Climatic and biotic extreme events moderate long‐term responses of above‐ and belowground sub‐Arctic heathland communities to climate change

Climate change impacts are not uniform across the Arctic region because interacting factors causes large variations in local ecosystem change. Extreme climatic events and population cycles of herbivores occur simultaneously against a background of gradual climate warming trends and can redirect ecosystem change along routes that are difficult to predict. Here, we present the results from sub‐Arctic heath vegetation and its belowground micro‐arthropod community in response to the two main drivers of vegetation damage in this region: extreme winter warming events and subsequent outbreaks of the defoliating autumnal moth caterpillar (Epirrita autumnata). Evergreen dwarf shrub biomass decreased (30%) following extreme winter warming events and again by moth caterpillar grazing. Deciduous shrubs that were previously exposed to an extreme winter warming event were not affected by the moth caterpillar grazing, while those that were not exposed to warming events (control plots) showed reduced (23%) biomass from grazing. Cryptogam cover increased irrespective of grazing or winter warming events. Micro‐arthropods declined (46%) following winter warming but did not respond to changes in plant community. Extreme winter warming and caterpillar grazing suppressed the CO₂ fluxes of the ecosystem. Evergreen dwarf shrubs are disadvantaged in a future sub‐Arctic with more stochastic climatic and biotic events. Given that summer warming may further benefit deciduous over evergreen shrubs, event and trend climate change may both act against evergreen shrubs and the ecosystem functions they provide. This is of particular concern given that Arctic heath vegetation is typically dominated by evergreen shrubs. Other components of the vegetation showed variable responses to abiotic and biotic events, and their interaction indicates that sub‐Arctic vegetation response to multiple pressures is not easy to predict from single‐factor responses. Therefore, while biotic and climatic events may have clear impacts, more work is needed to understand their net effect on Arctic ecosystems.     

Performance of dryland and wetland plant species on extensive green roofs

Background and Aims

Green roofs are constructed ecosystems where plants perform valuable services, ameliorating the urban environment through roof temperature reductions and stormwater interception. Plant species differ in functional characteristics that alter ecosystem properties. Plant performance research on extensive green roofs has so far indicated that species adapted to dry conditions perform optimally. However, in moist, humid climates, species typical of wetter soils might have advantages over dryland species. In this study, survival, growth and the performance of thermal and stormwater capture functions of three pairs of dryland and wetland plant species were quantified using an extensive modular green roof system.

Methods

Seedlings of all six species were germinated in a greenhouse and planted into green roof modules with 6 cm of growing medium. There were 34 treatments consisting of each species in monoculture and all combinations of wet- and dryland species in a randomized block design. Performance measures were survival, vegetation cover and roof surface temperature recorded for each module over two growing seasons, water loss (an estimate of evapotranspiration) in 2007, and albedo and water capture in 2008.

Key Results

Over two seasons, dryland plants performed better than wetland plants, and increasing the number of dryland species in mixtures tended to improve functioning, although there was no clear effect of species or habitat group diversity. All species had survival rates >75 % after the first winter; however, dryland species had much greater cover, an important indicator of green roof performance. Sibbaldiopsis tridentata was the top performing species in monoculture, and was included in the best treatments.

Conclusions

Although dryland species outperformed wetland species, planting extensive green roofs with both groups decreased performance only slightly, while increasing diversity and possibly habitat value. This study provides further evidence that plant composition and diversity can influence green roof functions.     

Roadside revegetation with native plants: Experimental seeding and transplanting of stem cuttings

Questions: Can we use local native plants for roadside revegetation? What cultural methods help enhance the process? Location: Trans Canada Highway, Terra Nova National Park, Newfoundland. Objectives: To (1) test stratification requirements for seed ger‐mination, (2) determine if germination, survival and growth of seedlings and stem cuttings of selected plants can be increased by mulching treatments and (3) identify native plants and cultural treatments useful for revegetation. Methods: We tested seed germination of Kalmia angustifoliaIris versicolorJuncus effususEriophorum vaginatum, Clintonia borealis and Cornus canadensis in a greenhouse experiment. We conducted field experiments of roadside revegetation using seeds of K. angustifolia, I. versicolorJ. effusus and E. vaginatum, as well as seedlings of I. versicolor and rooted stem cuttings of Emptrum nigrum and Juniperus communis after hay‐mat mulch and organic matter mulch application. Results: Stratified seeds of K. angustifoliaI. versicolor, J. effusus and E. vaginatum germinated successfully in the greenhouse, whereas C. borealis and C. canadensis seeds did not. Along roadsides, only I. versicolor seeds germinated. Iris versicolor cover increased significantly in organic matter mulch compared to hay‐mat mulch and control. Transplanted I. versicolor seedlings had high survival in all treatments but growth was reduced in organic matter mulch. Survival and growth of stem cuttings of E. nigrum and J. communis were significantly increased on hay‐mat mulch. Application: Rooted stem cuttings of E. nigrum and J. communis planted on hay‐mat mulch can be used as a practical method of roadside revegetation. These shrubs have low structure, are evergreen, and exhibit stress‐tolerance properties, which make them ideal species for roadside revegetation. They are also non‐palatable to wildlife. Roadside ditches can be revegetated by seeds or seedlings of I. versicolor. Robust roots and rhizomes of this plant may provide soil stability and dark green leaves and attractive flowers create aesthetically pleasing vegetation cover.     

Inhibition of growth,and effects on nutrient uptake of arctic graminoids by leaf extracts — allelopathy or resource competition between plants and microbes?

Previous research has shown that plant extracts, e.g. from boreal dwarf shrubs and trees, can cause reduced growth of neighbouring plants: an effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens ssp. tortuosa to three graminoid species, Carex bigelowii, Festuca vivipara and Luzula arcuata, grown in previously sterilized or non-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than those in sterilized soil, demonstrating larger nutrient deficiency. The difference in growth rate was probably caused by higher nutrient availability for plants in soils in which the microbial biomass was killed after soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nutrient limited graminoids in non-sterilized soil tended to reduce growth, whereas in the less nutrient limited sterilized soil it caused strong growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwarf shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P-rich Betula extract. In contrast to the adverse extract effects on plants, soil microbial respiration and soil fungal biomass (ergosterol) was generally stimulated, most strongly after addition of the Betula extract. Although we cannot exclude the possibility that the reduced plant growth and the concomitant stimulation of microbial activity were caused by phytochemicals, we believe that this was more likely due to labile carbon in the extracts which stimulated microbial biomass and activity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, depleting soil inorganic N alone, to the extent of reducing plant growth. This chain of events is supported by the negative correlation between plant growth and sugar content in the three added extracts, and the positive correlation between microbial activity, fungal biomass production and sugar content, and are known reactions when labile carbon is added to nutrient deficient soils.     

Morphological and anatomical studies on economically important Empetrum nigrum L. subsp.hermaphroditum (hagerup) bocher (empetraceae)

The study is based on anatomical and morphological investigations of Empetrum nigrum L. subsp. Hermaphroditum (Hagerup) Bocher, an important plant because of its value for medicine and food. Plant samples were collected from the northeastern part of Turkey. Morphological and anatomical features of various plant parts (e.g., stem, leaf, flower, and fruit) are illustrated and described in detail.     

Growth-ring variations of dwarf shrubs reflect regional climate signals in alpine environments rather than topoclimatic differences

Aim Our main aim is to determine if ring‐width variations in Empetrum hermaphroditum reflect regional or local topoclimate signals in an alpine environment. In the case that topoclimate provides the dominant signal, a secondary aim is to link these to spatial distribution patterns of different vegetation types. Location The study area is situated in the middle alpine belt in the Vågåmo region, Central Norwegian Scandes. Sampling sites cover different topoclimates: ridges, north‐facing slopes and south‐facing slopes. Methods We constructed ring‐width chronologies of E. hermaphroditum for each type of microsite for the common period 1951–2004. Climate data were prepared on an hourly, daily and growing‐season time scale. Climate–growth relationships were evaluated using bivariate correlations and regression tree methods for continuous time‐series analyses. In addition, extreme growth anomalies (pointer years) were compared with the climate conditions in those years. The impact of water supply on wood anatomy was determined by correlating the conductive area (percentage of vessel per growth ring) with a running mean (sum) of 10‐day intervals for temperature and precipitation. Results This study indicates that mean summer (June–August) temperatures determine the width of the growth rings of E. hermaphroditum irrespective of topoclimate. The length of the growing season, which is the most differentiating climatic factor between microsites, does not substantially alter the anatomical ring structure. Microsite differences in mean growth rates are attributed to the higher frequency of warm days. Extremely warm days limit ring‐width development at south‐facing slopes, while plants at ridges and north‐facing slopes still benefit from higher temperatures. As a consequence, pointer years are not developed synchronously at all microsites. Vessel formation is affected by available moisture, especially in the later part of the growing season. Main conclusions Topoclimate induces slight modifications of annual growth‐ring increments of E. hermaphroditum at different microsites. In contrast to the distribution patterns of vegetation types that are determined by snow cover, growth‐ring variations are related to summer temperature conditions, and the prominent regional climate signal is still reflected at all microsites. This offers the opportunity to reconstruct climatic change in alpine regions from dwarf shrub ring‐width chronologies.     

Vegetation shift from deciduous to evergreen dwarf shrubs in response to selective herbivory offsets carbon losses: evidence from 19 years of warming and simulated herbivory in the subarctic tundra

Selective herbivory of palatable plant species provides a competitive advantage for unpalatable plant species, which often have slow growth rates and produce slowly decomposable litter. We hypothesized that through a shift in the vegetation community from palatable, deciduous dwarf shrubs to unpalatable, evergreen dwarf shrubs, selective herbivory may counteract the increased shrub abundance that is otherwise found in tundra ecosystems, in turn interacting with the responses of ecosystem carbon (C) stocks and CO₂ balance to climatic warming. We tested this hypothesis in a 19‐year field experiment with factorial treatments of warming and simulated herbivory on the dominant deciduous dwarf shrub Vaccinium myrtillus. Warming was associated with a significantly increased vegetation abundance, with the strongest effect on deciduous dwarf shrubs, resulting in greater rates of both gross ecosystem production (GEP) and ecosystem respiration (ER) as well as increased C stocks. Simulated herbivory increased the abundance of evergreen dwarf shrubs, most importantly Empetrum nigrum ssp. hermaphroditum, which led to a recent shift in the dominant vegetation from deciduous to evergreen dwarf shrubs. Simulated herbivory caused no effect on GEP and ER or the total ecosystem C stocks, indicating that the vegetation shift counteracted the herbivore‐induced C loss from the system. A larger proportion of the total ecosystem C stock was found aboveground, rather than belowground, in plots treated with simulated herbivory. We conclude that by providing a competitive advantage to unpalatable plant species with slow growth rates and long life spans, selective herbivory may promote aboveground C stocks in a warming tundra ecosystem and, through this mechanism, counteract C losses that result from plant biomass consumption.     

Advanced snowmelt causes shift towards positive neighbour interactions in a subarctic tundra community

Positive and negative species interactions are important factors in structuring vegetation communities. Studies in many ecosystems have focussed on competition; however, facilitation has often been found to outweigh competition under harsh environmental conditions. The balance between positive and negative species interactions is known to shift along spatial, temporal and environmental gradients and thus is likely to be affected by climate change. Winter temperature and precipitation patterns in Interior Alaska are rapidly changing and could lead to warmer winters with a shallow, early melting snow cover in the near future. We conducted snow manipulation and neighbour removal experiments to test whether the relative importance of positive and negative species interactions differs between three winter climate scenarios in a subarctic tundra community. In plots with ambient, manually advanced or delayed snowmelt, we assessed the relative importance of neighbours for survival, phenology, growth and reproduction of two dwarf shrub species. Under ambient conditions and after delayed snowmelt, positive and negative neighbour effects were generally balanced, but when snowmelt was advanced we found overall facilitative neighbour effects on survival, phenology, growth and reproduction of Empetrum nigrum, the earlier developing of the two target species. As earlier snowmelt was correlated with colder spring temperatures and a higher number of frosts, we conclude that plants experienced harsher environmental conditions after early snowmelt and that neighbours could have played an important role in ameliorating the physical environment at the beginning of the growing season.     

Adsorption of allelopathic compounds by wood-derived charcoal: the role of wood porosity

In Swedish boreal forests, areas dominated by the dwarf shrub Empetrum hermaphroditum Hagerup are known for their poor regeneration of trees and one of the causes of this poor regeneration has been attributed to allelopathy (i.e. chemical interferences) by E. hermaphroditum. Fire-produced charcoal is suggested to play an important role in rejuvenating those ecosystems by adsorbing allelopathic compounds, such as phenols, released by E. hermaphroditum. In this study, we firstly investigated whether the adsorption capacity of charcoal of different plant species varies according to the wood anatomical structures of these, and secondly we tried to relate the adsorption capacity to wood anatomical structure. Charcoal was produced from eight boreal and one temperate woody plant species and the adsorption capacity of charcoal was tested by bioassays technique. Seed germination was used as a measurement of the ability of charcoal to adsorb allelochemicals. The charcoal porosity was estimated and the pore size distribution was then calculated in order to relate the wood anatomical features to the adsorption capacity. The results showed that the adsorption capacity of charcoal was significantly different between plant species and that deciduous trees had a significantly higher adsorption capacity than conifers and ericaceous species. The presence of macro-pores rather than a high porosity appears to be the most important for the adsorption capacity. These results suggest that fire-produced charcoal has different ability to adsorb phenols in boreal forest soil, and therefore may have differing effects on the germination of seeds of establishing tree seedlings.