High altitude and latitude environments are among those areas of the Planet that are experiencing the most significant changes of the climatic conditions due to the recent global change. Terrestrial ecosystems in these regions are extremely sensitive to climate, and for this reason, well suited for the comprehension, evaluation and monitoring of their responses, and their modelling under different climate change scenarios.
This thesis focuses on the impacts of climate change on terrestrial ecosystems of alpine (Foscagno valley, central Italian Alps) and Polar (both Continental Antarctica and Maritime Antarctica) tundra habitats. European Alps and Maritime Antarctica are two of the three areas of the world where have been recorded the greatest air temperature warming in the last 50 years, whereas simultaneously in Continental Antarctica, air temperature was almost stable. Due to different trends of climate and anthropogenic pressures around the world, contemporary global change is characterized by a large spatial variability that makes the planning of adaptation and mitigation strategies particularly complicate. Peculiar habitats have been protected by international, European and national programs (the Foscagno valley belongs to the Nature 2000 network, while the Antarctica ecosystems are protected under the Antarctic treaty and specially protected areas). However, analyzing the dynamics of terrestrial ecosystems in regions that are facing different climate change scenarios, as well as biological and anthropogenic constrains, could improve the knowledge of the dynamics of terrestrial ecosystems, that could be used for modelling future scenarios and to implement the adaptation plans for such protected areas.
The identification of conservation actions and monitoring plans is thus the priority for such threatened environments, to ensure a correct management of the biodiversity and of the ecosystem services that they can provide.
The alpine site is the Foscagno Valley, a high altitude site (>2500 m a.s.l.) located in the central Italian Alps, where since 2007 a field-based project of snow, ground surface temperature and plant phenology monitoring started. This PhD is part of this project, that since summer 2015 was implemented with manipulation experiments on two typical alpine vegetation communities (snowbed and grassland) to simulate potential future climate change related impacts on plant phenology and growth, including increase of nutrient availability (simulated by additions of urea, ammonium sulfate, NPK respectively), water availability (once or twice per week additions of water), lack of reproductive stages (flowers removal).
The primary aim of this PhD was to investigate alpine plant phenology and its relationships with climate change. We monitored phenology of 21 plants typical of alpine environments and representative of different growth forms types. In particular, we hypothesized that: a) the vegetative development (shoot appearance and leaf emergence) is regulated by snow melt timing, while the other phenological stages (i.e. flowering, seed development and ripening, and leaf senescence) are regulated mostly by photoperiod, which should indicate a conservative and adaptive strategy of alpine plants; b) plant phenology shows different plasticity depending on the growth form types and also on the phenological stage; c) extreme events can overwhelm the effect of photoperiod, and can lead to carry over effects in the phenological cycle and plant growth.
Our data indicated that the main predictor of the vegetative development stages was the snow melt, while the photoperiod was the best predictor of phenological stages from flowering peak up to leaf senescence. Therefore, the photoperiodic control on alpine plant phenology should be considered for the evaluation and modelling of the impacts of climate change in alpine region, not only concerning the reproductive stages, but also for the leaf senescence. This constitute a novelty concerning literature data about phenology of alpine plants (which have been always related to snow and/or temperature as triggering factors) because provides new insights on the capability of alpine plants to profit of potential future autumn warming.
Alpine plant phenology showed thus a strong conservative strategy, which differed among growth forms, according to our hypothesis. Over all the investigated phenological stages graminoids were the most plastic and responsive growth form and their higher adaptation capability could help to explain why this growth form is expanding more than forbs in alpine and Polar tundra habitats.
Extreme events showed impact on plant phenology, with differences among growth forms and phenological stage. In particular, we found the leaf senescence of herbaceous species to be highly sensitive to the combination of drought and heat, which led to an advance of almost one month in the season, while deciduous shrubs were not or less sensitive. Moreover, an extreme leaf senescence showed carry over effects on the next season growth rate.
Our results indicate also a statistically significant increase of the height of evergreen shrubs on the period 2010-2016, which was in agreement with the observed range expansion of shrubs in the Alps.
Relating to the long-term ground surface temperature (GST) monitoring, 13 temperature data loggers were installed in the Foscagno valley, under different surface types conditions, covering the most widespread growth forms of the area. In particular, we hypothesized that: a) GST was strongly influenced by the soil coverage types and snow cover, b) although the actual climatic conditions are less favorable, some vegetated soils could lead a ground cooling until to permafrost condition; c) the shrubland expansion could drive to an energetic disequilibrium of soils with thus positive feedbacks on the carbon cycle.
Our data confirmed that the snow cover duration was the main driver of the mean annual ground surface temperature, while the beginning of snow cover deeper than 80 cm influenced the freezing state of the soils during winter, and the timing of snow melt was related to the thawing degree days of the snow free period. Vegetated soils (shrublands and grasslands surface types) were warmer compared to bare ground, except for pioneer species (i.e. Cerastium uniflorum), that involved a cooling on soils leading to permafrost condition. Therefore, future changes in vegetation cover can lead to different soil thermal regimes and different spatial distribution of temperatures in alpine terrains. Potentially, the expansion of pioneer species could be related to longer persistence of permafrost conditions; on the contrary, if the shrublands expansion that we are facing in the Alps will continue in the future, soil temperatures will be warmer, which imply positive feedbacks to the carbon cycle.
The main purpose of the manipulation experiments started at the Foscagno Valley in 2015 was to investigate the responses of phenology and plant growth under different simulations of climate change. As many phenophases exhibited a strong photoperiodic control, we aimed to assess and quantify the eventual effects of the different manipulation treatments and identify the most responsive phenophases and communities types. Concerning the quantitative development, we hypothesized that: a) the vegetative development (shoot length, leaf length, maximum plant height) would be influenced by manipulations, with nutrients exerting the higher impacts, compared to flower removal and water addition (since terrestrial ecosystems are N and P limited); b) the above ground biomass (ABG) would be strongly influenced by the addition of nutrients, more than flowers removal or water addition.
Our data showed that manipulation experiments (in particular nutrient additions) involved strong impacts on plant phenology and growth, lengthening (with xNPK) or shortening (with ammonium sulfate) the duration of flowering and leaf senescence, and enhancing the vegetative quantitative phenology of alpine plants (with xNPK). One of the most important findings is that the photoperiodic limitation observed at the long-term phenology monitoring, could be overcome under future climate changes, with potentially consequences on niche competition within communities.
Above all the manipulation we found a strong N and P limitation, in particular concerning the vegetative development, which thus will imply strong consequences also on the carbon budget. Also the flower removal exerted visible effects especially on the quantitative growth, highlighting differences among the vegetation communities, with higher capability for snowbeds species to remobilize nutrients than grasslands species.
Another feedback of the manipulations experiments concerned the occurrence of carry over effects on the quantitative growth of the first shooting stage at the beginning of the season, provided by the enhanced nutrient availability from the previous year.
While in the alpine tundra vascular plants are dominant, in high latitude ecosystems, cryptogams (mosses and lichens) are the major components of terrestrial ecosystems. In particular, in Antarctica only few long-term data are available on the responses of mosses and lichens to climate change.
Comparable to Alps, the Maritime Antarctica is one of the regions of the planet recording the most rapid air warming, and Signy Island (South Orkney Islands) has been identified as a suitable context for the monitoring of biological changes. Here we analyzed the contemporary abundance and distribution of moss banks on the entire island, assessing their ecological requirements, and providing thus a baseline for future monitoring.
Thanks to the availability of previous field-based studies on the spatial distribution of moss banks carried out in the 1960s and 1970s, we assessed long-term and large-scale moss responses to climate change underlying the related ecological processes.
Differently to Alps and Maritime Antarctica, Continental Antarctica in the last 50 years showed a stable air temperature trend. This region is the last pristine environment on Earth, which provides a unique opportunity to assess the natural dynamics and responses to climate. In Victoria Land, in 2002 started a long-term monitoring project of the climate, permafrost and vegetation, of which we present here the results after 10 years of monitoring. Concerning this activity, we aim to: a) identify the patterns of spatial and temporal active layer variability; b) analyze the changes of the associated vegetation; c) identify the climatic forcing of active layer and vegetation changes.
In the Antarctic summer 2014/2015, we installed over a latitudinal gradient (73-77\ub0S) in Victoria Land, some manipulation experiments (additions of snow, water, urea, ammonium sulfate, NPK, guano respectively) coupled with the manipulation of snow accumulation, soil temperature and precipitation (installing respectively snow fences, open top chambers and snow shield) for the simulation of potential future climate change impacts. We thus briefly evaluated the effects on soil temperatures after the first year of the experiments, providing thus the start point for future further monitoring.
Among all the experiments, we found that the snow cover exerted the largest effect on the GST, thus highlighting its main role in regulating soil temperatures, which effects overcome the influence of air temperature, leading also to changes of soil moisture and water regim
