I’m always interested when someone reports that a plant is doing something odd. Corrêa and Fischer really help highlight something odd is happening with the title to their paper, Bizarre Cecropia pachystachya (Urticaceae) hemiepiphytic growth on palms in the “Pantanal” wetland (see also free version).
There are a few things to pull apart in that title. The Pantanal is a good starting point. If you know South America, then you’ll know the Pantanal, but it’s surprising how many people don’t. It’s the largest wetland on the planet, and if you’ve seen a wildlife documentary on South America, you’ve probably seen shots from the Pantanal, it’s easier to film in than it’s more famous neighbour, the Amazon Rainforest to the north. It’s a difficult place to pin down in Google Maps, but you’re looking at a good chunk of Mato Grosso do Sul, with bits in Mato Grosso, Bolivia and Paraguay.
If you do visit the Pantanal then it would be quite difficult to avoid Cecropia pachystachya. They’re excellent pioneer trees, moving rapidly into cleared areas. They’ve been studied a lot in the past because they’re myrmecophytic. In plain English, they get on really well with ants, encouraging them to protect the leaves from predators. What makes these C. pachystachya bizarre isn’t the myrmecophytism, it’s the hemiepiphytic growth.
An epiphyte is a plant that sits on another plant to grow, instead of in the ground. The hemi- prefix means that it’s not the whole story for C. pachystachya. Some of the trees germinate in palms, but most do not. Also the hemiepiphytic C. pachystachya plants look peculiar.
Corrêa and Fischer looked at Attalea phalerata, the Urucuri Palm. The roots reach around the tree from the stem of C. pachystachya intermesh and then hit the ground on the other side. It does look bizarre, but copyright laws being what they are, we can’t reproduce the images here. You can see them at ResearchGate, if you scroll down the page.
The paper follows up earlier work by Corrêa, Fischer and dos Santos on Seed banks on Attalea phalerata (Arecaceae) stems in the Pantanal wetland, Brazil (free access). They concluded:
Attalea phalerata stems contain a rich seed bank, comparable to soil seed banks of tropical forests. As most of these seeds are not adapted to grow in flooding conditions, palm stems might be regarded as safe sites for seeds (and seedlings) to escape from the seasonal flooding of the Pantanal.
The seeds arrive thanks to fruitbats, who eat the fruit of C. pachystachya while handing out under the branches of A. phalerata palms. The result is a regular deposition of seeds almost as a rain from the branches of the palm onto the stem. So given the regular seeding, why would C. pachystachya be relatively rare as a hemiepiphyte? That might be down to the Pantanal.
Corrêa and Fischer refer back to another recent paper on the Pantanal in Annals of Botany, Floral variation and environmental heterogeneity in a tristylous clonal aquatic of the Pantanal wetlands of Brazil. It’s a different plant, with the same problem, the flooding of the Pantanal which can be up to six months long, and up to four metres deep.
Looking closely at the Cecropia, Corrêa and Fischer have noticed that C. pachystachya is the only species that has really taken to the Pantanal plain, the other Cecropia preferring high, and less flooded, ground. In contrast C. pachystachya actually improves its germination with a period of waterlogging, but it can’t stay under too long. This might be one of a number of specific factors that C. pachystachya needs in order to germinate on a palm instead of in the soil.
Another factor is where on the palm stem the seed is. It seems that C. pachystachya has to be low on the stem to be secure, and it doesn’t yet climb the way other hemiepiphytes do.
However, if there is an evolutionary advantage in being able to germinate as a hemiepiphyte, then you’d expect natural selection – in the long term – to favour a population of plants that can do that. What we’re seeing is a snapshot of the Pantanal in evolutionary time. If favourable conditions continue, it might be that what Corrêa and Fischer have found is a plant taking its first steps to climbing up other trees.