Every spring, Europe’s forests are showered with far more than rain. Wind-pollinated trees release staggering quantities of pollen into the atmosphere, and much of it eventually settles on leaves, branches and the forest floor. New research now shows that this pollen is not merely biological debris — it is a measurable, taxon-specific driver of nutrient fluxes in the water that drips through forest canopies, a flow scientists call throughfall. The findings, published in Environmental Monitoring and Assessment, provide the first continental-scale evidence directly linking pollen deposition to forest water chemistry, and they suggest that decades of forest monitoring may have been quietly misattributing a slice of nutrient input to other sources.

The study was carried out by a large international team led by Elena Gottardini of the Fondazione Edmund Mach in Italy, working within the ICP Forests programme, a pan-European network that monitors air pollution effects on forests. The researchers analysed throughfall samples collected in 2018 from 60 intensive monitoring plots — known as Level II plots — across eight countries: Belgium, Finland, France, Germany, Italy, Norway, Switzerland and the United Kingdom. The plots ranged in altitude from 14 to 1,900 metres above sea level and were dominated by six tree genera: Abies, Fagus, Larix, Picea, Pinus and Quercus. In total, 196 throughfall samples were collected between February and July, covering the early vegetative season when most European trees flower.

The innovation of the study lies in its methodology. Rather than inferring pollen inputs from nearby aerobiological samplers, the team examined the very same water samples that were being chemically analysed. A known quantity of water from each sample was filtered through a 5-micrometre cellulose nitrate mesh, and a calibrated number of 10-micrometre polystyrene-divinylbenzene microspheres — roughly 155,000 per sample — were added as a counting standard. The filters were then dissolved in acetone, centrifuged, and the resulting pellet embedded in fuchsin jelly on microscope slides. Trained analysts identified and counted pollen grains under 400-times magnification, stopping when about 1 percent of the added microspheres had been counted. From these counts, the team calculated pollen deposition rates in grains per square metre per day, correcting for filtered volume, throughfall quantity and sampling duration.

The results were striking. Fifty-three pollen taxa were identified across the samples, with woody species accounting for 98 percent of all grains. Four genera — Pinus, Picea, Fagus and Quercus — together made up 91.4 percent of the total. Pine pollen alone represented more than half of everything collected, a dominance the researchers attribute to the buoyant air sacs on pine grains, which allow them to travel enormous distances from both local and distant sources. In beech, spruce and pine stands, the pollen of the dominant genus itself prevailed, making up 49.7, 59.1 and 84.3 percent of the local pollen spectrum respectively. In oak stands, oak pollen ranked second at 26.9 percent, behind pine. Data from fir and larch stands were excluded because sampling there did not align with those genera’s flowering periods.

By combining the measured pollen curves with published flowering phenology, the team defined main pollen seasons for each genus. Beech and oak flowering spanned April and May, with beech peaking in the last week of April and oak showing a double peak at weeks 19 and 23, likely reflecting multiple oak species flowering at slightly different times across the wide latitudinal and altitudinal spread of plots. Spruce pollen peaked around weeks 19 to 20, while pine peaked at week 19 with a secondary pulse at week 22. Overall, 57 percent of the throughfall samples fell within their stand’s flowering period for at least half of their collection window, providing a robust temporal overlap between pollen deposition and water chemistry measurements.

The chemical analysis, conducted at the Research Institute for Nature and Forest laboratory in Belgium, quantified major ions including potassium, calcium, magnesium, sodium, chloride, phosphate, sulphate, nitrate, nitrite and ammonium using ion chromatography, alongside total dissolved nitrogen and non-purgeable organic carbon measured on a TOC/TN analyser. Linear mixed-effects models then tested whether pollen deposition rates could predict the flux of these compounds, with plot identity included as a random effect and autocorrelation between repeated samples explicitly modelled.

The answer was a clear yes. Every statistically significant association between pollen and throughfall chemistry was positive, meaning more pollen corresponded to more nutrients reaching the forest floor. Potassium emerged as the star player: its flux was significantly explained by pollen from all four main genera, with pollen accounting for an average of 33 percent of the variance. This makes biochemical sense, because potassium is one of the predominant ions in pollen grains, and pollen washed from flowers and crowns by rain or deposited dry is a direct source of the ion during the flowering season — an idea first proposed decades ago but never confirmed at this scale. The strongest single relationships involved oak pollen, which explained 77.8 percent of the variance in ammonium fluxes and 55 percent in total dissolved nitrogen.

Broadleaved and coniferous pollen left different chemical fingerprints. Beech and oak pollen together significantly explained fluxes of non-purgeable organic carbon, dissolved organic nitrogen, total dissolved nitrogen, ammonium and magnesium, while the combined pollen of pine and spruce was linked with every measured compound except phosphate. Intriguingly, broadleaved pollen showed a weak negative association with nitrate fluxes. The authors urge caution in interpreting this pattern, since several non-exclusive mechanisms could be at play. Laboratory experiments have shown that pollen and its associated microbiota can transform extracellular nitrate into nitrite and nitrous oxide, an effect pronounced in beech and oak but weak or absent in pine and spruce. Canopy uptake of nitrogen and coincidentally lower atmospheric deposition during broadleaf flowering are also possible explanations that this single-year study cannot disentangle.

The dissolved organic carbon carried in throughfall also differed by forest type. Specific ultraviolet absorbance at 254 nanometres, a proxy for aromatic and recalcitrant carbon, was highest in spruce stands, whose needles release lignin derivatives and tannins. The humification index was significantly higher in oak throughfall, reflecting more decomposed, humic organic matter. Most revealing was the fluorescence index, which distinguishes microbial from plant-derived organic matter: beech and oak samples had significantly higher values, with medians near 1.9, than pine and spruce samples, which sat near 1.7. Values above 1.9 typically indicate microbially processed carbon, and the elevated readings under broadleaves suggest microbes play a larger role in transforming pollen-derived carbon in those forests — hinting that pollen inputs feed not just trees but entire soil microbial food webs.

For the authors, the implications reach well beyond academic curiosity. Routine throughfall monitoring has historically ignored pollen as a variable, meaning that chemical enrichment previously attributed solely to canopy leaching or atmospheric deposition may in fact be partly pollenic. As climate change pushes flowering earlier and increases airborne pollen amounts across arboreal taxa, these hidden inputs could grow. Masting behaviour in beech and oak — the episodic, synchronized production of enormous seed and pollen crops — introduces strong year-to-year variability that any nutrient budget must now reckon with.

The researchers acknowledge the limitations of a one-year, correlative dataset and call for multi-year studies incorporating canopy architecture, stand age, phenology and insect infestations, as well as stable isotope tracing or controlled exclusion experiments to nail down mechanisms. Still, the message is clear: pollen is not just about reproduction, or even allergies. It is a seasonal fertiliser delivered from the sky, and forests have been banking on it all along. Incorporating pollen surveys into long-term forest observation networks, the team argues, will be essential for accurate nutrient budgets in a changing climate.

Have you ever experienced a severe weather event, like a hurricane or a tornado? Researchers have shown that climate change is causing such weather events to become more extreme and more frequent, destroying communities and infrastructure. To determine which areas are most at risk, researchers from Canada studied the environmental effects of a powerful flooding event that occurred in southwestern British Columbia (BC) in November 2021. 

They focused on a type of severe flooding event that is characteristic of the North American Pacific coastal region, called an atmospheric river. An atmospheric river occurs when a long, narrow vapor cloud flows toward land from the Pacific Ocean, causing heavy rainstorms. In November 2021, an atmospheric river in southwestern BC destroyed numerous regional highways, pipelines, power lines, and railways within its path. Over 18,000 people were evacuated due to flooding, and 6 people died. 

The team explained that before the storm, BC experienced a severe drought and an unprecedented heatwave, followed by widespread thunderstorm activity. In 2021, lightning and human activity caused nearly 1,000 wildfires, which led to the third-worst wildfire season ever recorded in BC, partially because trees grow back slowly in forests that have been cut down or harvested. Poor vegetation recovery makes harvested forests more vulnerable to wildfires than old-growth forests. The November atmospheric river occurred shortly after this wildfire season. 

To determine how the atmospheric river impacted southwestern BC, the researchers analyzed approximately 70,000 square kilometers (27,027 square miles) of affected area. They compared satellite images of southwestern BC from Google Earth, Sentinel, and Landsat taken before and after the event to detect changes in river channels and identify geohazards, like landslides and debris flows. They also used satellite images and Canadian government data to identify which forests had been harvested within the study area. The researchers then traversed the area to confirm their remote analyses. 

The researchers used a type of 3D remote sensing, called light detection and ranging, or lidar, to measure how the 2021 atmospheric river redistributed sediment in the Coquihalla and Coldwater River valleys. With the help of lidar, the researchers discovered that both rivers had severely unstable banks due to erosion from the flooding event. Along the Coquihalla River, approximately 53,000 cubic meters (1,871,677 cubic feet) of sediment were eroded, while nearly 50,000 cubic meters (1,765,733 cubic feet) of sediment were eroded along the Coldwater River. 

The researchers also found that hillslope geohazards increased the volume of sediment in both rivers. And in some cases, upstream areas with a history of forest harvesting and wildfires also affected stable banks downstream. For example, they identified debris flows on an unstable hillslope that triggered a landslide on a riverbank downstream. They observed that the river and stream channels shifted due to a sudden addition of sediment from hillslope geohazards and floodwater during the event.

In total, the researchers documented over 1,300 geohazards along 47 kilometers (about 30 miles) of the Coquihalla River and 19 kilometers (about 12 miles) of the Coldwater River and nearby creeks. Nearly half of the geohazards originated in an area with a history of wildfires or forest harvesting. Of the 155 geohazards at the Coquilhalla River, 43% occurred below forest harvesting resource roads, and of the 117 geohazards observed at the Coldwater River, 29% originated in areas with past wildfires.

The researchers suggested that their findings could help scientists predict where severe geohazards will occur during extreme weather events, since both watersheds had geohazards originating in harvested forests. Before 2000, clear-cutting was a widespread practice in BC, and the team’s results demonstrate that unsustainable harvesting practices like this can have lasting environmental impacts.

The researchers concluded that both forest harvesting and wildfires contributed to the severe impacts of the November 2021 atmospheric river, causing catastrophic landslides and debris flows to occur in traditionally dry regions. They suggested that more accurate risk assessments can better equip communities for severe weather events like this. While the next atmospheric river that lands in the Pacific coastal region may be worse than the last, this study can inform scientists and communities about which areas it might affect most.

The post Wildfires and forest harvesting worsen severe weather impacts appeared first on Sciworthy.

A decade-long search through Taiwan’s remote forests led to the discovery of East Asia’s tallest tree and hundreds of other towering giants. An 84-meter-tall tree is surprisingly easy to miss. Hidden within Taiwan’s steep mountains and dense old-growth forests are some of the tallest trees on Earth, including giant conifers that rise higher than a [...]

As Earth’s climate system evolves with unprecedented rapidity, forests — vital guardians of global ecological balance — are undergoing complex transformations that extend far beyond the mere sequestration of carbon. Emerging research has begun to unravel the subtle yet profound ways in which climate change reshapes the biophysical interactions between forested landscapes and their surrounding environments, particularly regarding land surface temperature dynamics. A recent groundbreaking study delves deep into these dynamics, revealing starkly contrasting trends that underscore the dualistic nature of forests as climate regulators amidst rising atmospheric dryness. This nuanced revelation challenges long-held assumptions about the uniform benefits of forest cooling and opens new frontiers for understanding ecosystem-climate feedbacks in the Anthropocene.

Forests have long been recognized as crucial climate buffers, primarily through their capacity for carbon storage and evapotranspiration-driven cooling effects. The new research pivots attention to a complementary biophysical mechanism: the difference in growing-season land surface temperature (LST) between forested areas and adjacent open lands, designated as ∆LST_gs. By systematically quantifying ∆LST_gs globally across a span of more than two decades (2001–2023), the researchers uncovered contrasting temporal patterns that vary profoundly with geography and changing atmospheric conditions. Notably, these patterns defy a simplistic narrative of consistent forest cooling, instead revealing a dynamic response heavily modulated by atmospheric humidity levels.

Central to these divergent forest temperature dynamics is the rising vapor pressure deficit (VPD), a measure of atmospheric dryness that quantifies the difference between the amount of moisture in the air and its saturation point. The study identifies VPD as the predominant driver behind the observed variations in ∆LST_gs, exerting a stronger influence than other conventional climatic factors such as temperature, precipitation, or solar radiation. This finding shines light on the critical role that atmospheric moisture stress plays in forest surface energy exchanges, fundamentally altering the cooling potential traditionally attributed to forests under wetter conditions.

However, the forest response to this intensifying atmospheric dryness is far from uniform across global biomes. The research highlights a pivotal interaction between VPD fluctuations and plant hydraulic traits, particularly stomatal regulation strategies embodied by anisohydricity, a measure of plants’ ability to regulate water loss through stomata under drought stress. Forests exhibiting high anisohydricity maintain more open stomata even under dry conditions, enabling continued transpiration and evaporative cooling but at increased risk of hydraulic failure. In contrast, isohydric forests tightly conserve water by closing stomata earlier, reducing cooling at the leaf surface.

Intriguingly, this stomatal regulatory behavior correlates with latitude, delineating distinct forest cooling trajectories. Tropical forests near the equator tend to display more isohydric characteristics. Here, rising VPD often surpasses the hydraulic safety threshold of these ecosystems, leading to stomatal closure and a consequent weakening of forest cooling effects. This trend portends a diminished capacity of tropical forests to mitigate local heating as atmospheric dryness intensifies, potentially accelerating heat stress on these already vulnerable ecosystems.

Conversely, high-latitude forests manifest a more anisohydric strategy, maintaining open stomata under increasing VPD levels that nonetheless remain within their hydraulic safety margins. As a result, these boreal and temperate forests continue to sustain transpiration-driven cooling, which paradoxically intensifies with rising VPD. This phenomenon enhances the biophysical benefits of northern forests, amplifying their role as regional climate coolants and underscoring the heterogeneous nature of forest climate feedbacks across latitudes.

This nuanced physiological interplay yields profound implications regarding future forest-climate interactions under global warming. It underscores that elevated atmospheric dryness will not only influence ecosystem carbon dynamics but also significantly alter the biophysical feedback mechanisms by which forests regulate surface temperatures. As VPD continues to climb worldwide, the traditional ecological services of forests, particularly their cooling benefits, may become compromised in many parts of the globe, particularly within tropical zones critical for biodiversity and global climate regulation.

Moreover, the study’s integrative approach—which combines satellite observations of surface temperature with detailed climatological measurements and physiological trait data—provides a comprehensive framework to understand how climatic stressors mediate forest cooling effects. By leveraging large-scale data spanning two decades, the researchers offer robust evidence that forest cooling is not static but dynamically contingent on complex interactions among atmospheric moisture, plant hydraulics, and geographic distribution, emphasizing the need for ecosystem-specific climate mitigation strategies.

The findings challenge the prevailing optimism about forests’ capacity to offset warming through biophysical means alone. In vulnerable tropical regions, the erosion of cooling benefits linked to stomatal closure under heightened VPD may exacerbate heat stress, increase fire risk, and undermine forest resilience. This could trigger feedback loops accelerating tropical forest degradation and amplifying global warming, raising alarm over the future of these essential carbon sinks.

Conversely, the sustained or even enhanced cooling in high-latitude forests might partially offset regional warming trends, but the balance of such compensatory effects at the global scale remains uncertain. These complexities highlight an urgent need to integrate physiological and biophysical forest attributes into predictive climate models, allowing for more accurate assessments of forest contributions to local and global temperature regulation.

Crucially, the research points to the hydraulic safety margin as a vital threshold parameter dictating the tipping points at which forests transition from cooling to warming agents. This insight offers potential pathways for management interventions aimed at bolstering forest hydraulic resilience, such as selective species planting or conservation strategies tailored to optimize ecosystem-level responses to rising VPD.

In a broader context, the study underscores that atmospheric dryness—often overshadowed by temperature-centric perspectives on climate change—constitutes a formidable and multifaceted challenge for forest ecosystems worldwide. Rising VPD alters not only physiological processes at the leaf level but also cascades through landscape-scale energy budgets, with cascading impacts on regional climate patterns, hydrology, and ecosystem services.

As the global community contemplates reforestation and afforestation policies as climate mitigation tools, these findings call for a recalibrated understanding that accounts for the limits imposed by plant hydraulic behavior and atmospheric moisture constraints. The simplistic paradigm of “more trees equal cooler climate” must evolve into a sophisticated appreciation of when, where, and how forest ecosystems can be expected to maintain their biophysical cooling contributions in a drying and warming world.

Ultimately, this pioneering research adds a critical dimension to our grasp of climate-vegetation feedbacks, illuminating the complex, sometimes counterintuitive outcomes of drying atmospheres on forested landscapes. By highlighting the paramount role of VPD and stomatal regulation across global latitudinal gradients, it informs the scientific community, policymakers, and conservationists alike about the nuanced realities shaping the future of Earth’s green lungs and their vital climate services.

Understanding these relationships better will be indispensable to crafting adaptable and resilient conservation strategies capable of sustaining the biophysical cooling functions of forests. The study’s revelations lay groundwork for future research probing genetic, species-specific, and ecosystem-level hydraulic traits, as well as remote sensing advancements to monitor forest physiological stress and energy exchanges in real time under changing climates.

As atmospheric dryness intensifies in the coming decades, the fate of forests as biophysical climate modulators will hinge on the delicate balance between environmental stressors and intrinsic plant water regulation mechanisms. This research presents an urgent clarion call to incorporate these intricate biophysical and physiological insights into forest management and climate policy frameworks, lest the cooling benefits of the planet’s forests risk becoming relics of a less-dry past.

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Subject of Research:
Forest biophysical effects on land surface temperature under changing atmospheric dryness and climate conditions.

Article Title:
Globally constrained forest biophysical cooling benefits under rising atmospheric dryness.

Article References:
Zhang, C., Su, Y., Liao, Z. et al. Globally constrained forest biophysical cooling benefits under rising atmospheric dryness. Nat. Clim. Chang. (2026). https://doi.org/10.1038/s41558-026-02677-y

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41558-026-02677-y

Deep in Taiwan’s misty mountains, researchers have confirmed the tallest tree in the country: a thousand-year-old fir tree higher than a 20-story building, which they’ve named “the heaven sword of the Da’an River.” Climbers scaled the tree and dropped a measuring tape from the top to the forest floor during the Lunar New Year holiday in January 2023. The tree measured 84.1-meters (276-feet). The findings have been published in Frontiers in Forests and Global Change. A team of ecologists, geologists, remote-sensing specialists, professional climbers and Indigenous people that calls itself the “Taiwan tree seekers” began the search in 2014. “The common characteristics [of the team] are probably that we are all tree lovers and like adventures,” Rebecca Chia-Chun Hsu, lead author from Division of Forest Ecology, Institute of Taiwan Forestry Research, told CNN. ‘The Heaven Sword’, Taiwan’s tallest tree, measures 84.1 meters. Photo courtesy of Steven Pearce. Taiwan is one of the few places on Earth where trees can grow this tall. The island sits where the tropics meet the subtropics, and its mountains host several giant conifer species. The species behind the new record, Taiwania cryptomerioides, is known to the Indigenous Rukai people as “the tree that hits the moon.” Although nearly 60% of Taiwan is covered in forest, loggers cleared much of the island’s old-growth forest between 1912 and 1991. However, its steep slopes were too dangerous to reach, and pockets of ancient forest survived. Still, finding the tallest tree amid the rugged terrain was a task. Taiwan…This article was originally published on Mongabay