Sudden Oak Death as a novel mechanism of coarse woody debris dynamics in coast redwood forests

Background/Question/Methods

In coast redwood forests, coarse woody debris (CWD) are increased by the emerging forest disease Sudden Oak Death.  While P. ramorum is a generalist plant pathogen, it selectively kills tanoak (Lithocarpus densiflorus) with large tanoak trees killed significantly more rapidly than small trees. We expected rates of CWD recruitment to be highly dependent on pre-disease community structure, size distribution of tanoaks, and density of California bay laurel (Umbellularia californica). Tanoak and California bay laurel contribute to pathogen establishment and intensification by supporting the majority of P. ramorum sporulation, but bay laurel does not suffer mortaltiy due to infection. We conducted a census of course woody debris across 90 500m² meter plots in coast redwood forests of central California to document the mass, volume, and flux rate of CWD generated by Sudden Oak Death. We estimated the magnitude of changes caused by the disease, community influences on disease severity, and the period of increased CWD levels in disease impacted forests. We suggest that Sudden Oak Death is a novel disturbance in these forests; in contrast to fire (the dominant disturbance) the disease results in species specific mortality and selectively kills large trees.

Results/Conclusions

Sites impacted by Sudden Oak Death averaged 37.4 Mg ha^-1 of standing dead wood, an 817 fold increase compared to uninfected sites. Log mass averaged 10.2 Mg ha^-1, a 15.2 fold increase compared to pathogen free sites. Logistic regression models of tree fall suggest individuals with 20cm DBH remain standing for 10 years while individuals with DBH greater than 80cm may remain standing for 45 years. Stands with increasing densities of infected tanoak and bay laurel had more rapid rates of CWD recruitment compared to stands with lower densities of either sporulation supporting species. Tanoak snag and log decay rates were estimated by decay class density relationships and reconstructed mortality and tree-fall histories. Respective snag and log decay rates were 2.47% and 5.45% annually for the first five years following mortality and then declined to 2.36% and 1.91%, respectively, over the next 5 years. Together, these results show that Sudden Oak Death results in a large pulse of woody debris with relatively rapid decay rates in coast redwood forests. Total amounts of debris peaks within ten years of pathogen establishment but fire severity risk and carbon fluxes may remain elevated for the following 50 years.