Improving fire history reconstructions

Occurrence of forest fires in the past has been extensively studied, using biological archives such as tree rings or charcoal in organic sediments. These proxies both have their advantages and disadvantages. Tree ring records are spatially and temporally accurate, but of limited time span. For sedimentary charcoal, the origin of the charcoal is not necessarily clear, and the temporal resolution is limited. However, sedimentary charcoal records often extend back to millennia.

In a recent study, lead by Normunds Stivrins as part of the project EBOR (Long-term Ecological History of the Boreal Forest), we demonstrated the applicability of another potential proxy for fires: the spores of the Neurospora fungi. This fungi develops its fruiting bodies when temperatures exceed 65°C, which in the boreal forest typically occurs only during forest fires. As the production of charcoal and the formation of fire scars have partly different requirements compared to the conditions that lead to Neurospora spore release, this proxy could bring valuable new insight into fire history reconstructions. Indeed, statistical tests showed a positive relationship between other fire event indicators and Neurospora occurrence, allowing us to locate past fire-events at times when the sedimentary charcoal was absent. These findings thus pave way for improvements in the reliability of fire history reconstructions from multiple proxies, and help understanding the long-term disturbance history of the boreal forest.

The full paper is now published in the Holocene.

Fire frequency for Kämmekkä (A), Polttiais (B) and Naava (C) hollow. Dots at the top of boxes indicate the sampling density, a higher row of crosses – the inferred fires and the lower row of crosses – the fires in 120-year bins. Black curve – the posterior mean of the fire frequency and gray band – the 95% highest density interval. X-axis – age and y-axis fire frequency data in 120-year bins

Variability and Mixed-Severity Disturbances Characterize Unmanaged Southern Boreal Forests in Russian Karelia

By: Satu Kilpinen

Research on unmanaged forests provides understanding of the structure, dynamics and ecology of natural forest ecosystems. This understanding can be utilized for planning sustainable forest management, conservation measures, or for assessing human impact on forests. Natural disturbance dynamics can be used as a model in forest management, for example in the planning of restoration activities or commercial forest cuttings.

In this study, we quantified and analyzed the disturbance history of four different forest stands in southern boreal forests of Russian Karelia. Our aims were to determine the disturbance frequency, intensity and range in different spatial scales, and to explore the connection between disturbance quality and forest characteristics. We used standard dendroecological methods (i.e. tree-rings) to record growth releases and gap recruitment from individual trees, indicating past disturbance. With this information, we reconstructed a disturbance chronology for each stand over the past two or three centuries.

Two of the studied forest sites were spruce-dominated and one pine-dominated stand. One stand represented mixed-forests with quite an even mix of pine, larch, spruce and birch trees. The age structure of the forests was uneven-aged in three of the sites. The fourth site was an exception, characterized by dominance of Scots pine, and relatively even-aged trees (80-100 years). The disturbance chronologies showed significant variation in spatial and temporal scales between sites and sample plots. During the last 2-3 centuries, all the sites had low (0-20%) to moderate (20-40%) or low to high severity (>40%) disturbances (Fig.). All the sites had gone through a stand-scale disturbance (i.e. disturbance recorded throughout the stand), but no evidence on regional scale disturbance was found. We found no clear connection between forest characteristics and disturbance quality, albeit the data may have been too small to detect any strong interdependence.

The disturbance dynamics in the forests studied showed remarkable variability in disturbance frequency, intensity, and range. The results emphasize the natural heterogeneity and variance related to forest structure, composition and disturbance history. While these results back up the prior understanding of the disturbance regime in the Karelian region, they also give us a better possibility to reflect the changing future conditions and address the important questions for upcoming research. For example, if the past natural disturbance dynamics has been more driven by local conditions and events than regional climate conditions, will this remain with the ongoing climate change? And to what extent, is it possible (or desirable), to preserve the variable natural disturbance dynamics and characteristics of forests along with the diminishing natural forest area and increasing human impact?

See full paper here

Fig. Disturbance chronologies on a decadal scale. First, all four sites separately and then pooled together. In the x-axis years and in the (left) y-axis % of trees indicating a release or a gap recruitment. The sample size contributing to the chronology is shown as a dashed line with corresponding values on the right y-axis. The chronologies are truncated after sample size drops below 5 in individual sites and 10 in the pooled chronology. Red vertical lines represent the dated fire scar years from the site in question.

 

Legacies of past wildfires continue to influence the development of northern boreal forests long after the fires have disappeared

Forest fires were historically the most impor­tant disturbance agent in the northern European boreal forest, but have virtually disappeared in the 20th century. However, past fires have left persistent legacies in forest structure and, due to their strong influence on forest age structures, continue influencing forest dynamics long since their disappearance. It is hence clear that we need knowledge on past fire occurrence, to be able to attribute the changes we observe in the forest to their drivers.

We reconstructed past fires in three northern boreal landscapes (each 2 km x 2 km) in Värriö and Maltio strict nature reserves in northeastern Finland. For the reconstructions, we used dendrochronological methods (i.e., methods based on tree ring widths), and described the occurrence of fires in the past 300 years. For these three landscapes, the average fire cycles (a measure of fire activity) were 72 and 156 years in Scots pine-dominated landscapes, and 579 years in a Norway spruce-dominated land­scape. The numbers of fires during the past three centuries were clearly related with soil properties: forests on soils that retain water better had experienced fewer fires than forests on coarser soils with lower water holding capacity.

Tree age structures that we determined from 1800 live and dead trees showed strong age cohorts that were associated with large fires in two of the landscapes. The y-axis shows the cumulative number of trees – each horizontal line represents the life span of an individual tree. Years indicated fires that influenced most parts of the study landscape.

It is noteworthy that although tree growth and regeneration in these northern regions are considered highly climate-sensitive, fires have been a major driver of forest dynamics in these areas. It is also clear that the continued absence of fires will lead to considerable changes in the forest structure and species composition in the future, even if the reserves are strictly protected from direct human influence.

The study is published in Boreal Environment Research, and can be found here.

Variability in climate historically influenced forest fire occurrence at multiple time scales in Fennoscandia

By compiling all available tree ring based fire history reconstructions, and a set of temperature and precipitation reconstructions, we analyzed how climate had influenced the occurrence of forest fires in Finland and the adjacent regions in western Russia. Our analyses were based on Bayesian scale-space methods, that made it possible to assess these fire-climate linkages over different time scales. In the analyses, we saw the well-documented relationship between fires and weather conditions during individual summers (high fire activity was related to dry summers). However, and somewhat surprisingly, fires and climate (summer temperature, and especially summer precipitation) were also very clearly related to one another at longer, approximately decadal, time scales. The result of this work, as a collaboration between our group, Department of Geosciences and Geography of University of Helsinki, and the Department of Mathematical Sciences at the University of Oulu, is now available in Ecological Monographs.

Historically, the synchrony of fire occurrence in different parts of the region was related to drought.

Biological “archives” carry reliable information of past temperatures for all four seasons in Finland

In a joint study with several research groups at the University of Helsinki and Aalto University, we compiled a collection of records on different biological and physical phenomena, to assess their reliability as an indicator of past temperatures for different seasons. This collection included different characteristics of tree rings, and plant and ice phenology. Our assessment showed how different combinations of these temperature indicators provided reliable information on the temperatures of all four seasons, as well as annual temperatures. See the full paper in Plos one.

Determining tree stem volume in R, using taper curves

Taper curves (i.e., functions describing the shape of a tree stem) can be used for conveniently computing the volume of a tree stem, using volume integrals, but they are not very commonly used in ecological research. The curves and their volume integrals are particularly convenient for computing the volume of snapped trees, where the shape of the stem (taper) is determined by the tree diameter and the height of the tree before snapping, and the volume is computed from the base of the tree to the snapping height (or any other point along the stem). Same functions can be used to compute the volume of a fallen part of a snapped tree just as easily.

We’ve written an easy-to-use R-script for several tree species in regions we are working in, based on previously published taper curves: Scots pine, Norway spruce, and birch in Finland, as well as black spruce and balsam fir in Quebec, Canada. Take a look at the script here.

 

North Fennoscandian mountain forests: past, present and future

Mountain forests of northern Fennoscandia are often referred to as “Europe’s last wilderness”. In a recent paper, written jointly with Bengt-Gunnar Jonsson (Mid-Sweden University) and Annika Hofgaard (Norwegian Institute for Nature Research) we reviewed the history, current state and discuss the unpredictable future of these forests.

See the full paper here, or access the pre-print version here.

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New paper published in OIKOS

Our new paper on modeling tree crown asymmetry in high-latitude boreal forests, using circular statistics has just been published in OIKOS. This paper was a collaborative effort with Kenichiro Shimatani (Institute of Statistics and Mathematics, Japan), Toshihiro Abe (Nanzan University, Japan), and Yasuhiro Kubota (University of the Ryukyus, Japan) and our research group.

For more details, click here or see the full paper here.

 

Paper out in Science: Boreal forest health and global change

 

This paper is published in Science and it reviews the specific ecological nature of the boreal forests and the current status of forest health at the face of global change. The paper examines the trends of forest health across the circumboreal zone and suggests ways forward in sustaining more healthy forest ecosystems in the future.

Boreal forest health and global change, by S. Gauthier, P. Bernier, T. Kuuluvainen, A.Z.Shvidenko and D.G.Schepaschenko.

 

link to paper