Arthur Gessler

Swiss Federal Institute for Forest, Snow and Landscape Research WSL | WSL · Forest Dynamics Research Unit

Climate projections predict higher precipitation variability with more frequent dry extremes1. CO2 assimilation of forests decreases during drought, either by stomatal closure2 or by direct environmental control of sink tissue activities3. Ultimately, drought effects on forests depend on the ability of forests to recover, but the mechanisms control...

In trees, the interplay between reduced carbon assimilation and the inability to transport carbohydrates to the sites of demand under drought might be one of the mechanisms leading to carbon starvation. However, we largely lack knowledge on how drought effects on new assimilate allocation differ between species with different drought sensitivities...

There is considerable evidence that biodiversity promotes multiple ecosystem functions (multifunctionality), thus ensuring the delivery of ecosystem services important for human well-being. However, the mechanisms underlying this relationship are poorly understood, especially in natural ecosystems. We develop a novel approach to partition biodivers...

Climate models predict an increase in the intensity and frequency of drought episodes in the Northern Hemisphere. Among terrestrial ecosystems, forests will be profoundly impacted by drier climatic conditions, with drastic consequences for the functions and services they supply. Simultaneously, biodiversity is known to support a wide range of fores...

The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes...

Current increases in not only the intensity and frequency but also the duration of drought events could affect the growth, physiology, and mortality of trees. We experimentally studied the effects of drought duration in combination with fertilization on leaf water potential, gas exchange, growth, tissue levels of non-structural carbohydrates (NSCs)...

The vitality of Scots pine (Pinus sylvestris L.) is declining since the 1990s in many European regions. This was mostly attributed to the occurrence of hotter droughts, other climatic changes and secondary biotic stressors. However, it is still not well understood which specific atmospheric trends and extremes caused the observed spatio-temporal di...

Carbon (C) allocation plays a crucial role for survival and growth of the alpine treeline trees, which, however, is still poorly understood. Using in situ 13CO2 labeling, we investigated the leaf photosynthesis and the allocation of 13C labeled photoassimilates in various tissues (leaves, twigs and fine roots) in treeline trees and low-elevation tr...

Purpose Kettle holes are small inland water bodies known to be dominated by terrigenous material; however, the processes and structures that drive the enrichment and depletion of specific geochemical elements in the water column and kettle hole sediment remain unclear. We hypothesized that the mobile elements (Ca, Fe, K, P) behave different from ea...

Unprecedented tree dieback across Central Europe caused by recent global change‐type drought events highlights the need for a better mechanistic understanding of drought‐induced tree mortality. While numerous physiological risk factors have been identified, the importance of two principal mechanisms, hydraulic failure and carbon starvation, is stil...

We compiled hydrogen and oxygen stable isotope compositions (δ2H and δ18O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2H was more closely correlated with δ2H of xylem water or atmospheric vapour, whereas leaf water δ18O was more closely correlated with air relative humidity. This resulted from t...

Future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees' sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil mo...

Radial stem growth dynamics at seasonal resolution are essential to understand how forests respond to climate change. We studied daily radial growth of 160 individuals of seven temperate tree species at 47 sites across Switzerland over 8 years. Growth of all species peaked in the early part of the growth season and commenced shortly before the summ...

During the 20th century human activities drastically altered the natural environment at global and local scales by habitat destruction, urbanisation, intensive agriculture, and climate warming. This anthropogenic pressure has modified species distributions and abundances, and led to the increased spread of neophytes. However, the determination of t...

Organic matter (OM) can be protected from abiotic and biotic breakdown via its association with iron (Fe) and aluminum (Al) in both terrestrial and aquatic ecosystems. We hypothesized that the natural variation in sediment redox and pH regime govern how the two metals interact with OM in near-surface mineral sediments of 40 kettle holes of varying...

Most trees form symbioses with ectomycorrhizal fungi (EMF) which influence access to growth-limiting soil resources. Mesocosm experiments repeatedly show that EMF species differentially affect plant development, yet whether these effects ripple up to influence the growth of entire forests remains unknown. Here we tested the effects of EMF compositi...

Climate change exposes ecosystems to strong and rapid changes in their environmental boundary conditions mainly due to the altered temperature and precipitation patterns. It is still poorly understood how fast interlinked ecosystem processes respond to altered environmental conditions, if these responses occur gradually or suddenly when thresholds...

Stable carbon isotope ratios (δ ¹³ C) in organic matter convey important integrated and (if assessed in the tree ring archive) dateable information on plant physiology and related environmental drivers. While the generation of the δ ¹³ C signal in the primary assimilates in the leaves via photosynthetic carbon isotope fractionation is well understo...

Understanding the process of wood formation and its dynamics over the growing season is fundamental to interpret the isotopic signature of tree rings. Indeed, the isotopic signal recorded in wood does not only depend on the conditions influencing carbon, water, and nitrogen uptake in the leaves and roots, but also on how these elements are transloc...

The TreeNet research and monitoring network has been continuously collecting data from point dendrometers and air and soil microclimate using an automated system since 2011. The goal of TreeNet is to generate high temporal resolution datasets of tree growth and tree water dynamics for research and to provide near real-time indicators of forest grow...

Tree stems have been identified as sources of volatile organic compounds (VOCs) that play important roles in tree defence and atmospheric chemistry. Yet, we lack understanding on the magnitude and environmental drivers of stem VOC emissions in various forest ecosystems. Due to the increasing importance of extreme drought, we studied drought effects...

In the context of a recent massive increase of research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting edge, meaningful and integrated knowledge. Consideration of the belowground components in plant and ecosystem studies has been consistently called...

The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial. We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in...

Heatwaves combined with drought affect tree functioning with yet to be determined legacy effects on carbon (C) and nitrogen (N) allocation. We continuously monitored shoot and root gas exchange, δ13CO2 of respiration and stem growth in well‐watered and drought‐treated Pinus sylvestris (Scots pine) seedlings exposed to increasing daytime temperature...

The effects of plants on the biosphere, atmosphere, and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on literature in plant physiology,...

Abies alba (Mill.) has a high potential for mitigating climate change in European mountain forests; yet, its natural regeneration is severely limited by ungulate browsing. Here, we simulated browsing in a common garden experiment to study growth and physiological traits, measured from bulk needles, using a randomized block design with two levels of...

The analysis of the non-exchangeable hydrogen isotope ratio (δ²Hne) in carbohydrates is mostly limited to the structural component cellulose, while simple high-throughput methods for δ²Hne 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...

Many experiments have shown that biodiversity promotes ecosystem functioning and stability and that this relationship varies with resource availability. However, we still have a poor understanding of the underlying physiological and ecological mechanisms driving diversity effects and how they may interact with soil nutrient availability. We collect...

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 t...

Being able to persist in deep shade is an important characteristic of juvenile trees, often leading to a strong dominance of shade‐tolerant species in forests with low canopy turnover and a low disturbance rate. While leaf, growth, and storage traits are known to be key components of shade tolerance, their interplay during regeneration development...

Resource allocation to different plant tissues is likely to be affected by high investment into fruit production during mast years. However, there is a large knowledge gap concerning species-specific differences in resource dynamics. We investigated the influence of mast years on stem growth, leaf production, and leaf carbon (C), nitrogen (N), and...

Despite increasing reports of severe drought and heat impacts on forest ecosystems, community‐level processes, which could potentially modulate tree responses to climatic stress, are rarely accounted for. While numerous studies indicate a positive effect of species diversity on a wide range of ecosystem functions and services, little is known about...

The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day‐night conditions on tree growth remained uncertain. Here we present the first comprehensive field study of hourly‐resolved radial stem growth of seven temper...

Extreme droughts are expected to increase in frequency and severity in many regions of the world, threatening multiple ecosystem services provided by forests. Effective strategies to adapt forests to such droughts require comprehensive information on the effects and importance of the factors influencing forest resistance and resilience. We used a u...

1. The response of trees to intra‐annual environmental constraints varies temporally throughout a growing season and spatially across landscapes. A better understanding of these dynamics will help us anticipate the impacts of short‐term climate variability and medium‐term climate change on forests. Using the process‐based 3‐PG forest ecosystem mode...

Silver fir (Abies alba) has a high potential for mitigating climate change in European mountain forests, yet, its natural regeneration is severely limited by ungulate browsing. Here, we simulated browsing in a common garden experiment to study growth and physiological traits, measured from bulk needles, using a randomized block design with two leve...

Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of e...

Understanding the role of stored non-structural carbohydrates (NSC) in trees has been a long-term goal in tree physiology research. A study published in this issue strengthens the possible role of NSC storage as key to mitigate drought effects.

Above and belowground compartments in ecosystems are closely coupled on daily to annual timescales. In mature forests, this interlinkage and how it is impacted by drought is still poorly understood. Here, we pulse-labelled 100-year-old trees with 13 CO2 within a 15-year-long irrigation experiment in a naturally dry pine forest to quantify how droug...

The role of carbon (C) and nutrient uptake, allocation, storage and especially their interactions in survival and recovery of trees under increased frequencies and intensities of drought events is not well understood. A full factorial experiment with four soil water content regimes ranging from extreme drought to well-watered conditions and two fer...

Droughts and other rapid changes in abiotic environmental conditions can predispose trees to damage by pest insects and pathogens. For survival of coniferous trees, functional resin-based defences are essential, and it is important to know how they react to changes in environmental conditions at various time scales. We studied the effects of differ...

Over the last decades, spring leaf‐out of temperate and boreal trees has substantially advanced in response to global warming, affecting terrestrial biogeochemical fluxes and Earth’s climate system. However, it remains unclear whether leaf‐out will continue to advance with further warming because species’ effective chilling temperatures as well as...

The future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees' sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soi...

Changes in regional and global hydrological cycles and temperature regimes will affect forest health, functioning and growth, and therefore their role in carbon (C) sequestration. A severe spring-summer drought hit large parts of central Europe in 2018 leading locally to premature leaf senescence in deciduous tree species. In late summer 2018 and 2...

An extreme summer drought caused unprecedented tree dieback across Central Europe in 2018, highlighting the need for a better mechanistic understanding of drought-induced tree mortality. While numerous physiological risk factors have been identified, the principal mechanisms, hydraulic failure and carbon starvation, are still debated. We studied 9,...

This article is a Commentary on Dominguez et al. (2021), 229: 186–198.

Research on drought impact on tree functioning is focused primarily on water and carbon (C) dynamics. Changes in nutrient uptake might also affect tree performance under drought and there is a need to explore underlying mechanisms. We investigated effects of drought on a) in-situ nitrogen (N)-uptake accounting for both, N availability to fine-roots...

Drought alters allocation patterns of carbon (C) and nutrients in trees and eventually impairs tree functioning. Elevated soil nutrient availability might alter the response of trees to drought. We hypothesize that increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. To t...

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought-induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by alter...

The carbon and oxygen isotopic composition of water and assimilates in plants reveals valuable information on plant response to climatic conditions. Yet, the C and O uptake, incorporation, and allocation processes determining isotopic compositions are not fully understood. We carried out a dual-isotope labelling experiment at high humidity with 18O...

Früher Laubfall der Buche während der Sommertrockenheit 2018: Resistenz oder Schwächesymptom? Die Sommertrockenheit 2018 führte in verschiedenen Regionen der Schweiz zum vorzeitigen Laubfall bei Bu-chen (Fagus sylvatica L.). Es stellte sich die Frage, ob dieser Laubfall als Resistenzreaktion aufgefasst werden soll, die den Buchen einen physiologisc...

The circadian clock is a molecular timer of metabolism that affects the diurnal pattern of stomatal conductance (gs), amongst other processes, in a broad array of plant species. The function of circadian gs regulation remains unknown and here, we test whether circadian regulation helps to optimize diurnal variations in stomatal conductance. We subj...

Drought alters allocation patterns of carbon (C) and nutrients in trees and eventually impairs tree functioning. Elevated soil nutrient availability might alter the response of trees to drought. We hypothesize that increased soil nutrient availability stimulates root metabolism and carbon allocation to belowground tissues under drought stress. To t...

In the context of a recent massive increase into research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on producing cutting edge, meaningful and integrated knowledge. Consideration of the belowground components in plant and ecosystem studies has been consistently calle...

Conifer mortality rates are increasing in western North America, but the physiological mechanisms underlying this trend are not well understood. We examined tree‐ring based radial growth along with stable carbon and oxygen isotope composition (d13C and δ18O, respectively) of dying and surviving conifers at eight old‐growth forest sites across a str...