Drivers of annual variation in tree growth and forest sensitivity to storm damage in Finland

In this thesis, my aim is to study the drivers of tree growth variation and forest predisposition to storm disturbance in Finland. More specifically, the thesis aims to answer the following research questions: (1) What is the role of tree provenance in the climatic control of radial growth variation in Norway spruce? (2) How do weather conditions outside of growing season affect radial growth variation in Norway spruce and Scots pine? (3) How are forest properties, forest management and abiotic environmental factors connected to the storm damage probability of forest stands and individual trees? (4) Do the same factors affect stand-level damage probability in different storm types: autumn extra-tropical cyclones and summer thunder storms? (5) Is fine-scale topographic information connected to tree-level storm damage probability? The thesis addresses these questions by analyzing extensive empirical data sets. The different climatic drivers of Norway spruce provenances were studied using a tree-ring data from seven Norway spruce provenance experiments in Finland, established already in the 1930s and located in different climatic conditions and containing a large variety of provenances. The effects of non-growing season climatic conditions on tree-growth were studied by comparing tree-ring data from unmanaged forests with variables describing winter conditions and modelled tree frost hardiness levels. Storm damage probability on stand and tree levels was examined with storm damage data sets collected at Finnish National Forest Inventory plots after major storms. A statistical modeling approach was used throughout the thesis, utilizing methods such as generalized mixed effects models and statistical distributions of extreme values. The results revealed provenance differences in radial growth variation in Norway spruce. Provenances differed most in their growth response to winter temperature, as adaptation to low winter temperatures was weaker in Central European than in Northern European provenances. While cold winter temperatures were associated with frost damage and declined radial growth in Central European spruce provenances transferred north, simple temperature variables were not sufficient in studying the responses of trees to conditions outside of the growing season in natural forests. Instead, the results showed signs of reduced growth after events of insufficient frost hardiness levels and winters with high frost sum of snowless days. This indicates that accounting for a complexity of factors, such as frost hardiness of trees, snow cover and soil frost, is needed to understand the implications of weather conditions outside of growing season to tree growth. Stand-level damage probability was affected by stand characteristics and previous management operations. On tree-level, damage probability was connected to type of the tree species (conifer or broad-leaved) and tree height as well as recent changes in wind exposure and wood decay in the stand. Storm damage probability in autumn storms and summer thunder storms was affected by similar factors, and the similarities were clearest in the effects of forest management history and topography. However, due to the limitations of the data, the results may have missed subtler differences between the storm types. Topography was associated with storm damage probability on both stand and individual tree level. The results also show that high-resolution topographical information, describing the local topography near the tree, can improve models of tree-level storm damage probability.Väitöskirjassani tutkin puun kasvun vaihteluun sekä metsien myrskytuhoherkkyyteen vaikuttavia tekijöitä. Väitöskirja pyrkii vastaamaan seuraaviin tutkimuskysymyksiin: (1) Miten sääolojen vaikutus puun kasvuun eroaa kuusen eri eurooppalaisilla alkuperillä? (2) Miten kasvukauden ulkopuoliset sääolot vaikuttavat puun paksuuskasvuun kuusella ja männyllä? (3) Miten metsän ominaisuudet, metsänhoito ja ympäristötekijät vaikuttavat myrskytuhon todennäköisyyteen metsikön ja yksittäisen puun tasolla? (4) Vaikuttavatko samat tekijät myrskytuhoherkkyyteen erityyppisissä myrskyissä? (5) Onko pienen mittakaavan topografinen vaihtelu yhteydessä myrskytuhon todennäköisyyteen yksittäisen puun tasolla? Näihin tutkimuskysymyksiin paneudutaan analysoimalla kattavia empiirisiä aineistoja. Eri kuusen alkuperien erilaisia kasvuvasteita sääolojen vaihteluun tutkittiin analysoimalla puulustoaineistoa, joka on kerätty eri puolelle Suomea jo 1930-luvulla istutetuista alkuperäkokeista. Kasvukauden ulkopuolisten sääolojen vaikutuksia tutkittiin vertaamalla luonnontilaisista metsistä kerättyä puulustoaineistoa talviolosuhteita ja puiden mallinnettua kylmänkestävyyttä kuvaaviin muuttujiin. Metsien myrskytuhoja tutkimiseen käytettiin Valtakunnan metsien inventoinnin koealoilta dokumentoitua tuhotietoja voimakkaiden myrskyjen jälkeen. Aineistojen analysoinnissa hyödynnettiin tilastollisen mallintamisen menetelmiä. Tulokset paljastivat eroja kasvunvaihtelussa kuusen alkuperien välillä. Suurimmat erot alkuperien välillä liittyivät talvilämpötiloihin – keskieurooppalaiset alkuperät kasvoivat huonommin kylmien talvien jälkeen, kun taas suomalaiset alkuperät ovat paremmin sopeutuneita kylmään ilmastoon. Myös luonnontilaisten metsien puulustoaineistojen analysointi näytti, että kylmät talvet eivät olleet yhteydessä kasvun heikkenemiseen paikallista alkuperää olevilla puilla. Sen sijaan mahdollisesti riittämätön kylmänkestävyyden taso ja vähälumiset mutta kylmät talvet olivat joissain tapauksissa yhteydessä heikentyneeseen kasvuun. Tämä osoittaa, että pelkkien lämpötilamuutosten huomioiminen ei riitä, kun tutkitaan muuttuvien talviolosuhteiden vaikutuksia metsiin. Myrskytuhon todennäköisyys metsissä liittyi puuston ominaisuuksiin, aiempiin metsänhoitotoimenpiteisiin sekä paikallisiin tuuliolosuhteisiin vaikuttaviin tekijöihin. Tulokset osoittivat samojen tekijöiden vaikuttavan tuhon todennäköisyyteen tutkituissa myrskytyypeissä, eli kesäisissä ukkosmyrskyissä ja voimakkaisiin matalapaineisiin liittyvissä syysmyrskyissä. Etenkin metsänhoitohistorian ja topografian vaikutukset olivat eri myrskytyypeillä samanlaisia. Topografia oli yhteydessä myrskytuhon todennäköisyyteen niin metsikkö- kuin puutasolla. Tulokset osoittivat, että korkearesoluutioisten korkeusmallien avulla voidaan parantaa puutason myrskytuhomalleja

Topografian, maaperän ja geomorfologian vaikutukset putkilokasvien biomassan vaihteluun oroarktisella tunturikankaalla

Climate change is predicted to cause changes in vegetation productivity and biomass in northern areas. Understanding the factors affecting biomass helps us to estimate the future changes in plant communities and the amount of carbon stored in vegetation. In this thesis I examine the effects of soil properties (soil moisture, temperature, pH and calcium), geomorphological processes and topography on the small scale variation of aboveground vascular biomass. I also compare two biomass variables: the dry weight of a biomass sample and the vegetation volume. The data was collected during summer 2011 on the northern slopes of fell Saana, north western Finland. Modeling was done with generalized linear models (GLM) and also with generalized estimation equations (GEE) since they are able to take spatial autocorrelation into account. Variation partitioning was used to find out the single and combined effects of variable groups (soil, geomorphology and topography). The connections between the most common plant species and the biomass variables were also studied. Soil properties have a stronger impact on both biomass variables. Soil group alone explains 7 % of variation in the dry weight of biomass and 20 % of variation in vegetation volume. Geomorphology explains 4,7 % of variation in the biomass dry weight and 14,5 % in vegetation volume. Topography has the least effect on the biomass variables (2,2 % and 2,0 %). The combined effects of groups are also relatively strong, e.g. the combined effects of geomorphology and topography explain 9,7 % of variation in vegetation volume. Models with vegetation volume as a response variable were able to explain more of the variation (unexplained variation 48,7 %) than models of biomass dry weight (67,1 %). Also the relative importance of the variable groups differed for the two biomass variables, e.g. the soil group affected vegetation more than biomass dry weight. Both biomass variables correlated strongly with the two species with the largest coverage on the study site, crowberry (Empetrum nigrum ssp. hermaphroditum) and dwarf birch (Betula nana). The correlation of crowberry with vegetation volume was stronger than with biomass dry weight. Even though the models explain only about half of the variation, results suggest that soil properties and geomorphological processes have a clear impact on vegetation biomass. The impact of topography is strongly linked to other variable groups. A large amount of the unexplained variation and the importance of the combined effects of variable groups are due to the complexity of the factors controlling vegetation biomass. This makes it more complicated to predict the future changes of vegetation biomass. Changes will depend on how rising temperatures and increasing rainfall will affect not only soil properties and geomorphology but also all the factors that are behind the unexplained variation of biomass in the models.Ilmastonmuutoksen odotetaan muuttavan kasvillisuuden tuottavuutta ja biomassaa pohjoisilla alueilla. Kasvillisuuden biomassan taustalla olevien tekijöiden ymmärtäminen auttaa ennakoimaan kasvillisuuden ja sen sitoman hiilen määrän muutoksia tulevaisuudessa. Tässä tutkimuksessa selvitetään, miten maaperän ominaisuudet (kosteus, lämpötila, pH, kalsium), geomorfologiset prosessit sekä topografia vaikuttavat putkilokasvien maanpäällisen biomassan pienen mittakaavan vaihteluun. Tämän lisäksi vertaillaan kahta biomassaa kuvaavaa muuttujaa: biomassanäytteen kuivapainoa sekä kasvillisuuden tilavuutta. Aineisto kerättiin kesällä 2011 Käsivarren Lapissa, Saanan pohjoisrinteellä. Mallinnuksessa käytettiin yleistettyjä lineaarisia malleja (generalized linear models, GLM) sekä spatiaalisen autokorrelaation huomioon ottavia GEE-malleja (generalized estimation equations). Hajonnan osituksella selvitettiin maaperän, geomorfologian ja topografian yksittäis- ja yhteisvaikutuksia biomassalle. Biomassamuuttujia tarkasteltiin myös suhteessa tutkimusalueella peittävyydeltään merkittävimpiin kasvilajeihin. Maaperän ominaisuudet nousivat tärkeimmäksi muuttujaryhmäksi molemmilla vastemuuttujilla, maaperämuuttujien yksittäisvaikutuksen selittäessä biomassan kuivapainolla 7 % vaihtelusta ja kasvillisuuden tilavuudella 20 % . Geomorfologisten muuttujien merkitys oli toisiksi suurin (4,7 % ja 14,5 %) ja topografiaryhmän merkitys molemmilla muuttujaryhmillä pienin (2,2 % ja 2,0 %). Muuttujaryhmien yhteisvaikutukset olivat myös huomattavia, mm. topografian ja geomorfologian yhteisvaikutus selitti 9,7 % kasvillisuuden tilavuuden vaihtelusta. Biomassamuuttujien välillä oli selkeitä eroja. Kasvillisuuden tilavuuden ollessa vastemuuttujana jäi selittämättömän vaihtelun osuus huomattavasti pienemmäksi (48,7 %) kuin biomassan kuivapainon kohdalla (67,1 %). Myös muuttujaryhmien suhteellisessa tärkeydessä oli eroja biomassamuuttujilla maaperäryhmän merkityksen korostuessa kasvillisuuden tilavuudella. Molemmat biomassamuuttujat korreloivat voimakkaasti peittävyydeltään suurimpien lajien, pohjanvariksenmarjan (Empetrum nigrum ssp. hermaphroditum) ja vaivaiskoivun (Betula nana) kanssa. Pohjanvariksenmarja korreloi voimakkaammin kasvillisuuden tilavuuden kuin biomassan kuivapainon kanssa. Vaikka mallit selittävät kasvillisuuden tilavuudenkin kohdalla vain noin puolet vastemuuttujan vaihtelusta, voidaan tuloksista päätellä, että maaperän ominaisuuksilla ja geomorfologisilla prosesseilla on selvä merkitys kasvillisuuden biomassalle. Topografian merkitys biomassalle taas on vahvasti yhteydessä muihin muuttujaryhmiin. Selittämättömän hajonnan suuri osuus sekä muuttujaryhmien välisten yhteisvaikutusten tärkeys kertovat kasvillisuuden biomassan taustalla olevien tekijöiden monimutkaisuudesta, mikä vaikeuttaa ympäristön muutoksen vaikutuksien ennakointia. Kasvillisuuden biomassan muutokset tulevaisuudessa riippuvat siitä, miten nousevat lämpötilat ja sademäärät vaikuttavat maaperän ominaisuuksiin ja geomorfologisten prosessien aktiivisuuteen sekä kaikkiin niihin tekijöihin, jotka vastaavat mallinnuksessa selittämättömäksi jäävästä biomassan vaihtelusta

Geographical patterns in the radial growth response of Norway spruce provenances to climatic variation

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Arctic-alpine vegetation biomass is driven by fine-scale abiotic heterogeneity

During recent decades large changes in vegetation biomass have been observed in arctic and alpine areas. While these temporal trends have been clearly linked to changing climatic conditions, the drivers of local spatial variation in biomass are still relatively poorly understood. Thus, we examine the effects of abiotic conditions (as measured by ten variables representing topography, soil properties and geomorphological processes) on variation in aboveground vascular plant biomass to understand the determinants of contemporary fine scale heterogeneity in this variable. We also compare the results from one destructive biomass estimation method (clipharvesting) to three non-destructive biomass estimates: vegetation cover, height and volume. To investigate the local drivers of biomass we analysed an extensive data set of 960 1 m2 cells in arctic–alpine tundra using spatially-explicit generalized estimation equations to conduct variation partitioning. The abiotic environment had a clear impact on the fine scale distribution of biomass (variance explained 32.89 % with full model for sampled biomass). Soil properties (temperature, moisture, pH and calcium content) were most strongly related to aboveground biomass (independent effect in variation partitioning 7.03 % and combined effect including joined effects with topography and geomorphology 19.6 %). Topography had only a small influence after soil and geomorphology were taken into account (independent effect only 2.23 % and combined effect 18.73 %), implying that topography has only indirect effects on vegetation biomass. Of the three non destructive biomass estimates, the results for vegetation volume were most similar to those for clipharvested biomass samples. Thus, we recommend utilizing vegetation volume as a cost-efficient and robust non-destructive biomass estimate in arctic-alpine areas. Our results indicate that the fine scale environmental variation has to be taken into account more carefully when modelling vegetation biomass and carbon budget, especially under changing climatic conditions.Academy of Finland (Project Number 1140873).http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1468-04592015-04-30hb201

Connecting potential frost damage events identified from meteorological records to radial growth variation in Norway spruce and Scots pine

The aim of this study was to examine if temperature conditions potentially causing frost damage have an effect on radial growth in Norway spruce and Scots pine in Finland. We hypothesized that frost damage occurs and reduces radial growth after (1) extreme cold winter temperatures, (2) frost hardiness levels insufficient to minimum temperatures, and (3) the lack of insulating snow cover during freezing temperatures, resulting in increased frost and decreased temperatures in soil. Meteorological records were used to define variables describing the conditions of each hypothesis and a dynamic frost hardiness model was used to find events of insufficient frost hardiness levels. As frost damage is likely to occur only under exceptional conditions, we used generalized extreme value distributions to describe the frost variables. Our results did not show strong connections between radial growth and the frost damage events. However, significant growth reductions were found at some Norway spruce sites after events insufficient frost hardiness levels, and alternatively, after winters with high frost sum of snowless days. Scots pine did not show significant growth reductions associated with any of the studied variables. Thus, radial growth in Norway spruce may be more sensitive to future changes in winter conditions. Our results demonstrate that considering only temperature is unlikely to be sufficient in studying winter temperature effects on tree growth. Instead, understanding the effects of changing temperature and snow conditions in relation to tree physiology and phenology is needed.201

High-resolution topographical information improves tree-level storm damage models

Storms cause major forest disturbances in Europe. The aim of this study was to model tree-level storm damage probability based on the properties of a tree and its environment and to examine whether fine-scale topographic information is connected to the damage probability. We used data documenting effects of two autumn storms on over 17 000 trees on permanent Finnish National Forest Inventory plots. The first storm was associated with wet snowfall that damaged trees, while exceptionally strong winds and gusts characterized the second storm. During the storms, soils were unfrozen and deciduous trees were without leaves. Generalized linear mixed models were used to study how topographical variables calculated from digital elevation models (DEM) with resolutions of 2 and 10 m (TOPO2 and TOPO10, respectively) were related to damage probability, in addition to variable groups for tree (TREE) and stand (STAND) characteristics. We compared models containing different variable groups with Akaike information criteria. The best model contained the variable groups TREE, STAND, and TOPO2. Increase in slope steepness calculated from the high-resolution DEM decreased tree-level damage probability significantly in the model. This suggests that the local topography affects the tree-level damage probability and that high-resolution topographical data improves the tree-level damage probability models.Peer reviewe

Quantifying the impact of key factors on the carbon mitigation potential of managed temperate forests

Background: Forests mitigate climate change by reducing atmospheric CO2-concentrations through the carbon sink in the forest and in wood products, and substitution effects when wood products replace carbon-intensive materials and fuels. Quantifying the carbon mitigation potential of forests is highly challenging due to the influence of multiple important factors such as forest age and type, climate change and associated natural disturbances, harvest intensities, wood usage patterns, salvage logging practices, and the carbon-intensity of substituted products. Here, we developed a framework to quantify the impact of these factors through factorial simulation experiments with an ecosystem model at the example of central European (Bavarian) forests. Results: Our simulations showed higher mitigation potentials of young forests compared to mature forests, and similar ones in broad-leaved and needle-leaved forests. Long-lived wood products significantly contributed to mitigation, particularly in needle-leaved forests due to their wood product portfolio, and increased material usage of wood showed considerable climate benefits. Consequently, the ongoing conversion of needle-leaved to more broad-leaved forests should be accompanied by the promotion of long-lived products from broad-leaved species to maintain the product sink. Climate change (especially increasing disturbances) and decarbonization were among the most critical factors influencing mitigation potentials and introduced substantial uncertainty. Nevertheless, until 2050 this uncertainty was narrow enough to derive robust findings. For instance, reducing harvest intensities enhanced the carbon sink in our simulations, but diminished substitution effects, leading to a decreased total mitigation potential until 2050. However, when considering longer time horizons (i.e. until 2100), substitution effects became low enough in our simulations due to expected decarbonization such that decreasing harvests often seemed the more favorable solution. Conclusion: Our results underscore the need to tailor mitigation strategies to the specific conditions of different forest sites. Furthermore, considering substitution effects, and thoroughly assessing the amount of avoided emissions by using wood products, is critical to determine mitigation potentials. While short-term recommendations are possible, we suggest risk diversification and methodologies like robust optimization to address increasing uncertainties from climate change and decarbonization paces past 2050. Finally, curbing emissions reduces the threat of climate change on forests, safeguarding their carbon sink and ecosystem services

Tutkimus laittaa metsien tuulituhoriskit kartalle

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2. Myrskytuhot vuonna 2018 ja uusi työkalu tuulituhoille alttiiden metsien tunnistamiseen

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