Tree Physiology

- Norway spruce (Picea abies) dominates many European mountain forests, yet their seasonal water uptake strategies in high-elevation mono-specific natural stands remain poorly understood. We quantified contributions of shallow (0-10 cm) and deep (50-70 cm) soil layers to tree water uptake over three consecutive growing seasons (2020-2022) using stable water isotopes and Bayesian mixing analysis. - Contrary to the prevailing view of spruce as a shallow-rooted species relying primarily on water from the upper 10-20 cm of soil, our results showed more than 50% water uptake from deeper soil (50-70 cm), with deeper soil contributions crossing 80% in 2020. - During the dry and warm summer of 2022, positive soil recharge and elevated atmospheric demand increased evapotranspiration, with spruce trees taking up recently infiltrated rainfall from different soil depths, including >50% uptake from deeper layers. - Spruce water uptake shifted from cold-season-recharged soil water early in the growing season to warm-season precipitation in late summer. The timing of this shift in mid-summer can be explained by soil water recharge from recent rainfall infiltrated into the entire soil profile. This reliance on summer precipitation increases vulnerability of mono-specific spruce stands to more frequent droughts and heat waves under future climate change.

Compound droughts, i.e. the co-occurrences of heat and drought, represent a serious challenge for temperate forest trees leading to significant losses in forest biomass. We studied the physiological response of Norway spruce (Picea abies) saplings to heat and drought individually, and in combination. Continuous measurements of leaf gas exchange and VOC emission allowed us to identify fast-response reactions, while discrete VOC and root exudate samplings added qualitative information on compositional changes. Additionally, we used 13CO2 and 2H2O label pulses to investigate C-allocation and root water uptake in response to stress. Heat as well as drought reduced assimilation rates in the saplings, whereas transpiration, leaf VOC emission and root exudation rates increased in response to heat. Drought alone increased VOC emission but decreased exudation rates. Combined heat and drought triggered an amplified response in both processes despite negative net CO2 assimilation rates. Label incorporation showed compromised water uptake capacity of drought-stressed plants and illustrated de novo C-allocation to VOC emission and root exudates. The results point at the high susceptibility of Norway spruce saplings to drought and heat. Combined stress resulted in synergistic responses in VOC emissions and root exudates, showing the detrimental effect of compound droughts on Norway spruce. HighlightIn this study, we found synergistic effects of heat and drought on carbon losses from leaf VOC emission and root exudates despite negative assimilation rates in Norway spruce saplings.

Non-structural carbon compounds (NSCs) serve to buffer short-term imbalances between carbon supply and demand in trees; however, their seasonal dynamics throughout the entire tree remain inadequately understood. We quantified year-round non-structural carbohydrate storage and fluxes in a temperate pine forest by integrating monthly measurements of soluble sugars, starch, and lipids across five tissues with biometric scaling to ecosystem stocks. Soluble sugars were consistently highest in canopy tissues and maintained a relatively stable concentration, even as sugar fluxes exhibited pronounced seasonal variations and reversals. In contrast, starch showed clear seasonality, increasing during the mid-growing season and decreasing later, whereas lipid pools remained relatively stable and contributed minimally to short-term fluctuations. Ecosystem-scale analyses indicated that sugars predominantly contributed to NSC turnover, accounting for approximately 80% of the total annual flux, while stored pools exhibited slower changes. The net annual NSC flux, approximately 65 g C m-2 yr-1, was relatively modest in comparison to biomass production, which totaled around 522g C m-2 yr -1. These findings indicate that seasonal changes in carbon balance are primarily driven by rapid redistribution of soluble carbon rather than by significant changes in overall NSC storage.

Climatic and biogeographic variables are often used as a proxy for tree genetic diversity, but local factors can also influence it. We propose that woodland age, presence of ancient trees, and population size could impact genetic diversity. Using the RBG Kew UK National Tree Seed Project as a study case, we examined how these factors are accounted for during seed collection. We found 42% of tree seed collections come from ancient woodlands and that 8.4% overlap with ancient trees. Sampled forest patches size ranges from few individuals to several thousand. We then carried out a pilot to examine the role of population size on functional traits variation, testing the relationship between population size and seed germination and seedling thermal stress sensitivity in three populations of the Betula pubescens Ehrh. complex. We found that seeds and seedlings from larger populations showed higher fitness and stress resistance. Our results highlight the importance of local factors to predict variation in functional traits, relevant for tree resilience. Existing seed collections of native species stored in conservation seed banks offer a valuable resource to explore these factors and improve our understanding of genetic diversity in tree populations, with implications for biodiversity conservation and forestry production.

Forest regeneration depends not only on how many seeds trees produce, but on the physiological quality of those seeds. Yet while climate-driven shifts in seed quantity and masting have received sustained attention, the parallel question of whether climate change degrades seed quality remains poorly resolved. Using a nationwide dataset of seed mass and viability in European beech (Fagus sylvatica L.) collected between 1996 and 2024 (13,349 seed lots from 381 forest districts across Poland), with climate-quality analyses focused on 5,374 freshly harvested seed lots from 353 districts (2004-2023), we asked whether the two components of seed quality respond to different seasonal climatic windows, and whether harvest-year climate also shapes seed performance during long-term cold storage. Seed mass and seed viability were only weakly correlated (Spearmans {rho} = 0.15), acting as two independent dimensions of seed quality. Both revealed substantial temporal variation over the study period, but along distinct trajectories. Seed mass declined markedly between segmented-regression breakpoints in 2009 and 2019, more steeply at higher latitudes, coinciding spatially and temporally with the masting breakdown reported at the species northeastern range margin. Climatic associations were correspondingly divergent. Viability was positively associated with previous summer temperature, consistent with temperature-cued flower initiation, and negatively with spring temperature in the harvest year, plausibly reflecting thermal disruption of early embryogenesis. Seed mass showed no significant association with any seasonal climatic predictor, indicating control by slower or unmeasured processes. Storage duration progressively reduced viability, and this decline was further modulated by climate during seed development, with seeds developing under climatically suboptimal conditions losing viability faster. These results expose a hidden decoupling between seed quantity and seed quality under contemporary climate change, with direct consequences for forest regeneration and for ex situ conservation strategies that assume mast-year seeds will remain viable for decades.

Trees accumulate somatic mutations throughout their long lifespan, resulting in genetic mosaicism among branches. While recent genomic studies quantified these mutations, they were largely limited to describing static patterns of variation. In this study, we developed a mathematical model to infer the dynamic processes of somatic mutation accumulation from snapshot genomic data obtained from four tropical trees (Dipterocarpaceae), which dominate tropical rain forests in Southeast Asia. Our model focus on genetic differences between shoot apical meristems (SAMs) at branch tips and explicitly incorporate stem cell dynamics within SAMs during shoot elongation and branching, enabling us to quantify somatic genetic drift arising from stem cell lineage replacement. By comparing model predictions with empirical data from Dipterocarpaceae trees, we estimated key parameters governing stem cell dynamics and somatic mutation rates. Our results indicate that both shoot elongation and branching involve replacement of stem cell lineages, leading to a moderate degree of somatic genetic drift. Accounting for stem cell dynamics resulted in slightly lower mutation rate estimates than previous approaches that ignored these processes. Using the estimated parameters, we further performed stochastic simulations to predict patterns of somatic mutations, including features not directly observed in the sampled trees, such as occasional deviations of somatic mutation phylogenies from physical architecture. Together, our modeling framework provides insights into how genetic mosaicism is shaped within tropical trees and reveals the stem cell dynamics underlying their long-term growth and accumulation of somatic mutations. (236 words) Highlights- We built mathematical models to predict the genetic differences between branch tips by somatic mutations. - The model considers the varying dynamics of stem cells in shoot meristem during shoot elongation and branching. - We compared the model prediction with empirical data from tropical trees, Dipterocarpaceae, and estimated the dynamics of stem cells and mutation rate. - Somatic mutation dynamics were shaped by somatic genetic drift arising from stem cell lineage replacement during shoot elongation and branching. - Accounting for stem cell dynamics led to slightly smaller estimates of mutation rates compared with previous estimates that ignored the dynamics. - Our models offer insights into how genetic variability is shaped in the tropical trees and the stem cell dynamics underlying their long-term growth.

Winter warming is altering plant exposure to cold events, yet its effects on seasonal cold hardiness dynamics remain poorly understood. Here we quantified bud cold hardiness across four dormant seasons in a boreal peatland forest whole ecosystem warming experiment. Across a +0.00 to +9.00{degrees}C warming gradient, we semi-regularly measured cold hardiness in two overstory (Larix laricina and Picea mariana) and two understory species (Chamaedaphne calyculata and Rhododendron groenlandicum). Warming reduced cold hardiness in fall and spring by delaying acclimation and advancing deacclimation. However, risk was only increased in late winter and spring for three species. Warming reduced snow cover, increasing temperature variability and cold damage to understory shrubs. Together, our results show that cold damage risk depends on species traits, microclimate, and seasonal timing.

Heat tolerance determines the vitality of tree species under climate change independently of drought tolerance, but has been much less studied than tree water relations. We studied species-specific differences and the capacity for seasonal heat acclimation in Central Europes naturally most important tree species, Fagus sylvatica, in comparison with two exotic tree species (Fagus orientalis, Pseudotsuga menziesii) that are considered for silvicultural climate change adaptation in managed forests. Foliage of mature trees was incubated at temperatures from 35-50 {degrees}C for up to 4 h to simulate daily heat maxima during heat waves. The maximum quantum yield (Fv/Fm) of photosystem II (PS II) of dark-adapted leaves was measured, because the PS II is particularly sensitive to heat and its functionality can decide on plant survival under heat. Fagus sylvatica was much more tolerant to heat than Pseudotsuga menziesii, but weakly (albeit significantly) less tolerant than Fagus orientalis. Within its limits, Pseudotsuga menziesii showed high seasonal heat acclimation with constantly increasing tolerance during the growing season. Fagus orientalis, but practically not Fagus sylvatica, also acclimated to heat. This makes Fagus orientalis slightly superior over Fagus sylvatica in terms of heat tolerance, whereas the suitability of Pseudotsuga menziesii for silvicultural climate change adaptation is questionable. Strong heat acclimation, but also overall low heat tolerance, in Pseudotsuga menziesii might be the result of evergreenness, which requires the generation of both cold and heat tolerance during the year.

Pine needles contain two vascular cell types unique to gymnosperms: Transfusion parenchyma (tp) and tracheids (tt). Since they form the only connections between vascular bundles and bundle sheath, we hypothesised that they are involved in regulating the needles water import and photoassimilate export. Synchrotron-based tomography enabled us to quantify volume changes of tp and tt cells in Pinus pinea needles systematically along the needle and throughout a diurnal day cycle, as well as under rehydration. As a physiological indicator of tps carbohydrate status served their starch content. Segmentation of the comprehensive data uncovered dramatic volume changes during dehydration and showed a diurnal course of starch formation and degradation. These changes suggest a yet unknown osmotic water flux between tp and tt, balanced by the formers carbohydrate status. Confirming our hypothesis, excess of photoassimilates in tp cells went into starch synthesis during the day. Starch mobilisation during the night increased the osmotic potential in tp and led to water intake. According to the decreasing starch fraction from base to needle tip, this mechanism is predominant in the upper needle segments, particularly after rehydration of dehydrated needles. Mechanistically, osmolytes in tp cells maintain tension in tt for the needles water import. HighlightSynchrotron tomographic microscopy uncovers diurnal starch fluctuations and osmotic water pumping in inner tissues of pine needles that are utilised at night and when recovering from dehydration

Subalpine forests in the Alps are fragile ecosystems that play a crucial role in regional water resources and the local climate. These ecosystems are ecologically significant due to their unique biodiversity and vulnerability to climate change. While several components of the hydrological balance have been studied, the interplay between catchment-scale processes and plot-scale drivers such as fog presence and forest age remains insufficiently understood. To address this, we investigated the hydrological balance of a subalpine coniferous forest catchment at the Renon site in the Italian Alps, integrating observations across spatial scales. The study area includes a mosaic of mature and younger regrowth forest, where both interannual and seasonal variability in precipitation and fog presence are pronounced. At the catchment scale, we quantified above-canopy precipitation, evapotranspiration (ET, measured via eddy covariance at the ICOS tower), stream discharge, and soil moisture dynamics. Within the catchment, we characterised water partitioning using sap flow sensors for tree transpiration, throughfall and stemflow collectors with rain gauges above and below the canopy and epiphyte sampling. Mixed fog-rain events frequently coincided with higher throughfall. However, these changes had a minor effect on soil water storage and catchment discharge in the annual water balance, which was nearly closed. At the plot scale, our results show that tree transpiration was higher in the younger forest structure, while canopy interception is a dominant process in water partitioning in the older forest structure, where lichen abundance likely enhances interception. This study highlights the importance of multi-scale monitoring in temperate mountain forests, where forest age influences water partitioning, and fog presence, though not directly quantified, can still contribute to reducing evaporative processes. Such contributions may gain importance under changing climate conditions, albeit less prominently than in tropical or subtropical cloud forests.

The source{square}sink attenuation hypothesis suggests that plants regulate carbon fixation in response to fluctuations in sink demands. Many evergreen trees exhibit flushing growth patterns, where new shoot development generates a strong, transient demand for both carbon and nitrogen that may influence the function of mature leaves. This study examined the source-sink attenuation hypothesis in the context of vegetative sink growth by investigating the photosynthetic capacity and nitrogen dynamics in mature citrus leaves across three stages of flush development. In contrast to expectations, photosynthesis declined as flush growth progressed. Early flush initiation induced stomatal limitation in mature leaves, whereas as sink demand from further shoot growth continued carboxylation capacity and Rubisco abundance declined, despite relatively stable total leaf nitrogen. These results suggest that mature leaves undergo selective protein retooling under prolonged sink demand, constraining CO{square} fixation while maintaining C export. Overall, this study revealed that under strong combined N and C sink demands, mature citrus leaves function primarily as regulated carbon conduits rather than dynamically upregulating photosynthesis, providing new insight into source-sink coordination in woody perennial species. HighlightCitrus flush growth shows that mature leaves suppress photosynthesis through stomatal and biochemical regulation while reallocating carbon and nitrogen to support new shoot development, challenging classic source-sink theory.

O_LIStored seeds are crucial repositories of plant genetic diversity. However, long-term storage inevitably causes seed deterioration and loss of viability, and chemical processes within the seeds during storage can influence germination and seedling establishment. Emerging evidence suggests that seed ageing can also affect traits of adult plants, yet the extent to which this phenomenon is relevant across species, particularly for wild plant species with high genetic variation, remains unclear. C_LIO_LITo address this, we focused on 14 grassland species and subjected their seeds to simulated long-term storage by exposing them to artificial ageing conditions (60% rH, 45{degrees}C). We then compared plants grown from the aged seeds with plants from fresh seeds in a common garden experiment. C_LIO_LIArtificially aged seeds germinated later, the developing seedlings had lower survival rates and reduced growth. Adult plants grown from aged seeds flowered later, produced fewer flowers, and had less biomass by the end of the first vegetation period than those from fresh seeds. The effect of the ageing treatment varied between species, but the trend was overall significant across species, with minor differences between perennials and annuals. Interestingly, in perennial plants, the effects vanished or were inverted in the second growing season, with plants growing from aged seeds flowering earlier and producing more biomass. C_LIO_LISynthesis. Our results show that seed storage affects seedling performance, plant growth, and flowering phenology. These direct storage effects should be considered when using stored seeds for species conservation, ecosystem restoration, or evolutionary research relying on stored seeds. C_LI

Urban environments typically experience higher temperatures than surrounding natural landscapes, making urban vegetation crucial for cooling local areas and improving the health of city residents. Impervious urban surfaces limit the absorption and retention of precipitation, potentially limiting tree water access and threatening long-term survival. Here, we measured tree physiology and growth of Lophostemon confertus (Queensland brush box) trees to investigate how a passive irrigation system that stores stormwater affected the performance of young, establishing trees in a hot and dry suburb of western Sydney, Australia. During the 2024-2025 austral summer, three years after planting, the local climate was periodically hot and dry, with a total of 16 days above 35 {degrees}C. Irrigated L. confertus trees had higher water availability (i.e., higher predawn leaf water potential,{Psi} pre), lower water stress (i.e., higher midday leaf water potential,{Psi} mid, more frequently above turgor loss point), greater stomatal conductance (gs) on hot and dry summer days, and reduced leaf temperatures (Tleaf), compared to control trees. No significant differences in growth rates were observed between irrigated and control trees during the first three establishment years, but irrigated trees had greater crown survival during the hot, dry summer. Our results suggest passive irrigation may mitigate periods of short-term heat and drought stress in urban trees by increasing water access to support transpiration that prevents leaves from overheating, improving tree health. Higher tree transpiration may lead to greater ecosystem services by increasing cooling benefits, contributing to mitigation of urban heat island effects.

Background and AimsClimate gradients influence seed morphology, emergence, and early life-history traits with cumulative impacts to individual fitness. For ex situ seed collections, which represent an invaluable repository of potential trait information for species management and conservation, climate data can guide preservation of adaptive variation and inform deployment strategies for restoration. Here we leverage a range-wide ex situ seed collection of critically endangered black ash seeds (Fraxinus nigra) to evaluate how climatic gradients shape variation in morphology and early life-history. MethodsTo test how climate of origin, seed morphology, and early life-history interact to impact first year fitness, high-throughput X-ray imaging and neural network-based segmentation were used to quantify variation in seed morphology for 701 maternal lineages spanning 76 populations across the range of F. nigra. Following this, a subset of seeds were used to establish a common garden experiment and quantify variation in emergence, early life-history transitions, and their cumulative impact to first-year survival and growth. ResultsOn average, differences within-population explained [~]43% of the variability in seed morphology, while among-population differences explained [~]14%. This suggests that substantial genetic variation exists within populations for natural selection to act upon and differences have evolved among populations. Climate associations indicated warmer and drier environments predicted heavier seeds with faster developmental transitions and increased first-year height. Together, climate of origin, seed mass, and timing of developmental transitions best predicted cumulative fitness, with populations from more continental environments exhibiting greater survival and first-year height accumulation on average. ConclusionsOverall, these results highlight the importance of climate of origin, seed traits, and early developmental transitions to first-year fitness in a perennial tree species. This work demonstrates how ex situ collections can be used to identify climatically structured trait variation and guide conservation strategies aimed at maintaining adaptive potential under environmental change.

Forests play a crucial role in mitigating climate change as primary terrestrial carbon sinks. While some studies suggest that global warming enhances forest productivity, a growing body of evidence highlights detrimental impact primarily driven by increased water stress. Yet the extent to which positive effects of climate change offset its negative impacts on tree species productivity remains unclear at large spatial extents. We assessed forest growth and mortality for the 21 most abundant tree species in Europe using National Forest Inventory data from more than 50,000 plots and 700,000 trees to disentangle the relative importance of climate and forest structure. Specifically, we examined how vapor pressure deficit (VPD) anomalies across species climatic edges and stand developmental stages affect forest growth and mortality occurrence and intensity (i.e. whether mortality occurred and the amount of basal area lost). Then, we aggregated the responses across species and separately for broad-leaved and needle-leaved species to assess whether forest growth and mortality differed between major functional groups. Although the importance of forest growth and mortality drivers varied markedly among species, climate had a stronger influence on mortality than on growth, particularly in needle-leaved species. Forest growth declined and mortality increased along VPD anomaly in most species and forests studied. Responses were most pronounced at arid species edges in early-stage broad-leaved forests and at wet edges in late-stage needle-leaved forests, where differences between functional groups were also highest. We evidence the need to parametrise species-specific models of forest growth and mortality across large spatial extents to better understand and predict effects of climate change on forest productivity. In addition, our results emphasize the importance of improving the understanding of forest mortality processes given the strong influence of climate on mortality, while also further studying vulnerable populations to climate change in arid edges of species distributions.

During drought, numerous compounds accumulate in plant tissues, but their physiological roles remain unclear - they may function as osmolytes, osmoprotectants, or merely arise as by-products of stress-induced metabolic shifts. We developed an experimental approach to link accumulation patterns with specific functions, using Scots pine (Pinus sylvestris L.) saplings subjected to water deprivation and subsequent rewatering as a model system. We monitored changes in relative water content (RWC) and osmotic adjustment dynamics, employed untargeted primary metabolite profiling for preliminary screening of compounds correlated with water status, and performed quantitative GC-MS and LC-MS analyses of selected metabolites. Major inorganic cations (K, Ca{superscript 2}, Mg{superscript 2}) were also quantified to assess their potential roles. Our results revealed that tryptophan, valine, and lysine - though generally present in low abundance - exhibited selective accumulation under severely reduced RWC ([≤] 70%), suggesting their involvement as osmoprotectants. Major organic acids, particularly shikimic acid, showed trends consistent with osmotic adjustment. Notably, neither sucrose nor inorganic cations appeared to function as primary osmolytes in this context. The proposed approach offers a viable strategy for identifying compounds involved in plant adaptation to water deficit, with potential applications in breeding programs aimed at improving drought tolerance. HighlightsAn approach to identify osmolytes and osmoprotectants was implemented Accumulation of Trp, Val and Lys was consistent with their role in osmoprotection Osmotic adjustment relied predominantly on organic acids than on inorganic ions Monosaccharides but not sucrose correlates with changes in needle water status

Ongoing climate change will negatively impact tree populations unless they are able to acclimate to the changes in their local environment. Effective planning for climate adaptation management requires an understanding of the current state of local adaptation and physiological performance to assess whether populations are at risk of local extinction, to determine if seed movement is appropriate, and to select appropriate seed sources if intervention is needed. We established a new reciprocal transplant experiment (ACE, Adaptation to Climate and Environment) across a latitudinal gradient in North America to examine the impacts of warming on three bur oak (Quercus macrocarpa) populations across much of the species range. We established common gardens in Minnesota, Illinois, and Oklahoma with seedlings grown from seeds collected within 50 km of each of those locations from a total of sixty maternal families. We aimed to 1) assess local adaptation in each of the populations using survival and size as fitness metrics, and 2) evaluate physiological responses to different environments along the latitudinal gradient. We found that northern populations are maladapted to hotter climates as evidenced by their low survival, growth, and photosynthetic rates in the warmest common garden. The southernmost population had the highest survival rate, growth rate, and fitness of the three populations in the southernmost garden, providing evidence for local adaptation to the warmest site. However, conditions in the middle garden resulted in the highest fitness and best physiological performance for all populations. Growth and survival were correlated in the middle garden but were decoupled in the northern and southern gardens. This decoupling is likely due to stress associated with more extreme climates at the ends of the gradient that led to greater resource allocation to survival than to growth. Our results suggest that southern seed sources may perform well in warmer conditions in the north brought on by climate change, which has important implications for managers assisting broadly ranged tree species in adapting to climate change.

Soil salinization threatens global agriculture reducing yields, yet the metabolic signals controlling salt-sensitive root plasticity in alfalfa remain unclear. We hypothesize that salinity transiently uncouples the sucrose-trehalose-6-P (Tre6P)- Sucrose non-fermenting kinase 1 (SnRK1) nexus, aligning with a biphasic root metabolic response and altered root architecture. Alfalfa seedlings were grown in a hydroponic system and exposed to 200 mM NaCl, with root samples collected from 1 h to 7 d. While primary root growth and biomass remained unchanged, lateral root development was enhanced under salinity. Early response (1 h-1 d) was characterized by reduced carbon metabolites, low Tre6P, increased malondialdehyde, and SnRK1 activation, with a decline in glycolytic and TCA intermediates. During this phase, sucrose was negatively correlated with both Tre6P and SnRK1. Late response (3-7 d) showed a SnRK1 reactivation, Tre6P recovery, and osmoprotectant accumulation, including increased antioxidant capacity (+75% at 3dpt), proline (+178%), and sucrose (+18%) and starch depletion (-57%) at 7dpt respect to control. These metabolic changes coincided with the enhanced lateral root emergence. These findings indicate a two-phase response: early metabolic downscaling with transient Suc-Tre6P-SnRK1 disruption, followed by recovery with Tre6P restoration, SnRK1 reactivation, osmoprotection, and sustained root plasticity under salinity. HighlightSalinity triggers a temporary metabolic shift in alfalfa roots: plants first conserve energy, then adapt to stress, maintaining lateral root growth and flexible root architecture.

Recent studies have demonstrated the presence of kynurenine (KYN) and kynurenic acid (KYNA) in several plant species, but the metabolic function of these metabolites remains undefined. We hypothesized that KYN and KYNA are metabolites of auxin and play a role in plant morphogenesis. To test our hypothesis, we developed a plant tissue-culture-based bioassay using Hypericum perforatum (St. Johns wort; SJW), a model system for auxin and indoleamine metabolism and pharmacological inhibitors (PF-04859989, RO-61-8048, and KMO inhibitor II, JM6) of human kynurenine pathways enzymes. SJW is an interesting model system because explants root in the absence of plant growth regulators but supplementation of the culture media with 10 M IAA induces a callus response without de novo root organogenesis. Supplementation of the culture media with 10 M KYN increased root number and internodal length relative to basal media. We used a previously validated high-resolution mass spectrometry analytical method to quantify KYN, KYNA, and 3-hydroxyanthranilic acid (3-HAA). KYN, KYNA and 3-HAA were quantified in roots and shoots of SJW grown on basal media. Supplementation of the culture media with 10 M KYN increased the concentration of KYN, KYNA and 3-HAA in roots and shoots. Treatment with 10 M IAA increased KYN and 3-HAA concentration in shoots. Three pharmaceutical candidates that are kynurenine pathway inhibitors in humans were taken up into the tissues from the culture media and increased KYN content as compared to basal control. Together, these data propose a role for KYN in IAA metabolism, shoot and root organogenesis. HighlightsO_LIKynurenine metabolites are detected and accumulate in H. perforatum tissue culture C_LIO_LIIAA redirects metabolism towards accumulation of KYN and 3-HAA in shoots C_LIO_LIExogenous KYN promotes KYNA accumulation C_LIO_LIPharmacological inhibition alters kynurenine pathway metabolite profiles in a tissue-specific manner C_LIO_LIKynurenine and IAA differentially regulate root development C_LI

Large old trees are of eminent importance for organic carbon storage in forest ecosystems and thus play a role in mitigating climate change. Such trees also have an increased risk of internal stem decay and tree cavity formation, which promotes biodiversity, but complicates the prediction of their biomass and carbon stocks, which is usually done from stem diameter and tree height data applying allometric biomass functions. Since the extent of internal stem decay is known to vary widely between different forest ecosystems and data from moist temperate forests exhibited low significance of internal stem decay, we studied dry, frequently fire-exposed Pinus ponderosa forests in central Oregon to capture the other climatic extreme of temperate forests. We hypothesized high significance of internal stem decay for stand aboveground tree biomass, as we assumed widespread stem injury from fire. In addition, we tested the hypothesis that far more than the largest 1% of trees are necessary for 50% stand biomass, as this hypothesis is found in the literature, but has been challenged in other studies. We found low biomass loss due to internal stem decay by only ca. 1% suggesting that also for fire-prone temperate forests of western North America, biomass estimates based on allometric regression are reliable. The 1% largest trees-50% stand aboveground biomass hypothesis has to be rejection for our forests as long as only trees of a size are included that noteworthily contribute to stand biomass. This metrics strongly depends on regeneration density, which is not relevant for stand biomass.