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European Journal of Applied Sciences – Vol. 10, No. 2

Publication Date: April 25, 2022

DOI:10.14738/aivp.102.12174. Kullman, L., & Öberg, L. (2022). Treeline Ecotone Progression and Stability: Time Series Analysis of Individual Photographic Data

1973-2021 in the Swedish Scandes. European Journal of Applied Sciences, 10(2). 468-498.

Services for Science and Education – United Kingdom

Treeline Ecotone Progression and Stability: Time Series Analysis

of Individual Photographic Data 1973-2021 in the Swedish

Scandes

Leif Kullman

Department of Ecology and Environmental Science, Umeå University

SE-901 87 Umeå, Sweden

Lisa Öberg

Old Tjikko Photo Art & Science

Handöl 544 Fjällforsa, SE-837 71 Duved, Sweden

ABSTRACT

Positional and population change of the alpine treeline ecotone were studied at a

site (isolated low-fell) in the Swedish Scandes. Methods included early-20th century

records and subsequent re-visitations, including repeat photography, up to 2021.

Substantial temperature rise (summer and winter) during the past 100 years, has

left the elevational forest border virtually unchanged. However, the treelines, i.e.

scattered solitary trees >2 m tall, of mountain birch (Betula pubescens ssp.

czerepanovii) and Scots pine (Pinus sylvestris) have risen by maximum 70 and 65 m,

respectively since the early 20th century, in contrast to inertia of Norway spruce

(Picea abies). Hitherto, pine appears as the most successful species, particularly in

the light of the relatively large numbers of new trees established around the

elevated treeline and the proliferous regeneration in the treeline ecotone during

the past few decades. Presumably, a subalpine pine belt may eventually replace the

sparse birch belt in the snow-poor SE-facing slope, in accordance with regional

tendencies. Climate warming has reduced the snow cover duration on the NE-facing

slope, to the benefit of mountain birch. Possibly, closed birch stands will evolve

here. The study mountain displays a spectrum of different local habitat types, with

contrasting quantitative and qualitative responses to climate change,

representative of prevailing regional treeline ecotone dynamics. Important and

complex constraints to projections of treeline growth and rise, in response to

alleged future climate warming, include geomorphology, herbivory and snow pack.

INTRODUCTION

A distinguished Swedish ecologist, Hugo Sjörs, stated that “the aim of ecology is to see what

happens in nature” (Sjörs 1979). Accordingly, time is an essential dimension, particularly in

cold-marginal landscape-ecological research and perception, which urges for long-term

observational studies of individual and population responses (e.g. Däniker 1923; di Castri &

Hadley 1985; Franklin 1987; Vale & Vale 1994; Webb 1996; Wolkovich et al. 2014). In that

context, climate change with its real and imaginative consequences are signs in the time, with

great public and scientific concern. With that perspective, climate models suggest that the

proposed Post-Little Ice Age (15th to 19th century) climate warming may continue into the

future (IPCC 2013). It remains an open question whether such a course of climate evolution

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Kullman, L., & Öberg, L. (2022). Treeline Ecotone Progression and Stability: Time Series Analysis of Individual Photographic Data 1973-2021 in the

Swedish Scandes. European Journal of Applied Sciences, 10(2). 468-498.

URL: http://dx.doi.org/10.14738/aivp.102.12174

would mirror natural or anthropogenic recovery, or a combination of both, following the

discontinuation of the Little Ice Age (Humlum et al. 2011; Akasofu 2010, 2021). Anyhow, the

consequences for man and biota of future climate evolution (warming) are often projected in

terms of more or less dramatic landscape-scale transformation by the end of the present

century (Moen et al. 2004; ACIA 2005; Kaplan & New 2006; Kullman 2010a, 2019; Hagedorn et

al. 2014; Kullman & Öberg 2021; Schickhoff et al. 2022). In that context, real-world data are

urgently needed.

Up to the present day, after a century of rising summer and winter temperatures, climate and

biotic variability in northern regions have shifted, largely within the frames of renowned

natural Holocene variability (Bergman 2005; Kullman 2013; Paus & Haugland 2017; Ljungqvist

2017; Odland & Paus 2022). In that perspective, alpine treelines and treeline ecotones, in

regions with insignificant human impact, have emerged as particularly suitable objects of

sustained study with focus on past and recent environmental change (Tranquillini 1979;

Kullman 1998; Smithson et al. 2002; Holtmeier 2009; Behringer 2010; Körner 2012).

Globally, treeline upshift is a common and basically climate-driven phenomenon (Holtmeier &

Broll 2017), although modulated and constrained, to various degrees, by interrelated local

factors, such as geomorphology, wind, snow cover, soil fertility, allelopathy, herbivory and

human impacts (Kullman 1979; Hansen & di Castri 1992; Harsch et al. 2009; Leonelli et al. 2010;

Elliott 2011; Dufour-Tremblay et al. 2012; Holtmeier 2012; Östlund et al. 2015; Alatalo &

Ferrarini 2017; Hansson et al. 2021; Grigoriev et al. 2022; Fomin et al. 2022).

Along the entire Swedish Scandes, with relatively insignificant past and present human

disturbance to the natural plant cover, treelines have advanced by more than 200 altitudinal

meters during the past 100 years at optimal sites (Kullman & Öberg 2009; Kullman 2017a,

2021a). This process has locally brought tree growth back (foremost Pinus sylvestris) to the

relatively high positions, that prevailed in the early Holocene, when temperatures in northern

Europe are inferred to have been about 2-3 oC warmer than during the past few decades (Luoto

et al. 2014; Väliranta et al. 2015; Paus & Haugland 2017; Kullman 2017a). These high treeline

elevations refer to the treeline proper, i.e. the highest stations of solitary outpost individuals, at

least 2 m tall. Treeline and glacier history converge to integrate climate change over decades

and longer intervals and suggest that their recent changes may reflect significant and

progressive turning points in the relatively cool and instable Neoglacial epoch (cf. Luckman

1994; Kullman 2003).

It is important to stress that most sites along the treeline display smaller upshifts than the

obtained maximum values of >200 m, cited above (Kullman 1979, 1981; Kullman & Öberg

2009). From an analytical perspective it is particularly fruitful to compare treeline responses

on sites with large and minor shift, respectively (Kullman 1979). Peripheral low-alpine fells

form the lower end of such a continuum and often display a spectrum of different treeline

habitat types.

To a large extent, dynamic treeline studies rely on imprecise remote sensing and unvalidated

climate and ecological models, often with elusive range limit definitions (Kellomäki et al. 1997;

Moen et al. 2004; Kaplan & New 2006; Hofgaard et al. 2013; Frost & Epstein 2014; Mienna et al.

2022). Relatively little is known, based on direct in situ re-visitation studies of structural and

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European Journal of Applied Sciences (EJAS) Vol. 10, Issue 2, April-2022

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compositional changes and their climate controls within the treeline ecotone. The latter

represents the transitional zone between the closed forest and the treeless alpine tundra. In

particular, the balance of vegetative and sexual regeneration needs more attention (cf. Öberg &

Kullman 2012). In that context, it has been argued that future change will largely rely on

phenotypic plasticity (Kullman 1993; Macias-Fauria et al. (2012). The efficiency of that mode

gains historical support from the performance of mountain birch and spruce during the past

100 years of predominant climate warming (Kullman 1979, 1986). Documented multi- millennial longevity of the last-mentioned species, further strengthens such a contention

(Öberg & Kullman 2011, 2012).

Concerning the forest limits, i.e. the upper boundary of closed forest stands, below the treeline,

there are no indications, except for local local upshifts a few tens of metres, of significant

advancement towards higher elevations in the Scandes (e.g. Öberg & Kullman 2012; Rannow

2013).

Commonly, the treeline ecotone in the Scandes is a mosaic of solitary trees, small groves and

open patches with alpine tundra, mostly dwarf-shrub heath and mire. Prevailing tree species

are mountain birch (Betula pubescens ssp. czerepanovii, Norway spruce (Picea abies) and Scots

pine (Pinus sylvestris). As a rule, mountain birch forms a distinct subalpine belt above the mixed

coniferous forest (Kjällgren & Kullman 2002).

On low-alpine fells (this study), without an alpine relief, at the eastern and southern fringes of

the Scandes Mountains, the treeline ecotone is depressed in altitude and tree species zonation

manifests in compressed form, due to strong exposure. This feature is an expression of the so- called “Gipfelphänomen” and relates to various interrelated topoclimatic constraints to tree

growth, e.g. wind, snow cover, convex topography, soil depth and moisture (Scharfetter 1938;

Holtmeier 2009). These mountains display a variety of microhabitats (toposequences),

representative of the treeline ecotone in wider geographical context; snow cover, wind,

insolation, moisture and herbivory. These circumstances, within a limited area and a

homogenous climate, in addition to logistic facilitation, make them suitable objects, as natural

experiments in general treeline ecology.

An important constraint to tree establishment and survival in these habitats is so called winter

desiccation. This phenomenon is a complex action of temperature, wind, snow cover and

transpiration, when the water supply is blocked by frozen ground (Holtmeier 1974;

Tranquillini 1979; Kullman 1993, 2007a). In general, over the past 10-15 years, this stressor

has become alleviated in concert with climate improvement (Kullman 2007; Kullman & Öberg

2021).

The existence of these low fells is often related to climate cooling and “alpinization” during the

Little Ice Age since the 14th century (Payette et al. 1985; Kullman 2012, 2015b, 2017a).

Typically, they are characterized by small treeless caps, exposed to wind from all directions and

a thin and early disappearing snow cover. The ground layer flora uses to be a mixture of species

with alpine and boreal affinities, respectively. Characteristically, they and form an “archipelago”

in a matrix of predominant closed coniferous boreal forest at the eastern and southern and

fringes of the Scandes. In most cases these summits rise only 50-100 m above the treeline

(Wistrand 1962). This situation implies that they potentially receive plant propagules of most