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.


Naturally dynamic boreal forests show hierarchical structural variation patterns which are independent of the dominant tree species or disturbance regime

The spatial variability of forest structure (for example, tree sizes, distribution of stems and foliage, dead wood) is the result of multiple factors such as disturbances, succession, topography, and soil properties. However, the explicit scales at which these variation patterns occur are often described only qualitatively.

We studied three 2 km × 2 km landscapes in northeastern Finland and two in eastern Canada. We estimated canopy cover in contiguous 0.1-ha cells from aerial photographs and used scale-derivative analysis to identify characteristic scales of variation in the canopy cover data. We analyzed the patterns of variation at these scales using Bayesian scale space analysis.

We identified structural variation at three spatial scales in each landscape. Among landscapes, the largest scale of variation showed the greatest variability (20.1–321.4 ha), related to topography, soil variability, and long-term disturbance history. Superimposed on this large-scale variation, forest structure varied at similar scales (1.3–2.8 ha) in all landscapes. This variation correlated with recent disturbances, soil variability, and topographic position. We also detected intense variation at the smallest scale analyzed (0.1 ha, grain of our data), partly driven by recent disturbances.

Except for the large-scale variation, the identified scales were remarkably similar among the landscapes. This suggests that boreal forests may display characteristic scales of variation that occur somewhat independent of the tree species characteristics or the disturbance regime.

The study is published in Ecosystems, and lives here.