General reversal of N-decomposition relationship during long-term decomposition in boreal and temperate forests.

Decomposition of dead organic matter is fundamental to carbon (C) and nutrient cycling in terrestrial ecosystems, influencing C fluxes from the biosphere to the atmosphere. Theory predicts and evidence strongly supports that the availability of nitrogen (N) limits litter decomposition. Positive relationships between substrate N concentrations and decomposition have been embedded into ecosystem models. This decomposition paradigm, however, relies on data mostly from short-term studies analyzing controls on early-stage decomposition. We present evidence from three independent long-term decomposition investigations demonstrating that the positive N-decomposition relationship is reversed and becomes negative during later stages of decomposition. First, in a 10-y decomposition experiment across 62 woody species in a temperate forest, leaf litter with higher N concentrations exhibited faster initial decomposition rates but ended up a larger recalcitrant fraction decomposing at a near-zero rate. Second, in a 5-y N-enrichment experiment of two tree species, leaves with experimentally enriched N concentrations had faster decomposition initial rates but ultimately accumulated large slowly decomposing fractions. Measures of amino sugars on harvested litter in two experiments indicated that greater accumulation of microbial residues in N-rich substrates likely contributed to larger slowly decomposing fractions. Finally, a database of 437 measurements from 120 species in 45 boreal and temperate forest sites confirmed that higher N concentrations were associated with a larger slowly decomposing fraction. These results challenge the current treatment of interactions between N and decomposition in many ecosystems and Earth system models and suggest that even the best-supported short-term controls of biogeochemical processes might not predict long-term controls.

Different responses of oxygen and hydrogen isotopes in leaf and tree-ring organic matter to lethal soil drought.

The oxygen and hydrogen isotopic composition (δ18O, δ2H) of plant tissues are key tools for the reconstruction of hydrological and plant physiological processes and may therefore be used to disentangle the reasons for tree mortality. However, how both elements respond to soil drought conditions before death has rarely been investigated. To test this, we performed a greenhouse study and determined predisposing fertilization and lethal soil drought effects on δ18O and δ2H values of organic matter in leaves and tree rings of living and dead saplings of five European tree species. For mechanistic insights, we additionally measured isotopic (i.e. δ18O and δ2H values of leaf and twig water), physiological (i.e. leaf water potential and gas-exchange) and metabolic traits (i.e. leaf and stem non-structural carbohydrate concentration, carbon-to-nitrogen ratios). Across all species, lethal soil drought generally caused a homogenous 2H-enrichment in leaf and tree-ring organic matter, but a low and heterogenous δ18O response in the same tissues. Unlike δ18O values, δ2H values of tree-ring organic matter were correlated with those of leaf and twig water and with plant physiological traits across treatments and species. The 2H-enrichment in plant organic matter also went along with a decrease in stem starch concentrations under soil drought compared with well-watered conditions. In contrast, the predisposing fertilization had generally no significant effect on any tested isotopic, physiological and metabolic traits. We propose that the 2H-enrichment in the dead trees is related to (i) the plant water isotopic composition, (ii) metabolic processes shaping leaf non-structural carbohydrates, (iii) the use of carbon reserves for growth and (iv) species-specific physiological adjustments. The homogenous stress imprint on δ2H but not on δ18O suggests that the former could be used as a proxy to reconstruct soil droughts and underlying processes of tree mortality.

The role of Manganese in tree defenses against pests and pathogens.

Manganese (Mn) deficiency is a widespread occurrence across different landscapes, including agricultural systems and managed forests, and causes interruptions in the normal metabolic functioning of plants. The microelement is well-characterized for its role in the oxygen-evolving complex in photosystem II and maintenance of photosynthetic structures. Mn is also required for a variety of enzymatic reactions in secondary metabolism, which play a crucial role in defense strategies for trees. Despite the strong relationship between Mn availability and the biosynthesis of defense-related compounds, there are few studies addressing how Mn deficiency can impact tree defense mechanisms and the ensuing ecological patterns and processes. Understanding this relationship and highlighting the potentially deleterious effects of Mn deficiency in trees can also inform silvicultural and management decisions to build more robust forests. In this review, we address this relationship, focusing on forest trees. We describe Mn availability in forest soils, characterize the known impacts of Mn deficiency in plant susceptibility, and discuss the relationship between Mn and defense-related compounds by secondary metabolite class. In our review, we find several lines of evidence that low Mn availability is linked with lowered or altered secondary metabolite activity. Additionally, we compile documented instances where Mn limitation has altered the defense capabilities of the host plant and propose potential ecological repercussions when studies are not available. Ultimately, this review aims to highlight the importance of untangling the effects of Mn limitation on the ecophysiology of plants, with a focus on forest trees in both managed and natural stands.

The European trees phyllosphere characteristics and its potential in air bioremediation.

The air pollution remediation is naturally carried out by plants. Their overground parts called phyllosphere are a type of a natural filter on which pollutants can be adsorb. Moreover, microbial communities living in phyllosphere perform a variety of biochemical processes removing also chemical pollutants. As their pollution is nowadays a burning issue especially for highly developed countries, the development of effective and ecological technologies for air treatment are of the utmost importance. The use of phyllosphere bacteria in the process of air bioremediation is a promising technology. This article reviews the role of phyllospheric bacteria in air bioremediation processes especially linked with the moderate climate plants. Research results published so far indicate that phyllosphere bacteria are able to metabolize the air pollutants but their potential is strictly determined by plant-phyllospheric bacteria interaction. The European tree species most commonly used for this purpose are also presented. The collected information filled the gap in the practical use of tree species in air bioremediation in the moderate climate zone.

Microbial mechanisms for improved soil phosphorus mobilization in monoculture conifer plantations by mixing with broadleaved trees.

Replanting broadleaved trees in monoculture conifer plantations has been shown to improve the ecological environment. However, not much is known about the distribution properties of soil phosphate-mobilizing bacteria (PMB) under different mixed plantings or how PMB affects biometabolism-driven phosphorus (P) bioavailability. The phoD and pqqC genes serve as molecular markers of PMB because they regulate the mobilization of organic (Po) and inorganic (Pi) P. Differences in soil bioavailable P concentration, phoD- and pqqC-harboring PMB communities, and their main regulators were analyzed using biologically-based P (BBP) and high-throughput sequencing approaches after combining coniferous trees (Pinus massoniana) and five individual broadleaved trees (Bretschneidera sinensis, Michelia maudiae, Cercidiphyllum japonicum, Manglietia conifera, and Camellia oleifera). The findings revealed that the contents of litter P, soil organic carbon (SOC), available Pi (CaCl2-P), and labile Po (Enzyme-P) were significantly higher in conifer-broadleaf mixed plantations than those in the monospecific Pinus massoniana plantations (PM), especially in the mixed stands with the introduction of Cercidiphyllum japonicum, Michelia maudiae, and Camellia oleifera. Conifer-broadleaf mixing had little effect on the abundance of phoD and pqqC genes but significantly altered species composition within the communities. Conifer-broadleaf mixing improved soil microbial habitat mainly by increasing the pH, increasing carbon source availability and nutrient content, decreasing exchangeable Fe3+ and Al3+ content, and decreasing the activation degrees of Fe and Al oxides in acidic soils. A small group of taxa (phoD: Bradyrhizobium, Tardiphaga, Nitratireductor, Mesorhizobium, Herbaspirillum, and Ralstonia; pqqC: Burkholderia, Variovorax, Bradyrhizobium, and Leptothrix) played a key role in the synthesis of P-related enzymes (e.g., alkaline phosphomonoesterase, ALP) and in lowering the levels of mineral-occluded (HCl-P) and chelated (Citrate-P) Pi. Overall, our findings highlight that mixing conifers and broadleaves could change the PMB communities that produce ALP and dissolve Pi to make P more bioavailable.

Carbon budget at the individual-tree scale: dominant Eucalyptus trees partition less carbon belowground.

Large trees in plantations generally produce more wood per unit of resource use than small trees. Two processes may account for this pattern: greater photosynthetic resource use efficiency or greater partitioning of carbon to wood production. We estimated gross primary production (GPP) at the individual scale by combining transpiration with photosynthetic water-use efficiency of Eucalyptus trees. Aboveground production fluxes were estimated using allometric equations and modeled respiration; total belowground carbon fluxes (TBCF) were estimated by subtracting aboveground fluxes from GPP. Partitioning was estimated by dividing component fluxes by GPP. Dominant trees produced almost three times as much wood as suppressed trees. They used 25 ± 10% (mean ± SD) of their photosynthates for wood production, whereas suppressed trees only used 12 ± 2%. By contrast, dominant trees used 27 ± 19% of their photosynthate belowground, whereas suppressed trees used 58 ± 5%. Intermediate trees lay between these extremes. Photosynthetic water-use efficiency of dominant trees was c. 13% greater than the efficiency of suppressed trees. Suppressed trees used more than twice as much of their photosynthate belowground and less than half as much aboveground compared with dominant trees. Differences in carbon partitioning were much greater than differences in GPP or photosynthetic water-use efficiency.

Polycyclic aromatic hydrocarbon (PAH) phytoaccumulation in urban areas by Platanus × acerifolia, Celtis australis, and Tilia grandifolia leaves and branches.

Polycyclic aromatic hydrocarbon (PAH) concentrations in the leaves and 1-year-old branches of three common tree species growing in a middle-sized city located in a moderate climate zone were estimated. For this purpose, PAH phytoaccumulation in Platanus × acerifolia, Celtis australis, and Tilia grandifolia species from highly urbanized, traffic congested, and highly PAH-contaminated streets was compared with trees from non-contaminated parks in the same urban core. The gathered data was used to define 17 PAH profiles, identify the main PAH pollution emission sources, and determine the organ and species specificity of PAHs accumulation. Due to the direct absorption of polluted air via stomata, the leaves accumulated up to 30% more PAHs compared to the 1-year-old branches. As expected, PAH concentrations were much higher in street trees, while heavy weight PAHs (with five and six rings) were accumulated in the highest concentrations. The highest foliar Σ17 PAH concentrations were detected in street-grown C. australis, followed by P. acerifolia and T. grandifolia (502.68, 488.45, and 339.47 ng g-1 dry weight (DW), respectively). The same pattern was noted for Σ17 PAHs in branches (414.89, 327.58, and 342.99 ng g-1 DW, respectively). Thus, T. grandifolia emerged as the least effective PAH sink as it accumulated up to ~ 40% less PAHs than P. acerifolia and C. australis leaves/branches. Among the 17 tracked PAHs, benzo[a]anthracene, benzo[a]pyrene, dibenzo[a,h]anthracene, and pyrene were found to have accumulated in the highest concentrations in all analyzed species irrespective of the site, and accounted for more than 50% of the total detected PAHs. Finally, a "black box" about species and organ specificity, as well as specific drivers that limit PAHs uptake capacity by trees, was opened, while this work provides insights into further PAH phytoremediation strategies.

Isotope Distribution Analysis in H218O Pulse-Labeled Trees Frozen with Liquid Nitrogen.

Tracer injection has long been recognized as a valuable tool for delineating tree hydraulics and assessing water transport pathways. Recently, isotope tracers have emerged as innovative instruments for investigating tree hydraulics, providing new insights into tree water dynamics. Nevertheless, there is a critical need for further research to comprehensively grasp water movement and distribution within trees. A previously introduced technique for analyzing the isotopic ratio of water in wet tissues, offering millimeter-scale resolution for visualizing tracer movement, faces challenges due to its underdeveloped sample preparation techniques. In this study, we introduced an H2 18O tracer into S. gracilistyla samples, exclusively comprising indeterminate roots, stems, and leaves, cultivated through hydroponics and grown within the current year. Our objective was to assess the axial distribution of the tracer in the xylem. Additionally, we devised a novel method for preparing frozen wet tissue samples, enhancing the repeatability and success rate of experiments. The results demonstrated that all frozen wet tissue samples exhibited an average water loss rate of less than 0.6%. Isotopic analysis of these samples unveiled a consistent decline in tracer concentration with increasing height in all Salix specimens, with three out of five samples revealing a significant isotope gradient. Our findings affirm the efficacy and practicality of combining isotopic labeling with freezing, stabilization, and preparation techniques. Looking ahead, our isotopic labeling and analysis methods are poised to transcend woody plants, finding extensive applications in plant physiology and ecohydrology.

Response of carbon fixation, allocation, and growth to source-sink manipulation by defoliation in vegetative citrus trees.

Source-sink balance in plants determines carbon distribution, and altering it can impact carbon fixation, transport, and allocation. We aimed to investigate the effect of altered source-sink ratios on carbon fixation, transport, and distribution in 'Valencia' sweet orange (Citrus x sinensis) by various defoliation treatments (0%, 33%, 66%, and 83% leaf removal). Gas exchange parameters were measured on 0 and 10 days after defoliation using A/Ci response curves, and leaf export was measured two days after defoliation using radioisotope tracer techniques. Greater defoliation increased the maximum rate of carboxylation (Vcmax), electron transport rate (J1200), and triose-phosphate utilization rate (TPU). Leaf export was unaffected by defoliation but increased in leaves closer to the shoot apex. Basipetal translocation velocity in the trunk remained unaltered, indicating that more photosynthates remained in the shoot rather than being transported directly to the root sink. Defoliated plants initiated more new flush shoots but accumulated less shoot biomass per plant after 8 weeks. Carbon allocation to fine roots was smaller in defoliated plants, suggesting defoliation led to retention of carbohydrates in aboveground organs such as the trunk and other shoots from previous growing cycles. In conclusion, the low source-sink ratio increased carbon fixation without impacting individual leaf export in citrus. The results suggest that intermediate sinks such as the aboveground perennial organs play a role in mediating the translocation velocity. Further research is necessary to better understand the dynamics of source-sink regulation in citrus trees.

Tight coupling between leaf δ13 C and N content along leaf ageing in the N2 -fixing legume tree black locust (Robinia pseudoacacia L.).

N2 -fixing legumes can strongly affect ecosystem functions by supplying nitrogen (N) and improving the carbon-fixing capacity of vegetation. Still, the question of how their leaf-level N status and carbon metabolism are coordinated along leaf ageing remains unexplored. Leaf tissue carbon isotopic composition (δ13 C) provides a useful indicator of time-integrated intrinsic water use efficiency (WUEi). Here, we quantified the seasonal changes of leaf δ13 C, N content on a mass and area basis (Nmass , Narea , respectively), Δ18 O (leaf 18 O enrichment above source water, a proxy of time-integrated stomatal conductance) and morphological traits in an emblematic N2 -fixing legume tree, the black locust (Robinia pseudoacacia L.), at a subtropical site in Southwest China. We also measured xylem, soil and rainwater isotopes (δ18 O, δ2 H) to characterize tree water uptake patterns. Xylem water isotopic data reveal that black locust primarily used shallow soil water in this humid habitat. Black locust exhibited a decreasing δ13 C along leaf ageing, which was largely driven by decreasing leaf Nmass , despite roughly constant Narea . In contrast, the decreasing δ13 C along leaf ageing was largely uncoupled from parallel increases in Δ18 O and leaf thickness. Leaf N content is used as a proxy of leaf photosynthetic capacity; thus, it plays a key role in determining the seasonality in δ13 C, whereas the roles of stomatal conductance and leaf morphology are minor. Black locust leaves can effectively adjust to changing environmental conditions along leaf ageing through LMA increases and moderate stomatal conductance reduction while maintaining constant Narea to optimize photosynthesis and carbon assimilation, despite declining leaf Nmass and δ13 C.

The metabolic fingerprint of Scots pine-root and needle metabolites show different patterns in dying trees.

The loss of leaves and needles in tree crowns and tree mortality are increasing worldwide, mostly as a result of more frequent and severe drought stress. Scots pine (Pinus sylvestris L.) is a tree species that is strongly affected by these developments in many regions of Europe and Asia. So far, changes in metabolic pathways and metabolite profiles in needles and roots on the trajectory toward mortality are unknown, although they could contribute to a better understanding of the mortality mechanisms. Therefore, we linked long-term observations of canopy defoliation and tree mortality with the characterization of the primary metabolite profile in needles and fine roots of Scots pines from a forest site in the Swiss Rhone valley. Our results show that Scots pines are able to maintain metabolic homeostasis in needles over a wide range of canopy defoliation levels. However, there is a metabolic tipping point at around 80-85% needle loss. Above this threshold, many stress-related metabolites (particularly osmoprotectants, defense compounds and antioxidants) increase in the needles, whereas they decrease in the fine roots. If this defoliation tipping point is exceeded, the trees are very likely to die within a few years. The different patterns between needles and roots indicate that mainly belowground carbon starvation impairs key functions for tree survival and suggest that this is an important factor explaining the increasing mortality of Scots pines.

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.

Phytochrome-interacting factors play shared and distinct roles in regulating shade avoidance responses in Populus trees.

Plants adjust their growth and development in response to changing light caused by canopy shade. The molecular mechanisms underlying shade avoidance responses have been widely studied in Arabidopsis and annual crop species, yet the shade avoidance signalling in woody perennial trees remains poorly understood. Here, we first showed that PtophyB1/2 photoreceptors serve conserved roles in attenuating the shade avoidance syndrome (SAS) in poplars. Next, we conducted a systematic identification and characterization of eight PtoPIF genes in Populus tomentosa. Knocking out different PtoPIFs led to attenuated shade responses to varying extents, whereas overexpression of PtoPIFs, particularly PtoPIF3.1 and PtoPIF3.2, led to constitutive SAS phenotypes under normal light and enhanced SAS responses under simulated shade. Notably, our results revealed that distinct from Arabidopsis PIF4 and PIF5, which are major regulators of SAS, the Populus homologues PtoPIF4.1 and PtoPIF4.2 seem to play a minor role in controlling shade responses. Moreover, we showed that PtoPIF3.1/3.2 could directly activate the expression of the auxin biosynthetic gene PtoYUC8 in response to shade, suggesting a conserved PIF-YUC-auxin pathway in modulating SAS in tree. Overall, our study provides insights into shared and divergent functions of PtoPIF members in regulating various aspects of the SAS in Populus.

The mobilization and transport of newly fixed carbon are driven by plant water use in an experimental rainforest under drought.

Non-structural carbohydrates (NSCs) are building blocks for biomass and fuel metabolic processes. However, it remains unclear how tropical forests mobilize, export, and transport NSCs to cope with extreme droughts. We combined drought manipulation and ecosystem 13CO2 pulse-labeling in an enclosed rainforest at Biosphere 2, assessed changes in NSCs, and traced newly assimilated carbohydrates in plant species with diverse hydraulic traits and canopy positions. We show that drought caused a depletion of leaf starch reserves and slowed export and transport of newly assimilated carbohydrates below ground. Drought effects were more pronounced in conservative canopy trees with limited supply of new photosynthates and relatively constant water status than in those with continual photosynthetic supply and deteriorated water status. We provide experimental evidence that local utilization, export, and transport of newly assimilated carbon are closely coupled with plant water use in canopy trees. We highlight that these processes are critical for understanding and predicting tree resistance and ecosystem fluxes in tropical forest under drought.

Photochemical attributes determine the responses of plant species from different functional groups of ferruginous outcrops when grown in iron mining substrates.

Environments originating from banded iron formations, such as the canga , are important reference ecosystems for the recovery of degraded areas by mining. The objective of this work was to evaluate if the relationship between morphofunctional and photosynthetic attributes of native canga species from different functional group results in distinct responses when grown in iron mining tailings substrate. The experiment was carried out with species belonging to different functional groups: a widespread semi-deciduous tree-shrub, Myrcia splendens ; an endemic deciduous shrub, Jacaranda caroba ; and a nitrogen-fixing herbaceous species, Periandra mediterranea . The species were grown in two conditions, reference soil and iron ore tailing. Despite belonging to different functional groups when grown in tailings, the morphofunctional attributes presented similar responses between species. M. splendens was the species most affected by the conditions imposed by the iron ore mining tailings, with decreased light-use efficiency and electron transport. P. mediterranea had satisfactory growth and maintenance of photosynthetic attributes. J. caroba growing in the tailings increased the effective quantum yield of PSII. The photochemical and growth assessments were able to better explain the adaptive strategies developed by the species, guaranteeing a greater chance of success during the rehabilitation of mining substrates.

Modulating phyllosphere microbiome structure and function in Loropetalum chinense and Osmanthus fragrans: The impact of foliar dust and heavy metals.

Trees can effectively capture airborne particles and improve air quality. However, the specific response of phyllosphere microbiome (PMo) in different plant species to particulate matter (PM) and the heavy metals it contains are not yet fully understood. In this study, we investigated the impact of PM on the diversity and function of PMo in Loropetalum chinense and Osmanthus fragrans trees grown in industrial and clean zones with varying levels of PM pollution. Our findings revealed that leaf dust had a significant negative effect on microbial richness, with O. fragrans exhibiting higher microbial diversity than L. chinense. The dominant phylum of phyllosphere bacteria in all samples was Proteobacteria, and the dominant genera were Stenotrophomonas and Delftia. The relative abundance of these genera varied significantly among plant species and regions. Our results showed that PM had a significant impact on the community composition of PMo, with the presence of heavy metals exerting a greater effect than particle size. Moreover, the foliar microbial community of plants grown in industrial zones exhibited significantly higher metabolic functions related to stress resistance and disease resistance compared to plants in control zones. These findings highlight the structural and functional responses of PMo to PM and indicate their potential for enhancing plant adaptation to environmental stress.

Protecting an artificial savanna as a nature-based solution to restore carbon and biodiversity in the Democratic Republic of the Congo.

A large share of the global forest restoration potential is situated in artificial 'unstable' mesic African savannas, which could be restored to higher carbon and biodiversity states if protected from human-induced burning. However, uncertainty on recovery rates in protected unstable savannas impedes science-informed forest restoration initiatives. Here, we quantify the forest restoration success of anthropogenic fire exclusion within an 88-ha mesic artificial savanna patch in the Kongo Central province of the Democratic Republic of the Congo (DR Congo). We found that aboveground carbon recovery after 17 years was on average 11.40 ± 0.85 Mg C ha-1 . Using a statistical model, we found that aboveground carbon stocks take 112 ± 3 years to recover to 90% of aboveground carbon stocks in old-growth forests. Assuming that this recovery trajectory would be representative for all unstable savannas, we estimate that they could have a total carbon uptake potential of 12.13 ± 2.25 Gt C by 2100 across DR Congo, Congo and Angola. Species richness recovered to 33.17% after 17 years, and we predicted a 90% recovery at 54 ± 2 years. In contrast, we predicted that species composition would recover to 90% of old-growth forest composition only after 124 ± 3 years. We conclude that the relatively simple and cost-efficient measure of fire exclusion in artificial savannas is an effective nature-based solution to climate change and biodiversity loss. However, more long-term and in situ monitoring efforts are needed to quantify variation in long-term carbon and diversity recovery pathways. Particular uncertainties are spatial variability in socio-economics and growing conditions as well as the effects of projected climate change.

Plant secondary metabolites involved in the stress tolerance of long-lived trees.

Ancient trees are natural wonders because of their longevity, having lived for hundreds or thousands of years, and their ability to withstand changing environments and a variety of stresses. These long-lived trees have sophisticated defense mechanisms, such as the production of specialized plant metabolites (SPMs). In this review, we provide an overview of the major biotic and abiotic stresses that long-lived trees often face, as well as an analysis of renowned ancient tree species and their unique protective SPMs against environmental stressors. We also discuss the synthesis and accumulation of defensive SPMs induced by environmental factors and endophytes in these trees. Furthermore, we conducted a comparative genomic analysis of 17 long-lived tree species and discovered significant expansions of SPM biosynthesis gene families in these species. Our comprehensive review reveals the crucial role of SPMs in high resistance in long-lived trees, providing a novel natural resource for plant defense, crop improvement and even the pharmaceutical industry.