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The effects of drought and wildfire on forest structure and carbon storage in a resprouting eucalypt forest

Walden, Lewis L.ORCID: 0000-0001-9714-3603 (2020) The effects of drought and wildfire on forest structure and carbon storage in a resprouting eucalypt forest. PhD thesis, Murdoch University.

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The frequency and intensity of forest disturbances are projected to increase in many regions as climate changes, with an increased likelihood of multiple disturbance events occurring in short succession. The effects and importance of multiple disturbance events are gaining increasing attention but there is substantial uncertainty regarding the strength of interactions in relation to type, sequence, and relative timing. Forests play an integral role in climate change mitigation efforts, however, multiple disturbances could alter forest carbon dynamics, potentially reducing the storage capacity of impacted stands. Accurate accounting of carbon losses and transformations is thus imperative across forests globally, and particularly in non-conifer dominated types and with different responses such as resprouting.

This study investigated the effects of a global change-type drought (2010-11) and the Waroona Fire (2016), singularly on forest carbon dynamics in the regrowth Northern Jarrah (Eucalyptus marginata) Forest of southwestern Australia. This was followed by an investigation of the additive and interactive effects of these two disturbances on forest structure and recruitment. The affected forest stands were assessed using a plot-based forest inventory method combined with allometric equations and remote sensing metrics to determine carbon transformations and fire severity.

Following the drought die-off there was a significant loss of live standing carbon (49.3 t C ha-1), and a corresponding significant increase in the dead standing carbon pool six months after the drought. The Waroona Fire caused pyrogenic carbon emissions of between 10 and 21 t C ha-1, which corresponded to 5.5-12.2% of the mean pre-fire carbon storage, and the conversion of 59.2 – 72.8 t C ha-1 from the live to dead standing tree pool. This carbon, which comprises another 30-37% of the pre-fire carbon storage will be susceptible to loss over time both from decomposition and from future fires. Total immediate carbon emissions from the 360 km2 wildfire were estimated at 723,113 t C. When the disturbances were examined in combination there was no evidence of linked or compounding effects of these disturbances. Instead a strong effect of wildfire on live basal area (-20.7 ± 7.3 m2 ha-1) overwhelmed pre-existing drought impacts.

Eucalypt resprouting forests are particularly resilient, with focal species able to survive multiple stressors over a short time period. However, the newly regrowing burnt stands are vulnerable to future disturbance until they develop bark thick enough to withstand fire, and carbon stores to maintain resprouts following drought. The drought event preferentially removed large trees and the wildfire smaller trees from the live standing carbon pool. With a continually decreasing rainfall and projections of more frequent and severe fires in this area the trajectory of the regrowth in these stands is likely to be strongly tied to the future disturbance regime. Increases in disturbance frequency and intensity may therefore lessen the chances of these forests recovering to pre-fire structure and therefore carbon storage capacity.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Environmental and Conservation Sciences
Supervisor(s): Harper, Richard, Fontaine, Joe and Ruthrof, Katinka
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