The stream as a habitat templet for bryophytes: species' distributions along gradients in disturbance and substratum heterogeneity

1. The structure of bryophyte communities in streams in relation to habitat characteristics, especially disturbance, is described. Disturbance in rivers is quantified as movement of the stream bed, whereas in small streams water level fluctuation is used as an indicator of disturbance frequency. 2. Canonical correspondence analysis differentiated frequently disturbed sites from more stable ones. The existence of a disturbance gradient was confirmed in a subset of the study sites using long-term records of discharge variation. A parallel change was detected in the species composition of bryophyte communities with low-statured, potentially fast colonizers dominating the disturbed end and large perennial species the stable end of the gradient. 3. A consistent pattern of zonation of bryophyte species was found along the gradient from continually submersed to persistently dry conditions in small streams and lake outlets. An abrupt increase in species richness occurred at or just above the water line, where facultatively aquatic species tolerant of both conditions formed the bulk of the community. 4. The relationship between species richness and standing crop in stream bryophyte communities was consistent with the hump-backed model of Grime (1979), especially at the within-habitat scale. Quadrats of low and very high standing crop were characterized by low species richness, while peak richness was observed at intermediate standing crops. 5. A few perennial species (e.g. Fontinalis spp. and Rhynchostegium riparioides) capable of monopolizing space dominated the most stable habitats. Species composition in low biomass sites was more variable, yet only one basic growth-form (small-statured species with high allocation to spore production) seemed possible in these highly disturbed streams. In habitats of intermediate biomass, small-scale disturbances apparently allow the formation of a more varied bryophyte community. 6. A habitat templet for stream bryophyte life strategies and community structure is presented. Disturbance is proposed to be the factor filtering out traits unsuitable for a given environment. The potential of stream bryophytes for testing and developing general ecological theory is emphasized.     

The effect of canopy disturbance on species richness in a central Himalayan oak forest

Non-epiphytic species richness was studied in different disturbance classes within a Quercus semecarpifolia forest. Nine disturbance classes were defined according to the degree of biomass removal (lopping) and their spatial mixture. Six of these were observed in the study area. The species were divided into three functional groups: climbers, phanerophytes, and field-layer plants. The primary aim was to test if there is an elevated species richness under an intermediate disturbed canopy for (i) all vascular plants, (ii) lianas, (iii) phanerophytes and (iv) field-layer species. The richness of the different plant groups and all species were fitted against the disturbance gradient by means of Generalized Linear Models (GLM). Other environmental variables such as altitude, potential solar radiation, light intensity, canopy cover and soil parameters were also evaluated as predictors. Disturbance classes, canopy cover and light intensity were combined into a new variable, disturbance-complex, using Principal Component Analyses.Phanerophytes did not respond to any variable. Climbers were mostly related to pH and canopy cover, and were the only group related to altitude, nitrogen and loss-on-ignition. Herbaceous plants and total species richness showed a unimodal response to disturbance classes and the complex disturbance gradient, which supports the intermediate disturbance hypothesis. Relative radiation and slope also supported a unimodal response in herbaceous plants, but disturbance had a significant additional contribution to this pattern. The most significant predictor for these two groups was pH. The responses to organic carbon and phosphorus were not significant for any of the subsets.The results indicate that a small-scale lopping regime will enhance species richness of vascular plants; only a few species in the intermediate disturbed forest are weedy ruderals. In such a situation, the conservation policy may accept small-scale human impact as part of the forest landscape.     

Productivity and species richness across an environmental gradient in a fire-dependent ecosystem

The fire-dependent longleaf pine-wiregrass (Pinus palustris Mill.-Aristida beyrichiana Trin. & Rupr.) savannas of the southeastern United States provide a unique opportunity to examine the relationship between productivity and species richness in a natural ecosystem because of the extremely high number of species and their range across a wide ecological amplitude (sandhills to edges of wetlands). We used a natural gradient to examine how plant species richness and plant community structure vary with standing crop biomass (which in this system is proportional to annual net productivity) as a function of soil moisture and nitrogen mineralization rates in a frequently burned longleaf pine-wiregrass savanna. Highest ground cover biomass and highest species richness were found at the same position along the gradient, the wet-mesic sites. Relative differences in species richness among site types were independent of scale, ranging from 0.01 m(2) to 100 m(2). Nitrogen availability was negatively correlated with species richness. Dominance of wiregrass (in terms of biomass) was consistent across the gradient and not correlated with species richness. Regardless of site type, the community structure of the savannas was characterized by many perennial species with infrequent occurrences, a factor in the low temporal heterogeneity (percent similarity between seasons and years) and high within-site spatial heterogeneity (percent dissimilarity of vegetation composition). The coexistence of numerous species is likely due to the high frequency of fire that removes competing hardwood vegetation and litter and to the suite of fire-adapted perennial species that, once established, are able to persist. Our results suggest that soil moisture is an important factor regulating both the number of species present and community production within the defined gradient of this study.     

Effect of light stress from phytoplankton on the relationship between aquatic vegetation and the propagule bank in shallow lakes

1. The way light stress controls the recruitment of aquatic plants (phanerogams and charophytes) is a key process controlling plant biodiversity, although still poorly understood. Our aim was to investigate how light stress induced by phytoplankton, that is, independent from the aquatic plants themselves, determines the recruitment and establishment of plant species from the propagule bank. The hypotheses were that an increase in light stress (i) decreases abundance and species richness both of established aquatic plants and of propagules in the bank and (ii) decreases the recruitment success of plants from this bank. 2. These hypotheses were tested in 25 shallow lakes representing a light stress gradient, by sampling propagule banks before the recruitment phase and when the lakes are devoid of actively growing plants (i.e. at the end of winter), established vegetation at the beginning of the summer and phytoplankton biomass (chlorophyll a) during the recruitment and establishment phase. 3. The phytoplankton biomass was negatively correlated with the richness and abundance of established vegetation but was not correlated with the propagule bank (neither species richness nor propagule abundance). The similarity between the propagule bank and established vegetation decreased significantly with increasing phytoplankton biomass. 4. The contrast in species composition between the vegetation and the propagule bank at the highest light stress suggests poor recruitment from the propagule bank but prompts questions about its origin. It could result from dispersal of propagules from neighbouring systems. Propagules could also originate from a persistent propagule bank formerly produced in the lake, suggesting strong year‐to‐year variation in light stress and, as a consequence, in recruitment and reproductive success of plants.     

Alpine vascular plant species richness: the importance of daily maximum temperature and pH

Species richness in the alpine zone varies dramatically when communities are compared. We explored (i) which stress and disturbance factors were highly correlated with species richness, (ii) whether the intermediate stress hypothesis (ISH) and the intermediate disturbance hypothesis (IDH) can be applied to alpine ecosystems, and (iii) whether standing crop can be used as an easily measurable surrogate for causal factors determining species richness in the alpine zone. Species numbers and standing crop were determined in 14 alpine plant communities in the Swiss Alps. To quantify the stress and disturbance factors in each community, air temperature, relative air humidity, wind speed, global radiation, UV-B radiation, length of the growing season, soil suction, pH, main soil nutrients, waterlogging, soil movement, number of avalanches, level of denudation, winter dieback, herbivory, wind damage, and days with frost were measured or observed. The present study revealed that 82% of the variance in␣vascular species richness among sites could be explained by just two abiotic factors, daily maximum temperature and soil pH. Daily maximum temperature and pH affect species richness both directly and via their effects on other environmental variables. Some stress and disturbance factors were related to species richness in a monotonic way, others in an unimodal way. Monotonic relationships suggest that the harsher the environment is, the fewer species can survive in such habitats. In cases of unimodal relationships (ISH and IDH) species richness decreases at both ends of the gradients due to the harsh environment and/or the interaction of other environmental factors. Competition and disturbance seemed only to play a secondary role in the form of fine-tuning species richness in specific communities. Thus, we concluded that neither the ISH nor the IDH can be considered useful conceptual models for the alpine zone. Furthermore, we found that standing crop can be used as an easily measurable surrogate for causal factors determining species richness in the alpine zone, even though there is no direct causality.     

Recent vegetation changes at the high‐latitude tree line ecotone are controlled by geomorphological disturbance,productivity and diversity

Aim We test how productivity, disturbance rate, plant functional composition and species richness gradients control changes in the composition of high‐latitude vegetation during recent climatic warming. Location Northern Fennoscandia, Europe. Methods We resampled tree line ecotone vegetation sites sampled 26 years earlier. To quantify compositional changes, we used generalized linear models to test relationships between compositional changes and environmental gradients. Results Compositional changes in species abundances are positively related to the normalized difference vegetation index (NDVI)‐based estimate of productivity gradient and to geomorphological disturbance. Competitive species in fertile sites show the greatest changes in abundance, opposed to negligible changes in infertile sites. Change in species richness is negatively related to initial richness, whereas geomorphological disturbance has positive effects on change in richness. Few lowland species have moved towards higher elevations. Main conclusions The sensitivity of vegetation to climate change depends on a complex interplay between productivity, physical and biotic disturbances, plant functional composition and richness. Our results suggest that vegetation on productive sites, such as herb‐rich deciduous forests at low altitudes, is more sensitive to climate warming than alpine tundra vegetation where grazing may have strong buffering effects. Geomorphological disturbance promotes vegetation change under climatic warming, whereas high diversity has a stabilizing effect.     

Disturbance Winners or Losers? Plants Bearing Extrafloral Nectaries in Brazilian Caatinga

Plants bearing extrafloral nectaries (EFNs) often have traits typical of pioneer species, and may be expected to proliferate in disturbed habitats. However, a negative effect of disturbance on visitation by attendant ants could prevent EFN‐bearing plants from acting as disturbance winners. Here, we address the effects of chronic anthropogenic disturbance on the abundance of EFN‐bearing plants and their interactions with attendant ants in Caatinga vegetation of northeastern Brazil. We recorded the abundance of EFN‐bearing plants, proportion of plants visited by ants and composition of ant attendant species at 24 sites varying in levels of disturbance. EFN‐bearing plants as a whole did not behave as a disturbance winner group. The responses of the 13 species to increasing disturbance were highly variable, with three species declining in abundance (loser species). The richness of ant species attending EFNs did not vary with disturbance, but species composition did. The overall proportion of EFN‐bearing plants attended by ants per 5‐min period was not affected by disturbance. However, for the three loser species, attendance decreased from about 50 percent with low and moderate disturbance to half that with very high disturbance. We hypothesize that disturbed sites are more stressful for loser species compared with other EFN‐bearing plants, with physiological stress resulting in lower production of EFN secretions and reduced attraction of ants. This would make such species double losers, with physiological stress at disturbed sites not only directly influencing their performance but also indirectly affecting it through the disruption of a key mutualism.     

Variation in species composition and vegetation structure of succulent scrub on Tenerife in relation to environmental variation

Abstract. On Tenerife, the occurrence of environmental gradients over short distances provides a unique opportunity to investigate the relationship between vegetation and environmental factors. In the semi‐arid coastal region of Tenerife, floristic composition, species richness and vegetation structure of perennial plants have been studied in 67 locations covering the existing precipitation gradient. On the island as a whole, variation in species composition could be best explained by mean annual precipitation; at coastal sites, substrate age and soil characteristics also played a significant role. On the other hand, substrate chemistry and the type of eruptive material explained little of the floristic variation. Stand biomass was strongly correlated with mean annual precipitation and was, on the youngest lava flows studied, also affected by substrate age. The native stem succulent species made up the bulk of total biomass along the whole precipitation gradient. Disturbed and undisturbed sites differed significantly in stand biomass and cover. Species richness was correlated with precipitation and substrate age. Distribution of plant functional types was also related to the precipitation gradient. The relative abundance of hemicryptophytes and shrubs with non‐hairy leaves increased with increasing precipitation whereas the ratio of shrubs with hairy/non‐hairy leaves and succulent plants decreased. Some alien plants were quite frequent at disturbed sites but, on the whole, they contributed little to the species spectrum and to the stand biomass. Undisturbed sites remained almost free of introduced species not considering annuals.     

The relationship between species richness and standing crop in wetlands: the importance of scale

One of the few important empirical generalizations regarding herbaceous plant systems has been the demonstration that species richness is related to standing crop with maximum richness occurring at moderate levels of standing crop. This relationship is normally demonstrated by comparing among vegetation types (i.e., vegetation with different dominants). We undertook this study to test whether the species richness-standing crop relationship was evident at a finer-grained level of organization, the within vegetation type level. Fifteen wetland sites were sampled in eastern Canada and species richness and standing crop determined in each of 224 0.25 m² quadrats. Each site was relatively homogeneous in terms of the dominant species present and were therefore categorized as single vegetation types. However, as a group, the sites comprised a wide range of vegetation types.A second order polynomial regression indicated a significant bitonic relationship between species richness and standing crop at the among-vegetation types scale, that is, when all 15 sites were combined. At the within-vegetation type level, however, no significant relationships were observed (p>0.05). The results indicate that the model of species richness proposed by Grime has predictive power at a coarse-grained level of organization, among vegetation types, but does not survive the transition to a finer-grained level of organization, the within vegetation type level. Therefore, the higher level processes which structure species richness patterns among vegetation types are not the same processes which determine richness patterns within a vegetation type.     

Distribution of alien vs. native plant species in roadside communities along an altitudinal gradient in Tenerife and Gran Canaria (Canary Islands)

Roadside plant communities were studied along two roads following an altitudinal gradient in Gran Canaria and Tenerife (Canary Islands). Our aim was to investigate variation in plant species richness, particularly of the alien flora, along a gradient from coastal shrubland to summit vegetation (1950 m a.s.l. in Gran Canaria, 2300 m in Tenerife) in relation to variation in habitat factors (altitude, habitat structure, roadside disturbance, distance to urban nuclei). We compared different species groups that were classified in terms of their biogeographical status, origin and life form. Altitude was the most important factor determining species richness and composition along both roadside transects. Alien plants showed a unimodal distribution pattern along the altitudinal gradient, with less species and lower abundance at low and high altitudes, and highest abundance at intermediate altitude. Alien plant species were also relatively more frequent near urban centres. The number of native and alien species was significantly positively correlated along the altitudinal gradient. Both alien and native, non-endemic species showed differences in their distribution along the altitudinal gradient according to their biogeographical affinities and climatic tolerances. Despite considerable differences in species pools these patterns were consistent among the two islands. Environmental (abiotic) stress is proposed as a primary, altitude-related factor acting as a filter against most alien plants at coastal and high-mountain altitudes. A higher frequency or intensity of disturbance at intermediate altitudes may be a further causal factor promoting alien plants in this zone. Future management efforts to control alien plants along roads should, therefore, concentrate on intermediate altitudinal zones of the higher Canary Islands.     

Area, altitude and aquatic plant diversity

Several explanations have been given for the decline in species richness with altitude. However, separating the influences of altitude, area, and isolation is difficult because of the conical shape of mountains. We used species lists of aquatic plants from >300 lakes in a small geographical area to investigate the influence of altitude on species richness. Altitude and/or surface area were better predictors of species richness than any measure of water chemistry. The surface area and depth of individual lakes were not related to altitude, neither was the degree of isolation from other waterbodies. Although species range size increased with altitude, range sizes of all but the rarer species (in the data set) encompassed the lowest altitudes, indicating species loss rather than turnover and no influence of the Rapoport rescue effect. Nevertheless we found a decline in species richness with altitude, additive to the effect of area. Species were ascribed to attribute groups according to an a priori classification based on morphological and life-history traits. The number of attribute groups and number of species within each group increased with area, suggesting both increased diversity and coexistence within niches. With altitude, the number of attribute groups declined, but the number of species per group increased, consistent with a loss of richness and reduced competition. The species remaining at high altitudes were characterised by stress tolerant traits, associated with sites of low productivity.
Our results suggest an absolute effect of altitude on species richness, irrespective of other influences and consistent with a decline in productivity.     

Fine‐scale vascular plant species richness in different alpine vegetation types: relationships with biomass and cover

Abstract. Vascular plant species richness was related to biomass and vegetation cover in nine different alpine vegetation types on the Hardangervidda mountain plateau, western Norway. Each vegetation type was sampled within an 8m × 6m area, and the species‐richness pattern analysed. Evidence for a unimodal relationship between species richness and both biomass and cover was found at the within‐vegetation type scale. Cover was a better predictor for species richness than biomass, suggesting that light may be an important factor influencing species richness at this scale in alpine vegetation. The possibility that the results are an artefact of small grain size is also discussed, and several arguments for an ecological explanation of the humpback relationship between species richness and cover are discussed.     

Soil, biomass, and management of semi-natural vegetation – Part II. Factors controlling species diversity

Using a wide range of conditions and plant community types, species diversity was investigated in relation to edaphic and non-edaphic site conditions, management, and biomass characteristics. Both standing biomass and aboveground production were investigated, and their effects compared. Three taxonomic assemblages were studied: (1) vascular plants only, (2) bryophytes also included, (3) terrestrial lichens included as well. Using a multivariate approach, both species richness and evenness could best be explained when only vascular plants were considered, emphasizing the role of taxonomic restrictions. The models best explaining species richness merely required abiotic conditions. This supports recent theories emphasizing the importance of an environmental regulation of the pool of (adapted) species from which the actual species are recruited. Explanatory soil properties were moisture and pH (both unimodal), and the soil available N:P ratio. Plots with large perimeter:area ratios had significantly more species than those with low ratios, indicating the importance of consistency in quadrat shape. Hump-shaped species richness relationships could be identified for both standing biomass and productivity, but they explained only a small part of the variation and were apparent only if soil and management effects were not accounted for. Unimodality (and notably the decreasing phase) was most pronounced when using maximum standing biomass, suggesting that the key factor is competition for light. At intermediate levels of standing biomass, the positive effects of habitat productivity and the negative effects of standing biomass itself are in balance, and high species numbers may be expected. When soil or management variables were allowed in the models, hump-shaped biomass relationships were no longer confirmed, suggesting that such relationships may arise from the covariation of biomass with other factors. Management explained a much larger part of the variation than the hump-shaped biomass relationship, suggesting that mowing and hay removal (both showing independent positive effects) regulate species richness in ways other than solely through the control of maximum standing biomass. Significantly higher evenness values were obtained at sites with low maximum biomass values, but only for the vascular plants. In addition, species evenness was positively related to the frequency of mowing. The numbers of both rare and endangered species were strongly curtailed by high standing biomass values, suggesting that these species are more susceptible to competitive exclusion than others. Through direct as well as indirect effects, management is confirmed to be beneficial not only for general species richness, but also for the occurrence of rare and endangered species.     

Plant colonisation in small forest–floor patches: importance of plant group and disturbance traits

It has been proposed that the interaction between life–history attributes of different organisms and distrbance characteristics play an important role in determining the successional pattern following a disturbance event. We compared the responses of vascular plants and bryophytes (mosses and hepatics) to variation in disturbance size and severity in an old-growth boreal forest during a four–year period. The experiment included two patch sizes (0.25 and 2.5 m²) and two levels of severity: humus patches (removal of vegetation) and mineral soil patches (removal of both vegetation and humus layer). Treatments were chosen to reflect some aspects of disturbance by uprooting. Species richness was significantly affected by both disturbance size and severity but the response differed among plant groups. In vascular plants, species numbers were highest in humus patches while mosses were more numerous in mineral soil patches, the most severe disturbance. In contrast, severity had no effect on hepatics. Plant recovery was more rapid in bryophytes than in vascular plants. Species richness of bryophytes had exceeded that of adjacent, undisturbed vegetation after 2-3 yr. We attribute the contrasting response of the plant groups to differences in regeneration strategies. As a group, bryophytes had a greater variety of regeneration methods than vascular plants, with several types of asexual propagules and abundant production of spores in some species. In contrast, clonal ingrowth dominated in vascular plants while seedlings were rare. Thus, our analysis supports the view that plant response to patchy disturbance is strongly dependent on the interplay among disturbance traits and specific attribtites of different plant groups.     

Effects of litter accumulation on riparian vegetation: Importance of particle size

Abstract. The floristic effects of river‐borne litter that accumulates in riparian zones may vary in space and time depending on variations in mass and particle size of the deposited litter. To analyse the effects of litter mass and size we applied differentsized litter (natural uncut pieces and powder) to riparian vegetation at different quantities. Vegetation responses were analysed after one season at the community level (total biomass or richness for all species) and species traits (biomass or richness for groups of species). At the community level uncut litter, but not powder, reduced species richness and both uncut and ground litter reduced above‐ground biomass. At the species trait level uncut litter had a stronger effect than powder on species richness and biomass. The only positive effect of litter addition was that powder increased graminoid species richness. The topsoil conditions indicated that the major impact of deposited, river‐borne litter was that it acted as a physical barrier directly preventing established plants from penetrating the litter layer and reducing light and soil temperature.     

Forest structure,diversity, and primary production in relation to disturbance severity

1. Differential disturbance severity effects on forest vegetation structure, species diversity, and net primary production (NPP) have been long theorized and observed. Here, we examined these factors concurrently to explore the potential for a mechanistic pathway linking disturbance severity, changes in light environment, leaf functional response, and wood NPP in a temperate hardwood forest.
2. Using a suite of measurements spanning an experimental gradient of tree mortality, we evaluated the direction and magnitude of change in vegetation structural and diversity indexes in relation to wood NPP. Informed by prior observations, we hypothesized that forest structural and species diversity changes and wood NPP would exhibit either a linear, unimodal, or threshold response in relation to disturbance severity. We expected increasing disturbance severity would progressively shift subcanopy light availability and leaf traits, thereby coupling structural and species diversity changes with primary production.
3. Linear or unimodal changes in three of four vegetation structural indexes were observed across the gradient in disturbance severity. However, disturbance‐related changes in vegetation structure were not consistently correlated with shifts in light environment, leaf traits, and wood NPP. Species diversity indexes did not change in response to rising disturbance severity.
4. We conclude that, in our study system, the sensitivity of wood NPP to rising disturbance severity is generally tied to changing vegetation structure but not species diversity. Changes in vegetation structure are inconsistently coupled with light environment and leaf traits, resulting in mixed support for our hypothesized cascade linking disturbance severity to wood NPP.

     

Composition of grazed and cleared temperate grassy woodlands in eastern Australia: patterns in space and inferences in time

Abstract. A regional vegetation survey of the temperate grassy woodlands (temperate savanna) in Australia was designed to assess the effects of clearing and grazing on the composition of vegetation remnants and the adjacent pasture matrix. Vegetation was sampled across a range of habitats using 77 0.1024‐ha quadrats; the relative abundance of species was recorded. Classification analysis clustered the sites into three main groups that corresponded to intensity of grazing/clearing followed by groups based on underlying lithology (basalt, metasediment, granites). Using Canonical Correspondence Analysis, exogenous disturbance and environmental variables were related to the relative abundance of species; grazing intensity had the highest eigenvalue (0.27) followed by tree canopy cover (0.25), lithology (0.18), altitude (0.17) and slope (0.10). Based on two‐dimensional ordination scores, six species response groups were defined relating to intensity of pastoralism and nutrient status of the landscape. Abundance and dominance of native shrubs, sub‐shrubs, twiners and geophytes were strongly associated with areas of less‐intense pastoralism on low‐nutrient soils. The strongest effects on species richness were grazing followed by canopy cover. Continuously grazed sites had lower native species richness across all growth forms except native grasses. There was no indication that intermediate grazing intensities enhanced forb richness as a result of competitive release. Species richness for all native plants was lowest where trees were absent especially under grazed conditions. Canopy cover in ungrazed sites appeared to promote the co‐existence of shrubs with the herbaceous layer. Predicted declines in forb richness in treeless, ungrazed, sites were not detected. The lack of a disturbance‐mediated enhancement of the herbaceous layer was attributed to habitat heterogeneity at 0.1 ha sampling scale.     

Productivity–diversity patterns in arctic tundra vegetation

Productivity has long been argued to be a major driver of species richness patterns. In the present study we test alternative productivity–diversity hypotheses using vegetation data from the vast Eurasian tundra. The productivity–species pool hypothesis predicts positive relationships at both fine and coarse grain sizes, whereas the productivity–interaction hypothesis predicts unimodal patterns at fine grain size, and monotonic positive patterns at coarse grain size. We furthermore expect to find flatter positive (productivity–species pool hypothesis) or more strongly negative (productivity–interaction hypothesis) relationships for lichens and bryophytes than for vascular plants, because as a group, lichens and bryophytes are better adapted to extreme arctic conditions and more vulnerable to competition for light than the taller‐growing vascular plants. The normalised difference vegetation index (NDVI) was used as a proxy of productivity. The generally unimodal productivity–diversity patterns were most consistent with the productivity–interaction hypothesis. There was a general trend of decreasing species richness from moderately to maximally productive tundra, in agreement with an increasing importance of competitive interactions. High richness of vascular plants and lichens occurred in moderately low productive tundra areas, whereas that of bryophytes occurred in the least productive tundra habitats covered by this study. The fine and coarse grain richness trends were surprisingly uniform and no variation in beta diversity along the productivity gradient was seen for vascular plants or bryophytes. However, lichen beta diversity varied along the productivity gradient, probably reflecting their sensitivity to habitat conditions and biotic interactions. Overall, the results show evidence that productivity–diversity gradients exist in tundra and that these appear to be largely driven by competitive interactions. Our results also imply that climate warming‐driven increases in productivity will strongly affect arctic plant diversity patterns.     

Response of plant species and life form diversity to variable fire histories and biomass in the jarrah forest of south‐west Australia

Frequent fires reduce the abundance of woody plant species and favour herbaceous species. Plant species richness also tends to increase with decreasing vegetation biomass and cover due to reduced competition for light. We assessed the influence of variable fire histories and site biomass on the following diversity measures: woody and herbaceous species richness, overall species richness and evenness, and life form evenness (i.e. the relative abundance or dominance among six herbaceous and six woody plant life forms), across 16 mixed jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) forest stands in south‐west Australia. Fire frequency was defined as the total number of fires over a 30‐year period. Overall species richness and species evenness did not vary with fire frequency or biomass. However, there were more herbaceous species (particularly rushes, geophytes and herbs) where there were fewer shrubs and low biomass, suggesting that more herbaceous species coexist where dominance by shrubs is low. Frequently burnt plots also had lower number and abundance of shrub species. Life form evenness was also higher at both high fire frequency and low biomass sites. These results suggest that the impact of fire frequency and biomass on vegetation composition is mediated by local interactions among different life forms rather than among individual species. Our results demonstrate that measuring the variation in the relative diversity of different woody and herbaceous life forms is crucial to understanding the compositional response of forests and other structurally complex vegetation communities to changes in disturbance regime such as increased fire frequency.     

Distribution of plant functional types along gradients of disturbance intensity and resource supply in an agricultural landscape

Abstract. In this study, plant functional types are understood as groups of plants with similar biological traits displaying significant optima or maxima on a gradient plane of resource supply and disturbance intensity. The biological traits refer to expansion, vegetative regeneration, generative reproduction, dispersal and seed bank longevity. 129 vegetation samples were taken in an agricultural landscape in southwestern Germany, covering a wide range of terrestrial vegetation types – but with the exception of forests and wetlands. For each site, also soil data were recorded. Mean daily soil moisture was estimated with a simple model. Soil moisture, balanced nitrogen supply and available phosphorus were combined into a factor ‘resource supply’. In addition, disturbance intensity was estimated for each site. This factor was based on (1) frequency of disturbance, (2) disturbance depth below or above the soil surface, and (3) proportion of the area affected by a discrete disturbance event. 30 plant groups with similar biological characteristics resulted from a cluster analysis, based on a compilation of 19 biological traits for a regional species pool. Logistic regression on a gradient plane of disturbance intensity and resource supply yielded response curves for 28 groups. The dependent variable was defined as the probability of encountering all members of a group in a sample. 17 groups display a significant response curve on the gradient plane. Plants with a potential for long- range dispersal are concentrated on sites with low or high disturbance intensities (e.g. fallow land, fields, lawns). On sites with medium disturbance intensity (e.g. meadows) and low to medium resource supply, small-range dispersal predominates. There are no distinct trends concerning seed bank longevity. The potential for vertical and lateral expansion increases with decreasing disturbance intensity. Only at medium disturbance intensities does vertical expansion correlate positively with resource supply. Rapid detachment of daughter individuals occurs more often on productive sites than on less productive sites. Diversity of groups with similar biological traits is highest on sites with medium disturbance intensities.