COS 195-2 - Size-dependent mortality and species specific responses to varying degrees of fire severity in a California mixed hardwood system following the Tubbs Fire (Sonoma Co., October 2017)

Wednesday, August 17, 2022

3:45 PM – 4:00 PM EDT

Location: 516D

Presenting Author(s)

MK

Melina V. Kozanitas, PhD

Post Doctoral Researcher
University of California, Berkeley, California, United States

Coauthor(s)

MO

Meagan F. Oldfather, PhD

Post Doctoral Researcher
University of Colorado Boulder, Colorado, United States
PP

Prahlada D. Papper, PhD

University of California, Berkeley, California, United States
DA

David D. Ackerly

Dean of Rausser College of Natural Resources
University of California, Berkeley, California, United States

Background/Question/Methods

Recent years have shown increases in both frequency and intensity of wildfires in western North America as a result of accelerating climate change. In contrast to temperate coniferous forests, many species in mixed hardwood systems resprout prolifically post-fire. This leads to different survival outcomes for hardwoods with total canopy loss but vigorous basal resprouts and topkilled conifers. Resprouting combined with diverse regeneration and drought tolerance strategies create unique post-fire trajectories that require further research.

In Sonoma County, California the Tubbs Fire (Oct. 2017) burned ~13,351ha ( >25% high, >35% medium severity), including large areas of oak woodland and mixed hardwood forest. In 2018 we conducted a complete demographic resurvey of a pre-existing plot network (est. 2013) at the Pepperwood Preserve (38.57°N,-122.68°W), consisting of 54 plots (20x20m) stratified across varying topographic gradients and evenly represented by evergreen and deciduous oak sites (Oldfather 2016). All trees (dbh >1cm) and saplings ( >50cm tall, < 1cm dbh) were tagged and measured, and all seedlings (< 10cm) and juveniles (11-50cm) were tallied by species, both pre- and post-fire. We examined post-fire recovery in >30 species to quantify mortality, topkill (with resprouting), and crown survival in relation to pre-fire size (dbh), fire severity, and species.

Results/Conclusions

Fire severity was assessed using satellite-based remote sensing metrics and correlated with post-fire assessments of canopy damage in the field. Best-fit logistic models for post-fire status (dead, topkill, live-crown) included size, fire severity, species identification, and ‘sizeXseverity’ or ‘sizeXspecies’ interactions. Topkill rates are unimodal in relation to size, with higher mortality at small sizes and higher crown survival for larger trees. In both high and moderate severity fire California bay, madrone, coast live oak and black oak exhibited high levels of topkill combined with basal resprouting. Douglas-fir, which lacks the ability to resprout, exhibited the highest mortality, particularly saplings in high severity. D50, the critical size to achieve 50% survival, ranged from < 1cm to >15cm across species, on average, and decreased with fire severity, with the highest values for thinner bark species.

These results suggest that stands dominated by resprouting hardwoods are on a trajectory to return to pre-fire conditions, more so than Douglas-fir dominated sites, where there is potential for type conversion or invasion as understory species resprout-an important implication with regard to Douglas-fir encroachment. Quantification of post-fire responses, coupled with knowledge of drought-tolerance, will help project the impacts of future climate change and wildfire on these systems.