Invasive pampas grass prevails only when not shaded

When there is an empty space, such as an abandoned crop, the first colonizing plants are the least demanding or the most resistant. These adventurers create the right conditions –humidity, temperature, soil availability– for other species that, on occasions, can eliminate them due to competition. This process is known as ecological succession.

The ecologist Ramón Margalef said that evolution happened within the framework of succession. It has taken me a long time to understand what exactly that lapidary phrase meant. Now I think what he was trying to convey to us is that all stages of succession are equally important ecologically and evolutionarily. In other words: we cannot wish to have only forests because then the typical species of the pre-forest stages would disappear. There has to be everything. Bush-dwelling warblers are just as important as great spotted woodpeckers.

invasion and succession

Based on this idea (the importance of succession and mosaic landscapes) it could also be suggested that it seems a generalizable principle that plant invasion occurs within the framework of succession.

As I recently wrote, Pampas grass (Cortaderia selloana) occupies with relish the former cultivated fields now abandoned: open lands rich in light and nutrients. A fertilized substrate to be invaded by a plant that loves open, bright spaces.

Farmland abandoned for a short time where there are only herbaceous plants and ferns.
Alejandro Martinez Abrain, Author provided

But we have to think that if the abandonment of the fields becomes permanent, the plant succession will continue on its way. Cortaderia enters the abandoned fields of Galicia when there are only herbaceous plants or ferns (pteridium), but it starts to be devoured by the succession when the brambles appear.

file 20221130 8077 5qyocs.jpeg?ixlib=rb 1.1
A long-abandoned farmland where Pampas grass is in the process of encroaching.
Alejandro Martinez Abrain, Author provided

Finally, the Pampas grass ends up running out of light with the development of mature gorse, broom and codeso. Before the trees even arrive again. And not only are the existing feet eliminated, but the succession prevents the entry of new individuals.

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Farmland abandoned before the previous ones, next to them, where the plant succession is already in the bramble stage. Existing pampas grasses are ‘drowned out’ (no access to light) by brambles and new pampas grass stands have a hard time invading the field.
Alejandro Martinez Abrain, Author provided

The invasion is temporary

Thus, we must be aware that the invasion, in the case of plants that love open spaces, will not be forever. It has a built-in expiration date by default. Because whoever is in charge in the end is the one who has dominion over the heights, control over sunlight.

The same occurs in the holm oaks of Quercus ilex ilex, with large leaves and closed crowns typical of the Balearic Islands and few corners of the Iberian Peninsula. Before the oaks grow the beautiful and productive strawberry trees. But over time the latter will inevitably end up becoming mere corpses under the canopies of mature holm oaks, in a completely normal and absolutely natural process.

In summary, the invasion of plants that love open spaces can only be permanent in spaces that remain permanently open, either because the soils are very poor or because they are kept open on purpose, as occurs in the streets that the law obliges to generate under power lines in anticipation of fires.

For this reason, municipal environmental policies that penalize the owners of abandoned fields invaded by Pampas grass are not on the right track. Owners are forced to spend considerable amounts of money out of pocket to leave their fields open. However, in doing so they regenerate the open (light-filled) spaces that pampas grass loves. It is enough that a few feet have remained without removing them on the edges of the roads or in a garden for the grass to jump back into the fields. And start again.

With the science of ecology in hand we would do better to let the succession advance in those fields. In this way we would end the invasion without doing anything, that is, without investing money and energy in it.

Saving the distances, we could say that the succession is a relationship as ungrateful and asymmetrical as the one that sometimes occurs between parents and children. Everything is done to help those who come behind, and what is received in return is often darkness.

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