High mountain dwarf shrubs under climate change: effects of in situ experimental warming, CO2 enrichment and drought on their growth and functioning = Subarbusts d'alta muntanya davant el canvi climàtic. Efectes de l'escalfament, l'enriquiment de CO2 i la sequera experimentals in situ en el seu creixement i funcionament

[eng] High-elevation and high-latitude ecosystems are expected to be especially vulnerable to global change. Dwarf shrub species cover extensive areas in high mountain ecosystems, and warmer temperatures and land-use change have favoured their expansion. Co-occurring dwarf shrub species may respond differently to climate change, which can have major effects on their interactions, growth and performance, and ultimately lead to drastic changes in plant communities. In this Thesis, warming, CO2 enrichment and drought experiments have been carried out at the ecotone between the subalpine and alpine belts in different study sites across the Pyrenees and the Alps. The objective was to assess how these factors may influence dwarf shrub species interactions, growth, xylem anatomy and C allocation dynamics. Results of this Thesis show species-specific responses to the treatments, and these responses differed in magnitude and direction depending on the treatment applied. Vaccinium myrtillus responded positively to increased temperatures, whereas the co-occurring Vaccinium uliginosum and Empetrum hermaphroditum showed no response. Vaccinium myrtillus growth and functioning was similar across stands differing in neighbouring shrub species and we did not find evidence of alterations in plant-plant interactions with warming. Vaccinium myrtillus responded positively to CO2 enrichment with an increased stem basal area and a lagged increased vessel lumen size and xylem-specific hydraulic conductivity, but hydraulic efficiency decreased with warming. Although there was a correlation between growth and accumulated hydraulic conductivity, the growth-anatomical relationship changed with soil warming and CO2 enrichment, and growth stimulation occurred without immediate accompanying changes in the xylem anatomy. We did not find major effects of the experimental summer drought on V. myrtillus water potential and its allocation of recently assimilated C. However, V. uliginosum showed evidence of mild stress, since its stomatal conductance, water potential and speed of C transfer to belowground organs were reduced under drought. All these results suggest that climate change acts differently on co-occurring species, which may lead to future changes in plant community composition, structure and function.[cat] Els ecosistemes d'elevades altituds i latituds són especialment vulnerables al canvi global. Les espècies subarbustives poden cobrir àrees extenses en zones d'alta muntanya, i l'augment de la temperatura i els canvis en l'ús del sòl n'afavoreixen l'expansió. Les espècies subarbustives que coexisteixen podrien respondre de manera diferent al canvi climàtic, cosa que podria tenir grans efectes en les seves interaccions, així com en el seu creixement i el seu funcionament i, en última instància, comportar canvis dràstics en les comunitats vegetals. En aquesta Tesi Doctoral s'han dut a terme experiments d'escalfament, d’augment de la concentració de CO2 de l’aire i de sequera a l'ecotò entre els estatges subalpí i alpí en diferents zones del Pirineu i dels Alps. L'objectiu era comprendre com aquests factors afecten les interaccions entre espècies subarbustives, el seu creixement, l’anatomia del xilema i la distribució de carboni (C). Els resultats d'aquesta Tesi mostren respostes específiques als tractaments. A més, aquestes respostes diferien en magnitud i direcció en funció del tractament aplicat. Vaccinium myrtillus va respondre positivament a l'escalfament, mentre que les espècies coexistents Vaccinium uliginosum i Empetrum hermaphroditum no van mostrar cap resposta. El creixement i funcionament de V. myrtillus era similar en claps amb diferent composició d'espècies arbustives i, contràriament al resultat de nombrosos estudis previs, no es van trobar evidències d'alteracions en les interaccions planta-planta amb l'escalfament. Vaccinium myrtillus va respondre positivament a l'augment de CO2 amb un augment de l'àrea basal i un augment retardat en la mida dels vasos conductors i la conductivitat hidràulica específica. Tanmateix, l'eficiència hidràulica es va reduir amb l'escalfament. Tot i que es va trobar una correlació entre el creixement i l’anatomia en termes de conductivitat hidràulica acumulada en V. myrtillus, aquesta relació va canviar amb els tractaments d'escalfament i augment de CO2, i el creixement va augmentar sota aquests tractaments sense que es produïssin canvis immediats en l'anatomia del xilema que l'acompanyessin. La sequera experimental no va tenir grans efectes en el potencial hídric ni en la distribució de C recentment assimilat de V. myrtillus. Tanmateix, V. uliginosum va presentar lleugers signes d'estrès amb la sequera, ja que la seva conductància estomàtica i el seu potencial hídric es van veure reduïts, així com també la velocitat de transferència de C cap a òrgans subterranis. Tots aquests resultats suggereixen que el canvi climàtic actua de diferent manera sobre espècies coexistents, cosa que podria comportar canvis en la composició de les comunitats, així com en la seva estructura i funcionament en el futur

Iterative Next Boundary Detection for Instance Segmentation of Tree Rings in Microscopy Images of Shrub Cross Sections

We address the problem of detecting tree rings in microscopy images of shrub cross sections. This can be regarded as a special case of the instance segmentation task with several unique challenges such as the concentric circular ring shape of the objects and high precision requirements that result in inadequate performance of existing methods. We propose a new iterative method which we term Iterative Next Boundary Detection (INBD). It intuitively models the natural growth direction, starting from the center of the shrub cross section and detecting the next ring boundary in each iteration step. In our experiments, INBD shows superior performance to generic instance segmentation methods and is the only one with a built-in notion of chronological order. Our dataset and source code are available at http://github.com/alexander-g/INBD.Comment: CVPR 202

Traditional plant functional groups explain variation in economic but not size-related traits across the tundra biome

Aim Plant functional groups are widely used in community ecology and earth system modelling to describe trait variation within and across plant communities. However, this approach rests on the assumption that functional groups explain a large proportion of trait variation among species. We test whether four commonly used plant functional groups represent variation in six ecologically important plant traits. Location Tundra biome. Time period Data collected between 1964 and 2016. Major taxa studied 295 tundra vascular plant species. Methods We compiled a database of six plant traits (plant height, leaf area, specific leaf area, leaf dry matter content, leaf nitrogen, seed mass) for tundra species. We examined the variation in species‐level trait expression explained by four traditional functional groups (evergreen shrubs, deciduous shrubs, graminoids, forbs), and whether variation explained was dependent upon the traits included in analysis. We further compared the explanatory power and species composition of functional groups to alternative classifications generated using post hoc clustering of species‐level traits. Results Traditional functional groups explained significant differences in trait expression, particularly amongst traits associated with resource economics, which were consistent across sites and at the biome scale. However, functional groups explained 19% of overall trait variation and poorly represented differences in traits associated with plant size. Post hoc classification of species did not correspond well with traditional functional groups, and explained twice as much variation in species‐level trait expression. Main conclusions Traditional functional groups only coarsely represent variation in well‐measured traits within tundra plant communities, and better explain resource economic traits than size‐related traits. We recommend caution when using functional group approaches to predict tundra vegetation change, or ecosystem functions relating to plant size, such as albedo or carbon storage. We argue that alternative classifications or direct use of specific plant traits could provide new insights for ecological prediction and modelling

Seed production and dispersal limit treeline advance in the Pyrenees

Aims Pinus uncinata is the major treeline‐forming species in the Pyrenees. Yet, the role of its reproduction and dispersal as drivers of treeline dynamics remains unknown. Here we quantify seed production, dispersal and germination changes along the elevation gradient to assess whether they may constrain the foreseen treeline advance in the Pyrenees. Location Central Pyrenees, Catalonia, NE Spain. Methods We established four plots along an elevation gradient from the closed subalpine forest to the krummholz zone at five study sites. In each plot, we collected cones from five to six trees, measured their length, and triggered their opening in the laboratory to count the number of empty seeds and the number and weight of full seeds. We used the collected seeds in a germination experiment under controlled conditions in growth chambers. Additionally, we installed seed traps along the forest-alpine grassland transition to measure seed rain for three consecutive years in three of the study sites. Results The number of full seeds per cone decreased along the elevation gradient and was correlated with cone length. However, the proportion of full seeds per cone and their weight did not differ between elevation positions. Seed rain decreased drastically with elevation and no seeds arrived into the alpine grassland traps consistently across study years. Although germination success did not significantly differ between elevation provenances (i.e., elevation position of origin), we found significant differences in germination dynamics between study sites and between elevation provenances within sites. Conclusions Our results indicate that whereas the viability of Pinus uncinata seeds is not limited by elevation, seed production and dispersal are constraining the ongoing rates of treeline advance in the Pyrenee

No preferential C-allocation to storage over growth in clipped birch and oak saplings

Herbivory is one of the most globally distributed disturbances affecting C-cycling in trees, yet our understanding of how it alters tree C-allocation to different functions like storage, growth or rhizodeposition is still limited. We performed continuous 13C-labelling coupled to a clipping experiment to quantify the effects of simulated browsing on the growth, leaf morphology and relative allocation of stored vs. recently assimilated C to the growth (bulk biomass) and non-structural carbohydrate (NSC) stores (soluble sugars and starch) of the different organs of two tree species with contrasting wood anatomy: diffuse-porous (Betula pubescens) and ring-porous (Quercus petraea). Transfers of C from plants to bulk and rhizosphere soil were also evaluated. Clipped birch and oak trees shifted their C-allocation patterns above-ground as a means to recover from defoliation. However, such increased allocation to current-year stems and leaves did not entail reductions in the allocation to the rhizosphere, which remained unchanged between clipped and control trees of both species. B. pubescens and Q. petraea showed differences in their vulnerability and recovery strategies to clipping, the ring-porous species being less affected in terms of growth and architecture to clipping than the diffuse-porous. These contrasting patterns could be partly explained by differences in their C cycling after clipping. Defoliated oaks showed a faster recovery of their canopy biomass, which was supported by increased allocation of recently fixed C, but associated with large decreases in the fine root biomass of clipped trees of this species. Following clipping, both species recovered NSC pools to a larger extent than growth, but the allocation of 13C-labelled photo-assimilates into storage compounds was not increased as compared to controls. Despite their different response to clipping, our results indicate no preventative allocation into storage occurred during the first year after clipping in any of the two studied tree species

Four years of experimental warming do not modify the interaction between subalpine shrub species

Climate warming can lead to changes in alpine plant species interactions through modifications in environmental conditions, which may ultimately cause drastic changes in plant communities. We explored the effects of 4 years of experimental warming with open-top chambers (OTC) on Vaccinium myrtillus performance and its interaction with neighbouring shrubs at the Pyrenean treeline ecotone. We examined the effects of warming on height, above-ground (AG) and below-ground (BG) biomass and the C and N concentration and isotope composition of V. myrtillus growing in pure stands or in stands mixed with Vaccinium uliginosum or Rhododendron ferrugineum. We also analysed variations in soil N concentrations, rhizosphere C/N ratios and the functional diversity of the microbial community, and evaluated whether warming altered the biomass, C and N concentration and isotope composition of V. uliginosum in mixed plots. Our results showed that warming induced positive changes in the AG growth of V. myrtillus but not BG, while V. uliginosum did not respond to warming. Vaccinium myrtillus performance did not differ between stand types under increased temperatures, suggesting that warming did not induce shifts in the interaction between V. myrtillus and its neighbouring species. These findings contrast with previous studies in which species interactions changed when temperature was modified. Our results show that species interactions can be less responsive to warming in natural plant communities than in removal experiments, highlighting the need for studies involving the natural assembly of plant species and communities when exploring the effect of environmental changes on plant-plant interactions

Four years of experimental warming do not modify the interaction between subalpine shrub species

We thank Clara Borrull, Noelia Seguer, Estela Illa, Victoria Lafuente, Elena Lahoz and Santiago Pérez for their field and laboratory assistance. We are grateful to CCiT of the University of Barcelona for the use of their facilities and their technical assistance. This work was supported by Conselh Generau d'Aran and the project ARBALMONT/786-2012 (Organismo Autónomo Parques Nacionales, Ministerio de Agricultura, Alimentación y Medio Ambiente, Spain). AAR was funded by an FPU grant (Ministerio de Educación, Cultura y Deporte, Spain) and SP was funded by a Ramón y Cajal fellowship (RYC-2013-14164, Ministerio de Economía y Competitividad, Spain).Climate warming can lead to changes in alpine plant species interactions through modifications in environmental conditions, which may ultimately cause drastic changes in plant communities. We explored the effects of 4 years of experimental warming with open-top chambers (OTC) on Vaccinium myrtillus performance and its interaction with neighbouring shrubs at the Pyrenean treeline ecotone. We examined the effects of warming on height, above-ground (AG) and below-ground (BG) biomass and the C and N concentration and isotope composition of V. myrtillus growing in pure stands or in stands mixed with Vaccinium uliginosum or Rhododendron ferrugineum. We also analysed variations in soil N concentrations, rhizosphere C/N ratios and the functional diversity of the microbial community, and evaluated whether warming altered the biomass, C and N concentration and isotope composition of V. uliginosum in mixed plots. Our results showed that warming induced positive changes in the AG growth of V. myrtillus but not BG, while V. uliginosum did not respond to warming. Vaccinium myrtillus performance did not differ between stand types under increased temperatures, suggesting that warming did not induce shifts in the interaction between V. myrtillus and its neighbouring species. These findings contrast with previous studies in which species interactions changed when temperature was modified. Our results show that species interactions can be less responsive to warming in natural plant communities than in removal experiments, highlighting the need for studies involving the natural assembly of plant species and communities when exploring the effect of environmental changes on plant-plant interactions

Tundra Trait Team: A database of plant traits spanning the tundra biome

Published VersionMotivation:The Tundra Trait Team (TTT) database includes field‐based measurements of key traits related to plant form and function at multiple sites across the tundra biome. This dataset can be used to address theoretical questions about plant strategy and trade‐offs, trait–environment relationships and environmental filtering, and trait variation across spatial scales, to validate satellite data, and to inform Earth system model parameters. Main types of variable contained: The database contains 91,970 measurements of 18 plant traits. The most frequently measured traits (> 1,000 observations each) include plant height, leaf area, specific leaf area, leaf fresh and dry mass, leaf dry matter content, leaf nitrogen, carbon and phosphorus content, leaf C:N and N:P, seed mass, and stem specific density. Spatial location and grain: Measurements were collected in tundra habitats in both the Northern and Southern Hemispheres, including Arctic sites in Alaska, Canada, Greenland, Fennoscandia and Siberia, alpine sites in the European Alps, Colorado Rockies, Caucasus, Ural Mountains, Pyrenees, Australian Alps, and Central Otago Mountains (New Zealand), and sub‐Antarctic Marion Island. More than 99% of observations are georeferenced. Time period and grain: All data were collected between 1964 and 2018. A small number of sites have repeated trait measurements at two or more time periods.Major taxa and level of measurement:Trait measurements were made on 978 terrestrial vascular plant species growing in tundra habitats. Most observations are on individuals (86%), while the remainder represent plot or site means or maximums per species. Software format: csv file and GitHub repository with data cleaning scripts in R; contribution to TRY plant trait database (www.try-db.org) to be included in the next version release

From Understanding to Sustainable Use of Peatlands: The WETSCAPES Approach

Of all terrestrial ecosystems, peatlands store carbon most effectively in long-term scales of millennia. However, many peatlands have been drained for peat extraction or agricultural use. This converts peatlands from sinks to sources of carbon, causing approx. 5% of the anthropogenic greenhouse effect and additional negative effects on other ecosystem services. Rewetting peatlands can mitigate climate change and may be combined with management in the form of paludiculture. Rewetted peatlands, however, do not equal their pristine ancestors and their ecological functioning is not understood. This holds true especially for groundwater-fed fens. Their functioning results from manifold interactions and can only be understood following an integrative approach of many relevant fields of science, which we merge in the interdisciplinary project WETSCAPES. Here, we address interactions among water transport and chemistry, primary production, peat formation, matter transformation and transport, microbial community, and greenhouse gas exchange using state of the art methods. We record data on six study sites spread across three common fen types (Alder forest, percolation fen, and coastal fen), each in drained and rewetted states. First results revealed that indicators reflecting more long-term effects like vegetation and soil chemistry showed a stronger differentiation between drained and rewetted states than variables with a more immediate reaction to environmental change, like greenhouse gas (GHG) emissions. Variations in microbial community composition explained differences in soil chemical data as well as vegetation composition and GHG exchange. We show the importance of developing an integrative understanding of managed fen peatlands and their ecosystem functioning.

Recent updates and developments to plant genome size databases

Two plant genome size databases have been recently updated and/or extended: the Plant DNA C-values database (http://data.kew.org/cvalues), and GSAD, the Genome Size in Asteraceae database (http://www.asteraceaegenomesize.com). While the first provides information on nuclear DNA contents across land plants and some algal groups, the second is focused on one of the largest and most economically important angiosperm families, Asteraceae. Genome size data have numerous applications: they can be used in comparative studies on genome evolution, or as a tool to appraise the cost of whole-genome sequencing programs. The growing interest in genome size and increasing rate of data accumulation has necessitated the continued update of these databases. Currently, the Plant DNA C-values database (Release 6.0, Dec. 2012) contains data for 8510 species, while GSAD has 1219 species (Release 2.0, June 2013), representing increases of 17 and 51%, respectively, in the number of species with genome size data, compared with previous releases. Here we provide overviews of the most recent releases of each database, and outline new features of GSAD. The latter include (i) a tool to visually compare genome size data between species, (ii) the option to export data and (iii) a webpage containing information about flow cytometry protocols