Can we set a global threshold age to define mature forests?

Area of Science:

• Ecology
• Forestry
• Climate Science

Background:

• Global mature forests show increasing biomass density (BD), with debate on causes like CO2 rise versus land-use legacy.
• Previous studies suggested a 450-year threshold for mature forests but overlooked climate-age interactions.

Purpose of the Study:

• To determine how climate and forest age influence global forest BD.
• To assess if a universal threshold age for forest maturity can be established.

Main Methods:

• Utilized data from published studies to model forest BD using linear mixed-effects models.
• Analyzed dataset representativeness concerning global mature forests, climate space, and forest ages.
• Investigated biases in the dataset, particularly its northern hemisphere and young forest focus.

Main Results:

• Forest BD positively correlated with forest age, mean annual temperature, and annual precipitation.
• The influence of forest age on BD intensified with higher temperatures, while precipitation's effect diminished.
• The dataset exhibited biases towards cooler, drier northern hemisphere forests and those under 250 years old.

Conclusions:

• A single, universal threshold age for forest maturity is not supported; it is locally determined by climate-age interactions.
• Biomass density alone is insufficient to define forest maturity, as biodiversity and tree structure recovery times vary.
• Further research is needed on cost-effective methods for classifying forest maturity.