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

Does the age of fine root carbon indicate the age of fine roots in boreal forests?

To test the reliability of the radiocarbon method for determining root age, we analyzed fine roots (originating from the years 1985 to 1993) from ingrowth cores with known maximum root age (1 to 6 years old). For this purpose, three Scots pine (Pinus sylvestris L.) stands were selected from boreal forests in Finland. We analyzed root 14C age by the radiocarbon method and compared it with the above-mentioned known maximum fine root age. In general, ages determined by the two methods (root 14C age and ingrowth core root maximum age) were in agreement with each other for roots of small diameter (<0.5mm). By contrast, in most of the samples of fine roots of larger diameter (1.5-2mm), the 14C age of root samples of 1987-89 exceeded the ingrowth core root maximum age by 1-10 years. This shows that these roots had received a large amount of older stored carbon from unknown sources in addition to atmospheric CO2 directly from photosynthesis. We conclude that the 14C signature of fine roots, especially those of larger diameter, may not always be indicative of root age, and that further studies are needed concerning the extent of possible root uptake of older carbon and its residence time in roots. Keywords: fine root age, Pinus sylvestris, radiocarbon, root carbon, ingrowth cores, tree ringPeer reviewe

Fine root production and turnover of tree and understorey vegetation in Scots pine, silver birch and Norway spruce stands in SW Sweden

Fine roots contribute to net primary production in forests, but knowledge of fine root longevity and turnover is still incomplete and limited to few tree species. In this study, we used minirhizotrons to compare fine root biomass, longevity and turnover of Pinus sylvestris L., Betula pendula Roth and Picea abies (L) Karst. in southern Sweden. Minirhizotron tubes were installed in 2006 and root images were taken in 2007–2010. Soil cores were used to estimate fine root biomass. Soil samples were taken from the humus layer and from 0 to 10 cm, 10 to 20 cm and 20 to 30 cm depth in the mineral soil. Only images from the humus layer and the upper 10 cm of mineral soil were included in root analysis. Spruce has a higher aboveground production than pine and birch in southern Sweden and this was reflected in larger fine root biomass as well as higher fine root biomass production. The annual tree fine root biomass production (humus and 0–30 cm in mineral soil) was 73, 78 and 284 g m−2 in pine, birch and spruce stands, respectively. Thicker fine roots tended to live longer. The majority of the fine roots were thinner than 0.5 mm in diameter, with a turnover rate (KM) of 0.4 year−1. When comparing all fine roots, i.e. all roots 0–2 mm, pine had the highest longevity, 1120 days, compared with 900 days for spruce and 922 days for birch (KM)

Jämsän metsätiekokeissa käytettävän seostuhkan sisältämien aineiden huuhtoutuminen

Tutkimushankkeen tavoitteena oli selvittää liukoisten aineiden huuhtoutumista metsäteillä, joiden rakenteissa oli käytetty tuhkaa. Huuhtoutumista tutkittiin lysimetrien avulla tuhkan välittömässä läheisyydessä tien sisällä sekä syvemmällä tien rakenteissa. Lisäksi selvitettiin huuhtoutumisen vaikutusta tuhkaa sisältävän tien läpi kulkevan laskuojan veden laatuun. Tavoitteena oli saada tietoa tuhkan reagoimisesta tien läpi valuvan veden kanssa tierakenteessa, jotta tierakennetta voitaisiin mahdollisesti kehittää siihen suuntaan, että kemiallisia aineita vapautuisi tuhkakerroksista mahdollisimman vähän. Tuloksia voidaan jossain määrin käyttää taustatietona arvioitaessa laajemmin tuhkan käytön ympäristövaikutuksia, mutta tämä työ on ennen kaikkea ns. case-tutkimus ja edustaa vain tutkittuja aloja liittyen tierakenteiden tutkimukseen ja kehittämiseen. Tierakenteissa käytettiin lentotuhkaa, joka oli seostuhkaa. Tutkittavat metsätiet, joiden rakenteissa tuhkaa oli käytetty, sijaitsivat Jämsässä. Metsäteillä oli yhteensä 10 koealaa, joista viisi sijaitsi ensimmäisellä tiellä (Tie 1) ja viisi toisella tiellä (Tie 2). Metsätiekoealat koostuivat kontrollialasta, joka ei sisältänyt tierakenteessa tuhkaa, sekä neljästä erilaisesta tuhkarakenteesta: 15 cm:n paksuinen kerros tuhkaa ja mursketta suhteessa 33/66, 20 cm:n kerros tuhkaa ja mursketta suhteessa 50/50, 25 cm:n kerros tuhkaa, 50 cm:n kerros tuhkaa. Molemmilla teillä oli yksi ala kutakin käsittelyä. Teiden tuhkaa sisältävistä kerroksista vapautui ravinteita (B, Na, Ca, Mg, K) sekä sulfaattia ja kloridia tien sisällä alaspäin vajoavaan veteen. Näiden aineiden vapautuminen tuhkasta tulee huomioida tuhkaa käytettäessä. Tuhkan laadulla on vaikutusta ravinteiden ja suolojen vapautumiseen. Pitoisuuksien suuri vaihtelu tien sisäisissä vajovesissä vaikeutti yleistettävien tulosten saamista aineiden vapautumisesta vajoveteen. Myös erilaisten tuhkamäärien ja –rakenteiden eroja oli vaikea saada selville. Bariumin ja joidenkin raskasmetallien pitoisuudet olivat kuitenkin tien sisällä vajovesissä joissain tien kohdissa korkeita tuhka-aloilla verrattuna pelkkää mursketta sisältävään alaan. Toisaalta tiessä oli myös tuhka-aloilla samanlaisia pitoisuuksia kuin pelkän murskeen alalla, ja tällaiseen tilanteeseen olisi hyvä päästä tasaisesti kaikissa tien kohdissa. Tuhkan hyvä- ja tasalaatuisuus lienee avaintekijä matalille pitoisuuksille. Tuhkaa sisältävän tien vaikutukset ympäristöön riippuvat paitsi aineiden vapautumisesta vajoveteen, myös niiden uudelleen saostumisesta sekä vajoveden sekoittumisesta ympäröiviin vajo-, pohja- ja ojavesiin. Tässä työssä ympäristövaikutuksia ei tutkittu laajemmin, vaan huuhtoutumisen vaikutusta tutkittiin teiden läpi kulkevien laskuojien veden laadun osalta. Tuhkatien omat ojavedet yhtyivät näihin laskuojiin. Tuhkateiden läpi valuneen laskuojan veden laatuun tuhkatie ei vaikuttanut seurantajakson aikana bariumin ja mitattujen raskasmetallien osalta. Ravinnepitoisuudet (B, Na, Ca, Mg, K) sekä sulfaatin ja kloridin pitoisuudet sen sijaan nousivat laskuojassa tuhkatien vaikutuksesta, mutta veden laatu laskuojissa oli kuitenkin edelleen hyvä vaikutuksen jälkeenkin. Ojan hydrologiset olosuhteet vaikuttavat tuloksiin merkittävästi. Tuloksia voidaan pitää monilta osiltaan lupaavina tuhkarakenteiden kehittämisen kannalta, koska monien raskasmetallien keskimääräiset pitoisuudet olivat suhteellisen matalia useilla tuhkaa sisältävillä aloilla tai keskimääräiset pitoisuudet alenivat tierakenteessa veden kulkiessa alaspäin. Huomiota tulee kiinnittää niihin aineisiin, joissa esiintyi korkeita pitoisuuksia yksittäisinäkin arvoina tai yksittäisissäkin tien kohdissa (esim. Ba, As, Ni, Mo). Huuhtoutumisriski on mahdollisesti olemassa, mikäli korkeita pitoisuuksia tuottavia kohtia on tiessä monin paikoin, ja huuhtoutumista tapahtuu näistä paikoista suoraan ojiin tai pohjaveteen. Tierakenteita voidaan kehittää, jotta huuhtoutuminen olisi aina kaikista tien kohdista tasaista ja vähäistä. Tärkeitä kehittämis- ja tutkimuskohteita voivat olla mm. tuhkan tasalaatuisuuden ja tierakenteiden tiivistämisen merkitys huuhtoumille (tässä tutkimuksessa teitä ei tiivistetty erikseen rakentamisen yhteydessä). Rauta-, mangaani- ja kobolttipitoisuudet olivat korkeita teiden pohjatasoilla kaikilla aloilla, myös tuhkattomilla aloilla.201

Temperature and moisture dependence of daily growth of Scots pine (Pinus sylvestris L.) roots in southern Finland

Scots pine (Pinus sylvestris L.) is one of the most important conifers in northern Europe. In boreal forests, over one-third of net primary production is allocated to roots. Pioneer roots expand the horizontal and vertical root systems and transport nutrients and water from belowground to aboveground. Fibrous roots, often colonized by mycorrhiza, emerge from the pioneer roots and absorb water and nutrients from the soil. In this study, we installed three flatbed scanners to detect the daily growth of both pioneer and fibrous roots of Scots pine during the growing season of 2018, a year with an unexpected summer drought in southern Finland. The growth rate of both types of roots had a positive relationship with temperature. However, the relations between root elongation rate and soil moisture differed significantly between scanners and between root types indicating spatial heterogeneity in soil moisture. The pioneer roots were more tolerance to severe environmental conditions than fibrous roots. The pioneer roots initiated elongation earlier and ceased it later than the fibrous roots. Elongation ended when the temperature dropped below the threshold temperature of 4°C for pioneer roots and 6°C for fibrous roots. During the summer drought, the fibrous roots halted root surface area growth at the beginning of the drought, but there was no drought effect on the pioneer roots over the same period. To compare the timing of root production and the aboveground organs’ production, we used the CASSIA model, which estimates the aboveground tree carbon dynamics. In this study, root growth started and ceased later than growth of aboveground organs. Pioneer roots accounted for 87% of total root productivity. We suggest that future carbon allocation models should separate the roots by root types (pioneer and fibrous), as their growth patterns are different and they have different reactions to changes in the soil environment.Peer reviewe

Heterotrophic respiration and nitrogen mineralisation in soils of Norway spruce, Scots pine and silver birch stands in contrasting climates

Different tree species are often associated with different soil properties. Earlier studies have shown that Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), the two dominant tree species in Fennoscandia, often generate soils with larger carbon (C) and nitrogen (N) pools than silver birch (Betula pendula Roth.). Consequently, we hypothesised that spruce and pine would create soils with slower turnover rates than birch. To test this, C and N pools and C and N mineralisation rates were determined in different soil layers (humus, 0–10 cm, 10–20 cm mineral soil) at two sites with contrasting climatic conditions. One site (Tönnersjöheden) was located in the temperate zone in SW Sweden and one (Kivalo) in the north boreal zone in N Finland. At both sites, experimental plots with the three tree species had been established more than 50 years before the study. Samples from the different soil layers were incubated at 15 °C in the laboratory for 30 days, and C and N mineralisation rates were determined. In addition, earthworm abundance was estimated at Tönnersjöheden but not at Kivalo (no sign of bioturbation). At Tönnersjöheden, soil C and N pools (g C or N m−2) were ranked spruce > pine > birch. C mineralisation rate (mg CO2–C g−1 C d−1) was higher in the birch plots than in the other plots, but because of larger C pools in the spruce plots, field C mineralisation (g CO2–C m−2 year−1) was higher for spruce than for pine and birch. Field net N mineralisation (80–90 kg N ha−1 year−1) did not differ significantly between tree species, but nitrification rates (μg NO3–N g−1 C d−1) in the topsoil were higher in the birch plots than in the other plots. The birch plots had larger populations of earthworms and a higher degree of bioturbation than any of the coniferous plots, which probably explains the higher turnover rate of birch soil organic matter (SOM). At Kivalo, C and N soil pools were significantly larger in spruce than in birch plots, and C mineralisation rate was higher in birch and spruce humus than in pine humus. Net N mineralisation rate and annual field net N mineralisation (<4 kg N ha−1 year−1) were estimated to be very low, with no effect of tree species. Thus, the hypothesis of a ‘birch effect’ was supported at Tönnersjöheden, but only partly at Kivalo. The main difference seemed to be that the earthworms at Tönnersjöheden accelerated SOM decomposition under birch, whereas earthworm stimulation was negligible at Kivalo, probably because of climate-related limitations

Open-top chamber fumigation system for exposure of field grown Pinus sylvestris to elevated carbon dioxide and ozone concentration

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Impacts of timber forwarding on physical properties of forest soils in southern Finland

Forest harvesting activities can cause soil damage and disturbance through soil compaction, rut formation and soil mixing. These affect the soil structure and functions and forest productivity. Soil compaction results for instance in increased bulk density and decreased porosity, affecting soil moisture, water infiltration and aeration. The effects of timber forwarding on soil physical properties have gained little attention in boreal forests. These issues will become more important in the future since harvesting operations on unfrozen soils are getting more common due to the anticipated climate warming. In this study, changes of forest soil physical properties (bulk density, moisture content and porosity) after 1-10 forwarder passes on two fine-grained mineral soil sites in southern Finland were analysed. Penetration resistance and rut formation were also measured. The measurements were performed in three periods with different soil moisture conditions. The test drives were carried out with a conventional 8-wheeled forwarder with total mass of 29.8 tons. Soil bulk density increased and porosity decreased after the machinery passes. However, soil moisture content increased on one site and mainly decreased on another. The first three passes caused the greatest compaction and rutting, the first pass having the strongest impact. After the first and third pass 34-55% and over 70% of the total mean rut depth was formed, respectively. Further passes caused only minor rutting. The compaction and changes of soil physical properties appeared to be greater in dry conditions. Rut formation and soil mixing were greater in moist conditions. The results are, however, site-specific, and more research is needed to achieve a better understanding of the relationships between different factors affecting impacts of timber forwarding on soil.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