Long-term experimental drought combined with natural extremes accelerate vegetation shift in a Mediterranean holm oak forest

Chronic historical drought legacy exacerbates tree mortality and crown dieback during acute heatwave-compounded drought

“…This current study uses observational data to document chronic legacy effects during one of the best-documented tree mortality events that was triggered, in part, by a heatwave. Recently, Liu et al (2018) documented similar amplifying effects of chronic drought on tree mortality in a field experiment, where multiple heatwave-compounded droughts impacted their LTP exclusion experiment site. Collectively, these studies highlight the important interacting effects of chronic drought and acute heat events, both of which are expected to increase in Mediterranean and other climate regions in the coming decades (Andrys et al 2017).…”

“…While studies highlighting the interacting effects of drought and heat waves on forests are more recent, the importance of climate legacy, including the pattern of past drought, has been recognized as an important factor regulating tree response to future drought (Anderegg et al 2013, Anderegg et al 2015, Peltier et al 2016. For example, experiments from a groundwater dependent Mediterranean-type forest have shown that isohydric species experience increased crown defoliation and synergistic mortality effects during acute drought if they have experienced chronic drought (Barbeta et al 2015, Liu et al 2018. Retrospective dendrochronological studies have consistently detected legacy impacts from drought on subsequent growth and mortality patterns (Heres et al 2014, Macalady and Bugmann 2014, Berdanier Figure 5.…”

“…Several studies have shown the combined effects of chronic and acute drought conditions on trees, and that the response of trees to an acute drought, is partially determined by historical precipitation (Heres et al 2014, Macalady and Bugmann 2014, Barbeta et al 2015, Young et al 2017, Liu et al 2018. It remains unclear, however, whether the magnitude of chronic drought affects tree and forest responses to acute heatwave and drought events.…”

Stem Mortality and Forest Dieback in a 20-Years Experimental Drought in a Mediterranean Holm Oak Forest

“…General circulation models thus project an average decrease in soil moisture of 15% in the next 50 years and a return period of extreme droughts 10-fold shorter than in the twentieth century in Mediterranean regions (Bates et al, 2008). Forest ecosystems in these Mediterranean areas, seasonally exposed to water stress, may be particularly vulnerable to even slight increases in water deficits, which can induce reductions in tree growth (Barbeta et al, 2013;Liu et al, 2015Liu et al, , 2018 and crown conditions (Carnicer et al, 2011;Galiano et al, 2012) and increases in tree mortality (Breshears et al, 2005;Allen et al, 2010;Peng et al, 2011;Williams et al, 2012). Precipitation anomalies have also been strongly correlated with the subsequent occurrence of hot extremes in most areas of the world (Mueller and Seneviratne, 2012), and an increased frequency of heat waves is expected in Mediterranean regions, coinciding with summer drought and higher evapotranspiration and thus a lower availability of soil water (Fischer and Schar, 2010).…”

“…Several species of tall shrubs associated with holm oak (Quercus ilex L.) forests have lower growth rates but are more resistant to drought conditions than the holm oak, the current dominant species in these forests Ogaya and Peñuelas, 2003). Higher mortality rates and lower seed production and seedling survival of Q. ilex under future drier conditions could drive a progressive substitution of dominant oak species by the co-occurring species more resistant to low water availability (Lloret et al, 2004;Ogaya and Peñuelas, 2007;Liu et al, 2015Liu et al, , 2018. The selective thinning of stems is an ancient practice of forest management widely used to enhance the growth and development of the remaining stems, increasing the availability of resources per stem when stem density decreases.…”