Tuomas Aakala

Even-Aged and Uneven-Aged Forest Management in Boreal Fennoscandia: A Review

Since WWII, forest management in Fennoscandia has primarily been based on even-aged stand management, clear cut harvesting and thinning from below. As an alternative, uneven-aged management, based on selection cutting of individual trees or small groups of trees, has been proposed. In this review we discuss the theoretical aspects of ecology and economics of the two management approaches. We also review peer-reviewed studies from boreal Fennoscandia, which have aimed at comparing the outcomes of uneven-aged and the conventional even-aged forest management. According to a common view the main obstacle of practicing uneven-aged forestry is its low economic performance. However, the reviewed studies did not offer any straightforward support for this view and several studies have found uneven-aged management to be fully competitive with existing even-aged management. Studies on the ecological aspects indicated that selection cuttings maintain mature or late-successional forest characteristics and species assemblages better than even-aged management, at least at the stand scale and in the short term. We conclude that although the number of relevant studies has increased in recent years, the ecological and economic performance of alternative management methods still remains poorly examined, especially for those stands with multiple tree species and also at wider spatial and temporal scales. For future research we advocate a strategy that fully takes into consideration the interdisciplinary nature of forest management and is better connected to social goals and latest theoretical and methodological developments in ecology and economics.

Spatial distribution of dead wood and the occurrence of five saproxylic fungi in old-growth timberline spruce forests in northern Finland

Abstract Adequate understanding of the factors that determine the establishment and survival of dead-wood-dependent species in natural forests is a prerequisite to the successful maintenance of these species in managed forests. This study investigated the factors affecting the occurrence of five wood-inhabiting polypores in old-growth timberline spruce forests in northern Finland, including the substrate availability and the spatial arrangement of large-diameter logs. The volume of coarse woody debris (CWD) varied significantly between the sites (range 15–30 m3 ha−1), large-diameter logs comprising on average 35% of the total CWD volume. However, the within-site variation in CWD volume was 10–15 times larger than the between-site variation. The spatial distribution of large-diameter logs was aggregated or initially aggregated on four sites and random on one site, creating local patches of high CWD volume. An individual target species occurred on average on 8% of all large-diameter logs, and on 11% of optimal logs (as determined by the decay stage). The characteristics of the logs and stand variables explained partly the occurrence of the target species; furthermore, the mortality pattern of trees (stem breakage versus uprooting) and the presence of primary decomposers also significantly affected the occurrence of the target species. The results indicate that besides substrate availability and quality, local habitat factors and species interactions also play a role in the occurrence of wood-inhabiting fungi in boreal timberline forests.

The old Norway spruce forests of northern boreal Fennoscandia are alive and well: a review of Sirén (1955)

Abstract The long-term development and stability of the thick moss–conifer forests that occur throughout the circumpolar boreal zone have been a subject of debate since the early twentieth century. This study reviews Gustaf Siréns (1955) study “On the development of spruce forests on raw humus sites in north Finland and its ecology” (Acta Forestalia Fennica, 62), which has been one of the most influential investigations of these forests in Fennoscandia. Through a chronosequence of 71 stands and measurements of ecological conditions in two different stands over two summers, Sirén formulated a model of stand development, in which forests undergo successive cycles of evenly aged stands. Central to the model was the declining productivity with each tree generation, until the next stand-replacing disturbance. This development was driven by accumulation of the raw humus layer and the subsequent poor ecological conditions for growth and regeneration. However, more recent results contradict Siréns model. The methodology, the main results and their historical context in Siréns model are presented and evaluated here. The significance and shortcomings of the results are discussed in the light of current knowledge of the long-term development of northern boreal Picea abies stands.

Reliability of temperature signal in various climate indicators from northern Europe

We collected relevant observational and measured annual-resolution time series dealing with climate in northern Europe, focusing in Finland. We analysed these series for the reliability of their temperature signal at annual and seasonal resolutions. Importantly, we analysed all of the indicators within the same statistical framework, which allows for their meaningful comparison. In this framework, we employed a cross-validation procedure designed to reduce the adverse effects of estimation bias that may inflate the reliability of various temperature indicators, especially when several indicators are used in a multiple regression model. In our data sets, timing of phenological observations and ice break-up were connected with spring, tree ring characteristics (width, density, carbon isotopic composition) with summer and ice formation with autumn temperatures. Baltic Sea ice extent and the duration of ice cover in different watercourses were good indicators of winter temperatures. Using combinations of various temperature indicator series resulted in reliable temperature signals for each of the four seasons, as well as a reliable annual temperature signal. The results hence demonstrated that we can obtain reliable temperature information over different seasons, using a careful selection of indicators, combining the results with regression analysis, and by determining the reliability of the obtained indicator.

Effects of competition, drought stress and photosynthetic productivity on the radial growth of white spruce in western Canada

Understanding the complex interactions of competition, climate warming-induced drought stress, and photosynthetic productivity on the radial growth of trees is central to linking climate change impacts on tree growth, stand structure and in general, forest productivity. Using a mixed modeling approach, a stand-level photosynthetic production model, climate, stand competition and tree-ring data from mixedwood stands in western Canada, we investigated the radial growth response of white spruce [Picea glauca (Moench.) Voss] to simulated annual photosynthetic production, simulated drought stress, and tree and stand level competition. The long-term (~80-year) radial growth of white spruce was constrained mostly by competition, as measured by total basal area, with minor effects from drought. There was no relation of competition and drought on tree growth but dominant trees increased their growth more strongly to increases in modeled photosynthetic productivity, indicating asymmetric competition. Our results indicate a co-limitation of drought and climatic factors inhibiting photosynthetic productivity for radial growth of white spruce in western Canada. These results illustrate how a modeling approach can separate the complex factors regulating both multi-decadal average radial growth and interannual radial growth variations of white spruce, and contribute to advance our understanding on sustainable management of mixedwood boreal forests in western Canada.

Newtonian boreal forest ecology: The Scots pine ecosystem as an example

Isaac Newtons approach to developing theories in his book Principia Mathematica proceeds in four steps. First, he defines various concepts, second, he formulates axioms utilising the concepts, third, he mathematically analyses the behaviour of the system defined by the concepts and axioms obtaining predictions and fourth, he tests the predictions with measurements. In this study, we formulated our theory of boreal forest ecosystems, called NewtonForest, following the four steps introduced by Newton. The forest ecosystem is a complicated entity and hence we needed altogether 27 concepts to describe the material and energy flows in the metabolism of trees, ground vegetation and microbes in the soil, and to describe the regularities in tree structure. Thirtyfour axioms described the most important features in the behaviour of the forest ecosystem. We utilised numerical simulations in the analysis of the behaviour of the system resulting in clear predictions that could be tested with field data. We collected retrospective time series of diameters and heights for test material from 6 stands in southern Finland and five stands in Estonia. The numerical simulations succeeded to predict the measured diameters and heights, providing clear corroboration with our theory.

Spectral characteristics of pine needles at the limit of tree growth in subarctic Finland

Background: Forests in subarctic regions inhabit the transitional zone between the temperate region and the Arctic, where plant survival depends on the capability of individuals to adjust to changing environments. This transitional zone, generally falling between 50° N and 70° N, is becoming increasingly vulnerable under a changing climate, with treelines moving further north and also higher up slopes. Aims: The current study was carried out at 68° N, the most sensitive zone of the subarctic and northern limit of tree growth in Finnish Lapland. The physiology of trees in these conditions is still not fully understood, and an attempt is made here to contribute to a better understanding of the functionality of leaves by studying the spectral characteristics of needles of Pinus sylvestris along an altitudinal transect. Methods: A variety of spectral indices derived from high-resolution reflectance spectra were measured on freshly excised pine needles. The specific hypothesis is that spectral reflectance data can detect the stress that pine needles suffer at high elevations. We also wish to know which of the several possible indices is most indicative of stress. Results: All of the spectral indices, with the exception of NDVI, showed elevational as well as age-related trends consistent with the hypothesis that needles from the highest elevation suffer degradation of chlorophyll. Conclusions: We conclude that appropriate reflectance measurements provide useful information that may in future be used to detect stress effects on conifer foliage. The visual appearance of trees included in the study did also reflect in their spectral characteristics.

Spatial tree community structure in three stands across a forest succession gradient in northern boreal Fennoscandia

Development of species composition during succession is well studied in natural boreal forests, but empirical assessments of how within-stand spatial structure develops in late-successional stages arefew . �Here, �wequantifiedspatialpatternsinthreeunmanagedstandsconsistingofPicea abies (L.) Karst. and Betula pubescens Ehrh. and Betula pendula Roth (hereafter Betula spp.) in northern boreal Fennoscandia. We conducted a comprehensive analysis of small-scale spatial point patterns in three fully mapped 1.2-ha sample plots, representing different forest developmental stages: mid-successional, late-successional and old-growth forest. We used several variants of Ripleys K-function to analyze the spatial point patterns along the successional gradient. Univariate analyses showed that mature trees of both species were either randomly distributed or clumped. P. abies saplings were clumped, and Betula spp. saplings occurred in a random or clumped manner. In the bivariate analyses, saplings were more likely to be found in the surroundings of mature trees of the same species, but occurred independent of the individuals of other tree species. Mature treesshowedinterspecificrepulsion. �Onlymodestdif ferencesoccurredintheunivariatepatterns � between the three successional stages, but in the bivariate analyses the most evident patterns, i.e.�intra specificattractionandinterspecificrepulsion, �werestrongerintheoldersuccessional � stages.�Overall, �thestudiedstandsappearstructuredasspecies-specificmosaics. �Thesemosaics, � along with mixed species composition, seem to be maintained by species self-replacement, which contrastswithfindingsfromearlierstudies.

Dynamics of Carbon and Nitrogen Fluxes and Pools in Forest Ecosystem

Metabolism of trees, ground vegetation and microbes generate carbon and nitrogen fluxes in forest ecosystems. Carbon flows through the system, and nitrogen circulates between vegetation and soil. Trees synthesise sugars in photosynthesis and take nitrogen from soil. The biochemical regulation system allocates the annual amounts of synthesised sugars and nitrogen taken up to the growth of needles, wood and fine roots. The regularities in tree structure, generated by the action of the biochemical regulation system, determine the allocation to different tree components.

Dendroecological Applications to Coarse Woody Debris Dynamics

Coarse woody debris plays a crucial role in forest ecosystems. The current amount of woody debris on a given site represents a balance between additions (tree mortality) and depletions (wood decomposition, combustion, transport). Understanding woody debris dynamics has recently gained much attention, primarily because of the need to improve forest carbon accounting and modelling. Woody debris itself also holds great potential for use in dendrochronological studies, including those aimed at revealing forest stand dynamics. As such, tree-ring data from woody debris is at times used to make inferences about past stand dynamics; however, at other times tree-ring data from samples in close proximity to woody debris are used to make inferences about the dynamics of woody debris itself. Our case study provides an example of the latter application by addressing woody debris dynamics in three old-growth Picea rubens stands in Maine, USA. Our findings show striking fluctuations in woody debris mass over a 100-year period (1900–2000), with pulses in woody debris inputs corresponding to reconstructed forest disturbances. These fluctuations highlight the need to characterize woody debris dynamics for refining modelling efforts and developing restoration prescriptions in ecosystems with disturbance regimes dominated by gap- and meso-scale disturbances.