As treelines scale new mountain heights in response to a warming climate, research published in the journal Global Change Biology suggests factors such as unsuitable soils may be slowing their climb.

There have been a number of studies that point to warming mountain slopes enabling the spread of treelines upwards into new areas. However, this spread is not uniform, with local variation and lags in response to climate warming suggesting that other factors may also be at play.

Beyond the climate comfort zone
It was these other, non-climatic factors that captured the attention of researchers at the University of Guelph, Canada. To find out what could be contributing to variations in treeline spread, the researchers conducted a seed addition experiment; two subalpine species of spruce and fir trees were planted at various elevations beyond their typical climate limits at four study sites in the Rocky Mountains.

In particular, the researchers were interested in the role of soil. By controlling for variables such as climate, seed availability, and predation, they were able to zero in on soil properties at each site. Soil from each of the four study sites was also collected and used to plant seeds in growth chambers, allowing the researchers to observe soil effects in a parallel experiment.

“A more complicated story”
The study revealed that the healthy growth of trees was hindered by soils above the trees’ existing ranges. The researchers offer several potential explanations for this, including the presence of different fungi or soil microbes, and changes in soil chemistry caused by vegetation. They also suggest that climate warming may inhibit seedling germination or growth due to increased soil surface temperatures or reduced snow cover resulting in drier conditions.

These results, the researchers say, could have implications for other alpine areas beyond the Canadian Rocky Mountains, and support the assertion that treeline migration is limited by various climatic and non-climatic factors. “There’s a common belief about the impacts of climate change,” says University of Guelph researcher Emma Davis. “It’s actually a more complicated story than people believe.”

Discover more on the University of Guelph website.

Read the full journal article: Davis, E. L. and Gedalof, Z. ‘Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains.’ Global Change Biology, 2018; DOI: 10.1111/gcb.14338