Effects of Elevated Air Humidity and Soil Moisture on Carbon-Based Secondary Metabolites in Leaves and Fine Roots of Silver Birch Based on Tree Competitive Status

Plants being sessile organisms, cannot escape the changing climatic condition in which they live. Thus, they have to withstand various biotic and abiotic stresses during their life cycles. However, plants have evolved several defences against stresses including chemical defence, by producing compounds called "secondary metabolites." Plants allocate the available carbon either to growth, storage, or defence, and often trade-off occurs in allocating the carbon to growth or defence. The knowledge of plant defence responses against biotic stress such as competition and abiotic stress such as elevated air humidity and soil moisture, through alterations in secondary metabolites production, is still incomplete. We aimed to clarify the combined effect of changing climatic conditions projected for northern Europe (increasing precipitation), tree's competitive status, and light availability on leaf secondary metabolites (LSM) and fine-root secondary metabolites (RSM) of 8-year-old silver birch (Betula pendula Roth). The study was conducted at the Free Air Humidity Manipulation (FAHM) experimental site in Estonia, where relative air humidity and soil moisture are elevated. The leaf and fine-roots were sampled from the site, and secondary metabolites were analysed using a high-performance liquid chromatography-quadrupole-time of flight mass spectrometer (HPLC-qTOF-MS). Our result showed that exposure to irradiance (leaf position) strongly affected most LSM classes, whereas competitive status affected some LSM and RSM classes. The FAHM treatments had a more pronounced effect on RSM than on LSM. The content of RSM was lower in elevated air humidity and soil moisture conditions compared to ambient conditions (control). Redundancy Analysis (RDA) indicated an increase in the RSM content towards competitively suppressed trees. Carbon allocation to aboveground growth was similar across the treatments, but growth relationships with LSM and RSM classes varied among treatments. Our study suggests that young silver birches will allocate similar amounts of carbon to constitutive chemical leaf defence but less to root defence when environmental humidity increases.