Spatial patterns along the secondary trails
In support of the original DEF hypothesis along elevational gradients
(Alexander et al. 2010), we observed declining ruderal species richness
with increasing elevation on the trails branching off the Rallarvägen
(Fig. 6) (Alexander et al. 2010; Dainese et al. 2017; Lembrechts et al.
2014; Liedtke et al. 2020; Wedegärtner et al. 2022). Interestingly, only
few non-native ruderals were observed along these trails, especially
above 500 m a.s.l., rendering the decline in non-native richness less
obvious.
In the traditional view of the DEF-hypothesis, high anthropogenically
disturbed sites in the lowlands ensure quick and repeated introductions
of non-native ruderals, from where they move uphill until they reach
their (current and species-specific) elevational limits. This results in
decreasing richness with elevation. However, our results showed that
neither the non-native nor all ruderal species’ elevational maxima in
our system were correlated with their climatic affinity (EIV-T).
Instead, elevational limits unexpectedly related positively with their
first year of observation in the whole region (Fig. 7): newest
introductions were residing on average farther uphill. These patterns
evoke two possible interpretations. First, there has not been
incremental, linear uphill migration over time. This incremental
migration might here be overruled by human-mediated long-range
dispersal, which usually results in faster uphill migration than by
climate change (Hulme, 2014). This is in accordance with what we said
earlier and suggests that the DEF is often as much disturbance-driven as
it is climate-driven. These higher elevation populations might however
not be stable as climatic conditions might not allow interannual
survival, and long-term monitoring is thus needed to verify these
patterns (Lembrechts et al. 2016a). Second, our analysis could be
missing key variables responsible for the elevational distribution. The
goodness-of-fit for these regression analyses was very low (Fig. 7),
substantiating the fact that the first year of observation to the whole
region does not fit the elevational maximum very well.