# How Warmer and Drier Conditions Drive Forest Dieback and Tree Death: A Review and Conceptual Model for Silver Fir

## TL;DR

This paper reviews how hotter and drier conditions cause forest decline, focusing on silver fir trees and their physiological responses to drought stress.

## Contribution

The paper provides a conceptual model linking hotter droughts and vapor pressure deficit to growth decline and tree death in silver fir.

## Key findings

- Prolonged stomatal closure under elevated VPD in silver fir leads to reduced growth and impaired xylem development.
- Silver fir's vulnerability to hydraulic failure is linked to critical water potential thresholds under soil drought.
- Both atmospheric and soil drought stressors are key drivers of forest decline in silver fir populations.

## Abstract

Agricultural and ecological systems are threatened by extreme and compound climate extremes such as hotter droughts. These events are characterized by elevated maximum temperatures, leading to atmospheric drought, and reduced precipitation, leading to soil drought. Such conditions reduce plant productivity and are increasing mortality trees in forests worldwide. Some forest types are particularly vulnerable to hotter droughts such as some European mountain silver fir (Abies alba) forests. However, we still lack conceptual frameworks linking hotter droughts and rising VPD with growth decline and tree death. This review elucidates physiological responses to drought in conifers with a focus on silver fir. In silver fir declining populations, prolonged stomatal closure under elevated VPD can lead to reduced growth, and impaired xylem development, potentially triggering positive feedback that exacerbates hydraulic limitations. We also review the ecological significance of xylem vulnerability to embolism, identifying the critical water potential thresholds that determine silver fir survival and hydraulic failure risk under soil water deficit. These findings underscore the importance of both atmospheric and soil drought as physiological stressors causing forest decline, and highlight the need for further research into adaptive strategies and early warning indicators in tree species.