Abstract
Extreme drought events and plant invasions are major drivers of global change that can critically affect ecosystem functioning and alter ecosystem-atmosphere exchange. Invaders are expanding worldwide and extreme drought events are projected to increase in frequency and intensity. However, very little is known on how these drivers may interact to affect the functioning and resilience of ecosystems to extreme events. Using a manipulative shrub removal experiment and the co-occurrence of an extreme drought event (2011/2012) in a Mediterranean woodland, we show that native shrub invasion and extreme drought synergistically reduced ecosystem transpiration and the resilience of key-stone oak tree species. Ecosystem transpiration was dominated by the water use of the invasive shrub Cistus ladanifer, which further increased after the extreme drought event. Meanwhile, the transpiration of key-stone tree species decreased, indicating a competitive advantage in favour of the invader. Our results suggest that in Mediterranean-type climates the invasion of water spending species and projected recurrent extreme drought events may synergistically cause critical drought tolerance thresholds of key-stone tree species to be surpassed, corroborating observed higher tree mortality in the invaded ecosystems. Ultimately, this may shift seasonally water limited ecosystems into less desirable alternative states dominated by water spending invasive shrubs.
Highlights
Encroachment) or exotic woody plants in water-limited ecosystems can disturb the ecosystem water balance[11,14,15] with potential implications for the hydrological cycle in these areas[16]
We present the first study showing the synergistic effects of native shrub invasion and an extreme drought event
The winter precipitation (December to February) was as low as 12.4% of long-term mean over the southwest Iberian Peninsula (Fig. 1a)
Summary
We established six 25 × 2 5 m randomly selected stands paired by 3 sites in a Mediterranean cork-oak (Q. suber) savannah-type ecosystem, invaded by the native shrub C. ladanifer (Supplementary Table S1). This shrub species forms dense and extended mono-specific stands. Sap flux density was measured continuously from 25th of February 2011 to 30th September 2013, in uninvaded and invaded stands, in 4 randomly selected trees and 4 shrubs per stand. Transpiration of trees in invaded stands during pre-drought year was used as baseline condition for calculating resilience. Differences in resilience, resistance and recovery between trees in invaded and uninvaded stands were evaluated by a non-parametric Mann-Whitney Test. Full Methods are available in the online version of the paper
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