Dr. Valentina Vitali

Key message: Urban trees can acclimate to their growth environment through changes in vessel anatomy. Vessel lumen area and vessel frequency following a gradient from park trees to inner-city street trees.
Abstract: Urban trees stand in potentially stressful growth environments occurring along gradients of urban heat and impermeable surface cover and, to survive, can adjust their function and structure. The consequent tree-to-tree variations in hydraulic xylem traits can shed light on tree hydraulics and capacity to acclimate to diverse conditions, as well as identify limitations to tree growth and survival. Using microscopic analysis of increment cores, we compared early wood vessel traits of the ring-porous angiosperm Celtis occidentalis in three urban site types: central streets, residential streets and parks, within the city of Montreal. We explored differences in vessel traits (mean vessel lumen area, vessel frequency, vessel grouping index and derived variables) between site types, vessel trait intercorrelations and correlations with monthly temperature, precipitation and heat-moisture index over 10 years. The vessel traits significantly differed between site types. Park trees had the largest and central street trees had the smallest vessel lumen area and theoretical hydraulic conductivity; traits supporting efficient water transport. Central street trees had the largest vessel frequency and smallest theoretical vulnerability to cavitation; traits connected to hydraulic safety. Residential street tree traits were in between. Among central and residential street trees, water transport efficiency traits correlated positively with cool springs or arid summers, whereas among park trees, mainly vessel frequency and grouping index responded to climate variations. These results highlight the capacity of C. occidentalis to acclimate to urban environments and the potential of anatomical traits for quantifying the effects of urban environments on tree functioning.

Measurements of stable isotope ratios in organic compounds are widely used tools for plant ecophysiological studies. However, the complexity of the processes involved in shaping hydrogen isotope values (δ²H) in plant carbohydrates has limited its broader application.
To investigate the underlying biochemical processes responsible for ²H fractionation among water, sugars, and cellulose in leaves, we studied the three main CO₂ fixation pathways (C₃, C₄, and CAM) and their response to changes in temperature and vapor pressure deficit (VPD).
We show significant differences in autotrophic ²H fractionation (ε_A) from water to sugar among the pathways and their response to changes in air temperature and VPD. The strong ²H depleting ε_A in C₃ plants is likely driven by the photosynthetic H⁺ production within the thylakoids, a reaction that is spatially separated in C₄ and strongly reduced in CAM plants, leading to the absence of ²H depletion in the latter two types. By contrast, we found that the heterotrophic ²H-fractionation (ε_H) from sugar to cellulose was very similar among the three pathways and is likely driven by the plant's metabolism, rather than by isotopic exchange with leaf water.
Our study offers new insights into the biochemical drivers of ²H fractionation in plant carbohydrates.

Scots pine (Pinus sylvestris L.) is a common European tree species, and understanding its acclimation to the rapidly changing climate through physiological, biochemical or structural adjustments is vital for predicting future growth. We investigated a long-term irrigation experiment at a naturally dry forest in Switzerland, comparing Scots pine trees that have been continuously irrigated for 17 years (irrigated) with those for which irrigation was interrupted after 10 years (stop) and non-irrigated trees (control), using tree growth, xylogenesis, wood anatomy, and carbon, oxygen and hydrogen stable isotope measurements in the water, sugars and cellulose of plant tissues. The dendrochronological analyses highlighted three distinct acclimation phases to the treatments: irrigated trees experienced (i) a significant growth increase in the first 4 years of treatment, (ii) high growth rates but with a declining trend in the following 8 years and finally (iii) a regression to pre-irrigation growth rates, suggesting the development of a new growth limitation (i.e. acclimation). The introduction of the stop treatment resulted in further growth reductions to below-control levels during the third phase. Irrigated trees showed longer growth periods and lower tree-ring δ¹³C values, reflecting lower stomatal restrictions than control trees. Their strong tree-ring δ¹⁸O and δ²H (O–H) relationship reflected the hydrological signature similarly to the control. On the contrary, the stop trees had lower growth rates, conservative wood anatomical traits, and a weak O–H relationship, indicating a physiological imbalance. Tree vitality (identified by crown transparency) significantly modulated growth, wood anatomical traits and tree-ring δ¹³C, with low-vitality trees of all treatments performing similarly regardless of water availability. We thus provide quantitative indicators for assessing physiological imbalance and tree acclimation after environmental stresses. We also show that tree vitality is crucial in shaping such responses. These findings are fundamental for the early assessment of ecosystem imbalances and decline under climate change.

The contribution from foliage, roots, and fungi to soil organic matter (SOM) represents a key unknown in SOM dynamics. Since stable hydrogen isotope ratios (δ²H_n) can greatly differ among these sources, we explored δ²H_n to elucidate sources contribution to SOM. Moreover, we assessed whether addition of ²H-depleted water - providing ²H-signal that gets incorporated into organic matter - allows tracing of new organic H and thus assessing SOM turnover.
In a 17-year-long irrigation experiment in a dry pine forest, we measured δ²H_n and stable carbon and nitrogen isotope ratios (δ¹³C, δ¹⁵N) in SOM sources, bulk SOM and particle-size fractions. Relative source contribution to SOM was estimated by Bayesian mixing models including all three isotopes.
The δ²H_n increased from foliar litter (-153‰) to fine roots (-118‰) and fungal mycelia (-81‰). Mixing models indicated that foliar litter dominated organic layers (69 ± 15%) and coarse particulate organic matter (POM) at 0-5 cm (65 ± 12%). In contrast, roots dominated fine POM at 2-5 cm (58 ± 12%). Fungal mycelia contributed only to 1-5% of coarse and fine POM, but to 5-25% of mineral associated organic matter (MOM) at 0-5 cm.
The soil water ²H-depletion with irrigation (-76‰ vs -68‰ at 0-10 cm) was paralleled by lower δ²H_n in coarse POM of irrigated vs dry plots (-122‰ vs -111‰ at 0-5 cm), indicating organic H turnover of less than 17 years. In contrast, δ²H_n in fine POM and MOM decreased only by 3‰ with irrigation, implying mean residence times of approximately 30 years.
While δ¹³C and δ¹⁵N differed by a maximum of 1‰ between plant sources, δ²H_n were 30-39‰ higher in roots than foliar litter, thus improving SOM sources differentiation. Long-term ²H-labelling by irrigation indicated higher organic H turnover in coarse vs fine soil fractions, offering a novel tool to identify SOM cycling and microbial H incorporation into SOM pools.

Mixed-species plantations of trees with N-fixing species have the potential of promoting forest productivity and soil fertility. However, few studies in the literature have addressed the advantages of mixed-species plantations of leguminous trees over monocultures of leguminous trees based on in situ inventories over a long time period. Here, we monitored the dynamics of tree community composition, vegetation biomass, soil nutrients, and soil microbial phospholipid fatty acids (PLFAs), in an Acacia mangium monoculture plantation during 33 years of development and compared it with a mixed-species plantation of A. mangium associated with 56 native species which were underplanted 14 years after the initial establishment. Leaf N and phosphorus (P) concentrations of three main species in the overstory and understory of the A. mangium monoculture were measured. Our results showed that the soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (AP) concentrations significantly increased over time during the approximately thirty years of A. mangium monoculture plantation, while the disadvantages were associated with new species regeneration and the increment of vegetation biomass. In the A. mangium monoculture plantation, leaf N concentration of A. mangium, Rhodomyrtus tomentosa, and Dicranopteris dichotoma continuously increased from 21 to 31 years, while the leaf P concentration of A. mangium and R. tomentosa decreased. The mixed-species plantations of A. mangium with native tree species had lower SOC and soil TN concentrations, more new tree species recruitment in the understory, and faster vegetation biomass increment than the A. mangium monoculture. However, the PLFAs of soil microbial groups were slightly different between the two types of plantations. We conclude that improved soil N nutrient condition by A. mangium monoculture benefits N absorption by A. mangium, R. tomentosa, and D. tomentosa, while low soil AP limits P absorption by A. mangium and R. tomentosa. Meanwhile, transforming the A. mangium monoculture into a mixed-species plantation via the introduction of multiple native species into the A. mangium monoculture decreases SOC and TN concentrations but the advantages include improving forest regeneration and maintaining forest growth in a long-term sequence. These findings provide useful and practical suggestions for managing forest monocultures of A. mangium in subtropical regions.

Recent methodological advancements in determining the nonexchangeable hydrogen isotopic composition (δ²H_ne) of plant carbohydrates make it possible to disentangle the drivers of hydrogen isotope (²H) fractionation processes in plants. Here, we investigated the influence of phylogeny on the δ²H_ne of twig xylem cellulose and xylem water, as well as leaf sugars and leaf water, across 73 Northern Hemisphere tree and shrub species growing in a common garden. ²H fractionation in plant carbohydrates followed distinct phylogenetic patterns, with phylogeny reflected more in the δ²H_ne of leaf sugars than in that of twig xylem cellulose. Phylogeny had no detectable influence on the δ²H_ne of twig or leaf water, showing that biochemistry, not isotopic differences in plant water, caused the observed phylogenetic pattern in carbohydrates. Angiosperms were more ²H-enriched than gymnosperms, but substantial δ²H_ne variations also occurred at the order, family, and species levels within both clades. Differences in the strength of the phylogenetic signals in δ²H_ne of leaf sugars and twig xylem cellulose suggest that the original phylogenetic signal of autotrophic processes was altered by subsequent species-specific metabolism. Our results will help improve ²H fractionation models for plant carbohydrates and have important consequences for dendrochronological and ecophysiological studies.

Recent experiments have underlined the potential of δ²H in tree-ring cellulose as a physiological indicator of shifts in autotrophic versus heterotrophic processes (i.e., the use of fresh versus stored non-structural carbohydrates). However, the impact of these processes has not yet been quantified under natural conditions. Defoliator outbreaks disrupt tree functioning and carbon assimilation, stimulating remobilization, therefore providing a unique opportunity to improve our understanding of changes in δ²H. By exploring a 700-year tree-ring isotope chronology from Switzerland, we assessed the impact of 79 larch budmoth (LBM, Zeiraphera griseana) outbreaks on the growth of its host tree species, Larix decidua. LBM outbreaks significantly altered the tree-ring isotopic signature, creating a ²H-enrichment and an ¹⁸O- and ¹³C-depletion. Changes in tree physiological functioning in outbreak years are shown by the decoupling of δ²H and δ¹⁸O (O–H relationship), in contrast to the positive correlation in non-outbreak years. Across the centuries, the O–H relationship in outbreak years was not significantly affected by temperature, indicating that non-climatic physiological processes dominate over climate in determining δ²H. We conclude that the combination of these isotopic parameters can serve as a metric for assessing changes in physiological mechanisms over time.

The analysis of the non-exchangeable hydrogen isotope ratio (δ²H_ne) in carbohydrates is mostly limited to the structural component cellulose, while simple high-throughput methods for δ²H_ne values of non-structural carbohydrates (NSC) such as sugar and starch do not yet exist. Here we tested if the hot vapor equilibration method originally developed for cellulose is applicable for NSC, verified by comparison with the traditional nitration method. We set up a detailed analytical protocol and applied the method to plant extracts of leaves from species with different photosynthetic pathways (i.e., C₃, C₄, CAM). δ²H_ne of commercial sugars and starch from different classes and sources, ranging from -157.8 to +6.4‰, were reproducibly analysed with a precision between 0.2 and 7.7‰. Mean δ²H_ne values of sugar are lowest in C₃ (-92.0‰), intermediate in C₄ (-32.5‰), and highest in CAM plants (6.0‰), with NSC being ²H-depleted compared to cellulose and sugar being generally more ²H-enriched than starch. Our results suggest that our method can be used in future studies to disentangle ²H-fractionation processes, for improving mechanistic δ²H_ne models for leaf and tree-ring cellulose, and for further development of δ²H_ne in plant carbohydrates as a potential proxy for climate, hydrology, plant metabolism and physiology.

This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²H_c) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency climate signals in the δ²H_c chronologies were weaker than those recorded in carbon (δ¹³C_c) and oxygen isotope signals (δ¹⁸O_c) but similar to the tree-ring width ones (TRW). The δ²H_c climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. For both genera, years with extremely dry summer conditions caused a significant ²H-enrichment in tree-ring cellulose.
The δ²H_c inter-annual variability was strongly site-specific, as a result of the imprinting of climate and hydrology, but also physiological mechanisms and tree growth. To differentiate between environmental and physiological signals in δ²H_c, we investigated its relationships with δ¹⁸O_c and TRW. We found significant negative relationships between δ²H_c and TRW (7 sites), and positive ones between δ²H_c and δ¹⁸O_c (10 sites). The strength of these relationships was nonlinearly related to temperature and precipitation. Mechanistic δ²H_c models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year δ²H_c variations. Our results suggest that the information recorded by δ²H_c is significantly different from that of δ¹⁸O_c, and has a stronger physiological component independent from climate, possibly related to the use of carbohydrate reserves for growth. Advancements in the understanding of ²H-fractionations and their relationships with climate, physiology, and species-specific traits are needed to improve the modelling and interpretation accuracy of δ²H_c. Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.

1. Biodiversity effects on productivity and other ecosystem functions are strongly dependent on climate and resource availability. Based on the stress‐gradient hypothesis, under conditions of greater abiotic stress, diversity effects on plant performance are intensified due to the increased relative importance of positive plant interactions. However, whether this hypothesis is consistently applicable in forest systems remains unclear. A field trial was established to test the stress‐gradient hypothesis and examine diversity effects on above‐ground biomass production of young trees in mixtures exposed to different water availability.
2. Six native tree species of northern temperate forests (Acer saccharum, Betula papyrifera, Larix laricina, Picea glauca, Pinus strobus and Quercus rubra) were planted as monocultures and as mixtures of two, four and six species. For five growing seasons, four replicates of each community were exposed to conditions of either low‐ or high‐water availability created by rainfall exclusion and weekly irrigation, respectively. Growth‐years 4 and 5 were significantly different when the climatic water balance of the growing seasons was compared. We tested the effects of functional diversity on: (a) total growth of mixtures under low‐ and high‐water availability, and (b) annual growth in years 4 (higher water availability, 2017) and 5 (lower water availability, 2018).
3. Annual growth of most species in both years was greater under high‐ versus low‐water availability. Functional diversity had a significant positive effect on total biomass production and annual growth, and this effect was more strongly expressed under high‐water availability. Functional diversity effects on annual growth did not differ between years 4 and 5 regardless of their climatic water balance. Functional and species identity were key to understanding productivity responses to mixture and treatment effects.
4. Synthesis. Contrary to the stress‐gradient hypothesis, the positive effects of functional diversity on productivity were enhanced by high‐water availability and were independent of seasonal water balance.

Monitoring early tree physiological responses to drought is key to understanding progressive impacts of drought on forests and identifying resilient species. We combined drone-based multispectral remote sensing with measurements of tree physiology and environmental parameters over two growing seasons in a 100-y-old Pinus sylvestris forest subject to 17-y of precipitation manipulation. Our goal was to determine if drone-based photochemical reflectance index (PRI) captures tree drought stress responses and whether responses are affected by long-term acclimation. PRI detects changes in xanthophyll cycle pigment dynamics, which reflect increases in photoprotective non-photochemical quenching activity resulting from drought-induced photosynthesis downregulation. Here, PRI of never-irrigated trees was up to 10 times lower (higher stress) than PRI of irrigated trees. Long-term acclimation to experimental treatment, however, influenced the seasonal relationship between PRI and soil water availability. PRI also captured diurnal decreases in photochemical efficiency, driven by vapor pressure deficit. Interestingly, five years after irrigation was stopped for a subset of the irrigated trees, a positive legacy effect persisted, with lower stress responses (higher PRI) compared with never-irrigated trees. This study demonstrates the ability of remotely sensed PRI to scale tree physiological responses to an entire forest and the importance of long-term acclimation in determining current drought stress responses.

The analysis of stable carbon and oxygen isotopes in tree-rings is a widely applied tool which allows to retrieve information about past climatic conditions, as well as tree physiological responses to environmental changes. This is based on well-established mechanistic models and firm statistical relationships with climate variables. In contrast, the hydrogen isotopic signature (δ²H) of tree-rings has been reported to be poorly correlated to climate or difficult to explain, and as a consequence, hydrogen isotopes are far less utilized. However, recent plant-physiological experiments have highlighted the role of autotrophic versus heterotrophic processes affecting δ²H values, i.e. use of fresh assimilates versus stored carbohydrates, and have much improved our understanding of the role of post-photosynthetic ²H-fractionation. Using unpublished and literature δ²H data of tree-ring cellulose (δ²H_C) of 5 study sites in Europe and Asia, we systematically investigated the relationships between δ²H_C and tree-ring width (TRW), which, in contrast to previous research, could now be explained through post-photosynthetic ²H-fractionation. In most cases, these relationships were found to be negative (r² = 0.23 to 0.51, all P < 0.05) when the main growth limiting factors are precipitation and light, while in temperature-limited sites we observed a positive trend (r² = 0.14, P < 0.05). Our results suggest that, under stress conditions, trees use a surplus of carbon from reserves for wood formation. Therefore, in combination with TRW chronologies, δ²H_C may allow to infer about physiological information on stressful time periods independently of biotic and abiotic origin. Here, we discuss implications of these findings for tree-ring research, summarize them in a conceptual framework and suggest future research directions.

Trees that grow in urban areas are confronted with a wide variety of stresses that undermine their long-term survival. These include mechanical damage to the crown, root reduction and stem injury, all of which remove significant parts of plant tissues. The single or combined effects of these stresses generate a complex array of growth and ecophysiological responses that are hard to predict. Here we evaluated the effects of different individual and combined damage on the dynamics of non-structural carbohydrates (NSC, low weight sugars plus starch) concentration and new tissue growth (diameter increment) in young trees. We hypothesized that (i) tissue damage will induce larger reductions in diameter growth than in NSC concentrations and (ii) combinations of stress treatments that minimally alter the “functional equilibrium” (e.g., similar reductions of leaf and root area) would have the least impact on NSC concentrations (although not on growth) helping to maintain tree health and integrity. To test these hypotheses, we set up a manipulative field experiment with 10-year-old trees of common urban species (Celtis occidentalis, Fraxinus pennsylvanica, and Tilia cordata). These trees were treated with a complete array of mechanical damage combinations at different levels of intensity (i.e., three levels of defoliation and root reduction, and two levels of stem damage). We found that tree growth declined in relation to the total amount of stress inflicted on the trees, i.e., when the combined highest level of stress was applied, but NSC concentrations were either not affected or, in some cases, increased with an increasing level of stress. We did not find a consistent response in concentration of reserves in relation to the combined stress treatments. Therefore, trees appear to reach a new “functional equilibrium” that allows them to adjust their levels of carbohydrate reserves, especially in stems and roots, to meet their metabolic demand under stressful situations. Our results provide a unique insight into the carbon economy of trees facing multiple urban stress conditions in order to better predict long-term tree performance and vitality.

Picea abies and Fagus sylvatica are important tree species in Europe, and the foreseen increase in temperature and VPD could increase the vulnerability of these species. However, their physiological performance under climate change at temperate and productive sites is not yet fully understood, especially in uneven aged stands. Therefore, we investigated tree-ring width and stable isotope chronologies (δ¹³C/δ¹⁸O) of these two species at ten sites along a climate gradient in Central Europe. In these uneven-aged stands, we compared the year-to-year variability of dominant and suppressed trees for the last 80 years in relation to the sites’ spatial distribution and climate. δ¹⁸O and δ¹³C were generally consistent across sites and species, showing high sensitivity to summer VPD, whereas climate correlations with radial growth varied much more and depended on mean local climate. We found no significant differences between dominant and suppressed trees in the response of stable isotope ratios to climate variability, especially within the annual high-frequency signals. Additionally, we observed a strikingly high coherence of the high-frequency δ¹⁸O variations across long distances with significant correlations above 1,500 km, while the spatial agreement of δ¹³C variations was weaker (~700 km). We applied a dual-isotope approach that is based on known theoretical understanding of isotope fractionations to translate the observed changes into physiological components, mainly photosynthetic assimilation rate and stomatal conductance. When separating the chronologies in two time windows and investigating the shifts in isotopes ratios, a significant enrichment of either or both isotope ratios over the last decades can be observed. These results, translated by the dual-isotope approach, indicate a general climate-driven decrease in stomatal conductance. This improved understanding of the physiological mechanisms controlling the short-term variation of the isotopic signature will help to define the performance of these tree species under future climate.

Purpose of review Despite the rapidly increasing use of resilience indices to analyze responses of trees and forests to disturbance events, there is so far no common framework to apply and interpret these indices for different purposes. Therefore, this review aims to identify and discuss various shortcomings and pitfalls of commonly used resilience indices and to develop recommendations for a more robust and standardized procedure with a particular emphasis on drought events.
Recent findings Growth-based resilience indices for drought responses of trees are widely used but some important drawbacks and limitations related to their application may lead to spurious results or misinterpretation of observed patterns. The limitations include (a) the inconsistency regarding the selection and characterization of drought events and the climatic conditions in the pre- and post-drought period and (b) the calculation procedure of growth-based resilience indices.
Summary We discuss alternative options for metrics, which, when used in concert, can provide a more comprehensive understanding of drought responses in cases where common growth-based resilience indices are likely to fail. In addition, we propose a new analytical framework, the "line of full resilience," that integrates the three most commonly used resilience indices and show how this framework can be used for comparative drought tolerance assessments such as rankings of different tree species or treatments. The suggested approach could be used to harmonize quantifications of tree growth resilience to drought and it may thus facilitate systematic reviews and development of the urgently needed evidence base to identify suitable management options or tree species and provenances to adapt forests for changing climatic conditions.

New forest edges are continuously being created by forest management. In the Swiss Alps, silvicultural treatments have partly changed from the selection cutting widespread two decades ago to a more intensive strip cutting. However, little is known about the impact of such harvesting on tree growth and on the structural development of Alpine forest stands dominated by Norway spruce (Picea abies (L.) Karst.), which have high economic and protective value.
We therefore investigated the effect of strip cutting in four Alpine spruce stands differing in site and stand conditions through a dendrochronological analysis of 134 tree stems. The change in growth rate was assessed for the 10-year period before and after the cutting year, and rate changes in edge and non-edge trees were compared. The relative change in Hegyi's competition index before and after the cut was used as a proxy for the change in space and related resources. A linear model was developed to assess the effects of biotic and abiotic variables on changes in growth after strip cutting.
Radial growth responses varied greatly between the stands, with a significant increase only in edge trees in the two north-facing sites, i.e. 12% and 60%. Changes in tree competition had the strongest impact on tree growth, followed by site effects. With the same relative change in competition index, the radial growth of edge trees increased more strongly in reaction to cutting than that of non-edge trees. Additionally, small-diameter trees growing near edges benefited more from the strip cutting than larger trees.
Our results suggest that strip cutting on north-facing slopes can boost the growth of trees on the east and north-east-facing forest edges. Small spruce trees growing along newly created forest edges can be kept to enhance stand yield. As cutting often leads to long forest edges and may thus affect the growth of a significant proportion of the forest area, such effects should be considered in planning cutting layouts.