The drought‒dieback‒death conundrum in trees and forests

Abstract

Background: Climate warming is amplifying and exacerbating drought stress worldwide. Long-term trends of increasing evaporative demand and decreasing soil moisture availability occur superimposed on severe spells of drought. These rare, extreme droughts have triggered episodes of forest dieback that have led to reduced productivity and rising mortality rates, usually at small scales (dieback hotspots), but affecting biomes worldwide. Aims: This review summarizes and discusses the drivers, patterns and mechanisms of forest dieback caused by drought. Methods: I review studies on forest dieback and tree death linked to dry spells with a focus on tools to forecast dieback. Results: Several mechanisms have been described as physiological drivers of dieback, including hydraulic failure and carbon starvation, however hydraulics-based models have shown little predictive power of dieback and mortality. Field proxies of tree vigour, including changes in canopy defoliation and water content, combined with surrogates of tree functioning (tree-ring growth, wood anatomy, tree-ring δ 13 C or δ 18 O composition) may improve predictions of forest dieback or at least render early-warning signals of impending tree death. Conclusions: Drought-induced dieback and mortality are concerning phenomena which lack forecasting tools with sufficient predictive power. Surrogates of tree vigour, growth and functioning should be used to build more accurate models of tree death in response to extreme climate events linked to drought. Here, I argue for combining and comparing those surrogates to better forecast forest dieback.

Testing the limits of plant drought stress and subsequent recovery in four provenances of a widely distributed subtropical tree species

“…Besides, the vitality and functionality of meristematic cells (Mantova, Herbette et al, 2021; Mantova, Menezes‐Silva, et al, 2019) should also be considered to help identify the point of tree death. Fourth, drought‐induced tree mortality is a complex process and linked with many drivers, where hydraulic failure in the xylem is one of them (Camarero, 2021). Additionally, failure in soil–root water interactions and hydraulics may be a critical driver of tree death (Körner, 2019), and it deserves further investigation as an important step forward in mechanistic understanding of drought‐induced tree mortality.…”

Satellite data track spatial and temporal declines in European beech forest canopy characteristics associated with intense drought events in the Rhön Biosphere Reserve, central Germany

“…As such, the cloud presence, combined with long revisit time, likely led to misrepresentation of the full monthly NDVI response to the 2018 drought event (Philipp et al 2021). Consequently, satellite datasets with a shorter revisit time are essential to capture the canopy response to intense drought events and should be prioritized in the future to meet the requirement for early-warning indicators of forest dieback and mortality (Camarero 2021).…”

“…While the potential benefit of higher spatial resolution satellite imagery has been recognized, there are very few direct comparisions between sensors with different spatial and temporal characteristics (Buras et al 2020). Therefore, there is a lack of clarity on how the different attributes of these sensors will impact our ability to identify temporal and spatial impacts of intense drought events on forest canopies, hindering the development of early-warning indicators of forest dieback (Camarero 2021).…”

Post-drought conditions and hydraulic dysfunction determine tree resilience and mortality across Mediterranean Aleppo pine (Pinus halepensis) populations after an extreme drought event

“…Diminished tree growth and leaf development have been commonly considered adaptative mechanisms to water shortage as they would help trees to improve the xylem's capacity to provide leaves with water (Maherali et al 2002, Rosas et al 2013). However, growth reductions usually occur as a drought legacy effect (Gazol et al 2018, Rodriguez-Vallejo et al 2021) and can impair drought recovery in conifers (Brodribb et al, 2010), which may lead to tree mortality in forthcoming years (Camarero 2021).…”

“…The tolerance limits for species survival and legacy effects after drought events are outstanding factors that trigger changes in forest composition, species diversity, hydrological function and potential carbon storage (Brodribb et al 2020, Tramblay et al 2020. Thus identifying the variables associated with mortality and their corresponding thresholds should increase our capacity to forecast drought-induced mortality (Camarero 2021, Rowland et al 2021.…”