"Increase of shrubs causes increased competition for nutrients and water among plants and, as tall species, shrubs shade the low-growing Arctic species," said Heidi Mod of the University of Helsinki, Finland, and University of Lausanne, Switzerland. "Consequently, in addition to direct impacts of warming on plants, shrubification can further modify the original Arctic vegetation. This is something that should be acknowledged when forecasting the effects of warming on Arctic biodiversity."

Mod and colleague Miska Luoto from the University of Helsinki showed that in their models shrubification did indeed modify Arctic vegetation; what happened depended on the original species pool and environmental conditions.

"Our findings highlight that when estimating the effects of warming climate on Arctic vegetation, the mediating effects, such as shrubification, can be highly influential," said Mod. "Neglecting the role of shrubification can therefore lead to deficient predictions."

To come up with their results, Mod and Luoto looked at the outputs from a species distribution model for more than 100 plants for the present and the years 2050 and 2070 under the RCP4.5 scenario. The model incorporated data measured for 2292 study plots 1 square metre in size sited near the transition between mountain tundra and boreal mountain birch forest in Finland and Norway.

The researchers ran the model using only abiotic factors – growing season, over-wintering conditions, moisture, solar radiation and soil nutrients – and also including both abiotic factors and shrubification. Adding shrubification generally improved the model's performance.

Woody plants will expand their range, the models predicted, decreasing species richness, boosting species turnover and increasing the local extinction risk for ambient vegetation. Mean shrub cover in 2050 was roughly double today's value, and in 2070 the mean extinction rate predicted across the test area was almost 50%.

"Spatial modelling allows [us] to account for the cascading effects of both warming and shrubification on future vegetation, and to examine the role of shrubification across large spatial areas," said Mod.

Depending on location, particularly elevation, shrubification reversed or amplified the changes in plant growth generated by warming. Its impact also depends on species, the model predicted. "For example, shrubification decreases the number of Arctic species, but increases the number of boreal species in the area," said Mod. "This means that Arctic species will further lose ground in relation to boreal species in high latitude areas as climate continues to warm."

Although many studies have examined the impact of a warming climate on vegetation, research in the Arctic has tended to ignore the effects of shrubification on other plants, despite the increased competition and shading it creates.

"Indeed, the biotic interactions between species have also been intensively studied, yet they are rarely combined with climate change predictions," said Mod.

There are many ways to take the research forward, according to Mod, including repeating the analysis in another area. And structural changes to vegetation are occurring outside Arctic and alpine ecosystems too. "It is likely that mediating impacts similar to shrubification exist elsewhere, and these should be acknowledged in predictions to gain more realistic forecasts of future changes in world biota," said Mod.

There's still a need for basic research, Mod says, to investigate the effects of individual shrub species on other plants and find out the mechanism, as well as to study shrubification and its speed in more detail.

Mod and Luoto published their findings in Environmental Research Letters (ERL).

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