Spatial aspects of tree mortality strongly differ between young and old-growth forests

Area of Science:

• Forest Ecology
• patial Statistics
• Forest Dynamics

Background:

• Forest structural development influences tree mortality rates and spatial patterns.
• Young forests are expected to exhibit density-dependent mortality due to competition.
• Old-growth forests are predicted to experience mortality from contagious agents, increasing spatial aggregation.

Purpose of the Study:

• To test predictions of changing tree mortality patterns during forest structural development.
• To contrast mortality dynamics in young versus old-growth Abies amabilis forests over three decades.

Main Methods:

• Utilized three-decade records from mapped permanent plots in young (<60 years) and old-growth (>300 years) Abies amabilis forests.
• Analyzed tree mortality rates, spatial aggregation, and density dependence.
• Compared spatial patterns of surviving trees over time.

Main Results:

• Young forests experienced higher mortality rates (4.42%/year) than old-growth forests (0.60%/year).
• Mortality in young forests was aggregated and density-dependent, leading to more uniform tree spacing.
• Old-growth forest mortality was aggregated but density-independent, preserving spatial heterogeneity.

Conclusions:

• Density-dependent mortality in young forests promotes tree uniformity.
• A transition occurs in late succession to density-independent mortality, maintaining spatial heterogeneity.
• Forest age significantly alters tree mortality drivers and spatial outcomes.