Gas exchange of the lowest branches of young Scots pine: a cost-benefit analysis of seasonal branch carbon budget.

A cost-benefit approach was developed to analyze the carbon budget of the lowest Scots pine (Pinus sylvestris L.) branches subject to abscission. In addition to within-branch growth and respiratory costs, the budget included an estimation of a branch's share of the maintenance respiration of the stem and root. A branch was considered productive if the budget was positive. Foliar gas exchange and woody-tissue respiration were non-destructively measured at monthly intervals during the growing season on the six lowest branches of 10-year-old Scots pine trees, to the moment when the branches died naturally. Photosynthetic light response and temperature response of respiration, together with measurements of canopy light conditions and meteorological data, were used to calculate seasonal carbon budgets for the branches. Maintenance respiration of stems and roots was estimated from published data. All but one of the branches studied were found to be nonproductive over the growing season. Following a decrease in photosynthetic capacity in July, the cumulative budget became negative and the branches died, indicating that a negative carbon budget corresponds with the onset of abscission of the lowest branches.