Marie-Charlotte Nilsson

Understory vegetation as a forest ecosystem driver: evidence from the northern Swedish boreal forest

Vegetation research in boreal forests has tended to focus on the tree component, while little attention has been paid to understory components such as dwarf shrubs, mosses, and reindeer lichens. However, the productivity of understory vegetation is probably comparable to that of the trees. We review recent research in the boreal forest of northern Sweden to highlight the ecological importance of understory vegetation, both in the short term by influencing tree seedling regeneration, and in the longer term by affecting belowground processes such as decomposition, nutrient flow, and buildup of soil nutrients. Wildfire resulting from lightning strike is a primary determinant of understory vegetation, and as such is a major driver of forest community and ecosystem properties. Forest management practices that alter the fire regime and the composition of understory vegetation may have long-term consequences for both conservation goals and commercial forest productivity.

Fire-Derived Charcoal Causes Loss of Forest Humus

Fire is a global driver of carbon storage and converts a substantial proportion of plant biomass to black carbon (for example, charcoal), which remains in the soil for thousands of years. Black carbon is therefore often proposed as an important long-term sink of soil carbon. We ran a 10-year experiment in each of three boreal forest stands to show that fire-derived charcoal promotes loss of forest humus and that this is associated with enhancement of microbial activity by charcoal. This result shows that charcoal-induced losses of belowground carbon in forests can partially offset the benefits of charcoal as a long-term carbon sink.

Quantifying nitrogen-fixation in feather moss carpets of boreal forests

Biological nitrogen (N) fixation is the primary source of N within natural ecosystems, yet the origin of boreal forest N has remained elusive. The boreal forests of Eurasia and North America lack any significant, widespread symbiotic N-fixing plants. With the exception of scattered stands of alder in early primary successional forests, N-fixation in boreal forests is considered to be extremely limited. Nitrogen-fixation in northern European boreal forests has been estimated at only 0.5 kg N ha-1 yr-1; however, organic N is accumulated in these ecosystems at a rate of 3 kg N ha-1 yr-1 (ref. 8). Our limited understanding of the origin of boreal N is unacceptable given the extent of the boreal forest region, but predictable given our imperfect knowledge of N-fixation. Herein we report on a N-fixing symbiosis between a cyanobacterium (Nostoc sp.) and the ubiquitous feather moss, Pleurozium schreberi (Bird) Mitt. that alone fixes between 1.5 and 2.0 kg N ha-1 yr-1 in mid- to late-successional forests of northern Scandinavia and Finland. Previous efforts have probably underestimated N-fixation potential in boreal forests.

Key ecological function of charcoal from wildfire in the Boreal forest

Wildfire is a major disturbance factor in boreal forests where it is important in rejuvenating soil properties and encouraging tree regeneration and growth. However, the mechanisms behind these effects are poorly understood and little is known as to the ecological effects of charcoal produced by wildfire in these ecosystems. In this study we firstly quantified the mass of charcoal in a chronosequence of twelve forest sites in northern Sweden and found charcoal mass in soil to vary from 984 to 2074 kg ha -1 ; these levels appear to be sufficient for charcoal to have important ecological effect through its sorptive abilities. We then investigated the ability of charcoal from 32 forest stands (representing a range of ages from 1 to 350 yr since last fire) to adsorb phenolic compounds produced by the late successional dwarf shrub Empetrum hermaphroditum ; phenolic compounds from this species have previously been shown to have important phytotoxic effects in boreal forests. Charcoal in soil from forests younger than 100 yr was very effective at reducing these effects while older charcoal was not, suggesting that the sorptive ability is likely to be most important in earlier-successional forests. Experimental reheating of deactivated older charcoal from soil showed that temperatures above 450°C could reactivate charcoal. A microcosm experiment also revealed that soil microbes could effectively reactivate young charcoal that had been saturated with phenolics. Finally we investigated the effects of artificially made charcoal on soil microbial properties at six sites. The microbial biomass was consistently enhanced in humus when it was placed adjacent to charcoal particles. Decomposition of plant litter was sometimes also affected by being in the proximity of charcoal but the dircction of these effects was unpredictable. We conclude that charcoal might catalyse important ecological soil processes in early-successional boreal forests, effects that diminish as succession proceeds, and ultimately may have important long-term consequences for stand productivity and ecosystem function, especially in forests under strict fire control.

The charcoal effect in Boreal forests: mechanisms and ecological consequences

Abstract 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.

Ectomycorrhizal fungal communities in late‐successional Swedish boreal forests, and their composition following wildfire

This study was conducted to evaluate the effects of wildfires on ectomycorrhizal (EM) fungal communities in Scots pine (Pinus sylvestris) stands. Below‐ and above‐ground communities were analysed in terms of species richness and evenness by examining mycorrhizas and sporocarps in a chronosequence of burned stands in comparison with adjacent unburned late‐successional stands. The internal transcribed spacer (ITS)‐region (rDNA) of mycobionts from single mycorrhizas was digested with three restriction enzymes and compared with an ITS–restriction fragment length polymorphism (RFLP) reference database of EM sporocarps. Spatial variation seemed to be more prominent than the effects of fire on the EM fungal species composition. Most of the common species tended to be found in all sites, suggesting that EM fungal communities show a high degree of continuity following low‐intensity wildfires. Species richness was not affected by fire, whereas the evenness of species distributions of mycorrhizas was lower in the burned stands. The diversity of EM fungi was relatively high considering that there were only three EM tree species present in the stands. In total, 135 EM taxa were identified on the basis of their RFLP patterns; 66 species were recorded as sporocarps, but only 11 of these were also recorded as mycorrhizas. The species composition of the below‐ground community of EM fungi did not reflect that of the sporocarps produced. EM fungal species present in our ITS–RFLP reference database accounted for 54–99% of the total sporocarp production in the stands, but only 0–32% of the mycorrhizal abundance.

Separation of allelopathy and resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup

An experimental technique was used to separate and evaluate the magnitude of allelopathic interference relative to resource competition by the boreal dwarf shrub Empetrum hermaphroditum Hagerup (Empetraceae). To test for resource competition and allelopathy, respectively, Scots pine (Pinus sylvestris L.) seedlings were grown in both the greenhouse and in the field over a 3 year period, in four different treatments within E. hermaphroditum vegetation: (1) PVC tubes were used to reduce effects of interspecific below-ground competition; (2) activated carbon was spread on the soil to adsorb toxins leached from E. hermaphroditum litter and green leaves, thus reducing effects of allelopathic interference; (3) E. hermaphroditum vegetation was left untreated to evaluate inhibiting effects when both allelopathy and resource competition were present; (4) PVC tubes, placed in E. hermaphroditum vegetation spread with activated carbon were used to determine growth of seedlings when both allelopathy and resource competition were reduced. Scots pine seedlings grown in untreated vegetation (with both root competition and allelopathy present) had the lowest shoot length and dry weight; seedlings with both allelopathy and root competition reduced (activated carbon in tube) were the largest. Reducing either root competition alone (tube treatment) or allelopathy alone (carbon treatment) produced seedlings of intermediate size, but reduced competition had a greater effect than reduced allelopathy (although, in the greenhouse, significantly so only for root biomass). In the greenhouse experiment, biomass production of seedlings grown free of both interactions (carbon in tube) was greater than the simple sum of the growth response to the individual interactions (tube treatment and carbon treatment, respectively). Larger shoot:root ratios were also found when pine seedlings were grown without tubes (i.e. when resource competition was occurring). In the field, the removal of allelopathy (carbon treatments) increased shoot:root ratio when compared to the removal of resource competition. The study showed that two different interference mechanisms of E. hermaphroditum can be separated and quantified, and that below-ground competition and allelopathy by E. hermaphroditum are both important factors retarding growth of Scots pine.

NITROGEN FIXATION INCREASES WITH SUCCESSIONAL AGE IN BOREAL FORESTS

There is little understanding of successional dynamics of N fixation in northern boreal forests. Recent evidence suggests that N fixation by cyanobacteria in association with the common feather moss Pleurozium schreberi contributes to a significant proportion of the total N economy. The purpose of the work herein was to determine how time since last fire influences N fixation rates in boreal forests. We evaluated seasonal N fixation rates on a total of 12 natural forest preserves varying in time since last fire (35–355 years). Each site was monitored for N fixation activity using a calibrated acetylene reduction assay. Nitrogen fixation rates were found to increase linearly with time since fire. This increase in N fixation with succession is likely a function of degree of colonization by cyanobacteria and site factors such as presence of available N. Surface applications of 4.5 kg N·ha−1·yr−1 as NH4NO3 were found to eliminate N fixation while applications of P resulted in only a slight and temporary increase of N fixation rates. In contrast to common observation our findings suggest that N fixation in boreal forests becomes more important in late succession. Limited N availability in late succession is clearly one of the primary drivers of N fixation rates in boreal forest ecosystems. These findings may help to explain the origin of high rates of net N accumulation in soil unaccounted for at northern boreal sites.

Depth distribution of microbial production and oxidation of methane in northern boreal peatlands.

The depth distributions of anaerobic microbial methane production and potential aerobic microbial methane oxidation were assessed at several sites in both Sphagnum- and sedge-dominated boreal peatlands in Sweden, and compared with net methane emissions from the same sites. Production and oxidation of methane were measured in peat slurries, and emissions were measured with the closed-chamber technique. Over all eleven sites sampled, production was, on average, highest 12 cm below the depth of the average water table. On the other hand, highest potential oxidation of methane coincided with the depth of the average water table. The integrated production rate in the 0–60 cm interval ranged between 0.05 and 1.7 g CH4 m −2 day− and was negatively correlated with the depth of the average water table (linear regression: r2 = 0.50, P = 0.015). The depth-integrated potential CH4-oxidation rate ranged between 3.0 and 22.1 g CH4 m−2 day−1 and was unrelated to the depth of the average water table. A larger fraction of the methane was oxidized at sites with low average water tables; hence, our results show that low net emission rates in these environments are caused not only by lower methane production rates, but also by conditions more favorable for the development of CH4-oxidizing bacteria in these environments.

Effects of Plant Litter Species Composition and Diversity on the Boreal Forest Plant-Soil System

Although most plant products eventually enter the below-ground subsystem as plant litter, relatively little is known about the effects of plant litter diversity or composition on ecosystem processes and no study has considered the responses of plant growth to these factors. We conducted an experiment in which humus substrate was collected from three field sites in the boreal forest of northern Sweden. Litter was then placed on the humus surface, and the litter used consisted of monocultures of Empetrum hermaphroditum (dwarf shrub), Betula pendula (tree), and Pleurozium schreberi (feather moss), as well as mixtures containing all the possible (two-way and three-way) combinations of these species: the experiment was maintained in out-door conditions. Although decomposition rates of this surface-placed litter differed between species few effects of litter mixing on litter mass loss were apparent. Added litter of Pinus sylvestris litter broke down most slowly when placed in E. hermaphrodnum litter but sometimes showed elevated decomposition rates when placed in some of the multiple species litter mixes. Soil microbial biomass and activity was lowest when plant litter was absent, but as long as plant litter was present on the humus surface the species composition and diversity of the litter was irrelevant. There were few effects of litter treatments on growth of seedlings of either B. pendula or P. sylvestris planted into the humus. However, for one site there were significant effects of mixing litter of P. sclueberi and E. hermaphroditum in reducing growth of both seedling species. Litter treatments generally did not alter the competitive balance between B. pendula and P. sylvestris seedlings when grown together but for all sites litter treatments had significant effects on the overall intensity of competition, and mixing of litter of B. pendula and P. sehreheri had significant non-additive effects on competition intensity for two of the three sites. The abundance of mycorrhizae on seedlings was only weakly related to litter treatment but there were some positive effects of litter mixing on one of the most abundant mycorrhizal morphotypes on both species of seedlings for one of the sites. Our results suggest that litter presence was important in influencing a range of above ground and below ground properties and processes. In some instances individual species effects and litter mixing effects were also important but few general patterns emerged, and the nature of significant effects tended to be idiosyneratic. Ultimately our results show that plant litter has important afterlife effects which need to be considered in order to develop a more complete understanding of how biodiversity affects ecosystems.