A new study shows that during drought, it’s not how hot or how dry it is that determines gas emissions from plants – but how quickly conditions change. This discovery reshapes our understanding of the relationship between drought, vegetation, and air pollution.

New research led by Dr. Eran Tas from the Institute of Environmental Sciences in the Faculty of Agriculture, Food and Environment at Hebrew University in collaboration with Prof. Alex Genther from the University of California, Irvine and Prof. Pawel Misztal from the University of Texas at Austin, reveals a striking phenomenon: when the weather shifts rapidly – for example, a sharp increase in humidity or a sudden drop in temperature – vegetation responds immediately by changing the rate at which it emits naturally occurring biogenic volatile organic compounds (BVOCs) into the air.

On their own, these compounds are not pollutants, but they take part in forming tropospheric ozone and fine particles – both of which affect air quality and climate change.

What’s new in this research?
Until now, scientists assumed that humidity or temperature levels were what determined the intensity of emissions from plants.

The new study, conducted in the Beit Keshet Forest in northern Israel, shows for the first time that the rate of change in relative humidity (ΔRH/Δt) is the most reliable indicator for the BVOCs emission rate.

In simple terms: it’s not how hot or dry it is, but the short-term trend — how quickly those conditions are changing.

A sharp rise in humidity can trigger an increased release of water-soluble organic compounds such as formic and acetic acid.

According to Dr. Tas: “The forest responds in real time. Drought doesn’t just dry out the soil, it changes the chemistry of the air. Our findings offer a more accurate way to simulate the effect of vegetation on the feedback between the metrological conditions and air pollution, which becomes increasingly important under climate change, especially in dry regions like the Middle East.”

Broader implications

These insights will help improve global climate and air quality prediction models, which until now have not adequately accounted for the impact of rapid weather fluctuations.

The study is based on precise, high-frequency measurements of gas concentrations above vegetation using advanced instrumentation (PTR-ToF-MS), combined with meteorological analysis using climate models.