Introduction
Evidence is mounting that rapid human-caused environmental changes, such
as climate (e.g., temperature warming and shifting precipitation
patterns) and land use change, cause substantial species redistributions
in mountain areas (Elmendorf et al. 2012; Hedenås et al. 2016; Kowarik,
2003; Pearson et al. 2013; Thuiller et al. 2008). Climate change is four
times the global rate in Arctic regions (Rantanen et al. 2022) and is
especially pronounced in cold-climate mountain areas (Callaghan et al.
2013). Signs of the impact of warming on communities are already evident
in many of these mountain regions, where migration of species from lower
to higher elevations have been well-documented (Dainese et al. 2017;
Frei et al. 2010; Rixen & Wipf, 2017). The effects of land use changes
are usually less profound at higher elevations compared to lower
elevations, the rate of change in communities often lags that of climate
(Bertrand et al. 2011). This interchange between predominantly
disturbance-driven populations at lower elevations and the largely
climate-driven populations along elevational ranges, makes mountain
areas especially suitable for the study of the synergy between both.
Rising temperatures as a consequence of global change have favored
introductions of warm-adapted, non-native species to become established
in mountain areas, especially where disturbance was high (Elmendorf et
al. 2012; Heijmans et al. 2022; Pearson et al. 2013; Taylor et al. 2017;
Thuiller et al. 2005). The vegetation productivity and the length of the
growing season have also increased, as a result of both higher
temperatures in summer and on average a decrease in snow cover in winter
(Elmendorf et al. 2012; Pearson et al. 2013). For some species this
leads to increased growth rates or an extended distribution, while other
species may have adverse effects on fitness. The impact of climate
change on vegetation communities per se is thus quite difficult to
predict, as responses to the changing environment can vary widely in
speed and magnitude across species and functional groups (Klanderud &
Totland, 2005; Parmesan & Hanley, 2015).
Mountain regions are becoming increasingly accessible through improved
infrastructure. Roads and hiking trails are major conduits for
human-mediated dispersal in these regions (Dainese et al. 2017;
Lembrechts et al. 2014 and 2016a, Liedtke et al. 2020; Wedegärtner et
al. 2022), allowing for rapid uphill migration (Hulme, 2014). These
disturbed sites are often characterized by changes in soil conditions,
such as compaction and chemistry, which affect species diversity and
composition, by creating an environment that often promotes ruderal
species (Frenkel, 1977; Guo et al. 2018; Rendeková et al. 2019).
Roadside dispersion is related to traffic intensity and the size of the
road network (Chiuffo et al. 2018; Pauchard et al. 2009), while hiking
trails often facilitate ruderal plant dispersal from roads or
settlements further into the mountains (Liedtke et al. 2020).
Human-mediated dispersal facilitates non-native plant species influxes
from all over the world as tourists are often bringing in hitch-hiking
seeds that stick to their clothing, boots, or the tires of cars
(Frenkel, 1977). Most of these non-native species have a ruderal growth
strategy (Alexander et al. 2016; Chiuffo et al. 2018; Kowarik, 2003).
Consequently, non-native ruderals mostly appear first near train
stations (Brandes, 2002), parking lots (Frenkel, 1977), roadsides
(Lembrechts et al. 2014), and other places where human displacement is
most abundant (Guo et al. 2018; Liedtke et al. 2020). The degree of
invasion in a community is thus related to the intensity of human
activity (Kowarik, 2003).
Due to their long, harsh winters, and short, relatively cold summers,
subarctic mountain ecosystems were previously believed to be relatively
resistant to the influx of non-native species (Pauchard et al. 2009),
but climate change and increased anthropogenic disturbance are gradually
changing this view (Pauchard et al. 2009; Walther et al. 2009). Many
non-native ruderal species are known to be good dispersers that can
reach high elevations twice as fast as native species (Dainese et al.
2017), although their climatic tolerance may constrain their survival to
the next growing season (Rendeková et al. 2019). Nevertheless, a
widespread uphill migration of non-native species has been observed
along elevational gradients in response to climate change (Alexander et
al. 2016; Dainese et al. 2017; Kueffer et al. 2013; Pauchard et al.
2009). Indeed, introductions tend to take place in the lowlands
(Alexander et al. 2010; Guo et al. 2018; Liedtke et al. 2020; Pauchard
et al. 2009), and from these sites species either move through
human-mediated dispersal or spread out on their own.
The Directional Ecological Filtering (DEF) process describes the
unidirectional uphill expansion of non-native species (Alexander et al.
2010). Non-native species richness gradually declines with increasing
elevation. With their lower elevational limit consistently in the
lowlands, non-native species spread over an elevational range until they
reach their upper elevational limit. As a result of this directional
movement starting in the lowlands, only climatic generalists are likely
to reach high elevations. Non-native species are thus gradually filtered
out along the elevational gradient, probably due to increasing climatic
harshness (Alexander et al. 2010), although evidence shows that a longer
residence time also inevitably results in higher elevational limits
(Pyšek et al. 2011).
Testing the interactive effects of climate change and anthropogenic
disturbances on native and non-native ruderal species expansion requires
detailed knowledge on the history of disturbance events, as well as
long-term data on ruderal species distributions. Such data is available
for a mountain region in the north of Sweden, around Abisko – a small
village known for its hiking trails and the Abisko Scientific Research
Station (Andersson et al. 1996). The local climate is defined as
subarctic with cool summers and relatively mild winters with extensive
snow cover. The Scandes mountain range to the west, creates a rain
shadow effect directly over Abisko, making it the sunniest area in
northern Sweden (Callaghan et al. 2010 and 2013). However, similar to
other high latitude regions, Abisko has been subject to increasingly
severe climate warming in combination with substantial anthropogenic
disturbance since the early 1900s (Callaghan et al. 2013). This makes it
an ideal study area to test the interaction of these global change
drivers, specifically on the introduction and changes in ruderal species
compositions over time.
In 1903, a railroad was completed from Kiruna to Narvik, soon followed
by the first tourist hotel in Abisko (Callaghan et al. 2013). The
Rallarvägen trail - the focus of the underlying study - runs parallel to
the railroad and served as a transport road during construction. The
accessibility of the Abisko region was further improved with the opening
of the first paved road (the E10 highway) from Kiruna to Riksgränsen in
1982, which followed the existing Rallarvägen and the railroad line. The
effects of the E10 on roadside vegetation were studied in 1989 (Bäck &
Jonasson, 1998). Yet, in contrast to other studies that have examined
the role of roads on the influx of non-native species (e.g, Lembrechts
et al. 2014 and 2016a), the effects of the E10 construction were very
limited (Bäck & Jonasson, 1998). Since its opening, the upgraded
infrastructure has contributed to an increase in tourism in the region,
so it is possible that changes in vegetation composition resulting from
the road may have become noticeable only now. Additionally, over the
past decades, the average annual air temperature in the region has
gradually increased from 0 to 1°C (ANS, 2019; see also Callaghan et al.
2010). These rising temperatures already resulted in significant upward
shifts in the treeline and the distribution of a range of plant species,
as well as substantial changes in their phenology (MacDougall et al.
2021). The effects of the railroad in combination with the E10, tourism,
and climate change may have caused a steady increase in ruderal species
in the vegetation and dynamic changes in the ruderal composition over
the past 120 years.
Importantly, a unique historical time series is now available for the
region: we know the exact timing of major disturbance events (railroad
and road building), we have a clear view of the changes in climate in
the region (weather data has been continuously measured by the Abisko
Scientific Research Station) since 1913 (ANS, 2019), and we have
vegetation surveys along the Rallarvägen dating back till 1903
(Lewejohann & Lorenzen, 1983; Sylvén, 1904 and 1913-15). With these
datasets in hand, augmented with a recent resurvey of the Rallarvägen
trail and additional vegetation monitoring along trails leading from the
Rallarvägen into the mountains, we set out to answer three key research
questions about the history of ruderal species along the Rallarvägen and
in the broader Abisko region:
- What are the temporal dynamics of ruderal species introductions along
the Rallarvägen over the last century?
- Do these dynamics correlate most strongly with climate or land use
change?
- What drives the spatial expansion of ruderal species in the Abisko
region?
We hypothesized that railroad building at the beginning of the
20th century would have facilitated the establishment
of significant amounts of ruderal species – both native and non-native
– along the Rallarvägen trail. Additionally, we expected later
disturbance events, such as the building of the E10 in the early 1980s,
to have created a new influx of largely non-native ruderals, with in the
last decades an influx of mostly warm-adapted ruderal species as a
result of climate change.
We expected these ruderal species to be concentrated around points of
introduction with continuous disturbance, such as the main train
stations, with a progressive decline in richness with increasing
distance to these introductory points. Additionally, more recent
introductions and warm-adapted ruderals were expected to be restricted
to low-elevation and/or warmer environments.