How did forest trees respond when extreme drought followed years of drier conditions?

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Vegetation in water-limited ecosystems depends heavily on deep water reserves to endure dry periods, and most such ecosystems have shallow soils over deep groundwater reserves. To predict how seasonally dry ecosystems will respond to future climate, researchers need to understand how species-specific root systems function and adapt, and how water-source use changes or differs under more frequent or intense droughts.

The researchers used stable isotopes to investigate seasonal water uptake in a sclerophyll forest on sloped terrain with shallow soils. They assessed the effects of a 12-year experimental drought and the added impact of an extreme natural drought that caused widespread tree mortality and crown defoliation. The dominant species—Quercus ilex, Arbutus unedo and Phillyrea latifolia—all had dimorphic root systems that allowed them to access different water sources across space and time.

The plants drew water mainly from soil during cold and wet seasons, but increased their groundwater use during summer drought. Under the long-term experimental drought, plants shifted their wet-season uptake toward deeper soil layers, 10–35 cm deep, and reduced their summer groundwater uptake. This indicated flexibility in the functional distribution of fine roots that lessened the experimental drought's effect over the long term.

An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, resulting in greater crown defoliation in drought-affected plants. The study suggests that extreme droughts aggravate the moderate but persistent drier conditions simulated by the experiment and may deplete water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.