Pine (Pinus sylvestris L.) Penetration towards the Head of the Handölan Valley: Recent Reversal of Long-Term Retrogressional Trend – Contrasting Responses to Climate Change of Tree-and Forest Line.
Abstract
With a climate -change perspective, this study focuses on the recent history and performance of the much separated pine (Pinus sylvestris L.) tree and forest lines in a mountain valley in the southern Swedish Scandes. Historical records define quite accurately these “lines” by the early 20th century and mid -1970s. Their subsequent dynamic performances have been surveyed up to the present day. Both lines are currently positioned within the subalpine mountain birch forest belt (Betula pubescens ssp. czerepanovii). Consequently, their past, present and future changes rely on the evolution of the birch forest matrix. Between the early 20th century and the mid -1970s, the position of the pine forest - and tree lines remained fairly stable. Conspicuously, the forest line population densified in accord with early -20th century climate warming, although with a stagnant position. In great contrast, the tree line, i.e. scattered solitary, fast -growing and vigorous trees , has shifted up -valley 135 m through the birch forest belt to a position about 12 km south of the position by the mid-1970s. It is hypothesized that the vast separation of forest - and tree line relates to the presence of the subalpine birch forest belt. T his contention is supported by vigorous growth of outlier pine trees, predominantly in birch forest gaps and a treeline gradually rising towards the south in the valley. Accordingly, it is hypothesized that, with present -day climatic conditions, the potential pine forest line is much south (and higher) of its present position within the competing birch forest belt. In the case of future climate warming, this forest range is anticipated to be realized. That would be a return to the situation during the early - and mid - Holocene, when pine dominated the upper tree line ecotone and the birch belt was poorly developed.
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Introduction
On long- and short time scales, cold-marginal treeline stands, ultimately constrained by heat deficiency, may perform as sensitive biological indicators of climate change and variability (Kullman 1998; Holtmeier 2003; Körner 2007; Kharuk et al. 2009; Kullman & Öberg 2009). However, on local spatial scales, treeline responses to altered climatic conditions are modulated by topoclimatic, biotic and landuse factors as well as plant cover legacies (Kullman & Öberg 2009; Leonelli et al. 2011; Holtmeier & Broll 2012).
A fundamental response dichotomy may concern the treeline1 and forest line2, i.e. the uppermost peripheral single trees and the fringe of more or less closed forest, respectively (cf. Kullman 2010, 2014a). In the Scandes, extensive regional-scale elevational treeline upshifts of different species in close accord with post-Little Ice Age warming (Kullman & Öberg 2009) are not paralleled by analogously large advance of closed-canopy forest (Kullman1990, 2010, 2014a, b; Hofgaard et al. 2013; Rannow 2013). Despite substantial warming during the past 100 years, tree and forest stands have not become re-established at those high elevations were they grew during earlier warmer-than-present epochs of the Holocene, e.g. the Medieval Climate Optimum, around 1000 years ago and finally became extirpated during the subsequent Little Ice Age climate cooling (Kullman 2015b).
Most available data indicate that the treelines of all species have been positionally more compliant with climate change and variability than their forest lines (Kullman & Öberg 2009).The resilience of the forest line versus treeline was anticipated by early botanical explorers, stressing relatively subdued responses of the former and its great spatial variability in areas with virtually the same climate and more contiguous tree lines (Smith 1920; Enquist 1933; Kvamme 1993; Körner 2007). In this context it may be pertinent to reconsider a view flourishing among botanists working with treelines in the European Alps during the early 20th century claiming that a site where a solitary tree can grow, can also support the existence of closed forest (cf. Scharfetter 1938; Schiechtl 1967). This unvalidated contention has a particular relevance today, in connection with efforts to interprete “early warning” signs of changed balance between forest and alpine tundra in a hypothetical case of future climate warming. Currently, pine trees are commonly becoming established far beyond the limit of closed-canopy pine forest (Kullman & Öberg 2009; Kullman 2010, 2014b). A pertinent question is whether these new tree line markers also represent today´s potential limit of forest growth, although not yet realized due to lack of seed and dispersal constraints, imposed primarily by a dense and fairly stable birch forest belt (cf. Blűthgen 1960; Shiyatov 1993; Kullman 2010, 2014a, Holtmeier & Broll 2011).
Still, the enigma concerning differential nature and performance of tree and forest lines constitutes a caveat in the context of environmental monitoring (Kullman 2015c) and modelling future landscape ecological responses to climate change.
Models of the latter kind commonly envisage extensive forest encroachment on the alpine tundra, with profound consequences for flora, fauna, carbon cycling and radiation balance (Moen et al. 2004; ACIA 2005; Bernes 2007; Kaplan & New 2006). However, it has been pointed out that models projecting rapid forest expansion (e.g. Moen et al. 2004) over the alpine tundra are overstated and poorly supported by observational data (Moen & Lyngstad 2003; Rössler et al. 2008; Kullman 1986, 2009, 2014a; Hofgaard et al. 2013). Moreover, a sluggish pine forest advance up to the present, despite substantial climate warming, represents a climate-distribution disequilibrium state and appears as a more general option in the north (Davis 1989; Campbell & McAndrews 1993; Zackrisson et al. 1995; Hiller et al 2001; Holtmeier et al. 2003; Körner 2005; Payette 2007; MacDonald 2010; Paus 2013; Kullman 2014a). In Scandinavia and adjacent regions, this mechanism seems merely conditioned by dispersal constraints, related to the presence of a dense subalpine birch forest belt, functioning as a filter between the upper pine forest and the alpine tundra (Holtmeier 1974; Shiyatov 2003; Kullman 2010, 2015). Disequilibrium performance, in general, stands out as a particularly challenging aspect for the interpretation of tree and forest history at landscape and population scales. Proper forecasting of future biogeographic dynamics in general and treeline dynamics in particular, under prescribed climatic regimes in mountain regions, rely on a deeper understanding of this issue (cf. Zackrisson et al. 1995; Johnstone & Chapin 2003; Caccianiga & Payette 2006; Svenning & Sandel 2013; Normand et al. 2013).
Against the background outlined above, the present paper seeks to highlight the discrepancy between pine (Pinus sylvestris L.) treeline and forest line advance during the past 100 years or so. The reason for focusing on pine in particular is the fact that this species dominated the treeline ecotone during the earliest and warmest part of the Holocene (Kullman 2013). Moreover, pine is the only tree species in the region which reproduces solely by seed, which facilitates interpretation of stand history, since one has not to consider responses of a bank of old-growth krummholz specimens above the treeline. In addition, discernible onset of pine forest encroachment on alpine tundra during recent relatively warm decades is recorded in the southernmost Swedish Scandes (Kullman 2010, 2014b). This course of change coincided with and was conditional on recession of the competing birch forest belt, in response to earlier and more complete snow melt, which imposed drier soils (Kullman 2014b). Therefore it is reasonable to assume that in a substantially warmer future, pine will in due time regain some of the territory lost during the course of gradual Holocene climate cooling (Kullman 2013, 2015b). Accordingly, it may be hypothesized that a currently extended range of solitary pine trees may constitute an early positional indication and source of an extended cold limit of pine forest stands in a possibly warmer future. The realism of this option is tested with this field study of tree growth performance, carried out in a long mountain valley, trending towards large expanses of alpine tundra. This valley is particularly well-researched with respect to past and recent treeline and forest line history, which facilitates interpretation (Lundqvist 1969; Bergman et al. 2005; Kullman & Kjällgren 2006; Öberg & Kullman 2011; Kullman 2014a and literature cited therein).
Conclusion
- Concurrent with climate warming of all seasons over the past 100 years and up to the present day, the forest line of Scots pine ( Pinus sylvestris L.) has remained positionally static, although substantially densified.
- Since the mid -1970s, the tree line advanced by 135 m in elevation and 12 km southwards in the valley.
- The large spatial separation between the current forest and tree line likely relates to the intervening subalpine birch forest belt.
- Hypothetically, much of today´s subalpine birch forest belongs to a potential pine forest belt.
- Based on current changes and given a warmer future, an analogue to the early and mid -Holocene vegetation structure may evolve. As a main consequence, the birch forest belt may be replaced with a subalpine pine belt.
