Spatial risk assessment of silver fir and Norway spruce dieback driven by climate warming

Abstract

Context: A significant forest decline has been noticed these last years in Europe. Managers need tools to better anticipate these massive events.Objectives: We evaluated the efficiency of easily available data about environmental conditions and stand characteristics to determine stand vulnerability and map different levels of risk.Methods: We combined remote sensing images, photo-interpretation, and digital models describing environmental conditions within a modelling approach to achieve spatial risk assessment of stand vulnerability. We focused on silver fir and Norway spruce stands in the Vosges mountains (8.800 km², north-east of France), where severe symptoms of decline are visible.Results: Easily available factors describing environmental conditions were used to design relevant vulnerability maps: using an independent dataset, we predicted two-times (Norway spruce) and ten-times (silver fir) higher mortality rates in the units with a high level of risk compared to the others. Norway spruce were predicted highly vulnerable on 33% of their area versus 7% for silver fir. Mortality was higher where the soils displayed low water availability and had suffered severe drying events these last years; it was lower when the species belonged to uneven-aged stands, in mixture, and in the absence of forest edges. Furthermore, the stands acclimatised to drought conditions were more resilient. Conclusions: Vulnerability maps based on easily available geographic information describing climate, soil, and topography can efficiently discriminate canopy mortality patterns over broad areas, and can be useful tools for managers to mitigate the effects of climate change on forests.