Forest monitoring updates: physiological indicators of tree health for detecting and understanding a changing environment

Crown defoliation is the main parameter used to assess forest tree health. It has been adopted as indicator of tree vitality since the beginning of forest monitoring in Europe, in the ‘80s of the past century. In Italy defoliation assessment, carried out since 1996 in the national monitoring forest plot network, has made available data series on the health of the national forests, providing a robust representation over time and space. Recently, defoliation was found to be a sensitive indicator of impacts of climatic anomalies, such as recurrent harsh droughts, summer heat waves, wind storm and other abiotic disturbances, such us air pollution and atmospheric depositions, as well as biotic attacks (for ex., bark beetle in Picea abies (L.) Karst. stands), concomitant or consequential to severe climatic events. Events of this type have become frequent, intense and geographycally widespread. The national extensive forest monitoring network, with 260 permanent plots (5000 trees) distributed in all Italian regions, representative of the main forest types, allowed to know the extent and intensity of the damage (i.e. defoliation) by climatic events and other environmental factors, and the subsequent recovery processes put in place by some trees (and species) after the damage (i.e., reduction of the defoliation). In view of the fast climate changes, with increasing frequency of extreme events, and with short time for physiological recovery of trees, the need to consider the physiology of trees in forest health assessment, so in monitoring programmes, has emerged. The effects of defoliation on plant physiology play a central role in the tree-environment interactions and in plant species responses to climate changes. New indicators of tree health based on plant physiology, as photosynthetic efficiency, analysed by chlorophyll fluorescence, chlorophyll content, leaf functional traits, carbon reserves (i.e. non structural carbohydrates), tree growth and annual growth dynamic, measured by tree ring, are essential to evaluate functionalities and vitality of trees, the resilience of forest species to environmental changes and their recovery after a disturbance and damage. Several of these indicators can be assessed in specific cases of forest disturbances, for specific species or in particular environmental contexts. Some of them can act as markers of physiological damage suffered by plant, highlighting the severity of the damage and the risk of plant death. The extensive forest monitoring newtwork is an essential tool to know the health state of the Italian forests, to assess the entity of possible chages in species composition and community structure and functioning induced by climate impacts and other environmental factors. The adoption of physiological indicators, combined with the traditional ones based on visual assessment of tree condition, would make the monitoring system updated and with a greater efficiency to evaluate and monitor forest health in long time.