Dynamiques temporelles des communautés végétales forestières en réponse aux changements globaux. Approche comparative entre bryophytes et trachéophytes via l’écologie historique

Depuis environ deux siècles les activités humaines modernes ont profondément modifié les conditions environnementales sur la surface de la Terre. De nombreuses études ont mis en évidence une réponse de la végétation face à ces changements. Cependant, il persiste plusieurs incompréhensions. Premièrement, les réponses des communautés varient fortement entre les études, et les mécanismes responsables de cette variation sont encore mal connus. Deuxièmement, malgré leur importance écologique, les bryophytes restent largement sous étudiées dans les études temporelles, limitant notre compréhension de leur dynamique temporelle. Ce doctorat a pour objectif d’apporter des éléments de réponses à ces deux points. Les trois projets de recherche gravitent autour de la grande question : quels sont les effets des changements environnementaux sur la biodiversité? Les hypothèses que j’ai développées se construisent autour de ces deux grandes questions (i) Quels sont les effets des dépositions et du réchauffement de la température sur la végétation forestière? (ii) Il y a-t-il une différence de sensibilité entre bryophytes et trachéophytes face aux changements environnementaux? Le chapitre 2 teste l’hypothèse que les bryophytes sont plus sensibles que les plantes vasculaires face aux dépositions atmosphériques et au réchauffement de la température dans une région industrielle du nord-ouest de la France. Le chapitre 3 teste les mécanismes de réponse de la végétation forestière le long d’un gradient de réchauffement climatique dans l’est du Canada. Enfin, le chapitre 4 est une approche mixe entre les deux premiers chapitres, il teste la réponse des plantes vasculaires et des bryophytes face à différentes intensités de réchauffement de la température sur des gradients altitudinaux dans l’est de la province de Québec. Dans les trois chapitres, j’ai utilisé les méthodes de l’écologie historique. Après un long travail d’archive, j’ai rééchantillonné des relevés botaniques faits dans les années 1970. La sélection des sites à rééchantillonner suit un protocole finement détaillé afin de minimiser tous effets confondants. Les résultats des trois chapitres mettent en évidence le lien direct entre réponse de végétation et changements environnementaux. Premièrement, les bryophytes sont plus sensibles aux dépositions atmosphériques que les plantes vasculaires (chapitre 2). Secondement, les changements temporels de la végétation vasculaire sont plus grands dans les zones ou le réchauffement climatique fut le plus fort (chapitre 3). Dernièrement, face au réchauffement climatique, les réponses des bryophytes et plantes vasculaires diffèrent selon la propriété de la communauté qui est étudiée (chapitre 4). Les trois chapitres, montrent un changement systématique de la composition des communautés, sans pour autant de changement de la richesse spécifique. Ce doctorat fournit trois exemples de la force des méthodes historiques dans la compréhension des mécanismes de réponse de la végétation face aux changements globaux. Mes travaux supportent l’importance d’analyser la dynamique de la végétation avec une vision holistique. La compréhension des mécanismes liés à la dynamique temporelle de la végétation doit passer par l’étude de plusieurs groupes taxonomiques, avec différentes propriétés des communautés sur plusieurs échelles spatiales.Abstract: For at least the past two centuries, human activities have caused strong environmental changes in the biosphere. Many studies have shown responses of vegetation to global changes. However, many unknowns remain. First, most explicitly temporal studies have been conducted at a single site with a common intensity of environmental changes and historical land-use legacies. Results are highly variable among studies, and we have a very limited understanding of mechanisms underlying this variation. Second, despite the major contribution of bryophytes to ecosystem functioning, very few temporal studies have focused on bryophytes. This Ph.D. contributes to filling these two knowledge gaps. The overarching question for the three research projects presented here is: what is the impact of environmental change on biodiversity? We built a set of hypotheses around two main questions: (i) What is the effect of environmental changes on forest vegetation? (ii) Which taxon, bryophytes or vascular plants, is most sensitive to global changes? Chapter 2 tests the hypothesis that bryophytes are more sensitive than vascular plants to the combination of atmospheric deposition and warming in an industrial region in north-eastern France. Chapter 3 tests the hypothesis that forest vegetation changes have been greatest in regions with the strongest warming trends along a continental gradient in eastern Canada. The last chapter combines the two first approaches, quantifying temporal changes in bryophyte and vascular plant communities in sites with different warming intensities along elevational gradients in eastern Canada. To answer to these questions, I used an historical ecological approach by resurveying botanical plots initially surveyed in the 1970s. Plot selection followed a reproducible and detailed procedure to minimize confounding factors. Our results show a direct effect of global changes on forest vegetation. First, bryophytes appear more sensitive to atmospheric deposition than vascular plants (Chapter 2). Second, temporal changes in vascular plant communities were stronger in areas where warming has been greatest (Chapter 3). Third, in response to warming, changes in bryophyte and vascular plant communities show idiosyncratic differences, depending on the community property under study (Chapter 4). Results of the three chapters clearly show systematic changes in community composition, that are not necessarily accompanied by changes in local diversity. In sum, we provide empirical evidence that historical ecology is a powerful method to disentangling mechanisms of vegetation response to global changes. Only a holistic approach based on different biodiversity components, different spatial scales and wide variety of community properties permit an understanding of the complexity of temporal dynamics of vegetation

Different temporal trends in vascular plant and bryophyte communities along elevational gradients over four decades

Despite many studies showing biodiversity responses to warming, the generality of such responses across taxonomic groups remains unclear. Very few studies have tested for evidence of bryophyte community responses to warming, even though bryophytes are major contributors to diversity and functioning in many ecosystems. Here, we report an empirical study comparing long-term change in bryophyte and vascular plant communities in two sites with contrasting long-term warming trends, using "legacy" botanical records as a baseline for comparison with contemporary resurveys. We hypothesized that ecological changes would be greater in sites with a stronger warming trend and that vascular plant communities, with narrower climatic niches, would be more sensitive than bryophyte communities to climate warming. For each taxonomic group in each site, we quantified the magnitude of changes in species' distributions along the elevation gradient, species richness, and community composition. We found contrasted temporal changes in bryophyte vs. vascular plant communities, which only partially supported the warming hypothesis. In the area with a stronger warming trend, we found a significant increase in local diversity and dissimilarity (beta-diversity) for vascular plants, but not for bryophytes. Presence-absence data did not provide sufficient power to detect elevational shifts in species distributions. The patterns observed for bryophytes are in accordance with recent literature showing that local diversity can remain unchanged despite strong changes in composition. Regardless of whether one taxon is systematically more or less sensitive to environmental change than another, our results suggest that vascular plants cannot be used as a surrogate for bryophytes in terms of predicting the nature and magnitude of responses to warming. Thus, to assess overall biodiversity responses to global change, abundance data from different taxonomic groups and different community properties need to be synthesized.Peer reviewe

Diverging trends and drivers of Arctic flower production in Greenland over space and time

The Arctic is warming at an alarming rate. While changes in plant community composition and phenology have been extensively reported, the effects of climate change on reproduction remain poorly understood. We quantified multidecadal changes in flower density for nine tundra plant species at a low- and a high-Arctic site in Greenland. We found substantial changes in flower density over time, but the temporal trends and drivers of flower density differed both between species and sites. Total flower density increased over time at the low-Arctic site, whereas the high-Arctic site showed no directional change. Within and between sites, the direction and rate of change differed among species, with varying effects of summer temperature, the temperature of the previous autumn and the timing of snowmelt. Finally, all species showed a strong trade-off in flower densities between successive years, suggesting an effective cost of reproduction. Overall, our results reveal region- and taxon-specific variation in the sensitivity and responses of co-occurring species to shared climatic drivers, and a clear cost of reproductive investment among Arctic plants. The ultimate effects of further changes in climate may thus be decoupled between species and across space, with critical knock-on effects on plant species dynamics, food web structure and overall ecosystem functioning

Can bryophyte groups increase functional resolution in tundra ecosystems?

Funding Information: This study was supported by a grant to SL from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, Grant No. 797446 and by the Independent Research Fund Denmark, Grant no. 0135-00140B. Funding from the Academy of Finland (grant 322266), National Science Foundation (1504224, 1836839, PLR-1504381 and PLR-1836898), Independent Research Fund Denmark (9040-00314B), Moscow State University, (project No 121032500089-1), Natural Sciences and Engineering Research Council of Canada, ArcticNet, Polar Continental Shelf Program, Northern Science Training Program, Polar Knowledge Canada, Royal Canadian Mounted Police, Tomsk State University competitiveness improvement program and the Russian Science Foundation (grant No 20-67-46018) are gratefully acknowledged. Matthias Ahrens provided valuable insights on the cushion growth form, and we are most thankful. We thank Gaius Shaver and two anonymous reviewers for providing valuable critique and input to earlier versions of this manuscript. Publisher Copyright: © the author(s) or their institution(s).The relative contribution of bryophytes to plant diversity, primary productivity, and ecosystem functioning increases towards colder climates. Bryophytes respond to environmental changes at the species level, but because bryophyte species are relatively difficult to identify, they are often lumped into one functional group. Consequently, bryophyte function remains poorly resolved. Here, we explore how higher resolution of bryophyte functional diversity can be encouraged and implemented in tundra ecological studies. We briefly review previous bryophyte functional classifications and the roles of bryophytes in tundra ecosystems and their susceptibility to environmental change. Based on shoot morphology and colony organization, we then propose twelve easily distinguishable bryophyte functional groups. To illustrate how bryophyte functional groups can help elucidate variation in bryophyte effects and responses, we compiled existing data on water holding capacity, a key bryophyte trait. Although plant functional groups can mask potentially high interspecific and intraspecific variability, we found better separation of bryophyte functional group means compared with previous grouping systems regarding water holding capacity. This suggests that our bryophyte functional groups truly represent variation in the functional roles of bryophytes in tundra ecosystems. Lastly, we provide recommendations to improve the monitoring of bryophyte community changes in tundra study sites.Peer reviewe

Can bryophyte groups increase functional resolution in tundra ecosystems?

The relative contribution of bryophytes to plant diversity, primary productivity, and ecosystem functioning increases towards colder climates. Bryophytes respond to environmental changes at the species level, but because bryophyte species are relatively difficult to identify, they are often lumped into one functional group. Consequently, bryophyte function remains poorly resolved. Here, we explore how higher resolution of bryophyte functional diversity can be encouraged and implemented in tundra ecological studies. We briefly review previous bryophyte functional classifications and the roles of bryophytes in tundra ecosystems and their susceptibility to environmental change. Based on shoot morphology and colony organization, we then propose twelve easily distinguishable bryophyte functional groups. To illustrate how bryophyte functional groups can help elucidate variation in bryophyte effects and responses, we compiled existing data on water holding capacity, a key bryophyte trait. Although plant functional groups can mask potentially high interspecific and intraspecific variability, we found better separation of bryophyte functional group means compared with previous grouping systems regarding water holding capacity. This suggests that our bryophyte functional groups truly represent variation in the functional roles of bryophytes in tundra ecosystems. Lastly, we provide recommendations to improve the monitoring of bryophyte community changes in tundra study sites

Dynamiques temporelles des communautés végétales forestières en réponse aux changements globaux. Approche comparative entre bryophytes et trachéophytes via l’écologie historique

Depuis environ deux siècles les activités humaines modernes ont profondément modifié les conditions environnementales sur la surface de la Terre. De nombreuses études ont mis en évidence une réponse de la végétation face à ces changements. Cependant, il persiste plusieurs incompréhensions. Premièrement, les réponses des communautés varient fortement entre les études, et les mécanismes responsables de cette variation sont encore mal connus. Deuxièmement, malgré leur importance écologique, les bryophytes restent largement sous étudiées dans les études temporelles, limitant notre compréhension de leur dynamique temporelle. Ce doctorat a pour objectif d’apporter des éléments de réponses à ces deux points. Les trois projets de recherche gravitent autour de la grande question : quels sont les effets des changements environnementaux sur la biodiversité? Les hypothèses que j’ai développées se construisent autour de ces deux grandes questions (i) Quels sont les effets des dépositions et du réchauffement de la température sur la végétation forestière? (ii) Il y a-t-il une différence de sensibilité entre bryophytes et trachéophytes face aux changements environnementaux? Le chapitre 2 teste l’hypothèse que les bryophytes sont plus sensibles que les plantes vasculaires face aux dépositions atmosphériques et au réchauffement de la température dans une région industrielle du nord-ouest de la France. Le chapitre 3 teste les mécanismes de réponse de la végétation forestière le long d’un gradient de réchauffement climatique dans l’est du Canada. Enfin, le chapitre 4 est une approche mixe entre les deux premiers chapitres, il teste la réponse des plantes vasculaires et des bryophytes face à différentes intensités de réchauffement de la température sur des gradients altitudinaux dans l’est de la province de Québec. Dans les trois chapitres, j’ai utilisé les méthodes de l’écologie historique. Après un long travail d’archive, j’ai rééchantillonné des relevés botaniques faits dans les années 1970. La sélection des sites à rééchantillonner suit un protocole finement détaillé afin de minimiser tous effets confondants. Les résultats des trois chapitres mettent en évidence le lien direct entre réponse de végétation et changements environnementaux. Premièrement, les bryophytes sont plus sensibles aux dépositions atmosphériques que les plantes vasculaires (chapitre 2). Secondement, les changements temporels de la végétation vasculaire sont plus grands dans les zones ou le réchauffement climatique fut le plus fort (chapitre 3). Dernièrement, face au réchauffement climatique, les réponses des bryophytes et plantes vasculaires diffèrent selon la propriété de la communauté qui est étudiée (chapitre 4). Les trois chapitres, montrent un changement systématique de la composition des communautés, sans pour autant de changement de la richesse spécifique. Ce doctorat fournit trois exemples de la force des méthodes historiques dans la compréhension des mécanismes de réponse de la végétation face aux changements globaux. Mes travaux supportent l’importance d’analyser la dynamique de la végétation avec une vision holistique. La compréhension des mécanismes liés à la dynamique temporelle de la végétation doit passer par l’étude de plusieurs groupes taxonomiques, avec différentes propriétés des communautés sur plusieurs échelles spatiales.Abstract: For at least the past two centuries, human activities have caused strong environmental changes in the biosphere. Many studies have shown responses of vegetation to global changes. However, many unknowns remain. First, most explicitly temporal studies have been conducted at a single site with a common intensity of environmental changes and historical land-use legacies. Results are highly variable among studies, and we have a very limited understanding of mechanisms underlying this variation. Second, despite the major contribution of bryophytes to ecosystem functioning, very few temporal studies have focused on bryophytes. This Ph.D. contributes to filling these two knowledge gaps. The overarching question for the three research projects presented here is: what is the impact of environmental change on biodiversity? We built a set of hypotheses around two main questions: (i) What is the effect of environmental changes on forest vegetation? (ii) Which taxon, bryophytes or vascular plants, is most sensitive to global changes? Chapter 2 tests the hypothesis that bryophytes are more sensitive than vascular plants to the combination of atmospheric deposition and warming in an industrial region in north-eastern France. Chapter 3 tests the hypothesis that forest vegetation changes have been greatest in regions with the strongest warming trends along a continental gradient in eastern Canada. The last chapter combines the two first approaches, quantifying temporal changes in bryophyte and vascular plant communities in sites with different warming intensities along elevational gradients in eastern Canada. To answer to these questions, I used an historical ecological approach by resurveying botanical plots initially surveyed in the 1970s. Plot selection followed a reproducible and detailed procedure to minimize confounding factors. Our results show a direct effect of global changes on forest vegetation. First, bryophytes appear more sensitive to atmospheric deposition than vascular plants (Chapter 2). Second, temporal changes in vascular plant communities were stronger in areas where warming has been greatest (Chapter 3). Third, in response to warming, changes in bryophyte and vascular plant communities show idiosyncratic differences, depending on the community property under study (Chapter 4). Results of the three chapters clearly show systematic changes in community composition, that are not necessarily accompanied by changes in local diversity. In sum, we provide empirical evidence that historical ecology is a powerful method to disentangling mechanisms of vegetation response to global changes. Only a holistic approach based on different biodiversity components, different spatial scales and wide variety of community properties permit an understanding of the complexity of temporal dynamics of vegetation

Diverging trends and drivers of Arctic flower production in Greenland over space and time

Publisher Copyright: © 2023, The Author(s).The Arctic is warming at an alarming rate. While changes in plant community composition and phenology have been extensively reported, the effects of climate change on reproduction remain poorly understood. We quantified multidecadal changes in flower density for nine tundra plant species at a low- and a high-Arctic site in Greenland. We found substantial changes in flower density over time, but the temporal trends and drivers of flower density differed both between species and sites. Total flower density increased over time at the low-Arctic site, whereas the high-Arctic site showed no directional change. Within and between sites, the direction and rate of change differed among species, with varying effects of summer temperature, the temperature of the previous autumn and the timing of snowmelt. Finally, all species showed a strong trade-off in flower densities between successive years, suggesting an effective cost of reproduction. Overall, our results reveal region- and taxon-specific variation in the sensitivity and responses of co-occurring species to shared climatic drivers, and a clear cost of reproductive investment among Arctic plants. The ultimate effects of further changes in climate may thus be decoupled between species and across space, with critical knock-on effects on plant species dynamics, food web structure and overall ecosystem functioning.Peer reviewe

Honeybees’ foraging choices for nectar and pollen revealed by DNA metabarcoding

Abstract Honeybees are the most widespread managed pollinators of our food crops, and a crucial part of their well-being is a suitable diet. Yet, we do not know how they choose flowers to collect nectar or pollen from. Here we studied forty-three honeybee colonies in six apiaries over a summer, identifying the floral origins of honey and hive-stored pollen samples by DNA-metabarcoding. We recorded the available flowering plants and analyzed the specialized metabolites in honey. Overall, we find that honeybees use mostly the same plants for both nectar and pollen, yet per colony less than half of the plant genera are used for both nectar and pollen at a time. Across samples, on average fewer plant genera were used for pollen, but the composition was more variable among samples, suggesting higher selectivity for pollen sources. Of the available flowering plants, honeybees used only a fraction for either nectar or pollen foraging. The time of summer guided the plant choices the most, and the location impacted both the plants selected and the specialized metabolite composition in honey. Thus, honeybees are selective for both nectar and pollen, implicating a need of a wide variety of floral resources to choose an optimal diet from

Plant biodiversity change across scales during the Anthropocene

Plant communities have undergone dramatic changes in recent centuries, although not all such changes fit with the dominant biodiversity-crisis narrative used to describe them. At the global scale, future declines in plant species diversity are highly likely given habitat conversion in the tropics, although few extinctions have been documented for the Anthropocene to date (<0.1%). Nonnative species introductions have greatly increased plant species richness in many regions of the world at the same time that they have led to the creation of new hybrid polyploid species by bringing previously isolated congeners into close contact. At the local scale, conversion of primary vegetation to agriculture has decreased plant diversity, whereas other drivers of change—e.g., climate warming, habitat fragmentation, and nitrogen deposition—have highly context-dependent effects, resulting in a distribution of temporal trends with a mean close to zero. These results prompt a reassessment of how conservation goals are defined and justified

Can bryophyte groups increase functional resolution in tundra ecosystems?

Funding Information: This study was supported by a grant to SL from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, Grant No. 797446 and by the Independent Research Fund Denmark, Grant no. 0135-00140B. Funding from the Academy of Finland (grant 322266), National Science Foundation (1504224, 1836839, PLR-1504381 and PLR-1836898), Independent Research Fund Denmark (9040-00314B), Moscow State University, (project No 121032500089-1), Natural Sciences and Engineering Research Council of Canada, ArcticNet, Polar Continental Shelf Program, Northern Science Training Program, Polar Knowledge Canada, Royal Canadian Mounted Police, Tomsk State University competitiveness improvement program and the Russian Science Foundation (grant No 20-67-46018) are gratefully acknowledged. Matthias Ahrens provided valuable insights on the cushion growth form, and we are most thankful. We thank Gaius Shaver and two anonymous reviewers for providing valuable critique and input to earlier versions of this manuscript. Publisher Copyright: © the author(s) or their institution(s).The relative contribution of bryophytes to plant diversity, primary productivity, and ecosystem functioning increases towards colder climates. Bryophytes respond to environmental changes at the species level, but because bryophyte species are relatively difficult to identify, they are often lumped into one functional group. Consequently, bryophyte function remains poorly resolved. Here, we explore how higher resolution of bryophyte functional diversity can be encouraged and implemented in tundra ecological studies. We briefly review previous bryophyte functional classifications and the roles of bryophytes in tundra ecosystems and their susceptibility to environmental change. Based on shoot morphology and colony organization, we then propose twelve easily distinguishable bryophyte functional groups. To illustrate how bryophyte functional groups can help elucidate variation in bryophyte effects and responses, we compiled existing data on water holding capacity, a key bryophyte trait. Although plant functional groups can mask potentially high interspecific and intraspecific variability, we found better separation of bryophyte functional group means compared with previous grouping systems regarding water holding capacity. This suggests that our bryophyte functional groups truly represent variation in the functional roles of bryophytes in tundra ecosystems. Lastly, we provide recommendations to improve the monitoring of bryophyte community changes in tundra study sites.Peer reviewe