The sensitivity of root water uptake to cold root temperature follows species-specific upper elevational distribution limits of temperate tree species.

Physiological water stress induced by low root temperatures might contribute to species-specific climatic limits of tree distribution. We investigated the low temperature sensitivity of root water uptake and transport in seedlings of 16 European tree species which reach their natural upper elevation distribution limits at different distances to the alpine treeline. We used 2H-H2O pulse-labelling to quantify the water uptake and transport velocity from roots to leaves in seedlings exposed to constant 15°C, 7°C or 2°C root temperature, but identical aboveground temperatures between 20°C and 25°C. In all species, low root temperatures reduced the water transport rate, accompanied by reduced stem water potentials and stomatal conductance. At 7°C root temperature, the relative water uptake rates among species correlated positively with the species-specific upper elevation limits, indicating an increasingly higher sensitivity to lower root zone temperatures, the lower a species' natural elevational distribution limit. Conversely, 2°C root temperature severely inhibited water uptake in all species, irrespective of the species' thermal elevational limits. We conclude that low temperature-induced hydraulic constraints contribute to the cold distribution limits of temperate tree species and are a potential physiological cause behind the low temperature limits of plant growth in general.

Rapid hydraulic collapse as cause of drought-induced mortality in conifers.

Understanding the vulnerability of trees to drought-induced mortality is key to predicting the fate of forests in a future climate with more frequent and intense droughts, although the underlying mechanisms are difficult to study in adult trees. Here, we explored the dynamic changes of water relations and limits of hydraulic function in dying adults of Norway spruce (Picea abies L.) during the progression of the record-breaking 2018 Central European drought. In trees on the trajectory to drought-induced mortality, we observed rapid, nonlinear declines of xylem pressure that commenced at the early onset of xylem cavitation and caused a complete loss of xylem hydraulic conductance within a very short time. We also observed severe depletions of nonstructural carbohydrates, though carbon starvation could be ruled out as the cause of the observed tree death, as both dying and surviving trees showed these metabolic limitations. Our observations provide striking field-based evidence for fast dehydration and hydraulic collapse as the cause of drought-induced mortality in adult Norway spruce. The nonlinear decline of tree water relations suggests that considering the temporal dynamics of dehydration is critical for predicting tree death. The collapse of the hydraulic system within a short time demonstrates that trees can rapidly be pushed out of the zone of hydraulic safety during the progression of a severe drought. In summary, our findings point toward a higher mortality risk for Norway spruce than previously assumed, which is in line with current reports of unprecedented levels of drought-induced mortality in this major European tree species.

Rhizosphere activity in an old-growth forest reacts rapidly to changes in soil moisture and shapes whole-tree carbon allocation.

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests' resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied 13CO2 pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in above- and belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

Lack of hydraulic recovery as a cause of post-drought foliage reduction and canopy decline in European beech.

European beech (Fagus sylvatica) was among the most affected tree species during the severe 2018 European drought. It not only suffered from instant physiological stress but also showed severe symptoms of defoliation and canopy decline in the following year. To explore the underlying mechanisms, we used the Swiss-Canopy-Crane II site and studied in branches of healthy and symptomatic trees the repair of hydraulic function and concentration of carbohydrates during the 2018 drought and in 2019. We found loss of hydraulic conductance in 2018, which did not recover in 2019 in trees that developed defoliation symptoms in the year after drought. Reduced branch foliation in symptomatic trees was associated with a gradual decline in wood starch concentration throughout summer 2019. Visualization of water transport in healthy and symptomatic branches in the year after the drought confirmed the close relationship between xylem functionality and supported branch leaf area. Our findings showed that embolized xylem does not regain function in the season following a drought and that sustained branch hydraulic dysfunction is counterbalanced by the reduction in supported leaf area. It suggests acclimation of leaf development after drought to mitigate disturbances in canopy hydraulic function.

Climate Change Modulates Multitrophic Interactions Between Maize, A Root Herbivore, and Its Enemies.

How climate change will modify belowground tritrophic interactions is poorly understood, despite their importance for agricultural productivity. Here, we manipulated the three major abiotic factors associated with climate change (atmospheric CO2, temperature, and soil moisture) and investigated their individual and joint effects on the interaction between maize, the banded cucumber beetle (Diabrotica balteata), and the entomopathogenic nematode (EPN) Heterorhabditis bacteriophora. Changes in individual abiotic parameters had a strong influence on plant biomass, leaf wilting, sugar concentrations, protein levels, and benzoxazinoid contents. Yet, when combined to simulate a predicted climate scenario (Representative Concentration Pathway 8.5, RCP 8.5), their effects mostly counter-balanced each other. Only the sharp negative impact of drought on leaf wilting was not fully compensated. In both current and predicted scenarios, root damage resulted in increased leaf wilting, reduced root biomass, and reconfigured the plant sugar metabolism. Single climatic variables modulated the herbivore performance and survival in an additive manner, although slight interactions were also observed. Increased temperature and CO2 levels both enhanced the performance of the insect, but elevated temperature also decreased its survival. Elevated temperatures and CO2 further directly impeded the EPN infectivity potential, while lower moisture levels improved it through plant- and/or herbivore-mediated changes. In the RCP 8.5 scenario, temperature and CO2 showed interactive effects on EPN infectivity, which was overall decreased by 40%. We conclude that root pest problems may worsen with climate change due to increased herbivore performance and reduced top-down control by biological control agents.

Plant respiration: Controlled by photosynthesis or biomass?

Two simplifying hypotheses have been proposed for whole-plant respiration. One links respiration to photosynthesis; the other to biomass. Using a first-principles carbon balance model with a prescribed live woody biomass turnover, applied at a forest research site where multidecadal measurements are available for comparison, we show that if turnover is fast the accumulation of respiring biomass is low and respiration depends primarily on photosynthesis; while if turnover is slow the accumulation of respiring biomass is high and respiration depends primarily on biomass. But the first scenario is inconsistent with evidence for substantial carry-over of fixed carbon between years, while the second implies far too great an increase in respiration during stand development-leading to depleted carbohydrate reserves and an unrealistically high mortality risk. These two mutually incompatible hypotheses are thus both incorrect. Respiration is not linearly related either to photosynthesis or to biomass, but it is more strongly controlled by recent photosynthates (and reserve availability) than by total biomass.

High carbon storage in carbon-limited trees.

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.

Revisiting the relative growth rate hypothesis for gymnosperm and angiosperm species co-occurrence.

PREMISE OF THE STUDY: It is unclear to what extent the co-occurrence of angiosperm and gymnosperm species in some marginal ecosystems is explained by reduced growth in angiosperms due to carbon (C) limitation and by high stress tolerance in gymnosperms associated with lack of vessels and resource conservation. METHODS: We examined growth patterns and traits associated with C balance in four evergreen angiosperm species (including one vesselless species, Drimys winteri) and three gymnosperm tree species of a cold-temperate rainforest in southern Chile. We measured the mean basal area increment for the first 50 (BAI50 ) and the last 10 years (BAI10 ), wood density, leaf lifespan, and nonstructural carbohydrate (NSC) concentrations in different organs. KEY RESULTS: BAI50 was 6-fold higher in angiosperms than in gymnosperms and ca. 4-fold higher in Drimys than in the fastest-growing gymnosperm. BAI10 and aboveground NSC concentrations were significantly higher and leaf lifespan lower in angiosperms than in gymnosperms; these differences, however, were largely driven by the slow growth and low NSC concentrations of the Cupressaceae species (Pilgerodendron uviferum), while the two Podocarpaceae had BAI10 and NSC concentrations similar to angiosperms. In angiosperms, NSC and starch concentrations were generally higher in species with lower BAI10 , indicating no severe C limitation. CONCLUSIONS: The co-occurrence of angiosperms and gymnosperms in cold-temperate rainforests of southern Chile is not explained by growth disadvantages and C limitation in angiosperms. Long leaf longevity, but not lack of vessels, appeared to favor resource conservation and C balance in some gymnosperms (Podocarpaceae).

Living on next to nothing: tree seedlings can survive weeks with very low carbohydrate concentrations.

The usage of nonstructural carbohydrates (NSCs) to indicate carbon (C) limitation in trees requires knowledge of the minimum tissue NSC concentrations at lethal C starvation, and the NSC dynamics during and after severe C limitation. We completely darkened and subsequently released seedlings of two deciduous and two evergreen temperate tree species for varying periods. NSCs were measured in all major organs, allowing assessment of whole-seedling NSC balances. NSCs decreased fast in darkness, but seedlings survived species-specific whole-seedling starch concentrations as low as 0.4-0.8% per dry matter (DM), and sugar (sucrose, glucose and fructose) concentrations as low as 0.5-2.0% DM. After re-illumination, the refilling of NSC pools began within 3 wk, while the resumption of growth was delayed or restricted. All seedlings had died after 12 wk of darkness, and starch and sugar concentrations in most tissues were lower than 1% DM. We conclude that under the applied conditions, tree seedlings can survive several weeks with very low NSC reserves probably also using alternative C sources like lipids, proteins or hemicelluloses; lethal C starvation cannot be assumed, if NSC concentrations are higher than the minimum concentrations found in surviving seedlings; and NSC reformation after re-illumination occurs preferentially over growth.

Homeostatic levels of nonstructural carbohydrates after 13 yr of drought and irrigation in Pinus sylvestris.

Nonstructural carbohydrates (NSCs) are important for the growth and survival of trees. Drought may lead to a decrease in tree growth and to NSC depletion, whereas increased soil moisture in otherwise dry ecosystems may increase growth and NSC concentrations. A long-term (13 yr) irrigation experiment was conducted in a Pinus sylvestris-dominated forest located at the dry margin of the species in southern Switzerland. We measured the relative leaf area, growth, NSCs, needle δ13 C, [N] and [P] in trees on control and irrigated plots. Irrigation resulted in higher growth rates and carbon isotope discrimination, but did not alter NSC levels. Growth and NSC decreased with decreasing leaf area in both treatments, but NSC did not correlate with leaf-level gas exchange indices, such as foliar δ13 C, [N] or [P]. A legacy effect was shown, as trees with initially low leaf area had limited ability to respond to prolonged irrigation. The NSC constancy across treatments provides evidence that carbohydrate storage may stay constant when climate changes are sufficiently slow to allow acclimation. Moreover, we speculate that total leaf area, rather than leaf gas exchange per unit leaf area, drives the variation in whole-tree carbohydrate dynamics in this system.

Growth reduction after defoliation is independent of CO2 supply in deciduous and evergreen young oaks.

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.

Endogenous circadian rhythms in pigment composition induce changes in photochemical efficiency in plant canopies.

There is increasing evidence that the circadian clock is a significant driver of photosynthesis that becomes apparent when environmental cues are experimentally held constant. We studied whether the composition of photosynthetic pigments is under circadian regulation, and whether pigment oscillations lead to rhythmic changes in photochemical efficiency. To address these questions, we maintained canopies of bean and cotton, after an entrainment phase, under constant (light or darkness) conditions for 30-48 h. Photosynthesis and quantum yield peaked at subjective noon, and non-photochemical quenching peaked at night. These oscillations were not associated with parallel changes in carbohydrate content or xanthophyll cycle activity. We observed robust oscillations of Chl a/b during constant light in both species, and also under constant darkness in bean, peaking when it would have been night during the entrainment (subjective nights). These oscillations could be attributed to the synthesis and/or degradation of trimeric light-harvesting complex II (reflected by the rhythmic changes in Chl a/b), with the antenna size minimal at night and maximal around subjective noon. Considering together the oscillations of pigments and photochemistry, the observed pattern of changes is counterintuitive if we assume that the plant strategy is to avoid photodamage, but consistent with a strategy where non-stressed plants maximize photosynthesis.

Dying piece by piece: carbohydrate dynamics in aspen (Populus tremuloides) seedlings under severe carbon stress.

Carbon starvation as a mechanism of tree mortality is poorly understood. We exposed seedlings of aspen (Populus tremuloides) to complete darkness at 20 or 28 °C to identify minimum non-structural carbohydrate (NSC) concentrations at which trees die and to see if these levels vary between organs or with environmental conditions. We also first grew seedlings under different shade levels to determine if size affects survival time under darkness due to changes in initial NSC concentration and pool size and/or respiration rates. Darkness treatments caused a gradual dieback of tissues. Even after half the stem had died, substantial starch reserves were still present in the roots (1.3-3% dry weight), indicating limitations to carbohydrate remobilization and/or transport during starvation in the absence of water stress. Survival time decreased with increased temperature and with increasing initial shade level, which was associated with smaller biomass, higher respiration rates, and initially smaller NSC pool size. Dead tissues generally contained no starch, but sugar concentrations were substantially above zero and differed between organs (~2% in stems up to ~7.5% in leaves) and, at times, between temperature treatments and initial, pre-darkness shade treatments. Minimum root NSC concentrations were difficult to determine because dead roots quickly began to decompose, but we identify 5-6% sugar as a potential threshold for living roots. This variability may complicate efforts to identify critical NSC thresholds below which trees starve.

The amount of parenchyma and living fibers affects storage of nonstructural carbohydrates in young stems and roots of temperate trees.

PREMISE OF THE STUDY: Concentrations of nonstructural carbohydrates (NSCs) are used as proxies for the net carbon balance of trees and as indicators of carbon starvation resulting from environmental stress. Woody organs are the largest NSC-storing compartments in forest ecosystems; therefore, it is essential to understand the factors that affect the size of this important storage pool. In wood, NSC are predominantly deposited in ray and axial parenchyma (RAP); however, direct links between nutrient storage and RAP anatomy have not yet been established. Here, we tested whether the NSC storage capacity of wood is influenced by the amount of RAP. METHODS: We measured NSC concentrations and RAP fractions in root and stem sapwood of 12 temperate species sampled at the onset of winter dormancy and in stem sapwood of four tropical trees growing in an evergreen lowland rainforest. The patterns of starch distribution were visualized by staining with Lugol's solution. KEY RESULTS: The concentration of NSCs in sapwood of temperate trees scales tightly with the amount of RAP and living fibers (LFs), with almost all RAP and LFs being densely packed with starch grains. In contrast, the tropical species had lower NSC concentrations despite their higher RAP and LFs fraction and had considerable interspecific differences in starch distribution. CONCLUSIONS: The differences in RAP and LFs abundance affect the ability of sapwood to store NSC in temperate trees, whereas a more diverse set of functions of RAP might be pronounced in species growing in a tropical environment with little seasonality.

Tree carbon allocation dynamics determined using a carbon mass balance approach.

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(-1) was 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.

Stem growth phenology, not canopy greening, constrains deciduous tree growth.

Canopy phenology is a widely used proxy for deciduous forest growth with various applications in terrestrial ecosystem modeling. Its use relies on common assumptions that canopy greening and stem growth are tightly coordinated processes, enabling predictions on the timing and the quantity of annual tree growth. Here, we present parallel observations of canopy and stem growth phenology and annual stem increment in around 90 deciduous forest trees with diffuse-porous (Fagus sylvatica, Acer pseudoplatanus, Carpinus betulus) or ring-porous (Quercus robur × petraea) wood anatomy. These data were collected in a mixed temperate forest at the Swiss-Canopy-Crane II site, in 4 years with strongly contrasting weather conditions. We found that stem growth resumption lagged several weeks behind spring canopy greening in diffuse-porous but not in ring-porous trees. Canopy greening and stem growth resumption showed no or only weak signs of temporal coordination across the observation years. Within the assessed species, the seasonal timing of stem growth varied strongly among individuals, as trees with high annual increments resumed growth earlier and also completed their main growth earlier. The length of main growth activity had no influence on annual increments. Our findings not only challenge tight temporal coordination of canopy and stem growth phenology but also demonstrate that longer main growth activity does not translate into higher annual increments. This may compromise approaches modeling tree growth and forest productivity with canopy phenology and growth length.

Water loss after stomatal closure: quantifying leaf minimum conductance and minimal water use in nine temperate European tree species during a severe drought.

Residual canopy transpiration (Emin_canop) is a key physiological trait that determines trees' survival time under drought after stomatal closure and after trees have limited access to soil water. Emin_canop mainly depends on leaf minimum conductance (gmin) and vapor pressure deficit. Here we determined the seasonal variation of gmin and how gmin is related to interspecies variation in leaf cuticular and stomatal traits for nine European tree species in a mature forest. In addition, we determined the species-specific temperature responses of gmin. With this newly obtained insight, we calculated Emin_canop for the nine species for one day at our research site during the 2022 central European hot drought. Our results show that at ambient temperatures gmin ranged from 0.8 to 4.8 mmol m-2 s-1 across the nine species and was stable in most species throughout the growing season. The interspecies variation of gmin was associated with leaf cuticular and stomatal traits. Additionally, gmin exhibited strong temperature responses and increased, depending on species, by a factor of two to four in the range of 25-50 °C. For the studied species at the site, during a single hot drought day, Emin_canop standardized by tree size (stem basal area) ranged from 2.0 to 36.7 L m-2, and non-standardized Emin_canop for adult trees ranged from 0.3 to 5.3 L. Emin_canop also exhibited species-specific rapid increases under hotter temperatures. Our results suggest that trees, depending on species, need reasonable amounts of water during a drought, even when stomates are fully closed. Species differences in gmin and ultimately Emin_canop can, together with other traits, affect the ability of a tree to keep its tissue hydrated during a drought and is likely to contribute to species-specific differences in drought vulnerability.

Uniform carbon reserve dynamics along the vertical light gradient in mature tree crowns.

Understanding the within-tree variability of non-structural carbohydrates (NSC) is crucial for interpreting point measurements and calculating whole-tree carbon balances. Yet, little is known about how the vertical light gradient within tree crowns influences branch NSC concentrations and dynamics. We measured NSC concentrations, irradiance and key leaf traits in uppermost, sun-exposed and lowest, shaded branches in the crowns of mature, temperate trees from nine species with high temporal resolution throughout one growing season. Measurements from two additional years allowed us to test the generality of our findings among climatically contrasting years. Despite the vertical light gradient, we found very similar seasonal NSC dynamics and concentrations between sun and shade branches in most species. This can at least partially be explained by acclimations in SLA and photosynthetic leaf traits compensating the different light availability between the top and bottom canopy. Only in the ring-porous species Quercus and Fraxinus, starch refilling after budbreak was slower in lower branches. End-of-season NSC concentrations were similar between canopy positions and among observation years. Only Fagus had 40 and 29% lower starch concentrations by the end of the extremely dry year 2020, relative to the other two years. We show that NSC measured anywhere in a tree crown is often representative of the whole crown. Overall, our results suggest that carbon reserve dynamics in trees are largely insensitive to both microclimatic gradients and inter-annual climatic variation, and only deviate under severe carbon deficits, as was presumably the case with Fagus in our study.

European deciduous trees exhibit similar safety margins against damage by spring freeze events along elevational gradients.

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.

Tracing fresh assimilates through Larix decidua exposed to elevated CO2 and soil warming at the alpine treeline using compound-specific stable isotope analysis.

How will carbon source-sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO(2) and climate? We evaluated the effects of previously elevated CO(2) concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% (13)CO(2)) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO(2) did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum (13)C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations.