Abstract
Plant secondary metabolites have many important functions; they also determine the productivity and resilience of trees under climate change. The effects of environmental factors on secondary metabolites are much better understood in above-ground than in below-ground part of the tree. Competition is a crucial biotic stress factor, but little is known about the interaction effect of climate and competition on the secondary chemistry of trees. Moreover, competition effect is usually overlooked when analyzing the sources of variation in the secondary chemistry. Our aim was to clarify the effects of competitive status, within-crown light environment, and climate on the secondary chemistry of silver birch (Betula pendula Roth). We sampled leaves (from upper and lower crown) and fine roots from competitively dominant and suppressed B. pendula trees in plantations along a latitudinal gradient (56–67° N) in Fennoscandia, with mean annual temperature (MAT) range: −1 to 8°C. Secondary metabolites in leaves (SML) and fine roots (SMFR) were determined with an HPLC-qTOF mass spectrometer. We found that SML content increased significantly with MAT. The effect of competitive stress on SML strengthened in colder climates (MAT<4°C). Competition and shade initiated a few similar responses in SML. SMFR varied less with MAT. Suppressed trees allocated relatively more resources to SML in warmer climates and to SMFR in colder ones. Our study revealed that the content and profile of secondary metabolites (mostly phenolic defense compounds and growth regulators) in leaves of B. pendula varied with climate and reflected the trees’ defense requirements against herbivory, exposure to irradiance, and competitive status (resource supply). The metabolic profile of fine roots reflected, besides defense requirements, also different below-ground competition strategies in warmer and colder climates. An increase in carbon assimilation to secondary compounds can be expected at northern latitudes due to climate change.
Highlights
Plants’ ability to cope with global changes and withstand environmental stresses is strongly related to their production and accumulation of secondary metabolites
We found evidence supporting our research hypotheses: Both Secondary metabolites in leaves (SML) and Secondary metabolites in fine roots (SMFR) decreased northward with latitude; shade from over-topping neighbors and from lower canopy position initiated some similar responses in SML; and the relative allocation of C-based secondary metabolites in roots vs. leaves increased with latitude, especially in competitively suppressed trees
We found that individual leaf blade area and specific leaf area (SLA) decreased with mean annual temperature (MAT), especially in suppressed trees, while individual leaf weight was related to canopy position and a tree’s competitive status
Summary
Plants’ ability to cope with global changes and withstand environmental stresses is strongly related to their production and accumulation of secondary metabolites. The general ecological and evolutionary trends in plant secondary chemistry are explained by several theories, including the carbon/nutrient balance (CNB) hypothesis (Bryant et al, 1983) and the growth–differentiation balance (GDB) hypothesis (Herms and Mattson, 1992) These theories are often debated (Hamilton et al, 2001; Close and McArthur, 2002; Massad et al, 2014) and do not always reflect the responses of different metabolic pathways and individual compounds (Koricheva et al, 1998; Keinänen et al, 1999). In some environments or plant developmental stages, more resources can be invested in secondary metabolism without a direct cost on growth (Stamp, 2004; Massad et al, 2014)
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