New approaches for monitoring forest functioning in a changing climate - crossing scales from molecular mechanisms to stand wide processes

Long-term forest monitoring has a long tradition and provides important information for science-based decisions in forest management and policy. Until recently, mainly classical measurements of e.g., radial growth increment, defoliation and mortality have been performed and they are and in future will be core tools for describing forest vitality and functioning. However, new tools are now available that might provide additional information on mechanisms and environmental cause-effect relationships, and I will here give examples spanning from metabolomics via close-to-real-time assessments of growth and water relations to drone-based remote sensing. These methods help to understand the mechanisms which lead to impaired forest functioning and to inform forest adaptive management. We linked assessments of the leaf and root metabolic profile in Scots pine to defoliation and observed homoeostatic levels across a wide range of defoliation classes. Only at very strong defoliation, levels of metabolites related to defence, oxidative stress, osmoregulation and energy supply strongly decreased in the roots but where upregulated in needles indicating a negligence of root functioning at the expense of aboveground tissues. Close-to-real-time assessments with point dendrometers allow to dynamically monitor growth and tree water deficit responses to atmospheric and soil drought complementing annual stem diameter measurements. Species-specific risk assessments can be made and the seasonality of specific environmental factors impairing functioning can be quantified. In combination with xylem flow sensors and in situ water isotopologue monitoring systems, the origin of water taken up by trees can be quantified. With such methods it is possible to assess if trees can switch to deeper soil water resources during drought and how fast they recover their water use after a drought event. With ground-based monitoring it is sometimes not easy to see the forest for the trees. Drone based proximate sensing allows to receive overhead imagery of the crowns and with wavelength-specific sensors reflectance indices can be determined that show stress signals before any classical visual assessment. Drone-based imagery can thus provide early warning signals as well as a link to satellite remote sensing. Such new suites of methods combined with classical forest monitoring and satellite remote sensing are able to detect early warning signals of loss of forest functioning and might allow to determine areas at particular risk, where adaptive forest management needs to be implemented with high priority.