Effects of experimental warming on Betula nana epidermal cell growth tested over its maximum climatological growth range

Numerous long-term, free-air plant growth facilities currently explore vegetation responses to the ongoing climate change in northern latitudes. Open top chamber (OTC) experiments as well as the experimental set-ups with active warming focus on many facets of plant growth and performance, but information on morphological alterations of plant cells is still scarce. Here we compare the effects of in-situ warming on leaf epidermal cell expansion in dwarf birch, Betula nana in Finland, Greenland, and Poland. The localities of the three in-situ warming experiments represent contrasting regions of B. nana distribution, with the sites in Finland and Greenland representing the current main distribution in low and high Arctic, respectively, and the continental site in Poland as a B. nana relict Holocene microrefugium. We quantified the epidermal cell lateral expansion by microscopic analysis of B. nana leaf cuticles. The leaves were produced in paired experimental treatment plots with either artificial warming or ambient temperature. At all localities, the leaves were collected in two years at the end of the growing season to facilitate between-site and within-site comparison. The measured parameters included the epidermal cell area and circumference, and using these, the degree of cell wall undulation was calculated as an Undulation Index (UI). We found enhanced leaf epidermal cell expansion under experimental warming, except for the extremely low temperature Greenland site where no significant difference occurred between the treatments. These results demonstrate a strong response of leaf growth at individual cell level to growing season temperature, but also suggest that in harsh conditions other environmental factors may limit this response. Our results provide evidence of the relevance of climate warming for plant leaf maturation and underpin the importance of studies covering large geographical scales.Peer reviewe

Effects of experimental warming on Betula nana epidermal cell growth tested over its maximum climatological growth range

Numerous long-term, free-air plant growth facilities currently explore vegetation responses to the ongoing climate change in northern latitudes. Open top chamber (OTC) experiments as well as the experimental set-ups with active warming focus on many facets of plant growth and performance, but information on morphological alterations of plant cells is still scarce. Here we compare the effects of in-situ warming on leaf epidermal cell expansion in dwarf birch, Betula nana in Finland, Greenland, and Poland. The localities of the three in-situ warming experiments represent contrasting regions of B. nana distribution, with the sites in Finland and Greenland representing the current main distribution in low and high Arctic, respectively, and the continental site in Poland as a B. nana relict Holocene microrefugium. We quantified the epidermal cell lateral expansion by microscopic analysis of B. nana leaf cuticles. The leaves were produced in paired experimental treatment plots with either artificial warming or ambient temperature. At all localities, the leaves were collected in two years at the end of the growing season to facilitate between-site and within-site comparison. The measured parameters included the epidermal cell area and circumference, and using these, the degree of cell wall undulation was calculated as an Undulation Index (UI). We found enhanced leaf epidermal cell expansion under experimental warming, except for the extremely low temperature Greenland site where no significant difference occurred between the treatments. These results demonstrate a strong response of leaf growth at individual cell level to growing season temperature, but also suggest that in harsh conditions other environmental factors may limit this response. Our results provide evidence of the relevance of climate warming for plant leaf maturation and underpin the importance of studies covering large geographical scales.</p

Growing Season Changes over the Past Millennium in Northern High Latitudes: Tundra and Taiga Trough Time

In recent decades, it has become clear through meteorological observations that the climate is getting warmer. Particularly spring is getting warmer and seems to start earlier. Since in the far north the growing season is shorter than in the temperate latitudes, this advance of spring is more prominent. The ecosystem in the northern areas is more vulnerable to this disturbance in the spring, because many natural processes risk being out of balance with each other. In order to properly understand these current changes of the growing season, it is necessary to look back in time at the pre-industrial mode of climate, in order to understand how the dynamics of early spring manifested themselves before the modern period of warming. Since there is little to no reliable meteorological data available from before the industrial revolution, one is forced to derive the state of the climate of the past from so-called proxies, in this case biological thermometers. With the fossil remains of these biological thermometers, one can deduce what the temperature must have been in the past, for example. For these proxies to work properly, they must be calibrated and tested. In this PhD research, the degree of undulation in the circumference of the cell wall of the epidermal cells of the leaf of birch trees was used as a proxy for the intensity of the growing season, with the advance of spring as main influence. Dwarf birch cell circumference was previously known to work well as paleo-thermometer. During this PhD research, it was also established that the downy birch acts well as a proxy based on the circumference of the leaf cells. The proxy based on these birch species has been applied to experiments in Finland, Poland, and Greenland with the aim of demonstrating that the advance of spring will disrupt the ecology. The usefulness of the proxy has also been demonstrated by proving that it is sensitive to past fluctuations in the large-scale atmospheric system that affects the weather in the areas around the North Atlantic. This proxy has then been used to reconstruct the intensity of the growing season over roughly the past 1300 years in Denmark, in order to establish that a warming climate is often preceded by a warming spring. During the last millennium, the changes in the growing season have been closely linked to the state of prosperity of the population, a link that will also have to be taken into account in the future

Growing Season Changes over the Past Millennium in Northern High Latitudes: Tundra and Taiga Trough Time

In recent decades, it has become clear through meteorological observations that the climate is getting warmer. Particularly spring is getting warmer and seems to start earlier. Since in the far north the growing season is shorter than in the temperate latitudes, this advance of spring is more prominent. The ecosystem in the northern areas is more vulnerable to this disturbance in the spring, because many natural processes risk being out of balance with each other. In order to properly understand these current changes of the growing season, it is necessary to look back in time at the pre-industrial mode of climate, in order to understand how the dynamics of early spring manifested themselves before the modern period of warming. Since there is little to no reliable meteorological data available from before the industrial revolution, one is forced to derive the state of the climate of the past from so-called proxies, in this case biological thermometers. With the fossil remains of these biological thermometers, one can deduce what the temperature must have been in the past, for example. For these proxies to work properly, they must be calibrated and tested. In this PhD research, the degree of undulation in the circumference of the cell wall of the epidermal cells of the leaf of birch trees was used as a proxy for the intensity of the growing season, with the advance of spring as main influence. Dwarf birch cell circumference was previously known to work well as paleo-thermometer. During this PhD research, it was also established that the downy birch acts well as a proxy based on the circumference of the leaf cells. The proxy based on these birch species has been applied to experiments in Finland, Poland, and Greenland with the aim of demonstrating that the advance of spring will disrupt the ecology. The usefulness of the proxy has also been demonstrated by proving that it is sensitive to past fluctuations in the large-scale atmospheric system that affects the weather in the areas around the North Atlantic. This proxy has then been used to reconstruct the intensity of the growing season over roughly the past 1300 years in Denmark, in order to establish that a warming climate is often preceded by a warming spring. During the last millennium, the changes in the growing season have been closely linked to the state of prosperity of the population, a link that will also have to be taken into account in the future

A growing degree day inference model based on mountain birch leaf cuticle analysis over a latitudinal gradient in Fennoscandia

Cuticle analysis performed on fossil Betula nana (L.) leaves provides a strong proxy to reconstruct past growing season thermal properties expressed as growing degree days (GDD5). This proxy is so far available for the dwarf birch only and, therewith, restricted to regions or past periods of subarctic climatic conditions. In this study, we analysed modern leaf samples of mountain birch (Betula pubescens spp. czerepanovii (N. I. Orlova) Hämet-Ahti), which has a wider temperature range than the dwarf birch B. nana. The strong latitudinal climate gradient over Fennoscandia provides a unique opportunity to track growing season temperature imprints in the epidermis cell morphology of the modern mountain birch. We quantified the GDD5-dependent epidermal cell expansion, expressed as the undulation index (UI), over a 10° latitudinal transect translating to a range from ~1500°C to ~600°C GDD5 in 2016. Our results indicate that even in mountain birch the UI is positively correlated to GDD5 and, moreover, is largely independent of regional habitat conditions such as daylight length and precipitation. These results imply that in addition to the earlier studied (sub-)arctic dwarf birch, the closely related mountain birch can also be utilized in GDD5 reconstructions. The abundant presence of fossil mountain birch leaves in sediments from warmer than (sub)arctic palaeoclimates enables the reconstruction of growing season climate dynamics over past phases of climate change, overcoming earlier restrictions of the proxy related to spatial and temporal species occurrence as well as local light regimes

North Atlantic Oscillation seesaw effect in leaf morphological records from dwarf birch shrubs in Greenland and Finland

The North Atlantic Oscillation (NAO) determines wind speed and direction, seasonal heat, moisture transport, storm tracks, cloudiness and sea-ice cover through atmospheric mass balance shifts between the Arctic and the subtropical Atlantic. The NAO is characterized by the typical, yet insufficiently understood, seesaw pattern of warmer winter and spring temperatures over Scandinavia and cooler temperatures over Greenland during the positive phase of the NAO, and vice versa during the negative phase. We tested the potential to reconstruct NAO variation beyond the meteorological record through the application of a microphenological proxy. We measured the Undulation Index (UI) in Betula nana epidermal cells from herbarium leaf samples and fossil peat fragments dating back to 1865—exceeding most meteorological records in the Arctic—to estimate imprints of spring thermal properties and NAO in Greenland and Finland. We found negative relations between Greenland UI and late winter, spring and early summer NAO, and mostly positive, but not significant, relations between Finland UI and NAO in years with pronounced NAO expression. The direction of the UI response in this common circumpolar species is, therefore, likely in line with the NAO seesaw effect, with leaf development response to NAO fluctuations in northern Europe opposing the response in Greenland and vice versa. Increased knowledge of the UI response to climate may contribute to understanding ecological properties of key Arctic species, whilst additionally providing a proxy for NAO dynamics

North Atlantic Oscillation seesaw effect in leaf morphological records from dwarf birch shrubs in Greenland and Finland

The North Atlantic Oscillation (NAO) determines wind speed and direction, seasonal heat, moisture transport, storm tracks, cloudiness and sea-ice cover through atmospheric mass balance shifts between the Arctic and the subtropical Atlantic. The NAO is characterized by the typical, yet insufficiently understood, seesaw pattern of warmer winter and spring temperatures over Scandinavia and cooler temperatures over Greenland during the positive phase of the NAO, and vice versa during the negative phase. We tested the potential to reconstruct NAO variation beyond the meteorological record through the application of a microphenological proxy. We measured the Undulation Index (UI) in Betula nana epidermal cells from herbarium leaf samples and fossil peat fragments dating back to 1865—exceeding most meteorological records in the Arctic—to estimate imprints of spring thermal properties and NAO in Greenland and Finland. We found negative relations between Greenland UI and late winter, spring and early summer NAO, and mostly positive, but not significant, relations between Finland UI and NAO in years with pronounced NAO expression. The direction of the UI response in this common circumpolar species is, therefore, likely in line with the NAO seesaw effect, with leaf development response to NAO fluctuations in northern Europe opposing the response in Greenland and vice versa. Increased knowledge of the UI response to climate may contribute to understanding ecological properties of key Arctic species, whilst additionally providing a proxy for NAO dynamics

North Atlantic Oscillation seesaw effect in leaf morphological records from dwarf birch shrubs in Greenland and Finland

The North Atlantic Oscillation (NAO) determines wind speed and direction, seasonal heat, moisture transport, storm tracks, cloudiness and sea-ice cover through atmospheric mass balance shifts between the Arctic and the subtropical Atlantic. The NAO is characterized by the typical, yet insufficiently understood, seesaw pattern of warmer winter and spring temperatures over Scandinavia and cooler temperatures over Greenland during the positive phase of the NAO, and vice versa during the negative phase. We tested the potential to reconstruct NAO variation beyond the meteorological record through the application of a microphenological proxy. We measured the Undulation Index (UI) in&nbsp;Betula nana&nbsp;epidermal cells from herbarium leaf samples and fossil peat fragments dating back to 1865—exceeding most meteorological records in the Arctic—to estimate imprints of spring thermal properties and NAO in Greenland and Finland. We found negative relations between Greenland UI and late winter, spring and early summer NAO, and mostly positive, but not significant, relations between Finland UI and NAO in years with pronounced NAO expression. The direction of the UI response in this common circumpolar species is, therefore, likely in line with the NAO seesaw effect, with leaf development response to NAO fluctuations in northern Europe opposing the response in Greenland and vice versa. Increased knowledge of the UI response to climate may contribute to understanding ecological properties of key Arctic species, whilst additionally providing a proxy for NAO dynamics

A growing degree day inference model based on mountain birch leaf cuticle analysis over a latitudinal gradient in Fennoscandia

Cuticle analysis performed on fossil Betula nana (L.) leaves provides a strong proxy to reconstruct past growing season thermal properties expressed as growing degree days (GDD5). This proxy is so far available for the dwarf birch only and, therewith, restricted to regions or past periods of subarctic climatic conditions. In this study, we analysed modern leaf samples of mountain birch (Betula pubescens spp. czerepanovii (N. I. Orlova) Hämet-Ahti), which has a wider temperature range than the dwarf birch B. nana. The strong latitudinal climate gradient over Fennoscandia provides a unique opportunity to track growing season temperature imprints in the epidermis cell morphology of the modern mountain birch. We quantified the GDD5-dependent epidermal cell expansion, expressed as the undulation index (UI), over a 10° latitudinal transect translating to a range from ~1500°C to ~600°C GDD5 in 2016. Our results indicate that even in mountain birch the UI is positively correlated to GDD5 and, moreover, is largely independent of regional habitat conditions such as daylight length and precipitation. These results imply that in addition to the earlier studied (sub-)arctic dwarf birch, the closely related mountain birch can also be utilized in GDD5 reconstructions. The abundant presence of fossil mountain birch leaves in sediments from warmer than (sub)arctic palaeoclimates enables the reconstruction of growing season climate dynamics over past phases of climate change, overcoming earlier restrictions of the proxy related to spatial and temporal species occurrence as well as local light regimes