Janneke Hille Ris Lambers

Last Name

Hille Ris Lambers

First Name

Janneke

ORCID

0000-0001-5523-0354

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09716 - Hille Ris Lambers, Janneke / Hille Ris Lambers, Janneke

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Publications1 - 8 of 8

Rainwater uptake in conifer twigs: five experiments tell a story of absorption, storage, and transport
Chin, Alana R.O.; Gessler, Arthur; Guzmán-Delgado, Paula; et al. (2025)
Journal of Experimental Botany
While evidence supports the idea that a portion of the many raindrops that fall onto a forest canopy may be directly absorbed by the twigs they land on, we do not know how much is absorbed, how it enters the twig, or what internal path it might take on its way to the xylem. Using a diverse series of five experiments encompassing isotopic labelling, fluorescent tracers, rehydration kinetics, synchrotron-based X-ray tomographic microscopy, and thermal imaging, we follow the fate of rainwater from initial contact with the twig to its distribution to adjacent tissues. We provide conclusive, multi-pronged evidence of surface water-absorption into the xylem of 1-year-old conifer twigs with incomplete bark development. Additionally, we demonstrate a surface capillary phase, mixed apoplastic and symplastic internal routes, and the strong influence of intercellular airspace as a hydraulic capacitor across multiple tissues. We show that twigs are capable of rapid, large-volume water absorption, which may help trees take advantage of crown-wetting events and support the repair of hydraulic damage from frost and drought. Forecasting the impacts of climatic stress on different tree species will benefit from understanding the importance, and tissue-level specifics, of this critical water-acquisition pathway. Our work tells a detailed story of rain absorption by twigs and lays a foundation for future trait-based research into among-species differences in absorption capacity.
The Janzen-Connell hypothesis and seed masting
Davies, T. Jonathan; Hille Ris Lambers, Janneke; Wolkovich, E. M. (2025)
Trends in Ecology & Evolution
The Janzen-Connell hypothesis (JC effect) has been proposed as a mechanism for explaining high tropical tree diversity via negative conspecific density depen dence imposed by natural enemies. Seed masting describes the uneven invest ment in reproduction between years, and could be a mechanism for reducing seed predation by means of predator satiation. JC effects select for wide dispersal kernels, allowing species to escape enemies in space, while the predator-satiation model of seed masting assumes escape from predators in time. Although representing contrasting ecological dynamics, both models ar predicated on similar assumptions: that tree recruitment is limited by seed mor tality via natural enemies. We suggest that the individual fitness advantages c masting and JC dynamics would be better understood by considering bot mechanisms together.
Snow Height Sensors Reveal Phenological Advance in Alpine Grasslands
Zehnder, Michael; Pfund, Beat; Svoboda, Jan; et al. (2025)
Global Change Biology
Long-term phenological data in alpine regions are often limited to a few locations and thus, little is known about climate-change-induced plant phenological shifts above the treeline. Because plant growth initiation in seasonally snow-covered regions is largely driven by snowmelt timing and local temperature, it is essential to simultaneously track phenological shifts, snowmelt, and near-ground temperatures. In this study, we make use of ultrasonic snow height sensors installed at climate stations in the Swiss Alps to reveal the phenological advance of grassland ecosystems and relate them to climatic changes over 25 years (1998–2023). When snow is absent, these snow height sensors additionally provide information on plant growth at a uniquely fine temporal scale. We applied a two-step machine learning algorithm to separate snow- from plant-height measurements, allowing us to determine melt-out for 122 stations between 1560 and 2950 m a.s.l., and to extract seasonal plant growth signals for a subset of 40 stations used for phenological analyses. We identified the start of growth and calculated temperature trends, focusing particularly on thermal conditions between melt-out and growth initiation. We observed an advance of green-up by −2.4 days/decade coinciding with strong warming of up to +0.8°C/decade. Although the timing of snowmelt has not changed significantly over the study period in this focal region, phenological responses to early melt-out years varied due to differing influences of photoperiodic and thermal constraints, which were not equally important across elevations and communities. Phenological shifts of alpine grasslands are thus likely to become even more pronounced if snowmelt timing advances in the future as predicted. As climate change continues to reshape mountain ecosystems, understanding the interplay between phenological changes and species turnover will be essential for predicting future biodiversity patterns and informing conservation strategies in alpine regions.
Why longer seasons with climate change may not increase tree growth
Wolkovich , E.M.; Ettinger, Ailene K.; Chin, Alana R.; et al. (2025)
Nature Climate Change
Most climate change forecasts assume that longer growing seasons increase carbon storage through increased tree growth, but recent findings have challenged this assumption. Here we highlight divergent findings across studies, spanning diverse methods and disciplinary perspectives. Current hypotheses for why longer growing seasons may not always increase tree growth include drought-related effects and internal constraints. These hypotheses, however, are generally tested in different ways by different fields on different species, and rarely consider how external drivers and internal constraints interact. We outline how bridging these divides while integrating evolutionary history and ecological theory could help build a unified model across species for when longer seasons will—or will not—lead to greater tree growth, with major forecasting implications.
Evaluating floral resource availability in mountain habitats
Aji John, Pushpam Joseph; Tiusanen, Mikko; Richman, Sarah; et al. (2025)
Ecosphere
Climate-driven phenological mismatches have the potential to disrupt plant–pollinator interactions, emphasizing the need to uncover drivers behind spatial and temporal dynamics of floral resource availability. This is especially important in habitats such as mountain meadows, where climate change is not only likely to have outsized impacts, but topographic complexity creates a mosaic of microclimate and habitat heterogeneity. We investigated the impacts of elevation, canopy cover, and their interaction on the temporal availability of floral resources by deploying 35 trail cameras in open and forested habitats below and near the tree line in the Swiss Alps. We hypothesized that tree cover would lower species richness and floral abundance, especially at high elevations where low light might interact with harsh climates. However, we also hypothesized that a mosaic of open and forested habitats at any elevation may offer temporal benefits to pollinators by extending the flowering season and potentially providing complementary flower resources during critical life history phases. We applied machine learning approaches to images to extract first and last flowering dates, overall flowering duration, and flowering species richness, and then tested how these flowering metrics varied by site (low vs. high) and canopy categories (open vs. closed) and their interactions. We also explored temporal changes in species richness and the individual flowering phenology of the most abundant species. We found that canopy cover extended the entire flowering period while higher elevations shortened it, with both factors delaying the start of the flowering season. Flowering species richness was highest at the tree line, and floral abundance increased at and above the tree line relative to lower elevations. These results highlight the complex interactions between habitat structure and elevation in influencing flowering phenology and flower resource diversity. Understory wildflowers emerge as a potentially complementary resource for pollinators in mountain ecosystems, potentially benefiting them during the early season. This work also highlights the benefit of combining machine learning technologies with automated image capture (in our case, wildlife cameras) that allowed us to quantify phenology at an extremely fine temporal scale.
Continental Contrasts in Climate Extremes That Control Tree Fecundity
Clark, James S.; Andrus, Robert; Arianoutsou, Margarita; et al. (2026)
Global Change Biology
In 2023, more than half of olive harvests (Olea europaea) across Spain, Greece, and Türkiye were lost to drought. The same year late freeze destroyed 90% of the peach crop (Prunus persica) on the Georgia Piedmont and the apple crop (Malus domestica) in central New York, Vermont, and southern Quebec. Climate extremes now rank with the costliest threats to agriculture, but their role in forest recovery from diebacks that are happening globally is unknown for lack of tree fecundity estimates in forests. Tolerance of climate extremes could depend on past exposure but constrained by phylogenetic conservatism. We report a continental scale analysis of climate extremes and forest fecundity across North America and Europe showing that responses to late freeze and drought are happening now. Species differences are not explained by the traits typically included in ecological studies and they are weakly associated with phylogeny. Late freeze, that is, freezing temperatures that follow the onset of flower development in spring, is shown to be “normal” in North America, but not Europe, potentially explaining failed seed production due to delayed onset and the resultant shorter growing period by North American transplants dating back at least to the 18th century. Drought has thus far had the greatest impacts in dry forested regions, but here too, species differences are not explained by traditional trait values. If responses have been buffered from drought and late freeze by past exposure, acclimation and local adaptation prove inadequate as extremes intensify.
Warm-Loving Species Perform Well Under Limiting Resources: Trait Combinations for Future Climate
Levasseur , Sarah; de Melo , Vanessa; Hille Ris Lambers, Janneke; et al. (2025)
Global Change Biology
Ecosystems are warming alongside shifts in other abiotic factors, leading to interactive effects on populations and communities. This underscores the importance of studying how organisms respond to multiple environmental changes simultaneously. In pelagic ecosystems, as surface waters warm, longer and stronger periods of thermal stratification lead to changes in resource (light and nutrient) availability. We investigate the combined effects of temperature and resource availability on the growth rates of 19 populations (comprising 17 species) of freshwater phytoplankton in order to examine how temperature influences the minimum resource requirements (and Monod parameters) for light, nitrogen, and phosphorus. We also evaluate how resource availability affects each population's thermal traits (i.e., thermal performance curve - TPC - parameters). When averaged across all populations, the requirements for light and phosphorus tended to display a U-shaped relationship along temperatures. Individual populations varied greatly in their responses to temperature, leading to shifts in the identity of the best competitor across the thermal gradient, particularly for nitrogen and phosphorus. TPC responses to resource limitation were highly variable, but thermal optima and maxima of individual populations often decreased with resource limitation, and thermal breadths (range where growth is 80% or more of its maximum) often increased due to a flattening of TPCs. Across all populations and resource types, the maximum optimum temperature across resource levels (maximum Topt) tended to be positively correlated with the temperature at which populations had the lowest resource requirements (minimum R*). However, the temperature at which populations were the best competitors tended to be ~5°C colder on average than the temperature at which they grew the fastest. The populations with the highest thermal optima also had the lowest minimum resource requirements. Our findings reveal trait associations suggesting that some taxa already exhibit trait combinations that would support high performance under future warm and resource-limited conditions.
Trees use exogenous sugars for growth, but excess triggers negative feedback reducing photosynthetic carbon gain
Zhang, Yanli; Gessler, Arthur; Lehmann, Marco M.; et al. (2025)
Tree Physiology
Plants' non structural carbohydrates (NSCs) serve as their capital for growth, reproduction, defense and survival. To increase the NSC availability of carbon-limited trees, a recent study revealed the possibility of adding exogenous soluble sugars to carbon-starved trees. This provides an opportunity to investigate carbon allocation between source and sink, as well as the growth and physiological responses to external sugars. Using this method, we infused ¹³C-labeled glucose solution into the stem xylem of sycamore maple (Acer pseudoplatanus L.) trees (Experiment 1) and immersed branch cuttings of various tree species in a ¹³C-labeled glucose solution (Experiment 2). Our aim was to study whether infused sugars contribute to structural growth and how they influence photosynthesis. Specifically, we focused on whether trees can transport and utilize exogenous sugars for growth, and if sugar addition might trigger negative feedback mechanisms on carbon gain. We then traced the ¹³C label in bulk tissue and cellulose, and measured tissue NSC concentrations and leaf photosynthesis. Glucose addition consistently increased leaf NSC concentrations (Experiments 1 and 2), and exogenous sugars added were transported and incorporated into biomass formation in Experiment 1. However, excessive sugar addition triggered a negative feedback response, leading to leaf senescence (Experiments 1 and 2) and reduced photosynthesis (Experiment 2). Our findings validate the recently developed sugar addition method but emphasize the importance of carefully controlling the amount and rate of sugar addition to avoid negative feedback responses. This study will contribute to carbon physiological research, particularly in understanding carbon balance and source -sink relationships at the whole-plant level.

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Janneke Hille Ris Lambers

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