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European Journal of Applied Sciences – Vol. 10, No. 1
Publication Date: February 25, 2022
DOI:10.14738/aivp.101.11645.
Kullman, L. (2022). Praealpine Spruce (Picea Abies) Forest Dynamics During the Current Post-Little Ice Age Climate Era: A Case in
the Swedish Scandes. European Journal of Applied Sciences, 10(1). 246-259.
Services for Science and Education – United Kingdom
Praealpine Spruce (Picea abies) Forest Dynamics During the
Current Post-Little Ice Age Climate Era: A Case in the Swedish
Scandes
Leif Kullman
Department of Ecology and Environmental Science
Umeå University, SE-901 87 Umeå, Sweden
ABSTRACT
This study focuses on structural change of a praealpine stand of Norway spruce
(Picea abies), in a context of centennial climate warming, in the Swedish Scandes.
Progressive stand building followed on the end of the Little Ice Age by the late 19th
century. A distinct hiatus of this process occurred during the 1980s, as a
consequence of exceptionally cold winters with a sparse snow cover, particularly
1886/87. Extensive defoliation, caused by frost desiccation and fine root mortality,
followed and culminated 1994 and 1995. Subsequently, climate warming resumed
and stand foliation increased steadily up to the present day, when the forest
appears equally healthy as prior to the 1980s. Hereby, recent spruce stand
evolution adds to current progressive climate-mediated restructuring of upper
montane forests.
Key words. Praealpine forest, Picea abies, canopy structure, foliation dynamics, climate
variability, Swedish Scandes
INTRODUCTION
The current concern about climate change in the”Anthropocene” has highlighted the
importance of the time dimension in ecological research (Wolkovich et al. 2014). Time-series
of cold-marginal forest dynamics provide pertinent study objects in this context.
Worldwide, post-Little Ice Age climate warming has changed the face of the living and physical
world (Lamb 1982; Grove 1988). In that context, cold-marginal trees and tree stands are
generally renowned as efficient sentinels of past and present climate change and variability
(Tranquillini 1979; Kullman 1979, 1998; Holtmeier 2009; Körner 2007; Smith et al. 2009). This
circumstance motivates their continuous survey and analyses, as a basis of proper landscape
management and an independent cheque of meteorological records.
The altitudinal tree- and forest zonation in the Swedish Scandes is currently characterized by
an upper broadleaved subalpine birch forest belt (Betula pubescens ssp. czerepanovii). This belt
grades upslope to the alpine tundra and downslope into more or less closed coniferous stands
of Norway spruce (Picea abies), mixed with birch and scattered pine (Pinus sylvestris).
According to prevailing phytogeographical terminology, these forests with a high admixture of
birch, belong to the “northern boreal forest zone” or “mountain taiga” (Ahti et al. 1968; Kullman
2005), more specifically termed as the praealpine belt (Wistrand 1962). The ecology and long- term successional trajectories of these high-elevation forest have been analyzed and discussed
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Kullman, L. (2022). Praealpine Spruce (Picea Abies) Forest Dynamics During the Current Post-Little Ice Age Climate Era: A Case in the Swedish
Scandes. European Journal of Applied Sciences, 10(1). 246-259.
URL: http://dx.doi.org/10.14738/aivp.101.11645
by some authors (Sirén 1955; Hofgaard et al. 1991; Odland et al. 1992). Hofgaard et al. (1991)
evidenced that old-growth and moribund spruce stands, often predicted for autogenic gradual
senescence and virtual extirpation, have revitalized between the late 1930s and late 1980s,
apparently in response to post-Little Ice Age climate warming (Hofgaard et al. 1991; Kullman
& Öberg 2009). However, at more explicit treeline sites, modest crown thinning as well as stand
decline were well underway by the early 1970s in the southern Scandes, possibly in response
to transient climate cooling following the warm 1930s (Kullman 1988).
The late-1980s mark intensification of regressive stand-level processes, also below the treeline
ecotone. Primarily, this was a consequence of a decade with substantial early-winter cooling, in
combination with a sparse snow cover and even the spread of sporadic and occasional
permafrost, causing frost desiccation and fine root mortality (Jalkanen 1985; Eriksson 1987,
1988; Lindgren et al. 1989; Ritari 1990; Kullman 1989b). All over northern Fennoscandia, cold- marginal tree stands suffered heavy defoliation, canopy thinning and even minor treeline
retraction during these years (Raitio & Tikkanen 1988; Kullman & Högberg 1989; Kullman
1989a, 1991 a,b, 1996 a,b; Josefsson 1990; Tuovinen et al. 2005).
By the early 1990s and onwards, climate resumed its prior, centennial warming trend, which
halted prior modest regressive ecological processes, which had prevailed for some decades
after the 1930s. This most recent climate shift manifested firstly as resumed treeline advance
and associated population growth in concert with increased alpine plant species richness
(Kullman 2007 a, b; Odland et al. 2010; Michelsen et al. 2011; Felde et al. 2012). Moreover,
scattered spread and elevational upshifts of thermophilic tree species was recorded in the
subalpine and alpine belts (Kullman 2008, 2010, 2020), which heralds improved climate
conditions. Accordingly, recent resurveys of older vegetation records in the coniferous treeline
ecotone point to an ongoing structural reorganization and adjustment to a novel climatic
situation over the past few decades (Kullman 2019, 2021b).
In this context, little data exist concerning the most recent evolution of the upper closed,
praealpine conifer forest stands in the Scandes, right below the treeline ecotone. These forests
frequently suffered heavy canopy dieback during the cold phase of the 1980s (references
above), when temperatures transiently reverted to near-Little Ice Age conditions, i.e. the
coldest period of the entire Holocene (Grove 1988; Leonelli et al. 2010; Ljungqvist 2017).
The main objective of the present study is to display and analyze structural change of a typical
praealpine spruce forest stand (Picea abies) since the late 1980s and up to the present day. This
specifically focused time interval is characterized by climate warming, subsequent to a
relatively cold decade with extensive canopy decline locally over much of northern
Fennoscandia.
Hopefully, the results may serve as analogues in the context of trials to project future high- elevation landscape ecological evolution in future altered climates.
STUDY AREA
The concerned study site is close to the mouth of Handölan valley in the Swedish Scandes,
63°15 ́N; 12°26 ́E, 625 m a.s.l. (Fig. 1). The concerned spruce stand is the last outlier of closed
forest towards the south and higher elevations in the Handölan Valley (County of Jämtland).
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The tree cover is a mixture of spruce and mountain birch, 60 and 50 %, respectively (Fig. 2). A
few old-growth pines are interspersed in the stand together with scattered trees of Sorbus
aucuparia and Populus tremula. The average height of dominant polycormic spruces is 10-11
m, while the birches are 2-3 m shorter. The spruces occur mainly in clusters, as layering clonal
individuals, although monocormic and relatively young individuals occur at a lower frequency.
The nearest treelines for birch, spruce and pine are at 925, 860 and 800 m a.s.l., respectively,
high above the stand focused by this study.
The forest stand is characterized by distinct spruce tree clusters with open treeless glades,
maintained for long by abundant snow accumulation (Fig. 2). In these setting, scattered
mountain birches and bush-forming junipers (Juniperus communis) prevail.
Figure 1. Location map, showing the study area (X) at the mouth of the Handölan Valley in the
southern Swedish Scandes. The hatched area denotes the cover of mountain birch forest,
fringing treeless alpine tundra. The dashed line indicates the distribution limit of closed
conifer stands
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Kullman, L. (2022). Praealpine Spruce (Picea Abies) Forest Dynamics During the Current Post-Little Ice Age Climate Era: A Case in the Swedish
Scandes. European Journal of Applied Sciences, 10(1). 246-259.
URL: http://dx.doi.org/10.14738/aivp.101.11645
Figure 2. Late autumn aerial view of the core of the studied stand, displaying almost equal
proportions of spruce and mountain birch. Overall, spruces are currently in a healthy
condition, with fully foliated treetops. This appearance contrast with the situation by 1987-
1995, when the majority of extant spruces displayed more or less defoliated canopies. 2021-11-
01
The field-layer vegetation has the character of dwarf-shrub heath with Vaccinium vitis-idaea
(dominant), Vaccinium myrtillus, Empetrum hermaphroditum, Phyllodoce caerulea, Lycopodium
alpinum, Avenella flexuosa, Linnaea borealis and Cornus suecica. Predominant in the bottom
layer are Hylocomium splendens, Pleurozium schreberi, Dicranum spp., Barbilophozia
lycopodioides and Ptilium crista-castrensis. A more detailed description of the plant cover is
provided by Kullman (1996).
There are virtually no signs of major anthropogenic disturbance to this forest stand, although
fire and small-scale selective logging (pine in particular) is part of its history in the past
millennium. The stand history was extensively studied by radiocarbon-dated wood remnants,
preserved on the ground underneath the moss cover, in combination with static age structure
analysis (Kullman 1996).
Following at least six centuries of cold-climate conditions and high-elevation forest decline,
spruce dominance developed gradually after the 1850s, in line with post-Little Ice Age
warming. Prior to that, pine and birch seem to have dominated the scene for at least 1000 years.
The extant spruce-birch stand evolved following major disturbance by fire, and possibly a
devastating windstorm in 1837 (Kullman 1996). The latter cataclysmic episode affected high- elevation forests over large regions of northern Scandinavia (e.g. Hagemann 1905).
A prior spruce age structure analysis shows that initiation of new stems within the clones,
distinctly peaked in the warm 1930s, followed by gradual decline of stem acquisitions up to the
present day (Kullman 1996). The restocking process was ultimately facilitated by rapid growth
of new stems, belonging to old-established clones. This course of change has manifested the
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structure displayed at the present day and which constitutes the main object of the present
study.
Standard-level temperature data (1991-2020), relevant to this study, refer to
Storlien/Visjövalen meteorological station, 642 m a.s.l, 13 km NW. The temperatures for
January, July and the year are -5.5, 12.3 and 2.0 °C, respectively. Annual precipitation amounts
to c. 1000 mm/year.
METHODS
Based on extensive reconnaissance surveys, a representative sample plot (85 x 80 m) was
selected for long-term study, with particular focus on the vigor of extant spruces, specifically
crown condition. This endeavor was initiated in late 1985. Annually in September or October,
assessments were visually conducted as percentage needle loss, to the nearest 5%, relative to
a fully foliated tree. The total sample comprised 184 overstorey trees, chosen at random and
tagged for repeated monitoring. Multi-stemmed individuals were treated and judged
collectively as one entity. Conventionally, the estimates refer to the upper half of the crown (cf.
Westman & Lesinski 1986). The character of canopy dynamics was evidenced by repeat
photography of a subsample of representative trees. Some of which are presented here.
Soil temperatures were measured in an open space 3 m outside the canopy of a nearby spruce,
where relatively large amounts of snow use to accumulate. These measurements were carried
out once a week, 1986 to the present day, by a resistance thermistor (TO-03R), manufactured
by T. Johnsson Inc. The sensor was installed in the upper mineral soil at a depth of 20 cm and
annually calibrated. Because of the thermal inertia of the soil, the measured temperatures quite
truly represent thermal soil conditions and their annual variations (Deming 1995; Harris 2001;
Körner 2007).
Seed viability was tested annually on a composite sample of cones originating from all surveyed
spruces. Ten cones were sampled, representing all aspects of the crown. The germination tests
were carried out on moist filter paper in Petri dishes (5x100 seeds). For further details, see
Kullman (1984).
RESULTS
The present study takes place in a regional context of oscillating secular summer- and winter
climate warming, with a temporary and distinct dip in the 1980s (Fig 3). During that period,
local soil summer and winter temperatures were particularly low (Fig. 4 & 5), at the same as a
relatively sparse snow cover prevailed (Fig. 6). Subsequently, soil temperatures and snow
cover have increased substantially up to the present day. In late-1987, prevailing spruces
displayed a high degree of needle browning, with indications of pending canopy decline. During
the following years, culminating in 1994 and 1995, needles were extensively shed, with
consequent landscape-scale crown thinning. This process was followed by consistent recovery
up to the present day. Currently, foliation of the entire stand is even larger than in 1987 (Fig.
7). Overall, stand-level foliation (gain and loss) has changed in congruence with soil
temperature and snow cover evolution since 1985.
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Kullman, L. (2022). Praealpine Spruce (Picea Abies) Forest Dynamics During the Current Post-Little Ice Age Climate Era: A Case in the Swedish
Scandes. European Journal of Applied Sciences, 10(1). 246-259.
URL: http://dx.doi.org/10.14738/aivp.101.11645
The character of individual crown foliation dynamics in the study site is depicted in Figure 8-
10. An analogous course of change is perceivable also for praealpine stands in a larger
landscape-scale perspective (Fig. 11).
Over the entire surveying period, the germinability of spruce seed has increased steadily (Fig.
11), and a certain degree infill of stand gaps has occurred (Fig. 12). However, the spruce forest
has not expanded into the surrounding mire/subalpine birch forest mosaic during the past 100
years.
Figure 3. Mean annual regional air temperatures recorded at Storlien/Visjövalen
meteorological station (1901-2021). Upper. June-August. Lower. December-February
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Figure 4. Mean annual soil temperatures (June-August), 1985-2021
Figure 5. Mean annual soil temperatures (December-February), 1985-2021
Figure 6. Maximum snow depth, 1986-2021, in the stand gap close to the soil temperature
measuring spot. Annual records and trend
Figure 7. Annual estimates of cumulative needle loss, 1986-2021
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Kullman, L. (2022). Praealpine Spruce (Picea Abies) Forest Dynamics During the Current Post-Little Ice Age Climate Era: A Case in the Swedish
Scandes. European Journal of Applied Sciences, 10(1). 246-259.
URL: http://dx.doi.org/10.14738/aivp.101.11645
Figure 8. The same clonal group of spruces, captured at three occasions, 1987-2022. Typically,
needle loss was perceptible in 1987, but had culminated by 1994-1995. Thereafter, recovery is
strikting, although one stem is downed. A. 1987-10-15. B. 1994-06-24. C. 2022-01-01
Figure 9. Clonal spruce group with substantial needle loss in the autumn of 1987. Recovery up
to the present day is blatant. A.1987-10-15. B. 2021-10-25
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Figure 10. Typically, clonal spruces evolved severe defoliation between 1987 and 1994.
Subsequently and up to the present day, stunning recovery has taken place. The second stem
from the left (1994) is downed. A. 1994-04-15. B. 2021-09-04
Figure 11. The course of changes evidenced at the local stand-scale, represents a generic
regional pattern. Accordingly, this landscape view depicts an entire mountain slope in the
study region, showing canopy progression between a vigor nadir in 1987 and subsequent
regain of foliation. A. 1987- 08-14. B. 2021-08-12
Figure 11. Annual records of percentage spruce seed germinability, 1985-2021
Figure 12. During the past 30 years, with increasing seed viability, a tendency of gap-infilling
by young seed-regenerated spruces is evident. Most birches are old moribund, indicative of a
trend towards a more closed spruce stand structure. 2021-09-25
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Kullman, L. (2022). Praealpine Spruce (Picea Abies) Forest Dynamics During the Current Post-Little Ice Age Climate Era: A Case in the Swedish
Scandes. European Journal of Applied Sciences, 10(1). 246-259.
URL: http://dx.doi.org/10.14738/aivp.101.11645
DISCUSSION
The present study supports the contention (Kullman 1998) that praealpine spruce forests are
finely attuned (progressively and regressively) to climate change and variability at centennial
and decadal scales. Overall, the current state of condition, after a century of climate warming,
with short-term interruptions, is substantially improved. The landscape has gained a more lush
and healthy appearance than a century ago. That process is likely to be continued, given the
uncertain assumption that climate warming continues in the future.
By the late 1980s, the present stand and others alike in northern Sweden had declined in vigor,
as a proximate consequence of a decade early winter cooling, associated with a sparse snow
cover (Kullman 1996). Since 1994-1995, the regressive stand-level process has reverted, and
canopy conditions are back to their relatively healthy appearance prevailing prior to 1986. That
state related to 20th century climate warming since the late-1930s (Kullman 1986 a,b), as
evidenced also from analogous forests further north in Sweden (Hofgaard et al. 1991).
The displayed canopy progression by the mid-1990s, coincides with rising regional summer
and winter air and soil temperatures. The crucial importance of soil temperatures for sustained
growth in cold-stressed environments has been stressed previously (Körner & Paulssen 2004;
Leonelli et al. 2010; Kullman 2021a). The present results and their causalities should be
interpreted in that context.
Spruce has regenerated mainly by vegetative layering and individual height growth after the
Little Ice Age, during the past 100-150 years, with intermittent reversals (Kullman 1986a),
thereby stressing the climatic marginality of the concerned stand. Seed regeneration appears
as a recent phenomenon, related to substantially increased seed viability, which exceeded 50
% in recent years, to be compared with about 20 % by 1985. In general seed regeneration was
scant in high-elevation forests prior to the latter date, particularly during the warm 1930s
to1950s (Kullman 1986a, b). Hofgaard (1993), reported germinability figures of <1 % from
biogeographically similar forests in northern Sweden (1984-1992).
The present results highlight a novel functional aspect of the mountain taiga, stressing still
ongoing and possibly future transformation of high-altitude ecosystems in the post-Little Ice
Age era (Willis & MacDonald 2011; Macias-Fauria et al, 2011; Kullman 2019; Schickhoff et al.
2022). In the present case, this implies closure of the spruce stands, a process which eventually
may be enhanced by the accumulation of coarse woody debris, as older stems die and
accumulate on the ground. The importance of the latter aspect has been stressed by several
authors (Engelmark 1993; Hofgaard 1993a,b). Moreover, results from a nearby treeline
ecotone have evidenced recent canopy recovery, analogous the present study and
contemporary increase of seed viability (Kullman 2021b). Lack of seed regeneration may relate
to sparsity of coarse woody debris, provided by relatively young and tiny trees in sparse stands.
In this study, the spruce has demonstrated a remarkable ability recover by phenotypic plasticity
from severe physical stress, consistent with its extreme (multi-millennial) individual longevity
(Öberg & Kullman 2011).
In line with some prior case studies (Rössler et al. 2008; Hofgaard et al. 2013, the current results
do not support contentions of future forest area expansion into the alpine tundra, as envisioned
by certain authors (Moen et al. 2004; ACIA 2005; Kaplan & New 2006). This view of landscape-
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scale forest area resilience is supported also by case studies from subarctic Sweden (Kullman
1991; Kullman & Öberg 2017).
ACNOWLEDGEMENT
Dr. Lisa Öberg is thanked for competent and constructive comments on the manuscript and
skilful photo editing.
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