Predicting post-fire Douglas-fir beetle attacks and tree mortality in the Northern Rocky Mountains

PRINCIPAL INVESTIGATOR

Sharon Hood, Forester

Co-Investigators

Barbara Bentz, USDA Forest Service, Rocky Mountain Research Station, Logan, UT, bbentz@fs.fed.us; (435)755-3577

INTRODUCTION

Survival of conifers following wildfire depends on the type and degree of injuries sustained and the post-fire environment, which includes weather, stand attributes, and insect and disease population dynamics. Many species of wood-boring insects are attracted to fire-injured trees and may contribute to subsequent tree death and deterioration. Of particular concern to forest managers are the primary bark beetle species (Dendroctonus) which have the capacity to expand to outbreak levels in surrounding stands following buildup in fire weakened trees.

An accurate estimate of tree survival following fire injury is an important aspect of post-fire forest resource management. Research has shown that tree survival is species-specific and relative to root, stem, and crown damage. Although it is known that bark beetles can contribute to delayed tree mortality following wildfire, quantitative information necessary for more accurate predictions is unavailable, and little is known about bark beetle preference, brood production, and survival in fire-injured trees.

GOALS AND OBJECTIVES

1. Develop a model for predicting the probability of Douglas-fir beetle attack as a function of individual tree fire injury and stand characteristics
2. Develop a model for predicting the probability of Douglas-fir mortality within four years following fire that includes fire related injuries and the probability of attack by the Douglas-fir beetle.

METHODOLOGY

Three late summer wildfires were selected for the study: the Mussigbrod fire on the Beaverhead-Deerlodge National Forest in southwestern Montana, the Moose Fire on the Flathead National Forest and Glacier National Park in northwestern Montana, and the Green Knoll Fire on the Bridger-Teton National Forest in western Wyoming. We established randomly located, permanent plots within each fire boundary one year post-fire by selecting areas that burned under mixed-severity conditions and were dominated by Douglas-fir larger than 30 cm DBH. At each plot, all trees 12.7 cm DBH and believed alive before the fire were tagged. We recorded tree species, status (live or dead), DBH, crown scorch, cambium kill, bark char, ground char, and Douglas-fir beetle attacks for each tagged tree. We assessed each tree annually for additional mortality and bark beetle attacks for 4 years post-fire.

KEY RESULTS

Four variables and an interaction effect were significant in predicting Douglas-fir mortality 4 years post-fire: 1) percent crown volume scorched (scorch), 2) cambium kill rating (CKR), 3) diameter at breast height measured in inches (DBH), 4) Douglas-fir beetle attack level (attacked during first 4 years = 1, unattacked during first 4 years = 0), and 5) the interaction of DBH and attack level.

Four variables and an interaction effect were significant in predicting the probability of Douglas-fir beetle attack on fire-injured trees within 4 years post- fire: 1) percent crown volume scorched (scorch), 2) cambium kill rating (CKR), 3) DBH (measured in inches), 4) stand density index of Douglas-fir (SDIDF), and 5) interaction of crown scorch, CKR and SDIDF.

Unburned, light, and deep bark char categories could be used relatively accurately to predict cambium status. However, large errors could result if moderate bark char rating is used to determine cambium status. This is likely due to Douglas-fir having thick bark, where a moderate bark char rating reveals little in how much heating the cambium received.

The inclusion of beetle attacks in the delayed tree mortality model provides managers additional information for post-fire planning in Douglas-fir forests of the Northern Rocky Mountains. The field protocols we describe will also be useful for standardizing field-collected fire injury measurements most important for predicting post-fire tree mortality and Douglas-fir beetle attack.

FUNDING ORGANIZATIONS

This project was funded in part by the Forest Service, U.S. Department of Agriculture, Forest Health ProtectionRegion 1, the Beaverhead-Deerlodge National Forest (agreement number 0102-01-010), the Special Technology Development Program (R4-2004-02), and the Joint Fire Science Program (05-2-1-105).

PUBLICATIONS AND PRODUCTS

The mortality and beetle attack modeling results are presented in an article in the Canadian Journal of Forest Research:

Hood, S. M. and Bentz, B. 2007. Predicting post-fire Douglas-fir beetle attacks and tree mortality in the Northern Rocky Mountains. Canadian Journal of Forest Research. 37:1058-1069.

We also developed a guide to predicting post-fire Douglas-fir mortality and Douglas-fir beetle attacks. It is a two-part GTR. The main GTR describes the variables significant in predicting post-fire Douglas-fir mortality and bark beetle attacks and includes instructions and examples for using the models in post-wildfire management and prescribed burn planning. The supplement includes photographs and detailed methodology for measuring Douglas-fir beetle attacks and Douglas-fir fire-injury characteristics. It is small and printed on water-proof paper for ease of carrying in the field.

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.

Preliminary poster of RMRS-GTR-199.

A synopsis of the results was presented at a post-fire mortality workshop in 2006.