In situ seasonal patterns of root auxin concentrations and meristem length in an arctic sedge

Seasonal dynamics of root growth play an important role in large-scale ecosystem processes; they are largely governed by growth regulatory compounds and influenced by environmental conditions. Yet, our knowledge about physiological drivers of root growth is mostly limited to laboratory-based studies on model plant species. We sampled root tips of Eriophorum vaginatum and analyzed their auxin concentrations and meristem lengths biweekly over a growing season in situ in a subarctic peatland, both in surface soil and at the permafrost thawfront. Auxin concentrations were almost five times higher in surface than in thawfront soils and increased over the season, especially at the thawfront. Surprisingly, meristem length showed an opposite pattern and was almost double in thawfront compared with surface soils. Meristem length increased from peak to late season in the surface soils but decreased at the thawfront. Our study of in situ seasonal dynamics in root physiological parameters illustrates the potential for physiological methods to be applied in ecological studies and emphasizes the importance of in situ measurements. The strong effect of root location and the unexpected opposite patterns of meristem length and auxin concentrations likely show that auxin actively governs root growth to ensure a high potential for nutrient uptake at the thawfront.

Global maps of soil temperature

Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2 m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0-5 and 5-15 cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world's major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean = 3.0 ± 2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6 ± 2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (-0.7 ± 2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications.

Think globally, measure locally: The MIREN standardized protocol for monitoring plant species distributions along elevation gradients

Climate change and other global change drivers threaten plant diversity in mountains worldwide. A widely documented response to such environmental modifications is for plant species to change their elevational ranges. Range shifts are often idiosyncratic and difficult to generalize, partly due to variation in sampling methods. There is thus a need for a standardized monitoring strategy that can be applied across mountain regions to assess distribution changes and community turnover of native and non‐native plant species over space and time. Here, we present a conceptually intuitive and standardized protocol developed by the Mountain Invasion Research Network (MIREN) to systematically quantify global patterns of native and non‐native species distributions along elevation gradients and shifts arising from interactive effects of climate change and human disturbance. Usually repeated every five years, surveys consist of 20 sample sites located at equal elevation increments along three replicate roads per sampling region. At each site, three plots extend from the side of a mountain road into surrounding natural vegetation. The protocol has been successfully used in 18 regions worldwide from 2007 to present. Analyses of one point in time already generated some salient results, and revealed region‐specific elevational patterns of native plant species richness, but a globally consistent elevational decline in non‐native species richness. Non‐native plants were also more abundant directly adjacent to road edges, suggesting that disturbed roadsides serve as a vector for invasions into mountains. From the upcoming analyses of time series, even more exciting results can be expected, especially about range shifts. Implementing the protocol in more mountain regions globally would help to generate a more complete picture of how global change alters species distributions. This would inform conservation policy in mountain ecosystems, where some conservation policies remain poorly implemented.

How does a wetland plant respond to increasing temperature along a latitudinal gradient?

Global warming affects plant fitness through changes in functional traits and thereby ecosystem function. Wetlands are declining worldwide, and hence, ecosystem functions linked to wetlands are threatened. We use Caltha palustris "a common wetland plant" to study whether warming affects growth and reproduction differently depending on origin of source population, potentially affecting phenotypic response to local climate. We conducted a 2-year in situ temperature manipulation experiment using clone pairs of C. palustris in four regions, along a 1300-km latitudinal gradient of Sweden. Open-top chambers were used to passively increase temperature, paired with controls. Growth and reproductive traits were measured from 320 plants (four regions × five sites × two treatments × eight plants) over two consecutive seasons to assess the effect of warming over time. We found that warming increased plant height, leaf area, number of leaves, and roots. High-latitude populations responded more strongly to warming than low-latitude populations, especially by increasing leaf area. Warming increased number of flowers in general, but only in the second year, while number of fruits increased in low-latitude populations the first year. Prolonged warming leads to an increase in both number of leaves and flowers over time. While reproduction shows varying and regional responses to warming, impacts on plant growth, especially in high-latitude populations, have more profound effects. Such effects could lead to changes in plant community composition with increased abundance of fast-growing plants with larger leaves and more clones, affecting plant competition and ecological functions such as decomposition and nutrient retention. Effects of warming were highly context dependent; thus, we encourage further use of warming experiments to predict changes in growth, reproduction, and community composition across wetland types and climate gradients targeting different plant forms.

The role of arbuscular mycorrhizal fungi in nonnative plant invasion along mountain roads

Plant associated mutualists can mediate invasion success by affecting the ecological niche of nonnative plant species. Anthropogenic disturbance is also key in facilitating invasion success through changes in biotic and abiotic conditions, but the combined effect of these two factors in natural environments is understudied. To better understand this interaction, we investigated how disturbance and its interaction with mycorrhizas could impact range dynamics of nonnative plant species in the mountains of Norway. Therefore, we studied the root colonisation and community composition of arbuscular mycorrhizal (AM) fungi in disturbed vs undisturbed plots along mountain roads. We found that roadside disturbance strongly increases fungal diversity and richness while also promoting AM fungal root colonisation in an otherwise ecto-mycorrhiza and ericoid-mycorrhiza dominated environment. Surprisingly, AM fungi associating with nonnative plant species were present across the whole elevation gradient, even above the highest elevational limit of nonnative plants, indicating that mycorrhizal fungi are not currently limiting the upward movement of nonnative plants. We conclude that roadside disturbance has a positive effect on AM fungal colonisation and richness, possibly supporting the spread of nonnative plants, but that there is no absolute limitation of belowground mutualists, even at high elevation.

SoilTemp: A global database of near-surface temperature

Current analyses and predictions of spatially explicit patterns and processes in ecology most often rely on climate data interpolated from standardized weather stations. This interpolated climate data represents long-term average thermal conditions at coarse spatial resolutions only. Hence, many climate-forcing factors that operate at fine spatiotemporal resolutions are overlooked. This is particularly important in relation to effects of observation height (e.g. vegetation, snow and soil characteristics) and in habitats varying in their exposure to radiation, moisture and wind (e.g. topography, radiative forcing or cold-air pooling). Since organisms living close to the ground relate more strongly to these microclimatic conditions than to free-air temperatures, microclimatic ground and near-surface data are needed to provide realistic forecasts of the fate of such organisms under anthropogenic climate change, as well as of the functioning of the ecosystems they live in. To fill this critical gap, we highlight a call for temperature time series submissions to SoilTemp, a geospatial database initiative compiling soil and near-surface temperature data from all over the world. Currently, this database contains time series from 7,538 temperature sensors from 51 countries across all key biomes. The database will pave the way toward an improved global understanding of microclimate and bridge the gap between the available climate data and the climate at fine spatiotemporal resolutions relevant to most organisms and ecosystem processes.

Global plant trait relationships extend to the climatic extremes of the tundra biome

The majority of variation in six traits critical to the growth, survival and reproduction of plant species is thought to be organised along just two dimensions, corresponding to strategies of plant size and resource acquisition. However, it is unknown whether global plant trait relationships extend to climatic extremes, and if these interspecific relationships are confounded by trait variation within species. We test whether trait relationships extend to the cold extremes of life on Earth using the largest database of tundra plant traits yet compiled. We show that tundra plants demonstrate remarkably similar resource economic traits, but not size traits, compared to global distributions, and exhibit the same two dimensions of trait variation. Three quarters of trait variation occurs among species, mirroring global estimates of interspecific trait variation. Plant trait relationships are thus generalizable to the edge of global trait-space, informing prediction of plant community change in a warming world.

Dwelling in the deep - strongly increased root growth and rooting depth enhance plant interactions with thawing permafrost soil

Climate-warming-induced permafrost thaw exposes large amounts of carbon and nitrogen in soil at considerable depths, below the seasonally thawing active layer. The extent to which plant roots can reach and interact with these hitherto detached, deep carbon and nitrogen stores remains unknown. We aimed to quantify how permafrost thaw affects root dynamics across soil depths and plant functional types compared with above-ground abundance, and potential consequences for plant-soil interactions. A decade of experimental permafrost thaw strongly increased total root length and growth in the active layer, and deep roots invaded the newly thawed permafrost underneath. Root litter input to soil across all depths was 10 times greater with permafrost thaw. Root growth timing was unaffected by experimental permafrost thaw but peaked later in deeper soil, reflecting the seasonally receding thaw front. Deep-rooting species could sequester ¹⁵ N added at the base of the ambient active layer in October, which was after root growth had ceased. Deep soil organic matter that has long been locked up in permafrost is thus no longer detached from plant processes upon thaw. Whether via nutrient uptake, carbon storage, or rhizosphere priming, plant root interactions with thawing permafrost soils may feed back on our climate both positively and negatively.

Traditional plant functional groups explain variation in economic but not size-related traits across the tundra biome

AIM

Plant functional groups are widely used in community ecology and earth system modelling to describe trait variation within and across plant communities. However, this approach rests on the assumption that functional groups explain a large proportion of trait variation among species. We test whether four commonly used plant functional groups represent variation in six ecologically important plant traits.

LOCATION

Tundra biome.

TIME PERIOD

Data collected between 1964 and 2016.

MAJOR TAXA STUDIED

295 tundra vascular plant species.

METHODS

We compiled a database of six plant traits (plant height, leaf area, specific leaf area, leaf dry matter content, leaf nitrogen, seed mass) for tundra species. We examined the variation in species-level trait expression explained by four traditional functional groups (evergreen shrubs, deciduous shrubs, graminoids, forbs), and whether variation explained was dependent upon the traits included in analysis. We further compared the explanatory power and species composition of functional groups to alternative classifications generated using post hoc clustering of species-level traits.

RESULTS

Traditional functional groups explained significant differences in trait expression, particularly amongst traits associated with resource economics, which were consistent across sites and at the biome scale. However, functional groups explained 19% of overall trait variation and poorly represented differences in traits associated with plant size. Post hoc classification of species did not correspond well with traditional functional groups, and explained twice as much variation in species-level trait expression.

MAIN CONCLUSIONS

Traditional functional groups only coarsely represent variation in well-measured traits within tundra plant communities, and better explain resource economic traits than size-related traits. We recommend caution when using functional group approaches to predict tundra vegetation change, or ecosystem functions relating to plant size, such as albedo or carbon storage. We argue that alternative classifications or direct use of specific plant traits could provide new insights for ecological prediction and modelling.

Plant functional trait change across a warming tundra biome

The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature-trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.

Habitat properties and plant traits interact as drivers of non-native plant species' seed production at the local scale

To answer the long‐standing question if we can predict plant invader success based on characteristics of the environment (invasibility) or the invasive species (invasiveness), or the combination of both, there is a need for detailed observational studies in which habitat properties, non‐native plant traits, and the resulting invader success are locally measured. In this study, we assess the interaction of gradients in the environmental and trait space on non‐native species fitness, expressed as seed production, for a set of 10 invasive and noninvasive non‐native species along a wide range of invaded sites in Flanders. In our multidimensional approach, most of the single environmental gradients (temperature, light availability, native plant species diversity, and soil fertility) and sets of non‐native plant traits (plant size, photosynthesis, and foliar chemical attributes) related positively with invader seed production. Yet correlation with seed production was much stronger when several environmental gradients were assessed in interaction, and even more so when we combined plant traits and habitat properties. The latter increased explanatory power of the models on average by 25% for invasive and by 7% for noninvasive species. Additionally, we report a 70‐fold higher seed production in invasive than in noninvasive species and fundamentally different correlations of seed production with plant traits and habitat properties in noninvasive versus invasive species. We conclude that locally measured traits and properties deserve much more attention than they currently get in invasion literature and thus encourage further studies combining this level of detail with the generality of a multiregion and multispecies approach across different stages of invasion.

Lags in the response of mountain plant communities to climate change

Rapid climatic changes and increasing human influence at high elevations around the world will have profound impacts on mountain biodiversity. However, forecasts from statistical models (e.g. species distribution models) rarely consider that plant community changes could substantially lag behind climatic changes, hindering our ability to make temporally realistic projections for the coming century. Indeed, the magnitudes of lags, and the relative importance of the different factors giving rise to them, remain poorly understood. We review evidence for three types of lag: "dispersal lags" affecting plant species' spread along elevational gradients, "establishment lags" following their arrival in recipient communities, and "extinction lags" of resident species. Variation in lags is explained by variation among species in physiological and demographic responses, by effects of altered biotic interactions, and by aspects of the physical environment. Of these, altered biotic interactions could contribute substantially to establishment and extinction lags, yet impacts of biotic interactions on range dynamics are poorly understood. We develop a mechanistic community model to illustrate how species turnover in future communities might lag behind simple expectations based on species' range shifts with unlimited dispersal. The model shows a combined contribution of altered biotic interactions and dispersal lags to plant community turnover along an elevational gradient following climate warming. Our review and simulation support the view that accounting for disequilibrium range dynamics will be essential for realistic forecasts of patterns of biodiversity under climate change, with implications for the conservation of mountain species and the ecosystem functions they provide.

Potential contributions of root decomposition to the nitrogen cycle in arctic forest and tundra

Plant contributions to the nitrogen (N) cycle from decomposition are likely to be altered by vegetation shifts associated with climate change. Roots account for the majority of soil organic matter input from vegetation, but little is known about differences between vegetation types in their root contributions to nutrient cycling. Here, we examine the potential contribution of fine roots to the N cycle in forest and tundra to gain insight into belowground consequences of the widely observed increase in woody vegetation that accompanies climate change in the Arctic. We combined measurements of root production from minirhizotron images with tissue analysis of roots from differing root diameter and color classes to obtain potential N input following decomposition. In addition, we tested for changes in N concentration of roots during early stages of decomposition, and investigated whether vegetation type (forest or tundra) affected changes in tissue N concentration during decomposition. For completeness, we also present respective measurements of leaves. The potential N input from roots was twofold greater in forest than in tundra, mainly due to greater root production in forest. Potential N input varied with root diameter and color, but this variation tended to be similar in forest and tundra. As for roots, the potential N input from leaves was significantly greater in forest than in tundra. Vegetation type had no effect on changes in root or leaf N concentration after 1 year of decomposition. Our results suggest that shifts in vegetation that accompany climate change in the Arctic will likely increase plant‐associated potential N input both belowground and aboveground. In contrast, shifts in vegetation might not alter changes in tissue N concentration during early stages of decomposition. Overall, differences between forest and tundra in potential contribution of decomposing roots to the N cycle reinforce differences between habitats that occur for leaves.

Both seed germination and seedling mortality increase with experimental warming and fertilization in a subarctic tundra

Climate change is expected to force many species in arctic regions to migrate and track their climatic niche. This requires recruitment from seed, which currently shows very low rates in arctic regions, where long-lived and vegetatively reproducing plants dominate. Therefore, we pose the question whether recruitment (germination and seedling establishment) in arctic regions will significantly improve in a warmer world, and thus allow species to follow their climatic niche. We used a full factorial experiment to examine if realistic warmer temperatures (+3 °C; infrared radiation) and increased nitrogen availability (+1.4 g N m^-2 year^-1) affected germination, seedling survival and above- and below-ground seedling biomass in five species common in subarctic regions (Anthoxanthum odoratum, Betula nana, Pinus sylvestris, Solidago virgaurea, Vaccinium myrtillus). We found that warming increased seedling emergence in all species, but that subsequent mortality also increased, resulting in no net warming effect on seedling establishment. Warming slightly increased above-ground seedling biomass. Fertilization, on the other hand, did not influence seedling biomass, but it increased seedling establishment in B. nana while it reduced establishment in V. myrtillus. This may help B. nana dominate over V. myrtillus in warmer tundra. Surprisingly, no interactive effects between warming and fertilization were found. The lack of a general positive response of seedling establishment to warmer and more nutrient-rich conditions suggests that (sub)arctic species may experience difficulties in tracking their climatic niche. Predictions of future species distributions in arctic regions solely based on abiotic factors may therefore overestimate species' ranges due to their poor establishment. Also, the opposite response to fertilization of two key (sub)arctic dwarf shrubs, i.e. B. nana and V. myrtillus, could have important implications for the future development of arctic plant communities and argues for more research into the role of fertilization for plant establishment.

The hidden season: growing season is 50% longer below than above ground along an arctic elevation gradient

There is compelling evidence from experiments and observations that climate warming prolongs the growing season in arctic regions. Until now, the start, peak, and end of the growing season, which are used to model influences of vegetation on biogeochemical cycles, were commonly quantified using above-ground phenological data. Yet, over 80% of the plant biomass in arctic regions can be below ground, and the timing of root growth affects biogeochemical processes by influencing plant water and nutrient uptake, soil carbon input and microbial activity. We measured timing of above- and below-ground production in three plant communities along an arctic elevation gradient over two growing seasons. Below-ground production peaked later in the season and was more temporally uniform than above-ground production. Most importantly, the growing season continued c. 50% longer below than above ground. Our results strongly suggest that traditional above-ground estimates of phenology in arctic regions, including remotely sensed information, are not as complete a representation of whole-plant production intensity or duration, as studies that include root phenology. We therefore argue for explicit consideration of root phenology in studies of carbon and nutrient cycling, in terrestrial biosphere models, and scenarios of how arctic ecosystems will respond to climate warming.

Trade-off between competition and facilitation defines gap colonization in mountains

Recent experimental observations show that gap colonization in small-stature (e.g. grassland and dwarf shrubs) vegetation strongly depends on the abiotic conditions within them. At the same time, within-gap variation in biotic interactions such as competition and facilitation, caused by distance to the gap edge, would affect colonizer performance, but a theoretical framework to explore such patterns is missing. Here, we model how competition, facilitation and environmental conditions together determine the small-scale patterns of gap colonization along a cold gradient in mountains, by simulating colonizer survival in gaps of various sizes. Our model adds another dimension to the known effects of biotic interactions along a stress gradient by focussing on the trade-off between competition and facilitation in the within-gap environment. We show that this trade-off defines a peak in colonizer survival at a specific distance from the gap edge, which progressively shifts closer to the edge as the environment gets colder, ultimately leaving a large fraction of gaps unsuitable for colonization in facilitation-dominated systems. This is reinforced when vegetation size and temperature amelioration are manipulated simultaneously with temperature in order to simulate an elevational gradient more realistically. Interestingly, all other conditions being equal, the magnitude of the realized survival peak was always lower in large than in small gaps, making large gaps harder to colonize. The model is relevant to predict effects of non-native plant invasions and climate warming on colonization processes in mountains.

Alien roadside species more easily invade alpine than lowland plant communities in a subarctic mountain ecosystem

Effects of roads on plant communities are not well known in cold-climate mountain ecosystems, where road building and development are expected to increase in future decades. Knowledge of the sensitivity of mountain plant communities to disturbance by roads is however important for future conservation purposes. We investigate the effects of roads on species richness and composition, including the plant strategies that are most affected, along three elevational gradients in a subarctic mountain ecosystem. We also examine whether mountain roads promote the introduction and invasion of alien plant species from the lowlands to the alpine zone. Observations of plant community composition were made together with abiotic, biotic and anthropogenic factors in 60 T-shaped transects. Alpine plant communities reacted differently to road disturbances than their lowland counterparts. On high elevations, the roadside species composition was more similar to that of the local natural communities. Less competitive and ruderal species were present at high compared with lower elevation roadsides. While the effects of roads thus seem to be mitigated in the alpine environment for plant species in general, mountain plant communities are more invasible than lowland communities. More precisely, relatively more alien species present in the roadside were found to invade into the surrounding natural community at high compared to low elevations. We conclude that effects of roads and introduction of alien species in lowlands cannot simply be extrapolated to the alpine and subarctic environment.

Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe

Recent studies from mountainous areas of small spatial extent (<2500 km(2) ) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m(2) units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km(2) units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km(2) units. Ellenberg temperature indicator values in combination with plant assemblages explained 46-72% of variation in LmT and 92-96% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km(2) units peaked at 60-65°N and increased with terrain roughness, averaging 1.97 °C (SD = 0.84 °C) and 2.68 °C (SD = 1.26 °C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km(2) units was, on average, 1.8 times greater (0.32 °C km(-1) ) than spatial turnover in growing-season GiT (0.18 °C km(-1) ). We conclude that thermal variability within 1-km(2) units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

Plant community type and small-scale disturbances, but not altitude, influence the invasibility in subarctic ecosystems

Little of our knowledge about invasibility comes from arctic and alpine ecosystems, despite increasing plant migration and invasion in those regions. Here, we examine how community type, altitude, and small-scale disturbances affect invasibility in a subarctic ecosystem. Over a period of 4 yr, we studied seedling emergence and establishment in 17 species sown in gaps or undisturbed vegetation in four subarctic community types (Salix scrub, meadow, rich heath, poor heath) along an elevation gradient. Invasibility was lowest in rich heath and highest in Salix scrub. Small disturbances significantly increased the invasibility in most communities, thereby showing the importance of biotic resistance to invasion in subarctic regions. Unexpectedly, invasibility did not decrease with increasing elevation, and it was also not related to summer temperature. Our data suggest that biotic resistance might be more important than abiotic stress for invasibility in subarctic tundra and that low temperatures do not necessarily limit seedling establishment at high altitudes. High elevations are therefore potentially more vulnerable to invasion than was originally thought. Changes in community composition as a result of species migration or invasion are most likely to occur in Salix scrub and meadow, whereas Empetrum-dominated rich heath will largely remain unchanged.

Predicted changes in vegetation structure affect the susceptibility to invasion of bryophyte-dominated subarctic heath

BACKGROUND AND AIMS

A meta-analysis of global change experiments in arctic tundra sites suggests that plant productivity and the cover of shrubs, grasses and dead plant material (i.e. litter) will increase and the cover of bryophytes will decrease in response to higher air temperatures. However, little is known about which effects these changes in vegetation structure will have on seedling recruitment of species and invasibility of arctic ecosystems.

METHODS

A field experiment was done in a bryophyte-dominated, species-rich subarctic heath by manipulating the cover of bryophytes and litter in a factorial design. Three phases of seedling recruitment (seedling emergence, summer seedling survival, first-year recruitment) of the grass Anthoxanthum alpinum and the shrub Betula nana were analysed after they were sown into the experimental plots.

KEY RESULTS

Bryophyte and litter removal significantly increased seedling emergence of both species but the effects of manipulations of vegetation structure varied strongly for the later phases of recruitment. Summer survival and first-year recruitment were significantly higher in Anthoxanthum. Although bryophyte removal generally increased summer survival and recruitment, seedlings of Betula showed high mortality in early August on plots where bryophytes had been removed.

CONCLUSIONS

Large species-specific variation and significant effects of experimental manipulations on seedling recruitment suggest that changes in vegetation structure as a consequence of global warming will affect the abundance of grasses and shrubs, the species composition and the susceptibility to invasion of subarctic heath vegetation.

How do bryophytes govern generative recruitment of vascular plants?

Interactions between vascular plants and bryophytes determine plant community composition in many ecosystems. Yet, little is known about the importance of interspecific differences between bryophytes with respect to their effects on vascular plants. We compared the extent to which species-specific bryophyte effects on vascular plant generative recruitment depend on the following underlying mechanisms: allelopathy, mechanical obstruction, soil moisture and temperature control. We sowed 10 vascular plant species into monospecific mats of six chemically and structurally diverse bryophytes, and examined 1-yr seedling recruitment. Allelopathic effects were also assessed in a laboratory phyto-assay. Although all bryophytes suppressed vascular plant regeneration, there were significant differences between the bryophyte species. The lack of interactions indicated the absence of species-specific adaptations of vascular plants for recruitment in bryophyte mats. Differences between bryophyte species were best explained by alterations in temperature regime under bryophyte mats, mostly by reduced temperature amplitudes during germination. The temperature regime under bryophyte mats was well predicted by species-specific bryophyte cushion thickness. The fitness of established seedlings was not affected by the presence of bryophytes. Our results suggest that climatically or anthropogenically driven changes in the species' composition of bryophyte communities have knock-on effects on vascular plant populations via generative reproduction.

Effects of a warmer climate on seed germination in the subarctic

BACKGROUND AND AIMS

In a future warmer subarctic climate, the soil temperatures experienced by dispersed seeds are likely to increase during summer but may decrease during winter due to expected changes in snow depth, duration and quality. Because little is known about the dormancy-breaking and germination requirements of subarctic species, how warming may influence the timing and level of germination in these species was examined.

METHODS

Under controlled conditions, how colder winter and warmer summer soil temperatures influenced germination was tested in 23 subarctic species. The cold stratification and warm incubation temperatures were derived from real soil temperature measurements in subarctic tundra and the temperatures were gradually changed over time to simulate different months of the year.

KEY RESULTS

Moderate summer warming (+2.5 degrees C) substantially accelerated germination in all but four species but did not affect germination percentages. Optimum germination temperatures (20/10 degrees C) further decreased germination time and increased germination percentages in three species. Colder winter soil temperatures delayed the germination in ten species and decreased the germination percentage in four species, whereas the opposite was found in Silene acaulis. In most species, the combined effect of a reduced snow cover and summer warming resulted in earlier germination and thus a longer first growing season, which improves the chance of seedling survival. In particular the recruitment of (dwarf) shrubs (Vaccinium myrtillus, V. vitis-idaea, Betula nana), trees (Alnus incana, Betula pubescens) and grasses (Calamagrostis lapponica, C. purpurea) is likely to benefit from a warmer subarctic climate.

CONCLUSIONS

Seedling establishment is expected to improve in a future warmer subarctic climate, mainly by considerably earlier germination. The magnitudes of the responses are species-specific, which should be taken into account when modelling population growth and migration of subarctic species.

Factors associated with alien plants transitioning from casual, to naturalized, to invasive

To explain current plant invasions, or predict future ones, more knowledge on which factors increase the probability of alien species becoming naturalized and subsequently invasive is needed. We created a database of the alien plants in seminatural habitats in Ireland that included data on taxonomy, invasive status, invasion history, distribution, and biological and ecological plant characteristics. We used information from this database to determine the importance of these factors in increasing the ability of species to become naturalized and invasive. More specifically, we used two multiple logistic regressions to identify factors that distinguish naturalized from casual alien plant species and invasive from noninvasive, naturalized alien species. Clonal growth, moisture-indicator value, nitrogen-indicator value, native range, and date of first record affected (in order of decreasing importance) the probability of naturalization. Factors that distinguished invasive from noninvasive species were ornamental introduction, hermaphrodite flowers, pollination mode, being invasive elsewhere, onset of flowering season, moisture-indicator value, native range, and date of first record. Incorporation of phylogenetic information had little influence on the results, suggesting that the capacity of alien species to naturalize and become invasive evolved largely independently in several phylogenetic lineages. Whereas some of the variables were important for both transitions, others were only important for naturalization or for invasion. This emphasizes the importance of studying different stages of the invasion process when looking for mechanisms of becoming a successful invasive plant, instead of simply comparing invasive with noninvasive alien species. Our results also suggest that a combination of species traits and other variables is likely to produce the most accurate prediction of invasions.

Impact of field margin vegetation and herbage removal on ingrowing and anemochorous weeds

This research studies the effect of different ways to establish and to maintain unfertilised field margins on the development of potential weed plants and seed dispersal into adjacent crops. Plant communities in field margins either developed spontaneously or were sown with different seed mixtures of grasses and forbs. Margins were mown twice a year and the cuttings were either removed or not. Three years after establishment, the importance of important weeds Elymus repens, Cirsium arvense and Urtica dioica was significantly higher in the unsown community or when cuttings were not removed after mowing. Seed dispersal from the margin into adjacent crops was important in the unsown community during the first year after establishment. Between 82% and 99% of the seeds were disseminated within a distance of 4 m from the margin strip. In order to minimize the dissemination of weed species and invasion by noxious vegetatively propagated weeds it is recommended to establish a field margin by sowing and to remove the cuttings after each mowing cut.

Biodiversity and agro-ecology in field margins

This multidisciplinary study investigates agro-ecological functions (nature conservation, agriculture, environment) and implications of newly created, mown sown and unsown field margin strips installed on ex-arable land to increase biodiversity. From conservational concern, the development of species rich field margin strips was not strongly affected by the installed type of margin strip since species diversity converged over time, whether strips were sown or not. Convergence between unsown and sown margin strips occurred also in terms of species composition: unsown and sown strips became similar over time. Mowing without removal of cuttings significantly reduced species richness, yielded more grassy margin strips and delayed similarity in species composition between sown and unsown margin strips. Species richness on the longer term was not significantly affected by light regime nor by disturbance despite significant temporary effects shortly after the disturbance event. On the contrary vegetation composition in terms of importance of functional groups changed after disturbance: the share of spontaneous species within functional groups increased resulting in higher similarity between the sown and unsown vegetation. Furthermore risk of invasion was highest in the disturbed unsown community on the unshaded side of a tree lane. A positive effect of botanical diversity on insect number and diversity was found. However the effects of botanical diversity on insect number was mediated by light regime. At high light availability differences between plant communities were more pronounced compared to low light availablilty. The abundance of some insect families was dependent on the vegetation composition. Furthermore light availability significantly influenced insect diversity as well as the spatial distribution of families. From agricultural concern, installing margin strips by sowing a species mixture and a mowing regime with removal of cuttings are good practices to diminish the risk of species ingrowth into adjacent crops by creeping roots and rhizomes. Seed dispersal was only problematic one year after the installation of the field margin strips particularly nearby the unsown margin strip and wind-borne seeds were dispersed over limited distances, mainly within 4 m of field margins. Annual herbage yield was not affected by mowing management. DM yield of sown/unsown communities converged over time. Compared to herbage from an intensively managed fertilized grassland, field margin herbage revealed a low feeding value, owing to a low crude protein content, a low digestibility and a high crude fibre content. The unsown community had a higher forage quality than communities sown to bred commercially available grass varieties. Both digestibility and crude protein content decreased over time irrespective of plant community or location. Mid June cuts were more productive than mid September cuts but digestibility and crude protein content was lower. The use of herbage from field margins as hay for horses or as a component in farmland compost are good alternatives. A strong relation was found between the distribution of pest insects and their antagonist families along field margin strips indicating a status of biological equilibrium. From environmental concern, field margin strips buffered boundary vegetation and watercourses against cropped areas loaded with high levels of mineral nitrogen. Margin strips reduced the mineral nitrogen content of the soil in the margin and mineral nitrogen loss during wintermonths. Mineral nitrogen loss was not affected by field margin type but by distance from the field crop. A minimal width of 5 m is necessary to reach an optimal reduction in mineral soil N and N losses.

Vegetation development and floristical diversity of newly created sown and unsown field margin strips on ex arable land during the first 3 successional years

Early succesion of newly created sown/unsown margin strips on ex arableland, managed by two cuttings per year with/without removal of cuttings was characterised by the replacement of annuals in favour of perennials, a steadily increase in the importance of monocots and a decrease in non N-fixing dicots. Mowing with removal of cuttings delayed this succession pattern. Sorenson's qualitative similarity index (based solely on species occurrence) revealed that species composition of the sown communities (in terms of species occurrence) became increasingly similar to the unsown plots. Furthermore convergence in vegetation composition between sown and unsown plots occurred also in terms of species importance as assessed by Sorenson's quantitative index (based on the combination of species occurrence and importance). Similarity in species importance (but not of species occurrence) was significantly enhanced by cutting with removal of cuttings. During the first 3 successional years, species diversity of sown and unsown communities converged in time, irrespective of mowing regime or location. The decrease in species diversity, number of sown wildflower species and wildflower density of sown communities was more pronounced under a mowing regime without removal of cuttings. The annual addition of mown roadside herbage significantly enhanced species richness but not the importance of dicots.

Disentangling invasiveness and invasibility during invasion in synthesized grassland communities

•   An experiment with synthesized grassland communities was performed to identify plant traits that contribute to invasiveness and community traits that promote invasibility, and to study the relationship between, and the relative importance of, invasiveness and invasibility. •   Eight perennial grass species were used both as invasible monocultures and as potential invaders in gaps in these monocultures. Invasion success in the establishment phase, and invader and monoculture traits were assessed. •   Invasion success expressed as germination correlated significantly with germination time (invader trait), light penetration in the gaps and N acquisition by the edge plants (monoculture traits). Success expressed as leaf length correlated with seed mass, germination time (invader traits) and light penetration. Forty-six per cent of the variation in germination was explained by invader identity and 8% by monoculture identity, whereas, for leaf length, they explained 15% and 18%, respectively. •   Regenerative traits (seed mass and germination time) correlated with invasiveness, and resource availability (light and nitrogen) with invasibility. The results suggest that species characteristics would largely determine the extent of an invasion event (number of seedlings), while the success of individual invaders (growth and survival) is determined by both species and ecosystem characteristics.