Clonal distribution,fertility and sex ratios of the moss Plagiomnium affine (Bland.) T.Kop. in forests of contrasting age

Abstract

Six populations of the clonal forest floor moss Plagiomnium affine from forests of different age were screened for genetic variation at 23 allozyme loci, of which nine were polymorphic. Samples consisting of two adjacent unconnected shoots were taken at regular intervals along one transect from each population. A total of 602 shoots was analysed. Almost 80% of the shoots were sterile (i.e. not expressing male or female gender). Sex remained unknown for only 10% of shoots after identification of genets based on electrophoretic data. We identified a mean number of 3.7 fertile clones per population. The mean length of clones along transects in each population ranged between 2 and 3 m. The size distribution within populations was bimodal, with a few dominant clones and a varying number of much smaller clones. The overall sex ratio was slightly female biased at the ramet level, but balanced at the genet level. Forest age was negatively correlated with percentage of sterile shoots and positively correlated with frequency of sporophytes. In both cases correlations were significant only if population 1, which was subject to extreme soil disturbance by badgers, was excluded. We conclude that the effective population size is larger, and the susceptibility to genetic drift is lower, in old forests.     

Factors influencing reproductive success in the clonal moss, Hylocomium splendens

Female reproductive success in the unisexual perennial clonal moss Hylocomium splendens was examined by recording, if the segment was reproductive [produced sporophyte(s)] or not, together with several distance-to-male and male density variables, and segment size. This was done for every female segment in a population over a 5 year study period. A high fraction of the population could be sexed because we monitored the population in situ for 5 years, and thereafter harvested the population for electrophoretic analysis from which the clonal identity and expressed sex could be deduced. Fertilization distances in H. splendens were short, indicated by the fact that as many as 85% of the female segments with sporophytes were situated within a distance of 5.0 cm from the nearest male. The longest distance measured between a sporophytic female and the closest male was 11.6 cm. However, analysed within a generalized linear modelling (GLM) framework, the year was the best single predictor for the presence of H. splendens sporophyte although female-segment size and distance to the closest situated male were also strongly significant. The two latter factors explained larger fractions of variation in sporophyte presence in a GLM model with three predictors than in single-predictor models. This is because (i) the large variation in sporophyte production among years partly obscures the strong general increase in sporophyte production with increasing female-segment size and vitality, and (ii) the between-year variation and the size obscure the effect of the distance to the most proximate male. To our knowledge, this study is the first to incorporate into one model the relative importance of several factors for bryophyte reproductive success. Our results demonstrate the value of multiple-predictor approaches in studies of reproductive success.     

Fine-scale spatial patterns in populations of the clonal moss Hylocomium splendens partly reflect structuring processes in the boreal forest floor

Spatial patterns of important population or individual characteristics are expected to reflect structuring ecosystem processes to the extent that populations with strikingly different spatial patterns most likely have been structured by different processes. Few studies of spatial pattern in plant assemblages, bryophyte assemblages in particular, still exist and pattern has only vaguely been linked with process. In this study we describe fine-scale spatial variation in size and the occurrence of terminated (non-branching) segments in 21 Norwegian populations of the dominant boreal forest moss Hylocomium splendens, in search for general patterns of spatial structure. All Hylocomium splendens segments in the investigated plots were followed for a 10-year period; their size (dry mass, estimated from in situ measurements) and fate (terminated or ramifying) were recorded annually. The spatial structure of size and terminating segments at scales from 0.5 to 16 cm was described for each population as fractal dimension profiles, derived from semi-variograms. For about one half of the populations size and terminating segments could be assigned to a specific spatial pattern, the predicted outcome of one of three main structuring processes: (i) negative spatial dependence (fractal dimension F>3.0) on the scale of individuals (below 2 cm), indicative of negative interactions; (ii) positive spatial dependence (F<3.0) on the scale of individuals, indicative of positive interactions; and (iii) positive spatial dependence on broader (4–8 cm) scales, indicative of structuring by environmental factors or patchy disturbance. Patterns (i) and (iii) were observed both for size and terminated segments more often than expected. Fractal dimension profiles for size obtained separately for each year revealed temporal patterns of spatial structure that tended to be invariant over years. Negative spatial dependence of size, possibly due to large size difference between buried segments and other segments during self thinning, was typical of populations rapidly growing in number. Positive spatial dependence with a range of influence by the spatial process of 4–8 cm was observed in plots with sparse bryophyte cover and high cover of deciduous litter, probably due to accumulation of litter in depressions between shoots or groups of shoots. The main finding of this study is that different populations of the same species in the same type of ecosystem can be spatially structured in different ways. This accords with a model for the studied system as consisting of a mosaic of cells, of spatial extent of a few cm or larger, intergrading in space and temporally dynamic, the population characteristics of each cell being shaped by several structuring processes that vary, temporally and spatially, in their relative importance.     

Clonal structure and genet-level sex ratios suggest different roles of vegetative and sexual reproduction in the clonal moss Hylocomium splendens

The allozyme haplotype was determined for 157 ramets of the unisexual, perennial, clonal moss Hylocomium splendens within five 10×10 cm plots, which had been the subject of demographic studies over a 5-yr period. In addition, 25 shoots were analyzed from outside the plots and from four neighbouring patches. Only four haplotypes were encountered within the plots; one female type occurred in all plots and one male type in four plots, whereas two male haplotypes occurred in only one plot. Genets grew intermingled in all but one plot. The sex ratio within the five plots was female-biased at the ramet level (male:female=1:2.6), but male-biased at the genet level (3:1). Sporophytes were produced abundantly during the study period, but no signs of recruitment from spores were observed in the plots. Nine additional genets were encountered in neighbouring patches but from only one patch each. Four (44%) of these could potentially have been derived from spores generated within the plots. Our results suggest that each patch of H. splendens is colonized by a small number of genets, whereas different patches have different sets of genets, i.e. clonal diversity is determined by vegetative reproduction at within-patch scales and structured by sexual processes at among-patch scales.     

Between-year variation in climate-related growth of circumarctic populations of the moss Hylocomium splendens

1. The correlation between climatic variables and past (up to 20 years) growth was studied in seven circumarctic populations of the moss Hylocomium splendens, using retrospective analyses of growth. We hypothesized that relationships between growth and climate would be simpler in an ectohydric moss than in higher plants and that the moss could provide high signal-to-noise ratios of responses to climatic variation.
2. Growth parameters of the moss were strongly correlated with early summer temperatures and with the length of the growing season. Annual segment mass, growth rates and degeneration rates were highest at the mildest subarctic sites and lowest at the high arctic site. In contrast, 'longevity' (age of the oldest segment) increased at the climatically harsher sites.
3. Between-year growth variations at two contrasting sites were significantly correlated with June and July temperatures and, to a lesser extent, with early-season precipitation at one of the sites.
4. The moss currently tolerates a wide range of climates and large interannual variations in temperature and is likely to be at risk from climatic change only at the southern edge of its range.
5. The climate-change component most likely to affect the growth of H. splendens in the Arctic and Subarctic will be a lengthening of the growing season and in increase in early summer temperatures provided that moisture is not limiting.
6. Hylocomium splendens is a suitable species for monitoring climatic change at a circumarctic scale.     

CO2 environment,microclimate and photosynthetic characteristics of the moss Hylocomium splendens in a subarctic habitat

Summary In order to document the natural CO₂ environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO₂ concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July–September). Secondly, the photosynthetic responses of the species to controlled CO₂ concentrations, PAR, temperature, and hydration were measured in the laboratory. CO₂ concentrations around the upper parts of the plant, when PAR was above the compensation point (30 mol m^–2 s^–1), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 mol m^–2 s^–1), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO₂ uptake for most of the growing season. Our data suggest that the higher than normal ambient CO₂ concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO₂ predicted to occur over the next 50 years.     

Population biology of the clonal moss Hylocomium splendens in Norwegian boreal spruce forests. 5. Vertical dynamics of individual shoot segments

Demographic information was obtained for 12 528 mature segments (for which dry weight was estimated and vertical position in the bryophyte carpet recorded) and 3109 regenerated growing points for the perennial clonal moss Hylocomium splendens, recorded in Norwegian boreal spruce forests during a 6‐year period. Branching frequency varied with vertical position in the bryophyte carpet. Termination risk (probability of producing no offspring) was highest (44%) for buried segments, lowest (12%) for segments at intermediate vertical positions, and also high (26%) for emergent segments (due to increasing exposure to external mortality agents). Segment size increased from low levels in the bryophyte carpet to a maximum ca 2–10 mm below the top of the bryophyte carpet. This intermediate level was interpreted as the optimal compromise between incoming radiation (attenuating downwards) and microclimatic moisture conditions (improving downwards). Size‐corrected fitness, the number of offspring emerging from a mature segment within one year after maturation after allowance for differences in size, was lower for buried and emergent segments than for segments at intermediate positions. Small emergent segments were apparently liable to suffer from vitality reductions due to desiccation. The vertical position of a daughter segment depended on that of its parent segment, but also showed considerable stochastic variation. Burial acted as a strong sink for small segments regardless of vertical position. No evidence was found for species‐specific differences in the way pleurocarpous bryophytes interact, but reduced vertical mobility of H. splendens when growing among acrocarps indicated that growth‐form is an important determinant of bryophyte interactions. Evidence was found for vertical layering of the bryophyte carpet according to dominant type of interactions among individuals: none (environmental stress) above and at top, facilitation [a (+, +) interaction] at intermediate levels because of favourable water relationships in closed stands, and amensalism [a (0, ?) interaction] from higher‐situated segments that deprive lower‐situated segments access to light at lower relative levels. The intensity of amensalism increased downwards in the bryophyte carpet as indicated by a reinforced size hierarchy. The tendency for small H. splendens segments to become buried and lost from the population by amensalism is likely to represent a general mechanism for interactions between bryophyte species and succession in bryophyte‐dominated stands. Population effects of climatic and local environmental factors (favourability vs stress), disturbance and apparently random events are discussed with reference to their impact on the relative sizes of subpopulations acting as sources (due to facilitation) and sinks (due to amensalism).     

Temporal analysis of clonal structure in a moss bdelloid population

Clonal structure of a population of Macrotrachela quadricornifera (Rotifera, Bdelloidea) from a terrestrial moss in Northern Italy, was investigated over a 16 month period. Every month, 40–60 specimens of M. quadricornifera were collected from about 0.1 m² of moss. The individual animals were homogenized and their isozyme phenotypes analyzed by electrophoresis on vertical polyacrylamide gel. One enzyme, phosphoglucose isomerase (PGI), was used as a marker to distinguish the different clones present in the sample. A few clones were established from the rotifers sampled and patterns of esterases and , and malic enzyme were studied. Nine electrophoretic patterns for PGI were seen. One was dominant, a second was almost always present, but in lesser amounts. The remainder were present occasionally.There seemed to be no seasonal replacement of the clones and the composition of the population appeared to be unaffected by variations in temperature. Relative humidity seemed to be the more important factor in regulating the number of electromorphs of the rotifer population.     

Element concentrations in the boreal forest moss Hylocomium splendens: variation related to gradients in vegetation and local environmental factors

This study aims at assessing the suitability of Hylocomium splendens as a biomonitor for heavy metal deposition and contributing to the understanding of mineral nutrient uptake mechanisms in forest floor bryophytes. Concentrations of Ca, Mg, K, Cd, Cu and Pb were determined in Hylocomium splendens segment samples collected at 196 permanent vegetation plots from eleven monitoring areas, where the vegetation-environment relationships had previously been thoroughly studied. Element concentrations in Hylocomium splendens were related to environmentally interpreted vegetational gradients, cover of understorey vascular plants, and local environmental conditions, including element concentrations in humus, soil moisture and tree impact, by means of correlation analyses performed separately for each area. A combined test was performed for each Hylocomium splendens variable over all eleven areas, showing that concentrations of Ca, Cd and Mg in Hylocomium splendens were strongly correlated with the main vegetational gradient and with pH and concentrations of Ca and total N in humus, indicating considerable uptake of nutrients by Hylocomium splendens from water that has been in contact with humus. These relationships were less strong in the climatically most humid areas. Concentrations of all elements were more or less strongly correlated with one or more variables reflecting tree influence (tree density as measured by basal area and two crown influence indices) reflecting that throughfall precipitation, modified by leaching from the canopy, partly accounts for input of elements, especially for K, Cd and Mg. Only concentrations of Pb, Cd and Cu were significantly correlated, negatively, with vascular plant cover, indicating that supply of elements by leaching from understorey vascular plant foliage is negligible. The concentration of Cu in Hylocomium splendens was only weakly correlated, and the concentration of Pb unrelated to vegetational gradients and most environmental conditions. The results indicate that atmospheric deposition accounts for most of the input of Pb, confirming the suitability of Hylocomium splendens as a biomonitor for this element. For elements such as Cd local environmental conditions should be carefully considered, even when data from regional moss surveys are to be interpreted. This revised version was published online in June 2006 with corrections to the Cover Date.     

Costs of sporophyte production in the moss,Dicranum polysetum

We investigated the cost of sporophyte production in the moss Dicranum polysetum both by examining patterns of growth and reproduction in unmanipulated shoots and by experimentally manipulating sexual reproduction.The estimated proportion of total carbon investment allocated to sexual reproduction in sporophyte-producing shoots over the study period was 74.8%. Unmanipulated shoots that aborted all sporophytes had a significantly higher growth in the top shoots than shoots that produced sporophytes. In sporophyte-producing shoots, total apical growth decreased proportionately with the number of sporophytes.Experimental prevention of sporophyte development resulted in significantly higher total apical growth of the gametophytes. Shoots where current perichaetia were lacking when marked had a mass increase in the top shoots similar to manipulated sporophytic shoots whereas sporophytic control shoots grew significantly less than these two categories.The difference between control shoots and manipulated shoots in the mass of vegetative apical growth was mainly because of different length increments whereas mass per unit length was similar between groups. The probability to reproduce sexually in the year after the manipulation, and the biomass allocated to this reproduction, were not affected by the experimental treatment.     

Three modes of asexual reproduction in the moss Octoblepharum albidum

Abstract

Populations of Octoblepharum albidum growing in very moist and shaded habitats were found to have three kinds of asexual reproduction. First, the production of foliar gemmae along the upper half of leaf margins. Second, the production of protonema gemmae on protonemata originated from foliar gemmae. Third, the production of buds and new shoots at leaf tips. The formation of new shoots is further repeated at the leaf tips of the new shoots, giving the plant the character of a 'walking moss'.     

Age-related contents of polymerized lipids in the ectohydric forest mosses Pleurozium schreberi and Hylocomium splendens

Cell wall preparations of the ectohydric forest mosses, Pleurozium schreberi (Brid.) Mitt. and Hylocomium splendens (Hedw.) B. S. G. contain polymerized lipids consisting of hydroxy acids, dicarboxylic acids, fatty acids, fatty alcohols and unidentified components. The finding of polymerized lipids in ectohydric mosses, which have highly permeable cell walls, indicates that the polymers do not form an effective barrier against water and nutrients, at least not in the cell walls of these mosses. The youngest parts of P. schreberi and H. splendens contained 2.0 and 1.6 mg polymerized lipids, respectively, on a dry cell wall weight basis. In the senescent, greyish-green parts of P. schreberi and in the one-year-old shoot tissue of H. splendens the corresponding amounts were about 1.5-fold. In both species the increase was due to increases in hydroxy acids, particularly dihydroxyhexadecanoic acids, dicarboxylic acids, unknown components and, in the case of H. splendens , also an increase in fatty acids. The increase may be related to the maturation of the cell walls. In still older shoot parts the amounts of polymerized lipids decreased in both species, and remained low until final decay of the tissues into small particles. A slight increase in the amount of the polymerized lipid monomers was found in the oldest and most decomposed parts of H. splendens , probably indicating a better resistance to decay than for other cell wall components. These findings are discussed in relation to what is known from the ectohydric peat-forming Sphagnum mosses.     

Substratum preference,spore output and temporal variation in sporophyte production of the epixylic moss Buxbaumia viridis

Abstract

Occupancy and sporophyte numbers of Buxbaumia viridis on patches of decaying wood were investigated during a 4-year period (1996-99). Temporal variation in sporophyte number was investigated in relation to precipitation. Spore number per sporophyte was counted and spore number per substratum patch area and forest area were calculated. To predict the occupancy of B. viridis on decaying logs and stumps in a forest, the patch size (suitable wood area) was the most important variable. Decomposition stage was included in the model as a quality factor, expressing the higher and more stable humidity in late wood-decay stages. The suitable wood area of each patch was determined first and foremost by the stage of decomposition and diameter of the decaying wood. Only 16% of all wood patches considered suitable for production of sporophytes were occupied. The probability of a patch being occupied increased linearly with patch size to approximately 7 dm²; above this size the probability of being colonized was close to one. Both the number of sporophytes and the number of occupied patches were correlated with precipitation amounts during the summer months, with a reduction of occupied patches of 73% in the dry year 1999, compared with mean values for 1996-98. Spore number per sporophyte was correlated with length and width of the capsule, and varied between 1.4 and 9.0 million, with a mean value of 6.0 million. It is suggested that the rarity of B. viridis is caused by a low probability of patches being occupied because of their short longevity and small size, together with the facts that the species is dioicous, short-lived, sensitive to desiccation of the substratum, and has a gametophyte that is so minute it cannot compete with larger bryophytes.     

Sex ratio and sex expression in an urban population of the silver moss,Bryum argenteum Hedw.

ABSTRACT

Introduction. A well-supported pattern among dioicous bryophytes is male rarity. However, few assessments of bryophyte sex ratios have been made across environmental gradients to assess the role of environment in shaping population sex ratios.

Methods. We systematically surveyed 200 shoots from a 20?m² urban population of Bryum argenteum, and regenerated each shoot apex until sex expression occurred (up to 315 days).

Key results. Female shoots outnumbered male shoots 132 to 68, giving a sex ratio of 1.94♀: 1♂. The female bias was found in two transects in higher light environments but not in the third transect, which had a lower light level and an equal sex ratio. Female shoots took longer than male shoots to reach gametangial induction (122 vs. 60 days) and longer to produce 5 inflorescences (120 vs. 80 days). Male shoots produced an average of 10× the total number of inflorescences compared to female shoots (34 vs. 3.5 inflorescences). Despite producing more inflorescences, male plants also produced more regenerant shoots, thus contradicting the prediction that a higher prefertilisation reproductive effort in males trades off with vegetative proliferation. Female plants harboured significantly more associated microbes than male plants.

Conclusions. Our results support the role of light in influencing sex ratios in this species, suggest that trade-offs between reproduction and vegetative growth may not be strong for males, and indicate a potential role of a sex-specific microbiome in influencing sex ratios.     

Ecology of the invading moss species Orthodontium lineare in Sweden: Spatial distribution and population structure

The moss species Orthodontium lineare , originally from the southern hemisphere, has been spreading in Europe during this century. We have monitored the distribution patterns of the species in Sweden.
The age distribution differed between localities due to variation in the proportion of colonies in old age classes. We attributed this to differences in colony mortality. Although O. lineare had a high spore output in most localities, the colonies showed a clumped pattern, indicating strong neighbourhood effects in the colonization of new spots within the locality.
The regional distribution was uneven and markedly southwestern. The distribution was related to regional and local availability of suitable habitats (decaying wood in not-too-dense forests), and to a lesser extent to climatic variables.
Spore transport and establishment are suggested to be the limiting factors for colonization of suitable habitats of O. lineare .     

The frequency and distribution of thiopurine S-methyltransferase alleles in south Iranian population

Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of thiopurine drugs such as 6-mercaptopurine, 6-thioguanine, and azathiopurine. Variability in TPMT activity is mainly due to genetic polymorphism. The frequency of the four allelic variants of the TPMT gene, TPMT*2 (G238C), TPMT*3A (G460A and A719G), TPMT*3B (G460A) and TPMT*3C (A719G) were determined in an Iranian population from south of Iran (n = 500), using polymerase chain reaction (PCR)-RFLP and allele-specific PCR-based assays. Four hundred seventy four persons (94.8%) were homozygous for the wild type allele (TPMT*1/*1) and twenty five people were TPMT*1/*3C (5%). One patient was found to be heterozygous in terms TPMT*1 and *2 alleles with genotype of TPMT*1/*2 (0.2%). None of the participants had both defective alleles. The TPMT*3C and *2 were the only variant alleles observed in this population. The total frequency of variant alleles was 2.6% and the wild type allele frequency was 97.4%. The TPMT*3B and *3A alleles were not detected. Distributions of TPMT genotype and allele frequency in Iranian populations are different from the genetic profile found among Caucasian or Asian populations. Our findings also revealed inter-ethnic differences in TPMT frequencies between different parts of Iran. This view may help clinicians to choose an appropriate strategy for thiopurine drugs and reduce adverse drug reactions such as bone marrow suppression.