Dr. Patrick Fonti

Quantitative wood anatomy (QWA), which involves measuring wood cell anatomical characteristics commonly on dated tree rings, is becoming increasingly important within plant sciences and ecology. This approach is particularly valuable for studies that require processing a large number of samples, such as those aimed at millennial-long climatic reconstructions. However, the field faces significant challenges, including the absence of a publicly available comprehensive protocol for efficiently and uniformly producing high-quality wood thin sections for QWA along dated tree-ring series. This issue is especially critical for more brittle subfossil wood, in addition to fresh material from living trees. Our manuscript addresses these challenges by providing a detailed protocol for producing thin anatomical sections of wood and digital images, specifically tailored for long chronologies of tree-ring anatomy with an emphasis on conifer wood. The protocol includes step-by-step procedures for sample preparation, sectioning, and imaging, ensuring consistent and high-quality results. By offering this well-tried-and-tested protocol, we aim to facilitate reproducibility and accuracy in wood anatomical studies, ultimately advancing research in this field. It aims to serve as a reference for researchers and laboratories engaged in similar work, promoting standardized practices and enhancing the reliability of QWA data.

In the temperate zone, deciduous trees exhibit clear above-ground seasonality, marked by a halt in wood growth that represents the completion of wood formation in autumn and reactivation in spring. However, the growth seasonality of below-ground woody organs, such as coarse roots, has been largely overlooked. Here we use tree monitoring data and pot experiments involving saplings to examine the late-season xylem development of stem and coarse roots with leaf phenology in four common deciduous tree species in Western Europe. Coarse-roots wood growth continued throughout the winter whereas stem wood growth halted in autumn, regardless of the tree species, experimental setting or location. Our results do not indicate a clear temperature constraint on below-ground wood growth, even during prolonged periods with soil temperatures lower than 3 °C. The continuous differentiation of xylem root cells in autumn and winter suggests that the non-growing season does not exist sensu stricto for all woody organs of angiosperm deciduous tree species of the temperate zone. Our findings hold implications for understanding tree functioning, in particular the seasonal wood formation, the environmental controls of tree growth and the carbon reserves dynamics.

Stomata control plant water loss and photosynthetic carbon gain. Developing more generalized and accurate stomatal models is essential for earth system models and predicting responses under novel environmental conditions associated with global change. Plant optimality theories offer one promising approach, but most such theories assume that stomatal conductance maximizes photosynthetic net carbon assimilation subject to some cost or constraint of water. We move beyond this approach by developing a new, generalized optimality theory of stomatal conductance, optimizing any non-foliar proxy that requires water and carbon reserves, like growth, survival, and reproduction. We overcome two prior limitations. First, we reconcile the computational efficiency of instantaneous optimization with a more biologically meaningful dynamic feedback optimization over plant lifespans. Second, we incorporate non-steady-state physics in the optimization to account for the temporal changes in the water, carbon, and energy storage within a plant and its environment that occur over the timescales that stomata act, contrary to previous theories. Our optimal stomatal conductance compares well to observations from seedlings, saplings, and mature trees from field and greenhouse experiments. Our model predicts predispositions to mortality during the 2018 European drought and captures realistic responses to environmental cues, including the partial alleviation of heat stress by evaporative cooling and the negative effect of accumulating foliar soluble carbohydrates, promoting closure under elevated CO₂. We advance stomatal optimality theory by incorporating generalized evolutionary fitness proxies and enhance its utility without compromising its realism, offering promise for future models to more realistically and accurately predict global carbon and water fluxes.

Forests are undergoing increasing risks of drought-induced tree mortality. Species replacement patterns following mortality may have a significant impact on the global carbon cycle. Among major hardwoods, deciduous oaks (Quercus spp.) are increasingly reported as replacing dying conifers across the Northern Hemisphere. Yet, our knowledge on the growth responses of these oaks to drought is incomplete, especially regarding post-drought legacy effects. The objectives of this study were to determine the occurrence, duration, and magnitude of legacy effects of extreme droughts and how that vary across species, sites, and drought characteristics. The legacy effects were quantified by the deviation of observed from expected radial growth indices in the period 1940–2016. We used stand-level chronologies from 458 sites and 21 oak species primarily from Europe, north-eastern America, and eastern Asia. We found that legacy effects of droughts could last from 1 to 5 years after the drought and were more prolonged in dry sites. Negative legacy effects (i.e., lower growth than expected) were more prevalent after repetitive droughts in dry sites. The effect of repetitive drought was stronger in Mediterranean oaks especially in Quercus faginea. Species-specific analyses revealed that Q. petraea and Q. macrocarpa from dry sites were more negatively affected by the droughts while growth of several oak species from mesic sites increased during post-drought years. Sites showing positive correlations to winter temperature showed little to no growth depression after drought, whereas sites with a positive correlation to previous summer water balance showed decreased growth. This may indicate that although winter warming favors tree growth during droughts, previous-year summer precipitation may predispose oak trees to current-year extreme droughts. Our results revealed a massive role of repetitive droughts in determining legacy effects and highlighted how growth sensitivity to climate, drought seasonality and species-specific traits drive the legacy effects in deciduous oak species.

Frost drought refers to the chronic or acute desiccation of trees exposed to high evaporative pressures while being rooted in cold or frozen soils. This phenomenon has been known for more than a century but is still poorly characterized. Summer desiccation manifests itself as long-term stem contractions. Similar contractions have been reported in winter. In this study, we investigated the causes of total winter stem contraction (WSC) using 14 years of dendrometer data from evergreen (P.abies) and deciduous (L.decidua) mature trees growing along an elevational transect (from 800 to 2200 m asl) in the Swiss Alps. Results indicated that WSC varied between and and were strongly dependent on species, elevation, and tree height. Moreover, the magnitude of contractions was strongly associated with stem contractions subsequent to freeze–thaw events (F). We suggest that both F and WSC are the consequences of water losses due to ice blockage associated frost drought, occurring when the distal parts of the tree are thawed and transpiring, while the larger basal parts remain frozen, thus inhibiting water uptake and creating a hydraulic imbalance.

Cellulose of tree rings is often assumed to be predominantly formed by direct assimilation of CO₂ by photosynthesis and consequently can be used to reconstruct past atmospheric ¹⁴C concentrations at annual resolution. Yet little is known about the extent and the age of stored carbon from previous years used in addition to the direct assimilation in tree rings. Here, we studied ¹⁴C in earlywood and latewood cellulose of four different species (oak, pine, larch and spruce), which are commonly used for radiocarbon calibration and dating. These trees were still growing during the radiocarbon bomb peak period (1958-1972). We compared cellulose ¹⁴C measured in tree-ring subdivisions with the atmospheric ¹⁴C corresponding to the time of ring formation. We observed that cellulose ¹⁴C carried up to about 50% of the atmospheric ¹⁴C signal from the previous 1-2 years only in the earlywood of oak, whereas in conifers it was up to 20% in the earlywood and in the case of spruce also in the latewood. The bias in using the full ring of trees growing in a temperate oceanic climate to estimate atmospheric ¹⁴C concentration might be minimal considering that earlywood has a low mass contribution and that the variability in atmospheric ¹⁴C over a few years is usually less than 3‰.

As major terrestrial carbon sinks, forests play an important role in mitigating climate change. The relationship between the seasonal uptake of carbon and its allocation to woody biomass remains poorly understood, leaving a significant gap in our capacity to predict carbon sequestration by forests. Here, we compare the intra-annual dynamics of carbon fluxes and wood formation across the Northern hemisphere, from carbon assimilation and the formation of non-structural carbon compounds to their incorporation in woody tissues. We show temporally coupled seasonal peaks of carbon assimilation (GPP) and wood cell differentiation, while the two processes are substantially decoupled during off-peak periods. Peaks of cambial activity occur substantially earlier compared to GPP, suggesting the buffer role of non-structural carbohydrates between the processes of carbon assimilation and allocation to wood. Our findings suggest that high-resolution seasonal data of ecosystem carbon fluxes, wood formation and the associated physiological processes may reduce uncertainties in carbon source-sink relationships at different spatial scales, from stand to ecosystem levels.

Climate change is altering regional aridity and species composition in dryland ecosystems. Understanding the effects of long-term increasing aridity and species-specific stomatal behaviors on transpiration is therefore important for water-resource forecasting. To assess the effects of site aridity levels and species on stand transpiration rate (E_s), we monitored sap flux density (J_s) and relevant environmental parameters for 16 isohydric Picea crassifolia (spruce) and 14 anisohydric Juniperus przewalskii (juniper) trees over three growing seasons at four arid to semi-arid high-elevation sites on the Tibetan Plateau. Our results show that E_s was about 5–9 times higher in semi-arid sites than in arid sites, and about 6–9 times higher in spruce than in juniper. Spruce exhibited stronger stomatal regulation of transpiration than juniper. Soil water supply strongly promoted J_s only for spruce in the arid environment (R² = 0.23), while at the other sites, J_s was mostly controlled by atmospheric vapor pressure deficit (VPD) (R² > 0.82). However, increasing site aridity greatly reduced the sensitivities of both J_s and canopy conductance to VPD, especially for juniper. We expect that in the transition from semi-arid to arid alpine forests, E_s will initially rise due to increasing VPD. However, in the long term, there may be a stronger decline in E_s since both the sensitivity of J_s to VPD and the stand sapwood area will decrease. These findings could be used to reduce the impacts of climate change on water resources in high-elevation drylands.

Wood growth is key to understanding the feedback of forest ecosystems to the ongoing climate warming. An increase in spatial synchrony (i.e., coincident changes in distant populations) of spring phenology is one of the most prominent climate responses of forest trees. However, whether temperature variability contributes to an increase in the spatial synchrony of spring phenology and its underlying mechanisms remains largely unknown. Here, we analyzed an extensive dataset of xylem phenology observations of 20 conifer species from 75 sites over the Northern Hemisphere. Along the gradient of increase in temperature variability in the 75 sites, we observed a convergence in the onset of cell enlargement roughly toward the 5^th of June, with a convergence in the onset of cell wall thickening toward the summer solstice. The increase in rainfall since the 5^th of June is favorable for cell division and expansion, and as the most hours of sunlight are received around the summer solstice, it allows the optimization of carbon assimilation for cell wall thickening. Hence, the convergences can be considered as the result of matching xylem phenological activities to favorable conditions in regions with high temperature variability. Yet, forest trees relying on such consistent seasonal cues for xylem growth could constrain their ability to respond to climate warming, with consequences for the potential growing season length and, ultimately, forest productivity and survival in the future.

Climate change alters regional aridity and species composition in dryland ecosystems. However, the effect of varying aridity and species-specific responses on tree growth remains unclear. Here, we used high-resolution dendrometers to analyze the climatic factors controlling stem diameter variation in two alpine coniferous species, Qilian juniper (Juniperus przewalskii) and Qinghai spruce (Picea crassifolia), in their eastern- and western-most distributions (two species × two sites) in the Qilian Mountains during the 2018–2021 growing seasons. We observed species-, site-, and year-specific differences in growth phenology and radial growth dynamics. Juniper had early growth onset and cessation, but short growing season compared to spruce. Trees in the eastern sites showed earlier growth onset, later cessation, and a much longer growing season than those in the western sites for both species. Daily stem growth revealed similar responses for both species and site aridity, with daily vapor pressure deficit, maximum temperature, and solar radiation negatively, and relative humidity and precipitation positively affecting stem increments. However, when observation was extended to 7 or 21 days, the effect of precipitation on stem increments became weak or negligible in the wetter eastern sites, while continuing in the arid western sites. Further, soil water content appeared to have an impact on growth in the arid western site for both species. These results were confirmed by linear mixed-effects models, which revealed that site-aridity play an important role on tree stem variation, especially in junipers. Our results suggest that although short-term atmospheric water conditions influence radial stem variation, intra-annual tree growth still depends on soil water availability in drylands. Our findings provide new evidence of potential uses of dendrometers to measure environmental stress on tree growth and reveal that site aridity and species influence tree growth at different time scales in cold and arid ecosystems.

Earth system models and various climate proxy sources indicate global warming is unprecedented during at least the Common Era. However, tree-ring proxies often estimate temperatures during the Medieval Climate Anomaly (950-1250 ce) that are similar to, or exceed, those recorded for the past century, in contrast to simulation experiments at regional scales. This not only calls into question the reliability of models and proxies but also contributes to uncertainty in future climate projections. Here we show that the current climate of the Fennoscandian Peninsula is substantially warmer than that of the medieval period. This highlights the dominant role of anthropogenic forcing in climate warming even at the regional scale, thereby reconciling inconsistencies between reconstructions and model simulations. We used an annually resolved 1,170-year-long tree-ring record that relies exclusively on tracheid anatomical measurements from Pinus sylvestris trees, providing high-fidelity measurements of instrumental temperature variability during the warm season. We therefore call for the construction of more such millennia-long records to further improve our understanding and reduce uncertainties around historical and future climate change at inter-regional and eventually global scales.

Climate change poses a major threat to global forest ecosystems. In particular, rising temperatures and prolonged drought spells have led to increased rates of forest decline and dieback in recent decades. Under this framework, forest edges are particularly prone to drought-induced decline since they are characterized by warmer and drier micro-climatic conditions amplifying impacts of drought on tree growth and survival. Previous research indicated that forest-edge Scots pine trees have a higher growth sensitivity to water availability compared to the forest interior with consequent reduction of canopy greenness (remotely sensed NDVI) and higher mortality rates. Yet, the underlying physiological mechanisms remain largely unknown. Here, we address this knowledge gap by comparing stable carbon isotope signatures and wood anatomical traits in annual rings of trees growing at the forest edge vs. the forest interior and between trees that either survived or died during the extreme drought of 2015. Our analyses suggest that the exposure to drought of forest-edge Scots pine likely results in a reduction of stomatal conductance, as reflected by a higher δ¹³C of stem wood, thinner cell walls, and lower mean ring density. Moreover, we found dead trees to feature larger mean hydraulic lumen diameters and a lower cell-wall reinforcement, indicating a higher risk to suffer from cavitation. In conclusion, the typically drier micro-climatic conditions at the forest edge seem to have triggered a larger reduction of stomatal conductance of Scots pine trees, resulting in a lower carbon availability and significantly altered wood anatomical properties under an increasingly drier climate.

Dendrometers recording stem diameter variations (SDV) at high-resolution are useful to assess trees' water relation since water reserves are stored in the elastic tissue of the bark. These tissues typically shrink during the day as they release water when evaporative demand is high and swell during the night as they are replenished when evaporative demand is low, generating the typical SDV profile known as the diel SDV cycle. However, similar SDV cycles have been observed on dead trees due to the hygroscopic shrinking and swelling of the dead bark tissues. In order to remove this hygroscopic effect of the bark, dendrometers are applied as close as possible to the living bark tissues by removing the outer dead layer, however with questionable success. In this study, we used SDV time series from 40 point dendrometers applied on dead-bark-removed mature trees to assess and quantify the remaining hygroscopic effect on individual trees. To do so, we checked SDV behavior in the cold season and explored the relation between the diel SDV cycle and changes in relative humidity (RH). Our results showed that (a) the hygroscopic effect in SDV can be well-detected based on the amplitude of the diel SDV cycle (diel SDV_ampl) and the correlation between SDV and RH during both the cold and the warm season; (b) the level of the hygroscopic effect varies strongly among individuals; (c) diel SDV_ampl is proportional to both changes in RH and transpiration so that the hygroscopic effect on the diel SDV cycle can be quantified using a linear model where (diel SDV_ampl) is a function of RH changes and transpiration. These results allow the use of the model to correct the amplitude of the diel SDV cycles and suggest that this method can be applied to other ecological relevant water-related SDV variables such as tree water deficit.

Despite growing interest in predicting plant phenological shifts, advanced spring phenology by global climate change remains debated. Evidence documenting either small or large advancement of spring phenology to rising temperature over the spatio-temporal scales implies a potential existence of a thermal threshold in the responses of forests to global warming. We collected a unique data set of xylem cell-wall-thickening onset dates in 20 coniferous species covering a broad mean annual temperature (MAT) gradient (−3.05 to 22.9°C) across the Northern Hemisphere (latitudes 23°–66° N). Along the MAT gradient, we identified a threshold temperature (using segmented regression) of 4.9 ± 1.1°C, above which the response of xylem phenology to rising temperatures significantly decline. This threshold separates the Northern Hemisphere conifers into cold and warm thermal niches, with MAT and spring forcing being the primary drivers for the onset dates (estimated by linear and Bayesian mixed-effect models), respectively. The identified thermal threshold should be integrated into the Earth-System-Models for a better understanding of spring phenology in response to global warming and an improved prediction of global climate-carbon feedbacks.

New mutations provide the raw material for evolution and adaptation. The distribution of fitness effects (DFE) describes the spectrum of effects of new mutations that can occur along a genome, and is, therefore, of vital interest in evolutionary biology. Recent work has uncovered striking similarities in the DFE between closely related species, prompting us to ask whether there is variation in the DFE among populations of the same species, or among species with different degrees of divergence, that is whether there is variation in the DFE at different levels of evolution. Using exome capture data from six tree species sampled across Europe we characterized the DFE for multiple species, and for each species, multiple populations, and investigated the factors potentially influencing the DFE, such as demography, population divergence, and genetic background. We find statistical support for the presence of variation in the DFE at the species level, even among relatively closely related species. However, we find very little difference at the population level, suggesting that differences in the DFE are primarily driven by deep features of species biology, and those evolutionarily recent events, such as demographic changes and local adaptation, have little impact.

Tree rings form the backbone of high-resolution palaeoclimatology and represent one of the most frequently used proxy to reconstruct climate variability of the Common Era. In the European Alps, reconstructions were often based on tree-ring width (TRW) and maximum latewood density (MXD) series, with a focus on European larch. By contrast, only a very limited number of dendroclimatic studies exists for long-lived, multi-centennial Pinus cembra, despite the widespread occurrence of the species at treeline sites across the European Alps. This lack of reconstructions can be ascribed to the difficulties encountered in past studies in extracting a robust climate signal from TRW and MXD chronologies. In this study, we tested various wood anatomical parameters from P. cembra as proxies for the reconstruction of past air temperatures. To this end, we measured anatomical cell parameters and TRW of old-growth trees from the God da Tamangur forest stand, known for being the highest pure, and continuous P. cembra forest in Europe. We demonstrate that several wood anatomical parameters allow robust reconstruction of past temperature variability at annual to multidecadal timescales. Best results are obtained with maximum latewood radial cell wall thickness (CWTrad) measured at 40 μm radial band width. Over the 1920-2017 period, the CWTrad chronology explains 62 % and >80 % of interannual and decadal variability of air temperatures during a time window corresponding roughly with the growing season. These values exceed those found in past work on P. cembra and even exceed the values reported for MXD chronologies built with L. decidua and hitherto considered the gold standard for dendroclimatic reconstructions in the European Alps. The wood anatomical analysis of P. cembra records therefore unveils a dormant potential and opens new avenues for a species that has been considered unsuitable for climate reconstructions so far.

• The oxygen isotope composition (δ¹⁸O) of tree-ring cellulose is used to evaluate tree physiological responses to climate, but their interpretation is still limited due to the complexity of the isotope fractionation pathways.
• We assessed the relative contribution of seasonal needle and xylem water δ¹⁸O variations to the intra-annual tree-ring cellulose δ¹⁸O signature of larch trees at two sites with contrasting soil water availability in the Swiss Alps. We combined biweekly δ¹⁸O measurements of soil water, needle water, and twig xylem water with intra-annual δ¹⁸O measurements of tree-ring cellulose, xylogenesis analysis, and mechanistic and structural equation modeling.
• Intra-annual cellulose δ¹⁸O values resembled source water δ¹⁸O mean levels better than needle water δ¹⁸O. Large parts of the rings were formed under high proportional exchange with unenriched xylem water (p_ex). Maximum p_ex values were achieved in August and imprinted on sections at 50–75% of the ring. High p_ex values were associated with periods of high atmospheric evaporative demand (VPD). While VPD governed needle water δ¹⁸O variability, we estimated a limited Péclet effect at both sites.
• Due to a variable p_ex, source water has a strong influence over large parts of the intra-annual tree-ring cellulose δ¹⁸O variations, potentially masking signals coming from needle-level processes.

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance (G_c) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of G_c are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between G_c and the ability of stem tissues to rehydrate at night remains unclear.
We investigated whether species-specific G_c responses aim to prevent branch embolisms, or enable night-time stem rehydration, which is critical for turgor-dependent growth. For this, we used a unique combination of concurrent dendrometer, sap flow and leaf water potential measurements and collected branch-vulnerability curves of six common European tree species.
Species-specific G_c reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost (P₅₀). Instead, we found a stronger relationship with stem rehydration. Species with a stronger G_c control were less effective at refilling stem-water storage as the soil dries, which appeared related to their xylem architecture.
Our findings highlight the importance of stem rehydration for water-use regulation in mature trees, which likely relates to the maintenance of adequate stem turgor. We thus conclude that stem rehydration must complement the widely accepted safety–efficiency stomatal control paradigm.

Emerging diseases caused by both native and exotic pathogens represent a main threat to forest ecosystems worldwide. The two invasive soilborne pathogens Phytophthora cinnamomi and Phytophthora × cambivora are the causal agents of ink disease, which has been threatening Castanea sativa in Europe for several centuries and seems to be re-emerging in recent years. Here, we investigated the distribution, causal agents, and infection dynamics of ink disease in southern Switzerland. A total of 25 outbreaks were identified, 19 with only P. cinnamomi, 5 with only P. × cambivora, and 1 with both species. Dendrochronological analyses showed that the disease emerged in the last 20–30 years. Infected trees either died rapidly within 5–15 years post-infection or showed a prolonged state of general decline until death. Based on a generalized linear model, the local risk of occurrence of ink disease was increased by an S-SE aspect of the chestnut stand, the presence of a pure chestnut stand, management activities, the proximity of roads and buildings, and increasing annual mean temperature and precipitation. The genetic structure of the local P. cinnamomi population suggests independent introductions and local spread of the pathogen.