Resurgence of warmth-demanding tree species and a common pending thermophilization in subalpine and northern boreal Sweden-an ecological signal of post-Little Ice Age climate improvement

Largely consistent with general predictions and earlier empirical studies, it appears that post-Little Ice Age climate warming has started to affect large-scale biogeographic patterns in northern Sweden. Long-term monitoring in subalpine and adjacent regions reveals sparse spread of broadleaved thermophilic tree species. Saplings of Quercus robur, Ulmus glabra, Acer platanoides, Alnus glutinosa and Betula pendula have responded to recent climate warming by jump-dispersal in the order of 50-300 km northwards and 500-800 m upwards, relative to their natural range limits. Consistent with treeline rise by boreal tree species, the thermophilies have reinvaded regions where they grew during the warmest phase of the Holocene, 9500-8000 years ago, but were subsequently extirpated by the Neoglacial cooling. Confined to the past 20 years or so, the unique observations of recent termophilies comply with background climate data, i.e. warming of all seasons. These results may contribute to more realistic vegetation models by stressing that the distributions of certain plant species are able to track climate warming without substantial migrational lag. Hitherto, vegetation and climate evolution appear to be well within the frames of natural dynamics during the postglacial era, although mechanisms may differ.


Introduction
In a time coined by widespread concern and anxiety for proposed future climate warming, one of the most important tasks for contemporary vegetation ecology is to elucidate plant cover and landscape ecological changes over the past 100 years, following the climatic and ecological nadir of the Little Ice Age, approx. AD 1300-1900 (Lamb 1995 Schickhoff et al. 2022). This projection is contingent on the assumption that climate change scenarios (IPCC 2021) are borne out. In that perspective, detailed in situ biogeographic monitoring with a background of historical data is urgently needed.
The recent warming phase is reported to have already affected ecosystems and species in widely different parts of the world (Parmesan & Yohe 2003;Penuelas & Boada 2003;Kullman 2004aKullman ,b, 2010aWalter et al. 2005). High mountains at high latitudes play a key role in the early detection of ecological responses to the modern climate transformation. This is due to relatively large proposed future temperature changes hereabouts and since many resident plant and animal species exist at the limit of their climatic tolerance, i.e. responsive even to minor climatic shifts. Moreover, the pristine nature of extant biological communities enables interpretation in terms of climate change.
In the southern Swedish Scandes, summer (J.J.A.) and winter (D.J.F.) warming amount to about 1.5 °C for the period 1901-2022 (Kullman & Öberg 2022). This course of change has evoked upshifts of altitudinal (alpine) boreal treelines (trees at least 2 m tall) by maximum 200 m or slightly more, to positions that seem unsurpassed during the past 7000 years (Kullman & Kjällgren 2006;Kullman 2017aKullman ,b, 2021b. In addition, substantial upward migration of ground cover species, including herbs, sedges, grasses, ferns and dwarf-shrubs, by an average of 200 m, has occurred since the early 1950s. This course of change implies increased plant species richness on high alpine mountain summits in the Scandes (Kullman 2002(Kullman , 2007aKlanderud & Birks 2003;Odland et al. 2010;Michelsen et al. 2011;Felde et al. 2012). The marginal situation and volatile character of these new outposts, with particular respect to snow melt phenology, was highlighted by slight local extirpation and retreat during the cold summer of the year 2012, characterized by a short growth period and late snow melt at high alpine elevations (Kullman 2014).
It is important to stress, that the upper limit of closed forest has shifted upslope much less than the treeline during the past 100 years (Kullman 2022b), thereby contesting alarmistic model projections of a future substantial forest cover expansion and consequent major reduction of the alpine world and its constituent species in the Scandes (Moen et al. 2004).
In the above context, it has been predicted that thermophilic (boreonemoral) woody species, e.g. of the genera Quercus, Ulmus, Alnus, Corylus, Tilia, Acer and Betula, are likely to shift northwards into high-latitude and high-elevation boreal forests of Norway and Sweden, given that current climate amelioration prevails (Aas 1970;Boer et al. 1990;Dahl 1990;Holten & Carey 1992;Hafsten 1992;Angelstam & Svensson 1996;Vera 2000). These projections gain some local support from casual observations (Erkamo 1956;Aas 1970;Kullman 2002Kullman , 2003Kullman , 2006a. With this background, I here report and discuss in the perspective of modern climate change, the anomalous phenomenon of recent migration of broadleaved thermophilic tree species from distant low-lying sources into the subalpine belt of the south-central Swedish Scandes. In addition, the occurrence of planted trees and saplings of species belonging to this group is documented from the coniferous northern boreal forest where Picea abies and Pinus sylvestris prevail as dominants at the present day (Ahti et al. 1968).

Study area
The study is located mainly to the southern Swedish Scandes and adjacent boreal tracts to the east. Climate data relevant for treeline ecotone performance are derived from Storlien/Visjövalen meterological station, 642 m a.s.l., in the mountains close to the border between Sweden and Norway (Fig. 1). The standard level temperatures for January, July and the year are -5.

Methodological approach
The urgent need for real-world data, based on long-term systematic observations at the same locations is increasingly stressed. This is mandatory if we are to understand and tentatively foresee responses of species and communities to altered climatic conditions in the future ( The core of the present study relies on intentional search for thermophilic broadleaved deciduous trees within a regional network of sites (permanent transects) originally intended for long-term standardized treeline monitoring in the southern Swedish Scandes (Kullman 2001). In addition to positional treeline changes, casual records of the concerned group of species are reported from outside the surveyed transects, from exploratory travels in different parts of northern Sweden.
Present-day climate-mediated distributional shifts are not just restricted to high-mountain regions, but also concern lower elevations. From a dynamic biogeographical point of view, particular focus is devoted to the northern and elevational limit of Quercus robur. This species most distinctly marks the so-called Limes Norrlandicus (Fig. 1), by tradition held as an important biogeographical transition zone in northern Sweden below the treeline, separating biota with northern and southern affinities (Fransson 1965;Gustafsson & Ahlén 1996;Sjörs 1999;Gustafsson, 2008). A northward movement of Limes Norrlandicus appears imminent, but is not easily detected due to human interference with the plant cover (Kullman 2012).
Precise geographical locations and altitudes are obtained with a GPS navigator. Geographical coordinates are given as degree latitude and longitude. The present paper updates and extends a previous report (Kullman 2008).

Permanent line transects
Recent surveys of the treeline line transects have uncovered young specimens of Quercus robur, Ulmus glabra, Alnus glutinosa, Acer platanoides and Betula pendula, growing at unprecedented high elevations, well outside their previously known natural ranges and biogeographical zones in northern Sweden (Kullman 2008(Kullman , 2020a. These species have their main distribution limits in the boreonemoral zones of southern and mid Sweden, hundreds of altitudinal meters below the new colonists discovered by this study. Betula pendula grows closer to the high mountains than other species in the group focused here, although this is the most thermophilic birch species in Fennoscandia (Holm 1994;Kullman 2005). Quercus robur and Alnus glutinosa are supposed to be newcombers, dispersed by birds and wind from the Norwegian side of the border, just like the situation inferred for the early postglacial time, based on robust megafossil data (Kullman 2020a). Table 1 accounts for these records and their appearance. Photographs are given as Figures 2-7.
All recovered specimens are of low stature and judged to be quite young, and recently established at their growing sites.
There is nothing in their growth habitus, e.g. multiple stems or stools, to suggest that they have been growing at their present sites for lengthy periods as suppressed individuals.
Particular focus of search was on thermally favored mountains, renowned for a rich flora with southern affinities, as documented by competent botanists during the early-and mid-20th century (Smith 1920(Smith , 1957Kilander 1955). These localities contained spots with this kind of flora and are assumed to be isolated remnants of a generally richer flora prevailing more extensively during the warm early-Holocene (Smith 1951 (Fig. 8).    Ajuga pyramidalis (1050 m a.s.l.). Anthyllis is a late immigrant to this site, which is about 400 m higher than previously recorded in this region (Kilander 1955). Photos: 2008-07-04

A wider geographical context
The nearest present-day natural sites for the tree species accounted for above, are located 500-800 m lower and 50-300 km to the south. However, the exact positions of the potential (climatic) limits of tree-sized and reproducing individuals of these species are unknown. Possibly, their distribution limits have been pushed downwards and southwards by selective logging and pasturing during past centuries of the Little Ice Age (Andersson & Birger 1912;Aas 1970;Huldén 2001). This may explain why sown and planted individuals often thrive far outside their past empirical and assumed natural limits (Blomqvist 1933;Erkamo 1956). For example, Quercus robur grows in the form of large trees in interior parts of northern Sweden, as high as 300-500 m a.s.l. Initially, many of these putative parent specimens owe their existence to plantation trials in the warm 1930s. They have subsequently expanded in size during the past relatively warm decades, when they attained reproductive maturity. Accordingly, along the entire Bothnian coast of norhern Sweden and somewhat inland, Quercus and Acer are spreading centrifugally into seminatural rural coniferous forests (Johansson 2000;Kullman 2012), as illustrated by Figures 9-11).

Discussion
With focus on the Scandes, the prevailing post-Little Ice Age climate warming phase appears to be on the brink of evoking progressive distributional responses of thermophilic arboreal taxa. Accordingly, this paper accounts for one aspect of the ongoing qualitative restructuring of subalpine plant communities and a possible emergence of novel biogeographic zonation patterns (Kullman 2010a In the present context, some caution is needed. Findings of widely scattered saplings of warmth-demanding tree species, well beyond their previous natural ranges, should not be overstated as predictions of future trajectories, since many recovered specimens are still tiny and prone to extirpation (Fig. 8). Moreover, it is important to consider that the current spread of thermophiles represents an utterly sparse pattern in the mountainscape. Nevertheless, these circumstances are indicative of a potential to expand their distribution, abundance and biotic richness in the case of enhanced and sustained climate warming.
The reported occurrences of boreonemoral tree species are decidedly extra-limital to their known natural distributions in Sweden. In many cases, they may share the character of escapes from cultivations in nearby lower elevations. Analogous spread is recorded for exotic boreal tree species (Kullman 2013b(Kullman , 2020b. Taken together, the lastmentioned aspects add a complication to models of future high-mountains plant structure change in a frequently proposed warmer world. In that context, the need to take account of the existing pool of cultivated warmth-demanding plants in the northern landscape is obvious.

Conclusions
 Thermophilic deciduous tree species (boreonemoral) are currently spreading (still mostly saplings) to the alpine treeline ecotone and adjacent mountain forests in the southern Swedish Scandes.  Species particularly concerned are; Quercus robur, Ulmus glabra, Alnus glutinosa, Acer platanoides and Betula pendula. They have all emerged high above and further north of their traditional natural positions in the recent past. Given that the future climate will allow them to grow into tree-size, they may approach their highest tree positions during the thermal optimum by the early Holocene, 9500-8000 cal. yr BP. Even then, they would perform within the frames of inferred natural climate and vegetation evolution of the present interglacial era.  The obtained progressive distributional shifts comply with an initial displacement of the so-called Limes Norrlandicus, that is a biogeographic transition zone in northern Scandinavia, separating biota with northern and southern affinities, respectively.  In many cases, the origins of the newly thermophilies are cultivated trees further south, although beyond the natural distribution limits. This implies that models of future arboreal evolution will have to account for such outposts as dispersal nodes in a future warmer climate.  The current records are consistent with more general and ongoing treeline rise and restructuring of the plant cover in subalpine and upper boreal forests, in response to post-Little Ice Age climate warming (all seasons) since the early 20th century. That would be a resurgence to a stage that last prevailed during the Medieval Climate Optimum, 600-700 years ago, in many respects beneficial to society as well as nature.