A global satellite study reveals how drought and wildfire together disrupt the relationship between rainfall, plant growth, and carbon absorption.<\/p>\n
When drought and wildfire hit an ecosystem at the same time, something quietly breaks in the relationship between plants and water.\u00a0<\/p>\n
A global satellite study has found that vegetation hit by these compound events becomes markedly more sensitive to rainfall<\/a>. Yet, it is simultaneously less efficient at turning that water into carbon gain.\u00a0<\/p>\n The two processes, which normally move together, start to come apart. This finding\u2019s implications for the health of the world\u2019s forests are not reassuring.<\/p>\n The study was conducted by Huipeng Xi and Qunming Wang at Tongji University<\/a> in Shanghai.<\/p>\n The carbon sink under pressure<\/p>\n Every year, vegetation<\/a> absorbs enormous quantities of carbon dioxide through photosynthesis. <\/p>\n Much of that carbon eventually returns to the atmosphere, but the portion that remains stored in plants and soils helps slow climate change.<\/p>\n This service is not guaranteed, though. It depends on ecosystems remaining functional, which in turn depends heavily on water.\u00a0<\/p>\n Precipitation is the primary driver of plant growth across much of the globe. How sensitively vegetation photosynthesis<\/a> responds to rainfall variability is a key indicator of ecosystem health.<\/p>\n After drought and wildfire, however, a sharp increase in that sensitivity may signal that vegetation is struggling to recover.<\/p>\n Nearly 200,000 compound events<\/p>\n The researchers analyzed global satellite data from 2001 to 2022. They used three vegetation proxies to track photosynthetic activity at fine spatial resolution.\u00a0<\/p>\n These measurements included faint light emitted by plants during photosynthesis and two satellite-based indicators of vegetation greenness and growth.<\/p>\n Drought was identified using two complementary indices. One was based on precipitation deficits (meteorological drought). The other used terrestrial water storage anomalies derived from satellite gravimetry (hydrological drought).\u00a0<\/p>\n Fire was tracked using ESA<\/a>\u2019s burned-area dataset.<\/p>\n The team identified 72,088 compound meteorological drought\u2013fire events across the globe between 2001 and 2022, and 108,373 compound hydrological drought\u2013fire events.\u00a0<\/p>\n A compound event was defined as drought and fire striking the same location in the same month. To keep the signal clean, no other major disturbance could have occurred within a year before or after.<\/p>\n More rain sensitivity, less efficiency<\/p>\n Across the full study period, global vegetation sensitivity to precipitation declined. But in the one to three years after a compound drought\u2013fire event, that sensitivity jumped back up more sharply than after drought alone.\u00a0<\/p>\n Hydrological drought<\/a>\u2013fire events drove the strongest response: sensitivity increased by 7.89\u00d710\u207b\u2075 mm\u207b\u00b9 using the fluorescence measure, compared to 3.20\u00d710\u207b\u2075 mm\u207b\u00b9 for meteorological drought\u2013fire combinations.\u00a0<\/p>\n Around 54.6% of all vegetated areas showed increased sensitivity in the aftermath of compound events.<\/p>\n At the same time, water-use efficiency \u2013 the ratio of carbon gained to water consumed \u2013 declined across most of those same areas.\u00a0<\/p>\n The two processes, which in a healthy ecosystem track each other reasonably closely, were pulling apart.<\/p>\n \u201cOur findings reveal a critical vulnerability: after compound drought\u2013fire events, ecosystems become more dependent on rainfall but less efficient at converting water into carbon gain,\u201d Wang said.\u00a0<\/p>\n \u201cThis decoupling suggests that vegetation may be entering a fragile recovery state, where increased rain sensitivity does not translate into healthier carbon\u2013water balance \u2013 a warning signal for ecosystem resilience under escalating climate extremes.\u201d<\/p>\n What\u2019s driving the shift<\/p>\n To understand why some areas showed stronger responses than others, the team applied a random forest model to 22 environmental variables. It explained 79% of the spatial variation in sensitivity changes.\u00a0<\/p>\n The top predictors were atmospheric aridity and soil nutrients<\/a>, particularly nitrogen and phosphorus. Where the air was drier and soils more nutrient-depleted, post-disturbance sensitivity rose more steeply.<\/p>\n The finding points to a compounding feedback. Drought dries the air and depletes soil moisture. Fire<\/a> consumes organic matter and disrupts nutrient cycling.\u00a0<\/p>\n Recovering vegetation may therefore face drier atmospheric conditions and disrupted nutrient supplies. <\/p>\n Together, these pressures could help explain why rainfall becomes more influential even as water-use efficiency declines.<\/p>\n A warning for the carbon cycle<\/p>\n Compound drought\u2013fire events are not rare anomalies. The study catalogued nearly 200,000 of them across two decades.\u00a0<\/p>\n Climate projections suggest they will become more frequent as temperatures rise. <\/p>\n If each one leaves the affected ecosystem more dependent on precipitation while less capable of using it efficiently, the cumulative effect on the terrestrial carbon sink<\/a> could be substantial.<\/p>\n Vegetation that has decoupled its carbon and water processes is also harder to predict.\u00a0<\/p>\n Earth system models that assume a stable relationship between rainfall and photosynthesis may misrepresent these ecosystems.<\/p>\n They could potentially overestimate the carbon sink in regions with high compound disturbance frequency.\u00a0<\/p>\n The authors suggest that vapor pressure deficit and soil nutrient status could serve as early-warning indicators.<\/p>\n The satellite record reveals a troubling pattern: when drought and wildfire strike together, their effects on vegetation can persist for years after the immediate disturbance ends.<\/p>\n Even when plants begin to recover, they may become more dependent on rainfall while growing less efficient at using water to absorb carbon. <\/p>\n As climate extremes intensify, that imbalance could weaken the ability of forests and other ecosystems to help regulate Earth\u2019s climate.<\/p>\n The study is published in the Journal of Remote Sensing<\/a>.\u00a0<\/p>\n \u2014<\/p>\n Like what you read? 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