Look up at a forested mountainside and you will see what appears to be a line, beyond which no trees can grow because of the cold and lack of moisture. But closer inspection shows that the transition is more gradual as trees become shorter towards the inhospitable conditions, until they cease growing altogether. This transition area between two different adjacent plant communities is known as an ecotone and may contain a number of unique local species. Over recent decades tree lines have been reported as advancing, possibly as a result of increasing levels of carbon dioxide due to human activities. The ability of forests to thrive at higher altitudes can significantly increase the amount of carbon they contain, as well as reducing the available habitat for rare alpine plants. Therefore, changes to the tree lines around the world may have an important impact on the global carbon cycle and biodiversity. Mechanisms underlying the formation of tree lines are not yet fully understood by scientists. This is a major limitation for predicting the future evolution of tree lines under projected climate scenarios for this century. The EU-funded Pyrtreelinemod project can help scientists better understand tree line dynamics and predict its future evolution in response to climate change. Advanced computer modelling techniques and extensive datasets about individual trees will provide valuable information on the growth of the mountain pine (Pinus uncinata). This species is found at high altitudes across the Pyrenees. Models of tree line dynamics will include information about temperature and precipitation, especially snow, as well as seed viability and dispersal and tree growth. Data from the Pyrtreelinemod project will provide scientists with a greater understanding of the impact of climate change on mountain environments and the global carbon cycle. This information can be used to take mitigating action to protect these fragile ecosystems and provide more accurate climate change models.