Compound climate extremes driving recent sub-continental tree mortality in northern Australia have no precedent in recent centuries

Abstract

Compound climate extremes (CCEs) can have significant and persistent environmental impacts on ecosystems. However, knowledge of the occurrence of CCEs beyond the past ~ 50 years, and hence their ecological impacts, is limited. Here, we place the widespread 2015–16 mangrove dieback and the more recent 2020 inland native forest dieback events in northern Australia into a longer historical context using locally relevant palaeoclimate records. Over recent centuries, multiple occurrences of analogous antecedent and coincident climate conditions associated with the mangrove dieback event were identified in this compilation. However, rising sea level—a key antecedent condition—over the three decades prior to the mangrove dieback is unprecedented in the past 220 years. Similarly, dieback in inland forests and savannas was associated with a multi-decadal wetting trend followed by the longest and most intense drought conditions of the past 250 years, coupled with rising temperatures. While many ecological communities may have experienced CCEs in past centuries, the addition of new environmental stressors associated with varying aspects of global change may exceed their thresholds of resilience. Palaeoclimate compilations provide the much-needed longer term context to better assess frequency and changes in some types of CCEs and their environmental impacts.

Processes and mechanisms of coastal woody‐plant mortality

“…Observations of mangrove mortality have increased in recent decades, with some individual mortality events exceeding 1000 ha along coastlines (Duke et al, 2017). These mortality events have corresponded with periods of severe heat, leading to the hypothesis that rising temperature exacerbates impacts associated with elevated hypoxia and salinity (Allen et al, 2021; Lovelock et al, 2017). This is consistent with observations and predictions from upland forests, which suggest that rising temperature, and in particular rising vapor pressure deficit (VPD), is driving increasing tree mortality (McDowell et al, 2020; Williams et al, 2013).…”

The lunar nodal cycle controls mangrove canopy cover on the Australian continent

“…The largest mangrove dieback event recorded globally, involving more than 1000 km of mangrove shoreline in the Gulf of Carpentaria ( 11 ) in 2015/2016, has been linked to a weakening of the monsoon wet season, lower than average rainfall, and a temporary decrease in mean sea level ( 10 ). These conditions were driven by an extreme negative phase of the El Niño Southern Oscillation (ENSO) coincident with a positive phase of the Indian Ocean Dipole (IOD) ( 11 , 12 ). However, dieback in the adjacent Arnhem and Great Barrier Reef bioregions was far less severe, despite the similar imposition of the ENSO and IOD events on sea level and climate.…”

Multi-scale quantification of anthropogenic, fire, and drought-associated forest disturbances across the continental U.S., 2000–2014

“…Reports of tree mortality due to climatic extremes such as drought are increasing globally (Van Mantgem et al, 2009;Ganey and Vojta, 2011;Peng et al, 2011;Millar et al, 2012;Allen et al, 2015Allen et al, , 2021Bendixsen et al, 2015;Assal et al, 2016;Gazol and Camarero, 2022). These rising extreme events, along with fires, insects, and timber harvesting, can substantially threaten forest health.…”

“…Adams et al (2017) found that widely observed hydraulic failure tends to be one of the most important underlying physiological mechanisms that causes tree death under drought stress. Moreover, there have been many studies of forest declines and tree loss driven by insect outbreaks (e.g., mountain pine beetles) (Meddens and Hicke, 2014;Hicke et al, 2020), fire (Hicke et al, 2016), drought (Klos et al, 2009;Ganey and Vojta, 2011;Yang et al, 2021), and a combination of multiple causal agents (Allen et al, 2021;Knapp et al, 2021;Yang et al, 2021).…”