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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.

• The concentrations of nonstructural carbohydrates (NSCs) in plant tissues are commonly used as an indicator of total plant carbon (C) supply; but some evidence suggests the possibility for high NSC concentrations during periods of C limitation. Despite this uncertainty, NSC dynamics have not been investigated experimentally under long-term C limitation. • We exposed saplings of 10 temperate tree species differing in shade tolerance to 6% of ambient sunlight for 3 yr to induce C limitation, and also defoliated one species, Carpinus betulus, in the third season. Growth and NSC concentrations were monitored to determine C allocation. • Shade strongly reduced growth, but after an initial two-fold decrease, NSC concentrations of shaded saplings recovered to the level of unshaded saplings by the third season. NSC concentrations were generally more depleted under shade after leaf flush, and following herbivore attacks. Only under shade did artificial defoliation lead to mortality and depleted NSC concentrations in surviving individuals. • We conclude that, irrespective of shade tolerance, C storage is maintained under prolonged shading, and thus high NSC concentrations can occur during C limitation. Yet, our results also suggest that decreased NSC concentrations are indicative of C limitation, and that additional leaf loss can lead to lethal C shortage in deep shade.

Aim: Across all latitudes, high-elevation tree lines represent a drastic change in the dominant plant life-form, from upright trees to low-stature alpine plants. Although associated with low temperatures, the physiological mechanisms controlling this boundary are still not clear. The growth-limitation hypothesis assumes a direct low-temperature restriction of tissue formation at otherwise sufficient photoassimilation. In order to test this hypothesis, we present a global synthesis of previously published and new data on tree carbon supply status at high-elevation tree lines. Location: Global; 13 regions between 68° N and 45° S. Methods: Late-season concentrations of non-structural carbohydrates (NSC) in foliage and branch wood were measured at three elevations across the tree line ecotones, from upper tall forests (timber line) to the edges of aborescent tree growth (tree line). Year-round records of -10 cm soil temperatures were taken at the tree line. Results: Despite large differences in elevation and season length, the mean growing season temperature at the tree line was similar (approximately 6.6 °C) between all sites. NSC concentrations were not depleted at any of the elevation gradients between timber line and tree line, indicating no shortage of C at the uppermost tree stands. Tested across all sites, NSC concentrations in fact significantly increased with elevation. On average, NSC increased by 18% in leaves and 26% in branch wood from the lowest to the highest stands, primarily due to higher starch concentrations rather than free sugars. Hence, these responses do not reflect osmotic adjustments to lower temperatures at the end of the growing season. Main conclusions: This global data set contributes to a mechanistic understanding of tree line formation based on biological principles across climatic zones and tree genera. No evidence of C shortage was found at the high-elevation tree line in either seasonal and non-seasonal regions. The increasing trend of NSC concentrations with elevation is in line with the growth-limitation hypothesis.

Tree internal carbon (C) fluxes between compound and compartment pools are difficult to measure directly. Here we used a C mass balance approach to decipher these fluxes and provide a full description of tree C allocation dynamics. We collected independent measurements of tree C sinks, source and pools in Pinus halepensis in a semi‐arid forest, and converted all fluxes to g C per tree d⁻¹. Using this data set, a process flowchart was created to describe and quantify the tree C allocation on diurnal to annual time‐scales. The annual C source of 24.5 kg C per tree yr⁻¹was balanced by C sinks of 23.5 kg C per tree yr⁻¹, which partitioned into 70%, 17% and 13% between respiration, growth, and litter (plus export to soil), respectively. Large imbalances (up to 57 g C per tree d⁻¹) were observed as C excess during the wet season, and as C deficit during the dry season. Concurrent changes in C reserves (starch) were sufficient to buffer these transient C imbalances. The C pool dynamics calculated using the flowchart were in general agreement with the observed pool sizes, providing confidence regarding our estimations of the timing, magnitude, and direction of the internal C fluxes.

Commentary p 264

Summary Attempts at explaining range limits of temperate tree species still rest on correlations with climatic data that lack a physiological justification. Here, we present a synthesis of a multidisciplinary project that offers mechanistic explanations. Employing climatology, biogeography, dendrology, population and reproduction biology, stress physiology and phenology, we combine results from in situ elevational (Swiss Alps) and latitudinal (Alps vs. Scandinavia) comparisons, from reciprocal common garden and phytotron studies for eight European broadleaf tree species. We show that unlike for low‐stature plants, tree canopy temperatures can be predicted from weather station data, and that low‐temperature extremes in winter do not explain range limits. At the current low‐temperature range limit, all species recruit well. Transplants revealed that the local environment rather than elevation of seed origin dominates growth and phenology. Tree ring width at the range limit is not related to season length, but to growing season temperature, with no evidence of carbon shortage. Bud break and leaf emergence in adults trees are timed in such a way that the probability of freezing damage is almost zero, with a uniform safety margin across elevations and taxa. More freezing‐resistant species flush earlier than less resistant species. Synthesis: we conclude that the range limits of the examined tree species are set by the interactive influence of freezing resistance in spring, phenology settings, and the time required to mature tissue. Microevolution of spring phenology compromises between demands set by freezing resistance of young, immature tissue and season length requirements related to autumnal tissue maturation. We conclude that the range limits of the examined tree species are set by the interactive influence of freezing resistance in spring, phenology settings, and the time required to mature tissue. Microevolution of spring phenology compromises between demands set by freezing resistance of young, immature tissue and season length requirements related to autumnal tissue maturation.

Reduced productivity of trees after defoliation might be caused by limited carbon (C) availability. We investigated the combined effect of different atmospheric CO2 concentrations (160, 280 and 560 ppm) and early season defoliation on the growth and C reserves (nonstructural carbohydrates (NSC)) of saplings of two oak species with different leaf habits (deciduous Quercus petraea and evergreen Quercus ilex). In both species, higher CO2 supply significantly enhanced growth. Defoliation had a strong negative impact on growth (stronger for Q. ilex), but the relative reduction of growth caused by defoliation within each CO2 treatment was very similar across all three CO2 concentrations. Low CO2 and defoliation led to decreased NSC tissue concentrations mainly in the middle of the growing season in Q. ilex, but not in Q. petraea. However, also in Q. ilex, NSC increased in woody tissues in defoliated and low-CO2 saplings towards the end of the growing season. Although the saplings were C limited under these specific experimental conditions, growth reduction after defoliation was not directly caused by C limitation. Rather, growth of trees followed a strong allometric relationship between total leaf area and conductive woody tissue, which did not change across species, CO2 concentrations and defoliation treatments.

Summary Within the same forest stand, temperate deciduous trees generally exhibit a distinct pattern in leaf‐out timing, with some species flushing earlier than other species. This study aimed to explain the timing of leaf‐out of various temperate tree species in relation to the risk of freezing damage to leaves. We combined long‐term series of leaf‐out date (14–32 years) of five temperate tree species located in both low and high elevations in Switzerland, daily minimum temperatures recorded at the same sites and species‐specific freezing resistance (LT50) of emerging leaves. We calculated temperature safety margins (the temperature difference between absolute minimum temperature during leaf‐out and species‐specific LT50 values), and date safety margins (time lag between the last day when temperature falls below species‐specific LT50 values and the date of leaf‐out). Leaf‐out occurred when the probability to encounter freezing damage approaches zero, irrespective of climatic conditions (low vs. high elevation) and species (early‐ and late‐flushing species). In other words, trees leaf out precisely at the beginning of the probabilistically safe period. Interestingly, the temperature safety margins did not differ significantly between low and high elevation. Yet, the date safety margin was smaller at high elevation, presumably due to a faster increase in temperature during the leaf‐out period at high elevation. When species‐specific freezing resistance is taken into account, the time of leaf‐out converges among species towards a marginal risk of freezing damage. Thus, leaf‐out time has likely evolved in a way that the risk of freezing damage is minimized over a large spectrum of climatic conditions. Species with a small safety margin against freezing temperature, like Fagus sylvatica, appear to employ photoperiod co‐control of spring phenology, whereas species with a large safety margin depend largely on temperature for the right timing of leaf‐out. Our results offer a new avenue to explain the differences in leaf‐out timing among co‐occurring tree species. They further suggest that in a warming climate, tree species can expand their distribution range to the extent their phenology matches the stochasticity of freezing temperatures in spring. Lay Summary

Minimum temperature is assumed to be an important driver of tree species range limits. We investigated during which period of the year trees are most vulnerable to freezing damage and whether the pressure of freezing events increases with increasing elevation. We assessed the course of freezing resistance of buds and leaves from winter to summer at the upper elevational limits of eight deciduous tree species in the Swiss Alps. By reconstructing the spring phenology of these species over the last eight decades using a thermal time model, we linked freezing resistance with long-term minimum temperature data along elevational gradients. Counter-intuitively, the pressure of freeze events does not increase with elevation, but deciduous temperate tree species exhibit a constant safety margin (5–8.5 K) against damage by spring freeze events along elevational gradients, as a result of the later flushing at higher elevation. Absolute minimum temperatures in winter and summer are unlikely to critically injure trees. Our study shows that freezing temperatures in spring are the main selective pressure controlling the timing of flushing, leading to a shorter growing season at higher elevation and potentially driving species distribution limits. Such mechanistic knowledge is important to improve predictions of tree species range limits.