Roadsides host more plant species than the natural vegetation. That is the stunning conclusion I draw in my previous post. While this difference is clear on low elevations, it vanishes as we get higher in the mountains, ending in similar species richness in the alpine zone above the tree line. Surprisingly, as can be seen on the graph, this pattern is the result of a higher diversity of alpine species (dashed grey line, white dots) in the natural vegetation.
The alpine zone is a rocky, barren place without trees. That sounds as a bad thing for plants, but it also results in a higher availability of open places. The dominance of mosses and dwarf shrubs like the crowberries I wrote about, is less intense here. More open spots, less competition, more diverse habitats, all kinds of factors that could explain the higher plant diversity visible in the graph. All of this explains the higher species richness on high elevations. But why don’t we have the additional higher species richness here that we saw in the lowland roadsides?
Here is why: the higher amount of species in lowland roadsides comes from a bunch of typical roadside species, mostly highly competitive weeds (e.g. willowweed, see picture). They do not belong in the normal undisturbed subarctic mountains, but typically follow humans, agriculture and the availability of rich soils and mild conditions. This kind of culture followers forms an important part of the lowland roadside vegetation. These species are added on top of the baseline species richness of typical subarctic mountain vegetation. Therefore: higher roadside diversity.
These competitive weeds are rare in the roadsides on high elevations, where conditions are not mild at all. The roadsides there serve more as a refuge for stress-tolerant alpine species, because the difference with the natural alpine vegetation is much smaller: both contain open, low vegetation, with a lot of bare rock and bad protection against harsh weather. Perfect for stress-tolerant plants (like Saxifraga, see picture), a disaster for the competitive kind.
Conclusion: the subarctic mountain road has a much smaller effect on native plants than its lowland counterpart. Lowland roadsides suffer from the invading pressure of competitive weeds, while they serve on high elevations more as a refuge for a wide diversity of alpine species.