Remote sensing phenology at European northern latitudes - From ground spectral towers to satellites

Plant phenology exerts major influences on carbon, water, and energy exchanges between atmosphere and ecosystems, provides feedbacks to climate, and affects ecosystem functioning and services. Great efforts have been spent in studying plant phenology over the past decades, but there are still large uncertainties and disputations in phenology estimation, trends, and its climate sensitivities. This thesis aims to reduce these uncertainties through analyzing ground spectral sampling, developing methods for in situ light sensor calibration, and exploring a new spectral index for reliable retrieval of remote sensing phenology and climate sensitivity estimation at European northern latitudes. The ground spectral towers use light sensors of either nadir or off-nadir viewing to measure reflected radiation, yet how plants in the sensor view contribute differently to the measured signals, and necessary in situ calibrations are often overlooked, leading to great uncertainties in ground spectral sampling of vegetation. It was found that the ground sampling points in the sensor view follow a Cauchy distribution, which is further modulated by the sensor directional response function. We proposed in situ light sensor calibration methods and showed that the user in situ calibration is more reliable than manufacturer’s lab calibration when our proposed calibration procedures are followed. By taking the full advantages of more reliable and standardized reflectance, we proposed a plant phenology vegetation index (PPI), which is derived from a radiative transfer equation and uses red and near infrared reflectance. PPI shows good linearity with canopy green leaf area index, and is correlated with gross primary productivity, better than other vegetation indices in our test. With suppressed snow influences, PPI shows great potentials for retrieving phenology over coniferous-dominated boreal forests. PPI was used to retrieve plant phenology from MODIS nadir BRDF-adjusted reflectance at European northern latitudes for the period 2000-2014. We estimated the trend of start of growing season (SOS), end of growing season (EOS), length of growing season (LOS), and the PPI integral for the time span, and found significant changes in most part of the region, with an average rate of -0.39 days·year-1 in SOS, 0.48 days·year-1 in EOS, 0.87 days·year-1 in LOS, and 0.79%·year-1 in the PPI integral over the past 15 years. We found that the plant phenology was significantly affected by climate in most part of the region, with an average sensitivity to temperature: SOS at -3.43 days·°C-1, EOS at 1.27 days·°C-1, LOS at 3.16 days·°C-1, and PPI integral at 2.29 %·°C-1, and to precipitation: SOS at 0.28 days∙cm-1, EOS at 0.05 days∙cm-1, LOS at 0.04 days∙cm-1, and PPI integral at -0.07%∙cm-1. These phenology variations were significantly related to decadal variations of atmospheric circulations, including the North Atlantic Oscillation and the Arctic Oscillation. The methods developed in this thesis can help to improve the reliability of long-term field spectral measurements and to reduce uncertainties in remote sensing phenology retrieval and climate sensitivity estimation

An Optical Sensor Network for Vegetation Phenology Monitoring and Satellite Data Calibration

We present a network of sites across Fennoscandia for optical sampling of vegetation properties relevant for phenology monitoring and satellite data calibration. The network currently consists of five sites, distributed along an N-S gradient through Sweden and Finland. Two sites are located in coniferous forests, one in a deciduous forest, and two on peatland. The instrumentation consists of dual-beam sensors measuring incoming and reflected red, green, NIR, and PAR fluxes at 10-min intervals, year-round. The sensors are mounted on separate masts or in flux towers in order to capture radiation reflected from within the flux footprint of current eddy covariance measurements. Our computations and model simulations demonstrate the validity of using off-nadir sampling, and we show the results from the first year of measurement. NDVI is computed and compared to that of the MODIS instrument on-board Aqua and Terra satellite platforms. PAR fluxes are partitioned into reflected and absorbed components for the ground and canopy. The measurements demonstrate that the instrumentation provides detailed information about the vegetation phenology and variations in reflectance due to snow cover variations and vegetation development. Valuable information about PAR absorption of ground and canopy is obtained that may be linked to vegetation productivity

Focus on recent, present and future Arctic and boreal productivity and biomass changes

The reduction of cold temperature constraints on photosynthesis in recent decades has led to extended growing seasons and increased plant productivity (greening) in significant parts of Polar, Arctic and Boreal regions, here called northern lands. However, most territories within these regions display stable productivity in recent years. Smaller portions of Arctic and Boreal regions show reduced productivity (browning). Summer drought and wildfires are the best documented drivers causing browning of continental areas. Yet factors like winter warming events dampening the greening effect of more maritime regions have remained elusive, least monitored and least understood. ANorway-US network project called ArcticBiomass was launched in 2013 to further reveal both positive and negative effects of climate change on biomass in Arctic and Boreal regions. This focus collection named Focus on Recent, Present and Future Arctic and Boreal Productivity and Biomass Changes includes 24 articles and is an important outcome of this work and addresses recent changes in phenology, biomass and productivity and the mechanisms. These mechanisms include former human interactions (legacies) and drivers that control such changes (both greening and browning), along with consequences for local, regional and global scale processes.Wecomplete our synthesis by stressing remaining challenges and knowledge gaps, and provide an outlook on future needs and research questions in the study of climate and human driven interactions in terrestrial Arctic and Boreal ecosystems.publishedVersio

Mapping moth induced birch forest damage in northern Sweden, with MODIS satellite data

Large synchronous outbreaks of herbivory geometrids is regularly occurring at 9-10 years intervals when they reach peak densities in the Fennoscandian birch forest, in the northern part of Scandinavia (Tenow 1972, Bylund 1995). Climate change is likely to increase the frequency, intensity and extent of the outbreak due to increasing temperatures in the area (Callaghan 2010, Heliasz et al. 2011, Wolf et al. 2008). The consequence is a detrimental effect to the birch forest since the forest might not have enough time to recover between outbreaks, which will potentially decrease the proliferation and distribution of birch forest (Tenow et al. 2001, 2003, Karlsson et al 2004). This will have an ecological cost by making the forest inhabitable and non-resourceful for the animals and people that depend on it (Helle 2001). However, the effects on the Birch forest are not well known, therefore it is important to continue studying the distribution of forest damage, to gain a better understanding of its dynamics and the underlying spatio-temporal factors controlling the synchronous outbreaks. The most current year of infestation in the study area of the surroundings of lake Torneträsk in northern Sweden was 2012. To map the distribution and dynamics of the geometrids of the birch forest, time series data of MODIS 16-day NDVI composites were analyzed. To facilitate the analysis, a tree cover map with high resolution was created based on Lidar data. The Lidar based forest cover map was created to mask the forest. The topographical distribution of infested forest at four altitudinal intervals with 100 meter equidistance in between was also studied. A method was developed in this study to separate infested from non-infested forest with a threshold value based on z-score, which was successful at showing the distribution of the geometrid outbreak in 2012. The size of the infested area was 80km², equal to 54.3% of the forest in the area. If the forest classified as “likely infested” would have been included, the ratio of infested forest would increase to 64.4%. This is a significant proportion of the forest that will certainly affect the forest in future years. The topographical distribution of the infestation over the study area was relatively evenly distributed, without displaying any range of altitude that was more prone to infestation.Stora synkrona utbrott av björkmätare förekommer vart 9-10 år i de norra delarna av Skandinavien i den Fenoskandiska björkskogen. Björkmätaren konsumerar bladen och kan orsaka stora skador under ett toppår. Man misstänker att klimatförändringarna kommer att öka utbrottens omfattning och intensitet på grund av ökad temperatur i området. Konsekvenserna kan bli förödande för björkskogen, eftersom tiden mellan utbrotten riskerar att bli för korta för skogen att återhämta sig. Det kan resultera i att träden dör och skogen försvinner i de värst drabbade områdena. Om skogen minskar eller försvinner kan det få ekologiska konsekvenser för de djur som är beroende av den och människor som utnyttjar dess resurser. Hur skogen kommer att påverkas vet man inte säkert, därför är det viktigt att studera utbredningen av skogsskador och på så sätt få bättre kunskap om de underliggande faktorerna som kontrollerar utbrottens dynamik. Ett utbrott inträffade sommaren 2012 i studieområdet vilket var indelat i 2 olika stora överlappande område; Ett större som täcker området runt sjön Torneträsk i norra Sverige och ett mindre som täcker den västra delen av sjön runt Abisko. För att kartlägga omfattningen av björmatarutbrottet användes tidsserier av MODIS 16-dagars NDVI kompositer som analyserades men en utvecklad förändrings analys för projektet. Som en del i analysen skapades en högupplöst skogsutbredningskarta med Lidar data. Den topografiska utbredningen av utbrotten studerades också genom att dela in området i fyra höjdintervall med 100 meters ekvidistans mellan intervallen. För att identifiera skadade område utvecklades en metod baserat på standardiserade z-värde och definierade gränsvärde för klassificeringen av skogen. Det angripna området i Torneträsk var 251 km² stort, motsvarande 32% av skogen. I studieområdet i Abisko var utbrottet 80km2 stort, vilket motsvarar 54.3% av skogen i studieområdet. Om skogen klassad som ”troligen angripen” inkluderas i beräkningen, skulle den del av skogen som var angripen ökas ytterligare. Det är en signifikant andel skog som med stor sannolikhet kommer att påverka skogen i flera år. Den topografiska utbredningen av utbrottet i studieområdet var relativt jämnt fördelat, inget höjdintervall visade sig vara extra känsligt för att bli angripen

Development of a method for monitoring of insect induced forest defoliation - limitation of MODIS data in Fennoscandian forest landscapes

We investigated if coarse-resolution satellite data from the MODIS sensor can be used for regional monitoring of insect disturbances in Fennoscandia. A damage detection method based on z-scores of seasonal maximums of the 2-band Enhanced Vegetation Index (EVI2) was developed. Time-series smoothing was applied and Receiver Operating Characteristics graphs were used for optimisation. The method was developed in fragmented and heavily managed forests in eastern Finland dominated by Scots pine (Pinus sylvestris L.) (pinaceae) and with defoliation of European pine sawfly (Neodiprion sertifer Geoffr.) (Hymenoptera: Diprionidae) and common pine sawfly (Diprion pini L.) (Hymenoptera: Diprionidae). The method was also applied to subalpine mountain birch (Betula pubescens ssp. Czerepanovii N. I. Orlova) forests in northern Sweden, infested by autumnal moth (Epirrita autumnata Borkhausen) and winter moth (Operophtera brumata L.). In Finland, detection accuracies were fairly low with 50% of the damaged stands detected, and a misclassification of healthy stands of 22%. In areas with long outbreak histories the method resulted in extensive misclassification. In northern Sweden accuracies were higher, with 75% of the damage detected and a misclassification of healthy samples of 19%. Our results indicate that MODIS data may fail to detect damage in fragmented forests, particularly when the damage history is long. Therefore, regional studies based on these data may underestimate defoliation. However, the method yielded accurate results in homogeneous forest ecosystems and when long-enough periods without damage could be identified. Furthermore, the method is likely to be useful for insect disturbance detection using future medium-resolution data, e. g. from Sentinel-2.Peer reviewe

Klimabedingte Wachstumsreaktionen und Baumgrenzverlagerungen borealer Koniferen im alpinen und polaren Baumgrenzökoton Finnisch-Lapplands

At the northern margins of the boreal regions bordering the sub-Arctic, trees as a life form grow close to the limit of their ecological range and have to cope with low temperatures, low nutrient supply, and sparse light conditions during winter. The growing season lasts less than five months during which trees need to pass through all vegetative and reproductive stages. In the transition zone from closed forests to bare fell tops and open tundra, conifers form the outermost edge of their distribution area in the tree-line ecotone, which is characterized by harsh climatic conditions and disturbance regimes, challenging tree growth by frost, wind and snow load. The mortality rate is high and sequences of several favourable years are needed to grow and successfully establish sustainably high seed crops for natural regeneration. Here, temperature is found to be the limiting parameter for growth and regeneration, hence a temperature rise under current warming is expected to considerably improve the growing conditions for conifers (Kauppi et al. 2014; Salminen and Jalkanen 2015). Under mild winters, early springs and increasing summer temperatures, trees and shrubs are predicted to establish more successfully, regionally replacing graminoids in the alpine oroarctic tundra by higher vegetation (Juntunen et al. 2002; Jia et al. 2003; Goetz et al. 2011; Jeong et al. 2012; Walker et al. 2012; Pearson et al. 2013; Aakala et al. 2014). An expansion of conifers beyond the recent tree-line position may affect the microclimate and carbon fluxes of the ecosystem, potentially influencing large-scale circulation changes in Arctic regions (Jeong et al. 2012; Miller and Smith 2012; Pearson et al. 2013; Zhang et al. 2013). Estimating vegetation shifts in these regions is consequently of high scientific interest and is also in the focus of the present project. In Finnish Lapland, a monitoring project was established already in 1983 to monitor regeneration and growth of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) H. Karst.) regularly in five-year intervals along elevation gradients aligned along a North-South transect. Thereby, changes in the regeneration success, mortality and volume of growing stock of conifers in a changing environment were to be detected. The analysis was, however, restricted on the local plots and time frames, lacking a continuous and large-scale analysis of tree-line changes. The aim of the present study was therefore to supplement the monitoring study by dendroecological sampling and remote sensing. In a first step, the outcomes of the monitoring project during 1983–2009 were analysed and published in cooperation with the Natural Resources Institute Finland Luke (publication I). Afterwards, a dendroecological sampling was performed on six of the pine-dominated sites to measure growth rates and to create long-term site chronologies. Climate-growth responses of pine was analysed by computing Pearson correlation functions with relevant climatic parameters. Thereby, inter-annual growth variations and long-term growth trends were analysed and compared between mature and juvenile trees and between a northern and southern region (publication II). Finally, satellite images were acquired for the pine-dominated sites and analysed for large-scale vegetation changes around the monitoring sites (publication III). This was done by using the Normalized Difference Vegetation Index (NDVI) and a land cover classification using Random Forests. The monitoring study indicated increasing volumes of growing stock for both spruce- and pine-dominated stands in all of the studied sites and elevation zones. An increase in height and diameter of adult trees (> 2 m) was assumed to exceed the mortality rate, leading towards a densification of the established forest stands. The number of tree stems, saplings and seedlings increased also in nearly all spruce-dominated sites, predominantly in the open stands of higher elevations. The stem numbers of pine stagnated or even decreased in all locations and elevation zones, pointing towards a high mortality rate and high sensitivity of pine seedlings to abiotic and biotic disturbances. The dendroecological analysis revealed that stand structure and thermal conditions during the growing season affect pine-tree growth, especially during juvenile ages. Radial growth rates correlated highly negative with the occurrence of cold and frost days during the onset of the growing season predominantly in the north of the study region, while the impact of temperature diminished at the more rapidly warming southern sites. The site chronologies showed growth responses to climatic variations until the 1980s, but not during the current warming period. Increasing radial growth trends could be detected since 2000 in the juvenile trees of the southern sites, while the mature and northern trees did not respond significantly to the current warming. We assume that warmer and wetter conditions during winter, inducing high snow loads, wind damages, diseases and frost damage during spring, to possibly counteract the benefits of climate warming. The applied remote sensing approaches included NDVI change detection and land-cover classification. However, neither method revealed clear trends for advancing conifer tree lines towards open fell tops or treeless heath vegetation. Instead, we found evidence for densification of open forest stands at lower elevations and an expansion of deciduous vegetation at higher elevations into previously vegetation-free or sparsely covered fell tops. Increasing stand density was detected mostly in the southern, pine-dominated sites, while the northern sites indicated greening trends near the fell tops. Based on the evidence provided by the different applied approaches, we conclude that the pine forests first increase volume and seed production, before pine seedlings eventually may invade into the tundra. Under the current climatic conditions for pine, with low survival rates beyond the tree line, a high amount of seeds would be necessary to increase the survival rate of seedlings in open sites. However, we found climate warming and prolongation of the growing season predominantly in the southern regions, where forest densification and production of new seed trees might on long sight enable coniferous forests to expand towards open sites. In the north, warming rates are small, expanding the growing season for only a few days towards an earlier onset of spring. Here, the environmental conditions are still harsh enough to mask the benefits of climate warming and so far only promote the expansion of shrubs and mountain birch forests towards the open tundra. When climate warming continues also here, it is possible that pine seeds will survive sheltered in the forest-line zone before passing the critical stage of sapling size to gradually replace the deciduous vegetation.Die subpolare Zone der Nordhemisphäre bildet den Übergang vom borealen Nadelwald zur offenen Tundra und repräsentiert in der polaren und alpinen Baumgrenze die nördlichsten und höchstgelegenen Vorkommen baumförmiger Vegetation. Ganzjährig niedrige Temperaturen, ein geringes Licht- und Nährstoffangebot sowie eine kurze Vegetationsperiode erfordern eine hohe Adaption der Vegetation an widrige klimatische Bedingungen. Die Hauptbaumarten der Borealis sind in der Lage, lange Kälteperioden zu überdauern und sich während anhaltender klimatischer Gunstphasen zu reproduzieren. In diesem Rahmen sind die Länge der Vegetationsperiode sowie die Ausprägung der Juli-Temperatur für Zuwachs und Reproduktion ausschlaggebend, die sich bereits bei geringen Temperaturänderungen stark verändern können (Kauppi et al. 2014; Salminen and Jalkanen 2015). Der Fund subfossiler Kiefern nördlich und oberhalb der Baumgrenze weist auf ein ehemals größeres Verbreitungsgebiet borealer Gehölzvegetation und damit eine klimasensitive Reaktion der Koniferen hin. Milde Winter, verlängerte Vegetationsperioden und ganzjährig höhere Temperaturen erschaffen ökosystemare Bedingungen, wie sie sonst in niedrigeren Breiten- und Höhenlagen gefunden werden. Im derzeitigen Klimaoptimum wird daher eine Ausweitung der Nadelwaldbestände, insbesondere für das polare und alpine Baumgrenzökoton prognostiziert (Juntunen et al. 2002; Jia et al. 2003; Goetz et al. 2011; Jeong et al. 2012; Walker et al. 2012; Pearson et al. 2013; Aakala et al. 2014). Bisherige Forschungsergebnisse weisen bereits auf eine erhöhte Biomasseproduktion zuvor spärlich bewachsener Tundraareale hin, die jedoch als art- und standortspezifisch eingestuft werden. Externe Störgrößen, wie Wind- und Schneebruch, Schädlingsbefall sowie Rentierbeweidung limitieren den Verjüngungserfolg häufig und erschweren die Prognostizierung borealer Waldgrenzentwicklung (Juntunen und Neuvonen 2006; Heikkinen et al. 2002). Zudem kann aufgrund der großen Ausdehnung des borealen Nadelwaldgürtels bei Einzelstudien häufig nur auf die lokalen Bedingungen geschlossen werden, was einen räumlichen Vergleich erschwert. Ziel des Projektes ist es daher, anhand verschiedener Methoden Wachstum und Ausbreitung ausgewählter Nadelbaumarten im Untersuchungsraum Finnisch-Lappland zu betrachten, um Aussagen zum Wachstumsverhalten auf mehreren zeitlichen und räumlichen Ebenen treffen zu können. In Finnisch-Lappland wurden bereits im Jahr 1983 Monitoringflächen zur Erfassung des Bestandsvolumen und der Verjüngungsrate von Waldkiefer (Pinus sylvestris L.) und Fichte (Picea abies (L.) H. Karst.) errichtet. Im Abstand von fünf Jahren wurden beide Parameter entlang eines Nord-Süd-Transekts auf insgesamt dreizehn Standorten an Höhengradienten von der Waldzone bis zur Baumgrenzzone erfasst und auf standortspezifische Trends hin untersucht. Im ersten Teil des Dissertationsprojekts wurden die Ergebnisse des Bestandsmonitorings von 1983 bis 2009 ausgewertet und zusammen mit dem Natural Resources Institute Finland Luke publiziert (Publikation I). Sechs Kiefernstandorte des Monitorings wurden anschließend dendroökologisch analysiert und auf jährliche Zuwachsänderungen, Klimakorrelationen sowie langzeitliche Wachstumstrends untersucht. Vorteil der dendroökologischen Untersuchungen waren die jährliche Auflösung des Jahrringbreitenzuwachses sowie die zeitliche Abdeckung der Analysen bis zum Beginn des 19. Jahrhunderts. Das Radialwachstum wurde auf Unterschiede zwischen jungen und alten Bäumen sowie zwischen nördlichen und südlichen Standorten hin untersucht (Publikation II). Satellitenaufnahmen der Kiefernstandorte ermöglichten eine großräumige Analyse der Untersuchungsgebiete außerhalb der Monitoringplots (Publikation III). Anhand des Normalized Difference Vegetation Index (NDVI) und einer Random Forest-Oberflächenklassifizierung wurden Veränderungen in Art und Bedeckungsgrad der Vegetation bewertet. Das Bestandsmonitoring ergab einen signifikanten Zuwachs im Bestandsvolumen für beide Baumarten an allen Standorten. Dies lässt vermuten, dass der Zuwachs und Anteil gesunder adulter Bäume (> 2 m Höhe) die Mortalitätsrate übersteigt und die Ausweitung des Bestandsvolumens durch die derzeitigen klimatischen Bedingungen begünstigt wird. Im Gegensatz dazu zeigte die Individuenzahl ein art- und standortspezifisches Bild. Die Individuenzahlen der Sämlinge, junger und adulter Bäume stieg auf fichtendominierten Standorten kontinuierlich an, insbesondere in den offeneren Waldbeständen der oberen Höhenstufen. Bei der Kiefer konnten hingegen nur stagnierende oder sinkende Individuenzahlen festgestellt werden, die auf eine hohe Mortalitätsrate junger Kiefern zurückzuführen sind. Die Kiefer zeigte damit eine hohe Sensibilität gegenüber externen Störungen im Sämlingsstadium an. In der dendroökologischen Analyse wurden die Bestandsstruktur und thermische Bedingungen während der Vegetationsperiode als limitierende Faktoren ermittelt. Das Radialwachstum korrelierte insbesondere an den nördlichen Standorten des Untersuchungsgebietes negativ mit der Zahl kalter und frostiger Tage während der einsetzenden Vegetationsperiode im Frühjahr. Die Korrelation mit der Temperatur nahm hingegen an den wärmeren Südstandorten im Laufe der letzten Jahrzehnte ab. Die Standortchronologien bezeugten eine hohe Sensitivität des Radialwachstums für die Temperatur bis in die 1980er Jahre, die jedoch überraschenderweise während der derzeitigen Warmphase abnahm. Lediglich Kiefern jüngeren Alters zeigten eine signifikant positive Wachstumsänderung auf den Südstandorten, während eine vergleichbare Wachstumsreaktion bei adulten Bäumen und auf den Nordstandorten ausblieb. In Hinblick auf die in Teil II präsentierten Ergebnisse wird von einer Überlagerung des klimatischen Signals durch externe Störgrößen ausgegangen. Milde und nasse Winter gehen mit erhöhter Schneelast, Kronenbruch und Frostschäden während des Frühjahrs sowie der Ausbreitung von Pilzinfektionen einher und könnten einen klimabedingten Wachstumszuwachs überdeckt haben. Die Satellitenbildfernerkundung konnte keine Ausweitung der Nadelwaldbestände feststellen. Der NDVI belegte eine Vegetationszunahme auf bereits begrünten Flächen, insbesondere ursprünglich lichter Nadelwaldbestände im Süden und offener Fjellkuppen im Norden. Der Anstieg des NDVIs über Nadelwaldbeständen kann auf eine Zunahme der Vitalität und/oder Biomasse zurückgeführt werden und deckt sich mit einer durch das Monitoring festgestellten Zunahme des Bestandsvolumens. Eine Verschiebung der Baumgrenze nach Norden oder in höhere Höhenstufen konnte nicht beobachtet werden. Eine Begrünung der offenen Fjellkuppen wurde in Hinblick auf die Spektraldaten auf eine Ausweitung der Birkenwaldbestände und Strauchvegetation zurückgeführt, nicht aber auf den Vorstoß der Koniferen in die offene Tundra. Das Gesamtbild, das sich aus den Ergebnissen aller Teilstudien ergibt, beschreibt eine eher verhaltene Reaktion der Kiefer auf die regionale Klimaerwärmung. Die Fichte verzeichnet sowohl im Bestandsvolumen als auch hinsichtlich der Verjüngung eine positive Entwicklung und lässt eine Ausweitung der Fichtenwaldbestände in Finnisch-Lappland erwarten. Diese Annahmen sind jedoch ausschließlich auf die Ergebnisse einer einzelnen Untersuchungsmethode begründet. Der Verjüngungs- und Wachstumserfolg der Kiefer ist hingegen stark Standort- und Störungsabhängig, wie sowohl das Monitoring, die dendroökologische Analyse als auch die Satellitenbildauswertung erkennen lassen. Hier konnte bisher noch keine eindeutige Tendenz der Kiefernwaldentwicklung aus den Ergebnissen abgeleitet werden. Die Resultate der Studie belegen eine Verdichtung der Kiefernbestände, die nachfolgend adulte, vitale Samenbäume aus dem bislang noch recht jungen Baumgrenzökoton hervorbringen kann. Eine zeitverzögerte Baumgrenzverschiebung durch natürlich Verjüngung kann daher angenommen werden, sobald eine Verbesserung der klimatischen Bedingungen und eine Abnahme der externen Störungen eintritt und die Mortalitätsrate junger Kiefern verringert

Mapping the Birch and Grass Pollen Seasons in the UK Using Satellite Sensor Time-series

Grass and birch pollen are two major causes of seasonal allergic rhinitis (hay fever) in the UK and parts of Europe affecting around 15-20% of the population. Current prediction of these allergens in the UK is based on (i) measurements of pollen concentrations at a limited number of monitoring stations across the country and (ii) general information about the phenological status of the vegetation. Thus, the current prediction methodology provides information at a coarse spatial resolution only. Most station-based approaches take into account only local observations of flowering, while only a small number of approaches take into account remote observations of land surface phenology. The systematic gathering of detailed information about vegetation status nationwide would therefore be of great potential utility. In particular, there exists an opportunity to use remote sensing to estimate phenological variables that are related to the flowering phenophase and, thus, pollen release. In turn, these estimates can be used to predict pollen release at a fine spatial resolution. In this study, time-series of MERIS Terrestrial Chlorophyll Index (MTCI) data were used to predict two key phenological variables: the start of season and peak of season. A technique was then developed to estimate the flowering phenophase of birch and grass from the MTCI time-series. For birch, the timing of flowering was defined as the time after the start of the growing season when the MTCI value reached 25% of the maximum. Similarly, for grass this was defined as the time when the MTCI value reached 75% of the maximum. The predicted pollen release dates were validated with data from nine pollen monitoring stations in the UK. For both birch and grass, we obtained large positive correlations between the MTCI-derived start of pollen season and the start of the pollen season defined using station data, with a slightly larger correlation observed for birch than for grass. The technique was applied to produce detailed maps for the flowering of birch and grass across the UK for each of the years from 2003 to 2010. The results demonstrate that the remote sensing-based maps of onset flowering of birch and grass for the UK together with the pollen forecast from the Meteorology Office and National Pollen and Aerobiology Research Unit (NPARU) can potentially provide more accurate information to pollen allergy sufferers in the UK

Latitudinal gradient of spruce forest understory and tundra phenology in Alaska as observed from satellite and ground-based data

The latitudinal gradient of the start of the growing season (SOS) and the end of the growing season (EOS) were quantified in Alaska (61°N to 71°N) using satellite-based and ground-based datasets. The Alaskan evergreen needleleaf forests are sparse and the understory vegetation has a substantial impact on the satellite signal. We evaluated SOS and EOS of understory and tundra vegetation using time-lapse camera images. From the comparison of three SOS algorithms for determining SOS from two satellite datasets (SPOT-VEGETATION and Terra-MODIS), we found that the satellite-based SOS timing was consistent with the leaf emergence of the forest understory and tundra vegetation. The ensemble average of SOS over all satellite algorithms can be used as a measure of spring leaf emergence for understory and tundra vegetation. In contrast, the relationship between the ground-based and satellite-based EOSs was not as strong as that of SOS both for boreal forest and tundra sites because of the large biases between those two EOSs (19 to 26 days). The satellite-based EOS was more relevant to snowfall events than the senescence of understory or tundra. The plant canopy radiative transfer simulation suggested that 84–86% of the NDVI seasonal amplitude could be a reasonable threshold for the EOS determination. The latitudinal gradients of SOS and EOS evaluated by the satellite and ground data were consistent and the satellite-derived SOS and EOS were 3.5 to 5.7 days degree− 1 and − 2.3 to − 2.7 days degree− 1, which corresponded to the spring (May) temperature sensitivity of − 2.5 to − 3.9 days °C− 1 in SOS and the autumn (August and September) temperature sensitivity of 3.0 to 4.6 days °C− 1 in EOS. This demonstrates the possible impact of phenology in spruce forest understory and tundra ecosystems in response to climate change in the warming Artic and sub-Arctic regions

Polynomial trends of vegetation phenology in Sahelian to equatorial Africa using remotely sensed time series from 1983 to 2005

Popular science Our understanding of global warming can be achieved in different ways. One way is to study the phenological parameters of vegetation. Phenology or seasonality of vegetation can be identified from several parameters such as: the start of the growing season (SOS), end of the growing season (EOS), amplitude of the season (AMP), and length of the growing season (LOS). Changes of these parameters represent the cyclic changes of vegetation. Nowadays, imagery satellite data are reliable and widely-used sources to study the vegetation changes. Phenology parameters are derived from time series of vegetation indices (VI) that can be computed from satellite imagery. In this thesis, long-term dataset of GIMMS NDVI from 1983 to 2005 was used to extract and analyze vegetation phenology over Sahelian to equatorial areas. The TIMESAT software package was also used as an automated method to extract the parameters. Recent researches have shown the changes via analyzing the linear trends of the vegetation indices or lately through studying the linear trend of phenological parameters. Since changes of vegetation are not always simply linear, the overall aim of this thesis was to study vegetation changes through analysis of non-linear trends and more complex mathematical functions of phenology parameters, and via finding the relationship between the phenology parameters and soil moisture. Driving forces behind changes in phenology parameters including land cover, soil texture and rainfall were also taken in to consideration. The results illustrated that non-linear trends can detect notable proportions of vegetation changes in the study area. Not only significant portions of areas with linear trends could be represented using non-linear trends, but also these trends increased the precision of phenology change detection. Regarding the climate driver forces results showed that the vegetation phenology changes followed soil moisture variations. However the trends of vegetation changes has not especially followed land cover, soil texture and geographic characteristics although in some limited cases these driver forces are related to the changes.Global warming has both short and long term effects on seasonal phenological cycles of vegetation. Phenology parameters of vegetation such as start, end, length and amplitude of season can describe life cycle events of vegetation. In this thesis, long-term dataset of GIMMS NDVI time series from 1983 to 2005 was used to extract and analyze vegetation phenology over Sahelian to equatorial areas and TIMESAT software package was used as an automated method to extract the parameters. The overall aim of this thesis was to study vegetation changes through analysis of polynomial trends of phenology parameters. Phenology parameters were analyzed to detect hidden changes in vegetation dynamics. Through comparing polynomial trends of vegetation parameters and soil moisture, the relationship between the phenology parameters and soil moisture was detected and the role of climate driver forces (including land cover, soil texture and rainfall) behind the changes in phenology parameters were investigated. The results illustrated that polynomial trends can detect notable proportions of vegetation changes in the Sahel using remotely sensed data. Significant portions of areas with linear trends could be represented through quadratic and cubic trends, and these trends increased the precision of phenology change detection. Furthermore, in some areas vegetation changes were not detected neither through linear regressions nor polynomial trends. In such areas, polynomial hidden trends could be applied for detecting the fluctuations of vegetation parameters. In summation, applying polynomial trend analysis to time-series of satellite data is a powerful tool for investigating trends and variations in vegetation in semi-arid to sub-humid regions, like the Sahel. Regarding the climate driver forces, results showed that the vegetation phenology changes followed soil moisture variations, and in most occurrences, moderate correlations were found between SOS, EOS, and soil moisture. The trends of vegetation changes did not spatially follow land cover and soil types of the study area. However, in some limited cases, land cover, soil texture and geographic characteristics such as elevation were related to the changes