Trait-based selection and plant communities in high-latitude ecosystems

According to theory, the functional traits of species dictate how environmental selection affects them, and also the functioning of ecosystems that those species form. However, we lack a general understanding about how exactly environmental selection affects the trait composition of communities, and consequently, ecosystem functions. In this thesis, I study how the effects of environmental selection manifest in the functional composition of field-layer plant communities in the tundra and in boreal forests. My aims are 1) to sharpen our understanding about the effects of trait-based selection on plant communities by accounting for the microenvironment in models of trait composition, 2) to elucidate the effects of that selection on tundra carbon cycling, and 3) to reveal how forestry and reindeer husbandry, two forms of human land use, modulate long-term vegetation changes by favouring certain trait combinations over others. The study areas span four tundra landscapes in Finnish Lapland, Greenland, Svalbard, and the southern Indian Ocean, and hundreds of herb-rich boreal forest patches in Northern Finland. I use linear modelling to study how the results of vegetation surveys, visual, sensor-based and laboratory measurements of traits and the environment, and carbon flux chamber measurements relate to each other. My results suggest the following. 1) The environment strongly determines the functional composition of plant communities when accounting for microenvironmental conditions. Warm, ungrazed and unshaded conditions favor larger plants. Leaf traits that confer fast returns on invested resources are favoured in conditions of high soil resource availability, in ungrazed areas, and in the shade. 2) In the tundra, communities consisting of larger plants cycle carbon more rapidly and have larger above-ground carbon stocks. Communities with “fast” leaf traits also cycle carbon with higher intensity, but they have lower above-ground carbon stocks than communities with “slow” leaf traits. 3) In boreal forests, forestry modifies the functional composition of understory communities by decreasing the amount of light in the long term. While forestry seems to accelerate vegetation change, reindeer husbandry could be seen to counteract it by inhibiting the growth of average plant size observed in areas without reindeer. These results show that the functional traits of plants dictate how they are affected by environmental selection pressures. The effects of this selection are consistent at the community level across locations up to 15000 km apart. Furthermore, human land use is an important control of the functional composition of communities alongside natural environmental variation. This information will be useful in predicting which species will suffer and which benefit from global change, and what will be the consequences for ecosystem functioning.Lajeja on miljoonia ja jokainen niistä elää rajallisella alueella. Tutkijat tarvitsevat keinoja yleistää yksittäisiä lajeja koskevat tiedot käyttökelpoisiksi muiden lajien tutkimiseen. Yleistää voi esimerkiksi eliöiden mitattavien ominaisuuksien pohjalta. Voidaan esimerkiksi kysyä, onko jokin elinympäristö yhtä hyvä suurille ja pienille lajeille. Väitöskirjassani tutkin, voidaanko kasvilajien ominaisuuksia käyttää ennustamaan niiden suosimia elinympäristöjä ja vaikutusta ekosysteemitoimintoihin. Väitöskirjani tutkimusalueet ovat neljä tundramaisemaa Lapissa, Grönlannissa, Huippuvuorilla ja eteläisellä Intian valtamerellä, sekä lehtolaikuissa Pohjois-Suomen metsissä. Ilmastonmuutos ja ihmisen maankäyttö aiheuttavat nopeita ympäristömuutoksia näillä alueilla. Jos ymmärtäisimme, millaisia lajeja muuttuneet ympäristöt suosivat, voisimme ennustaa nykyistä paremmin tulevaisuuden kasvillisuuden koostumuksen. Tutkimukseni osoittavat, että kokoakseli ja lehtitalousspektri säätelevät lajeille suotuisia elinympäristöjä niin tundralla kuin metsissäkin. Lämpimät, valoisat ja laiduntamattomat ympäristöt ovat hyviä suuriksi kasvaville lajeille. Rehevät, varjoisat ja laiduntamattomat ympäristöt taas suosivat “sijoittajalajeja”, eli lajeja jotka laittavat kaikki resurssinsa peliin nopeiden voittojen toivossa. Tätä lajien ominaisuuksien säätelemää lajiston valikoitumista voidaan käyttää ennustamaan ekosysteemien muutosta. Ihmisen aiheuttamat ilmaston ja maankäytön muutokset ovat jo vaikuttaneet tundran ja boreaalisten metsien kasvillisuuteen ja ekosysteemitoimintoihin, ja muutokset tulevat jatkumaan. Lämpimämmissä olosuhteissa suuret kasvit syrjäyttävät pienemmät. Ihmisen muokkaamat rehevät metsät ovat varjoisempia kuin luonnontilaiset, mikä suosii sijoittajalajeja säästäjien kustannuksella. Toisaalta kasvaneet porotiheydet poronhoitoalueella saattavat hidastaa kasvillisuusmuutoksia viemällä suurilta kasveilta niiden valintaedun. Lajien ominaisuuksia voidaan siis käyttää ennustamaan niiden ympäristövaatimuksia ja vaikutusta ekosysteemitoimintoihin. Ominaisuudet ovat yhteisiä suureita, joiden avulla yksittäisten lajien vuorovaikutussuhteet ympäristönsä kanssa voidaan tehdä vertailukelpoisiksi. Tämä tieto on mielenkiintoista itsessään, mutta on myös hyödyksi lajien suojelun suunnittelussa sekä globaalimuutoksen seurausten ennustamisessa ekosysteemien toiminnalle

Relationships between above-ground plant traits and carbon cycling in tundra plant communities

The trait composition and trait diversity of plant communities are globally applicable predictors of ecosystem functioning. Yet, it is unclear how plant traits influence carbon cycling. This is an important question in the tundra where vegetation shifts are occurring across the entire biome, and where soil organic carbon stocks are large and vulnerable to environmental change. To study how plant traits affect carbon cycling in the tundra, we built a model that explained carbon cycling (above-ground and soil organic carbon stocks, and photosynthetic and respiratory fluxes) with abiotic conditions (air temperature and soil moisture), and the averages and within-community variabilities of three above-ground traits: plant height, leaf dry matter content (LDMC) and SLA. These functional parameters were represented by abundance-weighted means and standard deviations of species traits. The data were collected from an observational study setting from northern Finland. The explanatory power of the models was relatively high, but a large part of variation in soil organic carbon stocks remained unexplained. Average plant height was the strongest predictor of all carbon cycling variables except soil carbon stocks. Communities of larger plants were associated with larger CO2 fluxes and above-ground carbon stocks. Communities with fast leaf economics (i.e. high SLA and low LDMC) had higher photosynthesis, ecosystem respiration and soil organic carbon stocks. Within-community variability in plant height, SLA and LDMC affected ecosystem functions differently. Variability in SLA and LDMC increased CO2 fluxes and soil organic carbon stocks, while variability in height increased the above-ground carbon stock. The contributions of within-community trait variability metrics to ecosystem functioning within the study area were about as important as those of average SLA and LDMC. Synthesis. Plant height, SLA and LDMC have clear effects on tundra carbon cycling. The importance of within-community trait variability highlights a potentially important mechanism controlling the vast tundra carbon pools that should be better recognized. More research on root traits and decomposer communities is needed to understand the below-ground mechanisms regulating carbon cycling in the tundra.Peer reviewe

Dwarf Shrubs Impact Tundra Soils : Drier, Colder, and Less Organic Carbon

In the tundra, woody plants are dispersing towards higher latitudes and altitudes due to increasingly favourable climatic conditions. The coverage and height of woody plants are increasing, which may influence the soils of the tundra ecosystem. Here, we use structural equation modelling to analyse 171 study plots and to examine if the coverage and height of woody plants affect the growing-season topsoil moisture and temperature (< 10 cm) as well as soil organic carbon stocks (< 80 cm). In our study setting, we consider the hierarchy of the ecosystem by controlling for other factors, such as topography, wintertime snow depth and the overall plant coverage that potentially influence woody plants and soil properties in this dwarf shrub-dominated landscape in northern Fennoscandia. We found strong links from topography to both vegetation and soil. Further, we found that woody plants influence multiple soil properties: the dominance of woody plants inversely correlated with soil moisture, soil temperature, and soil organic carbon stocks (standardised regression coefficients = - 0.39; - 0.22; - 0.34, respectively), even when controlling for other landscape features. Our results indicate that the dominance of dwarf shrubs may lead to soils that are drier, colder, and contain less organic carbon. Thus, there are multiple mechanisms through which woody plants may influence tundra soils.Peer reviewe

Trait-based responses to land use and canopy dynamics modify long-term diversity changes in forest understories

Aim Land use is the foremost cause of global biodiversity decline, but species do not respond equally to land-use practices. Instead, it is suggested that responses vary with species traits, but long-term data on the trait-mediated effects of land use on communities are scarce. Here we study how forest understorey communities have been affected by two land-use practices during 4-5 decades, and whether changes in plant diversity are related to changes in functional composition. Location Finland. Time period 1968-2019. Major taxa studied Vascular plants. Methods We resurveyed 245 vegetation plots in boreal herb-rich forest understories, and used hierarchical Bayesian linear models to relate changes in diversity, species composition, average plant size, and leaf economic traits to reindeer abundance, forest management intensity, and changes in climate, canopy cover and composition. We also studied the relationship between species evenness and plant size across both space and time. Results Intensively managed forests decreased in species richness and had increased turnover, but management did not affect functional composition. Increased reindeer densities corresponded with increased leaf dry matter content, evenness and diversity, and decreased height and specific leaf area. Successional development in the canopy was associated with increased specific leaf area and decreased leaf dry matter content and height in the understorey over the study period. Effects of reindeer abundance and canopy density on diversity were partially mediated by vegetation height, which had a negative relationship with evenness across both space and time. Observed changes in climate had no discernible effect on any variable. Main conclusions Functional traits are useful in connecting vegetation changes to the mechanisms that drive them, and provide unique information compared to turnover and diversity metrics. These trait-dependent selection effects could inform which species benefit and which suffer from land-use changes and explain observed biodiversity changes under global change.Peer reviewe

Snow is an important control of plant community functional composition in oroarctic tundra

The functional composition of plant communities is a critical modulator of climate change impacts on ecosystems, but it is not a simple function of regional climate. In the Arctic tundra, where climate change is proceeding the most rapidly, communities have not shifted their trait composition as predicted by spatial temperature-trait relationships. Important causal pathways are thus missing from models of trait composition change. Here, we study causes of plant community functional variation in an oroarctic tundra landscape in Kilpisjarvi, Finland. We consider the community-weighted means of plant vegetative height, as well as two traits related to the leaf economic spectrum. Specifically, we model their responses to locally measured summer air temperature, snow conditions, and soil resource levels. For each of the traits, we also quantify the importance of intraspecific trait variation (ITV) for between-community functional differences and trait-environment matching. Our study shows that in a tundra landscape (1) snow is the most influential abiotic variable affecting functional composition, (2) vegetation height is under weak local environmental control, whereas leaf economics is under strong local environmental control, (3) the relative magnitude of ITV differs between traits, and (4) ITV is not very consequential for community-level trait-environment relationships. Our analyses highlight the importance of winter conditions for community functional composition in seasonal areas. We show that winter climate change can either amplify or counter the effects summer warming, depending on the trait.Peer reviewe

Geomorphological processes shape plant community traits in the Arctic

Aim Geomorphological processes profoundly affect plant establishment and distributions, but their influence on functional traits is insufficiently understood. Here, we unveil trait-geomorphology relationships in Arctic plant communities. Location High-Arctic Svalbard, low-Arctic Greenland and sub-Arctic Fennoscandia. Time period 2011-2018. Major taxa studied Vascular plants. Methods We collected field-quantified data on vegetation, geomorphological processes, microclimate and soil properties from 5,280 plots and 200 species across the three Arctic regions. We combined these data with database trait records to relate local plant community trait composition to dominant geomorphological processes of the Arctic, namely cryoturbation, deflation, fluvial processes and solifluction. We investigated the relationship between plant functional traits and geomorphological processes using hierarchical generalized additive modelling. Results Our results demonstrate that community-level traits are related to geomorphological processes, with cryoturbation most strongly influencing both structural and leaf economic traits. These results were consistent across regions, suggesting a coherent biome-level trait response to geomorphological processes. Main conclusions The results indicate that geomorphological processes shape plant community traits in the Arctic. We provide empirical evidence for the existence of generalizable relationships between plant functional traits and geomorphological processes. The results indicate that the relationships are consistent across these three distinct tundra regions and that geomorphological processes should be considered in future investigations of functional traits.Peer reviewe

Geomorphological processes shape plant community traits in the Arctic

Aim Geomorphological processes profoundly affect plant establishment and distributions, but their influence on functional traits is insufficiently understood. Here, we unveil trait-geomorphology relationships in Arctic plant communities. Location High-Arctic Svalbard, low-Arctic Greenland and sub-Arctic Fennoscandia. Time period 2011-2018. Major taxa studied Vascular plants. Methods We collected field-quantified data on vegetation, geomorphological processes, microclimate and soil properties from 5,280 plots and 200 species across the three Arctic regions. We combined these data with database trait records to relate local plant community trait composition to dominant geomorphological processes of the Arctic, namely cryoturbation, deflation, fluvial processes and solifluction. We investigated the relationship between plant functional traits and geomorphological processes using hierarchical generalized additive modelling. Results Our results demonstrate that community-level traits are related to geomorphological processes, with cryoturbation most strongly influencing both structural and leaf economic traits. These results were consistent across regions, suggesting a coherent biome-level trait response to geomorphological processes. Main conclusions The results indicate that geomorphological processes shape plant community traits in the Arctic. We provide empirical evidence for the existence of generalizable relationships between plant functional traits and geomorphological processes. The results indicate that the relationships are consistent across these three distinct tundra regions and that geomorphological processes should be considered in future investigations of functional traits.Peer reviewe

Dispersal-niche continuum index : a new quantitative metric for assessing the relative importance of dispersal versus niche processes in community assembly

Patterns in community composition are scale-dependent and generally difficult to distinguish. Therefore, quantifying the main assembly processes in various systems and across different datasets has remained challenging. Building on the PER-SIMPER method, we propose a new metric, the dispersal-niche continuum index (DNCI), which estimates whether dispersal or niche processes dominate community assembly and facilitates the comparisons of processes among datasets. The DNCI was tested for robustness using simulations and applied to observational datasets comprising organismal groups with different trophic level and dispersal potential. Based on the robustness tests, the DNCI discriminated the respective contribution of niche and dispersal processes in pairwise comparisons of site groups with less than 40% and 30% differences in their taxa and site numbers, respectively. In the observational datasets, the DNCI suggested that dispersal rather than niche assembly was the dominant assembly process which, however, varied in intensity among organismal groups and study contexts, including spatial scale and ecosystem types. The proposed DNCI measures the relative strength of community assembly processes in a way that is simple, easily quantifiable and comparable across datasets. We discuss the strengths and weaknesses of the DNCI and provide perspectives for future research.Peer reviewe

Geomorphological processes shape plant community traits in the Arctic

AIM : Geomorphological processes profoundly affect plant establishment and distributions, but their influence on functional traits is insufficiently understood. Here, we unveil trait–geomorphology relationships in Arctic plant communities. LOCATION : High-Arctic Svalbard, low-Arctic Greenland and sub-Arctic Fennoscandia. TIME PERIOD : 2011–2018. MAJOR TAXA STUDIED : Vascular plants. METHODS : We collected field-quantified data on vegetation, geomorphological processes, microclimate and soil properties from 5,280 plots and 200 species across the three Arctic regions. We combined these data with database trait records to relate local plant community trait composition to dominant geomorphological processes of the Arctic, namely cryoturbation, deflation, fluvial processes and solifluction. We investigated the relationship between plant functional traits and geomorphological processes using hierarchical generalized additive modelling. RESULTS : Our results demonstrate that community-level traits are related to geomorphological processes, with cryoturbation most strongly influencing both structural and leaf economic traits. These results were consistent across regions, suggesting a coherent biome-level trait response to geomorphological processes. MAIN CONCLUSIONS : The results indicate that geomorphological processes shape plant community traits in the Arctic. We provide empirical evidence for the existence of generalizable relationships between plant functional traits and geomorphological processes. The results indicate that the relationships are consistent across these three distinct tundra regions and that geomorphological processes should be considered in future investigations of functional traits.DATA AVAILABILITY STATEMENT: Data and code are openly available (Kemppinen et al., 2022; https://doi.org/10.5281/zenodo.6410638).Arctic Interactions at the University of Oulu and Academy of Finland.Nessling foundation and the Kone Foundation.Carl Tryggers Stiftelse.Academy of Finland Flagship fundingAcademy of FinlandFinnish Cultural FoundationArctic Interactions at the University of Oulu; Academy of Finland; Nessling Foundation; the Kone Foundation; Carl Tryggers Stiftelse and Finnish Cultural Foundation.http://www.wileyonlinelibrary.com/journal/gebPlant Production and Soil Scienc

Plant traits poorly predict winner and loser shrub species in a warming tundra biome

Climate change is leading to species redistributions. In the tundra biome, shrubs are generally expanding, but not all tundra shrub species will benefit from warming. Winner and loser species, and the characteristics that may determine success or failure, have not yet been fully identified. Here, we investigate whether past abundance changes, current range sizes and projected range shifts derived from species distribution models are related to plant trait values and intraspecific trait variation. We combined 17,921 trait records with observed past and modelled future distributions from 62 tundra shrub species across three continents. We found that species with greater variation in seed mass and specific leaf area had larger projected range shifts, and projected winner species had greater seed mass values. However, trait values and variation were not consistently related to current and projected ranges, nor to past abundance change. Overall, our findings indicate that abundance change and range shifts will not lead to directional modifications in shrub trait composition, since winner and loser species share relatively similar trait spaces