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Archive for October, 2015

Plotting vector data and running statistics really only shows part of the picture with Verticillium Wilt. We can tell from such data how many trees in a given area are effected and to what degree. However, when it comes to quantifying biomass loss as a result, it was necessary to look at raster data. For this I used the NASA developed Normalized Difference Vegetation Index (NDVI). NDVI is useful for assigning values to vegetation thickness. It uses the difference between the red band and near-infrared, to calculate a value. Since plants reflect about 50% of infrared light, the value is higher where there is plant cover. These measurements are calculated from +1 being a veritable jungle to -1 being barren rock.

I used zonal analysis in ArcGIS, with my test plot being the zone. I then averaged the zone for both 2010 (about when the blight started) and 2013 (the most recent available imagery of high enough resolution). For 2010 the mean was .17011 and for 2013 .16845. These are both pretty low, and reflect the developed nature of the suburban plots. However, the mean vegetation drop was .00166 over three years. This number is not incredibly high, but would be alarming if vegetation loss continues at such a pace. The following maps express the situation visually:

This map expresses NDVI levels from 2010.

This map expresses NDVI levels from 2010.

This map shows vegetation loss and decline in health in the 3 year period.

This map shows vegetation loss and decline in health in the 3 year period.

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Dying Norway Maple

Over the last few years I have noticed that the Norway Maples in the Lehigh Valley were not looking so great. The most immediately apparent problem ¬†was the crown die-back, and increasingly completely dead trees. This stands in stark contrast to the way things looked when I was a kid. The Maple lined streets were lush and green then… So, what happened?

After reading up on Norway Maples and Crown Die-Back, and listening to several episodes of You Bet Your Garden on NPR, I came up with a number of theories. First, I though, as Mike McGrath often suggests on the aforementioned show that over-mulching was causing root girdling. Basically, how this occurs is, when you over-mulch it concentrates the water over too small an area. The tree’s roots concentrate in that area, instead of spreading into a wider radius, and eventually strangulate themselves. Another issue that came up was decline from road salt. This relationship has long been noted in relationship with the New England Sugar Maple population, and the symptoms typically include crown die-back. Finally, I came across evidence of a fungal blight that is found throughout the northeast called Verticillium Wilt.

Verticillium Wilt is a fungus that starts in the soil, enters the trees through its roots, and causes a blight in the pith wood. Amongst the common symptoms are stunted growth in the tree’s new growth, discoloration of pith wood and, of course, crown die-back. Because the blight offers several diagnosis points, I was able to study the trees in the area, and found they exhibited all three tell-tale symptoms.

Discolored wood fibers around a rotten pith, characteristic of Verticillium Wilt.

Discolored wood fibers around a rotten pith, characteristic of Verticillium Wilt.

On the left is stunted new growth, characteristic of Verticillium Wilt. Compare to healthy growth on the right.

On the left is stunted new growth, characteristic of Verticillium Wilt. Compare to healthy growth on the right.

Various states of die-back.

Various states of die-back.

So how is Verticillium Wilt spread and why are so many trees dying. After reading that Verticillium Wilt can survive in soil or mulch for ten years without a host plant, I had an ahah! moment. I now have a hypothesis about the vector for this disease. When a tree contacts Verticillium Wilt it is typically fatal. When these trees that die are in people’s yards, they call the tree removal service, and they come by and fell and remove the tree. Typically the tree is then mulched, and the mulch is sold to contractors to then mulch people’s yards. This then spreads the disease to more suburban trees, which then die and are themselves mulched and so on.

I have started to study this with the aid of GIS. I have a suburban sample, which is mulched and an urban sample which is not mulched. In the suburban sample 54% of the trees examined show signs of Verticillium Wilt. Meanwhile, in the urban sample I found no trees that exhibit clear signs of the blight. I then ran a correlation on diameter, to see if the age of the tree could be affecting the sample. However, I found a -0.62 correlation, which is a weak correlation between an increase in diameter and decrease in occurrence of the crown die-back. This suggests that there may be a resistance in older trees, that older planting methods were more effective, or that mulching may indeed play a role. I did find a 0.41 correlation between mulching and die-back. Again this is a weak correlation and will need more data to flesh it out. I am hoping to add a  forest plot to expand the available data.

This is the sample of the suburban neighborhood, where Verticillium Wilt is present.

This is the sample of the suburban neighborhood, where Verticillium Wilt is present.

Test Plot at Kutztown Park. Verticillium Wilt is not present and there is not mulching around the trees.

Test Plot at Kutztown Park. Verticillium Wilt is not present and there is not mulching around the trees.

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