The metabolic 'toolkits' of temperate trees are species-specific and vary little with modest soil moisture variability.

During the process of photosynthesis, leaves capture CO₂ from the atmosphere and rapidly convert it into a diverse array of primary and secondary metabolites. Plants maintain a core set of metabolic pathways that ensure the basic building blocks of life that are available for each plant species to function. However, as plants evolved on land, they began to allocate this carbon (C) to innovative secondary metabolites and organs (cuticles, roots, wood) that protected them from abiotic stress (UV radiation, aridity, freezing) and biotic attack (fungal pathogens and insect/animal herbivory). As plants expanded over the land surface and occupied different niches, the amount of C fixed by plants varied and the types of secondary compounds synthesised by plants began to differ. Little is known about the metabolic profiles of the dominant European tree species and how variable the metabolomes of individual tree species are to changes in site conditions such as nutrient availability or soil moisture status. This study took advantage of recent advances in high-resolution mass spectrometry (HRMS) and bioinformatic tools to compare the leaf metabolomes of 14 commercially important tree species grown across three common gardens. Alongside the metabolic profiles, important chemical and morphological data were also collected from the trees during sampling, including targeted analysis of specific leaf metabolites such as proteins and phenolic compounds to obtain quantitative information on how their concentrations varied between tree species and site. Our analysis showed that the metabolomes of each tree species statistically differ from one another, and this dissimilarity was highly conserved at all three sites, even though tree growth and mortality rates varied between species and site. Our analysis also clearly highlighted distinct metabolome shifts between angiosperm and gymnosperm tree species, with angiosperms displaying greater concentrations of chlorophyll and amino acids alongside lower C/N ratios. These differences were also accompanied by discrepancies in an important set of secondary metabolites detected with the metabolomic technique. Furthermore, we also found that certain secondary compounds were essential in distinguishing between deciduous and evergreen species or families where targeted analysis could not detect significant differences. Our results indicate that temperate tree species may have conserved chemical 'fingerprints' that provide information on fundamental differences in the activity of certain plant metabolic pathways. This thus provides a promising tool to investigate how and why different plant species allocate C differently over the growing season and defend themselves against diverse abiotic and biotic pressures.