Scattered tree death contributes to substantial forest loss in California

Area of Science:

• Forest Ecology
• Remote Sensing
• Climate Change Impacts

Background:

• Global change drives large-scale tree mortality events worldwide.
• Existing forest monitoring methods often miss scattered or isolated dead trees, leading to uncertainty in mortality extent.
• Accurate assessment of individual tree mortality is crucial for understanding forest health and carbon dynamics.

Purpose of the Study:

• To map individual dead trees across California using high-resolution aerial imagery.
• To quantify the extent of both grouped and isolated tree mortality.
• To assess the implications of widespread individual tree death on forest ecosystems.

Main Methods:

• Utilized sub-meter resolution aerial photographs from 2020.
• Employed deep learning algorithms for automated dead tree detection.
• Compared results with field data to estimate underestimation bias.

Main Results:

• Identified 91.4 million dead trees across 27.8 million hectares of California's vegetated areas.
• Found that 19.5 million dead trees were isolated, and 60% occurred in small groups (≤3 trees per grid).
• Detected significant underestimation bias (16.7–24.7%) compared to field data.

Conclusions:

• Widespread individual tree mortality, including scattered and small groups, is a significant issue in California forests.
• This mortality impacts forest carbon budgets and sequestration capacity.
• Individual tree mortality poses a threat to forest health and increases wildfire risk.