Kuusen hienojuuridynamiikka ja karikkeen hiilisyöte metsämaahan erilaisissa ympäristöoloissa

Knowledge of the quantity of belowground litter carbon (C) input is scarce but highly valued in C budget calculations. Specifically, the turnover rate of fine roots is considered to be one of the most important parameters in the estimation of changes in soil C stock. In this thesis Norway spruce (Picea abies L. (Karst.)) fine root lifespan and litter production and their responses to nutrient availability and temperature were examined. Aboveground foliage and understory litter C inputs were also quantified. Furthermore, fine root isotopic C ages were compared to fine root lifespans. Increased nutrient availability and higher temperature shortened spruce fine root lifespan both in the long-term manipulation treatments and along a natural latitudinal gradient. Fertilization improved tree growth and the absolute amount of litter production, both below- and aboveground. Soil warming, by contrast, increased the belowground litter production in relation to aboveground foliage litterfall but did not lead to long-term increases in aboveground tree growth. In warmed soil, the changes in spruce fine root tip morphology pointed to nutrient deficiency. The results indicated that in nutrient limited forests climate warming is unlikely to increase the aboveground tree growth in the long-term. Fine root litter C input into the soil in relation to the aboveground litter C input was higher towards lower fertility, due particularly to the greater contribution of understory vegetation. The structural 14C age of fine roots was consistently 3 - 6 years older than the fine root lifespan determined with the minirhizotron method, indicating that root growth may use also use stored or recycled C. In almost all stands, fine root litter C input into the soil at least equalled the aboveground input, which confirms the significance of belowground litter production in the boreal forest C cycle. The importance of understory vegetation was also significant. In addition on understory vegetation, different stand age and tree species, more studies should also focus on the shift in the litter production pattern from above- to belowground along environmental change as this may have an impact on litter C quality and soil C storage in boreal forest soils.Ilmastosopimuksen mukaisesti Suomi raportoi vuosittain metsien kasvihuonekaasupäästöt ja -nielut. Metsien maaperän hiilivaraston muutoslaskelmiin liittyy kuitenkin suuri epävarmuus puiden hienojuurten eliniästä sekä vuotuisen juurikarikkeen myötä maahan kulkeutuvan hiilen määrästä. Tässä väitöskirjassa määritettiin kuusen (Picea abies L. (Karst.)) hienojuurten elinikä sekä maanalainen ja maanpäällinen kariketuotanto erilaisissa maaperän ravinteisuus- ja lämpötilaoloissa. Myös aluskasvillisuuden osuutta kariketuotannossa tarkasteltiin. Lisäksi väitöskirjassa verrattiin hienojuurten sisältämän isotooppisen hiilen (14C) ikää suhteessa hienojuurten elinikään. Ravinteisuuden lisääntyminen ja lämpötilan nousu lyhensivät kuusen hienojuurten elinikää sekä metsikkökokeessa että luonnollisella gradientilla. Lannoitus paransi puiden kasvua ja lisäsi sekä maanpäällistä että maanalaista kariketuotantoa. Maaperän lämpötilan kohottaminen sen sijaan lisäsi ainoastaan hienojuurten kariketuotosta, mutta puiden kasvu ei pitkällä aikavälillä lisääntynyt. Muutokset lämmitetyn maaperän hienojuurten juurenkärkien morfologiassa viittasivat puiden ravinnepuutokseen. Tulokset osoittavat, että ilmaston lämpeneminen ei välttämättä lisää puiden maanpäällistä kasvua pitkäaikaisesti mikäli metsämaan ravinteisuus sitä rajoittaa. Maaperän ravinteisuuden vähentyessä hienojuurikarikkeen määrä kasvoi suhteellisesti enemmän kuin maanpäällinen kariketuotanto, erityisesti aluskasvillisuuden suuremman osuuden vuoksi. Hienojuurten rakenteellinen hiili-14 oli isotooppimäärityksissä 3 - 6 vuotta vanhempaa kuin miniritsotronimenetelmällä määritetty hienojuurten elinikä, mikä osoittaa juurten käyttävän kasvuunsa äskettäin ilmakehästä yhteytetyn hiilen lisäksi myös varastoitua tai kierrätettyä hiiltä. Lähes kaikissa metsiköissä maanalaisen karikkeen määrä ulottui maanpäällisen kariketuotannon tasolle, mikä osoittaa juurikariketuotannon olevan merkittävä osa boreaalisten metsiemme hiilenkiertoa. Myös aluskasvillisuuden kariketuotanto oli merkittävä. Maanpäällisen ja maanalaisen kariketuotannon suhteiden muutos maaperän ympäristötekijöiden muuttuessa voi vaikuttaa maaperän hiilivarastoon ja sen pitkäaikaiseen pysyvyyteen. Tämä aihepiiri on tärkeä jatkotutkimuskohde

Fine root longevity and below- and aboveground litter production in a boreal Betula pendula forest

Abstract 1. Fine root turnover plays a critical role in carbon and nutrient cycling in forest ecosystems. In this study, we focused on the most abundant deciduous species in Nordic countries, silver birch (Betula pendula Roth) and its fine root dynamics, including the amount of litter produced by fine roots as well as by aboveground vegetation. 2. The minirhizotron method was used to quantify fine root longevity of silver birch and understory fine roots and rhizomes in northern Finland. Fine root biomass per basal area and ectomycorrhizal short root numbers per mg were also quantified. The fine root litter production was estimated by fine root biomass and longevity, and then compared with the aboveground litter collected with litter traps. 3. Birch fine root biomass was 1.4-fold higher than that of understory fine roots and rhizomes (234 ± 22, 171 ± 19 g m−2 respectively). Fine root longevity of birch (372 days) was significantly (P < 0.05) shorter than that of understory vegetation (643 days). The birch fine root longevity was positively related to root diameter and soil depth. Hazard analysis showed that thicker roots, long roots, roots produced late in the growing season, and roots growing deeper in the soil had relatively lower mortality hazard compared to the reference data. The total annual soil C input, including both birch and understory, was 283 g C m−2 yr−1. The proportion of understory annual C input was 35% of the total. Total annual belowground C input was 1.4-fold greater than that of aboveground. 4. Our study indicated that the total annual belowground litter production was greater than that of the aboveground litter in a boreal deciduous forest stand. Therefore, more emphasis should be put to quantify the C cycling of both above- and belowground parts of different tree species as well as understory in boreal forests.Peer reviewe

Kangasmaiden juuristosysteemi reagoi kasvuympäristön muutoksiin

Osa Metsäekosysteemien toiminta ja metsien käyttö muuttuvassa ilmastossa (MIL) -tutkimusohjelman loppuraporttia: http://urn.fi/URN:NBN:fi:metla-201210036195</a

Partitioning of forest floor CO2 emissions reveals the belowground interactions between different plant groups in a Scots pine stand in southern Finland

Changes in the climate may have unpredictable effects on belowground carbon processes and thus, the carbon balance of boreal forests. To understand the interactions of these processes in soil and to quantify the potential changes in the carbon cycle, partitioning of forest floor respiration is crucial. For this purpose, we used nine different treatments to separate the sources of forest floor carbon dioxide (CO2) emissions in a mature Scots pine (Pinus sylvestris L.) stand in southern Finland. To partition the belowground CO2 emissions, we used two different trenching methods: 1) to exclude roots and mycorrhizal fungal mycelia (mesh with 1-mu m pores) and 2) to exclude roots, but not mycorrhizal hyphae (mesh with 50-mu m pores). Additionally, we used 3) a control treatment that included roots and fungal hyphae. To partition the CO2 emissions from the forest floor vegetation, we 1) removed it, 2) left only the dwarf shrubs, or 3) left the vegetation intact. The forest floor CO2 emissions were regularly measured with a flux chamber throughout the growing seasons in 2013-2015. The total forest floor respiration was partitioned into respiration of tree roots (contributing 48%), heterotrophic soil respiration (30%) and respiration of ground vegetation other than shrubs (10%), dwarf shrubs (8%), and hyphae of mycorrhizal fungi (4%). Heterotrophic respiration increased in the trenched treatments without ground vegetation over time, due to the so-called 'Gadgil effect'. In the absence of tree mots, but when hyphal access was allowed, respiration in the dwarf shrub treatment increased throughout the experiment. This indicated that dwarf shrubs had fungal connections to outside the experimental plots via their ericoid mycorrhiza. At the same time, other ground vegetation, such as mosses, suppressed the dwarf shrub respiration in trenched treatments. Our results show that competition on the forest floor is intense between plant roots and soil microbes.Peer reviewe

Fine root longevity and carbon input into soil from below- and above ground litter in climatically contrasting forests

The major part of carbon (C) flow into forest soil consists of continually renewed fine roots and aboveground litterfall. We studied the belowground C input from the fine root litter of trees and understorey vegetation in relation to their aboveground litterfall in two Norway spruce (Picea abies L.) stands located in northern and southern Finland. The production of fine roots was estimated by using turnover and biomass data from minirhizotrons and soil cores. The foliage litter production of trees was estimated from litter traps, and that of the understorey vegetation from its annual growth and coverage. Finally, we augmented the data with four spruce plots in Sweden in order to study the above- and belowground litter ratios along latitudinal and soil fertility gradients. The fine root biomass of spruce trees per stand basal area was almost double in the northern site compared to the southern site. Furthermore, spruce fine roots in the north persisted significantly longer (97 ± 2 weeks) than spruce roots in the south (89 ± 2 weeks) or understorey fine roots at both sites. The annual production of tree foliage litter was higher in the southern stand, but the total amount of litter (including trees and understorey, above- and belowground) was similar at both sites, as was the ratio between the above- and belowground litter production. The role of understorey vegetation was greater in the northern site where it was responsible for 23% and 33% of below- and aboveground litter production, respectively, compared to 11% and 15% in the south. Thus, both below- and aboveground understorey C input is substantial and should be taken into account in ecosystem C cycle models. The regression between the aboveground:belowground litter production-ratio and the C:N-ratio of the organic layer (combined data from Finland and Sweden), showed that the share of belowground litter production increased when site fertility decreased. This shift in the litter production pattern from above- to belowground in the least fertile sites may have an impact on litter C quality and soil C storage

Unravelling the age of fine roots of temperate and boreal forests

Fine roots support the water and nutrient demands of plants and supply carbon to soils. Quantifying turnover times of fine roots is crucial for modeling soil organic matter dynamics and constraining carbon cycle–climate feedbacks. Here we challenge widely used isotopebased estimates suggesting the turnover of fine roots of trees to be as slow as a decade. By recording annual growth rings of roots from woody plant species, we show that mean chronological ages of fine roots vary from <1 to 12 years in temperate, boreal and sub-arctic forests. Radiocarbon dating reveals the same roots to be constructed from 10 ± 1 year (mean ± 1 SE) older carbon. This dramatic difference provides evidence for a time lag between plant carbon assimilation and production of fine roots, most likely due to internal carbon storage. The high root turnover documented here implies greater carbon inputs into soils than previously thought which has wide-ranging implications for quantifying ecosystem carbon allocation.Peer reviewe

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.Peer reviewe

Fine root dynamics in the boreal soil fertility gradient

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Hienojuurten elinikä ja kariketuotoksen hiili ilmastollisesti erilaisissa metsiköissä

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Distinct patterns of below- and aboveground growth phenology and litter carbon inputs along a boreal site type gradient

Forest ecosystem productivity is strongly linked to site nutrient availability, which is influenced by litter inputs and their decomposition rates. Fine roots and mycelia are key contributors in belowground soil carbon (C) accumulation, but studies have seldom reported how belowground litter C input is related to site types in boreal forests. In this study, three mature and one young Pinus sylvestris forests along a site type gradient in southern Finland were chosen for measurements of fine root biomass, fine root longevity, below- and aboveground growth phenology and annual litter input from tree and understorey vegetation. Site types were distinguished by understorey vegetation composition, which indicated the site fertility. Fine root biomass per tree stand basal area decreased significantly from nutrient-poor to nutrient-rich sites, the nutrient-poor sites with longer fine root longevity resulted in an equal belowground litter input with the nutrient-rich site. Above- and belowground annual litter inputs were 131–236 and 70–91 g m−2 year−1, respectively. Aboveground litter increased with site fertility, resulting into belowground litter having a decreasing trend from 37% to 23% of total litter inputs with increasing site fertility. Ectomycorrhizal mycelia and understory production contributed 8–13% and 18–41% of belowground production, respectively. Contribution of understorey vegetation to the belowground litter C input was lower than that of trees at xeric and sub-xeric sites but equaled to that of trees at the mesic site. Our study showed distinct dimensions of below- and aboveground litter inputs influenced by site types. Moreover, we emphasize that the belowground C inputs from ectomycorrhizal mycelia and the understorey in addition to those of trees should always be considered in C balances and C reporting in boreal conifers.Peer reviewe