Dear Colleagues,

Due to your high interest and the large number of papers published in Volume I, we are opening the second volume of this Special Issue.

The initial SI framework, as defined by the Volume I title and suggested topics of interest, has been broadened thanks to your valuable submissions, adding terms such as heavy metal-induced stress, carbon isotopes, water use efficiency, polyamines, antioxidants, or plant hormones to the list of subjects, which already included photosynthetic activity and other biochemical stress indicators; nutrients in different tree compartments; tree growth; tree leaf loss and mortality; visible symptoms of stress in foliage; and microscopical markers of stress. However, the initial concerns that instigate the use of indicators as means of measuring tree stress response are still with us, and will continue to cause a loss of tree vitality in the foreseeable future.

The changing climate and air pollution are among the greatest threats to the health and functioning of forest ecosystems, jeopardizing their ecological and economic functions and services. The impact of increasing temperatures and extreme weather events (droughts, storms, and temperature and precipitation extremes) on the vitality of forest trees is often difficult to separate from the impact of nitrogen deposition and tropospheric ozone, as they can exhibit synergistic effects. For example, forest soil acidification, atmospheric N deposition, and climate change are all partly responsible for the continuous decrease in foliar P concentrations in Europe, causing reduced tree growth.

A better understanding of physiological processes influencing tree vitality under the changing climate and air pollution pressures requires considerable research efforts and constant advancements in research methods and approaches.

The use of indicators is elementary in modern forest ecophysiology research, as they help us estimate the level of damage to trees and forest ecosystems.

For this Special Issue, we welcome original research papers dealing with ecophysiological indicators of the response of forest trees to environmental stress. Examples of such indicators are photosynthetic activity and other biochemical stress indicators; nutrients in different tree compartments; leaf loss, tree growth, and tree mortality; visible symptoms of stress in foliage; and microscopical markers of stress. We also welcome reviews of recent advances in forest ecophysiology. Thank you for your continued interest!

Dr. Nenad Potočić
Guest Editor

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• climate change
• air pollution
• dieback
• leaf injury
• growth
• water and nutrient uptake
• photosynthesis
• oxidative stress

Title: How Does Mistletoe Drive the Host’s Growth, Storage, and Water Use Efficiency? Response of the Scots Pine Growing in Mesotrophic Conditions
Authors: Radosław Jagiełło ¹; Janusz Olejnik ²; Marek Urbaniak ²; Kaludia Ziembińska ²; Paweł Szmyt ¹; Henrik Hartmann ³
Affiliation: ¹ Faculty of Forestry an Wood Technology, Poznań University of Life Sciences; ² Faculty of Environmental and Mechanical Engineering, Poznań University of Life Sciences; _³ Max Planck Institute for Biogeochemistry
Abstract: Hemiparasitic xylem-typing pine mistletoe attracts considerable interest because it spreads rapidly across Europe. Droughts, generally regarded as the primary cause of tree decline, are heightened under the additional impact of hemiparasite exacerbating problems with hydraulics and nutrition. This work aims to contribute to our knowledge of host growth, storage and water relations in response to mistletoe infestation. Thus we determined growth dynamics, concentration of non-structural carbohydrates and carbon isotope ratio (δ13C) in tree rings to estimate ecophysiological parameters related to tree vitality. We observed a sharp difference in growth between mistletoe-infested and healthy trees. We observed significantly higher levels of starch in the outermost rings of infested trees. No noteworthy differences were found for intrinsic water-use efficiency (iWUE). Although, there has been an apparent increasing trend of iWUE over the last fifteen years, indicating general water limitation to pine trees in the studied region. We indicated that downregulation in growth is a stress indicator of great importance. Our findings suggest that while grappling with water and nutrients leak, trees may prioritise carbon allocation to storage and keep stable iWUE or cannot use stored energy to grow due to water and nutrient limitations.

Title: Wood Production and Plasticity in Q.ilex and A.unedo Seedlings Subjected to Prolonged Drought and Simulated Heat Wave
Authors: Angela Balzano
Affiliation: Department of Wood Science and Technology, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva Ulica 101, SI-1000 Ljubljana, Slovenia

Title: Long-Term Monitored Norway Spruce Plots in the Ore Mts. – How Much Time Does Forest Regeneration After Heavy Air Pollution Calamity Take?
Authors: Radek Novotný; Věra Fadrhonsová; Vít Šrámek
Affiliation: Forestry and Game Management Research Institute, Strnady 136, 25202 Jíloviště, Czech Republic
Abstract: Region of so called “Black Triangle” on the border between Germany, Poland and Czech Republic belonged historically to the most air polluted areas in Europe. Brown coal combusting power plants were the main source of pollution (sulphur dioxide, fluorine, dust), which resulted in severe damage to trees and forest decline over large areas. Sulphur emissions and deposition had increased steadily during 20th century with the peak in the end of 1970ies and beginning of 1980ies, when sulphur bulk deposition load reached even more than 100 kg.ha-1.year-1. In this paper we evaluate almost 30 years long period of monitoring of Norway spruce plots in the Ore Mts., during which the health state, nutrition and soil chemistry has been evaluated. While air quality improved quickly after desulfurization of main pollution sources, soil chemistry and nutrition level are changing very slowly. Forest soil still remains acidified and base saturation of sorption complex is still very low despite liming which has been applied in some areas of the Ore Mts. Slight improvement is visible in upper soil layers, mainly in organic FH horizon. On the other hand, the health state assessed as crown defoliation and height increment as well as nutrition level of Norway spruce has improved during last three decades and forest stands in this region are regarded as stable. Thus, the quality of forest soils and its development is very important factor which can be one of limits to stability of forest ecosystems within this area.