Aerial Detection Surveys
Damage to trees caused by insects, disease, animals, and weather, are a natural and common occurrence in the region's forests. Such disturbances can result in changes to biodiversity and species composition, and allow for cycling of nutrients from trees to soil, but can also negatively affect timber quality and other important ecosystem services. There is also concern that climate change and further introduction of non-native pests and pathogens may alter disturbance patterns.
Aerial detection surveys (ADS) have been used to map the cause and extent of forest disturbances in Vermont for nearly 50 years.
Statewide annual sketch-mapping surveys are collected by Vermont Department of Forests, Parks, and Recreation (VT FPR), and the US Forest Service over the Green Mountain National Forest and other federal lands. The US Forest Service Forest Health Monitoring Program sets survey methods and standards for ADS across the US.
In most years, assessments cover the entire state (>2.5 million hectares). Mapped polygons include the disturbance cause, type, size, and severity which are confirmed with ground assessments. Causal agents of disturbance can range from insects and disease, to weather events (ice, wind, and frost), wild animals, and humans. Surveys are a cost-effective and vital tool for detecting emerging forest health issues and tracking trends, but are not comprehensive of all forest damage.
2016 in Summary
In 2016, 32,737 hectares (80,897 acres) of forest disturbance were mapped in Vermont, which is less than 1% of Vermont's forestland. This is a decrease from 2015 when 46,425 ha were mapped, and is much lower than the long-term (1995-2016) average of 100,843 ha/year.
White pine needle damage was more widespread (Figure 4) and severe than in previous years. Due to the extent of damage and the earlier timing of aerial surveys in 2016, it accounted for the most area mapped (12,510 ha) for the first time in the record (Figure 5). Understanding the cause of this damage is ongoing. For more information, see the document from University of Massachusetts Extension (2016).
In 2016, we saw the first year of a forest tent caterpillar (Malacosoma disstria) outbreak, primarily in northern Vermont (Figure 4). Outbreaks result in defoliation of hardwood trees and usually last several years. It was the second most mapped agent (9,917 ha) (Figure 5), yet amounted to less than 1% of Vermont's hardwood forests.
Symptoms attributed to drought accounted for 3,209 ha, amounting to 0.1% of Vermont's forestland (Figure 5). On-the-ground surveys found that stressed trees – such as those already affected by insects or diseases – showed more signs of drought stress. Dry conditions may also explain why fall foliage was earlier and showy (VT FPR 2016).
Some regionally destructive, non-native pests, like gypsy moth (Lymantria dispar), browntail moth (Euproctis chrysorrhoea), and emerald ash borer (Agrilus planipennis), were not detected in Vermont in 2016 although they caused damage elsewhere in the northern Forest region (MA, ME, NH, NY, and VT)
More complete results of the 2016 Vermont ADS effort can be found in the 2016 report, Forest Insect and Disease Conditions in Vermont (VT FPR, 2016).
Long Term Trends
Summing all disturbances per year (1997-2016) reveals substantial year to year variability (Figure 6). This is partially due to shifting monitoring and assessment priorities year to year, but also depends on the nature of the disturbance. Several disturbances are episodic, particularly abiotic weather events (e.g., late spring frost events, drought) and many insect outbreaks. The year of the highest disturbance area occurred in 1998 with a severe ice storm that caused widespread damage to trees (381,843 ha). Only two agents have been detected every year in the 22-year period: beech bark disease and birch defoliator complex (Figure 7).
In total, 66 different damage agents have been mapped in Vermont since 1995. When the maximum extent of damage caused by specific damage agents is compared to number of years they were mapped, agents have varying impacts in the landscape (Figure 7). In general, insects and abiotic agents have had the largest effect on the region's forests. The most damaging agents overall have been ice and snow damage (394,838 ha), birch defoliator complex (324,541 ha) and forest tent caterpillar (277,202 ha).
Abiotic disturbance agents, like ice and frost events and drought, can indiscriminately affect trees regardless of species (although there can be reasons why specific species may be more harmed in abiotic events, due to branching structure, wood density, or habitat, for example) and as a result can cause widespread disturbance. Most other disturbance agents have only affected a small area of regional forestland. Only seven agents out of 66 have resulted in total damage greater than 100,000 ha in the 22-year period. Many tree diseases identified in the region have not caused large disturbance extents despite frequent occurrence (Figure 7). Of diseases, beech bark disease and anthracnose have resulted in the largest disturbance area. Forest fire is an infrequent event regionally, and when it does occur, the extent is small.
Regional Context & Implications
Aerial detection surveys data provides the longest statewide annual record of forest disturbances. Relatively low levels of total forest disturbance have been mapped. Most disturbances cause small damage extents and minor total damage.
The annual rate of disturbance in Vermont is comparable to the rest of northern Forest region (3% of forestland/year). Many, if not all, of the disturbances affecting Vermont's forests are regional issues. Disturbances do not know where state boundaries lie, and as a result pests and pathogens, as well as abiotic stressors, like hurricanes, ice, and drought can affect the whole northern Forest region.
As our climate continues to change, it is projected that extreme weather events will become more frequent, which may mean more storms, wind, ice, frost, or flood events. Elevated summer temperatures, along with changes to rainfall patterns, could lead to more severe and frequent droughts. Such abiotic events can cause large areas of damage to multiple tree species (Figure 7). It is only as we continue to monitor disturbances over time can we begin to understand the patterns of various types of events and how they may be changing.
Many invasive insects and diseases have been detected in Vermont, or in neighboring states. Overall, introduced pests and pathogens have caused much more disturbance to the region's forests than those of native origin, and we could see widespread declines of specific species, such as ash (Fraxinus spp.) with the continued spread of emerald ash borer. The good news is that we are not seeing widespread outbreaks of introduced pests and pathogens in Vermont, nor are we seeing increases in total disturbance over time, in Vermont or regionally. The high species diversity in many of Vermont's forest stands and continued vigilant monitoring may be helping to mitigate widespread issues and to identify problems before they become widespread.
In 2016, there was less disturbance than the Vermont average (1995-2016) and the regional average (1997-2016). The most damaging disturbance agents in 2016 were white pine needle damage, forest tent caterpillar, and drought.
- Vermont Forest Parks and Recreation (VT FPR). 2016. Forest Insect and Disease Conditions in Vermont: 2016. Report. Available online at: http://fpr.vermont.gov/sites/fpr/files/ForestandForestry/ForestHealth/Library/2016%20Forest%20Insect%20%26%20Disease%20Conditions%20in%20Vermont.pdf.
- UMass Extension, Plant Diagnosis Laboratory. 2016. Dramatic needle browning and canopy dieback of eastern white pine (Pinus strobus) in southern New England. Available online at: https://ag.umass.edu/sites/ag.umass.edu/files/content-files/alerts-messages/2016whitepineupdate.pdf.
- Vermont Agency of Natural Resources. 2017. Climate Change in Vermont. Available online at: http://climatechange.vermont.gov/vermonts-changing-climate.