Abstract
We investigated seasonal variations in δ13C values and concentrations of carbohydrates and organic acids in needles of declining and healthy mountain pine (Pinus mugo ssp. uncinata (DC.) Domin) trees from the Swiss National Park (SNP), using compound-specific isotopes analysis (CSIA). Our goal was to study the impact of climatic drivers on the individual compounds and understand the reasons of partial tree declines in relation to healthy mountain pine trees under seasonal weather patterns. We found that temperature is the main climatic driver determining the seasonal carbon dynamics at the needle level. Lower seasonal δ13C variability and lower concentration levels of sucrose in needles suggest less photosynthetic activity and sink carbon demand in declining compared to healthy mountain pine trees. Higher concentration levels of hexose (glucose and fructose) can play a reserve function for surviving mechanisms of mountain pine trees. Seasonal patterns of organic acid (malate and citrate) suggest an increasing investment in maintenance and repair mechanisms. The seasonal course of carbohydrates and organic acids can therefore be considered an indicator for a modified carbon metabolism within the leaves and possibly within the other tree tissues, partially explaining the decline of mountain pine trees.
Highlights
Many forest ecosystems are declining globally due to increasing heat and drought-induced tree mortality [1,2,3,4,5]
We applied unpaired t-test analysis to reveal whether δ13 C values and concentrations of carbohydrates and organic acids differ between aspects and between declining and healthy mountain pine trees
We merged the data from both aspects for each individual compound separately (n = 8) (Figure 3a,c,e,g). δ13 C of individual soluble carbohydrates in needles of declining mountain pine trees (Figure 3a) showed less clear seasonal patterns compared to healthy mountain pine trees (Figure 3b)
Summary
Many forest ecosystems are declining globally due to increasing heat and drought-induced tree mortality [1,2,3,4,5]. While δ13 C values and concentrations of individual sugars may respond to climatic changes, they are only an indirect indicator of specific metabolic responses Organic acids, such as citrate and malate, can function as carbon storage, they are known to support various other physiological functions in plant metabolism such as respiration, stomatal regulation, nutrient transport, pH adjustments in plant and soils, and root-soil interactions [20,25,29]. We present a first study of seasonal variations in δ13 C values and concentrations of individual carbohydrates (sucrose, glucose, fructose and sugar alcohol—pinitol) and organic acids (malate and citrate) in needles of declining and healthy mountain pine trees from the SNP. We (1) investigated if δ13 C values and concentrations of individual compounds in declining mountain pine trees responded differently over the growing season to infer on potential physiological and metabolic changes; (2) compared the compound responses in declining with healthy mountain pine trees from the same site to better understand which specific compound in needles could be indicative of tree decline; and (3) determined if potential environmental drivers (i.e., air temperature and vapor pressure deficit) are related to changes in δ13 C and concentrations in individual compounds
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