About

Janneke Hille Ris Lambers is an affiliate professor in the Department of Biology at the University of Washington and a full professor of Plant Ecology at ETH Zurich. Her research is motivated by basic and applied questions in plant community ecology, focusing on forecasting the impacts of climate change on species distributions, population dynamics, phenology, and community structure, as well as understanding the processes that drive species coexistence. Her work employs observational studies, manipulative experiments, and statistical modeling, with a significant portion conducted at Mt. Rainier National Park, serving as a natural laboratory for climate change research. She received her Ph.D. from Duke University in 2001, working with James S. Clark, and has conducted extensive research at LTER sites such as the Coweeta Hydrologic Laboratory and Cedar Creek Ecosystem Reserve. Her studies have included examining differences among temperate tree species in seed dispersal, seed banking, and mortality, as well as investigating how declining diversity influences productivity and the impacts of global change on seed production. During her postdoctoral work at the University of California, Santa Barbara, she focused on the dominance of Mediterranean annual grasses over diverse California annual grasses and forbs, analyzing the roles of niche and neutral processes in species coexistence. Since arriving at the University of Washington in 2006, she earned tenure in 2010 and became a full professor in 2014. In 2020, she moved to ETH Zurich, where she continues her research and maintains an affiliation with the University of Washington, including involvement in the MeadoWatch community science program.

Research topics

• Sociology
• Biology
• Zoology
• Mathematics
• Geography
• Climatology
• Demography
• Physical geography
• Evolutionary biology
• Economic geography
• Geology
• Statistics
• Genetics
• Ecology

Selected publications

• Correction: Positive and negative plant–plant interactions influence seedling establishment at both high and low elevations

  Alpine Botany · 2026-02-18

  articleOpen access
  PublisherOA PDFDOI

• Assessing heatdome effects on forest dynamics in the Pacific Northwest using Planet imagery

  bioRxiv (Cold Spring Harbor Laboratory) · 2026-01-13

  articleOpen accessSenior author

  Abstract Increasingly frequent extreme heat events are imposing significant stress on forest ecosystems, with implications for forests and the organisms that live in them. This study evaluates the effects of the June 2021 Pacific Northwest (PNW) heatdome (a prolonged high-pressure system that generated record-breaking temperatures) on forest canopy health and its subsequent recovery within the conifer-dominated Ellsworth Creek Preserve in Washington state, USA, using high-resolution Planet Labs satellite imagery. We analyzed normalized difference vegetation index (NDVI) trends from spring to fall from 2017 to 2024 to identify canopy stress and quantify changes in canopy greenness. Experimental forest restoration thinning treatments implemented to test their effectiveness in restoring old-growth forest characteristics at our sites allowed us to assess impacts of the heatdome event relative to these restoration treatments. Our results show a marked decline in summer NDVI following the 2021 heatdome, signaling substantial canopy stress. This decline, different than patterns in the other seven years, was particularly strong in dense, unmanaged (control) forest stands, which exhibited more pronounced NDVI declines than thinned (treatment) stands. These more pronounced stress signals during and after the heat event were associated with greater tree canopy height, canopy complexity, and tree density. The summer NDVI trends over the three post-heatdome years suggest a gradual recovery in tree canopy greenness, with thinned stands showing faster NDVI recovery than controls. In all, these results suggest that restoration thinning may improve canopy resilience to extreme heat. However, recovery trajectories varied widely within stand types, with persistent evidence of stress in high-density areas that suggest incomplete recovery as of 2023. Broadly, these findings show the vulnerability of dense forest canopies to extreme heat events, suggesting that forests with high canopy density may be less adaptable to recurring heat events. Nonetheless, managing forests through restoration thinning may enhance forest resilience to these climate-induced stressors – providing potential guidance for adaptive management strategies to maintain forest health in a world with more extreme climates.

  PublisherDOI

• The Janzen–Connell hypothesis and seed masting

  Trends in Ecology & Evolution · 2025-10-18

  article
  PublisherDOI

• Warm-loving species perform well under limiting resources: trait combinations for future climate

  bioRxiv (Cold Spring Harbor Laboratory) · 2025-07-07 · 1 citations

  preprintOpen access

  Abstract 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 aquatic ecosystems, as surface waters of lakes and oceans warm, longer and stronger periods of thermal stratification lead to changes in resource (light and nutrient) availability. We investigate the combined effect of temperature and resource availability on 19 populations (comprising 17 species) of freshwater phytoplankton 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 they 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.

  PublisherOA PDFDOI

• Forest canopy cover affects microclimate buffering during an extreme heat event

  Environmental Research Communications · 2024-09-01 · 7 citations

  articleOpen accessSenior author

  Abstract Increasing temperatures and extreme heat episodes have become more common with climate change. While forests are known to buffer increasing temperatures (relative to non-forested areas), whether this buffering is maintained under extreme temperature events is relatively unknown. Here we assess whether forests continue to buffer microclimate (specifically temperatures) during an extreme heat event: the Pacific Northwest (PNW) heat dome in June 2021. We use a combination of ground-based and regional climate data and find that forest understories were 3 °C cooler than a clear-cut area and 4 °C cooler than regional temperatures during the PNW heat dome. By examining forests with different levels of canopy cover we also found that the buffering capacity of forests is greater under denser canopies even under extreme heat events. Additionally, we found vertical variation in thermal buffering, with the greatest amount of buffering at the surface of the forest floor. Overall, our findings suggest that temperate coastal forests, that are known to buffer average temperatures, can also act as microclimate buffers during extreme heat events like the heat dome that occurred in the PNW in 2021. This could be good news for forest dwelling organisms that are sensitive to such extreme heat events.

  PublisherOA PDFDOI

• The importance of regeneration processes on forest biodiversity in old-growth forests in the Pacific Northwest

  Philosophical Transactions of the Royal Society B Biological Sciences · 2024-04-07 · 8 citations

  articleOpen access

  Forest diversity is the outcome of multiple species-specific processes and tolerances, from regeneration, growth, competition and mortality of trees. Predicting diversity thus requires a comprehensive understanding of those processes. Regeneration processes have traditionally been overlooked, due to high stochasticity and assumptions that recruitment is not limiting for forests. Thus, we investigated the importance of seed production and seedling survival on forest diversity in the Pacific Northwest (PNW) using a forest gap model (ForClim). Equations for regeneration processes were fit to empirical data and added into the model, followed by simulations where regeneration processes and parameter values varied. Adding regeneration processes into ForClim improved the simulation of species composition, compared to Forest Inventory Analysis data. We also found that seed production was not as important as seedling survival, and the time it took for seedlings to grow into saplings was a critical recruitment parameter for accurately capturing tree species diversity in PNW forest stands. However, our simulations considered historical climate only. Due to the sensitivity of seed production and seedling survival to weather, future climate change may alter seed production or seedling survival and future climate change simulations should include these regeneration processes to predict future forest dynamics in the PNW. This article is part of the theme issue 'Ecological novelty and planetary stewardship: biodiversity dynamics in a transforming biosphere'.

  PublisherOA PDFDOI

• Seed classification with random forest models

  Applications in Plant Sciences · 2024-05-01 · 5 citations

  articleOpen access

  Premise: To improve forest conservation monitoring, we developed a protocol to automatically count and identify the seeds of plant species with minimal resource requirements, making the process more efficient and less dependent on human operators. Methods and Results: Seeds from six North American conifer tree species were separated from leaf litter and imaged on a flatbed scanner. In the most successful species-classification approach, an ImageJ macro automatically extracted measurements for random forest classification in the software R. The method allows for good classification accuracy, and the same process can be used to train the model on other species. Conclusions: This protocol is an adaptable tool for efficient and consistent identification of seed species or potentially other objects. Automated seed classification is efficient and inexpensive, making it a practical solution that enhances the feasibility of large-scale monitoring projects in conservation biology.

  PublisherOA PDFDOI

• Author response for "Forest canopy cover affects microclimate buffering during an extreme heat event"

  2024-04-23

  peer-reviewSenior author
  PublisherDOI

• Leaf‐level resistance to frost, drought and heat covaries across European temperate tree seedlings

  Journal of Ecology · 2024-01-12 · 16 citations

  articleOpen accessSenior author

  Abstract Most trees die as seedlings, with harsh environmental conditions, such as early‐spring frosts and summer heat waves, being important drivers of early mortality. However, it remains unclear whether tolerance to different environmental extremes (e.g. frost vs. heat) trades‐off, or covaries synergistically and how stress tolerances relate to growth rates and life history strategies. Given the likely role of extreme environmental conditions as environmental filters, the ability to tolerate different stressors at the seedling stage could shape the occurrence and composition of present and future forests. We explored the relationships between different leaf‐level stress tolerances, functional traits and geographic distributions across 22 species of temperate European tree seedlings. We measured indicators of tolerance to frost, drought and heat and related these values to growth rates and to important functional traits (e.g. leaf mass per area and stem‐specific density). Finally, we explored the links between measured seedling stress tolerances and climatic niche limits inferred from adult trees' distributions. We found that seedlings of most species were either moderately tolerant to all three stressors, or susceptible to all of them. Moreover, higher stress tolerances were associated with traits describing slower growth and lower competitive ability. However, seedling tolerances to climatic factors were unrelated to the environmental limits of their adult geographic distributions. Synthesis. Our results suggest that temperate tree seedlings might not experience trade‐offs when facing an increase in multiple extreme climate stressors, but may experience trade‐offs related to growth rate and competitive ability in the establishment phase. The lack of correlation between leaf‐level stress tolerances and the environmental limits of adult geographic distributions suggests that predicting species' current or future geographic distributions in Europe will require a more nuanced understanding of how climatic tolerances at juvenile and adult stages influence range limits. A better understanding of the interaction between survival in extreme climate, leaf‐level stress tolerances of seedlings and the factors driving species distributions is needed to understand future forest responses to climate change.

  PublisherOA PDFDOI

• Fire sparks upslope range shifts of <scp>N</scp>orth <scp>C</scp>ascades plant species

  Ecology · 2024-01-25 · 7 citations

  article

  As ongoing climate change drives suitable habitats to higher elevations, species ranges are predicted to follow. However, observed range shifts have been surprisingly variable, with most species differing in rates of upward shift and others failing to shift at all. Disturbances such as fires could play an important role in accelerating range shifts by facilitating recruitment in newly suitable habitats (leading edges) and removing adults from areas no longer suited for regeneration (trailing edges). To date, empirical evidence that fires interact with climate change to mediate elevational range shifts is scarce. Resurveying historical plots in areas that experienced climate change and fire disturbance between surveys provides an exciting opportunity to fill this gap. To investigate whether species have tended to shift upslope and if shifts depend on fires, we resurveyed historical vegetation plots in North Cascades National Park, Washington, USA, an area that has experienced warming, drying, and multiple fires since the original surveys in 1983. We quantified range shifts by synthesizing across two lines of evidence: (1) displacement at range edges and the median elevation of species occurrences, and (2) support for the inclusion of interactions among time, fire and elevation in models of species presence with elevation. Among species that experienced fire since the original survey, a plurality expanded into new habitats at their upper edge. In contrast, a plurality of species not experiencing fire showed no evidence of shifts, with the remainder exhibiting responses that were variable in magnitude and direction. Our results suggest that fires can facilitate recruitment at leading edges, while species in areas free of disturbance are more likely to experience stasis.

  PublisherDOI

Recent grants

• DISSERTATION RESEARCH: Climate change and biome range shifts at high elevations

  NSF · $15k · 2012–2014

• CAREER: Life on the Edge: The Effects of Climate, Competition and History on Range Limits

  NSF · $783k · 2011–2017

Frequent coauthors

• James S. Clark

  Duke University

  25 shared

• Aji John

  University of Washington

  18 shared

• Elli J. Theobald

  University of Washington

  13 shared

• Kavya Pradhan

  13 shared

• Claire Fortunel

  12 shared

• Benoı̂t Courbaud

  Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement

  12 shared

• Georges Künstler

  12 shared

• Meera Lee Sethi

  University of Washington

  11 shared

Education

• Ph.D.

  Duke University

  2001

• Other, Seed dispersal, seed banking and density-dependent mortality in temperate tree species

  Coweeta Hydrologic Laboratory

• Other, Productivity and global change impacts on seed production

  Cedar Creek Ecosystem Reserve

• Other, Factors allowing Mediterranean annual grasses to dominate and niche-neutral processes in Serpentine annuals

  University of California, Santa Barbara