Forest & Grassland Health

Alaska's Aerial Detection Survey

2020 Forest Damage Detection

In 2020, aerial surveys to detect active forest damage from insects, diseases, declines, and abiotic agents were grounded for the first time in decades due to the pandemic. In a typical year, our team aerially surveys around 20 million acres, or 15% of the forested area of the Alaska. An extensive ground survey approach in forests along roads and trails, along with remote-sensing techniques utilizing high-resolution satellite imagery, enabled our team to gather the best forest health information possible. We also created an Alaska Forest Health project in iNaturalist to solicit observations from citizen scientists. The remote-sensing methods and crowd-sourcing techniques developed to meet current challenges will undoubtedly enhance our forest health surveys into the future.

Combining Ground Surveys & Remote Sensing

We conducted ground surveys along roads and trails, mapping major damage at regular intervals. These ground surveys covered approximately 2.5 million acres. Our goal was to capture major damage observations, approximating what would be mapped during our annual aerial survey, thereby providing damage locations to hone our remote-sensing tools and techniques. As in recent years, we also recorded damage that is indecipherable from the air using the Survey123 app (displayed in the cumulative and interactive ground survey dashboard).
 

High-resolution satellite imagery used to facilitate forest damage detection in Alaska  in 2020.

High-resolution satellite imagery of spruce beetle damage on the Kenai Peninsula (left) and spruce beetle damage observed on the ground (right).

 

Based on locations with known forest damage, we evaluated damage signatures in high-resolution satellite imagery. This approach enabled us to map similar damage across broader and less accessible swaths of the landscape. High-resolution (< 1m) Worldview 2 and Worldview 3 satellite imagery captured June to September 2020 was requested through both Digital Globe and the USGS using their CRSSP Imagery Derived Requirements (CIDR) imagery request tool. Available imagery was mosaicked (overlaid and positioned) in ArcPro software to create basemaps, which were then imported into our standard aerial survey mapping software on mobile tablets.

Finally, surveyors systematically scanned 4.8 million forested acres of imagery for forest damage. Using the same methods as aerial survey, they circled damage areas, attributing them with the damage agent, plant host, and damage severity. Imagery quality varied. Overall, damage was more difficult to see in imagery compared to what can be seen from the plane at 1000-1500ft above the ground. Some agents that cause relatively homogenous color change to the tree canopy (e.g., spruce beetle and hemlock sawfly) were easier to pick up in the imagery compared to more subtle or scattered damage that can be mapped from a survey plane. Using both road and remote-sensing surveys, we mapped about 350,000 acres of damage across the 7.3 million acres surveyed, presented at the conclusion of this StoryMap. 

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Click images for larger versions.

Hemlock sawfly damage observed during aerial survey.

Spruce needle rust in western Alaska in 2019. Credit: Jason Moan.

Aerial view of birch leaf miner damage in 2017. Photo credit Jason Moan AK-DOF.
Aerial view of spruce beetle mortality in 2018.

Aerial view of hemlock sawfly defoliation damage.

Spruce needle rust from the air in western AK (Photo credit: Jason Moan) and from the ground near Juneau. 

Aerial view of birch leafminer defoliation damage in 2017 (Photo credit: Jason Moan).

Spruce killed by spruce beetle in Southcentral Alaska in 2018.

Aerial detection survey flightlines 2019.

Alaska Aerial Detection Survey Flightlines 2019.

Aerial survey view in Ernest Sound, Southeast Alaska. Isaac Davis aerially sketchmaps in SE Alaska. Camping on Upnuk Lake in Wood-Tikchik State Park in 2018. Hemlock sawfly defoliation of western hemlock on Etolin Island in 2018. Spruce beetle outbreak in the Mat-Su Valley seen during the 2017 aerial detection survey.

Aerial survey view of Ernest Sound in Southeast Alaska (2018).

Isaac Davis uses a tablet to map and record forest damage in Southeast Alaska.

Camping on Upnuk Lake in Wood-Tikchik State Park during the western survey (2018).

Hemlock sawfly defoliation of western hemlock on Etolin Island in 2018.

Spruce beetle outbreak in the Mat-Su Valley during the 2017 aerial detection survey.

About Alaska's Aerial Detection Survey

Prior to each annual aerial survey, we plan tentative survey dates and routes. These plans sometimes change to accomodate survey requests and damage observed during the survey, and also due to weather conditions and other constraints. Contact our team to request locations for survey or to report recent forest damage. Karen Hutten is the Aerial Survey Program Manager for Alaska (karen.hutten@usda.gov). Learn more about Alaska Forest Health on our homepage.

Aerial surveys are an effective and economical means of monitoring and mapping insect, disease and other forest disturbance at a coarse scale. In Alaska, Forest Health Protection (FHP) and the Alaska DNR Division of Forestry, monitor about 30 million acres of forest annually at a cost of less than a penny per acre. Much of the damage acreage referenced in annual Forest Health Conditions of Alaska reports is generated from aerial detection surveys, so it is important to understand how these data are collected and the data’s inherent strengths and weaknesses. While there are limitations, no other method is currently available to detect subtle differences in vegetation damage signatures within a narrow temporal window at such low costs.

Each year approximately 15% of Alaska’s 126 million forested acres are surveyed, which equates to approximately 3% of all forested land in the United States. Unlike some regions of the United States, surveys in Alaska do not cover 100% of the forested lands. Availability of trained personnel, short summers, vast land area, airplane rental costs, and limited time of all involved require adapting survey strategies to efficiently cover the highest priority areas.

No two observers will interpret and record an outbreak or damage signature in exactly the same way, but the essence of the event should be captured. While some observations are ground checked, most are not. Many times, the single opportunity to verify the data on the ground by examining affected trees and shrubs is during the survey mission, and this can only be done when the terrain will allow the plane to land and take off safely. Due to the nature of aerial surveys, the data provides estimates of the location and intensity of damage, but only for damage agents with signatures that can be detected from the air during the survey period. Many root diseases, dwarf mistletoe, stem decays and other destructive pathogens are not represented in aerial survey data because these agents are not detectable from an aerial view. Signs and symptoms of some pathogens may not coincide with the timing of the survey.

For the most part, surveys provided a non-systematic sampling via flight transects. Due to survey priorities, client requests, known outbreaks, and several logistical considerations, some areas are rarely or never surveyed, while other areas are surveyed annually. The reported data should only be used as a partial indicator of insect and disease activity for a given year. When viewing resultant survey maps, keep in mind that the survey flightlines on published maps indicate where data was collected. Although general trends in non-surveyed areas could be similar to those in surveyed areas, this is not always the case. Establishing trends from aerial survey data is possible, but care must be taken to ensure that multi-year projections compare the same areas, and that sources of variability are considered.

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Aerial Sketch-Mapping & Damage Signatures

Aerial detection surveys employ a method known as aerial sketch-mapping to observe and document forest damage from an aircraft. When an observer identifies an area of forest damage, a polygon or point is drawn on a computer touch screen. The touch screen displays the plane's location over a topographic map with a variety of features and layers that can improve mapping accuracy. Trained observers have learned to recognize and associate damage patterns, discoloration, tree species, and other clues to distinguish specific types of forest damage from surrounding undamaged forest. Damage attributable to a known agent is a “damage signature” and is often pest-specific. Only actively occurring, recent damage is mapped; old damage or mortality is not mapped during the aerial detection survey unless there are specific extenuating circumstances.

Knowledge of these damage signatures allows trained surveyors to not only identify damage caused by known pests, but also to be alerted to new or unusual signatures, such as those that may be caused by uncommon or invasive species. Detection of novel damage signatures caused by newly invasive species is an important component of Early Detection Rapid Response monitoring.

Aerial sketch-mapping offers the added benefit of allowing the observer to adjust their perspective to study a damage signature from multiple angles and altitudes, but is challenged by time limitations, fuel availability and other factors. Survey aircraft typically fly at about 100 knots (115 mph) and 1,000-1,500 feet above ground level with variable atmospheric conditions. Low clouds, high winds, precipitation, smoke, and poor light conditions can inhibit the detection of damage signatures. Terrain, distance, and weather conditions prevent some areas from being surveyed altogether.

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History of Aerial Survey Mapping Efforts in Alaska

Prior to 1999, sketch-mapping was done on 1:250,000 (1 inch = 4 miles) USGS quadrangle maps. Today, forest damage information is sketched on 1:63,000 scale (1 inch = 1 mile) digital USGS quadrangle maps or imagery on a digital sketch-mapping system. This system displays the plane’s location via GPS and has many advantages over paper maps including greater accuracy and resolution in polygon placement and shorter turnaround time for processing and reporting data. The sketch-map information is then entered into a computerized Geographic Information System (GIS) for more permanent storage and retrieval by users. Over 40 years of aerial survey data has been collected in Alaska, giving a unique perspective of Alaska’s dynamic and changing forests.

How to Request Surveys & Survey Data

We encourage interested parties to request aerial surveys. These requests and other information are used to determine which areas should be prioritized for survey. Whenever possible, aerial survey requests should include specific location information (waypoints of distinct landmarks) and any observations about the type, extent and severity of the damage, the host/s affected, and when the damage was first noted. For aerial survey requests or aerial survey data, contact Karen Hutten at karen.hutten@usda.gov or Garret Dubois at garret.d.dubois@usda.gov. Alaska Region Forest Health Protection can also produce customized pest maps and analyses tailored to projects conducted by partners.

Areas that have several years’ worth of data collected are surveyed annually to facilitate analysis of multi-year trends. In this way, general damage trend information for the most significant, visible pests is assembled and compiled in this annual report. It is important to note that for much of Alaska’s forested land, the aerial detection surveys provide the only information collected on an annual basis. Forest insect and disease data can be downloaded through the FHP Mapping and Reporting Portal, Insect and Disease Survey (IDS) Explorer. Other applications available on the Portal include Forest Pest Conditions, Data Summaries, Alien Forest Pest Database, Forest Disturbance Monitor (not available for Alaska), National Insect and Disease Risk Maps, and more. All available information within the FHP Mapping and Reporting Portal is on a national scale and often lists data by US Forest Service Region; Alaska is Region 10. Some available products may not include Alaska.

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Aerial Detection Survey Maps (2002-present)

Aerial Detection Survey Maps and annual Forest Health Conditions in Alaska reports are found on our webpage. For 2017- present, this includes interactive webmaps of aerial flightlines and damage.

Citing Maps and Survey Data: Please cite “USDA Forest Service, Forest Health Protection and its partners” as the source of this data in maps and publications.

Aerial Detection Survey Data Disclaimer: Forest Health Protection and its partners strive to maintain an accurate Aerial Detection Survey (ADS) dataset, but due to the conditions under which the data are collected, FHP and its partners shall not be held responsible for missing or inaccurate data. ADS data are not intended to replace more specific information. An accuracy assessment has not been done for this dataset; however, ground checks are completed in accordance with local and national guidelines (http://www.fs.fed.us/ foresthealth/aviation/qualityassurance.shtml). Maps and data may be updated without notice.


Content prepared by Robin Mulvey and Karen Hutten, Forest Health Protection, Juneau, AK.

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