Warming is not pushing all mountain trees in the same direction. Some species are rapidly gaining elevation, while others seem instead to be progressing toward lower areas. A global study links this difference to the way each species transports water and withstands a lack of moisture.
To understand this movement, the authors brought together several datasets. They analyzed tree-ring series from 121,743 trees belonging to 45 species. They also used observations of elevational shifts for 102 species, then compared this information to 11 traits related to the hydraulic functioning of trees.
Pexels illustration image
These hydraulic traits describe in particular how water circulates in leaves and stems. They also provide indications of a plant’s resistance to drought. Two species living on the same mountain can therefore react differently to a comparable rise in temperatures.
The result shows a clear contrast. Species whose growth reacts strongly to heat or lack of water tend to follow the warming toward higher elevations. Conversely, species that are more resistant to these constraints display more characteristics associated with expansion toward the lower parts of slopes.
Temperature alone is therefore not enough to predict the direction taken by a forest.
The tree-ring analysis provides another piece of information. For nearly a third of the studied species, the relationship between elevation, drought, and growth changed over time. A population located higher or lower on a slope therefore does not necessarily respond to the climate in the same way several decades later.
The authors also tested whether regional differences in warming alone explained the observed movements. For the 45 species with corresponding data, this relationship was not statistically significant. The trees’ own characteristics therefore provide additional information for understanding why their distribution ranges evolve differently.
This point matters for anticipating the future of mountain forests. Moving upward does not offer an unlimited reserve of space, because the available surface generally decreases as altitude increases. Researchers now propose integrating species’ hydraulic traits into models designed to predict the reorganization of forests under a climate that continues to warm.