Forest stand structure, productivity, and age mediate climatic effects on aspen decline

Area of Science:

• Ecology
• Forestry
• Climate Science

Background:

• Climate change impacts forest ecosystems, with quaking aspen (Populus tremuloides) experiencing significant mortality events.
• Previous studies often overlooked forest stand structure, age, and productivity, limiting understanding of aspen decline drivers.

Purpose of the Study:

• To investigate the relationship between forest stand structure, climate variables, and quaking aspen mortality.
• To determine how forest stand characteristics mediate the effects of climate on aspen decline in the Western United States.

Main Methods:

• Utilized data from 841 forest inventory plots across the Rocky Mountains and Intermountain Western United States.
• Assessed recent aspen mortality using the proportion of dead aspen trees as an index.
• Correlated mortality indices with forest structure metrics (density, basal area, diameter, productivity) and climate data (vapor pressure deficit, winter precipitation).

Main Results:

• Forest structure was the primary driver of mortality patterns, explaining a larger proportion of variation than climate.
• Growing-season vapor pressure deficit and winter precipitation over the past 20 years were significant climate factors.
• Mortality sensitivity to precipitation increased with aspen density and size, while sensitivity to vapor pressure deficit was higher in young stands.

Conclusions:

• Forest stand development, competition from encroaching conifers, and tree physiological traits influence drought-induced mortality.
• Understanding forest succession is crucial for accurately assessing the causes and consequences of sudden aspen decline.
• Integrating stand-scale factors with climate data provides a more comprehensive view of forest ecosystem dynamics.