Impacts of Phloem Chilling on Mature Red Maples at Harvard Forest 2019

Whether tree growth is limited by carbon supply or demand is a crucial question due to wide-ranging repercussions for projections of carbon sequestration on land. By temporarily restricting phloem transport using stem chilling, which increases phloem sap viscosity to create local bottlenecks to phloem transport, we created gradients of carbon supply in stems of mature red maples during the first half of the growing season. These carbon supply gradients had clear effects on tree physiology with radial growth in particular varying up to almost seven-fold with carbon supply. Local bulk nonstructural carbon concentrations in stems and roots remained relatively stable, suggesting that they are not rapidly modulated in response to changes in supply and demand. However, phloem and leaf nonstructural carbon accumulated above chilling-induced bottlenecks and were associated with reductions in photosynthetic capacity as well as the advancement of leaf coloration and fall, supporting the idea of within-tree feedbacks reducing carbon supply when supply exceeds demand. Most strikingly, radial growth varied systematically with carbon supply up to almost seven-fold, indicating that growth of red maple during the early growing season is strongly carbon-supply limited. The code to process these data and reproduce our results is available at https://github.com/TTRademacher/Exp2019Analysis. For more details pertaining to the methods see Rademacher et al. (2021) and contact the investigator.