I’d like to introduce you to a very distinctive tree, straight out of Dr. Seuss. In the Puerto Rican rainforest here, you will often see Cecropia schreberiana (Urticaceae) with its long spindly branches and ringed white trunk spiraling up to the highest points of the canopy. It reminds me of a long metal hose with frayed ends, and looks like no other plant you see here. Its spindly form reflects its pioneer strategy. This species grows best in disturbed areas where its ability to deploy leaves on narrow branches lets it grow rapidly and reproduce before being eventually out-competed by slower but more mechanically robust plants. In this way, Cecropias dance between patches of disturbed forest, alighting in each forest patch for just a few decades.
The species has some strange adaptations that make this lifestyle possible. First, it grows very large leaves with a very distinctive shape. Above you can see an example branch that we cut from 11 meters height in the canopy (the lowest branch we could find, and hard work too). These leaves are able to capture sunlight well in the high canopy without shading each other, thanks to their long petioles. The leaves don’t last forever, though – a common sound in the forest is a crashing, falling noise, as one of these giants hits the forest floor. It sounds at least as loud as a squirrel falling out of a tree.
On the subject of falling, another adaptation is weak wood. Having only a few leaves at the very top of the tree means that its wood doesn’t need to support much weight, or require much investment. of course this means that even small disturbances can greatly damage the plant. Here you can see a very large branch that fell on our field station after an overnight rainstorm. I woke up one morning and was very surprised to see the path into the kitchen completely blocked by this pile!
This wood is built in a very curious way. Rather than growing radially and leaving a solid heartwood, like most trees, Cecropia grows these jointed woody segments that are hollow. Hollow wood preserves much of the strength of solid wood (think of scaffolding made from steel tubing) without much of the cost. Above you can see a branch I cut in half.
In that photo, you can also see some strange curved structures. These are male flowers – Cecropia individuals are either male or female, and the male plants disperse their pollen through the wind (which is plentiful in the highest reaches of the canopy). The dangling open structure of these flowers helps the wind catch the pollen.
And finally, one thought on daily life for this species. In the canopy, there are many risks – bright sun can damage young leaves, and insects can easily chew holes through valuable tissue. Cecropia leaves develop in a protective sheath, after which the young leaf is allowed to unfurl. It’s a beautiful and highly functional structure.
This species commonly reaches heights exceeding thirty meters, so it’s rare to get a glimpse at its flowers and leaves. You may imagine that directly climbing a tree like this would be an unadvisable adventure! As we work our way through this forest, we have certainly found that this is the most difficult species to work with, but also the strangest and most fun. Imagine Cecropia the next time you read Dr. Seuss!