While the importance of old trees and ancient forests as reservoirs of carbon is well established, their capacity to respond to changing climate and rising atmospheric carbon dioxide is poorly understood. We used a combined dendrochronological and tree mapping approach to reconstruct whole-tree wood production for the past two centuries for individual redwood (Sequoia sempervirens) trees in a range of height and size classes in the tallest known forest, at Humboldt Redwoods State Park, California. We used the historical climate record to examine the influence on tree growth of climate variables and CO2. We tested the role of CO2 by regressing the magnitude of growth response in paired dry or wet years against the difference in CO2 concentration for the same pair of years.
Individual tree wood volume growth was greatest in the largest trees. When scaled to the stand level, wood production has increased by at least 15% from the first half of the 19th century to the recent half century, with the most pronounced increase occurring since 1960. Variation in atmospheric CO2 explained 65% of the growth difference between paired recent and past dry years, and 50% of the growth difference between paired recent and past wet years. Consistent with elements of increasing hydraulic limitation in taller trees, the relative CO2 enhancement of growth increased with tree height and was greater in dry years than wet years. Our results demonstrate that extremely large and old trees continue to grow rapidly and retain high growth sensitivity to external factors.