Tree crown defoliation as an early warning signal of increased mortality risk

Crown condition is considered as one of the most important indicators of a tree’s vitality. As part of the monitoring program Long-term Forest Ecosystem Research, the defoliation of tree crowns in Swiss forests has been monitored on an annual time scale since 1985. This long-term data set makes it possible to track the progress of defoliation until the trees die and to take into account a variety of stress factors that may have played a role in this process.

In Swiss forests, the average defoliation of trees and tree mortality has increased in the past decades. However, this only occurred in areas at lower altitude, where climate change has particularly intensified the atmospheric water demand. The importance of water stress as a driver for this development is also confirmed by some of the highest annual increases of defoliation that directly followed exceptionally dry and hot summers.

The probability that individual trees die within a few years starts to increase when the crown defoliation exceeds about 30%. Around 75-85%, most trees seem to reach a point of no return, from which they cannot recover, and which leads to death within a few years, even if no further stress occurs. In the needles of such strongly defoliated Scots pines (Pinus sylvestris L.), we found elevated levels of many stress-related metabolites (particularly osmoprotectants, defense compounds and antioxidants), whereas the levels of these metabolites were homeostatic in the needles of trees in lower defoliation classes. In contrast to the needles, these metabolites were reduced in fine roots of the strongly defoliated trees, suggesting that mainly belowground carbon starvation may impair key functions for tree survival, consequently leading to early death.