Wildfires have become more frequent and severe in the western United States over the past several decades. These fires threaten forests, wildlife, homes, and human lives. Many people understand that rising temperatures, dry air, and climate change make the land more likely to burn.
But a new scientific study adds a powerful new idea to our understanding. It focuses on how plants themselves react to dry weather. This trait, called plant-water sensitivity or PWS, may help explain why some areas burn more often and more severely than others, even when the weather is the same.
What Is Plant-Water Sensitivity and Why It Matters
Plant-Water Sensitivity (PWS) is a crucial metric that describes how much the moisture content in live plants changes in response to variations in climate-derived moisture balance. It essentially indicates how sensitive vegetation moisture is to water limitation.
A high PWS means that a decrease in moisture balance will cause a larger drop in live fuel moisture content, which can lead to higher flammability and a larger burned area.
PWS is calculated as the sum of slopes from a linear regression between live fuel moisture content anomalies and lagged climate-derived moisture balance anomalies. It helps in understanding and forecasting wildfire vulnerability, as it accounts for the ecophysiological controls of vegetation on fire hazard.
When LFMC is high, the vegetation is safer. When it drops, fire risk rises. One of the main climate variables that affects plant moisture is Vapour Pressure Deficit (VPD), which shows how dry the air is. The higher the VPD, the more moisture is pulled from the plants into the air.
However, not all plants lose water at the same rate. Some are more resistant to dry air, while others are more sensitive. This difference is what the scientists call plant-water sensitivity, or PWS. Plants with high PWS lose water quickly when the air is dry.
Their LFMC falls sharply, and they become flammable. Plants with low PWS can hold on to their moisture for longer periods. This study shows that the amount of area burned during a wildfire season depends greatly on the PWS of the plants in that region, not just on the weather. To measure PWS, the researchers looked at how much LFMC changed in response to both recent and past climate conditions.
They used satellite data from 2016 to 2020 to track LFMC at a fine resolution of 4 kilometers.
They compared LFMC to a climate-derived moisture balance, which includes both rainfall and VPD. Importantly, they also included moisture conditions from the past because soils and plants store water.
They used a time lag of up to 150 days to capture these effects. For each 15-day step, they calculated how strongly LFMC dropped in response to dryness, and then added up all these responses. The final number is the plant-water sensitivity. The higher the number, the more quickly a plant’s water content drops when it gets dry.
How Plant-Water Sensitivity Relates to Wildfires
The key question was whether PWS could help explain how much land burns during fire season. To test this, the researchers studied wildfire data from 2001 to 2020 using NASA’s MODIS burned area product.
They also gathered VPD data over the same period. Then, they grouped the landscape into 15 equal vegetation zones based on PWS levels. In each group, they calculated how much burned area increased for each unit increase in VPD.
The results were clear and powerful. There was a strong positive relationship between PWS and the sensitivity of burned area to VPD. In other words, the more sensitive the plants were to dryness, the more area burned as the air became drier.
The correlation was very strong, with a value of R² = 0.71 and a significance of P < 0.0001. This means that 71% of the variation in how burned area responds to dryness can be explained by plant-water sensitivity alone.
This was true in both forested and shrubland areas. In grasslands, however, the relationship was weaker, likely because other factors such as wind and fuel availability play a larger role in grassland fire behavior.
Based on the results, the scientists grouped areas into three categories of fire hazard: low hazard, medium hazard, and high hazard. Low-hazard areas had PWS values below 1.
In these places, burned area stayed mostly stable, even when the air got drier. Medium-hazard areas had PWS values between 1 and 1.5, and burned area increased moderately with VPD. High-hazard areas had PWS above 1.5, where even a small increase in dryness led to large increases in fire extent.
Why Some Regions Are More Sensitive Than Others
After seeing how strongly PWS influenced fire risk, the next question was: what causes one area to have high PWS and another to have low PWS? The researchers used machine learning tools to explore this.
They tested 14 different plant and soil traits using a random forest model. Eight traits came from plants, including root depth, canopy height, and xylem properties that control water movement.
Six traits came from soil, including texture (sand, silt, clay), porosity, and how fast water can move through the soil. They found that soil traits explained slightly more of the variation in PWS than plant traits.
In total, soil traits made up 55% of the predictive power, and plant traits made up 45%.
The most important soil trait was saturated hydraulic conductivity, which shows how quickly water can move through soil. Next was the shape of the soil water retention curve, which affects how much water the soil holds at different tensions.
Root depth was the most important plant trait. Plants with deeper roots can reach water stored far underground and are less likely to dry out quickly. This means that both what is growing in an area and what it is growing in — the vegetation and the soil — work together to control how flammable the area becomes under dry conditions.
PWS and Climate Change Are Combining to Increase Fire Risk
An important part of this research was mapping how PWS overlaps with climate trends. The study looked at changes in VPD from 1980 to 2020. They found that VPD increased across 91% of the western United States, with a mean increase of 0.05 hPa per year.
Interestingly, areas where VPD increased the most also had some of the highest PWS values. This overlap creates what the researchers called “double-hazard zones”. These are areas where both the climate is drying rapidly and the plants are very sensitive to that drying.
About 28% of the western U.S. falls into this double-hazard category. Some key regions include the Sierra Nevada in California, the Great Basin in Nevada, eastern Oregon, and the Mogollon Rim in Arizona.
These areas are expected to see the greatest increases in fire size and frequency in the coming decades if current trends continue. Some areas, like the southern Sierra Nevada, have very high PWS, while nearby areas like the northern Sierra have low PWS, showing that these risks can vary dramatically even over short distances.
Population Growth in High-Risk Areas
It’s not just nature that is increasing the risk. Human decisions are making the problem worse. The Wildland-Urban Interface, or WUI, is the zone where human homes and buildings meet natural vegetation.
Between 1990 and 2010, the number of people living in the WUI nearly doubled, growing from 10 million to 20.8 million. But the most rapid growth happened in the highest-risk areas.
In high-PWS zones, the WUI population increased by 160% over those two decades. In comparison, medium-hazard areas grew by 95%, and low-hazard zones grew by 107%. Around 1.5 million people moved into high-hazard WUI areas.
This is about the same as the combined current population of San Francisco and Seattle. These people are now living in places where vegetation dries out quickly and the air is getting drier every year. The overlap of high population and high fire risk increases the potential for damage to homes, health, and the environment.
The study’s maps showed that these areas also experienced the greatest increase in burned area per unit rise in VPD. This means that wildfire danger to people is not only rising because the climate is changing, but also because people are moving into the most dangerous regions.
Why Fire Models Need to Change
Most current wildfire danger systems focus only on meteorological conditions, especially temperature, humidity, and wind. These models usually estimate fuel moisture content based on these weather factors alone.
However, this study shows that such models may miss a critical part of the equation: how different plants respond to those weather conditions. Two areas with the same weather can have very different fire risks depending on the local vegetation and soil.
By including PWS in fire danger models, scientists and managers can better predict when and where fires will become severe. For example, the National Fire Danger Rating System in the United States could improve its predictions by using maps of PWS along with standard weather data. This would help firefighters, land planners, and emergency managers prepare better and reduce risk.
Conclusion
The final message of this study is that wildfire danger is not just about hot weather and dry air. It is also about how plants react to that dryness and how quickly they become flammable. Some ecosystems are more sensitive than others, and this sensitivity depends on both plant traits and soil traits.
Human activity adds another layer to the risk, especially when people move into the most sensitive regions. If we want to manage wildfire risk effectively, we need to understand and consider plant-water sensitivity.
This means using better fire models, planning smarter land development, and perhaps even adjusting the kinds of vegetation we encourage in high-risk areas. By taking a full picture of climate, ecology, and human behavior, we can reduce the danger and protect both people and the planet.
Key Terms and Concepts
What is Wildfire: Wildfire refers to an uncontrolled fire that burns in wildland areas, often impacting human health and the environment. These fires can grow rapidly and are influenced by various factors such as increased atmospheric aridity and changes in vegetation. Wildfires threaten human lives and structures, and alter ecosystem functions and vegetation growth patterns. The article focuses on understanding factors that influence the extent of areas burned by wildfires.
What is Vapour Pressure Deficit (VPD): Vapour Pressure Deficit (VPD) is a measure of how dry the air is and how much more water the air can hold before becoming saturated. It’s calculated using the wetness of dead foliage and twigs in the litter. An increase in VPD indicates drier atmospheric conditions, which has been linked to a chronic increase in wildfire areas in the western United States. The article shows that a greater increase in VPD leads to a larger increase in burned area, especially in regions where plants are more sensitive to water limitations.
What is Burned Area: Burned area refers to the total extent of land that has been consumed by wildfires. The article investigates how different factors, especially Vapour Pressure Deficit and Plant-Water Sensitivity, contribute to the increase in burned area. Understanding the dynamics of burned area is critical for assessing wildfire risk and its impacts on the environment and human populations. The sensitivity of burned area to atmospheric aridity can vary significantly across different regions.
What is Wildland-Urban Interface (WUI): The Wildland-Urban Interface (WUI) is a transitional zone where wildland areas meet urban environments. This area is significant for wildfire risk because it often has abundant vegetation (fuel loads) and a high density of human populations, leading to frequent human-caused ignitions. The expansion of WUI, particularly into regions with high plant-water sensitivity, disproportionately increases human exposure and vulnerability to wildfires.
What is Atmospheric Aridity: Atmospheric aridity refers to the dryness of the atmosphere. It is a key climate factor that contributes to a decline in fuel moisture and an increase in burned areas. Anthropogenic climate change and natural climate variability have led to a substantial increase in atmospheric aridity, which is a major driver of increased wildfire activity.
What are Fuel Loads: Fuel loads refer to the amount of combustible material, such as vegetation (live or dead), available to burn in a wildfire. Historic fire suppression practices have increased fuel loads in many areas, contributing to more intense and extensive fires. The presence of large fuel loads in the Wildland-Urban Interface, combined with frequent human-caused ignitions, poses a high risk to human lives and structures.
What is Live Fuel Moisture Content (LFMC): Live Fuel Moisture Content (LFMC) is a measure of the amount of water present in live plants relative to their dry biomass. It is a critical indicator of fuel flammability. Generally, as atmospheric aridity increases, LFMC decreases, making vegetation more susceptible to burning. The effect of atmospheric aridity on LFMC is influenced by various factors, including soil water availability and plant hydraulic traits.
What are Plant Hydraulic Traits: Plant hydraulic traits are characteristics of plants that affect how they absorb, transport, and lose water. These traits play a significant role in regulating the Live Fuel Moisture Content (LFMC) and, consequently, fuel flammability. Examples include root water uptake efficiency and transpirational water loss mechanisms. Variations in these traits can cause significant differences in LFMC even under the same meteorological conditions, influencing wildfire hazard at large scales.
What is Climate-derived Moisture Balance: Climate-derived moisture balance is an integrated measure that considers both precipitation and Vapour Pressure Deficit (VPD) to represent the water available to vegetation. It is used to understand how climate influences Live Fuel Moisture Content (LFMC). Plant-Water Sensitivity (PWS) is defined as the sensitivity of LFMC to this climate-derived moisture balance, indicating how well ecosystems buffer against climatic water limitations.
What is Wildfire Hazard: Wildfire hazard, as defined in the article, refers to the potential for an increase in burned area due to Plant-Water Sensitivity (PWS). The article categorizes PWS into three hazard levels: low, medium, and high, based on how sharply burned area increases with VPD. High hazard areas are those where even a moderate increase in VPD can lead to a significant increase in burned area due to the high sensitivity of vegetation to water limitation. It is important to note that hazard here specifically refers to the increase in burned area and not to losses or benefits from the burned area.
What is Wildfire Risk: Wildfire risk is a broader concept than wildfire hazard. In the context of this article, “risk” is used when wildfire hazard coincides with human exposure and vulnerability. This means that wildfire risk increases not only due to conditions that lead to larger burned areas (hazard) but also when human populations and structures are present in those vulnerable areas. The article highlights that the expansion of human populations into the Wildland-Urban Interface (WUI) has disproportionately increased wildfire risk.
What is Saturated Soil Hydraulic Conductivity (Ks): Saturated soil hydraulic conductivity () is a measure of how easily water can move through saturated soil. It is one of the most important drivers of Plant-Water Sensitivity (PWS), accounting for 20% of its explained variance. Higher conductivity means water infiltrates quickly, which can influence how quickly plants experience water limitation and thus impact wildfire vulnerability.
What are Soil Water Retention Curves (n): The shape parameter of soil water retention curves, denoted by ‘n’, describes how soil holds and releases water. This parameter is another significant driver of Plant-Water Sensitivity (PWS), explaining 13% of its variance. It dictates how much water is available to plants at different soil moisture levels, directly affecting the plant’s ability to maintain its live fuel moisture content during dry periods.
What is Root Depth: Root depth refers to how deep plant roots penetrate into the soil. It is an important factor influencing Plant-Water Sensitivity (PWS), contributing 9% to its explained variance. Deeper roots can access water from lower soil profiles, potentially buffering the plant against surface-level moisture limitations and thus affecting its overall water sensitivity.
What is Isohydricity: Isohydricity is a plant stomatal regulation trait, meaning how plants control the opening and closing of their stomata (tiny pores on leaves) to regulate water loss. Isohydric plants tend to keep their stomata relatively closed to conserve water, which can affect their live fuel moisture content and, consequently, their plant-water sensitivity.
What is Xylem Capacitance: Xylem capacitance refers to the ability of the xylem (the water-conducting tissue in plants) to store and release water. This hydraulic trait is important because it can buffer against sudden changes in water availability, influencing how quickly a plant’s live fuel moisture content drops during dry periods and thus affecting its plant-water sensitivity.
What is Maximum Xylem Conductance: Maximum xylem conductance is a plant hydraulic trait that refers to the maximum rate at which water can flow through the xylem. This trait directly impacts the plant’s ability to transport water from its roots to its leaves, thereby influencing its overall water balance and live fuel moisture content, which in turn affects its plant-water sensitivity.
What is Stomatal Conductance Slope Parameter (g1): The stomatal conductance slope parameter () is a plant hydraulic trait that is inversely proportional to the square root of water use efficiency. It relates to how much the stomata open or close in response to environmental conditions, thereby controlling water vapor exchange. This parameter is a key indicator of how plants regulate water loss and affects their overall plant-water sensitivity.
What is (Xylem water potential at 50% loss in xylem conductivity): represents the xylem water potential at which there is a 50% loss in xylem conductivity. It is a critical plant hydraulic trait indicating the plant’s vulnerability to cavitation (formation of air bubbles in the xylem) and water stress. A lower indicates a greater resistance to water stress. This trait significantly influences a plant’s ability to maintain its live fuel moisture content under dry conditions, thereby impacting plant-water sensitivity.
What are Double-hazard Regions: Double-hazard regions are specific geographic areas where both Plant-Water Sensitivity (PWS) is high (typically ) and the Vapour Pressure Deficit (VPD) has increased faster than the average. The co-occurrence of these two factors amplifies increases in burned area, leading to a significantly higher wildfire hazard. These regions are particularly vulnerable to increased wildfire activity due to the combined effect of vegetation sensitivity and rising atmospheric dryness. Examples include parts of the Sierra Nevada in California, eastern Oregon, the Great Basin in Nevada, and the Mogollon Rim in Arizona.
What is Antecedent Precipitation: Antecedent precipitation refers to the rainfall that occurs before a specific event, in this case, before the peak fire season (specifically, December-May). In arid ecosystems, antecedent precipitation can spur vegetation growth, which in turn increases fuel loads, potentially leading to larger burned areas during the fire season. However, the study found no positive correlation between PWS and the sensitivity of fuel availability to antecedent precipitation.
What is Normalized Difference Vegetation Index (NDVI): Normalized Difference Vegetation Index (NDVI) is a widely used satellite-derived index that measures vegetation greenness and density. In the context of this study, NDVI is used to represent fuel availability. The study investigated the correlation between PWS and the sensitivity of fuel availability (represented by NDVI) to antecedent precipitation across different land covers.
What is Dead Fuel Moisture Content (DFMC): Dead Fuel Moisture Content (DFMC) is a meteorological estimate of the wetness of dead vegetation, such as twigs and branches. It serves as an indicator of wetness and combines precipitation, temperature, and humidity over lagged timescales. The article uses 100h DFMC, calculated from 24h antecedent precipitation and equilibrium moisture content, to represent climate-derived moisture balance, which is crucial for determining Plant-Water Sensitivity.
What are Plant Functional Types: Plant functional types are classifications of plants based on shared physiological and structural characteristics, which are often used in global fire models to parameterize vegetation controls on burned area. However, the article suggests that relying solely on plant functional types may lead to errors in vegetation-fire relationships because the specific plant and soil hydraulic traits that control Plant-Water Sensitivity can vary significantly even within these functional types. The PWS metric offers a more direct and scalable way to quantify vegetation-climate sensitivity.
Reference:
Rao, K., Williams, A.P., Diffenbaugh, N.S. et al. Plant-water sensitivity regulates wildfire vulnerability. Nat Ecol Evol 6, 332–339 (2022). https://doi.org/10.1038/s41559-021-01654-2
