Monday, September 18, 2023

Second update on the rapid expansion of an invasive grass in Florida

Hillside with cogon grass. From Amazing Lingsat

The previous 2 posts on this topic:

Update on the rapid expansion of an invasive grass (December 30, 2021)

Rapid expansion of an invasive grass as viewed using Google StreetView (April 4, 2021)

About 20 months after the last update (December 30, 2021), I again visited the same location and checked on the pace of expansion of the Imperata cylindrica cluster.

The cogon grass cluster had been expanding at a rate of 0.6 meters per month to one side, and during the last check, it had moved 6 meters to the west (left) of a green optic cable marker (see pic below).

When I visited yesterday, the cluster had expanded laterally another 6 meters before encountering a large low lying tree (see pic below).

Cogon has continued lateral expansion all the way to tree on west (left). September, 2023

The cogon ramets had crowded to the right of the crown of the low lying tree, but has not yet been able to bridge the shadow gap and cross to the other side.

However, I found isolated ramets huddling under the tree's shadow, with the one furthest west about 3 meters in from the crown edge. In addition, the cogon cluster is at the same time growing around the tree from behind. The result of these two activities means that sooner or later, the cogon will make its way to the unshadowed west side of the tree. at which point it should again commence its rapid lateral movement.

Cogon slogging under the tree's shadow towards the other side. September 2023.

The expansion rate of 0.6-0.7 meters per month that I had calculated in previous posts was based on an unshaded environment. It's been 20 months since I checked in Dec 2021, and in that time, the cluster has moved around 9 meters sideways, which comes out to 0.45 meters per month. This is noticeably slower than before, but obviously understandable given the shaded territory that it is trying to cross.

An interesting side topic is the ability of the species to traverse shaded environments. Although there are some indications that cogon grass is able to tolerate some shading, the presence of ramets deep into the shade of the tree might be explained by something more interesting.

As a rhizomatous species, cogon grass may have clonal integration. That is, researchers have found that clusters of interconnected ramets have attributes that cannot be found in non-clonal species. Not only does information flow between the members of the genet, but nutrients and water can be translocated throughout the entire network of ramets. This ability to actively and deliberately move information and other materials allows these clonal grasses to withstand stresses in heterogeneous environments that separated individual plants cannot. 

Thus in this case, the ramets under the crown of the tree might be sustained by photosynthates flowing to it from the unshaded and larger clones. 

More information on clonal integration is discussed here:

I've been rather lax in checking up on this fascinating invasion recently (has 20 months gone already!!), but in future I'll try to update it every 6 months or so. 

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