WICOMICO'S GREEN ASH
In Wicomico County, climate change is adding a new layer of complexity to other on-going, unrelated environmental problems. Wicomico's forests are in the midst of an unfolding catastrophe that is completely reshaping ecosystem dynamics. Until recently, the Green Ash was one of our four most abundant tree species, but now it is being destroyed by an invasive insect. Because they can thrive in both dry and wet landscapes, and can tolerate a range of temperatures, pH, and salinities, Green Ash would normally be expected to prosper in a changing climate [1]. However, scientists expect that zero Green Ash trees will remain in Wicomico County by the early 2020s [4].
Even without climate change, this would result in a complete restructuring of Wicomico County forests. But now our forests are being forced to adapt to the loss of a key species within the context of a rapidly warming planet where sea levels are rising, salt water is intruding, and flood events are increasing. These events decrease the vitality of other forest species that would normally take the place of Green Ash trees. Frankly, no-one knows what this will mean for the forests of Wicomico County in the near future, or for the many species that inhabit them. This story is explained below.
In Wicomico County, climate change is adding a new layer of complexity to other on-going, unrelated environmental problems. Wicomico's forests are in the midst of an unfolding catastrophe that is completely reshaping ecosystem dynamics. Until recently, the Green Ash was one of our four most abundant tree species, but now it is being destroyed by an invasive insect. Because they can thrive in both dry and wet landscapes, and can tolerate a range of temperatures, pH, and salinities, Green Ash would normally be expected to prosper in a changing climate [1]. However, scientists expect that zero Green Ash trees will remain in Wicomico County by the early 2020s [4].
Even without climate change, this would result in a complete restructuring of Wicomico County forests. But now our forests are being forced to adapt to the loss of a key species within the context of a rapidly warming planet where sea levels are rising, salt water is intruding, and flood events are increasing. These events decrease the vitality of other forest species that would normally take the place of Green Ash trees. Frankly, no-one knows what this will mean for the forests of Wicomico County in the near future, or for the many species that inhabit them. This story is explained below.
Green Ash (Fraxinus pennsylvanica) is a key species that provides many services to ecosystems and systematically colonizes vast stretches of forest. It is a medium-sized, deciduous tree native to the eastern United States and can extend along waterways as far west as Wyoming and Colorado below elevations of 6,000 ft. Its habitat lies along floodplains of rivers and streams, in wetlands, occasionally along the banks of swamps, and has been planted extensively in yards and along streets as an ornamental, shade-providing tree. Its winged seeds are food for many forms of wildlife, including birds, wild turkey, and some rodents, and its foliage is perused by white-tailed deer. Once established, this fast-growing species can adapt to climactic stresses as it can tolerate a wide range of soil types, moisture, pH ranges, and seasonal flooding. Thus, theoretically, it is exceptionally resilient to the forces of climate change. In fact, as the salt water intrudes and pushes the fresh water lens further up stream and inland, Green Ash habitat in Wicomico County would be expected to increase with climate change. Instead, Green Ash populations are plummeting as a result of human interference. For example, the damming of rivers (built to protect Wicomico's private property) prevents the spread of wetland habitat for the Green Ash. Moreover, humans have introduced an Asian beetle that feeds aggressively on Green Ash trees in the Eastern United States: The Emerald Ash Borer [2,3].
Ecosystems constantly operate in a state of flux, adjusting and evolving in response to various environmental changes. Therefore, the resilience of a certain species is determined by whether it can adapt to, or migrate from, the stressor in time to reestablish its population. The following story exposes the vulnerability of one unsuspecting species, the Green Ash tree, to a newly introduced environmental factor, the Emerald Ash Borer.
The Emerald Ash Borer arrived in Detroit, Michigan in the 1990s and made its way to the Eastern Shore of Maryland by the summer of 2015. Ash trees were once among the top four most common trees in the Wicomico County area, especially along the Pocomoke River. Now, municipalities like ours are spending billions of dollars removing dead ash trees from communities across the US and Canada. State parks are taking proactive measures at their campsites, ripping down any existing ash trees so as not to invite infestation and allow dead trees to fall on unsuspecting campers [4].
Adult Emerald Ash Borers lay their eggs deep within the phloem of the tree, and when the larvae hatch, they chew through the phloem, effectively girdling the trees from the inside out (photo below). Although ash seedlings may remain viable in the soil for a few years after a major population crash, the Emerald Ash Borer can also remain in the forests at low population densities and thus quickly infest any new saplings that arise after the mature trees have been girdled. This beetle has been known to colonize trees as small as 2.5 cm in diameter at breast height (dbh), and death comes within five years of infestation. So, because Green Ash is unable to survive to maturity and reproduce, the Emerald Ash Borer causes 100% mortality among Green Ash populations. It spreads quickly, infesting both healthy and stressed trees, and has been known to kill 99% of ash trees within six years of colonization. Though the Emerald Ash Borer feeds on all types of ash, it has a particular preference for Green Ash and has subsequently killed millions of trees within the Eastern US [4].
Adult Emerald Ash Borers lay their eggs deep within the phloem of the tree, and when the larvae hatch, they chew through the phloem, effectively girdling the trees from the inside out (photo below). Although ash seedlings may remain viable in the soil for a few years after a major population crash, the Emerald Ash Borer can also remain in the forests at low population densities and thus quickly infest any new saplings that arise after the mature trees have been girdled. This beetle has been known to colonize trees as small as 2.5 cm in diameter at breast height (dbh), and death comes within five years of infestation. So, because Green Ash is unable to survive to maturity and reproduce, the Emerald Ash Borer causes 100% mortality among Green Ash populations. It spreads quickly, infesting both healthy and stressed trees, and has been known to kill 99% of ash trees within six years of colonization. Though the Emerald Ash Borer feeds on all types of ash, it has a particular preference for Green Ash and has subsequently killed millions of trees within the Eastern US [4].
Climate change has introduced warmer temperatures that have brought, and will continue to bring, more Green Ash habitat into the range of the invasive Emerald Ash Borer. During years with cold winters, the Emerald Ash Borer larvae can survive subzero temperatures using a method called supercooling, which is characteristic of many insects. Supercooling refers to insects' ability to produce specialized sugars, alcohols or antifreeze proteins during that allow liquids to cool below their normal freezing points. Once these insects make these molecular compounds and become acclimated to the cold, the temperature must drop below the supercooling point before large portions of the populations die. Research from China suggests that the supercooling point for Emerald Ash Borer ranges between -9.4 and 15.5 degrees Fahrenheit. However, small portions of the population still survive if they are insulated well enough inside the bark of the tree. We do not see these types of temperatures in Wicomico County, so nearly all Emerald Ash Borer larvae hatch every year anyway. However, in colder states, like Minnesota, warming temperatures would mean that more larvae survive the winters, increasing populations of Emerald Ash Borer and decreasing the populations of Green Ash Trees at a significantly quicker pace in these cooler areas [5].
Something else must take the place of all these dying trees, and this is how climate change will determine the future of Wicomico County's forests. As temperatures warm and saltwater intrudes inland to where the Ash used to sit, the new colonizing species must be more resistant to the changing environment. Though this may make the forest more resilient overall, it may completely restructure habitats and alter the functioning of ecosystems depending on what becomes established in this new niche [6].
Consequently, the future of Wicomico's forests is uncertain.
EMERALD ASH BORER
The Chesapeake Bay Program : Published on Aug 24, 2016
"The emerald ash borer is an invasive insect that is killing ash trees as it spreads across the United States. Colleen Kenny, a forester with the Maryland Forest Service, and Paula Shrewsbury, a professor at the University of Maryland, talk about the efforts being used to slow down the emerald ash borer and save ash trees—which have an important role in protecting water quality."
"The emerald ash borer is an invasive insect that is killing ash trees as it spreads across the United States. Colleen Kenny, a forester with the Maryland Forest Service, and Paula Shrewsbury, a professor at the University of Maryland, talk about the efforts being used to slow down the emerald ash borer and save ash trees—which have an important role in protecting water quality."
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References
Green Ash Background Photo: Mary PK Burns, flickr
- The Morton Arboretum. (2017). Green ash (Not recommended). Retrieved from http://www.mortonarb.org/trees-plants/tree-plant-descriptions/green-ash-not-recommended
- Dickerson, J. (n.d.) Green Ash Fraxinus pennsylvanicus Marsh. Retrieved from https://plants.usda.gov/factsheet/pdf/fs_frpe.pdf
- Maryland Forest Service. Maryland Department of Natural Resources. (2007). Sustainable Forest Management Plan for Chesapeake Forest Lands: Sustainable Forests for People and the Bay (Report No. 11). Sustainable Forestry Initiative. Retrieved from http://dnr.maryland.gov/forests/Documents/chesapeake/CF-SFMP_2018.pdf
- Westwood, M., Oldfield, S., Jerome, D. & Romero-Severson, J. (2017). Fraxinus pennsylvanica. The IUCN Red List of Threatened Species. Retrieved from http://www.iucnredlist.org/details/61918934/0
- Maryland Department of Natural Resources. (2018). How Cold is too Cold for Emerald Ash Borer? Retrieved from https://www.dnr.state.mn.us/invasives/terrestrialanimals/eab/winter.html
- Rogers, C. E., & McCarty, J. P. (2000). Climate change and ecosystems of the Mid-Atlantic region. Climate Research (14), 235-244. Retrieved from http://www.int-res.com/articles/cr/14/c014p235.pdf
Green Ash Background Photo: Mary PK Burns, flickr