SUMMARY

Mountain regions are characterized by the occurrence of extreme hydrogeomorphic events (HEs, debris flow/floods), with the potential to affect infrastructure, cultural and economic assets as well as to put the future welfare of people living in mountain valleys at risk. Because of the quick onset of these extreme events, early warning system are often inappropriate. Land-use planning is the most efficient tool to reduce economic damages and death tolls caused by extreme floods. Yet, land-use planning depends on reliable baseline data on historical extreme events to: (i) define the expected hazard level; (ii) calibrate simplified model-based approaches; (iii) evaluate the reliability of existing structural measurements; and (iv) detect and attribute the impact of anthropogenic climate change signal. Moreover, long-term records on historical extreme events enable the testing of theories and hypothesis in mountain hydrology. The EXTreeM project is designed to tackle these limitations by developing new analytical tools to date and quantify long-term HEs. Innovatively, EXTreeM proposes the use of trees as sensors of extreme HEs to expand the existing historical measurements records in the headwater catchments of the Pyrenees. By doing this, the EXTreeM aims at: (i) understanding long-term climate-environmental-HEs linkages; and (ii) improving hazard delimitation using calibrated model based on truth evidence on extreme events. In particular, the EXTreeM project focused in four challenges: (i) How can trees be better used to analyze the occurrence of past and recent HEs in mountain watershed? (ii) To what extent long-term HEs reconstruction at local and regional scale will contribute to understand trends of changes related to climate and environmental changes? (iii) Can energy-based botanical evidence provide relevant information about the magnitude of past HEs? (iv)  How a more accurate evaluation of the hazard related to HEs can be attained in mountain catchments? To achieve these challenges, the EXTreeM is structured in 4 interconnected workpackages (WP) (project duration: 4 years). WP1 aims to identifying new growth responses indicators of Pyrenean trees and shrubs to HEs. The WP2 focuses on developing robust long-term regional HEs chronologies and identify linkages with climate and environmental changes. The WP3 deals with exploring the potential use of new family of energy-based botanical evidence to determine the magnitude of HEs. Finally, the WP4 aims at using the gathered knowledge to improve the hazard evaluation in poorly gauged catchments. Outcomes of the project aspire to address the priority calls: (i) Civil security for society (improving hazard assessment in the headwater catchment of the Central Pyrenees); (ii) Climate, energy, and mobility (improving the understanding of long-term HEs activity in relation to climate variability); (iii) Food, bioeconomy, natural resources and environment (understanding long-term HEsenvironmental linkages, and valuing vegetation as unique source of long-term records). Additionally, this project is aligned with relevant international and national initiatives, such as Sendai Framework for Disaster Risk Reduction (UNDRR), the 2030 Agenda for Sustainable Development (contributing to SDGs #1,3,9,10,11 and 15), the IPCC; and the National Climate Change Adaptation Strategy and Civil Protection Plans, among others.