Phenological and reproductive responses of tundra plants to enhanced snow cover in high arctic Adventdalen, Spitsbergen

Arktische Regionen gehören zu den Gegenden der Erde welche vom Klimawandel am meisten betroffen sind. Insbesondere Wintertemperaturen haben sich in den letzten Jahrzehnten verändert und es werden längerfristige Veränderungen hydrologischer Prozesse wie z.B. Niederschlagsmengen und -formen (flüssig, fest) und Verteilung von Schneemassen diskutiert (z.B. Serreze et al., 2000; Dormann and Woodin, 2002; Acia, 2005; Lemke et al., 2007). Messfehler, inkonsequente Mess-Methoden, lückenhafte Messreihen, natürliche Witterungsschwankungen und nicht ausreichend bekannte makroklimatische Prozesse führen zu unsicheren Trendberechnungen und Klimawandelszenarios (Groisman and Easterling, 1994; Serreze et al., 2000; Forland and Hanssen-Bauer, 2003; Acia, 2005). Dementsprechend ungenau sind Modelle zur Berechnung der Schneedecke und die Vorhersagen für die nächsten Jahrzehnte reichen von höheren Schneetiefen mit entsprechend verzögertem Abschmelzen und dadurch verkürzten Vegetationsperioden bis zu reduzierten Schneetiefen mit verfrühtem Abschmelzen. Hocharktische Regionen sind aufgrund niedriger mittlerer Jahrestemperaturen ausgezeichnet durch späte Schneeschmelze und daher durch kurze Wachstumsperioden (Schmidt et al., 2006), geringe Nährstoffumsätze (Nadelhoffer et al., 1992) und ausgeprägte intra-saisonale Feuchtigkeitsschwankungen (Scott and Rouse, 1995). Die Dauer der Schneedecke steuert viele ökophysiologische Prozesse und damit letztlich Fitness und Reproduktionserfolg, welche die Artenzusammensetzung auf Landschaftsniveau entscheidend mitbestimmen. Es wurde gezeigt, dass der Reproduktionserfolg hocharktischer Arten mit steigenden Temperaturen (Arft et al., 1999) und Nährstoffverfügbarkeit (Wookey et al., 1994; 1995) zunimmt. Walker (1999) und Borner (2008) haben festgestellt, dass Diasporenproduktion stattfand, obwohl die Phänologie verzögert und die photoaktive Periode durch experimentell erhöhte Schneedecke verkürzt war. Es wurde allerdings nicht getestet, ob die Reproduktionseinheiten fruchtbar waren oder nicht. Wir vermuten, dass eine verkürzte Wachstumsperiode zu einem Mangel an Photosyntheseprodukten führt und Fruchtreifungsprozesse hemmt und dadurch Reproduktionserfolg verringert. In dieser Arbeit wurde die Schneehöhe mittels Zäunen erhöht und der Einfluss dieser Veränderung auf Phänologie und Reproduktionserfolg im Vergleich zu nicht manipulierten Umgebungsbedingungen getestet. Unsere Arbeitshypothesen waren, dass erhöhte Schneetiefe und damit einhergehender verzögerter Start der Wachstumsperiode (1) die phänologische Entwicklung verzögert, (2) die Anzahl an Diasporen verringert und (3) die Keimfähigkeit von Reproduktionseinheiten (Samen und Brutknollen) verringert. Die Studie wurde in Adventdalen, Spitzbergen (N78°10’, E16°06’) durchgeführt. Die lokalen klimatischen Eckdaten sind Jahresmitteltemperaturen von -6.7°C und jährliche mittlere Niederschlagsmengen von 190mm. Schneezäune wurden senkrecht zur vorherrschenden Windrichtung aufgestellt, um ca. 1.5m hohe Schneeansammlungen auf der Leeseite der Zäune zu produzieren. Die Phänologie von insgesamt 13 Arten wurde wöchentlich aufgenommen, wovon fünf in der vorliegenden Arbeit präsentiert werden (Bistorta vivipara, Cassiope tetragona, Dryas octopetala, Luzula arcuata ssp. confusa, Salix polaris) (Figure 1). Gleichzeitig wurden Blüten von fünf Arten gezählt (Stellaria crassipes, Saxifraga oppositifolia, Pedicularis hirsuta, Dryas octopetala, Cassiope tetragona) (Figure 3). Samen von sechs Arten wurden gesammelt, sobald sie reif erschienen, und drei Arten wurden direkt (Bistorta vivipara, Luzula arcuata ssp. confusa, Salix polaris), drei Arten nach Stratifizierung (Alopecurus magellanicus, Cassiope tetragona, Dryas octopetala) zum Keimen gebracht. Alle zwei bis sieben Tage über einen Zeitraum von 12 Wochen wurde die Anzahl der gekeimten Samen überprüft, und gekeimte Samen/Brutknollen entfernt (Figure 2). Flächen hinter den Zäunen wurden 11±4.48 Tage später schneefrei als unbeeinflusste Kontrollflächen, was dazu führte, dass die meisten Phenophasen hinter den Zäunen verzögert zu den Kontrollflächen begannen (Figure 1). Samenreife bzw. Blütenseneszenz einiger Arten trat jedoch gleichzeitig hinter den Zäunen und in Kontrollflächen ein, was einen Verlust an Reifungszeit impliziert: Es wurde hinter den Zäunen weniger Zeit nach Schneeschmelze, also nach Beginn der Wachstumsperiode, zum Reifen der Diasporen benötigt als in Kontrollflächen. Das Erscheinen individueller Phenophasen in Tagen nach der Schneeschmelze zeigte unterschiedliche Reaktionsformen für verschiedene Arten: (1) Individuen von Dryas, Cassiope und Luzula hinter den Zäunen brauchten in der ersten Hälfte der Wachstumsperiode (von Schneeschmelze bis Blütenbildung) mehr Zeit, um bestimmte Phenophasen zu erreichen als in der zweiten Hälfte. Produktion von photosynthetisch aktiven Blättern und Blüten nahm mehr Zeit in Anspruch als in Kontrollflächen und verstärkte somit den Effekt der Verkürzung der Wachstumsperiode und gab Samenreifungsprozessen noch weniger Zeit. (2) Die gleiche Anzahl an Tagen für Phenophasen vor Blütenproduktion hinter Zäunen und in Kontrollen wurde von Bistorta benötigt, für die Seneszenzphasen und Reifung der Brutkörper wurden allerdings weniger Tage benötigt. (3) Salix brauchte länger für Phenophasen vor Blütenproduktion, spätere Phenophasen waren allerdings nicht beschleunigt. Die Bluetenanzahl hinter den Zäunen war stark reduziert und der Anfang, Höhepunkt und Schluss der Blühperiode war verzögert, benötigte aber die gleiche Anzahl an Tagen nach Schneeschmelze (Figure 3). Bistorta Brutknospen und Dryas Samen von Versuchsflächen mit erhöhter Schneedecke hatten geringere Keimungsraten als Brutknospen/Samen von Kontrollflächen. Luzula zeigte insignifikante geringere Keimungsraten hinter Zäunen und Salix, Alopecurus und Cassiope zeigten keine reduzierte Keimungsfähigkeit (Figure 2). Die vorliegende Studie zeigt ähnliche Ergebnisse wie frühere Studien (Walker et al., 1999; Borner et al., 2008), und zwar dass eine erhöhte Schneedecke und der daraus resultierende verspätete Start der Wachstumsperiode zur Verkürzung der photoaktiven Periode führt: Frühe Phenophasen benötigen die gleiche Zeit bzw. teilweise länger, während späte Phenophasen der meisten Arten nach kürzerer Zeit einsetzen. Das weist darauf hin, dass frühe Phenophasen wie Produktion photosynthetisch aktiver Blätter und Blüten unabhängig von externen Einflüssen ist, also womöglich genetisch determiniert sind, während spätere Phenophasen wie Fruchtreife und Seneszenz von äußeren Signalen wie Temperatur und spektraler Zusammensetzung des Lichts beeinflusst werden (Marchand et al., 2004; Aerts et al., 2006). Der Reproduktionserfolg wurde durch eine erhöhte Schneedecke stark reduziert. Zwar wurde die Keimfähigkeit von Samen bzw. Brutknospen von nur zwei der sechs getesteten Arten beeinträchtigt, aber die Anzahl der Blüten, und damit der Samen, aller Arten war hinter Zäunen weit geringer als in Kontrollflächen. Das könnte eine Reaktion auf eine verkürzte Wachstumsperiode der vorigen Saison sein, da einige arktische und alpine Arten Blütenknospen für das nächste Jahr bereits im Vorjahr ansetzen. Die Blühphenologie war desweiteren zeitlich versetzt, und das könnte langfristige Folgen für die Symbiose mit bestäubenden Insekten haben, insbesondere wenn die Blütenentwicklung nicht mit der Bestäuberentwicklung synchronisiert ist (Olesen et al., 2008). Langfristig gesehen könnte eine durch erhöhte Schneedecke herbeigeführte Verkürzung der Wachstumsperiode die Artenzusammensetzung der betroffenen Gebiete verändern. Arten, welche auf geringe Schneetiefen angewiesen sind (z.B. Dryas) könnten an die Grenze ihrer physiologischen Wachstumsfähigkeit geraten und von anderen Arten, welche flexiblere Wachstumsschemata haben und an hohe Schneetiefen angepasst sind (z.B. Cassiope) abgelöst werden. Positive und negative Interaktionen zwischen benachbarten Individuen der gleichen und anderer Arten könnten diesen Prozess verstärken (Heegaard and Vandvik, 2004; Wipf et al., 2006).Climate change scenarios suggest, among others, an increase of solid winter precipitation in high latitude sites, leading to enhanced and more stable temperatures under a resulting deeper snowpack and later melt out dates of affected areas. Snow accumulations behind snow fences have been used to simulate an increase of snow cover in high arctic Svalbard and resulted in a delay of growing season of 11±4.48 days (mean±sd) in 2008. Phenology of five species was assessed visually throughout the season; flowers were counted for five species and germination rates of propagules of six species were investigated. Phenology behind fences was delayed for all species, whereas green-up of leaves was more delayed than (1) senescence of leaves for all but one species and (2) propagule dispersal for some species. Flowering rates were reduced due to delayed snow melt for all species. Germination rates were lower for two out of six species and to be very low for three stratified species compared to three non-stratified species from treatment and control areas. Different responses to enhanced snow cover have been found for different species. The evergreen shrub Dryas octopetala and the forb Bistorta vivipara experienced significant reduction of germination rates. The snow bed species Cassiope tetragona has shown a more flexible phenological development than other species making it more likely to dominate in an environment with later melt out dates. It has been suggested that species interactions will lead to positive feedbacks and increase the observed short-term responses in the long term leading to substantial community changes in the affected areas

Comparison of methods for revegetation of vehicle tracks in High Arctic tundra on Svalbard.

Natural regeneration after anthropogenic disturbance is slow in the tundra biome, but assisted regeneration can help speed up the process. A tracked off-road vehicle damaged a High Arctic dwarf shrub heath in Svalbard in May 2009, drastically reducing vegetation cover, soil seed bank and incoming seed rain. We assisted regeneration the following year using six different revegetation treatments, and monitored their effects one month-, and one- and eight years after their application. By 2018, all treatments still had a lower vegetation cover and limited species composition than the undamaged reference vegetation. The fertiliser treatment was the most effective in restoring vegetation cover (71 % vegetation cover, of which 62 % were bryophytes and 38 % vascular plant species). Compared to the reference plots (98 % vegetation cover, of which 32 % were bryophytes and 66 % were vascular plant species), the composition of the disturbed vegetation was still far from regenerated to its original state nine years after the tracks were made. The slow regrowth demonstrated in this study underlines the importance of avoiding disturbance of fragile tundra, and of implementing and upholding regulations restricting or banning such disturbance.publishedVersio

Distinct summer and winter bacterial communities in the active layer of Svalbard permafrost revealed by DNA- and RNA-based analyses

The active layer of soil overlaying permafrost in the Arctic is subjected to dramatic annual changes in temperature and soil chemistry, which likely affect bacterial activity and community structure. We studied seasonal variations in the bacterial community of active layer soil from Svalbard (78°N) by co-extracting DNA and RNA from 12 soil cores collected monthly over a year. PCR amplicons of 16S rRNA genes (DNA) and reverse transcribed transcripts (cDNA) were quantified and sequenced to test for the effect of low winter temperature and seasonal variation in concentration of easily degradable organic matter on the bacterial communities. The copy number of 16S rRNA genes and transcripts revealed no distinct seasonal changes indicating potential bacterial activity during winter despite soil temperatures well below −10°C. Multivariate statistical analysis of the bacterial diversity data (DNA and cDNA libraries) revealed a season-based clustering of the samples, and, e.g., the relative abundance of potentially active Cyanobacteria peaked in June and Alphaproteobacteria increased over the summer and then declined from October to November. The structure of the bulk (DNA-based) community was significantly correlated with pH and dissolved organic carbon, while the potentially active (RNA-based) community structure was not significantly correlated with any of the measured soil parameters. A large fraction of the 16S rRNA transcripts was assigned to nitrogen-fixing bacteria (up to 24% in June) and phototrophic organisms (up to 48% in June) illustrating the potential importance of nitrogen fixation in otherwise nitrogen poor Arctic ecosystems and of phototrophic bacterial activity on the soil surface

The impact of land use on non-native species incidence and number in local assemblages worldwide.

While the regional distribution of non-native species is increasingly well documented for some taxa, global analyses of non-native species in local assemblages are still missing. Here, we use a worldwide collection of assemblages from five taxa - ants, birds, mammals, spiders and vascular plants - to assess whether the incidence, frequency and proportions of naturalised non-native species depend on type and intensity of land use. In plants, assemblages of primary vegetation are least invaded. In the other taxa, primary vegetation is among the least invaded land-use types, but one or several other types have equally low levels of occurrence, frequency and proportions of non-native species. High land use intensity is associated with higher non-native incidence and frequency in primary vegetation, while intensity effects are inconsistent for other land-use types. These findings highlight the potential dual role of unused primary vegetation in preserving native biodiversity and in conferring resistance against biological invasions

Experimental warming differentially affects vegetative and reproductive phenology of tundra plants

Rapid climate warming is altering Arctic and alpine tundra ecosystem structure and function, including shifts in plant phenology. While the advancement of green up and flowering are well-documented, it remains unclear whether all phenophases, particularly those later in the season, will shift in unison or respond divergently to warming. Here, we present the largest synthesis to our knowledge of experimental warming effects on tundra plant phenology from the International Tundra Experiment. We examine the effect of warming on a suite of season-wide plant phenophases. Results challenge the expectation that all phenophases will advance in unison to warming. Instead, we find that experimental warming caused: (1) larger phenological shifts in reproductive versus vegetative phenophases and (2) advanced reproductive phenophases and green up but delayed leaf senescence which translated to a lengthening of the growing season by approximately 3%. Patterns were consistent across sites, plant species and over time. The advancement of reproductive seasons and lengthening of growing seasons may have significant consequences for trophic interactions and ecosystem function across the tundra.publishedVersio

The tundra phenology database: more than two decades of tundra phenology responses to climate change

Observations of changes in phenology have provided some of the strongest signals of the effects of climate change on terrestrial ecosystems. The International Tundra Experiment (ITEX), initiated in the early 1990s, established a common protocol to measure plant phenology in tundra study areas across the globe. Today, this valuable collection of phenology measurements depicts the responses of plants at the colder extremes of our planet to experimental and ambient changes in temperature over the past decades. The database contains 150 434 phenology observations of 278 plant species taken at 28 study areas for periods of 1\u201326 years. Here we describe the full data set to increase the visibility and use of these data in global analyses and to invite phenology data contributions from underrepresented tundra locations. Portions of this tundra phenology database have been used in three recent syntheses, some data sets are expanded, others are from entirely new study areas, and the entirety of these data are now available at the Polar Data Catalogue (https://doi.org/10.21963/13215)

The tundra phenology database: More than two decades of tundra phenology responses to climate change

Observations of changes in phenology have provided some of the strongest signals of the effects of climate change on terrestrial ecosystems. The International Tundra Experiment (ITEX), initiated in the early 1990s, established a common protocol to measure plant phenology in tundra study areas across the globe. Today, this valuable collection of phenology measurements depicts the responses of plants at the colder extremes of our planet to experimental and ambient changes in temperature over the past decades. The database contains 150,434 phenology observations of 278 plant species taken at 28 study areas for periods of 1 to 26 years. Here we describe the full dataset to increase the visibility and use of these data in global analyses, and to invite phenology data contributions from underrepresented tundra locations. Portions of this tundra phenology database have been used in three recent syntheses, some datasets are expanded, others are from entirely new study areas, and the entirety of these data are now available at the Polar Data Catalogue (https://doi.org/10.21963/13215)

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km² resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e., offset) between in-situ soil temperature measurements, based on time series from over 1200 1-km² pixels (summarized from 8500 unique temperature sensors) across all the world’s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in-situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0–5 and 5–15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world\u27s major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (−0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications

Global maps of soil temperature.

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0-5 and 5-15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world's major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications