Sunday, April 4, 2021

Rapid expansion of an invasive grass as viewed using Google StreetView

Cogon grass flowering
Last month I talked about using Google Street View as a tool to view the progress of invasive species over time. The example I used showed how a row of Neyraudia reynaudiana (Burma Reed) appears and fills the side of a road over the course of several years, but one cannot tell from the images whether it was outcompeting other plants or simply filling an empty niche.

In contrast, the series of images that I viewed in Google Street View recently clearly shows another invasive grass actively displacing other species as it expands. The location in this case is along a roadside in Kissimmee, Florida, and the invasive species is Imperata cylindrica (Cogon grass).

Cogon grass in 2011
Cogon grass is much smaller than Burma Reed, but it shows a characteristic form when seen even from a distance. It forms dense clusters of relatively straight blades, and the earliest image from 2011 has this invasive grass occupying only around 25% of the distance between two electrical poles.

Cogon grass in 2014 (in red)
However, images from 2014 show that the dense cluster of I. cylindrica had expanded to cover perhaps 50% of the distance, and by 2015 the invasive grass had seemingly already bridged the gap. By 2016, there is no doubt that the Cogon grass had managed to reach the opposite electrical pole and had displaced other plants.

Cogon grass in 2015 (in red)
I visited the area recently, and I measured the distance between the two poles as approximately 50 meters. This means that the Cogon grass cluster expanded laterally at around 7.5 meters per year.

Cogon grass in 2016 (in red)
Interestingly enough, this cluster continues to expand along the roadside even today. In 2018, the vegetation that lies to the left of the images above does not seem to show any Cogon grass infestation (see below).
No clearly visible cogon grass in 2018 between these 2 poles

But when I visited the location in March 2021, the cogon grass had occupied the area all the way to an optical cable line marker, which was placed about halfway between the poles. 

Cogon grass reaches halfway point by 2021
It would be interesting to continue to monitor the expansion of this invasive cluster later, when I again drop by the location in late 2021.

No comments:

Post a Comment