COS 235-4 - An experimental test of the range-limiting drivers of Cedrus libani suggests an unexpectedly wide altitudinal range and a likely high resilience to future climate warming

Abstract: Understanding the range-limiting drivers of species is essential to predict the distributional changes that are expected to result from climate warming. However, despite sustained theoretical interest and recent observational or experimental works, the ecological constraints that impose species’ range boundaries remain poorly understood. In this work, we seek to assess whether the range limits of the Cedar of Lebanon in natural conditions coincide with the limits of its climatic niche by comparing key fitness parameters of the species within and outside its altitudinal range and by investigating the ecological causes of the Cedar's disappearance outside its limits.

We recorded the presence/absence of Cedar in 1241 plots along a 2800 m altitudinal gradient encompassing the Cedar natural range in Lebanon to assess its altitudinal range. We established in this region a set-up of 8 experimental sites spread over 2200 m of elevation, in order to extend it far beyond the altitudinal warm and cold limits of the Cedar's natural distribution. The constraints conditioning the climatic limits of the Cedar were investigated by establishing different watering modalities and by comparing Cedar performances with that of 3 competitor tree species. We measured the growth and survival at 3 years of the 1360 cedars planted in the set-up and those of competitor species.

The experimental set-up revealed optimal survival and growth rates of the Cedar over an altitude range of 1500 m, three times higher than the altitudinal range of its natural distribution in Lebanon, which extends from 1300 to 1830 m altitude. Water stress limits the survival of the Cedar below 500 m, at an altitude much lower than its low altitudinal limit. Competition explains the absence of Cedar below 900 m altitude due to a superior growth of competitor species. Three thousand years of intensive exploitation of Cedar likely explain its absence between 900 and 1300 m altitude. Finally, cold temperature and water stress limit the survival of Cedar above 2000 m, an altitude slightly above the species cold limit under natural conditions.

The high survival rate experimentally observed below the warm limit of the Cedar natural distribution gives hope for its resilience to future climate warming within its current range. If the pattern observed for the Cedar is common to many mountainous species, it would challenge predictions of extinction of these species under climate change and pave the way for adaptive actions, such as competition management, to improve their long-term survival.