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.     

Relationships between ericaceous vegetation and soil nutrient status in a post-fire Kalmia angustifolia-black spruce chronosequence

Post-fire nutrient flushes are an important precursor to secondary succession in fire-driven boreal forest. We studied the magnitude of changes in post-fire soil nutrient status across a chronosequence of ericaceous shrub-dominated boreal forest stands in eastern Newfoundland, Canada. The chronosequence comprised nine stands burned between 1 and 38 years prior to the study. These sites have resisted tree reestablishment following forest fire-induced mortality of black spruce and a concomitant increase in dominance of the ericaceous dwarf shrub Kalmia angustifolia L. Our objectives were: (1) to identify the factors driving soil nutrient status in these post-fire stands dominated by ericaceous plants, and (2) to test hypotheses that specific relationships exist among environmental factors, dominant vegetation and indicators of soil nutrient status. Macronutrients such as NH₄⁺, total organic N and mineral soil P concentrations showed non-linear declines with time since fire. These parameters were also negatively associated with cover of ericaceous plants. Potential phytotoxins such as total phenolics and aluminium concentrations increased with increasing cover of K. angustifolia. Variability in net ammonification, total P and total phenolic acids in organic soils were strongly related to ericaceous dominance even when the effect of time since fire was partialled out using regression analysis. These findings suggest a strong capacity for ericaceous vegetation to have top-down effects on soil chemical property particularly in the organic horizon with the increase in its post-fire dominance.     

Conifer Regeneration Problems in Boreal and Temperate Forests with Ericaceous Understory: Role of Disturbance,Seedbed Limitation,and Keytsone Species Change

Conifer regeneration failure in the presence of dense ericaceous cover resulting from the removal of canopy trees by forest harvesting observed in boreal and temperate forest has been attributed to allelopathy, competition, and soil nutrient imbalance. Ecosystem-level alleopathic effect has been argued as a cause for conifer regeneration failure by citing examples from a species-poor boreal forest in northern Sweden with ground vegetation dominated by crowberry (Empetrum hermaphroditum, Ericales) and New Zealand dairy pastures invaded by nodding or musk thistle (Carduus nutans). This article aims to explain the phenomenon of vegetation shift from conifer forest to ericaceous heath by extending the argument of ecosystem-level impact of ericaceous plants and linking the disturbance-mediated regeneration strategies of the dominant conifer species and the understory ericaceous species with the quality of seedbed substrate that influence the direction of secondary succession. It has been argued that fire severity plays a pivotal role in controlling seedbed quality and the regeneration mechanisms of conifers, which in turn determines the direction of post-disturbance succession. The post-fire-dominated ericaceous plants and their habitat-modifying effects have been explained from the point of view of keystone species concept and their role as ecosystem engineers. In the absence of high severity natural fires the canopy keystone species (conifer) fails to regenerate successfully mainly due to limitation of favorable seedbed. On the other hand, the understory ericaceous plants regenerate vigorously by vegetative methods from the belowground components that survived the fire. Forest harvesting by clearcutting or selective cutting also create similar vigorous vegetative regrowth of ericaceous plants, but conifer regeneration suffers from the lack of a suitable seedbed. Thus in the absence of successful conifer regeneration, the vigorously growing understory ericaceous plants become the new keystone species. The new keystone ericaceous species bring about a significant long-term habitat change by rapid accumulation of plyphenol-rich humus. Ericaceous phenolic compounds have been found to inhibit seed germination and seedling growth of conifers. By forming protein-phenol complexes they cause a further reduction of available nitrogen of the already nutrient-stressed habitat. A low pH condition in the presence of phenolic compounds causes the leaching of metallic ions and forms hard iron pans that impair soil water movement. The phenolic allelochemicals of ericaceous humus are also inhibitory to many conifer ectomycorrhizae. On the other hand, ericaceous plants perpetuate in the community by their stress-tolerating strategies as well as their ability to acquire nutrients through ericoid mycorrhizae. Three mechanisms working at the ecosystem level can be suggested as the cause of vegetation shift from forest to ericaceous heath. These are (1) the absence of high severity natural fire and the limitation of suitable conifer seedbed in the presence of thick humus, (2) increased competition resulting from the rapid vegetative regeneration of understory ericaceous plants after forest canopy opening by harvesting or nonsevere fire, and (3) habitat degradation by phenolic allelochemicals of ericaceous plants causing a soil nutrient imbalance and iron pan formation. Thus, a shift in keystone species from conifer to ericaceous plant in the post-disturbance habitat may induce a retrogressive succession due to ecosystem-level engineering effects of the new keystone species. Vegetation management in conifer-ericaceous communities depends on land management objectives. If the objective is to produce timber and other forest products then the control of ericaceous plants and site preparation is necessary after forest harvesting. Ploughing and liming followed by conifer planting and repeated N fertilization has been applied successfully to promote afforestation of Calluna heathlands in Britain. However, such practice has not been proven successful in the reforestation of Kalmia-dominated sites in eastern Canada. If, on the other hand, the land management objective is to maintain heathlands for herbivore production or conservation of cultural landscape, as in the case of certain Calluna-dominated heathland in Western Europe, then moderately hot prescribed burning is useful as a management tool.     

Postglacial vegetation history in Luxembourg: new charcoal data from the cave of la Karelslé (Waldbillig, eastern Gutland)

The archaeological site of La Karelslé (Waldbillig, Luxembourg) provides the first important charcoal sequence in Luxembourg for the development of Holocene plant cover. During the Pre-boreal, the results show open woodland vegetation dominated by Pinus sylvestris type in association with Betula sp. and Rosaceae. After an important gap covering the Boreal and the early Atlantic, the data reveal the dominance of a deciduous Quercus woodland with Tilia sp., Fraxinus excelsior and Corylus avellana. Finally during the sub-Boreal, the assemblages indicate a cooler and wetter climate and Quercus woods with Fagus sylvatica and Taxus baccata. There is no clear evidence for human impact. Received June 5, 2001 / Accepted December 3, 2001     

Conspecific versus heterospecific litter effects on seedling establishment

Plant litter is an important determinant of seed germination and seedling establishment. Positive effects of litter have received considerable attention, but few studies have explicitly tested whether seedlings are more facilitated by conspecific litter compared to heterospecific litter. In order to contrast conspecific and heterospecific facilitative effects on seedling establishment, we used Anthriscus sylvestris, Angelica sylvestris, Pimpinella saxifraga and different combinations of their seeds and litter seedbeds as a model system. Although litter had a significant species-specific effect on seedling emergence, we found no evidence of strictly conspecific facilitation. Anthriscus sylvestris displayed a positive response to all types of litter. In contrast, there was a clear negative effect of conspecific litter in Pimpinella saxifraga. Activated carbon did not modify the negative effect, indicating that chemical compounds were not the cause. Our study suggests a high level of idiosyncrasy in response to litter at the species level.     

Aboveground and belowground responses to quality and heterogeneity of organic inputs to the boreal forest

Leaf litter and other organic resources returned to the soil are important regulators of ecological processes in forest ecosystems, and their ecological impacts may be strongly influenced both by their quality and by interactions between coexisting resource types. To date, most studies on effects of resource identity and mixing have only involved leaf litter, despite the fact that other resource types constitute a major input to the soil. We investigated how quality and heterogeneity of organic substrates found in boreal forests affects the activity and community structure of soil microbes, and plant growth. Six organic substrates (wood, charcoal, berries, sporocarps, vertebrate faeces and leaf litter) were added singly or in mixtures of two, three and six resource types to pots containing forest soil (with or without tree seedlings of Betula pendula Roth). The largest positive effects of single substrates on microbial basal respiration (BR), substrate-induced respiration (SIR) and microbial metabolic quotient (qCO₂) were found for nutrient-rich substrates (faeces and sporocarps) or substrates with high sugar-content (berries). Mixing of substrates had no effect on BR or SIR, but decreased qCO₂ or altered the microbial community structure for specific combinations of substrates. In contrast to the niche complementarity hypothesis, microbial catabolic diversity was not stimulated by greater diversity of resources. Seedling growth responses to single substrates were neutral or negative; the inhibition of growth probably resulted largely from microbial competition for nutrients. Substrate mixing enhanced seedling nutrient-uptake and growth for all mixtures containing sporocarps and leaf litter. Overall, plants responded more strongly to resource heterogeneity than microbes, and synergistic effects only occurred when nutrient-rich substrates were present within the substrate mixtures. In particular, our results demonstrate a role for complex and non-additive interactions among previously overlooked resource types returned to the soil in influencing ecosystem functions such as nutrient cycling and plant productivity.     

Long‐term forest structure and regeneration after wildfire in Russian Karelia

We examined forest structure and regeneration in a 350‐ha forest dominated by Pinus sylvestris 31 yr after a wildfire in the Vienansalo wilderness, Russian Karelia. In most parts of the area, the 1969 fire was not stand replacing but had left larger trees alive so that the area generally remained forest covered. In some localities, however, all trees apparently died and distinct gaps were formed, suggesting that the fire severity varied considerably, contributing to increased variation in stand structure. Living and dead wood volumes were similar, 112 and 96 m³.ha^‐1, respectively. The tree species proportions of dead vs living wood indicated that prior to fire disturbance Picea was more common in the area. Regeneration was abundant (saplings, ca. 14 000 ind.ha^‐1, height 20 ‐200 cm) and tree seedling recruitment had occurred over a long period of time. Regeneration density was highest on the mesic Vaccinium‐Myrtillus forest site type, decreasing towards nutrient‐poor site types. The most common regeneration microsites were level ground (56% of saplings), immediate surroundings of decayed wood (23%) and depressions (11%). The high proportion of saplings on level ground suggests that after the fire regeneration conditions have been favourable across the whole forest floor. Nevertheless, the areas in the vicinity of decayed wood have been particularly important microsites for seedling establishment. The results provide an example of the effects of wildfire on forest structure in a natural Pinus sylvestris dominated forest, demonstrating the non stand replacing character of fire, high variability in stand structure and the abundance of post‐fire regeneration.     

Does soil determine the boundaries of monodominant rain forest with adjacent mixed rain forest and maquis on ultramafic soils in New Caledonia?

Aim To determine the soil characteristics of Nothofagus‐dominated rain forests in an ultramafic region (i.e. soils having high concentrations of metals including Mg, Fe and Ni), and whether soil characteristics may explain the location of monodominant rain forest in relation to adjacent mixed rain forest and maquis (shrub‐dominated vegetation). Location New Caledonia. Methods Soil characteristics were compared among six Nothofagus‐dominated rain forests from a range of altitudes and topographic positions. At four of these sites, comparisons were made with soils of adjacent mixed rain forest and maquis. Results Soil characteristics varied among the monodominant Nothofagus forests, largely due to differences between ultramafic soils and soils influenced by non‐ultramafic intrusions. The soils of all vegetation types had low concentrations of nutrients, particularly P, K and Ca (both total and extractable/exchangeable), and high total concentrations of Ni, Fe, Cr and Mn. There were significant differences between the rain forests and adjacent maquis in soil concentrations of several elements (N, P, Ca, Mg and Mn), more so in surface soils than at depth, but much of this pattern may be caused by effects of vegetation on the soil, rather than of soil on the vegetation. However, there were no significant differences in soil concentrations of any mineral elements between Nothofagus forest and adjacent mixed rain forest. Main conclusions We found no evidence for soil mediation of boundaries of Nothofagus rain forest with mixed rain forest, and little evidence for the boundaries of either forest type with maquis. We suggest that the local abrupt boundaries of these monodominant Nothofagus forests are directly related to temporal factors, such as time since the last wildfire and frequency of wildfire, and that disturbance is therefore a major causal factor in the occurrence of these forests.     

Les feux préhistoriques holocènes en montagne sub-méditerranéenne : premiers résultats sur le Causse Méjean (Lozère,France)

Charcoal remains, evidence of ancient fire, have been discovered in the dolines of the asylvatic or deforested sub Mediterranean area, from the ‘Grands Causses’. First ¹⁴C AMS dating on Pinus cf. sylvestris charcoal, cover the period 4805–2990 yr BP. This testifies to the development of a pine forest in the transition Middle–Late Holocene. The existence of several phases of fires, affecting the forests in the northwestern areas of the Causse during the second part of the Holocene is also proved. The origin of present-day herbaceous vegetation seems to date back to the Bronze Age. To cite this article: D. Quilès et al., C. R. Palevol 1 (2002) 59–65.     

6000 years of forest dynamics in Suserup Skov, a seminatural Danish woodland

• 1 The history of a forest stand over the last 6000 years has been reconstructed by studying pollen, macrofossils and charcoal from a small, wet hollow in Suserup Skov on the island of Sjælland in eastern Denmark.
• 2 The earliest recorded forest was Tilia‐dominated but contained an intimate mixture of many different tree species that included Acer campestre, A. platanoides, Alnus glutinosa, Betula pubescens, Corylus avellana, Frangula alnus, Fraxinus excelsior, Malus sylvestris, Populus tremula, Pinus sylvestris, Quercus robur, Q. petraea, Salix spp., Sorbus aucuparia, Tilia cordata and T. platyphyllos. The preserved fruits of T. platyphyllos confirm its hitherto doubtful status as a native member of the Danish flora.
• 3 The present‐day woodland developed after a period of intensive anthropogenic disturbance between ≈ 600 bc and ad 900, during which time open canopy conditions prevailed at Suserup. Fagus sylvatica and Fraxinus excelsior are the dominant trees at present, together with some Quercus robur and Ulmus glabra. 4 Charcoal was present in the sediments from most time periods except at the Ulmus decline. In the last 1000 years of the sequence — the period of Fagus dominance — charcoal counts were consistently low.
• 5 Pinus sylvestris was a natural component of this primarily deciduous forest, and the last macrofossil find dates from c. ad 900. Macrofossil Pinus cone scales recorded c. ad 1800 originate from planted individuals. Prior to Fagus dominance, the forest had an open structure partly caused by frequent, low‐intensity fires associated with the presence of Pinus sylvestris.
• 6 The replacement of Tilia by Fagus in this forest was catalysed by human activity. If the forest had not been so disturbed, the rich diversity of trees would most probably have persisted up to the present time, with only a moderate‐sized Fagus population.

     

Effects of fertilization on understorey vegetation in a Norwegian Pinus sylvestris forest

Abstract. Boreal coniferous forests have been impacted by long distance airborne pollutant deposition for most of the 20th century. Changes in forest understorey vegetation attributable to N‐deposition have been observed in southern Sweden, but not so far in southern Norway. We recorded the quantity of all species of vascular plants, bryophytes and lichens in 144 plots in a fertilization experiment in a 35‐yr old Pinus sylvestris forest in Aust Agder County, southernNorway initiated 6 yr before our study. Each plot represented a combination of three levels of nitrogen, two levels of magnesium and two levels of phosphorus addition. Effects of fertilization on species quantity were tested by Kruskal‐Wallis one‐way analysis by ranks. For vascular plants, only small and hardly significant differences were found between treatments and control. Significant negative effects of N‐fertilization were found on both mosses and lichens. To some extent, these effects could be attributed to direct effects of application of the fertilizer, but were more likely to be due to a negative feedback response to the faster growth of pine trees in fertilized stands, reducing throughfall precipitation and increasing litter fall. Significant differences between Mg‐ and P‐fertilized sites and respective controls were found for too few species to be likely to represent an overall trend.     

Macro-Particle Charcoal C Content following Prescribed Burning in a Mixed-Conifer Forest,Sierra Nevada,California

Fire suppression and changing climate have resulted in increased large wildfire frequency and severity in the western United States, causing carbon cycle impacts. Forest thinning and prescribed burning reduce high-severity fire risk, but require removal of biomass and emissions of carbon from burning. During each fire a fraction of the burning vegetation and soil organic matter is converted into charcoal, a relatively stable carbon form. We sought to quantify the effects of pre-fire fuel load and type on charcoal carbon produced by biomass combusted in a prescribed burn under different thinning treatments and to identify more easily measured predictors of charcoal carbon mass in a historically frequent-fire mixed-conifer forest. We hypothesized that charcoal carbon produced from coarse woody debris (CWD) during prescribed burning would be greater than that produced from fine woody debris (FWD). We visually quantified post-treatment charcoal carbon content in the O-horizon and the A-horizon beneath CWD (> 30 cm diameter) and up to 60 cm from CWD that was present prior to treatment. We found no difference in the size of charcoal carbon pools from CWD (treatment means ranged from 0.3–2.0 g m^-2 of A-horizon and 0.0–1.7 g m^-2 of O-horizon charcoal) and FWD (treatment means ranged from 0.2–1.7 g m^-2 of A-horizon and 0.0–1.5 g m^-2 of O-horizon charcoal). We also compared treatments and found that the burn-only, understory-thin and burn, and overstory-thin and burn treatments had significantly more charcoal carbon than the control. Charcoal carbon represented 0.29% of total ecosystem carbon. We found that char mass on CWD was an important predictor of charcoal carbon mass, but only explained 18–35% of the variation. Our results help improve our understanding of the effects forest restoration treatments have on ecosystem carbon by providing additional information about charcoal carbon content.     

Diversity of forest vegetation across a strong gradient of climatic continentality: Western Sayan Mountains, southern Siberia

Southern Siberian mountain ranges encompass strong climatic contrasts from the relatively oceanic northern foothills to strongly continental intermountain basins in the south. Landscape-scale climatic differences create vegetation patterns, which are analogous to the broad-scale vegetation zonation over large areas of northern Eurasia. In their southern, continental areas, these mountains harbour forest types which potentially resemble the full-glacial forests recently reconstructed for Central Europe. To identify forest vegetation–environment relationships in the southern Siberian mountain ranges, forest vegetation of the Western Sayan Mountains was sampled on a 280 km transect running from the northern foothills with oceanic climatic features to the continental Central Tuvinian Basin in the south. Based on the species composition, vegetation was classified into hemiboreal forests, occurring at drier and summer-warm sites with high-pH soil, and taiga, occurring at wetter, summer-cool sites with acidic soil. Hemiboreal forests included Betula pendula-Pinus sylvestris mesic forest, Larix sibirica dry forest and Pinus sylvestris dry forest. Taiga included Abies sibirica-Betula pendula wet forest, Abies sibirica-Pinus sibirica mesic forest and Pinus sibirica-Picea obovata continental forest. Hemiboreal forests were richer in vascular plant species, while taiga was richer in ground-dwelling cryptogams. Vegetation–environment relationships were analysed by indirect and direct ordination. Winter and summer temperatures and precipitation exerted a dominant influence on species composition. Soil pH was also an important correlate of species composition, but this factor itself was probably controlled by precipitation. At a more local scale, the main source of variation in species composition was topography, producing landscape patterns of contrasting plant communities on slopes of different aspects and valley bottoms. The response of tree species to major environmental factors was expressed with Huisman–Olff–Fresco models. Larix sibirica appeared to be most resistant to drought and winter frosts, Pinus sibirica was adapted to low temperatures both in winter and summer, and Picea obovata had an intermediate response to climate. Betula pendula, Pinus sylvestris and Populus tremula were associated with the warmest sites with intermediate precipitation, while Abies sibirica was the most moisture-demanding species, sensitive to deep winter frosts.     

Decomposition of secondary compounds from needle litter of Scots pine grown under elevated CO2 and O3

Elevated atmospheric carbon dioxide (CO₂) and ozone (O₃) concentrations have both been shown to affect plant tissue quality, which in turn could affect litter decomposition and carbon (C) and nutrient cycling. In order to evaluate effects of climate change on litter chemistry, needle litter was collected from Scots pine (Pinus sylvestris L.) saplings exposed to elevated CO₂ or O₃ concentration and their combination over three growing seasons in open‐top chambers. The decomposition of needle litter was followed for 19 months in a pine forest. During decomposition, needle samples for secondary compound analysis were collected and the mass loss of needles was followed. Main nutrients and total phenolics were analysed from litter in the beginning and at the end of the experiment. After 19‐month decomposition, the accumulated mass loss was about 34%; however, no significant differences were found in decomposition rates of needle litter between various treatments. Concentrations of total monoterpenes were about 4%, total resin acids 21% and total phenolics 14% of the initial concentrations in litter after 19‐month decomposition. In the beginning of litter decomposition, concentrations of individual monoterpenes –α‐pinene and β‐pinene – were significantly higher in needle litter grown under elevated CO₂. However, concentrations of total monoterpenes during the whole decomposition period were not significantly affected by CO₂ or O₃ treatments. Concentrations of some individual and total resin acids were higher in needle litter grown under elevated CO₂ or O₃ than under ambient air. There were no significant differences in concentrations of total phenolics as well as nitrogen (N) and the main nutrient concentrations between treatments during decomposition. High concentrations of monoterpenes and resin acids in needles might slightly delay C recycling in forest soils. It is concluded that elevated CO₂ and O₃ concentrations do not have remarkable impacts on litter decomposition processes in Scots pine forests.     

Physical and chemical effects of litter on plant establishment in semi-natural grasslands

A field experiment was conducted in a semi-natural grassland to study the interspecific variation in the effect of litter on seedling emergence and establishment and separate physical from chemical effects. Seeds of seven forb species were sown in plots subjected to either litter amendment (0, 400 or 900 g m⁻²) or water extracts of litter (corresponding to 400 and 900 g litter m⁻²). In addition, an extract was treated with activated carbon to estimate the possible effects of secondary chemical compounds. The response to plant litter differed amongst species: negative, neutral and positive responses were observed. Anthriscus sylvestris was the only species with a strong positive response to litter. We found no consistent relation between seed size and response to plant litter. Physical effects of litter were generally stronger than chemical effects. However, water extract of litter inhibited emergence in three species. Activated carbon removed the negative effect of the litter extract, which suggests that the effect was caused by an inhibitory chemical compound rather than by increased competition in response to nutrients added via the extract. The balance between facilitative and inhibitory effects of litter depended on species identity and litter quantity. Facilitative effects dominated at low and intermediate quantities of litter, and inhibitory effects at high litter quantities. One species, Campanula rotundifolia, showed a switch from positive to negative responses with increasing quantities of litter. However, we found no general threshold for litter quantity valid across species.     

High-elevation ground-layer plant community composition across environmental gradients in spruce-fir forests

We examined the influence of vegetation structure and soil chemistry on post-adelgid, ground-layer plant communities in high-elevation forests of the southern Appalachian Mountains. Specifically, we hypothesized that post-disturbance community composition and diversity would vary along a gradient of soil acidity and other soil characteristics influenced by acid deposition. Ground-layer vegetation and soils were sampled on 60 randomly located nested vegetation plots in the spruce-fir (Picea abies) zone of Great Smoky Mountains National Park, TN and NC, USA. To capture a range of deposition levels, plot placement was stratified based on modeled acid deposition classes. Ordination and multiple regression results showed that ground-layer composition and diversity were negatively associated with acidity of the A horizon and the presence of ericaceous shrubs (i.e., Rhododendron spp.). A strong correspondence between soil acidity and ericaceous shrub cover was also observed, suggesting that soil acidity may be, in conjunction with overstory disturbance resulting from chronic acid deposition and adelgid induced mortality, an important driver of ericaceous shrub thicket expansion. Slow-decaying, acidic ericaceous litter may also induce a positive feedback resulting in enhanced acidification.     

The effect of nutrient supply and light intensity on tannins and mycorrhizal colonisation in Dutch heathland ecosystems

(1) Increased atmospheric nitrogen deposition has shifted plant dominance from ericaceous plants to grass species. To elucidate the reduced competitiveness of heather, we tested the hypothesis that additions of nitrogen reduce the concentrations of phenolics and condensed tannins in ericaceous leaves and retard mycorrhizal colonisation in ericaceous plants. We also tested the negative effects of reduced light intensity on carbon-based secondary compounds and mycorrhizal colonisation in ericaceous plants. (2) We performed a field inventory at three heathland sites in the Netherlands varying in nutrient supply and light intensity. Leaves of ericaceous plants and grasses were collected and analysed for concentrations of tannins, phenolics and nutrients. Similarly, we took root samples to record mycorrhizal colonisation and soil samples to measure the soil mineralisation. In addition, we conducted two-factorial experiments with Calluna vulgaris plants, in which we varied fertiliser and shade levels under greenhouse and field conditions. (3) The field inventory revealed that nitrogen addition and shading both negatively affected the concentration of total phenolics. The total phenolics and condensed tannin concentrations were positively correlated (P < 0.001), but in the field experiment, the condensed tannins were not significantly affected by the treatments. Our results provide the first evidence that the carbon nutrient balance can be used to predict the amount of total phenolics in the dwarf shrub C. vulgaris. (4) In the field experiments, shading of plants resulted in significantly less mycorrhizal colonisation. Only in the greenhouse experiment did addition of nitrogen negatively affect mycorrhizal colonisation. (5) Our results imply that increased atmospheric nitrogen deposition can depress the tannin concentrations in ericaceous plants and the mycorrhizal colonisation in roots, thereby reducing the plants’ competitiveness with respect to grasses. Additionally, if ericaceous plants are shaded by grasses that have become dominant due to increased nitrogen supply, these effects will be intensified and competitive replacement will be accelerated.     

Invasion of Calluna heath by native and non-native conifers: the role of succession, disturbance and allelopathy

The coastal heathlands of North-western Europe are treeless anthropogenic landscape of high conservation value, now threatened by encroachment by both native (Pinus sylvestris L.) and non-native (Picea sitchensis (Bong) Carr.) conifers. This study wants to gain insight into the different external drivers which interact with the natural successional processes to determine the future development of the heathland landscape. We tested how seedling emergence and establishment is related to competitive effects from the standing vegetation. Heathlands are nutrient-stressed habitats and we tested the seedling emergence and establishment along a nutrient gradient using mean Ellenberg values. In addition we tested if phytotoxic substances are affecting the seedling emergence and establishment related to different successional stages. Study species differed in seed size and we expected different colonization rates and competitive effects from the heathland vegetation. A factorial experiment was carried out in three different successional stages to isolate the effects of competition versus phytotoxins on seedling emergence and establishment by combining vegetation cutting and charcoal addition treatments. Both conifers were able to invade all successional stages, but invasibility decreased during succession. This was largely attributable to competitive effects from the standing vegetation, although phytotoxic effects were also detected, especially in late-successional stages. Seedling emergence and establishment was higher in the larger-seeded P. sylvestris than in P. sitchensis. These results present a management challenge as fire is an important component of the traditional management of the coastal heathlands, while at the same time increasing invasibility. Managers at sites susceptible to invasion should ensure that resources for tree seedling control are available before fire or other management-related disturbances are applied.     

Wildfire and forest harvest disturbances in the boreal forest leave different long-lasting spatial signatures

Aims

Natural disturbances leave long-term legacies that vary among landscapes and ecosystem types, and which become integral parts of successional processes at a given location. As humans change land use, not only are immediate post-disturbance patterns altered, but the processes of recovery themselves are likely altered by the disturbance. We assessed whether short-term effects on soil and vegetation that distinguish wildfire from forest harvest persist over 60 years after disturbance in boreal black spruce forests, or post-disturbance processes of recovery promote convergence of the two disturbance types.

Methods

Using semi-variograms and Principal Coordinates of Neighbour Matrices, we formulated precise, a priori spatial hypotheses to discriminate spatial signatures following wildfire and forest harvest both over the short- (16–18 years) and long-term (62–98 years).

Results

Both over the short- and the long-term, wildfire generated a wide spectrum of responses in soil and vegetation properties at different spatial scales, while logging produced simpler patterns corresponding to the regular linear pattern of harvest trails and to pre-disturbance ericaceous shrub patches that persist between trails.

Conclusions

Disturbance by harvest simplified spatial patterns associated with soil and vegetation properties compared to patterns associated with natural disturbance by fire. The observed differences in these patterns between disturbance types persist for over 60 years. Ecological management strategies inspired by natural disturbances should aim to increase the complexity of patterns associated with harvest interventions.