Decomposing the mid-Holocene Tsuga decline in eastern North America

Area of Science:

• Paleoecology
• Quaternary Science
• North American Flora

Background:

• The mid-Holocene decline of Tsuga canadensis (hemlock) in eastern North America is often considered a synchronous event.
• Previous research faced limitations due to low-resolution dating and lack of paired paleoclimate and vegetation records.

Purpose of the Study:

• To reconstruct a high-resolution, precisely dated record of Tsuga decline and associated environmental changes.
• To investigate the synchronicity and potential drivers of the Tsuga decline across its range.

Main Methods:

• Intensive pollen analysis (1-cm intervals) and testate amoebae analysis for water table depth from a single sediment core at Irwin Smith Bog, Lower Michigan.
• High-precision chronology using radiometric dating.
• Intensive pollen analysis from a second core at Tower Lake, Upper Michigan.

Main Results:

• The terminal Tsuga decline at both Michigan sites is dated to approximately 5000 cal yr BP, ~400 years later than previously estimated.
• A transient decline around 5350 cal yr BP may correlate with declines at other eastern North American sites.
• At Irwin Smith Bog, the hemlock decline preceded a significant drop in water table depth by ~200 years, decoupling it from abrupt climate change.

Conclusions:

• The terminal Tsuga decline was heterochronous across its range.
• The decline was not directly triggered by abrupt climate change at Irwin Smith Bog.
• The Tsuga decline likely involved multiple phases and scales, requiring further high-resolution paleoclimate and vegetation studies for full understanding.