Delayed tree mortality following fire in western conifers

PRINCIPAL INVESTIGATOR

Sharon Hood, Forester

Co-Investigators and Cooperators

Sheri Smith, Danny Cluck, Elizabeth Reinhardt, Kevin Ryan, Charles McHugh, Michael Landram

INTRODUCTION

Accurate prediction of post-fire tree mortality is critical for making sound land management decisions such as developing burning prescriptions and post-fire salvage marking guidelines. Numerous post-fire mortality models have been developed for western U.S. conifers. Ryan and Reinhardt (1988) developed the original logistic regression mortality model used in today’s U.S. fire behavior and effects models, making it perhaps the most widely used post-fire mortality model in the U.S. Ryan and Amman (1994) updated the original model to the form currently used in FFE-FVS, FOFEM, and BehavePlus.

The Ryan and Amman mortality model includes a bark thickness term based on species and diameter at breast height (DBH) and percent crown volume scorched, an easily and quickly determined fire-injury variable. While the Ryan and Amman model is now widely used as a silvicultural tool in the western U.S., it was developed from a relatively small sample of seven western coniferous species (n=2,356) and only from prescribed fires in the Pacific Northwest and Northern Rockies. The predictive accuracy of the model has not been assessed for fires outside the original study’s geographic area, for wildfires, or for other tree species except ponderosa pine.

Post-fire tree mortality models use a multitude of crown, bole, and root injury variables to predict mortality. This lack of standardized methods makes model comparison difficult and hard for managers to know what variables best predict mortality. Various methods have been developed to estimate cambium injury on the tree bole to avoid direct sampling. Ryan (1982) first developed categories of bark char severity to indicate stem injury resulting from fire. His codes have since been used as a surrogate for stem injury in post-fire tree mortality models. While these studies have found various measures of bark char to be a statistically significant variable to predict delayed tree mortality, it is unknown how well bark char actually relates to cambium injury for many species.

GOALS AND OBJECTIVES

We pooled data amassed from numerous post-fire tree mortality studies across the western U.S. in order to:

1. Assess the predictive accuracy of the tree mortality models currently used in FOFEM, BehavePlus, and FFE-FVS both at the stand and individual tree level,
2. Assess the relationship between bark char codes and cambium status (live/dead) to determine when it is appropriate to use bark char codes in place of direct sampling, and
3. Develop new 3-year post-fire mortality models to improve the predictive accuracy of FOFEM and BehavePlus. Models were developed for white fir, red fir, subalpine fir, incense cedar, western larch, lodgepole pine, whitebark pine, ponderosa pine, Jeffrey pine, sugar pine, Engelmann spruce, and Douglas-fir.

KEY RESULTS

Evaluation of FOFEM tree mortality model

For prescribed burn planning purposes, the model proved to be a useful and relatively accurate method for predicting stand level post-fire tree mortality. It correctly predicted overall mortality within ±20% of the observed mortality for the majority of species tested. These species were lodgepole pine, whitebark pine, Engelmann spruce, subalpine fir, white fir, ponderosa pine, Jeffrey pine, Douglas-fir, and sugar pine. However, correctly predicted mortality was quite variable when individual fires were examined and model accuracy may be lower for some fires, as indicated by the data. Red fir, incense cedar, and western larch stand level mortality was over-predicted. Western hemlock was the only species tested where stand level mortality was greatly under-predicted.

When using the Ryan and Amman model, managers can expect less mortality than the model predicts when burning in incense cedar, western larch, and red fir forests. Managers can also expect higher mortality than the model predicts when planning prescribed burns in stands of western hemlock if tree boles are charred.

The Ryan and Amman model was less accurate for predicting individual tree mortality. Individual tree mortality predictions are used to develop post-fire salvage marking guidelines. For this purpose, other species specific mortality models developed from individual geographic areas may be more accurate. Species specific models often include other variables, such as stem injury and insect attacks, that can increase predictive accuracy. The species we tested that provided excellent discrimination (ROC≥0.8) were subalpine fir, incense cedar, and yellow pine. The Ryan and Amman model was especially poor at classifying Engelmann spruce, red fir, and very large diameter yellow pine.

Evaluation of Bark Char Codes to Predict Cambium Status

Bark char codes were relatively accurate for predicting cambium status after fire for many species. However, moderate bark char was not clearly associated with either live or dead cambium for thicker bark species. For these species (white fir, incense cedar, yellow pine, Douglas-fir, and sugar pine), cambium should be sampled directly to determine injury when bark char is moderate.
Results from this study show that tree injury from direct sampling of the cambium does not contribute to additional post-fire ponderosa pine tree mortality. We expect that other tree species would have a similar response. This research indicates that direct cambium sampling is a better variable than bark char codes for use in post-fire tree mortality modeling for species with thick bark and that direct sampling can be performed without causing additional tree mortality.

Tree Mortality Modeling

The species specific models developed for this project offer improved prediction over the current mortality model in FOFEM. Two sets of models were created, one for use in pre-fire planning where only crown injury and DBH were potential variables, and a second, optimal model for use in post-fire planning that used all significant variables. Crown scorch was the most important variable in predicting mortality. CKR and beetle attacks also were consistently significant in the models. However, tree size was not significant in predicting mortality for most species.

For most species, the optimal post-fire model increased accuracy by approximately 2-4%. This is likely not enough to justify the extra time needed to assess these additional variables. Assessing cambium injury for Engelmann spruce, whitebark pine, and lodgepole pine however, greatly increases model accuracy. This is most likely because these three species all have very thin bark and fire around the tree bases will kill most trees even with little to no crown scorch.

Management Implications

• When using the Ryan and Amman model (FOFEM prior to version 5.7), managers can expect less mortality than the model predicts when burning in incense cedar, western larch, and red fir forests.
• When using the Ryan and Amman model (FOFEM prior to version 5.7), managers can expect higher mortality than the model predicts when planning prescribed burns in stands of western hemlock if tree boles are charred.
• Moderate bark char was not clearly associated with either live or dead cambium for thicker bark species (white fir, incense cedar, ponderosa pine, Jeffrey pine, Douglas-fir, and sugar pine). Cambium should be sampled directly to determine injury when bark char is moderate for these species.
• Tree injury from direct sampling of the cambium does not contribute to additional post-fire ponderosa pine tree mortality.
• FOFEM 5.7 offers improved accuracy in predicting 3-year post-fire tree mortality for white fir, subalpine fir, red fir, incense cedar, western larch, lodgepole pine, whitebark pine, Engelmann spruce, sugar pine, Douglas-fir, ponderosa pine, and Jeffrey pine.
• FOFEM 5.7 now allows users to directly enter crown scorch, cambium injury, and beetle attacks to improve model accuracy.

FUNDING ORGANIZATION

We acknowledge funding from the Joint Fire Science Program under program #05-2-1-105



PRODUCTS

The new models are in FOFEM 5.7 (http://www.fire.org/). They will be added to BehavePlus by Spring 2009.
Hood, S. M.; Cluck, D. R.; Smith, S. L.; Ryan, K. C. 2008. Using bark char codes to predict post-fire cambium mortality. Fire Ecology. 4: 57-73.

Hood, S. M. 2008. Delayed Tree Mortality following Fire in Western Conifers. JFSP Final Report 05-2-1-105, US Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula, MT. 35 p.

Hood, S. M.; McHugh, C.; Ryan, K. C.; Reinhardt, E.; Smith, S. L. 2007. Evaluation of a post-fire tree mortality model for western US conifers. International Journal of Wildland Fire. 16: 679-689.

Hood, S. M.; Bentz, B.; Gibson, K.; Ryan, K. C.; DeNitto, G. 2007. Assessing post-fire Douglas-fir mortality and Douglas-fir beetle attacks in the northern Rocky Mountains. Gen. Tech. Rep. RMRS-GTR-199, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fort Collins, CO. Includes Supplement.

Final Report

http://www.firescience.gov/projects/05-2-1-105/project/05-2-1-105_05-2-1-105_final_report.pdf