5 years later, we are getting ready for a re-survey of our longterm observational plots along the roads in the Norwegian mountains. The perfect moment to summarize for a second what we learned from our first trip.
Roadsides host more plant species than the natural vegetation. That is the conclusion I drew 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, versus the black line and dots representing the roadside) 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 mentioned in the previous post), is less intense here. More open spots, less competition, more diverse habitats, all kinds of factors that could explain the higher plant diversity as revealed by the graph. All of this explains the higher species richness on high elevations. But why don’t we have the additional higher species richness in roadsides here as well, as 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, Epilobium angustifolium, see picture). They do not belong in such numbers in the ‘traditional’ undisturbed subarctic mountain vegetation, but typically follow humans, agriculture and the availability of rich soils and mild conditions. Such culture followers form 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 a lot harsher. The roadsides there serve more as a refuge for stress-tolerant alpine species, because the difference in environmental conditions with the surrounding undisturbed areas is much smaller: both contain open, low vegetation, with a lot of bare rock, exposed to the harsh climate. Ideal circumstances for stress-tolerant plants (like Saxifraga stellaris, see picture), yet 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.
You want to know the exact scientific story? Here it is!
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