Mechanisms behind species-specific water economy responses to water level drawdown in peat mosses

Variations in water economy traits in two Sphagnum species across their distribution boundaries

PREMISE

We assessed changes in traits associated with water economy across climatic gradients in the ecologically similar peat mosses Sphagnum cuspidatum and Sphagnum lindbergii. These species have parapatric distributions in Europe and have similar niches in bogs. Sphagnum species of bogs are closely related, with a large degree of microhabitat niche overlap between many species that can be functionally very similar. Despite this, ecologically similar species do have different distributional ranges along climatic gradients that partly overlap. These gradients may favor particular Sphagnum traits, especially in relation to water economy, which can be hypothesized to drive species divergence by character displacement.

METHODS

We investigated traits relevant for water economy of two parapatric bryophytes (Sphagnum cuspidatum and S. lindbergii) across the border of their distributional limits. We included both shoot traits and canopy traits, i.e., collective traits of the moss surface, quantified by photogrammetry.

RESULTS

The two species are ecologically similar and occur at similar positions along the hydrological gradient in bogs. The biggest differences between the species were expressed in the variations of their canopy surfaces, particularly surface roughness and in the responses of important traits such as capitulum mass to climate. We did not find support for character displacement, because traits were not more dissimilar in sympatric than in allopatric populations.

CONCLUSIONS

Our results suggest that parapatry within Sphagnum can be understood from just a few climatic variables and that climatic factors are stronger drivers than competition behind trait variation within these species of Sphagnum.

Variation in Water-Holding Capacity in Sphagnum Species Depends on Both Plant and Colony Structure

Peatlands have become a focal point in climate mitigation strategies as these ecosystems have significant carbon sequestration capacities when healthy but release CO2 and other greenhouse gases when damaged. However, as drought episodes become more frequent and prolonged, organisms key to the functioning of some peatlands are increasingly under pressure from desiccation. The Sphagnum mosses, which tend to keep their ecosystem waterlogged and many of whom promote peat formation, are only mildly desiccation-tolerant in comparison to other mosses. The role of Sphagnum anatomy and colony structure is poorly understood in the context of desiccation resilience. Using four different Sphagnum species belonging to four different subgenera and positions along the gradient of the water table, we show that plant morphological traits and colony density are important determinants of water storage capacity. Our results show that, as previously postulated, the majority of the water is stored in an easily exchangeable form, probably extracellularly, and that plant morphological traits, specifically the type and presence of branches, are major contributors to water storage and can explain some of the interspecies variation. We also show that plant density is another important determinant for water storage capacity as higher densities hold larger quantities of water per unit of biomass for all four species, which increases resilience to desiccation. The results presented here suggest that species choice and planting density should receive more attention when considering peatland restoration strategies.

Moss functional trait ecology: Trends, gaps, and biases in the current literature

Functional traits are critical tools in plant ecology for capturing organism-environment interactions based on trade-offs and making links between organismal and ecosystem processes. While broad frameworks for functional traits have been developed for vascular plants, we lack the same for bryophytes, despite an escalation in the number of studies on bryophyte functional trait in the last 45 years and an increased recognition of the ecological roles bryophytes play across ecosystems. In this review, we compiled data from 282 published articles (10,005 records) that focused on functional traits measured in mosses and sought to examine trends in types of traits measured, capture taxonomic and geographic breadth of trait coverage, reveal biases in coverage in the current literature, and develop a bryophyte-function index (BFI) to describe the completeness of current trait coverage and identify global gaps to focus research efforts. The most commonly measured response traits (those related to growth/reproduction in individual organisms) and effect traits (those that directly affect community/ecosystem scale processes) fell into the categories of morphology (e.g., leaf area, shoot height) and nutrient storage/cycling, and our BFI revealed that these data were most commonly collected from temperate and boreal regions of Europe, North America, and East Asia. However, fewer than 10% of known moss species have available functional trait information. Our synthesis revealed a need for research on traits related to ontogeny, sex, and intraspecific plasticity and on co-measurement of traits related to water relations and bryophyte-mediated soil processes.

More is not always better: peat moss mixtures slightly enhance peatland stability

Terrestrial wetland ecosystems challenge biodiversity-ecosystem function theory, which generally links high species diversity to stable ecosystem functions. An open question in ecosystem ecology is whether assemblages of co-occurring peat mosses contribute to the stability of peatland ecosystem processes. We conducted a two-species (Sphagnum cuspidatum, Sphagnum medium) replacement series mesocosm experiment to evaluate the resistance, resilience, and recovery rates of net ecosystem CO2 exchange (NEE) under mild and deep water table drawdown. Our results show a positive effect of mild water table drawdown on NEE with no apparent role for peat moss mixture. Our study indicates that the carbon uptake capacity by peat moss mixtures is rather resilient to mild water table drawdown, but seriously affected by deeper drought conditions. Co-occurring peat moss species seem to enhance the resilience of the carbon uptake function (i.e. ability of NEE to return to pre-perturbation levels) of peat moss mixtures only slightly. These findings suggest that assemblages of co-occurring Sphagnum mosses do only marginally contribute to the stability of ecosystem functions in peatlands under drought conditions. Above all, our results highlight that predicted severe droughts can gravely affect the sink capacity of peatlands, with only a small extenuating role for peat moss mixtures.

Sphagnum mosses, the impact of disturbances and anthropogenic management actions on their ecological role in CO2 fluxes generated in peatland ecosystems

Mosses of the genus Sphagnum are the dominant vegetation in most pristine peatlands in temperate and high-latitude regions. They play a crucial role in carbon sequestration, being responsible for ca. 50% of carbon accumulation through their active participation in peat formation. They have a significant influence on the dynamics of CO2 emissions due to an efficient maximum potential photosynthetic rate, lower respiration rates, and the production of a recalcitrant litter whose decomposition is gradual. However, various anthropogenic disturbances and land use management actions that favor its reestablishment have the potential to modify the dynamics of these CO2 emissions. Therefore, the objective of this review is to discuss the role of Sphagnum in CO2 emissions generated in peatland ecosystems, and to understand the impacts of anthropogenic practices favorable and detrimental to Sphagnum on these emissions. Based on our review, increased Sphagnum cover reduces CO2 emissions and fosters C sequestration, but drainage transforms peatlands dominated by Sphagnum into a persistent source of CO2 due to lower gross primary productivity of the moss and increased respiration rates. Sites with moss removal used as donor material for peatland restoration emit twice as much CO2 as adjacent undisturbed natural sites, and those with commercial Sphagnum extraction generate almost neutral CO2 emissions, yet both can recover their sink status in the short term. The reintroduction of fragments and natural recolonization of Sphagnum in transitional peatlands, can reduce emissions, recover, or increase the CO2 sink function in the short and medium term. Furthermore, Sphagnum paludiculture is seen as a sustainable alternative for the use of transitional peatlands, allowing moss production strips to become CO2 sink, however, it is necessary to quantify the emissions of all the components of the field of production (ditches, causeway), and the biomass harvested from the moss to establish a final closing balance of C.

The dynamics of external water conduction in the dryland moss Syntrichia

Syntrichia relies on external water conduction for photosynthesis, survival, and reproduction, a condition referred to as ectohydry. Capillarity spaces are abundant in Syntrichia, but the link between function and morphology is complex. The aim of this study was to provide a better understanding of species-specific morphological traits underlying the functions of water conduction and storage. We used an environmental scanning electron microscope and confocal microscopy for observing anatomical characters in the leaves of Syntrichia species. We also measured hydration/dehydration curves to understand the rate of conduction and dehydration by experimental approaches. Syntrichia is an ectohydric moss that can externally transport and store water from the base of the stem using capillary action. We propose a new framework to study ectohydric capabilities, which incorporates three morphological scales and the timing of going from completely dehydrated to fully hydrated. Characters of interest in this model include cell anatomy (papillae development, hyaline basal cells and laminar cells), architecture of the stem (concavity and orientation) and whole clump characteristics (density of stems). We report significant variations in the speed of conduction, water holding capacity and hydration associated with each species studied (11 in total). All Syntrichia species are capable of external water conduction and storage, but the relevant traits differ among species. These results help to understand potential evolutionary and ecological trade-offs among speed of water conduction, water holding capacity, ontogeny, and differing habitat requirements. An integrative view of ectohydry in Syntrichia contributes to understanding the water relationships of mosses.

Molecular and physiological responses to desiccation indicate the abscisic acid pathway is conserved in the peat moss, Sphagnum

Mosses of the genus Sphagnum are the main components of peatlands, a major carbon-storing ecosystem. Changes in precipitation patterns are predicted to affect water relations in this ecosystem, but the effect of desiccation on the physiological and molecular processes in Sphagnum is still largely unexplored. Here we show that different Sphagnum species have differential physiological and molecular responses to desiccation but, surprisingly, this is not directly correlated with their position in relation to the water table. In addition, the expression of drought responsive genes is increased upon water withdrawal in all species. This increase in gene expression is accompanied by an increase in abscisic acid (ABA), supporting a role for ABA during desiccation responses in Sphagnum. Not only do ABA levels increase upon desiccation, but Sphagnum plants pre-treated with ABA display increased tolerance to desiccation, suggesting that ABA levels play a functional role in the response. In addition, many of the ABA signalling components are present in Sphagnum and we demonstrate, by complementation in Physcomitrium patens, that Sphagnum ABI3 is functionally conserved. The data presented here, therefore, support a conserved role for ABA in desiccation responses in Sphagnum.

The moss traits that rule cyanobacterial colonization

BACKGROUND AND AIMS

Cyanobacteria associated with mosses represent a main nitrogen (N) source in pristine, high-latitude and -altitude ecosystems due to their ability to fix N2. However, despite progress made regarding moss-cyanobacteria associations, the factors driving the large interspecific variation in N2 fixation activity between moss species remain elusive. The aim of the study was to identify the traits of mosses that determine cyanobacterial colonization and thus N2 fixation activity.

METHODS

Four moss species varying in N2 fixation activity were used to assess cyanobacterial abundance and activity to correlate it with moss traits (morphological, chemical, water-balance traits) for each species.

KEY RESULTS

Moss hydration rate was one of the pivotal traits, explaining 56 and 38 % of the variation in N2 fixation and cyanobacterial colonization, respectively, and was linked to morphological traits of the moss species. Higher abundance of cyanobacteria was found on shoots with smaller leaves, and with a high frequency of leaves. High phenol concentration inhibited N2 fixation but not colonization. These traits driving interspecific variation in cyanobacterial colonization, however, are also affected by the environment, and lead to intraspecific variation. Approximately 24 % of paraphyllia, filamentous appendages on Hylocomium splendens stems, were colonized by cyanobacteria.

CONCLUSIONS

Our findings show that interspecific variations in moss traits drive differences in cyanobacterial colonization and thus, N2 fixation activity among moss species. The key traits identified here that control moss-associated N2 fixation and cyanobacterial colonization could lead to improved predictions of N2 fixation in different moss species as a function of their morphology.

Global CO2 fertilization of Sphagnum peat mosses via suppression of photorespiration during the twentieth century

Natural peatlands contribute significantly to global carbon sequestration and storage of biomass, most of which derives from Sphagnum peat mosses. Atmospheric CO₂ levels have increased dramatically during the twentieth century, from 280 to > 400 ppm, which has affected plant carbon dynamics. Net carbon assimilation is strongly reduced by photorespiration, a process that depends on the CO₂ to O₂ ratio. Here we investigate the response of the photorespiration to photosynthesis ratio in Sphagnum mosses to recent CO₂ increases by comparing deuterium isotopomers of historical and contemporary Sphagnum tissues collected from 36 peat cores from five continents. Rising CO₂ levels generally suppressed photorespiration relative to photosynthesis but the magnitude of suppression depended on the current water table depth. By estimating the changes in water table depth, temperature, and precipitation during the twentieth century, we excluded potential effects of these climate parameters on the observed isotopomer responses. Further, we showed that the photorespiration to photosynthesis ratio varied between Sphagnum subgenera, indicating differences in their photosynthetic capacity. The global suppression of photorespiration in Sphagnum suggests an increased net primary production potential in response to the ongoing rise in atmospheric CO₂, in particular for mire structures with intermediate water table depths.

Development of an Image Analysis Pipeline to Estimate Sphagnum Colony Density in the Field

Sphagnum peatmosses play an important part in water table management of many peatland ecosystems. Keeping the ecosystem saturated, they slow the breakdown of organic matter and release of greenhouse gases, facilitating peatland’s function as a carbon sink rather than a carbon source. Although peatland monitoring and restoration programs have increased recently, there are few tools to quantify traits that Sphagnum species display in their ecosystems. Colony density is often described as an important determinant in the establishment and performance in Sphagnum but detailed evidence for this is limited. In this study, we describe an image analysis pipeline that accurately annotates Sphagnum capitula and estimates plant density using open access computer vision packages. The pipeline was validated using images of different Sphagnum species growing in different habitats, taken on different days and with different smartphones. The developed pipeline achieves high accuracy scores, and we demonstrate its utility by estimating colony densities in the field and detecting intra and inter-specific colony densities and their relationship with habitat. This tool will enable ecologists and conservationists to rapidly acquire accurate estimates of Sphagnum density in the field without the need of specialised equipment.

Interlinking moss functional traits. A commentary on: 'Mechanisms behind species-specific water economy responses to water level drawdown in peat mosses'

This article comments on: Fia Bengtsson, Gustaf Granath, Nils Cronberg and Håkan Rydin, Mechanisms behind species-specific water economy responses to water level drawdown in peat mosses, Annals of Botany, Volume 126, Issue 2, 01 August 2020, Pages 219–230, https://doi.org/10.1093/aob/mcaa033.