Vegetation can respond to drought through different mechanisms, including changes in the plants' structure and physiology.
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A new study led by Dr. Wantong Li and Dr. Rene Orth at the Max Planck Institute for Biogeochemistry, Germany, has now disentangled drought response pathways across the globe.
By analyzing satellite data they showed that the vegetation's physiology in many ecosystems has deviated from its structure under drought on a global scale. Their findings, recently published in Nature Communications, represent a significant leap forward in our understanding of how the Earth's ecosystems respond to water scarcity.
By focusing on severe droughts, the team successfully characterized physiological responses to water scarcity related to photosynthesis, evaporation and water content. They found that two plant functions are effected most.
First, the exchange rates of CO2 and water vapor regulated by the stomata, tiny fine-controlled openings on the leave surface, and second the efficiency of how solar radiation is used for photosynthesis or absorbed otherwise by the leaves.
"Plant physiologists have shown before that individual plant species respond to drought in ways that go beyond simple changes in structural appearance," explains Wantong Li. "Our study now confirmed this for the vegetation as a whole, on a global scale and for many species included."
The advantage of the new approach is several-fold.
First, including the global effects of drought stress on the biological functioning of the plants provides a better picture of the vegetation drought response than just looking at the structural or outer appearance. Also, the former approach likely underestimates the timing, the scale and the severity at which vegetation is actually affected by drought.
Secondly, the vegetation's physiology typically responds faster to stress than its structural appearance. Drought impacts may thus be recognized earlier. For example, while a brownish meadow may easily be recognized as suffering from drought, other plants, in particular trees, may look normal but in fact already experienced serious drought-induced physiological changes.
The results also showed that vegetation in semi-arid and arid regions were affected the most, indicating a strong relationship between background climate and physiological responses.
In wetter regions the scientists observed that vegetation is still green or even greener during drought events. Nevertheless, their functioning was reduced, leading to a clear discrepancy between functional and structural changes that can be detected during severe drought. ■