Carbon consequences of drought differ in forests that resprout

Abstract

Prolonged drought and intense heat‐related events trigger sudden forest die‐off events and have now been reported from all forested continents. Such die‐offs are concerning given that drought and heatwave events are forecast to increase in severity and duration as climate change progresses. Quantifying consequences to carbon dynamics and storage from die‐off events are critical for determining the current and future mitigation potential of forests. We took stand measurements five times over 2+ years from affected and unaffected plots across the Northern Jarrah Forest, southwestern Australia, following an acute drought/heatwave in 2011. We found a significant loss of live standing carbon (49.3 t ha−1), and subsequently a significant increase in the dead standing carbon pool by 6 months post‐die‐off. Of the persisting live trees, 38% experienced partial mortality contributing to the rapid regrowth and replenishment (82%–88%) of labile carbon pools (foliage, twigs, and branches) within 26 months. Such regrowth was not substantial in terms of net carbon changes within the timeframe of the study but does reflect the resprouting resilience of this forest type. Dead carbon generated by the die‐off may persist for centuries given low fragmentation and decay rates resulting in low biogenic emission rates relative to other forest types. However, future fire may threaten persistence of both dead and live pools via combustion and mortality of live tissue and impaired regrowth capacity. Resprouting forests are commonly regarded as resilient systems, however, a changing climate could see vulnerable portions of forests become carbon sources rather than carbon sinks.

Patterns of post‐drought recovery are strongly influenced by drought duration, frequency, post‐drought wetness, and bioclimatic setting

“…The modeling experiments proposed that both the temporal change in stem size for surviving species and the replacement of vulnerable species with more drought‐tolerant ones could help increase ecosystem resilience to severe droughts, especially for ecosystems dominated by croplands with some patch grasslands/savanna ecosystems. Current field investigation also showed that drought‐induced partial mortality of living trees could stimulate the regrowth of foliage, twigs, and branches for forests that resprout (Walden et al, 2019), leading to more rapid recovery than non‐resprouters (Zeppel et al, 2015). Thus, ecosystem resilience depends on both drought severity and species composition (Gazol et al, 2018), while the speed of recovery is associated with the efficiency of repair, regrowth, and the severity of functional damage (Ruehr et al, 2019).…”

“…Current field investigation also showed that drought-induced partial mortality of living trees could stimulate the regrowth of foliage, twigs, and branches for forests that resprout (Walden et al, 2019), leading to more rapid recovery than non-resprouters (Zeppel et al, 2015). Thus, ecosystem resilience depends on both drought severity and species composition (Gazol et al, 2018), while the speed of recovery is associated with the efficiency of repair, regrowth, and the severity of functional damage (Ruehr et al, 2019).…”

Forest regeneration within Earth system models: current process representations and ways forward

“…Many landscape‐scale disturbances are predicted to become more frequent and severe in the coming decades (Turner, 2010; Trumbore et al ., 2015; Seidl et al ., 2017), likely driving changes in the relative abundance of seed and nonseed‐based regeneration strategies. Epicormic or apical resprouting by mature trees also limits the short‐term loss of ecosystem C relative to what occurs with mature tree mortality followed by recruitment from seed (Zeppel et al ., 2015; Walden et al ., 2019).…”

Underappreciated plant vulnerabilities to heat waves

“…Heat waves can also have secondary or indirect effects on ecosystems (Matusick et al, 2018; Nowicki et al, 2019). For example, a heat‐wave‐and‐drought‐induced forest die‐off in southwestern Australia triggered significant loss of live standing biomass (49.3 t carbon ha −1 ; Walden et al, 2019). That forest die‐off event resulted in adult mortality and altered regeneration of a key midstorey species, Banksia grandis , indirectly affecting an important food source of an endangered cockatoo (Steel et al, 2019).…”