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 wikipediawhich 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!

First, though, about that awkward family name – Malpighiaceae. I’ve heard botanists shorten it to something that sounds like “Malpiggy,” which I suppose is correct enough for pronunciation, but which sounds, in English, unjust to the Italian scientist for whom the family was named: Marcello Malpighi, a 17th century anatomist. We humans have a Malpighi skin layer, and we have Malpighian corpuscles in our kidneys and spleen. So why is a plant family named for him? Because he also looked at “plant anatomy” under the microscope and wrote a book about what he saw – the Anatomia Plantarum.

One plant feature he must have described in some detail are plant hairs that are attached to a plant by the middle of the hair, not at either end. There’s a sketch at Plant Families of the Dominican Republic, a site, by the way, that has an excellent description of the Malpighiaceae family.

The sketch shows the characteristic clawed petals of the flower on the left, the Malpighian Hairs at the top right, and the winged fruit, like the maple wing, at the lower right. It’s worth taking a closer look at the vine flower and the nance flower to see these details.

Malpighi flowers have claws

Here’s a closer look at the vine flower,

compared with nance flower.

Look at the bumps between the claws of the petals – they’re green in the vine flower and yellow in the nance flower. These are oil glands. Many flowers in the Malpighiaceae family are pollinated by bees that collect oil from the flower while pollinating it.

The flowers occur in clusters, although the clusters differ between the vine (at the left, below) and the nance tree (right, below).

Malpighi fruits are variable

We could not use the fruits of these plants to place them in any particular family. The fruit of the vine is a winged “samara,” which is the fruit of elms and maples (the vine samara, which occurs in 3s, is shown in the two images on the left, below). The nance fruit is a fleshy fruit formed from a single flower, a drupe, with one seed inside (the image on the right – the nance fruits are the ones in the bottle).

Malpighi leaves are simple and opposite.

Leaves may be simple or compound. Malpighi leaves are simple, consisting of one blade; each leaf bud produces only one blade.

Leaves also may be opposite, alternate, whorled, or spiral. If opposite, the blades are exactly opposite each other on the stem. Malpighi leaves are opposite, as you can see here. The vine leaves are on the left, the nance leaves on the right.

Malpighi plants may be lianas or trees

Lianas are woody vines that start at ground level and climb trees to reach as much light as possible. They are common in rainforests, but also are found in temperate zones. Clematis, for example, is a liana (wikipedia). Today’s plant is growing on the ground in a disturbed area and has started climbing nearby shrubs. Nance is a tree. The vine, or liana, is on the left, the nance tree on the right.

Malpighi plants may have stipules

Linneaus himself, “Father of Taxonomy,” came up with the term stipule. A stipule is an outgrowth on either side of the base of the leaf stalk. In today’s plant, the stipules are small, scale-like, and triangular. You’ll need to click on the image at the left to enlarge it enough to see the stipules. Zero in at the point where the leaf stalks meet the stem. The nance stipules, by contrast, are relatively easy to see although you’ll probably want to enlarge the image on the right to see them better.

Finally, Malpighi plants have Malpighian hairs

You may have noticed the reddish hairs on the nance stem and leaf stalk in the image above, but it may not have been easy to see the hairs on the vine plant. Here’s a larger version of the underside of the leaf, but even so you may want to enlarge it to see the hairs well. While you’re looking, you may notice the swelling of the leaf stalk just at the base of the leaf blade. Those are leaf glands, and are common in the Malpighiaceae family.

To summarize the Malpighiaceae family characteristics that this vine and the nance share, then are:

• Flowers with clawed petals
• Simple, opposite leaves
• Stipules
• Malpighian hairs

We also looked at their fruits, which are not a family characteristic, and we saw that one is a vine, the other a tree.

Now to classify, as far as possible, the vine itself.

I used Gentry‘s field guide/key to the woody plants of Northwest South America to get a handle on the species. Gentry divides the Malpighiaceae family (about 60 genera) into two large groups:

1. Trees and shrubs with unwinged fruits [the nance tree (genus Byrsonima) goes here]
2. Lianas with wind or water-dispersed fruits [today's vine plant goes here]

He then breaks down each group into smaller groups. The small group into which this vine falls is the group with maple-like wings. He puts four common genera into this group. I’ll only discuss the one that seem most feasible to me: Stigmaphyllon.

Genus name

It’s because of Stigmaphyllon that this post has the title it does. The name Stigmaphyllon comes from “stigma,” which is the part of the female structure that receives pollen during pollination, and “-phyllon” which means “leaf.” According to Arthur Gibson, Garden Director for the UCLA Botanical Garden, … each stigma bears a very conspicuous leafy portion next to the sticky stigmatic tip. Once I started looking for the three leaves, I found them. Here’s a close-up of the center portion of the flower:

You can easily see those leafy structures at the right. On the left you see the green oil glands of the sepals.

Gentry states that the leaves of Stigmaphyllon usually have a heart-shaped base and those that don’t have such leaves do have unusually large glands on the leaf stem. These descriptions make me a little uneasy, because the leaves on this vine are elliptical, without a heart-shaped base. Further, I don’t have enough experience to know whether the glands on the leaf stem are unusually large. Nevertheless, Stigmaphyllon is the best fit I have for a genus.

Species name

As far as species… Discover Life lists 18 species in the genus Stigmaphyllon (do a search for Stigmaphyllon to see the list). Only 4 species have illustrations, but one of them is S. ellipticum, which seems awfully similar to this plant, and nearly satisfies my concern about leaf shape. Certainly the species name is encouraging!

Having been stung on species identification before, however, I’m assigning this plant an extremely tenuous identification: Stigmaphyllon (probable genus) ellipticum (possible species). The proper botanical name would be Stigmaphyllon ellipticum (Humb., Bonpl. & Kunth) A. Juss. In the meantime, I’ve saved a sample in my plant press and hope to get a real identification one of these days.

The nance, Byrsonima crassifolia, is one of the three conspicuous trees in this savanna. It’s often a host for orchids,

and its fruit is used to make juice and to flavor ice cream. Different parts of the plant are used throughout the American tropics for medicinal purposes such as treating snake bites, diarrhea and dysentery, bad coughs, and reducing fever.

I’ve worked through the taxonomy of nance, before, showing that it is in the family Malpighiaceae. But what features of this plant put it in that family, relating it to acerola and about 1300 other species, most of whom are native to the new world?

For on thing they often have reddish hairs.

Not only do we see in this image the reddish hairs, but we see three other characteristics of Malpighiaceae:

1. simple (not compound) leaves
2. opposite (rather than alternate) leaves
3. stipules

The stipules are those small appendages at the base of the leaf stalk. If you read about the stipules that characterize the coffee family (Rubiaceae), you may remember that those were interpetiolar stipules – ones that occur between the bases of the petioles. The stipules of the Malpighiaceae may be either interpetiolar or intrapetiolar stipules – ones that occur at the base of the petioles. Here in Byrsonima, the stipules are intrapetiolar – at the base of the petiole.

The flowers, though, are most distinctive. They occur in clusters either at the ends of stalks or branches (terminal) or at the angle between the leaf stalk and stem (axillary). Nance inflorescences are terminal. The young flowers are yellow and as they age, they turn orange in color.

Each flower within the cluster has

1. three carpels (female organs)
2. “several” fertile stamens (male organs)
3. five petals with clawed bases, commonly fringed or toothed.

Most interesting to me are those clawed, fringed petals. That’s what I’ve been trying to get a picture of. I’ve scanned, taken macros indoors and out, and have had abominable results.

You can get an idea, I think, with this one:

You can certainly see the 5 clawed, fringed petals. The carpels and stamens are there in the center, but you could not prove it by me which are the three carpels and which of the remainder are the stamens.

Byrsonima flowers produce lipids instead of nectar. In the next image you can see the glands that produce this oil. They are located over the sepals. (You can also see those reddish “malpighian” hairs on the flower stalk.)

Bees of the genus Centris obtain both pollen and oil at each visit to the flower. The mixture of oil and pollen is fed to bee larvae. Adult bees feed themselves with nectar from other species of plants.

I’ve gone a little beyond the family characteristics here, once I zeroed in on the flower. So let me just summarize what I now know to look for to recognize the Malpighiaceae family.

1. reddish hairs
2. simple, opposite leaves
3. stipules
4. flowers with 5 clawed, fringed petals

These should suffice in the field to at least tell me the plant is likely to be a member of the family.

Now what sets the nance tree apart from the other members of the family? I found an excellent description of Byrsonima crassifolia written by Mireya D. Correa A. at the University of Panama and Smithsonian Tropical Research Institute. My botanical skills are still too naive to pluck the key species features from the description, but much in that document is of interest. All the information in the next two paragraphs is gleaned from that paper.

The nance tree is resistant to fire and therefore has a mode of growth that makes it appear twisted. [I wonder, but haven't been able to find out, whether this twistedness creates nice little niches for the orchids. I also see more epiphytes on the nance than on other trees in the relatively dry climate of a savanna. Hmm.] It is found in both wet and dry tropical forests but usually grows in barren soils at elevations up to 1500 m. The bark is fissured, gray to dark chocolate color, with lenticels.

It flowers from November through July, primarily from March through June, for approximately 6 weeks. One fruit (drupe) is produced from each flower. They ripen primarily in August and September and are dispersed by birds. Humans harvest the fruits by collecting them from the ground, by hand. Fruits can be made to fall from branches by shaking them or, if the branches are not accessible, by throwing a piece of wood and hitting the branches. The fruits are tightly stuffed into previously cleaned bottles with water and are sold this way.

All this discussion is creating a craving for nance ice cream. I’ve heard it described as tasting walnut-flavored, but I don’t taste it that way. Nance does add a richness to the taste, though. Now I’m going to have to wait until August when nance ice cream will reappear in the stores!