Temporal patterns
Native and non-native ruderal species temporal dynamics deviated
surprisingly much from our hypotheses. Indeed, we expected to find an
increase in ruderal species over time as a result of the joint impact of
climate and land use change on the vegetation composition along the
Rallarvägen, since both have been shown to be advantageous for ruderal
species (Chiuffo et al. 2018; Guo et al. 2018; Thuiller et al. 2005).
Following this hypothesis, we can indeed assume that already shortly
after the building of the railroad at the beginning of the 20th century
substantial numbers of ruderal species were already present along the
Rallarvägen trail - both native and non-native to the region, as the
bare ground that remained accommodated up to 82 ruderal species, of
which 44 were non-native, according to the first survey in 1903 (Fig.
2). Contrary to expectations, however, ruderal species (both the total
and the non-native ruderals separately) were most numerous along the
trail during the time of the railroad building and settlement expansion
(1903-1913). The building of the E10 road (completed in 1983) did not
result in an additional influx of ruderal species, nor did the
substantial climate warming since the late 1980s. In 2021, total ruderal
species richness was just below the levels of 1913, while non-native
ruderal richness was only half of the numbers in 1903 and 1913. The drop
in 1983 must be at least partially a methodological anomaly, but even
then, we must consider the total number of ruderal species to have
remained roughly constant through time, and the number of non-native
ruderals to be decreasing (see ‘limitations’ below).
What is more, patterns in the community temperature index (expressed as
the Ecological
Indicator Values for Temperature; EIV-T) revealed that most warm-adapted
ruderal species were already present in the early 20thcentury, after the building of the railroad, with little evidence of an
influx of warm-adapted species due to climate change in recent years
(Fig. 3). In fact, we even observed a steady decline in mean EIV-T of
the non-native ruderals up until 2021. Our data showed that 36 (mostly
non-native) ruderal species with high EIV-Ts (≥ 4) in the historical
years, had disappeared by 1983, and did not reappear in 2021, while
species observed for the first time more recently had on average lower
EIV-T values. It is likely that those warm-adapted ruderal species were
introduced along the Rallarvägen with its initial construction (and
likely the addition of non-native soil introduced into gardens in
settlement areas). After this initial introduction, however, these
species did not manage to sustain a stable population, potentially due
to a combination of harsh winters and the lack of continuous disturbance
after 1913 (Lembrechts et al. 2016b; Niittynen & Luoto, 2017; Rendeková
et al. 2019). Interestingly, the lack of additional significant
disturbance events along the trail in recent years, seems also to have
prevented these species from re-establishing now that the climate has
warmed to an average 1.0°C (ANS, 2019; Lembrechts et al. 2016b). In
time, many of the ruderal species along the trail were thus again
outcompeted by the native species that thrive in subarctic mountain
environments – demonstrating community resilience. Nevertheless, as
climate change progresses, we expect that warmer temperatures might
still tip the scale, allowing subsequently introduced warm-adapted
non-native species to gradually establish successfully in areas where
disturbance is prominent (Dainese et al. 2017).