The 2020 windstorm forest damage in the Colorado Rocky Mts. - satellite-based mapping automation and environmental insights

Wind is among one of the most frequent drivers of forest disturbance around the world. Wind disturbance (blowdown, windthrow) results from particular meteorological conditions, where wind gust speed is a key factor. Blowdown is conditioned also by wind direction together with local topography, and influence the soil disturbance due to tree uprooting.

We mapped and analyzed the large-scale 2020 blowdown in the spruce-fir subalpine forest on the western slope of the Front Range, Rocky Mountains (Colorado, US). The area of interest (AOI) is a 9 x 29 km rectangle (39.80° N, 105.77° W and 40.06° N, 105.67° W) located south of the Rocky Mountain National Park and north of the Berthoud Pass. The mapping focused on developing and automating the workflow based on Sentinel 2 data and Change Vector Analysis (CVA) and comparing its output with the Global Forest Change (GFC) data. The CVA mapping is based on 1) the difference image computed using post- and pre-event images, and 2) the parameters calculated using two bands of the difference image: magnitude (mgt) representing the amount of change, and direction (drct) referring to the type of change. To create the CVA output, we used bands 11 and 12, together with 40° < drct < 47° and mgt > 0.1. Both CVA output and GFC data have a true positive rate (TPR) of 66-67%, with a false positive rate (FPR) of 0.9% and 3%, respectively. The CVA can be adjusted to achieve TPR up to 75.5%, of which FPR was 5.8%. Our approach is based on an unsupervised method, and open-source data, and is fully automated using R. Using CVA, the blowdown area was estimated to 1379.7 ha. The comparison between GFC data and CVA output revealed the higher efficiency of CVA for small patches with intensive damage. GFC data were better for indicating the location of patches with lower damage intensity.

We also aimed to capture different environmental insights related to the meteorological conditions causing the blowdown, soil disturbance patterns, and the impact of topography. Large-scale blowdowns are infrequent in the Rockies and are usually associated with the occurrence of unusual meteorological conditions. The blowdown was caused by strong easterly winds (gusts of 30 m•s⁻¹) blowing on September 7th - 9th, 2020, associated with the passage of a cold front causing exceptionally early late-summer cooling. The blowdown patches distribution generally followed the run of the valleys and ridges (SE-NW), with large patches in southern and central parts, and smaller ones in the northern part. The blowdown caused soil disturbance, with root plate volumes of 0.1 – 0.8 m³. The bearings of uprooted tree stems followed the direction of the main wind currents reported in the climate time series. Our approach can be valuable for research on blowdown mapping and triggering factors, GFC data assessment, soil disturbance, and interplays with relief.

The study has been supported by the Polish National Science Centre (project no. 2019/35/O/ST10/00032) and by the Polish National Agency for Academic Exchange (agreement no. PPN/STA/2021/1/00081/U/00001).