A couple of years ago someone in a comment to my post on cecropias asked whether cecropia trees were deciduous or evergreen. I had mentioned that the leaves, being big, could become unsightly when many of them dropped from the tree. The image to the left. is of one such leaf. My response was based on my own temperate-climate viewpoint, supported by some Cecropia references and an entry in wikipedia which states that (in botany and horticulture) deciduous plants “…are those that lose all their leaves for part of the year.” I said that “my” cecropia was an evergreen tree because it never lost all its leaves.

The answer did not satisfy me, though, and the question has been nagging at the back of my brain ever since. What is going on with deciduous trees in the tropics? At first I thought that maybe deciduous trees here lost their leaves in the dry season, which would make sense for water conservation. And indeed I did see some trees without leaves at that time. But other trees would lose their leaves at other times of the year, and some trees would lose some of their leaves, but not all of them, seemingly throughout the year.

Finally, I’ve located a paper that is readable, a classic in tropical biology, and that explains what’s going on with deciduous leaves in the tropics. I’ll be quoting and paraphrasing from it extensively throughout this post. The paper is by D.H. Janzen, written in 1975, and it’s called Ecology of Plants in the Tropics (Studies in Biology). To give you a sense both of his style and of the tropical environment, here’s a quote from the Introduction:

…in the same habitat there are tree species that are totally deciduous during a six-month dry season, species that are completely evergreen, and species that drop their leaves in the rainy season and bear them during the dry season.

In temperate climates, you’ll find a few conifers mixed in, say, an beech-maple forest, but you would consider such a forest deciduous because most of the trees there drop all their leaves every winter. None of the trees would lose all their leaves every summer.

Such information begs the question: why do trees drop their leaves?

Why do trees drop their leaves?

Janzen says that leaf drop may be a response to

• increased shading due to the more leaves growing overhead within the tree’s crown or in other trees’ crowns
• damage by herbivores, wind, or falling debris
• dry weather

Increased shading. If there is a gap in the forest or if the area is disturbed, pioneer species of trees will come in, and for these species here is a premium on rapid vertical growth. In these circumstances,

“…leaves tend to be shed almost as soon as a shadow is cast upon them. If minerals can be thoroughly extracted from leaves before discarding…the cellulose skeleton that is lost means relatively little to the plant in a light-rich habitat.” [Janzen]

Aha! This seems to me the perfect explanation for the Cecropia behavior.

Here’s a leaf about to drop from a young Cecropia – zooming in on it in the adjacent image.

Doesn’t that leaf look like it’s had all its minerals and other goodies extracted?

The petioles (leaf stalks) are quite long in Cecropias. Here’s where this one is attached to the tree. Notice the bud coming in just above where the petiole is about to let go. Also, you can see the triangular scars lower down, where other leaves have already been dropped.

Cecropias growing in a forest emerge above the surrounding forest canopy when mature. At this point rapid vertical growth is no longer important and leaf drop slows down.

Janzen notes that once mature, “…it is probably only the crown margins [that] lose some leaves through shading.”

Damage. Here’s an image from a couple of years ago where a young Cecropia suffered severe herbivore damage to its leaves. At the top of the plant you can see the characteristic Cecropia stipule at the top of the stem and young red leaves emerging around it, but the two leaves that run diagonally across the image are practically transparent from the damage.

In the next images, some of the herbivore-damaged leaves have already dropped off another Cecropia plant (left) although some heavily damaged ones are still attached toward the top. New leaves are also coming in.

Janzen asks the intriguing question “…why [do] plants actively discard old leaves instead of simply allowing them to be eaten off[?]” We know that abscission – the intentional dropping of a leaf – is an active process. Leaves stay attached to dead trees. So the plant is dropping its leaves “on purpose” after herbivore damage. Janzen‘s answer is that it must be that

“…when a leaf is discarded, the plant has the chance to extract all possible nutrients from it.”

Further, he notes, dropping leaves may be a way of escaping from herbivores.

“Otherwise, the herbivore population could easily build up on the new leaves if they were produced at a rate of a few per day over a long time.”

Dry weather. Here’s and example of a tree that loses its leaves in the dry season (left), flowers, and then grows new leaves when the rains start (right).

It’s Tabebuia rosea, called roble or oak locally because of its strong wood. Here’s a zoom in on one of its flowers.

Besides dropping leaves for water conservation, the plant may find an advantage in dropped leaves for pollination – its flowers are much more visible to pollinators when the leaves are absent, or nearly so.

As already noted, not all deciduous trees lose their leaves in the dry season, but the longer the dry season, the higher the proportion of trees that do drop their leaves. However, even here there are variations from habitat to habitat, for a tree living along a river may not lose its leaves while a tree of the same species elsewhere will do so.

Further, there are trees such as the nance (Byrsonima crassifolia) and others that drop old leaves and produce new ones during much of the year. Right now we’re getting pretty seriously into the rainy season this year and several trees have actively dropped leaves on the ground beneath them. If you click to enlarge the images you can see no herbivore damage. From left to right: the leaves of Miconia argentea (called dos caros locally), Miconia rubiginosa, (called canillo locally) and Byrsonima crassifolia (nance).

So, how long can a leaf live?

All this discussion about leaves dropping has to make one wonder just how long it is possible for a single leaf to live. Janzen says the understory is a place where leaves are especially resistant to herbivore and mechanical damage and therefore

“…there is no obvious reason why an understory leaf should not live for many years (as indeed do the 3-7-year-old leaves of ericaceous [heath family plants] evergreen shrubs in the understory of deciduous forests in the south-eastern United States).”

Leaves not living in the understory have shorter life spans – along the order of 7 months [Coley and Aide 1991, cited in Santos 2000], perhaps because they can maintain a high growth rate and therefore have high leaf turnover rates.

What, then, is a deciduous plant?

At this point I had to re-think the definition of a deciduous tree or plant being one that loses all its leaves during a certain period of time. Janzen shows throughout his article that there is a continuum of leaf-drop behavior, especially in the tropics, that depend on habitat, herbivores, and many other conditions. The Cecropias mentioned in several of my reference books are described as “evergreen” yet, as we’ve seen from many examples, they lose leaves often. Some deciduous trees, on the other hand, may drop all their leaves in one day and within two or three days be flush with new leaves!

In the wikipedia discussion of “deciduous,” it is pointed out that intermediate plants may be called semi-deciduous or semi-evergreen. At this point, I’ve begun to think that all the plants in this neotropical savanna are semis!

I started surveying the others in the area, and found this one is not unique. So what’s going on?

Here was one of the happy little cecropias along the roadside in August.

Here it is now, in October, after Herbivore Wars.

And here’s that peculiar formation at its tip (rectangle area in previous image).

Here we see an obvious baby leaf on the right, a battered gray leaf on the left, and a sharp-pointed green something-or-other at the tip.

Further, this something-or-other is present on both the young cecropias that I transplanted earlier this year:

In this last image we can begin to see what’s happening. New leaves, or a new leaf, is forming under this sharp-pointed green something-or-other.

I did a web search on Google Scholar for cecropia biology, and found this extremely helpful article on cecropia and its ants in Costa Rica. (More on those ants another time.) It turns out that the green something-or-other is a “sheathing stipule.” The botanical definition of a stipule is a “small leaflike appendage to a leaf.” In this case, it looks like the stipule is protecting the young leaf as it develops.

So, what happened to the cecropia crown? Here’s what I’m guessing, based partly on material I read in the article just mentioned. Cecropia grows taller by adding internodes – those sections that make the cecropia trunk look like bamboo. Each internode bears a single leaf. When the leaf is fully developed it resides at the top of the tree and, because the leaves are so large, nothing looks unusual. As a cecropia matures, branches develop, and internodes are added at the end of each branch, so the tree as a whole has an umbrella-like crown. In the image of the mature cecropia below, at least one internode is visible with its sheathing stipule.

I never would have noticed it, had it not been for the peculiar look of the immature tree.

As a group, cecropias (Cecropia spp.) are one of the most conspicuous genera of trees in the Neotropics….They are thin-boled, spindly trees with bamboolike rings surrounding a gray trunk. Their leaves are large, deeply lobed, and palmate, somewhat resembling a parasol.” (p. 71) Kricher, J., 1997. A Neotropical Companion: An Introduction to the Animals, Plants, and Ecosystems of the New World Tropics (2nd ed – 1999), Princeton University Press, 451 pp.

From my kitchen window, I see a cecropia tree. From my bathroom window, when I take a shower, I see a cecropia tree. From the end of my driveway, when returning home, I see a cecropia tree. The cecropia tree, more than any other, reminds me that I’m in the tropics.

There’s a gap in the vegetative screen between our house and our neighbor’s house. I’ve long wanted to plant a cecropia shoot or two in that gap. I know that it will grow quickly (up to 3 m per year, according to Kricher), that it will grow tall relative to the other trees here (up to 18 m), and that it attracts birds and even sloths (though I’ve not yet seen a sloth here). Kricher has cited research that shows that 48 animal species, including leafcutting ants, iguanas, birds, and mammals make direct use of a particular species of cecropia, Cecropia obtusifolia. Further, a “…total of thirty-three bird species from ten families, including some North American migrants, feed on cecropia flowers or fruit.” In Kricher’s words, “Such trees function for tropical birds as fast-food restaurants.” [p. 72]

I see birds most frequently when looking at the cecropia tree out the kitchen window. Two of the more interesting species I’ve seen there are the Crested Oropendola and the Lineated Woopecker.

The first bird image is of the Crested Oropendola was taken by Glen Tepke on a trip to Panama. And here’s a neat description of the behavior of the Crested Oropendola.

The second bird image is of the Lineated Woodpecker was taken in Mexico and it appears to be clutching to the side of a cecropia. No coincidence, probably, because these woodpeckers are eating the ants associated with the cecropia (see below for more on the ants).

Last year I asked our gardener to help me find a cecropia shoot to transplant to grow into the vegetative gap. He immediately looked under the tree of an adult cecropia, to no avail. But he did tell me that the cecropia is called “guarumbo” in Panama. For the past year, I’ve kept my eyes peeled underneath the guarumbo trees bordering our property.

A couple of months ago one of the trees began producing fruit. I blocked off a large area underneath the tree so I wouldn’t accidentally mow down a baby cecropia. I saw a couple of fruit catkins on the ground. I put one in a pot of dirt and just watched it for a week, but then our young Labrador discovered it and that was the end of my experiment. Then I read somewhere on the web (I can’t find the source now – when I do, I’ll provide a link) that birds were responsible for seed dispersal of cecropia.

Then, in late July, I began to notice small cecropia shoots along the side of the road.

And also in our driveway. So I began to mow under the cecropia tree again and took my spade to the driveway. I’ve now planted two young shoots in the area near the neighbor-vegetation-gap (here’s one of them), and I’ll keep close track of how quickly they grow.

A couple of notes on the drawbacks to purposefully growing cecropia trees.

One is aesthetic. Because the leaves (and stems) are so large, when they drop and shrivel, they are unsightly. Earlier this year the leaves seemed to drop in great abundance, perhaps just before the tree was about to flower.

The second drawback is a warning. Most cecropias are hosts to a biting ant, genus Azteca, and because the ants live in the hollow stems of the tree, cutting one down can be treacherous!

Nevertheless, cecropia trees are also known for their medicinal qualities, and in Cuba, nearly every part of the tree is used in some form of medicine. Crecropia has recently been reclassified and place into the family Urticaceae, the Nettle family, which, for those of us from temperate climates, is also a family known for its medicinal qualities.

Finally, for the classically minded, yes, Cecropia is named for Cecrops, first King of Athens.