No treeline shift despite climate change over the last 70 years

Background: The recent rise in temperature and shifting precipitation regimes threaten ecosystems around the globe to different degrees. Treelines are expected to respond to climate warming by shifting to higher elevations, but it is unclear whether they can track temperature changes. Here, we integrated high-resolution aerial imagery with local climatic and topographic characteristics to study the treeline dynamic from 1945 to 2015 on the semi-arid Mediterranean island of Crete, Greece. Results: During the study period, the mean annual temperature at the treeline increased by 0.81 degrees C, while the average precipitation decreased by 170 mm. The treeline is characterized by a diffuse form, with trees growing on steep limestone slopes (>50 degrees) and shallow soils. Moreover, the treeline elevation decreases with increasing distance from the coast and with aspect (south > north). Yet, we found no shift in the treeline over the past 70 years, despite an increase in temperature in all four study sites. However, the treeline elevation correlated strongly with topographic exposure to wind (R-2 = 0.74, p < 0.001). Therefore, the temporal lag in treeline response to warming could be explained by a combination of topographic and microclimatic factors, such as the absence of a shelter effect and a decrease in moisture. Conclusion: Although there was no treeline shift over the last 70 years, climate change has already started shifting the treeline altitudinal optimum. Consequently, the lack of climate-mediated migration at the treeline should raise concerns about the threats posed by warming, such as drought damages, and wildfire, especially in the Mediterranean region. Therefore, conservation management should discuss options and needs to support adaptive management.

Automated summary

This study examined the dynamic of treelines on the Mediterranean island of Crete from 1945 to 2015. Although there was no shift in the treeline over the past 70 years, the increased temperature has started to affect the optimal altitude of the treeline. The treeline elevation was found to be strongly correlated with topographic exposure to wind.