Landscape-scale characteristics of forest tornado damage in mountainous terrain

Context

Landscape patterns created by natural disturbance such as windstorms can affect forest regeneration, carbon cycling, and other ecological processes.

Objectives

We develop a method for remotely measuring tornado damage severity and describe landscape-scale patterns of tornado damage. We examine the extent and distribution of damage severity and gaps created by tornadoes, and examine how topographic variation can influence tornado damage severity.

Methods

Focusing on two April 2011 tornadoes that struck the Chattahoochee National Forest (CNF) in Georgia and the Great Smoky Mountains (GSM) in Tennessee, we used supervised classification of aerial photographs to map damage severity. We report the extent and distribution of damage severity from each track and characterize patterns of damage using FragStats. Using topographic overlays, we test hypotheses regarding how physiographic features such as valleys and ridges affect tornado damage severity.

Results

Tornado damage severity estimates were significantly correlated with ground-truth measurements. The 64-km CNF track damaged 1712 ha (>25 % severity), while the 26-km GSM track damaged 1407 ha. Tornado damage severity was extremely variable and frequency of gap sizes drastically decreased with size, with many small gaps and few very large gaps, consistent with other types of wind damage. Damage severity declined as tornadoes ascended ridges and increased as they descended ridges. This effect was more consistent on shallow slopes relative to steeper slopes.

Conclusions

This study outlines an objective methodology for remotely characterizing tornado damage severity. The results from this study fill an important gap in ecological understanding of the spatial components of the forest tornado disturbance regime.