OOS 56-3 - Ecophysiological drivers of the mixing of new and old reserve carbon in tree sapwood

Tree nonstructural carbon (NSC) can be decades old, and remobilization of old NSC represents one physiological mechanism of resilience. In mature trees, decades-old NSC is often present in deep sapwood, but the process by which reserves are built up over time, and the “mixing” of new reserves into existing pools are poorly understood. We asked: how deeply do trees mix new reserve carbon? Is mixing depth constant across time? If not, how does stress such as drought and wildfire influence mixing depth?

We synthesized published and unpublished ∆14C measurements (n=280) of tree sapwood carbon reserves in temperate trees. This dataset represents 52 trees of 7 species (Populus tremuloides, Acer rubrum, Fraxinus americana, Pinus edulis, Pinus strobus, Sequoia sempervirens, and Quercus rubra,), though only one tree each of P. strobus and F. americana. This includes trees in varying states of either experimental or natural drought stress, severe wildfire recovery, or terminal decline following lethal bark beetle (Ips confusus) attack. We measured both the age of NSC (∆14C ) by reference to the atmospheric bomb-spike as well as NSC concentrations. We then apply a ring-resolved mixing model to simulate the patterns we observed.

We found mixing of young NSC ( <5 years old) was generally shallow, occupying the youngest 6-15 sapwood rings across species. The shallowest mixing depths were found in eastern trees in Q. rubra (6 3 rings) and A. rubrum (6 4 rings); the deepest was P. edulis (15  8 rings). However there was large among-tree variability, where some very old S. sempervirens mixed young NSC to depths exceeding 20 rings. Severe stress from fire or beetle attack increased the age of NSC in some rings by more than 15 years, likely through exhaustion of younger reserves. In contrast, long term but moderate drought reduced the age remaining by 5-10 years, perhaps through sustained exhaustion of older reserves and/or coincident failure to replace them.

This synthesis suggest a “rule” of carbon reserves, where the oldest NSC is only remobilized under greatest stress and disturbance. They also suggest mixing depth is temporally dynamic, reflecting overflow of carbon reserves into deep sapwood when conditions are favorable, and limited mixing and/or exhaustion when conditions are unfavorable. We suggest ∆14C measurements enable unique inference on the turnover time of carbon reserves over long time periods in response to stress.