Kajar Köster

Fungal Community Shifts in Structure and Function across a Boreal Forest Fire Chronosequence

ABSTRACT Forest fires are a common natural disturbance in forested ecosystems and have a large impact on the microbial communities in forest soils. The response of soil fungal communities to forest fire is poorly documented. Here, we investigated fungal community structure and function across a 152-year boreal forest fire chronosequence using high-throughput sequencing of the internal transcribed spacer 2 (ITS2) region and a functional gene array (GeoChip). Our results demonstrate that the boreal forest soil fungal community was most diverse soon after a fire disturbance and declined over time. The differences in the fungal communities were explained by changes in the abundance of basidiomycetes and ascomycetes. Ectomycorrhizal (ECM) fungi contributed to the increase in basidiomycete abundance over time, with the operational taxonomic units (OTUs) representing the genera Cortinarius and Piloderma dominating in abundance. Hierarchical cluster analysis by using gene signal intensity revealed that the sites with different fire histories formed separate clusters, suggesting differences in the potential to maintain essential biogeochemical soil processes. The site with the greatest biological diversity had also the most diverse genes. The genes involved in organic matter degradation in the mature forest, in which ECM fungi were the most abundant, were as common in the youngest site, in which saprotrophic fungi had a relatively higher abundance. This study provides insight into the impact of fire disturbance on soil fungal community dynamics.

Regeneration in windthrow areas in hemiboreal forests: the influence of microsite on the height growths of different tree species

AbstractNatural regeneration of windthrow areas is an important issue when planning forestry measures after forest disturbances. Seedling recruitment was investigated in storm-damaged hemiboreal mixed forests in eastern Estonia. The establishment and growth of seedlings from natural regeneration was registered for tree species in soil pits and in mounds of uprooted trees in stands that were either heavily or moderately damaged. Seedling growth is expected to be better in large but shallow soil pits created by uprooted Norway spruce [Picea abies (L.) Karst.] and poorer in small but deep pits created by the hardwoods in the area, silver birch (Betula pendula Roth.) and European aspen (Populus tremula L.). The most abundant regenerating species was birch. Pits hosted larger seedling numbers than mounds, due to soil instability in mounds. Rowan (Sorbus aucuparia L.) showed significantly faster growth than the other seedling species. Norway spruce pits were preferred to pits of other species by both birch and spruce seedlings. Black alder [Alnus glutinosa (L.) J. Gaertn.] did not show a preference for pits of a certain species of uprooted tree. Both spruce and rowan preferred hardwood mounds over spruce mounds. Storm severity also affected species composition: birch predominantly occurred on pits and mounds in heavily disturbed areas, while spruce was more abundant in the moderately damaged areas. The effects of advance regeneration and surrounding stands on seedling microsite preferences should be considered in future research and subsequent management recommendations.

The influence of storm-induced microsites to tree regeneration patterns in boreal and hemiboreal forest

We reviewed studies dealing with regeneration under variable conditions in boreal and hemiboreal forests as affected by different microsite types by tree species functional groups. Generally, the importance of storm-induced microsites for regeneration dynamics in boreal forests depends on several factors: (1) distribution and type of microsites (generated by storm characteristics and stand conditions); (2) viable seed supply (stand history, species dispersal traits and status of surviving trees) and their species’ life history strategy; (3) climatic and site conditions (pre-storm conditions and storm-induced changes); and (4) delayed storm effects, such as retarded falling of trees, favoured vegetation growth, etc. Studies acknowledging the significance of microsites were mostly related to intermediate or severe events, causing sufficient changes in resource levels and growth conditions, and influencing extrinsic factors such as frost heaving, erosion and browsing. Also, the dispersal traits of available tree species, including sprouting and response of surviving trees, such as canopy expansion, should be considered in evaluating microsite importance in individual cases. In intermediate to severe windstorm events, pioneer species are generally profiting most from the additional offer in microsites, requiring bare mineral soil and elevated locations for their establishment and growth. Under gap dynamics, shade-tolerant species benefit from dead wood and elevated locations as these offer safe sites in stands with abundant understorey vegetation.

Variation and ecological characteristics of coarse woody debris in Lahemaa and Karula National Parks, Estonia

Abstract The coarse woody debris (CWD) was inventoried in two boreal Estonian conifer-dominated forest landscapes/national parks, Lahemaa and Karula, with different forestry history and management intensity. The inventoried areas in both national parks consisted of a core with a strict nature reserve (unmanaged) and the surrounding protected special and restricted management zones (management activities in the past). Stands with no records of silvicultural activity since the 1920s (unmanaged) were compared with traditionally harvested stands. CWD was measured as standing dead trees, logs and snags >10 cm in diameter and >1.3 m in length in 304 circular plots (Lahemaa 134 plots, Karula 170 plots; r=11.28 m, 400 m2). The volumes of CWD varied considerably between individual stands. The mean volume of CWD (standing and down combined) in Lahemaa was 48.5 m3 ha−1, ranging from 0.6 to 148.6 m3 ha−1. The mean volume of CWD in Karula was 27.6 m3 ha−1, ranging from 0.2 to 193.7 m3 ha−1 from stand to stand. On average, 19.5 m3 ha−1 (40.2%) of CWD in Lahemaa was standing dead wood and 29.1 m3 ha−1 (59.8%) down dead wood. In Karula standing dead wood formed 15.2 m3 ha−1 (55.7%) and down dead wood 12.2 m3 ha−1 (44.3%). Variation in CWD volumes was clearly dependent on the management history of the stands. Stands with a documented history of management (e.g. cuttings and thinnings) had significantly lower CWD volume than natural stands found mainly in strict nature reserves. Stands selectively logged a long time ago (more than approximately 60 years) did not differ considerably from natural stands in the amount of CWD. The amount of CWD in managed stands (Lahemaa 14.1 m3 ha−1 and Karula 10.6 m3 ha−1) was comparable to other studies in silviculturally managed forests in the boreal zone. The study shows that CWD amounts in Estonian conditions are similar to previous studies in this region.

Assessment of tree mortality after windthrow using photo-derived data.

We used sequential surface photography and photo-derived data to evaluate tree mortality in a windthrow area in eastern Estonia, where a storm occurred in 2001. The study is based on photographs taken from the edge of three completely destroyed areas with total canopy destruction in which wind-felled spruce trees (Picea abies) were left after disturbance. In total, 137 spruce trees were observed over a five-year period. We used a transition matrix to examine tree mortality dynamics and patterns. At the end of the five-year period, only 25% of the spruce trees survived in areas surrounding the windthrow. The mortality was highest in the second year after disturbance and the probability of a tree falling was surprisingly high over the entire study period. According to local observations, Ips typographus caused most of the tree deaths, but the co-influences of other factors were also important as there was a large proportion of falling trees in the area.

Influences of Reindeer Grazing on Above- and Belowground Biomass and Soil Carbon Dynamics

Abstract Reindeer (Rangifer tarandus L.) are the most important large mammalian herbivores in Lapland, strongly affecting the dynamics of vegetation by grazing and trampling, and this is likely in turn to have consequences for the soil processes. We have investigated the changes occurring in above- and belowground biomasses, and soil C dynamics (CO2 efflux, soil C content, soil microbial biomass) among areas grazed and not grazed by reindeer. Our study areas are located in the northern boreal subarctic coniferous forest (undisturbed Scots pine [Pinus sylvestris L.] forests that are naturally lichen-dominated). Our study showed that grazing by reindeer decreased the biomass and cover of lichens in the area significantly. Also the tree regeneration was affected by grazing, as we had much less tree regeneration in the grazed areas. In subarctic mature pine forest, grazing did not affect the soil temperature or the soil moisture. We found no statistically significant effect of grazing on the soil CO2 efflux, soil C stock, and the soil microbial C biomass. Soil microbial N biomass was significantly lower in the grazed areas compared to the non-grazed areas. Our results indicate that in the northern boreal subarctic coniferous forests, grazing by reindeer can be considered as “C neutral.”

Bacterial community structure and function shift across a northern boreal forest fire chronosequence

Soil microbial responses to fire are likely to change over the course of forest recovery. Investigations on long-term changes in bacterial dynamics following fire are rare. We characterized the soil bacterial communities across three different times post fire in a 2 to 152-year fire chronosequence by Illumina MiSeq sequencing, coupled with a functional gene array (GeoChip). The results showed that the bacterial diversity did not differ between the recently and older burned areas, suggesting a concomitant recovery in the bacterial diversity after fire. The differences in bacterial communities over time were mainly driven by the rare operational taxonomic units (OTUs < 0.1%). Proteobacteria (39%), Acidobacteria (34%) and Actinobacteria (17%) were the most abundant phyla across all sites. Genes involved in C and N cycling pathways were present in all sites showing high redundancy in the gene profiles. However, hierarchical cluster analysis using gene signal intensity revealed that the sites with different fire histories formed separate clusters, suggesting potential differences in maintaining essential biogeochemical soil processes. Soil temperature, pH and water contents were the most important factors in shaping the bacterial community structures and function. This study provides functional insight on the impact of fire disturbance on soil bacterial community.

The long-term impact of low-intensity surface fires on litter decomposition and enzyme activities in boreal coniferous forests

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.

Nitrogen balance along a northern boreal forest fire chronosequence

Fire is a major natural disturbance factor in boreal forests, and the frequency of forest fires is predicted to increase due to climate change. Nitrogen (N) is a key determinant of carbon sequestration in boreal forests because the shortage of N limits tree growth. We studied changes in N pools and fluxes, and the overall N balance across a 155-year non stand-replacing fire chronosequence in sub-arctic Pinus sylvestris forests in Finland. Two years after the fire, total ecosystem N pool was 622 kg ha-1 of which 16% was in the vegetation, 8% in the dead biomass and 76% in the soil. 155 years after the fire, total N pool was 960 kg ha-1, with 27% in the vegetation, 3% in the dead biomass and 69% in the soil. This implies an annual accumulation rate of 2.28 kg ha-1 which was distributed equally between soil and biomass. The observed changes in N pools were consistent with the computed N balance +2.11 kg ha-1 yr-1 over the 155-year post-fire period. Nitrogen deposition was an important component of the N balance. The biological N fixation increased with succession and constituted 9% of the total N input during the 155 post-fire years. N2O fluxes were negligible (≤ 0.01 kg ha-1 yr-1) and did not differ among post-fire age classes. The number and intensity of microbial genes involved in N cycling were lower at the site 60 years after fire compared to the youngest and the oldest sites indicating potential differences in soil N cycling processes. The results suggest that in sub-arctic pine forests, the non-stand-replacing, intermediate-severity fires decrease considerably N pools in biomass but changes in soil and total ecosystem N pools are slight. Current fire-return interval does not seem to pose a great threat to ecosystem productivity and N status in these sub-arctic forests.

Carbon dioxide, methane and nitrous oxide fluxes from a fire chronosequence in subarctic boreal forests of Canada

Forest fires are one of the most important natural disturbances in boreal forests, and their occurrence and severity are expected to increase as a result of climate warming. A combination of factors induced by fire leads to a thawing of the near-surface permafrost layer in subarctic boreal forest. Earlier studies reported that an increase in the active layer thickness results in higher carbon dioxide (CO2) and methane (CH4) emissions. We studied changes in CO2, CH4 and nitrous oxide (N2O) fluxes in this study, and the significance of several environmental factors that influence the greenhouse gas (GHG) fluxes at three forest sites that last had fires in 2012, 1990 and 1969, and we compared these to a control area that had no fire for at least 100years. The soils in our study acted as sources of CO2 and N2O and sinks for CH4. The elapsed time since the last forest fire was the only factor that significantly influenced all studied GHG fluxes. Soil temperature affected the uptake of CH4, and the N2O fluxes were significantly influenced by nitrogen and carbon content of the soil, and by the active layer depth. Results of our study confirm that the impacts of a forest fire on GHGs last for a rather long period of time in boreal forests, and are influenced by the fire induced changes in the ecosystem.