Professor Northern Arizona University, United States
Nonstructural carbon can be many years old, and the remobilization of tree nonstructural carbon represents one potential mechanism of physiological resilience to stress and disturbance. In August 2020, the CZU Lightning Complex fires started following weather that produced 11,000 lightning strikes throughout California. This fire complex burned into Big Basin Redwoods State Park, containing one of the largest continuous stands of old-growth redwoods in the world. Coast redwood (Sequoia sempervirens) is extremely fire adapted, and surviving trees subsequently produced new leaf area through massive epicormic re-sprouting. This process is supported by nonstructural carbon, the age (time-since-fixation) of which can be estimated by comparing its radiocarbon (14C) signature to the record of the atmospheric 14C bomb-spike. Nonstructural carbon is a mixed pool, and so this age estimate represents the mean age of a given pool (e.g. sapwood nonstructural carbon). In the year following the fire, we covered portions of burned stems to exclude light and thus collected epicormic sprout samples grown in the dark. We also collected CO2 respired from increment cores. We then used 14C to estimate the mean age of the nonstructural carbon reserves that were being drawn on for regrowth and cellular respiration.
We sampled 18 trees from 77 – 457 cm DBH showing varying degrees of post-fire vigor, including individuals with over 100 sapwood tree rings. ∆14C of re-sprout carbon ranged from ~0 – 40 ‰, representing ages of roughly 0-15 years. Around 40% of this variation in sprout carbon age could be explained by the age of nonstructural carbon in adjacent phloem tissue (p< 0.01), which also contained nonstructural carbon up to ~15 years old in some trees. Large variability in the observed ∆14C of nonstructural carbon in the sapwood and re-sprout carbon likely arose from the large range of tree sizes and ages we sampled, where larger trees also had older phloem nonstructural carbon (p< 0.01, R2=0.49). To understand the large variation in sapwood nonstructural carbon ∆14C, we fit an allocation model to study how past mixing led to the observed ∆14C. Sapwood nonstructural carbon of deep sapwood tree rings in some individuals contained pre-bomb carbon (assimilated before peak of the bomb spike in 1963), more than 60 years old. Despite this, carbon allocated to sprouts was younger (0-15 years). These redwood trees’ nonstructural carbon pools are very large, and perhaps exceed the demand for re-sprouting.