Roots and associated fungi drive long-term carbon sequestration in boreal forest.

Boreal forest soils function as a terrestrial net sink in the global carbon cycle. The prevailing dogma has focused on aboveground plant litter as a principal source of soil organic matter. Using (14)C bomb-carbon modeling, we show that 50 to 70% of stored carbon in a chronosequence of boreal forested islands derives from roots and root-associated microorganisms. Fungal biomarkers indicate impaired degradation and preservation of fungal residues in late successional forests. Furthermore, 454 pyrosequencing of molecular barcodes, in conjunction with stable isotope analyses, highlights root-associated fungi as important regulators of ecosystem carbon dynamics. Our results suggest an alternative mechanism for the accumulation of organic matter in boreal forests during succession in the long-term absence of disturbance.

Empirical and theoretical challenges in aboveground-belowground ecology.

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground-belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground-belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.

Biodiversity and ecosystem functioning: current knowledge and future challenges.

The ecological consequences of biodiversity loss have aroused considerable interest and controversy during the past decade. Major advances have been made in describing the relationship between species diversity and ecosystem processes, in identifying functionally important species, and in revealing underlying mechanisms. There is, however, uncertainty as to how results obtained in recent experiments scale up to landscape and regional levels and generalize across ecosystem types and processes. Larger numbers of species are probably needed to reduce temporal variability in ecosystem processes in changing environments. A major future challenge is to determine how biodiversity dynamics, ecosystem processes, and abiotic factors interact.

The charcoal effect in Boreal forests: mechanisms and ecological consequences.

Wildfire is the principal disturbance regime in northern Boreal forests, where it has important rejuvenating effects on soil properties and encourages tree seedling regeneration and growth. One possible agent of this rejuvenation is fire-produced charcoal, which adsorbs secondary metabolites such as humus phenolics produced by ericaceous vegetation in the absence of fire, which retard nutrient cycling and tree seedling growth. We investigated short-term ecological effects of charcoal on the Boreal forest plant-soil system in a glasshouse experiment by planting seedlings of Betula pendula and Pinus sylvestris in each of three humus substrates with and without charcoal, and with and without phenol-rich Vaccinium myrtillus litter. These three substrates were from: (1) a high-productivity site with herbaceous ground vegetation; (2) a site of intermediate productivity dominated by ericaceous ground vegetation; and (3) an unproductive site dominated by Cladina spp. Growth of B. pendula was stimulated by charcoal addition and retarded by litter addition in the ericaceous substrate (but not in the other two), presumably because of the high levels of phenolics present in that substrate. Growth of P. sylvestris, which was less sensitive to substrate origin than was B. pendula, was unresponsive to charcoal. Charcoal addition enhanced seedling shoot to root ratios of both tree species, but again only for the ericaceous substrate. This response is indicative of greater N uptake and greater efficiency of nutrient uptake (and presumably less binding of nutrients by phenolics) in the presence of charcoal. These effects were especially pronounced for B. pendula, which took up 6.22 times more nitrogen when charcoal was added. Charcoal had no effect on the competitive balance between B. pendula and P. sylvestris, probably due to the low intensity of competition present. Juvenile mosses and ferns growing in the pots were extremely responsive to charcoal for all sites; fern prothalli were entirely absent in the ericaceous substrate unless charcoal was also present. Charcoal stimulated active soil microbial biomass in some instances, and also exerted significant although idiosyncratic effects on decomposition of the added litter. Our results provide clear evidence that immediately after wildfire fresh charcoal can have important effects in Boreal forest ecosystems dominated by ericaceous dwarf shrubs, and this is likely to provide a major contribution to the rejuvenating effects of wildfire on forest ecosystems.

Microbe-plant competition, allelopathy and arctic plants.

Michelsen et al. (1995) present results of an experiment in which aqueous leaf extracts of three arctic woody plant species were found to inhibit growth and nutrient acquisition of three graminoid species, and suggested that microbial nutrient immobilisation, rather than allelopathy, was responsible for the observed trends. In doing this they also question previous work proposing that the Arctic dwarf shrub Empetrum hermaphroditum is allelopathic. We suggest that their conclusions are not unequivocally supported by their data. Firstly we indicate that the approaches used for estimating microbial nutrient immobilisation are questionable. Secondly we indicate that most of the trends that they discussed are based on data in which the treatments and controls are not significantly different for the majority of cases. Finally we respond specifically to their criticisms of previous work on E. hermaphroditum. While the question of how arctic plants interact is an interesting one, we conclude that this question cannot be answered by their data.

Use of a comparative approach to identify allelopathic potential and relationship between allelopathy bioassays and "competition" experiments for ten grassland and plant species.

Various allelopathy bioassays were used to evaluate the allelopathic potential of 10 grassland forage species against a common test (phytometer) species,Carduus nutans L. Aqueous extracts did not influenceC. nutans germination, although radicle elongation was often severely inhibited.C. nutans was strongly affected by shoot, but not root, leachates. Decomposing ground tissue had mixed effects, and often stimulated shoot production ofC. nutans. Calculation ofR (2) (coefficient of determination) values between these results, and the results of previous experiments investigating the effects of the same 10 species onC. nutans emergence and development in field plots and glasshouse competition experiments frequently revealed strong, statistically significant relationships. Our results therefore provide correlative evidence for the importance of allelopathy in field conditions.

The dual importance of competition and predation as regulatory forces in terrestrial ecosystems: evidence from decomposer food-webs.

The relative importance of predation and competition (resource limitation) in influencing the components of a below-ground food-web consisting of three trophic levels (bacteria and fungi; bacterial-feeding and fungal-feeding nematodes; and top predatory nematodes) was estimated using microbial biomass and nematode frequency data collected throughout a 1-year period in two agro-ecosystems. The study suggested that bacterial and fungal biomass were likely to be regulated by grazing and competition respectively, and that these differences were likely to be attributed to the biological (probably morphological) differences between bacteria and fungi, in contrast to the predictions of the hypothesis of Hairston et al. (1960). Top predatory nematodes were sometimes strongly related to the microbial but not microbial-feeding trophic levels, indicating that microbial biomass may directly influence top predator numbers, and that the intermediate level may simply serve as a conduit by which resources pass from the bottom to top trophic levels. This study also suggests that the detritus food-web acts as two distinct (bacterial-and fungal-based) compartments.

Interspecific competitive interactions between pairs of fungal species in natural substrates.

The role of interspecific competition in fungal communities in natural substrates is poorly understood because fungi do not form easily definable populations. A new approach to investigating fungal competition, using natural substrates containing a range of known biomass concentrations of each of two species, is described. Relative competitive success of each species is assessed over time in terms of propagule production and substrate colonisation by each species. In an agricultural soil Mucor hiemalis usually out-competed Trichoderma harzianum. After 27 days, the success of both species in the mixtures was independent of the initial biomass concentration of either species, although the success of T. harzianum in these mixtures was substantially inhibited relative to the T. harzianum monocultures. In a forest soil, T. polysporum maintained a competitive advantage over M. hiemalis, and induced M. hiemalis to produce propagules rather than mycelia. Coexistence of both species always occurred in both experiments, and in the forest soil experiment the two-species mixtures all contained a higher total microbial biomass than the monocultures of either species by day 47, suggesting some niche differentiation.