How much warming can tundra ecosystems withstand?

In a new doctoral thesis from the Swedish University of Agricultural Sciences, PhD-student Kohsuke Tanigawa investigated whether warming could trigger threshold effects in tundra ecosystems. These triggers can be understood as critical points where small temperature increases suddenly lead to large ecological changes. 

– The question we are asking is whether ecosystems respond linearly to rising temperatures, or whether there will be a sudden shift once the temperature passes a certain threshold. If environmental changes cause even one species to disappear, it could disrupt the functioning of the whole system, says Kohsuke Tanigawa. 

Testing extreme levels of warming

The research focused on dry heath tundra near Abisko in northern Sweden. To test for threshold effects, Kohsuke combined laboratory warming experiments on soils and intact tundra communities with field experiments in which soils were transplanted along mountain slopes. 

The controlled experiments included five temperature levels, ranging from current conditions to 9 °C warmer, allowing him to examine the effects across a broad warming gradient.

The experiments showed that plants, microbes and soil animals remained surprisingly unaffected – even the strongest warming treatment did not lead to threshold-induced disruptions of ecosystem functions.

– I was surprised by how much warming these communities could withstand. Even at 9 °C above current temperatures, we did not observe a drastic reorganization of the ecosystem within one growing season, says Kohsuke.

Abrupt changes in soil microbes

But this lack of widespread non-linear changes did not mean that all ecosystem processes remained unchanged or responded linearly.

Some of the clearest threshold responses occurred in the soil microbes. At certain warming levels, the efficiency with which these microbes used soil carbon changed abruptly, suggesting that important changes may occur underground before obvious changes become visible in the aboveground vegetation.

There are still pieces of the puzzle to put together. Kohsuke Tanigawa is waiting for data on the composition of soil microbial communities, which he hopes will explain the observed threshold responses and give a clearer indication of what further changes can be expected as the tundra continues to warm.

– We are asking very fundamental questions, and although the pattens are not always clear, there is value in testing our hypothesis thoroughly, says Tanigawa. Our results show why it is useful to examine several levels of warming on different combinations of plants and soil organisms: ecosystem properties may appear relatively stable, but important ecological processes could actually be changing, he adds.

Thesis and defence

Kohsuke Tanigawa successfully defended his thesis on Friday 18 September 2026 in Umeå, Sweden. The non-linearity of tundra warming responses and their thresholds.