Sequence of plant responses to droughts of different timescales: lessons from holm oak (Quercus ilex) forests

Abstract

The functional traits of plants in regions of the world with a Mediterranean climate have been shaped to tolerate periods of water deficit. These species are adapted to summer droughts but may not be able to cope with future increases in drought intensity, duration, and/or frequency. Here, we review the mechanisms and traits of drought resistance and recovery of the well-studied holm oak (Quercus ilex), which we propose as a model species for Mediterranean-type ecosystems. Our aim was to understand the differences and links between the responses of Q. ilex to summer droughts, extreme droughts, and long-term drought experiments. A main goal was to provide an integral picture of drought responses across organisational and temporal scales for identifying the most relevant processes that are likely to contribute to determining the future of Mediterranean vegetation. Evidence from long-term drought experiments showed that acclimation processes from the molecular (e.g. epigenetic changes) to the ecosystem level (e.g. reductions in stand density) mitigate the effects of drought. Changes in leaf morphology and hydraulics, leaf-to-shoot allometry, and root functioning are among the key mechanisms for overcoming increasing drought. The duration of drought determines its severity in terms of canopy loss and stem mortality. Although Q. ilex can vigorously resprout after such episodes, its resilience may be subsequently reduced. In the future, higher frequency of return of extreme droughts will challenge thus the capacity of these forests to recover. The insights provided by this review of the complex interplay of processes that determine the response of trees to droughts of different duration, intensity, and frequency will also help us to understand the likely responses of other resprouting angiosperms in seasonally dry ecosystems that share similar functional traits with Q. ilex.

Fire‐induced deforestation in drought‐prone Mediterranean forests: drivers and unknowns from leaves to communities

“…This is because, for instance, oak decline is already occurring over a large proportion of the Mediterranean (Peguero‐Pina et al. , Barbeta and Peñuelas , Gómez‐Aparicio et al. ) and resprouting tree species are being displaced by seeder and resprouting shrub species under drought (Peñuelas et al.…”

“…Additionally, although the bulk of post-fire deforestation so far has occurred in pine forests, we cannot ignore deforestation in resprouting forests also under a warmer and drier future. This is because, for instance, oak decline is already occurring over a large proportion of the Mediterranean (Peguero-Pina et al 2015, Barbeta and Peñuelas 2016, G omez-Aparicio et al 2017) and resprouting tree species are being displaced by seeder and resprouting shrub species under drought (Peñuelas et al 2017). Consequently, fire may further promote forest-toshrub state transitions in oak or resprouter forests.…”

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

“…Climatological drought and warming events are categorized in a variety of ways based on their duration, frequency, and intensity, and these have varying effects on trees and forests (McDowell et al 2008, Barbeta and Peñuelas 2016). Chronic water stress commonly increases background mortality rates in tree populations (Pangle et al 2015, Berdanier andClark 2016), and incites tree decline (Bigler et al 2006), especially when opportunistic pests and pathogens are present (Hart et al 2014).…”

Widespread Crown Defoliation After a Drought and Heat Wave in the Forests of Tuscany (Central Italy) and Their Recovery—A Case Study From Summer 2017

“…Recent research underlines the role of non-structural carbohydrates in plant growth (Mason et al, 2014): after the loss of the shoot tip, sugars are rapidly redistributed over large distances between different parts and organs of the plant and accumulated in axillary buds within a timeframe compatible with the resumption of their activity. Causing the depletion of non-structural carbohydrates in plant tissues, however, recurrent drought episodes can dramatically reduce the resprouting ability and the resilience of the whole plant (Barbeta and Peñuelas, 2016). In turn, this is likely to result in a shift from forest to Mediterranean steppe if severe drought will persist in the long-term (Jacobsen and Pratt, 2018).…”