The interaction between trees’ water needs during drought and the signals that appear in their canopies is not fully understood. The first visually detectable signs, which we describe as early warning signals in tree canopies, are often not noticeable at first glance. When these signs become widely apparent, tree decline is already underway. In this study, we focus on identifying early visible signs of drought stress in the tree crowns, such as very small leaves, premature needle/leaf discolouration and abscission, and defoliation. We provide guidance on recognising initial signs, offer specific examples, and comprehensively analyse each signal. Our focus is on signs in the tree crowns that appear during intense and prolonged droughts, which we confirmed by calculating the Standardised Precipitation Evapotranspiration Index (SPEI). Our findings are based on 20 years (2004–2024) of continuous fieldwork and data collection from permanent sample plots in Serbia, which was conducted as part of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). We also conducted a comprehensive review of the literature and key findings related to the early signs we address. This research was further motivated by the signs observed in the tree crowns during the summer of 2024 due to extreme climatic events, which classify this year as one of the hottest recorded in Serbia. However, we still cannot conclusively determine which specific trees will die back based solely on these early warning signals, as some trees manage to withstand severe drought conditions. Nonetheless, the widespread appearance of these indicators is a clear warning of significant ecosystem instability, potentially leading to the decline of individual trees or larger groups.

Primary parasitism, endoparasite could be solitary and gregarious, as so ectoparasitic. Ectoparasites could be hyperparasitoid, superparasite, then those are: 1) polymorphic, panmictic populations; from beginning: wasps Dinarmus acutus (Chalc.: Pteromalidae) (called Oedaulae sp. at 2006 when study begun) and Eupelmus urosonus as well as Eupelmus vesicularis (Chalc.: Eupelmidae). 2) further reduction of gene flow between host races until the diverging populations, sympatric and panmictic. The prey-larvae system. These are the ones that make the final circle, the smallest: family: Eulophids (Tetrastichus sp. and Aprostocetus sp.). That would be: i) Larvae of seed beetle in the Legume pod ii) Pteromalid or Eupelmids- primary ones iii) On theirs’s larvae there’s hyper and supers: Eurytomid and Braconid. iv) Theirs solitary ecto’s are one of those: Tetrastichus sp. and Aprostocetus sp.; they are also like other pairs almost symbionts, when you look at some circumstances: Adults leave seed-pods in the same decades, March and April, in the spring, and September-October-November, in the fall. So, Eurytomidae: Chalcids are not herbivores, like superparasitoids (Braconidae: Hymenoptera) – all circle is made of primary, secondary, tertiary, quaternary - 4+1 = 2+2+1= 5 (five ranges of parasitoids in eco-niches). But nothing is sure as an event which is not obligatory in these systems.