Differential effect of warming and drought on the production-resorption dynamics of metabolites in temperate trees: Implications for carbon cycling
The process of nutrient resorption from plant leaves during senescence subsequently affects both plant productivity and soil nutrient cycling; changes in either of these could potentially feed back to climate change. Although elemental nutrient resorption has been shown to respond modestly to temperature and precipitation, we know remarkably little about the influence of increasing intensities of drought and warming on the resorption of different classes of plant metabolites. We studied the effect of a factorial combination of warming (+≤4oC) and altered precipitation [ambient, dry (-50%) wet (+50%)] on the production and resorption of metabolites in Quercus rubra, a dominant tree species across North American temperate forest ecosystems. The green and senesced leaf tissues were analyzed for both polar and non-polar metabolites using gas chromatography mass spectrometry platforms. We also analyzed the influence of climatic factors on total elemental nitrogen and the extractable and non-extractable proteins in leaf tissues.
Green tissues of trees exposed to combined warming and drought had more amino acids, organic acids and sugars, which can function as osmoregulators and antioxidants, helping to mitigate climatic stress. Surprisingly, drought and warming had opposite effects on the resorption efficiency (RE) of extractable metabolites. However, RE of total N differed markedly from that of extractable metabolites; for instance, under drought, the RE of N was negligible, but that of amino acids exceeded 90%. Thus, leaves that grow and senesce in drier conditions may have a higher elemental N content, but soil heterotrophs may be able to access a smaller proportion of the N in the resultant leaf litter due to the extended resorption of labile nitrogenous compounds. These results will be discussed in further detail elucidating the effects of tissue polyphenols produced under climatic stress that could interfere with N resorption through complexation reactions. Our results suggest that N resorption may be controlled not only by plant demand, but also by climate, through the regulation of metabolic processes during senescence. As climate-carbon models incorporate increasingly sophisticated nutrient cycles, these results highlight the need to adequately understand nutrient cycling responses to climatic variables.