Giant stem rosettes have strong facilitation effects on alpine plant communities in the tropical Andes

ABSTRACT

Background: Giant rosettes constitute one of the most distinctive growth-forms in tropical alpine ecosystems. However, their interactions with other plant species remain unexplored in high Andean páramos.

Aims: Quantify the effect of a dominant rosette (Coespeletia timotensis) on soil and microclimatic conditions and relate the impacts to plant community structure.

Methods: We analysed topsoil temperatures, soil organic matter (SOM) and plant species cover in areas adjacent to C. timotensis and paired areas outside, at three sites (4250–4360 m a.s.l.) in La Culata National Park, Venezuela. Species richness, total vegetation cover and percent cover of each species near and away from the rosettes were compared.

Results: Topsoil temperature amplitudes were lower and SOM greater near C. timotensis stems, compared to areas away from them. C. timotensis had a consistent positive effect on species richness, vegetation cover and the cover of many abundant species (including cushions and herbs).

Conclusions: The facilitation effects of C. timotensis on abiotic conditions and community structure indicate they play a key role as nurse plants, pointing to giant rosettes as foundation species for the maintenance of plant diversity in the alpine tropics.     

Effects of environmental heterogeneity on species diversity and composition of terrestrial bryophyte assemblages in tropical montane forests of southern Ecuador

Background: Most studies on tropical bryophytes deal with epiphytic species. This is the first ecological study of tropical forests that focuses specifically on terrestrial bryophytes.

Aim: To investigate the differences between slope and ridge environments in upper montane forests of southern Ecuador in terms of species diversity (richness, abundance), species composition and life forms of terrestrial bryophytes.

Methods: We used Non-metric Multidimensional Scaling (NMDS) to group bryophyte relevés by study location, habitat type and exposure class. Species indicator values were calculated and compared for different habitats.

Results: In total, 140 species were recorded, the majority being liverworts. NMDS analyses and Mantel correlations clearly separated between slope and ridge relevés, and between sunny and shaded microhabitats on ridges. Bryophyte life forms also showed different distribution patterns in slope and in ridge habitats. Mosses were more prominent in sunny than in shaded microhabitats.

Conclusions: Environmental differentiation between ridges and slopes, and small-scale variation in microclimatic conditions caused by differences in exposure, were stronger predictors of species richness and composition than geographical distance between study sites.     

The consequences of multiple resource shifts on the productivity and composition of alpine tundra communities: inferences from a long-term snow and nutrient manipulation experiment

Background: Gradients in the amounts and duration of snowpack and resulting soil moisture gradients have been associated with different plant communities across alpine landscapes.

Aims: The extent to which snow additions could alter plant community structure, both alone and in combination with nitrogen (N) and phosphorus (P) additions, provided an empirical assessment of the strength of these variables on structuring the plant communities of the alpine tundra at Niwot Ridge, Colorado Front Range.

Methods: A long-term snow fence was used to study vegetation changes in responses to snowpack, both alone and in conjunction with nutrient amendments, in plots established in dry and moist meadow communities in the alpine belt. Species richness, diversity, evenness and dissimilarity were evaluated after 20 years of treatments.

Results: Snow additions, alone, reduced species richness and altered species composition in dry meadow plots, but not in moist meadow; more plant species were found in the snow-impacted areas than in nearby controls. Changes in plant community structure to N and N + P additions were influenced by snow additions. Above-ground plant productivity in plots not naturally affected by snow accumulation was not increased, and the positive responses of plant species to nutrient additions were reduced by snow addition. Plant species showed individualistic responses to changes in snow and nutrients, and indirect evidence suggested that competitive interactions mediated responses. A Permanova analysis demonstrated that community dissimilarity was affected by snow, N, and P additions, but with these responses differing by community type for snow and N. Snow influenced community patterns generated by N, and finally, the communities impacted by N + P were significantly different than those affected by the individual nutrients.

Conclusions: These results show that changes in snow cover over a 20-year interval produce measureable changes in community composition that concurrently influence and are influenced by soil nutrient availability. Dry meadow communities exhibit more sensitivity to increases in snow cover whereas moist meadow communities appear more sensitive to N enrichment. This study shows that the dynamics of multiple limiting resources influence both the productivity and composition of alpine plant communities, with, species, life form, and functional traits mediating these responses.     

Contrasting long-term alpine and subalpine precipitation trends in a mid-latitude North American mountain system,Colorado Front Range,USA

Background: Long-term climate trends in mountain systems often vary strongly with elevation.

Aims: To evaluate elevation dependence in long-term precipitation trends in subalpine forest and alpine tundra zones of a mid-continental, mid-latitude North American mountain system and to relate such dependence to atmospheric circulation patterns.

Methods: We contrasted 59-year (1952–2010) precipitation records of two high-elevation climate stations on Niwot Ridge, Colorado Front Range, Rocky Mountains, USA. The sites, one in forest (3022 m a.s.l.) and the other in alpine tundra (3739 m), are closely located (within 7 km horizontally, ca. 700 m vertically), but differ with respect to proximity to the mountain-system crest (the Continental Divide).

Results: The sites exhibited significant differences in annual and seasonal precipitation trends, which depended strongly on their elevation and distance from the Continental Divide. Annual precipitation increased by 60 mm (+6%) per decade at the alpine site, with no significant change at the subalpine site. Seasonally, trends at the alpine site were dominated by increases in winter, which we suggest resulted from an increase in orographically generated precipitation over the Divide, driven by upper-air (700 hPa) north-westerly flow. Such a change was not evident at the subalpine site, which is less affected by orographic precipitation on north-westerly flow.

Conclusions: Elevation dependence in precipitation trends appears to have arisen from a change in upper-air flow from predominantly south-westerly to north-westerly. Dependence of precipitation trends on topographic position and season has complex implications for the ecology and hydrology of Niwot Ridge and adjacent watersheds, involving interactions among physical processes (e.g. snowpack dynamics) and biotic responses (e.g. in phenologies and ecosystem productivity).     

Uncoupled changes in tree cover and field layer vegetation at two Pyrenean treeline ecotones over 11 years

Background: The alpine treeline ecotone is regarded as a sensor of the effects of global change on alpine plant communities. However, little is known about how treeline dynamics influence the diversity and composition of alpine plant communities. Such information is necessary to forecast how ascending montane forests may affect the composition of alpine flora.

Aims: We analysed the temporal variations in tree cover, plant diversity and composition, and the effect of tree cover dynamics on field layer vegetation over a period of 11 years, at two alpine treeline ecotones in the central Pyrenees, Spain.

Methods: Tree and field layer vegetation was sampled in permanent transects in 1998 and 2009, using the point-intercept method. Temporal changes in tree cover, plant species richness and abundance were characterised along the ecotone by using a randomisation approach, rarefaction curves, and a non-parametric multivariate test, respectively.

Results: Tree cover increased significantly at one of the sites, whereas plant species richness only increased at the other site where tree cover had not changed. Vegetation composition changed significantly at both sites, but it was not spatially coupled with changes in tree cover along the ecotone.

Conclusions: A change of tree cover does not necessarily trigger changes in the ground flora at the treeline over relatively short periods (decade scale). The results challenge our ability to infer short-term biodiversity impacts from upslope advance of forests. Integrated tree and field layer monitoring approaches are necessary to produce a better understanding of the impact of ongoing global change on treeline ecotones.     

Plant community response to nitrogen and phosphorus enrichment varies across an alpine tundra moisture gradient

Background: The extent to which nutrient availability influences plant community composition and dynamics has been a focus of ecological enquiry for decades.

Aims: Results from a long-term nitrogen (N) and phosphorus (P) addition experiment in alpine tundra were used to evaluate the importance of the two nutrients in structuring plant communities in three communities that differed in their snow cover amounts and duration and soil moisture characteristics.

Methods: A factorial N and P experiment was established in three meadows differing in initial vegetation composition and soil moisture. Plant and soil characteristics were measured after 20 years, and the dissimilarity among meadows and treatments were measured using permutational analysis of variance.

Results: Plant species richness declined uniformly across the three meadow types and in response to N and N + P additions, while both evenness and the Shannon diversity index finding indicated that nutrient additions had the highest impact on moister habitats. Overall, N impacts overshadowed changes attributed to P additions, and the N and N + P plots in wet meadow sites were the least diverse and scored the lowest dissimilarity averages among treatments. Dissimilarity estimates indicated that the control and P plots in the dry meadow community were more distinct in composition than all other plots, and especially those in the moist or wet meadows. Above-ground biomass of grasses and sedges (graminoids) increased with N additions while forbs appeared to show responses dictated in part by the graminoid responses. The most abundant grass species of moist and wet meadow, Deschampsia cespitosa, dominated N and N + P plots of the wet sites, but did not show a N response in moist areas in spite of its general abundance in moist meadow. Competition from other plant species in the moist areas likely diminished the D. cespitosa response and contributed to the resilience of the community to nutrient enrichment.

Conclusions: Initial community composition, as influenced by the specific moisture regime, appears to control the extent to which changes in nutrient resources can alter plant community structure. Long-term fertilization tends to support most but not all findings obtained from shorter-termed efforts, and wet meadows exhibit the largest changes in plant species numbers and composition when chronically enriched with N.     

Positive effects of an extremely hot summer on propagule rain in upper alpine to subnival habitats of the Central Eastern Alps

Background: Propagule production and dispersal largely determine the distribution and potential migration ability of alpine plant species. Variation in reproductive success caused by year-to-year variation in climate may critically influence these processes.

Aims: To obtain estimates for the propagule rain in high-alpine plant communities and detect potential dispersal events from lower elevations.

Methods: The magnitude and composition of the propagule rain was studied in different plant communities along an elevation gradient from the upper alpine to subnival zone. Propagules were trapped at eight elevations from 2760 to 3070 m a.s.l. for three years from July to September 2003–2005. Vascular plant species and their cover were recorded in an area with a radius of 10 m surrounding the traps.

Results: A five- to 10-fold higher propagule rain was observed in 2003, a year with an exceptionally hot summer, compared to 2004 and 2005. Propagule and species numbers varied highly among years and community types. Few propagules of non-local origin were recorded in any year.

Conclusions: Extremely hot summers are likely to greatly magnify the propagule rain size of species in alpine habitats. Such ‘mast years’ may contribute to enhanced and accelerated vegetation changes in alpine habitats in the absence of limiting factors.     

Diversity of forest plant species at the community and landscape scales in Switzerland

Abstract

Conservation strategies increasingly refer to indicators derived from large biological data. However, such data are often unique with respect to scale and species groups considered. To compare richness patterns emerging from different inventories, we analysed forest species richness at both the landscape and the community scales in Switzerland. Numbers of forest species were displayed using nationwide distributional species data and referring to three different definitions of forest species. The best regression models on a level of four predictor variables ranged between adj. R ² = 0.50 and 0.66 and revealed environmental heterogeneity/energy, substrate (rocky outcrops) and precipitation as best explanatory variables of forest species richness at the landscape scale. A systematic sample of community data (n = 729; 30 m², 200 m², 500 m²) was examined with respect to nationwide community diversity and plot species richness. More than 50% of all plots were assigned to beech forests (Eu-Fagion, Cephalanthero-Fagion, Luzulo-Fagion and Abieti-Fagion), 14% to Norway spruce forests (Vaccinio-Piceion) and 13% to silver fir forests (Piceo-Abietion). Explanatory variables were derived from averaged indicator values per plot, and from biophysical and disturbance factors. The best models for plot species richness using four predictor variables ranged between adj. R ² = 0.31 and 0.34. Light (averaged L-indicator, tree canopy) and substrate (averaged R-indicator and pH) had the highest explanatory power at all community scales. By contrast, the influence of disturbance variables was very small, as only a small portion of plots were affected by this factor. The effects of disturbances caused by extreme events or by management would reduce the tree canopies and lead to an increase in plant species richness at the community scale. Nevertheless, such community scale processes will not change the species richness at the landscape scale. Instead, the variety of different results derived from different biological data confirms the diversity of aspects to consider. Therefore, conservation strategies should refer to value systems.     

Microtopographic heterogeneity constrains alpine plant diversity, Glacier National Park, MT

Theoretical and empirical evidence exists for a positive relationship between environmental heterogeneity and species diversity. Alpine plant communities can exhibit exceptional diversity at a fine scale, which niche theory would suggest is the result of fine scale spatial heterogeneity of the environment. To test if species diversity of alpine plants is driven by environmental heterogeneity, we sampled vascular plant species composition, microtopography, and ground cover within 1?m² plots with and without solifluction forms in Glacier National Park, MT. We analyzed the relationship between microtopographic heterogeneity and species richness at the plot and sub-plot scale with linear and quantile regression, respectively. Species richness does not differ between the plots varying in cover type. Species richness is negatively related to the fractal dimension (D) of the ground surface and non-vegetated ground cover within 1?m² plots. At a finer scale, the standard deviation of elevation and slope appear to impose a limit on species richness such that more variable sub-plots have lower species richness. Contrary to our expectations, microtopographic heterogeneity does not promote the diversity of alpine plants. The negative relationship between topographic heterogeneity and species richness is contrary to the theoretical prediction that environmental heterogeneity generally results in greater species diversity. It is possible that microtopographic variability represents a measure of soil disturbance, which would be expected to have a negative effect on species diversity in alpine tundra due to its low productivity.     

Obituaries

Background: Although impacts of edge effects on forest ecosystems are well known, their consequences on savannas have rarely been explored.

Aims: To investigate the influence of edge effects on the plant community and microclimate of a cerrado fragment in south-eastern Brazil.

Methods: Several plant community variables (density, basal area, richness and cover by each vegetation layer) and microclimatic variables (light, air temperature and humidity), were measured in 10 transects across a savanna fragment surrounded by exotic grasses, and were used to fit semi-parametric models relating these variables with the distance from the habitat edge.

Results: Differences in microclimate and tree communities were poorly related to distance from the edge. On the other hand, there were detectable edge effects on the ground layer community (i.e. plants less than 50 cm in height). Edges had a negative effect on native plants of this layer (density and richness of all species and cover of native grasses), while favouring invasive grasses.

Conclusions: Unlike reports for edge effects in forest ecosystems, microclimate does not explain changes in this cerrado fragment. The most significant edge effect threatening the conservation of cerrado vegetation is the widespread invasion by African grasses. Starting from the fragment borders, this invasion causes changes in the structure and composition of the native plant community, thus jeopardising the population dynamics and persistence of native species.     

Functional group dominance and not productivity drives species richness

Background: There is a lack of consensus about the productivity–richness relationship, with several recent studies suggesting that it is not productivity but other factors that are the important drivers that determine species richness.

Aims: Here, we examine the relationship between productivity, functional group dominance and plant species richness at the plot scale in Tibetan Plateau meadows. These alpine meadows are ideal to examine the species productivity-richness relationship because they have a very high species richness, a large gradient in productivity, and can be dominated by either graminoids (grasses and sedges) or forbs.

Methods: We measured plant species richness and above-ground biomass along a natural gradient of functional group abundance in 44 plots distributed across five natural, winter-grazed but otherwise undisturbed sites in the eastern part of the Qing-Hai Tibetan Plateau, in Gansu province, China in 2008.

Results: Graminoid abundance (i.e. graminoid biomass as percent of the total above-ground biomass) explained 39% of plot differences in species richness while neither productivity nor the biomass of the three most abundant plant species, either individually or combined, were a significant predictor of species richness.

Conclusions: Our results show that within these alpine meadows, a shift from graminoid to forb dominance, rather than the individual dominant species or productivity itself, is strongly correlated with species richness. Thus, differences in functional group abundance can be a strong driver of observed plant species richness patterns.     

Seedling assemblages and the alternative successional pathways experienced by Atlantic forest fragments

Background: Habitat loss and fragmentation have been argued to drastically alter the composition of tree assemblages inhabiting small forest fragments but the successional trajectory experienced by such edge-affected habitats remains controversial.

Aims: Here we examine whether small fragments (3.4–91.2 ha) support seedling assemblages more similar to those in 10–70-year-old secondary forests than to those in mature forests, in order to infer to what extent fragments move toward early successional systems.

Methods: Using 59 0.1-ha plots distributed in a fragmented landscape of Brazilian Atlantic forest, we evaluated species richness and functional and taxonomic composition of seedling assemblages in 20 small forest fragments, 19 stands of secondary forest and 20 stands of mature forests in the interior of an exceptionally large fragment (ca. 3500 ha).

Results: Small fragments presented the least species-rich seedling assemblages (17.2 ± 5.7 species), followed by secondary (22.5 ± 5.3), and mature forest (28.4 ± 5.3). Small fragments had seedling assemblages with functional and taxonomic composition more similar to those in secondary than in mature forest. Small fragments had a greater relative richness and abundance of pioneer trees (ca. 40% more), vertebrate-dispersed (6–25%), and those bearing medium-sized seeds (30–70%), while large-seeded species and individuals were reduced (>50% decrement) in comparison to seedling assemblages in mature forest.

Conclusions: By comparing seedlings across a wide range of successional habitats we offer evidence that small forest fragments are experiencing an alternative successional pathway towards an early-successional system with reduced plant diversity.     

Multi-scale comparison of topographic complexity indices in relation to plant species richness

Topographic complexity is a key component of habitat, which has been linked to increased species richness in many ecological communities. It can be measured in various ways and it is unclear whether these different measurements are mutually comparable when they relate to plant species richness at different spatial scales. Using a densely sampled set of observations for Rhododendrons (406 species and 13,126 georeferenced records) as a test case, we calculated eight topographic complexity indices from a 250-m resolution digital elevation model and examined their correlations with Rhododendron species richness in China at seven spatial scales: grain sizes 0.05°, 0.1°, 0.25°, 0.5°, 1.0°, 1.5°, and 2.0°. Our results showed that the eight topographic complexity indices were moderately to highly correlated with each other, and the relations between each pair of indices decreased with increasing grain size. However, with an increase in grain size, there was a higher correlation between topographic complexity indices and Rhododendron species richness. At finer scales (i.e. grain size ≤ 1°), the standard deviation of elevation and range of elevation had significantly stronger correlations with Rhododendron species richness than other topographic complexity indices. Our findings indicate that different topographic complexity indices may have positive correlations with plant species richness. Moreover, the topographic complexity–species richness associations could be scale-dependent. In our case, the correlations between topographic complexity and Rhododendron species richness tended to be stronger at coarse-grained macro-habitat scales. We therefore suggest that topographic complexity index may serve as good proxy for studying the pattern of plant species richness at continental to global levels. However, choosing among topographic complexity indices must be undertaken with caution because these indices respond differently to grain sizes.     

The oldest monitoring site of the Alps revisited: accelerated increase in plant species richness on Piz Linard summit since 1835

Background: High-altitude ecosystems in the Alps have experienced severe environmental changes over the past decades, such as strong warming and increasing numbers of visitors and grazers. Few studies have followed the effects of such changes on the alpine flora over a period longer than a few decades. The summit of Piz Linard (3410 m, south-eastern Switzerland) is the oldest site in the Alps whose flora has been recorded, on average every 20 years since the Little Ice Age (1835).

Aims: We re-surveyed the summit flora of Piz Linard to trace its floristic changes and identify their patterns and possible drivers.

Methods: We mapped each species' highest location, distribution and abundance in the uppermost 30 m of the Piz Linard summit in 2011, and compared species composition and species’ altitudinal distribution over time.

Results: Species richness increased at an accelerated rate since 1992 and rose from 12 to 16 species since the previous record in 2003. Most already present species increased in abundance and colonised new areas of the summit, while new arrivals mainly established at sites with already high species richness. Species appeared after 1992 differed from species already present previously by having had lower maximum altitudes elsewhere in south-eastern Switzerland.

Conclusions: Temporal and spatial patterns of colonisations and former altitudinal ranges of species all point to climate warming as the principal driver of floristic change on Piz Linard.     

Soil pH and phosphorus drive species composition and richness in semi-natural heathlands and grasslands unaffected by twentieth-century agricultural intensification

Background: Increased soil phosphorus (P) caused by agricultural intensification has been associated with decreased plant species richness (SR) in central Europe. How plant communities and soil P gradients are related in unimproved open habitats remains unclear.

Aims: The aim of this article was to characterise the relationship between soil chemical parameters and plant species composition and richness in unimproved open habitats.

Methods: The influence of soil chemical parameters (pH, P, K, Mg) on species composition was assessed, using data from 40 heathland and 54 grassland plots, by non-metric multidimensional scaling and permutational multivariate analysis of variance. The relationship between soil chemical parameters and SR was tested by linear mixed effects models.

Results: A direct relationship between heathland community composition and pH was observed, explaining 10% of variation in species composition, while P, Mg and pH together explained 17% of variation in grassland composition. In heathlands, SR increased with increasing pH, whereas in grasslands, SR decreased with increasing soil P.

Conclusions: Soil chemical parameters were substantially related to plant community composition and richness. In an area spared from a century of agricultural intensification, reduced pH appeared to constrain SR in heathlands, while even slight P increases (<10 mg kg^?1) depressed plant SR in semi-natural grasslands.     

Conifer seedling recruitment across a gradient from forest to alpine tundra: effects of species,provenance, and site

Background: Seedling germination and survival is a critical control on forest ecosystem boundaries, such as at the alpine–treeline ecotone. In addition, while it is known that species respond individualistically to the same suite of environmental drivers, the potential additional effect of local adaptation on seedling success has not been evaluated.

Aims: To determine whether local adaptation may influence the position and movement of forest ecosystem boundaries, we quantified conifer seedling recruitment in common gardens across a subalpine forest to alpine tundra gradient at Niwot Ridge, Colorado, USA.

Methods: We studied Pinus flexilis and Picea engelmannii grown from seed collected locally at High (3400 m a.s.l.) and Low (3060 m a.s.l.) elevations. We monitored emergence and survival of seeds sown directly into plots and survival of seedlings germinated indoors and transplanted after snowmelt.

Results: Emergence and survival through the first growing season was greater for P. flexilis than P. engelmannii and for Low compared with High provenances. Yet survival through the second growing season was similar for both species and provenances. Seedling emergence and survival tended to be greatest in the subalpine forest and lowest in the alpine tundra. Survival was greater for transplants than for field-germinated seedlings.

Conclusions: These results suggest that survival through the first few weeks is critical to the establishment of natural germinants. In addition, even small distances between seed sources can have a significant effect on early demographic performance – a factor that has rarely been considered in previous studies of tree recruitment and species range shifts.     

Nurse effect of the native cushion plant Azorella monantha on the invasive non-native Taraxacum officinale in the high-Andes of central Chile

Positive interactions among native plant species are common in alpine habitats, particularly those where one species (nurse plant) generates microclimatic conditions that are more benign than the surrounding environment, facilitating the establishment of other species. Nonetheless, these microclimatic conditions could facilitate the establishment of non-native species as well. A conspicuous component of the alien alpine flora of the central Chilean Andes is the perennial herb Taraxacum officinale agg. (dandelion). In contrast to other alien species that are restricted to human-disturbed sites, T. officinale is frequently observed growing within native plant communities dominated by cushion plants. In this study we evaluated if T. officinale is positively associated with the cushion plant Azorella monantha. Via seedling survival experiments and gas-exchange measurements we also assessed the patterns of facilitation between cushions and dandelions, and explore the potential mechanisms of invasion by dandelions. T. officinale grows spatially positively associated with cushions of A. monantha. Survival of seedlings, as well as their net-photosynthetic rates and stomatal conductance, were higher within cushions than in open areas away from them, suggesting that the microclimatic modifications generated by this native cushion facilitates the establishment and performance of a non-native invasive species. Our results, as well as other recent studies, highlight the role of native communities in facilitating rather than constraining non-native plant invasions, particularly in stressful habitats such as alpine environments.     

The impact of red deer on liverwort-rich oceanic heath vegetation

Background: There is concern about increasing numbers of large herbivores including red deer (Cervus elaphus), but little is known about their impact on bryophytes.

Aims: This study set out to determine the effect of different localised densities of red deer on the internationally important Northern Atlantic hepatic mat, characteristic of oceanic heath vegetation, at four locations in the Scottish Highlands where sheep have been absent for decades.

Methods: Thirty 7 m × 7 m plots were randomly located in each study area. The standing crop dung pellet group count method was used to estimate red deer density. Species richness, diversity and cover of hepatic mat liverworts were obtained from 1 m × 1 m quadrats placed at random within the sample plots. Calluna vulgaris cover, ericoid height, rock cover, gradient and altitude were also recorded.

Results: Model simplification in analysis of covariance revealed a consistent pattern of decreasing cover of hepatic mat and Calluna with increasing red deer density at all four study areas. Northern Atlantic hepatic mat cover, diversity and species richness were positively correlated with Calluna cover.

Conclusions: The data suggest that Calluna cover is reduced (through trampling and browsing) at high local densities of red deer which has had cascading effects on the Northern Atlantic hepatic mat. Alternative explanations are discussed.     

Enhanced sexual reproduction in bryophytes at high latitudes in the maritime Antarctic

Abstract

Contrary to the generally accepted hypothesis that bryophyte fertility decreases with increasing latitude and therefore climatic severity, a detailed study of bryophyte reproductive strategies at sites in the southern maritime Antarctic (68–72°S) has revealed that an unexpectedly high proportion of species is capable of producing sporophytes. Of the regional bryoflora, 43% (19 species; 17 mosses, 2 liverworts) in Marguerite Bay and 47% (17 species; 16 mosses, 1 liverwort) in Alexander Island are known to produce sporophytes, although the number fruiting at comparable latitudes on the colder and more arid Antarctic continent is less (33%). These numbers and proportions are much greater than were previously known at such relatively high southern polar latitudes. Sporophytes of several species are recorded for the first time within the Antarctic biome, while those of two liverworts (Lophozia excisa and Cephaloziella varians) are reported for the first time south of 62°S. High incidence of sporophyte production is attributed to locally favourable microclimatic conditions producing small-scale 'oases'. The large majority of fertile mosses are monoecious short acrocarps growing on rather calcareous soils. Spore production and size data for a number of species are compared with previously published studies of the same or closely related species from the northern maritime Antarctic and sub-Antarctic, but no consistent trends are revealed between species over the latitudinal gradient.     

Grain size affects the relationship between species richness and above-ground biomass in semi-arid rangelands

ABSTRACT

Background: Discrepancies in the shape of the productivity–diversity relationship may arise from differences in spatial scale. We hypothesised that there is a grain size effect on the productivity–diversity relationship.

Aims: To determine the effect of three sampling grain sizes on the productivity–diversity relationship.

Methods: We applied generalised linear mixed effect models on community data from 735 vegetation plots in the Taleghan rangelands, Iran, sampled at three grain sizes (0.25, 1 and 2 m²) to ascertain plant productivity-diversity patterns, while accounting for the effects of site, plant community type, disturbance, and life form.

Results: Overall, relationships between biomass and plant species richness were unimodal at grain sizes of 0.25 and 1 m², and asymptotical at 2 m². The spurious occurrence of a single large shrub may overwhelm a small-sized sampling unit, resulting in a high estimate of the sample’s biomass relative to species richness. However, the relationship between biomass and species richness at larger grain sizes is more likely to reach an asymptote.

Conclusions: Shrubs are partly responsible for driving the relationship between plant biomass and species richness. Given that the frequency of shrubs is highly variable between small plots but not so in large plots, their presence may result in unimodal productivity–diversity relationships at small but not at large grain sizes.