Northern Boreal Zone Research Articles

Vole cycles and predation in temperate and boreal zones of Europe

Summary Graham & Lambin (2002) have reported on a weasel‐reduction experiment, concluding that the impact of weasel predation on field vole survival was neither sufficient nor necessary to initiate and drive the cyclic decline of field vole populations in Kielder Forest, northern England. They also stated that their findings contradict conclusively the specialist predator hypothesis put forward to explain population cycles of voles in North Europe. Straightforward inferences from Kielder Forest to the northern boreal zone are misleading, because the population cycles of voles in Kielder Forest differ essentially from North European vole cycles. The low amplitude of the vole cycles in Kielder Forest, their restricted spatial synchrony in comparison to northern Europe and the virtual lack of interspecific synchrony in Kielder Forest suggest that there are essential differences between the mechanisms responsible for the two types of cyclic fluctuations of voles. The weasel‐reduction experiment may provide a misleading picture on the role of predators, even in the Kielder Forest cycle. The experimental reduction of weasels alone may not stop the population decline of voles, because competing larger predators are expected to increase their hunting in the weasel‐reduction areas. The small spatial scale of the experiment, which produced only slight, short‐term differences in weasel densities between reduction and control areas, also suggests that other predators could have compensated easily for the weasels that were removed. We propose a new version of the predation hypothesis to explain low‐amplitude population cycles of voles in temperate Europe, including the Kielder Forest. The interaction between generalist predators and vole populations might account for these cycles because generalists can have a functional response that is destabilizing in the neighbourhood of the equilibrium point. As most generalists are orders of magnitude larger than weasels, and thus need much more food for survival, generalist‐driven cycles should be characterized by high prey minima, as observed in Kielder Forest.

Holocene solifluction, climate variation and fire in a subarctic landscape at Pippokangas, Finnish Lapland, based on radiocarbon‐dated buried charcoal

AbstractA large number of radiocarbon dates from charcoal layers buried beneath stacked solifluction lobes at Pippokangas, in the northern boreal zone of Finnish Lapland, are used to reconstruct a Holocene history of solifluction. Although the site is surrounded by Scots pine forest, the solifluction lobes occur on the lower slopes of a kettle hole, the microclimate of which prevents the growth of trees. Samples from the upslope end of charcoal layers have enabled the recognition of four synchronous phases of solifluction lobe initiation: 7400–6700, 4200–3400, 2600–2100 and 1500–500 cal. yr BP. Rates of lobe advance are shown to be lobe‐dependent and age‐dependent: initially, average rates were commonly 0.14–0.19 cm yr−1, later falling to 0.02–0.07 cm yr−1 or less as the lobes approached the bottom of the slope. The absence of charcoal prior to 8000 cal. yr BP, together with single IRSL and TL dates, indicate a relatively stable early Holocene landscape. The onset of solifluction around 7400 cal. yr BP. appears to have followed the immigration of pine around the site, which increased the frequency of forest fires. Phases of solifluction activity seem to have been triggered by millennial‐scale variations in effective moisture (the climatic hypothesis), rather than episodic burning of the surface vegetation cover (the geoecological hypothesis), although climate may also have affected fire frequency and severity. Copyright © 2005 John Wiley & Sons, Ltd.

Growing season changes in Fennoscandia and Kola peninsula during the period 1982 to 1999 - Implications for reindeer husbandry (In Norwegian with Summary in English)

Growing season changes in Fennoscandia and Kola peninsula during the period 1982 to 1999 - Implications for reindeer husbandry (In Norwegian with Summary in English)

Photoperiodic Regulationof Apical Growth Cessation in Northern Tree Species

Summary Tree species adapted to the climatic conditions of the northern boreal and subarctic vegetation zones have a capacity to develop a very high level of frost hardiness, even to survive the temperature of liquid nitrogen in midwinter. Proper timing of hardening, as well as of dehardening, is crucial for winter survival of these species. In northern tree species, cessation of apical elongation growth and bud set is a prerequisite for developmental and metabolic processes leading to hardening, and this chain of events is induced by photoperiod. The northern tree species are closely adapted to the local light climate and display photoperiodic ecotypes. The critical photoperiod is under genetic control and increases with increasing latitude of origin of the eco-type. The photoperiod is probably perceived by the phytochrome system, but the role of other pigment systems, like cryptochrome, has not been studied in woody plants. Phytochrome genes have been cloned from both conifers and deciduous species, but so far we do not have any information about possible differences between photoperiodic ecotypes at the phytochrome level. Northern and southern ecotypes have different responses to red:far red ratios, which could indicate differences in composition of their phytochrome systems, for example, the proportions of phytochrome A and B. Both phytochrome A and phytochrome B can be involved in photoperiodic responses. Experiments with transgenic hybrid aspen suggest that responses to photoperiod could be affected by the amount of phytochrome A present in plants. In deciduous species, the plant hormone gibberellin A1 (GA1) can completely substitute for a long photoperiod, and short day induced cessation of growth is preceded by a significant reduction of GA1 levels, particularly in the elongation zone. Photoperiodic control of GA metabolism is supported by several studies, but very little is known about the interaction between phytochrome and GA metabolism and/or responsiveness to GA1. Although our knowledge is still very fragmentary, available results suggest that cessation of growth and initiation of hardening in trees can be controlled both through the phytochrome and the GA mediated systems. Research with tree species is a tedious and slow process, but with the emerging new methods and approaches, we may expect exciting new results in the near future.

A modern pollen–climate calibration set from northern Europe: developing and testing a tool for palaeoclimatological reconstructions

AbstractAims and location The potential of pollen records in quantitative climate reconstructions has been widely debated but seldom tested. Our aim is to develop a pollen–climate transfer function for northern Europe and test its performance and inference power by numerical cross‐validation with modern climate data. Annual mean temperature (Tann) was assessed as the critical climatic variable becauseTannhas a distinct south–north gradient (5.5 to −4.7 °C) in the study region with a corresponding zonal vegetation gradient from the hemiboreal zone in the south to the northern boreal zone in the north.Methods We collected 137 pollen surface samples from small‐ to medium size lakes from southern Estonia to northern Finland. The transfer function forTannwas developed with weighted averaging partial least squares (WA‐PLS) regression. All 102 terrestrial pollen and spore types were included in the calculation sum and all 137 surface samples and all 102 taxa were included in the transfer function. The performance of the WA‐PLS transfer function was evaluated by leave‐one‐out cross‐validation.Results A cross‐validated root mean square error of prediction (RMSEP) of our model is 0.89 °C and the coefficient of determination (r2) between the observed meteorologicalTannvalues and those predicted by the model in leave‐one‐out cross‐validation is 0.88. The RMSEP as a percentage of the gradient length ofTannis 8.8%. These figures indicate high performance statistics for our transfer function compared with other inference models. This is probably because of standardization of our surface‐sampling and pollen‐analytical procedures, careful selection of the surface sample sites with consideration of the relevant pollen source area, the simple patterns of vegetation zones and climate in the study area, and the mostly natural floristic composition of the forests in northern Europe. However, we also demonstrate the limitations of our model in reliably detecting fine‐scale climatic variability.Main conclusions The study shows the strong influence ofTannon modern pollen composition and demonstrates the potential of pollen data for long‐term climate reconstructions in northern Europe. It also provides evidence against simple interpretations of fine‐scale variations in a single climate reconstruction. In particular, our results highlight the importance of careful study design and implementation in the construction of pollen–climate transfer functions.

Phellodon secretus (Basidiomycota), a new hydnaceous fungus from northern pine woodlands

Phellodon secretus Niemela' & Kinnunen (Basidiomycota, Thelephorales) resembles Phellodon connatus (Schultz : Fr.) P. Karst., but differs in having a thinner stipe, cottony soft pileus, and smaller and more globose spores. Its ecology is peculiar: it is found in dry, old-growth pine woodlands, growing in sheltered places under strongly decayed trunks or rootstocks of pine trees, where there is a gap of only a few centimeters between soil and wood. Basidiocarps emerge from humus as needle-like, ca. 1 mm thick, black stipes, and the pileus unfolds only after the stipe tip has contacted the overhanging wood. In its ecology and distribution the species resembles Hydnellum gracilipes (P. Karst.) P. Karst. It seems to be extremely rare, found in Northern boreal and Middle boreal vegetation zones, in areas with fairly continental climate.

Methane emissions from natural peatlands in the northern boreal zone in Finland, Fennoscandia

Methane (CH 4) emissions were measured over two snow-free seasons from ten natural minerotrophic peatlands in the northern boreal zone of Finland using a static chamber technique. The daily CH 4 fluxes at the sites ranged from −2.9 to 790 mg m −2 d −1. At nine wet fens, representing three ecological peatland classes, the seasonal mean CH 4 emissions were 8.1–250 mg m −2 d −1 during a dry summer, and 15–330 mg m −2 d −1 during a wet summer. At these sites, the mean CH 4 emission correlated positively with the mean water table level ( r 2=0.64). The driest site (herb-grass spruce mire) showed the lowest CH 4 emissions, with the seasonal averages being below 1 mg m −2 d −1. The CH 4 emissions from these natural northern boreal peatlands in the main aapa-mire zone were similar or higher than those reported from other boreal peatlands in Finland and elsewhere, probably attributable to the high fertility and moisture of the northern boreal sites, characteristics typical of peatlands in the aapa-mire zone. Thus, the aapa mires in the boreal zone in Fennoscandia, Canada and Russia should be a larger natural source for atmospheric CH 4 than estimated in the global inventories for wetland CH 4 emissions.

The effect of a north-facing forest edge on tree water use in a boreal Scots pine stand

Tree water use and growth increment were studied in a north-facing forest edge of a 70-year-old monospecific Scots pine (Pinus sylvestris L.) stand growing on poor sandy soils in the northern boreal zone of Sweden. The forest edge in this study bordered a 20-year-old clearcut. There were differences in water use and growth increment during the growing season between trees growing at the forest edge and trees growing in the forest interior. These differences were likely related to soil conditions, such as access to soil moisture, soil temperature, and soil frost conditions, whereas an effect of aboveground microclimate was not found. The estimated tree water use and growth increment over one growing season tended to be greater for trees at the edge zone relative to those from the interior. The variability of the measured tree water fluxes was high, especially for the edge-zone trees. There were also structural differences between the two groups of trees, most notably in the radial profile of conductive xylem, in tree height, and in green crown length, but these differences were on the limits of statistical significance. The estimated seasonal transpiration was low, about 70 mm when estimated exclusively for trees in the forest interior and 107 mm when estimated exclusively for trees at the forest edge. This illustrates the likely magnitude of water use enhancement resulting from the conditions specific to the forest edge.

Geochemistry and mineralogy of saprolite in Finnish Lapland

An ancient saprolite has developed on the Palaeoproterozoic granulite, granite gneiss and amphibolite bedrock of the Vuotso–Tankavaara area of central Finnish Lapland. The present day climatic regime in Finnish Lapland lies within the northern boreal zone and so the saprolite there can be regarded as fossil. Cores of saprolite were collected from 4 sections (42 samples) and analyzed chemically and mineralogically. In the study area, progressive weathering of the rocks has been marked by gradual enrichment in Al, Fe and Ti; and depletion of Na, K and Ca. The higher concentration of Fe(III) and water and reduced Na and Ca in weathered bedrock in the 4 sections are indicative of oxidation, hydration and leaching processes involved during weathering. The primary minerals in the saprolite are plagioclase feldspar, K-feldspar, quartz, garnet (almandine) and hornblende; the common secondary minerals are kaolinite, halloysite, and vermiculite in addition to minor amounts of sericite. Intense weathering is indicated by: (1) the presence of kaolinite and halloysite in 4 sections of different bedrock types, and (2) the comparatively lower SiO 2/Al 2O 3 (wt.%) ratio (2.30) of weathered granulites (3 sections) as compared to fresh granulite (4.33) and that of weathered amphibolite (2.68) as compared to fresh amphibolite (3.56). In general, kaolinite and halloysite have formed through the weathering of feldspars, garnet, and biotite. Vermiculite is the most probable alteration product of biotite. The formation of kaolinite and halloysite in Finnish Lapland indicates wetter and warmer climatic conditions during the time of their formation than at present. The possible time for formation of the saprolite is early Cretaceous–early Tertiary into Middle Miocene.

Fluxes of CH4, CO2, and N2O in hydroelectric reservoirs Lokka and Porttipahta in the northern boreal zone in Finland

Concentrations and fluxes of greenhouse gases methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) were measured during open water conditions in two hydroelectric reservoirs, Lokka and Porttipahta, in the northern boreal zone in Finland. These reservoirs were located on peat and forest soils and were built in 1967 and 1970, respectively. Over 20 years after their flooding, the reservoirs were still largely supersaturated with dissolved CH4and CO2. Measured with floating static chambers, the stations in Lokka released more CH4(means of 5.3–119 mg m−2d−1) during the open water periods in 1994 and 1995 than the stations in Porttipahta (means of 2.5–4.8 mg m−2d−1), measured in 1995. The mean CO2emission rates in Lokka (21–133 mg m−2h−1) and Porttipahta (36–95 mg CO2m−2h−1) were more similar. The chamber measurements could not detect episodic CH4ebullition in Lokka Reservoir, indicated by generally higher CH4fluxes (means of 2.6–660 mg m−2d−1) obtained with bubble gas collectors than with the chambers. The seasonal mean N2O fluxes were generally low, ranging from −89 to 270 μg m−2d−1. There was no clear association between the CH4emissions and the bottom type, including mineral soils and old peat deposits. The flooded vegetation, higher nutrient content, and primary production in the water column rather than old peat could account for the higher CH4emissions from the stations in Lokka. This conclusion is supported by the high content of modern carbon (C) in methane (percent modern C of 92–104%) that was extracted from the sediment of Lokka. The results suggested that if there is a good long‐term availability of phosphorus and nitrogen, the intensive internal C cycle associated with the primary production could maintain high CH4and CO2production for decades, similar to the situation in eutrophied natural lakes.

Exchange of CO2, CH4 and N2O between the atmosphere and two northern boreal ponds with catchments dominated by peatlands or forests

Concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the water column and their exchange at the water/air interface were studied during the open water period in two freshwater ponds with different catchment characteristics in the northern boreal zone in Finland; either peatlands or coniferous upland forests dominated the catchment of the ponds. Both ponds were supersaturated with dissolved CO2 and CH4 with respect to the equilibrium with the atmosphere, but were close to the equilibrium with N2O. The mean CO2 efflux from the pond was higher in the peatland-dominated catchment (22 mg m−2 h−1) than in the forested catchment (0.7 mg m−2 h−1), whereas the mean CH4 emissions were similar (7.6 and 3.5 mg m−2 d−1, respectively). The fluxes of N2O were generally negligible. The higher CO2 concentrations and efflux in the pond with the peatland-dominated catchment were attributed to a greater input of allochthonous carbon to that pond from its catchment due to its higher water colour and higher total organic carbon (TOC) concentration. The water pH, which also differed between the ponds, could additionally affect the CO2 dynamics. Since the catchment characteristics can regulate aquatic carbon cycles, catchment-scale studies are needed to attain a deeper understanding of the aquatic greenhouse gas dynamics.

Scots pine needle longevity and gradation of needle shedding along pollution gradients

Scots pine (Pinus sylvestris L.) needle longevity and gradation of needle shedding (the time span with 90–10% needle survival) in the middle and northern boreal vegetation zones were studied. The duration of the first half of gradual shedding (the time span with 90–50% needle survival) was of special interest, because it represents the number of youngest cohorts that have lost needle biomass. Branches of adult trees along transects with an air pollution gradient were sampled. Mean longevity (the sum of survival proportions) decreased 15–40% as pollution sources were approached. To rank or compare the values of a single needle age parameter, we prefer to use mean longevity. It gives an estimate of the whole needle biomass, and it was the parameter with the least variation. The first half of gradual shedding proved to be a valid indicator of changes in needle age structure. The most marked variation in needle age structure was seen in slightly polluted areas, which continued to pose a risk of acute pollution injuries. This study confirmed the feasibility of the needle age structure of branch samples as an objective and reliable vitality indicator for Scots pine.

A lysimeter system for monitoring soil solution chemistry

In northern temperate and boreal zones, long-term sampling of the soil solution using lysimeter systems requires (i) a lysimeter that can be installed with a minimum of soil disturbance, (ii) a lysimeter that is small enough to install in stony soil but large enough to representatively sample the soil solution, (iii) a system with a constant tension that integrates soil solution sample collection over time, and (iv) a system that will not break under freezing conditions. A lysimeter system that meets these requirements was designed, and consists of a lysimeter (a fritted glass plate mounted on a glass casing) connected in series to a sample bottle and a tension-generating unit. The glass casing of the lysimeter has two ports: a drainage port for continuous flow of soil solution to the sample bottle, and an access port that can be used (i) for air entry to facilitate complete sample retrieval, (ii) for cleaning, (iii) for re-wetting the fritted glass plate, or (iv) for winterizing the lysimeter with alcohol. A 1-m hanging water column was used to supply a constant tension of approximately 10 kPa. A potential 93% efficiency rate of sample collection can be achieved, and the systems are capable of sampling soil solution beneath organic horizons, as well as in the rooting zone of mineral horizons. The system is presently being used to evaluate the effects of different harvesting and silvicultural treatments on nutrient cycling processes in the soil. Almost 700 of these lysimeter systems are in use in the Atlantic provinces of Canada. The lysimeters have been functioning satisfactorily for over 10 yr and are suitable for long-term monitoring of the soil solution. Key words: Lysimeter design, soil monitoring methodology, nutrient cycling

Anthropogenic extinction of top carnivores and interspecific animal behaviour: implications of the rapid decoupling of a web involving wolves, bears, moose and ravens.

The recent extinction of grizzly bears (Ursus arctos) and wolves (Canis lupus) by humans from 95-99% of the contiguous USA and Mexico in less than 100 years has resulted in dramatically altered and expanded prey communities. Such rampant ecological change and putative ecological instability has not occurred in North American northern boreal zones. This geographical variation in the loss of large carnivores as a consequence of anthropogenic disturbance offers opportunities for examining the potential consequences of extinction on subtle but important ecological patterns involving behaviour and interspecific ecological interactions. In Alaska, where scavengers and large carnivores are associated with carcasses, field experiments involving sound playback simulations have demonstrated that at least one prey species, moose (Alces alces), is sensitive to the vocalizations of ravens (Corvus corax) and may rely on their cues to avoid predation. However, a similar relationship is absent on a predator-free island in Alaska's Cook Inlet and at two sites in the Jackson Hole region of the Rocky Mountains (USA) where grizzly bears and wolves have been extinct for 50-70 years. While prior study of birds and mammals has demonstrated that prey may retain predator recognition capabilities for thousands of years even after predation as a selective force has been relaxed, the results presented here establish that a desensitization in interspecific responsiveness can also occur in less than ten generations. These results affirm (i) a rapid decoupling in behaviour involving prey and scavengers as a consequence of anthropogenic-caused predator-prey disequilibriums, and (ii) subtle, community-level modifications in terrestrial ecosystems where large carnivores no longer exist. If knowledge about ecological and behavioural processes in extant systems is to be enhanced, the potential effects of recently extinct carnivores must be incorporated into current programmes.

Seasonal and interannual influences of the terrestrial ecosystems on atmospheric CO 2: a model study

The prognostic CARAIB (Carbon Assimilation In the Biosphere) model has been used in conjunction with the Max-Planck Institut TM2 atmospheric transport model to calculate the atmospheric CO 2 fluctuations at the global scale. Two applications are briefly described. In the first one, the seasonal CO 2 variation is calculated and a Fourier analysis is performed to determine the relative contributions of the various vegetation types. It is found that the seasonal signal is dominated by the grasslands and needle leaf forests in the northern boreal and temperate zones. In the southern hemisphere, tropical deciduous forests and grasslands make the primary contribution. In the second application, the net primary productivity (NPP), soil heterotrophic respiration (SHR) and net ecosystem productivity (NEP) are calculated for years 1987 and 1988 with the model driven by observed climatic variables. Preliminary results indicate that the NEP variations between these two years are strongly dominated by tropical ecosystems. However, it is shown that the results are strongly dependent on the dataset used for the 1987-88 temperature record, raising the question of reliability of such modelling studies of the interannual variability of the biosphere.

Accumulation rates of carbon in mires in Finland and implications for climate change

Stratigraphical analyses based on 1028 dated Holocene peat cores were used to estimate the rate of carbon accumulation in Finnish peatlands. Results were compared with data from other Boreal areas. The basal age of peat columns was found to be the best predictor of carbon accumulation and a significant correlation between depth and age of peat was evident. When normalized for the mean depth of peatlands in Finland ( c. 1.5 m), the average long-term carbon accumulation was 26.1 g C m-2 yr-1. In individual cores the values ranged from 2.8 to 88.6 g (average 22.5 ± 11.6 g) being much higher in bogs than in fens and almost double in southern mires as compared with those in the northern Boreal zone. The modelled rate of actual carbon accumulation is about 2/3 of that above. Boreal mires are actual sinks of carbon for thousands of years ahead in the present climate. The predicted greenhouse warming may shift the present Sphagnum mires northward and the net effect may be to restrain the radiative forcing. Poorly known 3D mass accumulation in mires and unpredictable functioning of northern mire ecosystems to climatic changes (for example via increased mire fires) complicate this interpretation.

Stand dynamics, regeneration patterns and long‐term continuity in boreal old‐growth Picea abies swamp‐forests

Abstract. The occurrence of macrofossil charcoal (long axis > 0.5 mm) and Picea abies (Norway spruce) pollen in peat stratigraphies, in combination with size and age data from 2976 P. abies trees were used to analyse ecosystem continuity and stand‐structure in ten old‐growth swamp‐forests in northern Sweden. All stands were dominated by P. abies, a species whose abundance increased westwards in Sweden between 3000 and 2000 yr B.P. In three stands no macrofossil charcoal was found and the maximum age of the peat, determined by 14C dating, varied from 1800 to 3600 yr B.P. In the other seven stands the number of levels containing charcoal varied from 1 to 23, but only between 1 and 7 levels were found after the appearance of spruce. Here the maximum age of the peat varied from 400 to 7900 yr B.P. The ten stands had an all‐sized stand structure and a stand continuity of ca. 300 yr. The shape of the age structure was similar to an inverse J‐curve. This indicates a continuous recruitment over time in a self‐perpetuating ecosystem. In a short‐term perspective (< 300 yr), the swamp‐forests are characterized by individual trees continually emerging while others are dying. it is suggested that internal dynamics of continuous small‐scale disturbances in combination with local site‐specific factors determine the structure of these forests. in a long‐term perspective, some of the present spruce swamp‐forests within the northern boreal zone have functioned as true fire‐free refugia since the establishment of P. abies populations while others have been affected by recurring fires, although not as frequently as forests on surrounding drier sites. The hypothesis that Scandinavian spruce swamp‐forests in general have functioned as true longterm fire‐free refugia is thus modified by the present results.

Remote sensing, a tool for reindeer range land management

AbstractThe Finnish reindeer-herding area lies in the northern boreal forest zone. The herding area is divided into 57 reindeer management districts with a total area of 114,355 km2 (excluding water areas). The official number of reindeer in the year 1992 was 263,789, or 2.3 reindeer per square kilometre. In 1946 the density was only about one reindeer per square kilometre. The increased pressure on the mid-winter lichen pastures has reduced the volume of lichen. Also the decreased area of old coniferous forests with arboreal lichens has diminished the supply of food during the late-winter months. The lack of suitable winter pastures has led to the need for supplementary feeding with hay.The intention of this study was to develop and test methods for cost-effective reindeer range land inventory and to assess the present state of pastures in seven districts. Use was made of Landsat 5 TM satellite imagery that was processed with remote sensing and GIS software. The analysis of well-known field sites proved the image classification to be between 80 and 90% accurate. By comparing field data and classification results, estimates can be made on the available fodder reserves in the different pasture types. The results show that mid-winter pastures are both scarce and degraded in six of the seven areas investigated. Only one area (Muotkatunturi) has mid-winter pastures in reasonably good condition. Late-winter pastures with arboreal lichens are more widespread than mid-winter pastures and determine the amount of additional winter feeding required. Summer pastures are abundant in all seven reindeer-herding districts and are therefore not a limiting factor. The method used is objective, inexpensive, and fast when compared to conventional methods. Information can be produced on the quantity and quality of reindeer pastures. This knowledge can be used to make better estimates of the natural grazing capacity of reindeer districts in Finland.

Simulation of tree species composition and organic matter accumulation in Finnish boreal forests under changing climatic conditions

A model simulating the regeneration, growth and death of trees and the consequent carbon and nitrogen dynamics of the forest ecosystem was applied to determine the effect of expected temperature rise on tree species composition and the accumulation of organic matter in the boreal forest ecosystem in Finland (between latitudes 60°–70° N). In the southern and middle boreal zones a temperature rise of 2–3° C (temperature for 2 x CO2) over a period of one hundred years increased the competitive capacity of Scots pine (Pinus sylvestris) and birch species (Betula pendula and B. pubescens), and slowed down the invasion by Norway spruce (Picea abies). In the northern boreal zone a corresponding rise in temperature promoted the invasion of sites by Norway spruce. The accumulation of organic matter was promoted only slightly compared to that taking place in the current climatic conditions. A further doubling of temperature (temperature for 4 x CO2) over an additional period of two hundred years led to the replacement of coniferous stands with deciduous onesin the southern and middle boreal zones. In the northern boreal zone an admixture of coniferous and deciduous species replaced pure coniferous stands with the latter taking over sites formerly classified as tundra woodland. In the southern and middle boreal zones the replacement of coniferous species induced a substantial decrease in the amount of organic matter; this returned to its former level following the establishment of deciduous species. In the northern boreal zone there was no major change in the amount of organic matter such as occurred in the case of the tundra woodland where the amount of organic matter accumulated was nearly as high as in the northern boreal zone.

Long-term development of needle mass, radiation interception and stemwood production in naturally regenerated Pinus sylvestris stands on Empetrum-Vaccinium site type in the northern boreal zone in Finland: an analysis based on an empirical study and simulation

The development of stand needle mass, interception of photosynthetically active radiation (PAR) and production of stemwood were examined in Scots pine ( Pinus sylvestris L.) stands growing on Empetrum-Vaccinium site type in the northern boreal zone in Finland. In the analysis, Ilvessalo's classical study on the growth and development of Scots pine stands was completed by: (1) a needle biomass equation to facilitate the analysis of the structural development and stemwood production of the stands in relation to needle biomass, and (2) a process model of radiation interception to simulate the pattern of PAR interception by the stands with age. The analysis covered a period of 150 years, from stand age 50 to 150 years. According to the simulations, the stand needle biomass culminated around age 90 years and decreased thereafter. The stage of maximum stemwood production was reached at the age of 50 years, when the stand needle mass had increased to approximately two-thirds of the maximum. The amount of intercepted PAR changed in parallel with stand needle mass, but the amount of PAR intercepted per unit weight of foliage was reciprocally related to stand needle mass. With the increase in stand needle mass, and the concomitant increase in the rate of self-thinning, the needle efficiency in stemwood production decreased. After the culmination of needle mass the stemwood production efficiency of needles settled to a nearly stable level, in spite of the later decline in total needle mass and the correspondence increase in needle efficiency in PAR interception. The estimated proportion of intercepted PAR stored to stemwood decreased throughout the period of analysis. Possible mechanisms underlying the observed patterns of stand development are discussed with reference to the literature.