The current carbon supply status of temperate forest trees was assessed by analysing the seasonal variation of nonstructural 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 CO 2 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 CO 2 concentrations over the last 150 years.

The mechanisms of carbon starvation: how, when, or does it even occur at all?Recent observations of increasing vegetation mortality events appear to be a result of changing climate, in particular, an increase in the frequency, length and intensity of droughts (e.g. Allen et al., 2010). The threat of widespread increases in future mortality has rekindled interest in the mechanisms of plant mortality and survival because we do not yet understand them well enough to confidently model future vegetation dynamics (Sitch et al., 2008). In this issue of New Phytologist, provide a viewpoint on the 'carbon (C) starvation hypothesis ' (McDowell et al., 2008). Their viewpoint is invaluable for stimulating our field to explicitly refine our definitions and identify the key experiments needed to understand mechanisms of vegetation survival and mortality. Two important conclusions of their paper were that mortality can occur at nonzero carbohydrate levels and that careful experiments focused on the explicit mechanisms of C starvation, as well as on partitioning the roles of hydraulic failure and C starvation, are needed to understand the physiological underpinnings of how plants die. We applaud these conclusions, and agree that hasty acceptance of any hypothesis before adequate testing is foolish. In this commentary, we highlight some of the valuable ideas from Sala et al. and provide additional comments that we hope will prompt careful future tests on the mechanisms of plant mortality.When the C-starvation hypothesis was proposed (McDowell et al., 2008), it represented an attempt to summarize and interpret the existing literature on vegetation mortality, of which there was a wealth of indirect studies, but a paucity of true, mechanistic tests. The original formulation of the hypothesis suggested that stomatal closure minimizes hydraulic failure during drought, causing photosynthetic C uptake to decline to low levels, thereby promoting carbon starvation as carbohydrate demand continues for maintenance of metabolism and defense. The plant either starves outright, or succumbs to attack by insects or pathogens, whichever occurs first. By contrast, failure to maintain xylem water tension lower than its cavitation threshold results in embolisms, which, if unrepaired, can eventually lead to widespread hydraulic failure, desiccation and mortality. We hoped that the C-starvation and hydraulic failure hypotheses would generate discussion and new ideas; and 'The paucity of studies that quantified mortality forces scientists to use data from nonmortality studies to develop hypotheses … we do this at the risk of confusing stress responses with mortality mechanisms.' , as summarized by Sala et al., active discussion is taking place. A primary conclusion from the discussion is that we need clarification of the various mechanisms by which C starvation can occur, if it occurs at all.Plants maintain metabolism through respiratory processes that consume carbohydrates, and in doing so their C budgets must obey the law of conservation of energ...

Plants store large amounts of non-structural carbohydrates (NSC). While multiple functions of NSC have long been recognized, the interpretation of NSC seasonal dynamics is often based on the idea that stored NSC is 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 high NSC concentrations in some plants have been interpreted to reflect excess supply relative to demand. An alternative view, however, is that NSC accumulation reflects the relatively high NSC levels required for plant survival; an important issue that remains highly controversial. Here, we assembled a new global database to examine broad patterns of seasonal NSC variation 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, NSC account for ~10% of dry plant biomass and are highest in leaves and lowest in stems, whereas belowground organs show intermediate concentrations. Total NSC, 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, NSC concentrations were highest and most variable in herbaceous species and in conifer needles. Conifers showed the lowest stem and belowground NSC concentrations. Minimum NSC values were relatively high (46% of seasonal maximums on average for total NSC) and, in contrast to average values, were similar among biomes and functional types. Overall, although starch depletion was relatively common, seasonal depletion of total NSC or SS was rare. These results are consistent with a dual view of NSC function: 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 of NSC will have important implications for the way we understand and model plant carbon allocation and survival under stress.

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The carbon charging of pines across the treeline ecotone of three different climatic zones (Mexico 19 degrees N Pinus hartwegii, Swiss Alps 46 degrees N P. cembra and northern Sweden 68 degrees N P. sylvestris) was analyzed, to test whether a low-temperature-driven carbon shortage can explain high-elevation tree limits, and whether the length of the growing season affects the trees' carbon balance. We quantified the concentrations of non-structural carbohydrates (NSC) and lipids (acylglycerols) in all tree organs at three dates during the growing seasons across elevational transects from the upper end of the closed, tall forest (timberline) to the uppermost location where groups of trees > or =3 m in height occur (treeline). Mean ground temperatures during the growing season at the treelines were similar (6.1+/-0.7 degrees C) irrespective of latitude. Across the individual transects, the concentrations of NSC and lipids increased with elevation in all organs. By the end of the growing season, all three species had very similar total mobile carbon (TMC) concentrations at the treeline (ca. 6% TMC in the aboveground dry biomass), suggesting no influence of the length of the growing season on tree carbon charging. At a temperate lowland reference site P. sylvestris reached only ca. 4% TMC in the aboveground dry biomass, with the 2% difference largely explained by higher lipid concentrations of treeline pines. We conclude that carbon availability is unlikely to be the cause of the altitudinal tree limit. It seems rather that low temperatures directly affect sink activity at the treeline, with surplus carbon stored in osmotically inactive compounds.