While researching a topic, a student has taken the following notes:

• In North America, woodlands have expanded into areas that were once grasslands.
• Thomas Rogers and F. Leland Russell of Wichita State University investigated whether woodland expansion is related to changes in climate.
• Rogers and Russell analyzed core samples from oak trees on a site that was not wooded in the past and indexed the age of the trees with historical climate data to see if tree populations and climate were correlated.
• Tree population growth was associated with dry intervals.
• Droughts may have played a role in woodland expansion.