Altitudinal changes in temperature responses of net photosynthesis and dark respiration in tropical bryophytes

Characteristics of photosynthetic rates in different vegetation types at high-altitude in mountainous regions

Mountains play an important role in the carbon cycle of the terrestrial ecosystem and are one of the most sensitive ecosystems to climate change. However, our current knowledge regarding the physiological responses of alpine plants to environmental changes remains limited due to the severe climatic conditions prevailing in these high-altitude regions. Therefore, this study quantified the variations in photosynthetic rates (An) and identified their driving factors of herbaceous plants, shrubs, and trees along an elevation gradient (2200 m asl to 3200 m asl) on Mount Gongga. Elevation emerged as a significant determinant of An, with a general increase observed, albeit followed by a decline above 3000 m asl. In high-altitude regions, trees displayed more significant fluctuations in An compared to herbaceous plants and shrubs. The lower levels of atmospheric carbon dioxide concentration (eCO2) and temperature in high-altitude regions resulted in a 16 % increase in An for herbaceous plants, 60 % increase for shrubs, and 43 % increase for trees compared to the low-altitude areas. Structural equation modeling (SEM) analyses underscored the considerable impact of environmental factors on An. Notably, photosynthetically active radiation, eCO2, and stomatal conductance were identified as positive influencers, while other factors exerted negative effects. Our results further highlighted that trees were subject to greater constraints from multiple factors compared to herbs and shrubs, aligning with the outcomes of our variance analysis. In summary, our study presents a comprehensive assessment of vegetation responses to environmental factors along elevational gradients. The significance of An in plants at high altitude to external factors suggests the potential adaptability of alpine plants, and also indicates that changes in photosynthetic physiological functions at high altitude should be paid more attention to in the study of climate change.

Modelling the carbon balance in bryophytes and lichens: Presentation of PoiCarb 1.0, a new model for explaining distribution patterns and predicting climate-change effects

PREMISE

Bryophytes and lichens have important functional roles in many ecosystems. Insight into their CO2 -exchange responses to climatic conditions is essential for understanding current and predicting future productivity and biomass patterns, but responses are hard to quantify at time scales beyond instantaneous measurements. We present PoiCarb 1.0, a model to study how CO2 -exchange rates of these poikilohydric organisms change through time as a function of weather conditions.

METHODS

PoiCarb simulates diel fluctuations of CO2 exchange and estimates long-term carbon balances, identifying optimal and limiting climatic patterns. Modelled processes were net photosynthesis, dark respiration, evaporation and water uptake. Measured CO2 -exchange responses to light, temperature, atmospheric CO2 concentration, and thallus water content (calculated in a separate module) were used to parameterize the model's carbon module. We validated the model by comparing modelled diel courses of net CO2 exchange to such courses from field measurements on the tropical lichen Crocodia aurata. To demonstrate the model's usefulness, we simulated potential climate-change effects.

RESULTS

Diel patterns were reproduced well, and the modelled and observed diel carbon balances were strongly positively correlated. Simulated warming effects via changes in metabolic rates were consistently negative, while effects via faster drying were variable, depending on the timing of hydration.

CONCLUSIONS

Reproducing weather-dependent variation in diel carbon balances is a clear improvement compared to simply extrapolating short-term measurements or potential photosynthetic rates. Apart from predicting climate-change effects, future uses of PoiCarb include testing hypotheses about distribution patterns of poikilohydric organisms and guiding conservation strategies for species.

The trait co-variation regulates the response of bryophytes to nitrogen deposition: A meta-analysis

The nitrogen deposition has the potential to alter the trait composition of plant communities by affecting the fitness and physiological adaptation of species, consequently exerting an influence on ecosystem processes. Despite the importance of bryophytes in nutrient and carbon dynamics across different ecosystems, there is a lack of research examining the relationship between nitrogen deposition and the co-variation of bryophyte traits. To address this gap, a meta-analysis was conducted using data from 27 independent studies to investigate potential associations between trait co-variation of bryophytes and nitrogen deposition. The results revealed that interspecific variability regulates the influence of nitrogen deposition on bryophytes by affecting trait co-variation. Multiple correspondence analysis identified six combinations of closely related traits. For example, species with unbranched main stems frequently exhibit robust leaf midribs, leading to leaf wrinkling and leaf clasping around the stem as a response to water loss. Some weft or mat species tend to obtain resources (nitrogen) through their scale hairs on the main stem. Some species with narrow leaves require leaf teeth to maintain a normal leaf shape. The subgroup analyses indicated that certain traits, including unbranched main stem, changes in leaf morphology, robust leaf midrib, main stem without scale hairs, narrow leaf, leaf margin with teeth, undeveloped apophysis, and erect capsule minimize interaction with pollutants and represent a resource strategy. Conversely, functional traits representing a resource acquisition strategy, such as branched main stem, no changes in leaf morphology, short and weak leaf midrib, main stem with scale hairs, broad leaf, leaf margin without teeth, developed apophysis, and non-erect capsule increase pollutant exposure. Overall, our results suggest that anthropogenic global change may significantly impact bryophytes due to changes in their individual physiology and colony ecological indicators caused by increased nitrogen deposition.

Physiological Plasticity as a Strategy to Cope with Harsh Climatic Conditions: Ecophysiological Meta-Analysis of the Cosmopolitan Moss Ceratodon purpureus in the Southern Hemisphere

Determining the physiological tolerance ranges of species is necessary to comprehend the limits of their responsiveness under strong abiotic pressures. For this purpose, the cosmopolitan moss Ceratodon purpureus (Hedw.) Brid. is a good model due to its wide geographical distribution throughout different biomes and habitats. In order to disentangle how this species copes with stresses such as extreme temperatures and high radiation, we designed a meta-analysis by including the main photosynthetic traits obtained by gas exchange measurements in three contrasting habitats from the Southern Hemisphere. Our findings highlight that traits such as respiration homeostasis, modulation of the photosynthetic efficiency, adjustment of the optimal temperature, and switching between shade and sun-adapted forms, which are crucial in determining the responsiveness of this species. In fact, these ecophysiological traits are in concordance with the climatic particularities of each habitat. Furthermore, the photosynthetic trends found in our study point out how different Livingston Island (Maritime Antarctica) and Granite Harbour (Continental Antarctica) are for plant life, while the population from the Succulent Karoo Desert (South Africa) shares traits with both Antarctic regions. Altogether, the study highlights the high resilience of C. purpureus under abrupt climate changes and opens new perspectives about the wide spectrum of physiological responses of cryptogams to cope with climate change scenarios.

When time is not of the essence: constraints to the carbon balance of bryophytes

The data available so far indicate that the photosynthetic and relative growth rates of bryophytes are 10% of those reported for tracheophytes. By examining the existing literature and reanalysing data published in over 100 studies, this review examines the ecophysiological, biochemical, and structural reasons behind this phenomenon. The limiting Rubisco content and surface for gas exchange are the internal factors that can explain the low photosynthetic and growth rates of bryophytes. The role of the thicker cell walls of bryophytes in limiting CO2 diffusion is unclear, due to the current uncertainties regarding their porosity and permeability to CO2. From this review, it is also evident that, despite bryophytes having low photosynthetic rates, their positive carbon balance is tightly related to their capacity to deal with extreme conditions. Contributing factors include their capacity to deal with large daily temperature oscillations, and their capacity to delay the cessation of photosynthesis under water deficit (or to tolerate desiccation in extreme situations). Although further studies on bryophytes are needed before more solid conclusions can be drawn, it seems that their success relies on their remarkable tolerance to a highly variable environment, possibly at the expense of their maximum photosynthetic rate.

Limitations to photosynthesis in bryophytes: certainties and uncertainties regarding methodology

Bryophytes are the group of land plants with the lowest photosynthetic rates, which was considered to be a consequence of their higher anatomical CO2 diffusional limitation compared with tracheophytes. However, the most recent studies assessing limitations due to biochemistry and mesophyll conductance in bryophytes reveal discrepancies based on the methodology used. In this study, we compared data calculated from two different methodologies for estimating mesophyll conductance: variable J and the curve-fitting method. Although correlated, mesophyll conductance estimated by the curve-fitting method was on average 4-fold higher than the conductance obtained by the variable J method; a large enough difference to account for the scale of differences previously shown between the biochemical and diffusional limitations to photosynthesis. Biochemical limitations were predominant when the curve-fitting method was used. We also demonstrated that variations in bryophyte relative water content during measurements can also introduce errors in the estimation of mesophyll conductance, especially for samples which are overly desiccated. Furthermore, total chlorophyll concentration and soluble proteins were significantly lower in bryophytes than in tracheophytes, and the percentage of proteins quantified as Rubisco was also significantly lower in bryophytes (<6.3% in all studied species) than in angiosperms (>16% in all non-stressed cases). Photosynthetic rates normalized by Rubisco were not significantly different between bryophytes and angiosperms. Our data suggest that the biochemical limitation to photosynthesis in bryophytes is more relevant than so far assumed.

Diversity, phylogeny, and adaptation of bryophytes: insights from genomic and transcriptomic data

Bryophytes including mosses, liverworts, and hornworts are among the earliest land plants, and occupy a crucial phylogenetic position to aid in the understanding of plant terrestrialization. Despite their small size and simple structure, bryophytes are the second largest group of extant land plants. They live ubiquitously in various habitats and are highly diversified, with adaptive strategies to modern ecosystems on Earth. More and more genomes and transcriptomes have been assembled to address fundamental questions in plant biology. Here, we review recent advances in bryophytes associated with diversity, phylogeny, and ecological adaptation. Phylogenomic studies have provided increasing supports for the monophyly of bryophytes, with hornworts sister to the Setaphyta clade including liverworts and mosses. Further comparative genomic analyses revealed that multiple whole-genome duplications might have contributed to the species richness and morphological diversity in mosses. We highlight that the biological changes through gene gain or neofunctionalization that primarily evolved in bryophytes have facilitated the adaptation to early land environments; among the strategies to adapt to modern ecosystems in bryophytes, desiccation tolerance is the most remarkable. More genomic information for bryophytes would shed light on key mechanisms for the ecological success of these 'dwarfs' in the plant kingdom.

Light and Water Conditions Co-Regulated Stomata and Leaf Relative Uptake Rate (LRU) during Photosynthesis and COS Assimilation: A Meta-Analysis

As a trace gas involved in hydration during plant photosynthesis, carbonyl sulfide (COS) and its leaf relative uptake rate (LRU) is used to reduce the uncertainties in simulations of gross primary productivity (GPP). In this study, 101 independent observations were collected from 22 studies. We extracted the LRU, stomatal conductance (gs), canopy COS and carbon dioxide (CO2) fluxes, and relevant environmental conditions (i.e., light, temperature, and humidity), as well as the atmospheric COS and CO2 concentrations (Ca,COS and Ca,CO2). Although no evidence was found showing that gs regulates LRU, they responded in opposite ways to diurnal variations of environmental conditions in both mixed forests (LRU: Hedges’d = −0.901, LnRR = −0.189; gs: Hedges’d = 0.785, LnRR = 0.739) and croplands dominated by C3 plants (Hedges’d = −0.491, LnRR = −0.371; gs: Hedges’d = 1.066, LnRR = 0.322). In this process, the stomata play an important role in COS assimilation (R2 = 0.340, p = 0.020) and further influence the interrelationship of COS and CO2 fluxes (R2 = 0.650, p = 0.000). Slight increases in light intensity (R2 = 1, p = 0.002) and atmospheric drought (R2 = 0.885, p = 0.005) also decreased the LRU. The LRU saturation points of Ca,COS and Ca,CO2 were observed when ΔCa,COS ≈ 13 ppt (R2 = 0.580, p = 0.050) or ΔCa,CO2 ≈ −18 ppm (R2 = 0.970, p = 0.003). This study concluded that during plant photosynthesis and COS assimilation, light and water conditions co-regulated the stomata and LRU.

Seasonal Dynamics of Photochemical Performance of PS II of Terrestrial Mosses from Different Elevations

Mosses are critical components of tropical forest ecosystems and have multiple essential ecological functions. The drying and rehydrating and often hot environments in tropical regions present some of the greatest challenges for their photosynthetic activities. There is limited knowledge available on the physiological responses to the changing environments such as temperature and water pattern changes for terrestrial mosses. We examined the seasonal dynamics of photochemical performance of PS II through the measuring of chlorophyll fluorescence of 12 terrestrial mosses in situ from five different elevations by Photosynthesis Yield Analyzer MINI-PAM-II, along with the seasonal changes of climatic factors (air temperature, dew point, relative humidity and rainfall), which were collected by local weather stations and self-deployed mini weather stations. The results showed a great seasonality during observing periods, which, mainly the changes of rainfall and relative humidity pattern, presented significant impacts on the photochemical performance of PS II of terrestrial mosses. All these tested moss species developed a suitable regulated and non-regulated strategy to avoid the detrimental effect of abiotic stresses. We found that only Hypnum plumaeforme, Pterobryopsis crassicaulis and Pogonatum inflexum were well adapted to the changes of habitat temperature and water patterns, even though they still experienced a lower CO₂ assimilation efficiency in the drier months. The other nine species were susceptible to seasonality, especially during the months of lower rainfall and relative humidity when moss species were under physiologically reduced PS II efficiency. Anomobryum julaceum, Pogonatum neesii, Sematophyllum subhumile, Pseudotaxiphyllum pohliaecarpum and Leucobryum boninense, and especially Brachythecium buchananii, were sensitive to the changes of water patterns, which enable them as ideal ecological indicators of photosynthetic acclimation to stressed environments as a result of climate change.

It Is Hot in the Sun: Antarctic Mosses Have High Temperature Optima for Photosynthesis Despite Cold Climate

The terrestrial flora of Antarctica's frozen continent is restricted to sparse ice-free areas and dominated by lichens and bryophytes. These plants frequently battle sub-zero temperatures, extreme winds and reduced water availability; all influencing their ability to survive and grow. Antarctic mosses, however, can have canopy temperatures well above air temperature. At midday, canopy temperatures can exceed 15°C, depending on moss turf water content. In this study, the optimum temperature of photosynthesis was determined for six Antarctic moss species: Bryum pseudotriquetrum, Ceratodon purpureus, Chorisodontium aciphyllum, Polytrichastrum alpinum, Sanionia uncinata, and Schistidium antarctici collected from King George Island (maritime Antarctica) and/or the Windmill Islands, East Antarctica. Both chlorophyll fluorescence and gas exchange showed maximum values of electron transport rate occurred at canopy temperatures higher than 20°C. The optimum temperature for both net assimilation of CO2 and photoprotective heat dissipation of three East Antarctic species was 20-30°C and at temperatures below 10°C, mesophyll conductance did not significantly differ from 0. Maximum mitochondrial respiration rates occurred at temperatures higher than 35°C and were lower by around 80% at 5°C. Despite the extreme cold conditions that Antarctic mosses face over winter, the photosynthetic apparatus appears optimised to warm temperatures. Our estimation of the total carbon balance suggests that survival in this cold environment may rely on a capacity to maximize photosynthesis for brief periods during summer and minimize respiratory carbon losses in cold conditions.

Effects of moisture dynamics on bryophyte carbon fluxes in a tropical cloud forest

Bryophytes play key roles in the ecological function of a number of major world biomes but remain understudied compared with vascular plants. Little is known about bryophyte responses to different aspects of predicted changes in moisture dynamics with climate change. In this study, CO₂ fluxes and photosynthetic light responses were measured within bryophyte mesocosms, being subjected to different amounts, frequencies, and types (mist or rainfall) of water addition, both before and after different periods of complete desiccation. Bryophyte carbon fluxes and photosynthetic light response were generally affected by the magnitude and type, but not frequency, of watering events. Desiccation suppressed bryophyte carbon uptake even after rehydration, and the degree of uptake suppression progressively increased with desiccation duration. Estimated ecosystem-level bryophyte respiration and net carbon uptake were c. 58% and c. 3%, respectively, of corresponding fluxes from tree foliage at the site. Our results suggest that a simplified representation of precipitation processes may be sufficient to accurately model bryophyte carbon cycling under future climate scenarios. Further, we find that projected increases in drought could have strong negative impacts on bryophyte and ecosystem carbon storage, with major consequences for a wide range of ecosystem processes.

Anatomical constraints to nonstomatal diffusion conductance and photosynthesis in lycophytes and bryophytes

Photosynthesis in bryophytes and lycophytes has received less attention than terrestrial plant groups. In particular, few studies have addressed the nonstomatal diffusion conductance to CO₂ g_nsd of these plant groups. Their lower photosynthetic rate per leaf mass area at any given nitrogen concentration compared with vascular plants suggested a stronger limitation by CO₂ diffusion. We hypothesized that bryophyte and lycophyte photosynthesis is largely limited by low g_nsd . Here, we studied CO₂ diffusion inside the photosynthetic tissues and its relationships with photosynthesis and anatomical parameters in bryophyte and lycophyte species in Antarctica, Australia, Estonia, Hawaii and Spain. On average, lycophytes and, specially, bryophytes had the lowest photosynthetic rates and nonstomatal diffusion conductance reported for terrestrial plants. These low values are related to their very thick cell walls and their low exposure of chloroplasts to cell perimeter. We conclude that the reason why bryophytes lie at the lower end of the leaf economics spectrum is their strong nonstomatal diffusion conductance limitation to photosynthesis, which is driven by their specific anatomical characteristics.

The double role of pigmentation and convolute leaves in community assemblage of Amazonian epiphytic Lejeuneaceae

Background

Epiphytic bryophyte communities in the Amazon forest show a vertical gradient in species composition along the trunk of the host trees. The investigation of species traits related to this pattern has focused on the physiology of selected taxa with a clear preference for one of the extremes of the gradient. Although some species are indeed only found on the tree base or in the outer canopy, the vertical gradient is composed mainly by the variation in the abundances of species with a broader occurrence along the height zones. Therefore, this study approaches the differences among community assemblages, rather than among species, to test the role of morphological and dispersal traits on the establishment of the vertical gradient in species composition.

Methods

A character state matrix was built for 104 species of the family Lejeuneaceae recorded as epiphytes in the Amazonian terra firme forests, and six binary traits supposed to influence species occurrence: dark pigmentation on leaves; ability to convolute leaves when drying; possession of thickened cell walls; reproduction mode (monoicous or dioicous); occurrence of asexual reproduction; and facultative epiphyllous habit. Based on a previous dataset on community composition along the vertical gradient, trait occurrences in random draws of the metacommunity was compared to trait occurrences in field data, in order to detect significant deviations in the different height zones.

Results

Four out of the six traits tested showed significantly higher or lower occurrence in the species composition of canopy and/or understory communities. Traits related to high dispersal ability did not vary much along the vertical gradient; although facultative epiphylls were overrepresented on tree base. Dark pigmentation and convolute leaves were significantly more frequent in the canopy communities, but also significantly less frequent in communities at the base of the tree.

Discussion

Dark pigmentation and convolute leaves seem to be advantageous for the establishment in the canopy zones. They may, respectively, prevent light damage and allow longer periods of photosynthesis. Interestingly, these traits occur randomly along the trunk, but are wiped out of communities on the tree base. In the relatively deep shade of the first meters of the understory, they possibly hamper net carbon gain, the first by darkening the leaf surface and the second by delaying desiccation—which can be damaging under high temperatures and low light. The fact that production of asexual propagules is not overrepresented in the most dynamic microenvironment along the gradient, the canopy, challenges current views of bryophyte life strategy theory.

Seasonal variation of secondary metabolites in nine different bryophytes

Bryophytes occur in almost all land ecosystems and contribute to global biogeochemical cycles, ecosystem functioning, and influence vegetation dynamics. As growth and biochemistry of bryophytes are strongly dependent on the season, we analyzed metabolic variation across seasons with regard to ecological characteristics and phylogeny. Using bioinformatics methods, we present an integrative and reproducible approach to connect ecology with biochemistry. Nine different bryophyte species were collected in three composite samples in four seasons. Untargeted liquid chromatography coupled with mass spectrometry (LC/MS) was performed to obtain metabolite profiles. Redundancy analysis, Pearson's correlation, Shannon diversity, and hierarchical clustering were used to determine relationships among species, seasons, ecological characteristics, and hierarchical clustering. Metabolite profiles of Marchantia polymorpha and Fissidens taxifolius which are species with ruderal life strategy (R-selected) showed low seasonal variability, while the profiles of the pleurocarpous mosses and Grimmia pulvinata which have characteristics of a competitive strategy (C-selected) were more variable. Polytrichum strictum and Plagiomnium undulatum had intermediary life strategies. Our study revealed strong species-specific differences in metabolite profiles between the seasons. Life strategies, growth forms, and indicator values for light and soil were among the most important ecological predictors. We demonstrate that untargeted Eco-Metabolomics provide useful biochemical insight that improves our understanding of fundamental ecological strategies.

Ciprofloxacin vs. temperature: Antibiotic toxicity in the free-floating liverwort Ricciocarpus natans from a climate change perspective

The physiological responses of the aquatic liverwort Ricciocarpus natans to ciprofloxacin (Cipro) exposure under different growth temperatures were investigated. Cipro appears to act as an inhibitor of mitochondrial Complex III by blocking the oxidation of quinol, resulting in the formation of hydrogen peroxide (H₂O₂). H₂O₂ accumulation upon Cipro exposure is responsible for decreased photosynthesis in plants. The amount of H₂O₂ in plants is kept under control by antioxidant enzymes, whose activities are central to the responses of plants to Cipro yet are influenced by temperature. Increased temperature favored Cipro uptake by plants as well as its deleterious effects on mitochondrial activity; however, it also favored the activity of antioxidant enzymes, thereby preventing the exacerbation of the deleterious effects of Cipro. The uptake of Cipro by plants appears to be largely a passive process, although some uptake must be driven by an energy-consuming process. Ricciocarpus natans should be considered for programs aimed at the reclamation of Cipro since this plant exhibits high Cipro-tolerance, the capacity for accumulation and increased uptake rates of the antibiotic with increasing temperatures (from 20 to 30 °C).

Associations between shoot-level water relations and photosynthetic responses to water and light in 12 moss species

In vascular plants, there is a clear coupling between traits related to water and traits related to carbon economics. For bryophytes this coupling has been little studied but is expected to be strong, because in these poikilohydric plants photosynthesis varies strongly with water availability. We hypothesized that there is a trade-off between water-holding and photosynthetic capacities for mosses, resulting in a limited spectrum of possible trait combinations. At one end of this spectrum, mosses would tend to stay wet and active for long periods but would have slow photosynthetic rates. At the other end, mosses would avoid external water and dry out quicker but would have high photosynthetic capacities. We determined the water relations (water-holding and -retention capacities), photosynthetic water- and light-response curves of shoots of 12 moss species and explored the associations between these traits and their distributions among the studied species. The results partly support our hypotheses, in that the water-holding and water-retention capacities of mosses are positively related to each other and to the value and width of the optimal water-content range for photosynthesis. However, the photosynthetic capacities were specific to taxonomic groups, and the relationships between the water relations and the photosynthetic capacity are weak or inconsistent and depend strongly on the species used for analysis. The positive relationships between water-holding, water-retention and photosynthetic water-use capacities suggest two contrasting adaptations to avoid damage during dehydration: taking more time to 'prepare' or quick photosynthetic adjustment. However, the spectrum we hypothesized cannot be generalized for all mosses and defining a broader spectrum will require the extension of this study to a much larger number of species and including stand-level measurements of water loss and photosynthesis.

Bryophyte gas-exchange dynamics along varying hydration status reveal a significant carbonyl sulphide (COS) sink in the dark and COS source in the light

Carbonyl sulphide (COS) is a potential tracer of gross primary productivity (GPP), assuming a unidirectional COS flux into the vegetation that scales with GPP. However, carbonic anhydrase (CA), the enzyme that hydrolyses COS, is expected to be light independent, and thus plants without stomata should continue to take up COS in the dark. We measured net CO₂ (A^C ) and COS (A^S ) uptake rates from two astomatous bryophytes at different relative water contents (RWCs), COS concentrations, temperatures and light intensities. We found large A^S in the dark, indicating that CA activity continues without photosynthesis. More surprisingly, we found a nonzero COS compensation point in light and dark conditions, indicating a temperature-driven COS source with a Q₁₀ (fractional change for a 10°C temperature increase) of 3.7. This resulted in greater A^S in the dark than in the light at similar RWC. The processes underlying such COS emissions remain unknown. Our results suggest that ecosystems dominated by bryophytes might be strong atmospheric sinks of COS at night and weaker sinks or even sources of COS during daytime. Biotic COS production in bryophytes could result from symbiotic fungal and bacterial partners that could also be found on vascular plants.

Short-Term Responses in Maximum Quantum Yield of PSII (Fv/Fm) to ex situ Temperature Treatment of Populations of Bryophytes Originating from Different Sites in Hokkaido, Northern Japan

There is limited knowledge available on the thermal acclimation processes for bryophytes, especially when considering variation between populations or sites. This study investigated whether short-term ex situ thermal acclimation of different populations showed patterns of site dependency and whether the maximum quantum yield of PSII (Fv/Fm) could be used as an indicator of adaptation or temperature stress in two bryophyte species: Pleurozium schreberi (Willd. ex Brid.) Mitt. and Racomitrium lanuginosum (Hedw.) Brid. We sought to test the hypothesis that differences in the ability to acclimate to short-term temperature treatment would be revealed as differences in photosystem II maximum yield (Fv/Fm). Thermal treatments were applied to samples from 12 and 11 populations during 12 or 13 days in growth chambers and comprised: (1) 10/5 °C; (2) 20/10 °C; (3) 25/15 °C; (4) 30/20 °C (12 hours day/night temperature). In Pleurozium schreberi, there were no significant site-dependent differences before or after the experiment, while site dependencies were clearly shown in Racomitrium lanuginosum throughout the study. Fv/Fm in Pleurozium schreberi decreased at the highest and lowest temperature treatments, which can be interpreted as a stress response, but no similar trends were shown by Racomitrium lanuginosum.

Water relations and gas exchange of fan bryophytes and their adaptations to microhabitats in an Asian subtropical montane cloud forest

Fan life forms are bryophytes with shoots rising from vertical substratum that branch repeatedly in the horizontal plane to form flattened photosynthetic surfaces, which are well suited for intercepting water from moving air. However, detailed water relations, gas exchange characteristics of fan bryophytes and their adaptations to particular microhabitats remain poorly understood. In this study, we measured and analyzed microclimatic data, as well as water release curves, pressure-volume relationships and photosynthetic water and light response curves for three common fan bryophytes in an Asian subtropical montane cloud forest (SMCF). Results demonstrate high relative humidity but low light levels and temperatures in the understory, and a strong effect of fog on water availability for bryophytes in the SMCF. The facts that fan bryophytes in dry air lose most of their free water within 1 h, and a strong dependence of net photosynthesis rates on water content, imply that the transition from a hydrated, photosynthetically active state to a dry, inactive state is rapid. In addition, fan bryophytes developed relatively high cell wall elasticity and the osmoregulatory capacity to tolerate desiccation. These fan bryophytes had low light saturation and compensation point of photosynthesis, indicating shade tolerance. It is likely that fan bryophytes can flourish on tree trunks in the SMCF because of substantial annual precipitation, average relative humidity, and frequent and persistent fog, which can provide continual water sources for them to intercept. Nevertheless, the low water retention capacity and strong dependence of net photosynthesis on water content of fan bryophytes indicate a high risk of unbalanced carbon budget if the frequency and severity of drought increase in the future as predicted.

The temperature acclimation potential of tropical bryophytes

Bryophyte biomass and diversity in tropical moist forests decrease dramatically from higher altitudes towards the lowlands. High respiratory carbon losses at high temperatures may partly explain this pattern, if montane species are unable to acclimatise their metabolic rates to lowland temperatures. We transplanted ten bryophyte species from two altitudes (1200 and 500 m a.s.l.) to lower (warmer) altitudes (500 m and sea level) in Panama. We studied short-term temperature acclimation of CO2 exchange for 2.5 months, and survival and growth for 21 months following transplantation. Short-term acclimation did not occur, and on a longer time scale mortality was highest and growth lowest in the transplanted samples. A few transplanted samples of most species, however, survived the whole experiment and finished with growth rates similar to controls. This recovery of growth rate suggests temperature acclimation, in spite of no measurable metabolic changes in smaller random samples. This acclimation even compensated for shorter periods of CO2 uptake due to more rapid drying. Nevertheless, these species are not abundant in lowland forests, perhaps due to dispersal or establishment limitation. The apparent heterogeneity of the acclimation potential within species may allow populations to adapt locally and avoid being forced uphill under climatic warming.