Tracing five decades of junipers’ responses to global changes in Mediterranean high mountains

Abstract

Persistent and long-lived species, such as Juniperus communis and Juniperus sabina, base their population survival strategy on a great individual longevity under extreme environmental conditions. In the high Mediterranean mountain, global change stressors may challenge such persistence strategy by reducing growth and survival. This study identifies how global change is affecting the growth of juniper shrubs over five decades in Sierra Nevada (southern Spain). For this, we analysed the growth rates and foliage damages of 900 juniper shrubs by manual digitalization of their crowns in historical orthoimages from 1970’s to 2020’s. Almost all small junipers (96 %) and most larger junipers (86 %) increased their size, particularly at lower elevations, favoured by the lengthening of the growing season and by the abandonment of traditional land uses. Such finding supports the persistence strategy hypothesis, highlighting not only junipers’ survival, but also the maintenance of growth even under harshening conditions. Despite the loss of snow cover protection, we also found good juniper foliage health, with a low proportion (~12 %) of damaged shrubs. Damages only occurred at the highest elevations, particularly on larger shrubs (> 7 m²). However, these findings should be seen as an early warning of increasing risks associated with ongoing climate change, such as heightened exposure to mechanical damage due to the increased wind speeds at high elevations and severe droughts affecting species recruitment. Using historical aerial and satellite orthoimages, we have been able for the first time to track the fate of hundreds of juniper individuals over the last 50 years.