Plants store large amounts of non‐structural carbohydrates (NSC). While multiple functions ofNSChave long been recognized, the interpretation ofNSCseasonal dynamics is often based on the idea that storedNSCis a reservoir of carbon that fluctuates depending on the balance between supply via photosynthesis and demand for growth and respiration (the source–sink dynamics concept). Consequently, relatively highNSCconcentrations in some plants have been interpreted to reflect excess supply relative to demand. An alternative view, however, is thatNSCaccumulation reflects the relatively highNSClevels required for plant survival; an important issue that remains highly controversial. Here, we assembled a new global database to examine broad patterns of seasonalNSCvariation across organs (leaves, stems, and belowground), plant functional types (coniferous, drought‐deciduous angiosperms, winter deciduous angiosperms, evergreen angiosperms, and herbaceous) and biomes (boreal, temperate, Mediterranean, and tropical). We compiled data from 121 studies, including seasonal measurements for 177 species under natural conditions. Our results showed that, on average,NSCaccount for ~10% of dry plant biomass and are highest in leaves and lowest in stems, whereas belowground organs show intermediate concentrations. TotalNSC, starch, and soluble sugars (SS) varied seasonally, with a strong depletion of starch during the growing season and a general increase during winter months, particularly in boreal and temperate biomes. Across functional types,NSCconcentrations were highest and most variable in herbaceous species and in conifer needles. Conifers showed the lowest stem and belowgroundNSCconcentrations. MinimumNSCvalues were relatively high (46% of seasonal maximums on average for totalNSC) and, in contrast to average values, were similar among biomes and functional types. Overall, although starch depletion was relatively common, seasonal depletion of totalNSCorSSwas rare. These results are consistent with a dual view ofNSCfunction: whereas starch acts mostly as a reservoir for future use, soluble sugars perform immediate functions (e.g., osmoregulation) and are kept above some critical threshold. If confirmed, this dual function ofNSCwill have important implications for the way we understand and model plant carbon allocation and survival under stress.

The current carbon supply status of temperate forest trees was assessed by analysing the seasonal variation of non‐structural carbohydrate (NSC) concentrations in leaves, branch wood and stem sapwood of 10 tree species (six deciduous broad‐leafed, one deciduous conifer and three evergreen conifer trees) in a temperate forest that is approximately 100 years old. In addition, all woody tissue was analysed for lipids (acylglycerols). The major NSC fractions were starch, sucrose, glucose and fructose, with other carbohydrates (e.g. raffinose and stachyose) and sugar alcohols (cyclitols and sorbitol) playing only a minor quantitative role. The radial distribution of NSC within entire stem cores, assessed here for the first time in a direct interspecific comparison, revealed large differences in the size of the active sapwood fraction among the species, reflecting the specific wood anatomy (ring‐porous versus diffuse‐porous xylem). The mean minimum NSC concentrations in branch wood during the growing season was 55% of maximum, and even high NSC concentrations were maintained during times of extensive fruit production in masting Fagus sylvestris. The NSC in stem sapwood varied very little throughout the season (cross species mean never below 67% of maximum), and the small reductions observed were not significant for any of the investigated species. Although some species contained substantial quantities of lipids in woody tissues (‘fat trees’; Tilia, Pinus, Picea, Larix), the lipid pools did not vary significantly across the growing season in any species. On average, the carbon stores of deciduous trees would permit to replace the whole leave canopy four times. These data imply that there is not a lot of leeway for a further stimulation of growth by ongoing atmospheric CO₂ enrichment. The classical view that deciduous trees rely more on C‐reserves than evergreen trees, seems unwarranted or has lost its justification due to the greater than 30% increase in atmospheric CO₂ concentrations over the last 150 years.