Limitations
The core challenge in this study lies in the quality and comparability
of the historical surveys. While we here focused on overarching trends
in species richness, these trends mask a substantial turnover in species
composition between surveys, which is likely at least partially the
result of methodological differences. These methodological differences
seem to have been most substantial for the 1983 effort, yet these can be
interpreted when considering the following aspects: 1) monitoring
differences, 2) weather conditions, and 3) disturbance intensity.
While the vegetation survey of Lewejohann & Lorenzen (in 1983) was
conducted during the same month (July) as Sylvén’s surveys (in 1903 and
1913) and our own in 2021, the covered perimeter (20 areas in total) was
much larger. As a result, they presumably had the time to survey all
areas only once, therefore likely missing ruderal species that appear
later in the season. Additionally, their data missed perennial species
such as Trifolium pratense L., Trifolium repens L., andVicia cracca L., which were present in 1903 and 1913 and are
currently very abundant along the Rallarvägen trail. Even when these
ruderal species are not flowering, they are easily identified,
suggesting some selectivity in documenting species during their survey.
An unexpected discovery, as Lewejohann & Lorenzen set out to survey
vegetation changes following the disturbance caused by the building of
the E10 road, and thus should have shown particular interest in ruderal
species (Lewejohann & Lorenzen 1983; Lembrechts et al. 2014). We did
learn that tourism in the area declined substantially in the early 1980s
due to a temporarily reduced availability of tourist accommodation
during that time. Consequently, we cannot exclude a recovery period from
ruderal dominance in the vegetation. Nevertheless, the train stations
would have seen continuous disturbance even in those days, and we thus
deem it unlikely that these common ruderals mentioned above would have
disappeared entirely. We do believe that all survey locations are still
comparable as the landmarks in the surveyed villages have remained the
same since 1903: the train stations have always been an anchor point for
the villages.
Importantly, 1983 was also an anomalous year in terms of weather
conditions (supplementary Table 16). Summer temperatures were on average
1°C lower than in 1913, and summer precipitation was exceptionally high
(238.8 mm). Snow cover and snowmelt timing are among the most important
drivers of structuring subarctic community composition and distribution
(Wipf, 2010). Depending on the thickness, snow cover insulates species
from the harsh winter conditions as it decouples the soil surface
temperature from the air temperature (Niittynen & Luoto, 2017). If the
snow layer disappears too early, the species underneath will be exposed
to the spring frost. Snowmelt timing determines the growing season
length (Niittynen & Luoto, 2017; Wipf, 2010). Moderate rainfall can
speed-up the snow-melting process, expediting the onset of the growing
season. Heavy rainfall, on the other hand, causes earlier snowmelt, yet
then subsequent cold spells could potentially kill off ruderals, or
reduce the surviving species’ ability to exploit the whole growing
season. Such negative effects are especially disadvantageous for annual
species as they do not get the chance to adapt their physiology or
morphology in similar ways to perennial plants (Li et al. 2019). While
we do not have information on the spring snow cover in 1983, the
anomalous precipitation value could suggest that such a scenario of
early snowmelt followed by spring forest events might have happened.
Annual species are rare in tundra ecosystems (Weidema, 2000), but their
distribution in 1983 is rather noteworthy. Seven of 57 (about 13%) of
the observed ruderal species in 1983 had an annual life cycle, which was
substantially lower than in the other observational years: 54% in 1903,
49% in 1913, and 16% in 2021. Five out of those seven occurred only at
Björkliden. Interestingly, this was also the subregion in 1983 that
accommodated species with highest average EIV-Ts (Fig. 3). This, in
addition to the observed degree of heterogeneity between the subregions
in 1983 (supplementary Fig. 4), is probably caused by the timing of
observation, which seems of particular importance during that summer.
Another aspect that could have contributed to the low number of ruderal
observations in 1983 was the low disturbance intensity during the E10
highway building between 1976 and 1982 (Bäck & Jonasson, 1998). We
found that ruderal species distributions along the Rallarvägen were in
the most recent survey still clearly correlated to the railroad, rather
than the E10 (Fig. 4). In contrast to the railroad construction, the E10
was not built using the Rallarvägen as a transport road, and hence its
direct impact on the Rallarvägen vegetation during that time was
substantially smaller. Since then, the highway facilitated easier
movement in and around the area by cyclists, cars, and trucks, while
parking lots now enable hikers to enter the trail at numerous locations,
which were less accessible in the past (Bäck & Jonasson, 1998; Frenkel,
1977; Lembrechts et al. 2014). Nevertheless, the resulting influx of
ruderals along the Rallarvägen was possibly not yet detectable in 1983.
Indeed, ruderal species numbers increased again since then, yet whether
this was simply methodological, disturbance, or climate related we do
not know. The proportionally rising number of non-native ruderal species
is potentially an indication of a delayed response, yet the above
discussion should make clear that caution regarding species patterns in
1983 is warranted.