.

The long-term impact of low-intensity surface fires on litter decomposition and enzyme activities in boreal coniferous forests

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.

Winter ecology of a subalpine grassland: Effects of snow removal on soil respiration, microbial structure and function

Seasonal snow cover provides essential insulation for mountain ecosystems, but expected changes in precipitation patterns and snow cover duration due to global warming can influence the activity of soil microbial communities. In turn, these changes have the potential to create new dynamics of soil organic matter cycling. To assess the effects of experimental snow removal and advanced spring conditions on soil carbon (C) and nitrogen (N) dynamics, and on the biomass and structure of soil microbial communities, we performed an in situ study in a subalpine grassland in the Austrian Alps, in conjunction with soil incubations under controlled conditions. We found substantial winter C-mineralisation and high accumulation of inorganic and organic N in the topsoil, peaking at snowmelt. Soil microbial biomass doubled under the snow, paralleled by a fivefold increase in its C:N ratio, but no apparent change in its bacteria-dominated community structure. Snow removal led to a series of mild freeze-thaw cycles, which had minor effects on in situ soil CO2 production and N mineralisation. Incubated soil under advanced spring conditions, however, revealed an impaired microbial metabolism shortly after snow removal, characterised by a limited capacity for C-mineralisation of both fresh plant-derived substrates and existing soil organic matter (SOM), leading to reduced priming effects. This effect was transient and the observed recovery in microbial respiration and SOM priming towards the end of the winter season indicated microbial resilience to short-lived freeze-thaw disturbance under field conditions. Bacteria showed a higher potential for uptake of plant-derived C substrates during this recovery phase. The observed temporary loss in microbial C-mineralisation capacity and the promotion of bacteria over fungi can likely impede winter SOM cycling in mountain grasslands under recurrent winter climate change events, with plausible implications for soil nutrient availability and plant-soil interactions.

Effect of climate warming on the annual terrestrial net ecosystem CO2 exchange globally in the boreal and temperate regions

The net ecosystem CO2 exchange is the result of the imbalance between the assimilation process (gross primary production, GPP) and ecosystem respiration (RE). The aim of this study was to investigate temperature sensitivities of these processes and the effect of climate warming on the annual terrestrial net ecosystem CO2 exchange globally in the boreal and temperate regions. A database of 403 site-years of ecosystem flux data at 101 sites in the world was collected and analyzed. Temperature sensitivities of rates of RE and GPP were quantified with Q10, defined as the increase of RE (or GPP) rates with a temperature rise of 10u2009°C. Results showed that on the annual time scale, the intrinsic temperature sensitivity of GPP (Q10sG) was higher than or equivalent to the intrinsic temperature sensitivity of RE (Q10sR). Q10sG was negatively correlated to the mean annual temperature (MAT), whereas Q10sR was independent of MAT. The analysis of the current temperature sensitivities and net ecosystem production suggested that temperature rise might enhance the CO2 sink of terrestrial ecosystems both in the boreal and temperate regions. In addition, ecosystems in these regions with different plant functional types should sequester more CO2 with climate warming.

Changes in very fine root respiration and morphology with time since last fire in a boreal forest

AimsWe examined the physiological and morphological responses of individual fine root segments in boreal forests stands with different age since the last fire to determine changes in specific fine root respiration and morphological traits during forest succession.MethodsWe investigated the respiration of fine roots divided into three diameter classes (<0.5, 0.5–1.0, and 1.0–2.0xa0mm) in a Finnish boreal Pinus sylvestris L. in forest stands with 5, 45, 63, and 155xa0years since the last fire.ResultsSpecific respiration rates of <0.5xa0mm roots in 155-year-old stands were 74xa0%, 38xa0%, and 31xa0% higher than in 5-, 45-, and 63-year-old stands, respectively. However, the respiration rates of thicker diameter roots did not significantly change among stands with respect to time after fire. Similarly, fire disturbance had a strong impact on morphological traits of <0.5xa0mm roots, but not on thicker roots. Root respiration rates correlated positively with specific root length (length per unit mass) and negatively with root tissue density (mass per unit volume) in all stand ages. The linear regression lines fitted to the relationships between root respiration and specific root length or root tissue density showed significantly higher intercepts in 63- and 155-year-old than in 5-year-old stands.ConclusionSignificant shifts in the intercept of the common slope of respiration vs. morphology indicate the different magnitude of the changes in physiological performance among the fire age class. Despite a specific small geographic area, we suggest that the recovery of boreal forests following wildfire induces a strategy that favors carbon investment in nutrient and water exploitation efficiency with consequences for higher respiration, length, and lower tissue density of very fine roots.

Newtonian boreal forest ecology: The Scots pine ecosystem as an example

Isaac Newtons approach to developing theories in his book Principia Mathematica proceeds in four steps. First, he defines various concepts, second, he formulates axioms utilising the concepts, third, he mathematically analyses the behaviour of the system defined by the concepts and axioms obtaining predictions and fourth, he tests the predictions with measurements. In this study, we formulated our theory of boreal forest ecosystems, called NewtonForest, following the four steps introduced by Newton. The forest ecosystem is a complicated entity and hence we needed altogether 27 concepts to describe the material and energy flows in the metabolism of trees, ground vegetation and microbes in the soil, and to describe the regularities in tree structure. Thirtyfour axioms described the most important features in the behaviour of the forest ecosystem. We utilised numerical simulations in the analysis of the behaviour of the system resulting in clear predictions that could be tested with field data. We collected retrospective time series of diameters and heights for test material from 6 stands in southern Finland and five stands in Estonia. The numerical simulations succeeded to predict the measured diameters and heights, providing clear corroboration with our theory.

The effect of roots and easily available carbon on the decomposition of soil organic matter fractions in boreal forest soil

Summary The priming effect induced by carbon (C) that is easily available to microbes has been shown to increase the mineralization of soil organic matter (SOM) that is resistant to decomposition, but the combined effects of easily available carbon and the living root system have rarely been studied. The aim of this research was to study the decomposition of SOM fractions of different solubility in water and their 13C, 14C and 15N abundance with and without the presence of a living root system and easily available carbohydrate in the form of glucose. The SOM collected from the organic horizon of a boreal forest soil in H yytiälä, southern F inland (61°51′N, 24°17′E), and exposed to laboratory incubations with and without the presence of P inus sylvestris L. seedlings and glucose, was separated into three chemical fractions with accelerated solvent (ASE) and pressurized hot water extractions (PHWE). Changes in the natural abundance of 13C, 14C and 15N, spectral properties assessed by F ourier transform infrared spectroscopy (FTIR) and the C and N pools of SOM fractions were studied after incubation for 6u2009months. The extractions separated SOM into fractions with distinctive isotopic composition. The most easily soluble SOM fraction showed the largest abundance of 15N and 14C, and the living root system induced changes in the abundance of 15N and FTIR spectra. Our research suggests that plant roots may induce SOM degradation and N uptake from soluble SOM fractions, but 13C, 14C, 15N or FTIR alone cannot be used to describe the recalcitrance of SOM and its accessibility to microorganisms. It is better to use several methods in parallel to study the decomposability of SOM. HighlightsWe studied the effects of tree roots and addition of glucose on the solubility of soil organic matter (SOM).SOM can be separated into pools with distinctive isotopic concentrations with pressurized hot water.Roots may induce organic matter degradation and N uptake from soluble SOM fractions.Larger 14C abundance in soluble SOM indicated it contained older C than the recently assimilated C.