Crowding, climate, and the case for social distancing among trees

Area of Science:

• Ecology
• Forestry
• Climate Change Science

Background:

• Western forests face increasing megadisturbances like wildfire, insects, and drought.
• Climate change significantly impacts forest health, but climate-vegetation-disturbance interactions are complex.
• Traditional restoration relies on historical conditions, which are often unattainable in novel forest ecosystems.

Purpose of the Study:

• To examine nonlinear relationships between fire, climate, forest spatial structure, and bark beetle impacts.
• To understand how local forest structure influences tree mortality risk under different climate and disturbance regimes.
• To provide a mechanistic basis for developing effective forest restoration and resilience strategies.

Main Methods:

• Utilized two extensive longitudinal datasets of spatially explicit, annual tree mortality.
• Employed a post-hoc factorial design to analyze interactions between fire, climate, and forest structure.
• Quantified nonlinear relationships influencing tree mortality and bark beetle attack risk.

Main Results:

• Prefire drought increased mortality risk, but favorable local forest neighborhoods could mitigate this.
• Higher mortality occurred in crowded stands during wet years (42%) compared to sparse stands during drought (30%).
• Drought increased bark beetle risk, yet lower tree density limited fire-beetle interactions; antecedent fire had varied effects on drought mortality over time.

Conclusions:

• Local forest spatial structure plays a critical role in mediating climate and disturbance impacts on tree mortality.
• Managed fire and silvicultural practices can potentially counteract climate change effects and enhance forest resilience.
• Novel forest conditions necessitate adaptive strategies that consider complex disturbance interactions for effective restoration.