So I was curious. It’s certainly not the Salvia in the mint family that we know from the Northern Hemisphere, but the leaves are a little reminiscent of sage to the touch. What plant is it, then?

After I saw the one that fell, I learned to recognize the mature tree. It has a trunk that reminds me of one of my favorite savanna trees – Miconia rubiginosa.

Having the images here side by side, I see that the trunks aren’t all that similar. The grooves of the “salvia” on the left are lighter in color and straighter than those of the Miconia on the right. Nevertheless, the bark in both cases is deeply grooved and the trunks of the mature trees are roughly similar diameters.

In an abandoned pasture, the “salvia” is likely one of the tallest of the trees there, but if it’s growing along side a quebrada (a spring, sometimes seasonal) it’s just one among other tall ones. When in bloom, it’s possible to see that the flowers are white, but that’s about all you can tell from the ground. The flowers are high up in the crown of the tree, impossible to reach.

I decided my only hope of seeing a flower close-up would be to wait until our sapling matured enough to bloom, with luck while it was still short enough for me to reach the flowers.

This year, 21 months after the first picture of the sapling was taken, it looked like this, and the top is out of sight – threaded into the nance tree (Byrsonima crassifolia) the sapling is beneath.

Quite a rapid growth, in my opinion, but not rapidly enough to produce flowers. Nice leaves, though, don’t you think?

Fortunately, another young tree nearby did have some flowers:

Enough to confirm my suspicion that this tree belongs to the aster family (Asteraceae).

I admit I groaned a little. The Asteraceae family is huge and its flowers are tiny. The flowers are so special they have their own terminology. The terms I’m about to use were gleaned from a wonderfully helpful page on the Flowers of Asteraceae at PlantzAfrica. What is shown in the image above is a cluster of synflorescences. One of them needs to be broken down a little so you can see what that means.

Here is a single synflorescence on the left. From it I plucked a head, or in Asteraceae terminology, a capitulum, which is next to the ruler. The capitulum is a small inflorescence or flower cluster. In this case, the cluster of flowers consists of two tiny flowers called florets. So a synflorescence is, you might say, an inflorescence of inflorescences. Perhaps a “compound inflorescence” would be a better term.

There’s a great deal more to the terminology, but this is enough to confirm that the plant belongs to the Asteraceae family. If I had to identify this plant based on a dissection of these tiny heads, I’d be in trouble. But I figured I could possibly get around the problem by holding in my head that this plant is a tree.

There aren’t that many trees in the Asteraceae family.

So I took a deep breath and pulled out my copy of Gentry’s field guide and plowed into Asteraceae. He listed 19 genera that contained trees, and I was preparing myself to read up on all 19 when I came across this description for the genus Pollalesta on page 349:

Medium-sized second growth and savanna trees with corymbose panicles. Leaves densely gray-pubescent below.

The accompanying illustration was promising. And yes, this is second growth in these pastures of the savanna.

Botanical side note: [definitions from the New Oxford American Dictionary]

• corymb = a flower cluster whose lower stalks are proportionally longer so that the flowers form a flat or slightly convex head. corymbose = adjective.
• panicle = a loose, branching cluster of flowers, as in oats.
• pubescent = covered with short soft hair; downy

This is what it’s often like for me – look up every third word or so in a description! Some of the terms are beginning to stick, but I despair of ever knowing them all. I do use a reference other than a standard dictionary, though. The book by Harris and Harris gives real botanical definitions and illustrates every term. It’s wonderful, but their definitions are a little too complex for my posts.

Next, I checked the Tree Atlas of Panama, and yes, trees of the genus Pollalesta are found here.

Next reference to check was Flora of Panama: Compositae [Compositae is the former name of the Asteraceae family]. Ah, success! Pollalesta is in Panama and it is represented by only one species: P. discolor. It’s enough to say that the description of the head (capitulum) fits and that there are two tiny flowers (florets) in the capitulum.

So, Pollalesta discolor it is. But wait. What’s this about Piptocoma discolor, the name in the title of this post?

Well, as happens fairly often in the plant identification world, a qualified botanist decided that Pollalesta was not really a separate genus but, on the traditional basis of the characteristics of the plants, should really be lumped together with the genus called Piptocoma. The only species in Costa Rica and Panama affected by this decision was Pollalesta discolor now called Piptocoma discolor. [Pruski] The complete official name now is Piptocoma discolor (Kunth) Pruski.

What is known about this tree, this Piptocoma discolor?
For one thing, it is found in abandoned pastures in Panama and Costa Rica, in dry to moist areas. It flowers in January and April and in November and December; it fruits in June, July, and December. [INBio]

It occurs from Costa Rica south into northwestern South America

In the province of Herrera here in Panama, it has the common name of negro [Spanish for "black"]. I suspect that name comes from the color of the inside of the bark, which is extremely dark. The Miconia rubiginosa tree that I mentioned earlier is known locally as canela negra, also, presumably because of the dark interior of the bark. In Herrera, at least, the Piptocoma is recognized as having some value in the pasture and so is not removed. It is said that its bark, pounded and mixed with water, gives a drink against snake bite. Don’t use that remedy on my say-so, though!

That’s about all I’ve been able to find about it. Not a popular tree, I suppose, except perhaps with our former gardener, Dario, and with me. I probably like it because Dario did, but you must admit the leaves are pretty, the bark is interesting, and it makes a nice tall tree in the pasture. I’ll probably never learn why Dario called it salvia silvestre, though.

Back in May, I noted the early blooming of Miconia rubiginosa after the heavy rains in April.

Now, in July, I’ve seen two plants in bloom that I normally see bloom in August: a tree with large bunches of yellow flowers, Senna hayesiana, and a miniature relative of the Elephant Ear herb, Xanthosoma mexicanum. I’m also seeing flower buds on an unidentified Miconia that usually blooms in September. I’m sure more’s going on along this line than I’m seeing.

Maybe these large swings in time of blooming are relatively normal for the tropics. But I’m starting to think that it’s more than coincidence that different plants from three unrelated families (Senna = Fabaceae or bean family, Xanthosoma = Araceae or arum family, Miconia = Melastomataceae or “black mouth” family)  are blooming immediately after uncommonly large monthly rainfalls.

When it blooms, in June as I said, the whole grove lights up.

But this year it bloomed in early May, nearly six weeks earlier than usual. (Sorry, but my pictures from that day are horrible. The blooms, though, looked like these, above and below, taken in June last year.)

Why such an early bloom?
Plants may not bloom at exactly the same time every year, but it struck me that this year’s early blooms might possibly have something to do with the heavy rains in April.

It has been argued that either moisture availability or sunlight could limit photosynthesis in trees and thus limit the time of year when there is enough energy in the plant available to produce flowers, fruits, and seeds. A study by Zimmerman and colleagues found that, on a community level in tropical forests, sunlight was more important than moisture in triggering the blooms. But at a species level, things were different. Some species found in both seasonal and nonseasonal forests bloomed at roughly the same time of year whereas other species did not. Overall, most species in both places produced flowers, fruits, and seeds when irradiance was highest, but there were some that bloomed when less light and more moisture were available.

My take-away point from this study is that, in general, sunlight provides the energy necessary for a plant to start the reproductive process. However, different species may use different levels of sunlight, whether determined by day length or cloud cover, as a trigger to set flowering in motion. So, does M. rubiginosa look for June day length or June light level to determine when it will bloom?

We are close to the equator. Day length does change throughout the year, but by relatively little. So, for instance, the total day length for the first week in May is about 12 hours and 48 minutes (calculated at the sunrisesunset). The total day length for the first full week in June is about 13 hours. The average difference in day length, then, from early May to early June is 12 minutes. A relatively small difference. Perhaps the flower-triggering clock in M. rubiginosa was a little sloppy this year.

Maybe it’s not so early
However, it is also true that the heavy rains in April caused a reduction in light level for the month. Enough to bring it down nearly to the level that it was in May of 2009. If light level, rather than day length is the trigger, maybe the clock was not so sloppy. Could it be that April’s low light level triggered an early May bloom?

I’ve found no literature on when M. rubiginosa usually blooms, and probably many conditions must be met for the tree to bloom. Maybe it’s just a coincidence that we had much more rain than usual, and much less light than usual, in April of this year and that M. rubiginosa bloomed in early May.

Or perhaps, after all, the clock was simply off. Hey, it happens in this house all the time.

When I first thought about identifying this tree, I zoomed in on those leaves. The color of their undersides reminded me of many of the Miconia leaves around here. At least one species in Panama is called dos caras – named for the “two faces” of the leaves.

But a quick look shows that the sangrillo (left image below) lacks the longitudinal veins of the Miconia (right image below).

      

Since those longitudinal veins are characteristic of the entire family to which Miconia belongs – the Melastomataceae – and the sangrillo doesn’t have them, then I thought I’d better start the identification from scratch.

One of the most valuable parts of the book by Alwyn Gentry (A Field Guide to the Families and Genera of Woody Plants of Northwest South America …) is that he starts his keys with leaf arrangement, something quite easy to determine. In the case of sangrillo you can see the leaf arrangement in the image on the left, above – the leaves are opposite and they are also simple.

When I looked at Gentry’s book, I saw that if I could check just one more characteristic – the presence of latex – then I could really narrow down the families under consideration. So I tore off a leaf, looked at the leaf stalk, and knew immediately where the tree got its common name. There was a small amount of reddish-orange latex oozing from the stalk. That “little red” could have two meanings: there was not much of it, and it was more orange than red, so it was only “a little red.” I could wipe it onto my finger and see its color but I was completely unable to get a reasonable picture of it with my point-and-shoot camera. I had what I needed, though. I went back to Gentry’s key and found that trees in our area having these three characteristics:

• simple leaves
• opposite leaves
• presence of latex

fall into one of two families: 1) the dogbane or Apocynaceae family and 2) the clusia or Clusiaceae family.

Sangrillo belongs to the Clusiaceae family

I knew immediately to which family the sangrillo belonged – but it surprised me. It was not the Apocynaceae family, which tends to have large, beautiful flowers such as Plumeria, and their reproductive parts – the male stamens and the female pistils – are usually hidden by their petals. So it had to be that sangrillo belongs to the Clusiaceae family.

Why the surprise? Because we have many trees of the Clusia genus here, and their leaves are so smooth and leathery that I tended to generalize that feature onto the entire family. To do so is wrong, of course. The Clusiaceae family consists of about 37 genera and more than 1600 species. It includes, besides Clusia, St. John’s wort (the entire genus of Hypericum) and the delicious mangosteen (Garcinia mangostana) (wikipedia).

Once I knew the family, I bypassed keys and went straight for pictures. The book Trees and Shrubs of Panama, by Carrasquilla, is arranged by family, so I opened that book to the Clusiaceae and immediately found pictures of a Vismia latisepala that looked good, but I couldn’t see the details well enough to peg it.

So I turned to the Tree Atlas of Panama where I found three other Vismia species listed. What images that were there were good, but without supporting text, I could not be confident of my identification based on them. 

At last, I found a key for the Vismia of Panama. It was in the Annals of the Missouri Botanical Garden (Vol 65, 1978) and it was written at a time when Vismia was considered to belong to a different family, the Hypericaceae.* Even the species name I came up with is no longer used, but the plant is still the same plant, and by tracking down synonyms, I was able to give a name to the sangrillo.

The features to look for, then, are:

Leaf blade usually < 20 cm long; base wedge-shaped or rounded

The length of the sangrillo leaves that I measured ranged from 11 to 17 cm, none even approached 20 cm. This feature immediately distinguishes the sangrillo from one other species of Vismia that is in Panama – Vismia macrophylla, which has leaf blades generally much longer than 20 cm.

Leaf underside lightly covered with short, soft, matted wooly hairs.

I simply could not photograph this feature, but from the description you can guess what short, soft, matted wooly hairs should feel like – and it does.

Outer sepals oblong to elliptic or lanceolate

Here is the sangrillo flower. I’ve never seen those globular buds open much further than this. The sepals are the outer covering for this flower and they have separated enough to show the petals and a little tuft, which I’ll come to shortly. These sepals are lance-shaped rather than oval-shaped, thus separating  sangrillo in the key from the species Vismia japurensis (and also from Vismia latisepala, the species I saw pictured in Carrasquilla’s book).

That tuft?

That tuft is part of the shaggy-haired lining of the inner surface of the petals. In the following image of the flower from Discover Life, the flower has been opened to reveal the shaggy lining of the petals and also the reproductive parts of the flower. There are five (female) pistils. The (male) stamens, in five tight bundles or fascicles, lie just inside the petals.

Sepals spread and may bend backward in fruit

This last feature helps separate sangrillo from the species Vismia guianensis, in which the sepals are pressed against the fruit.

The result: Vismia baccifera.

Vismia baccifera has been known by six other names. The name in the key I used was Vismia panamensis, which certainly suggests that it is a common tree here. The origin of the name Vismia has eluded me, but the (current) species name, baccifera, means “bearing berries,” which of course is true but which also would apply to any other member of the genus Vismia. I should also point out that the common name sangrillo, even though I’ve been using it as if it referred only to the one species, is in practice applied to most if not all the Panamanian species of Vismia.

Vismia baccifera is distributed throughout Central and Northern South America.

In Costa Rica, Vismia baccifera prefers open areas and occasionally proliferates rapidly, accelerating the natural recovery of a scrubland or secondary forest. The seeds are dispersed mainly by bats (Zamora et al.).

Once I read about the involvement of bats in the ecology of the tree, I went back and looked at it to see how easy it would be for bats to find these little fruits. Remember the position of the flowers in the bat-pollinated balsa tree? Well, here in Vismia baccifera, at least some of the fruits are sticking right up there at the very tops of the trees where bats can find them. A click to enlarge the image will make this clear.

Since the fruits are all over the tree, though, and not just at the tops of the crowns, it is likely that birds or other animals partake of the fruits as well. Although I have not found literature on birds eating the fruits of Vismia baccifera, birds are often seen in association with the trees. The top five birds noted are (ZipCode Zoo):

• a kingbird (Tyrannus melancholicus)
• Blue-gray Tanager (Thraupis episcopus)
• Turkey Vulture (Cathartes aura)
• Black Vulture (Coragyps atratus)
• Clay-colored Thrush (robin) (Turdus grayi)

All are listed in A Guide to the Birds of Panama except for the particular species of kingbird, Tyrannus melancholicus. I would guess that the vultures use the sangrillo as a perch, not as a food source, but Blue-gray Tanagers and Clay-colored Thrush are definitely fruit eaters. 

In summary, then, this particular sangrillo is Vismia baccifera because it has

• leaves shorter than 20 cm
• short, wooly, matted hairs on its leaf undersides
• lance-shaped sepals that bend backward in fruit

One of its distinctive features is the shaggy-haired lining of the inner surface of its petals.

It’s in the Clusiaceae family because it has

• simple, opposite leaves
• latex

That little red latex is what gives sangrillo its common name in Panama. Bats are important in dispersing its seeds but fruit-eating birds are seen with it as well.

All in all, a fine small tree for a neotropical savanna.

_______________

*At the time of the publication of the key, the family Hypericaceae had only two genera in Panama: Vismia and Hypericum. The Vismia was distinguished from the Hypericum by having a petiole whereas Hypericum does not. (There were other differences, too, but this would have been the easy-to-spot difference I would have chosen.)

The wind damage is compounded by the contrast between the conditions at the end of the rainy season and conditions two months into the dry season. Before I go into that, I should explain the sticks leaning against the banana plants. These are crutches put there to support the plant once a bunch of bananas develops. If the plant is not supported, the bananas will pull the plant down. The bag is placed over the ripening bananas in an attempt to keep the birds away (Black-chested Jays are especially noisy and destructive).

Okay, here’s the damage report. (Click on image to enlarge.)

In the upper left corner is a leaf-less tree (deciduous in the dry season) that I have not yet identified. A large limb split off it and fell onto an already fallen Miconia and onto one of the banana plants. The Miconia has been mostly cleared away, but it also damaged banana plants and for awhile provided a highway for squirrels to get to the ripening bananas under the bag. The Miconia also pretty much demolished the fern stand. As noted, banana leaves normally take on a shredded appearance in the dry (and normally windy) season. They are designed to do that without causing damage to their vascular systems.

And here’s the comparison between the seasons.

At the end of the rainy season the grass is green; in February it is brown. The banana leaves are fairly intact in the rainy season and are shredded in the dry season.

The bijao (the large-leafed plant at the extreme right of both images, Calathea lutea) is bright green in the rainy season, and not all of the leaves are erect. In the dry season, the leaves are grayish, shredded like the bananas, and nearly all are completely erect to expose themselves to less sun than if they were more parallel to the ground (this movement of the leaves is a characteristic of this groups of plants and for this reason they are known as “prayer” plants).

Between the bijao and the bananas is an Eugenia biflora, which has just finished blooming and so looks a little paler than it normally would because it is studded with pale green developing berries.

All in all, with the wind damage and the normal drying impact, the pleasant little area looks pretty devastated. However, it’s a miniature ecological event as well. At the edge of our tiny forest, a new “gap” has been created. It will be interesting to watch as it responds to this new situation.

We’ve been through four days so far of high winds, and they haven’t stopped yet, although they lessened for a few hours this morning. Today for the first time we started seeing small trees felled. They are mostly this Miconia rubiginosa, called canillo, and a tree in the Aster family called palo blanco. Both are fairly brittle, and palo blanco branches break off the tree at the slightest excuse. But the Miconia rubininosa is one of my favorite savanna trees – partly because they make nice groves in our area, partly because they are so pretty when they bloom, and partly because it was one of the first trees that I sought professional botanical help to identify.

The winds were caused by a plunge of the North American Jet Stream down into the Caribbean, wreaking all sorts of havoc for us in Central America, and worse for our Caribbean friends. If you really have nothing better to do for the next few minutes, you can watch the wind play with our blurry nance trees topped by cecropia trees and our sigua tree (which is about to bloom). The fenced area is the dog run, the yellow building is the bodega (storage shed), and the clothes lines are the clothes lines. The brown grass shows you just how dry this dry season is.

P.S. Berry Go Round # 13, a carnival of blogs about plants, is up, with some seriously interesting posts.

Others have not been so remiss, and I’d like to point you to the current Berry Go Round – Issue #12 – that’s being hosted at Foothills Fancies. Plenty of good reading about plants, nicely put together.

Way back at Thanksgiving this year, the Trees, Climate and People blog posted their botanical guide to Thanksgiving dinner. It’s a list of 42 plants with their common names, their scientific names and families, and the dish(es) they appeared in. I missed it at the time, but I love the concept and am passing it along in case you’re interested in compiling your own botanical guide to any holiday meal.

John Bennett sent a message about the tree known in Panama as algarrobo. There’s a town named Algarrobos on the road that goes from Dolega to David, and I’ve been meaning to stop there and look around, assuming I’ll find some of those trees. They’re probably also here in Potrerillos, but I thought my chances of actually encountering one would be better in a town named after them.

Here’s what John had to say back on December 8, 2008:

“Dry season or our winter is almost here, time to shed leaves and prepare for the dry spell. Unless you happen to be an “algarroba” tree. Or Hymenaea courbaril L. – Family: Fabaceae-caesalpinioideae – Sinonims: Nombres Comunes: Algarrobo, algarroba, guapinol, in which case you simply change to dry season leaves, which I suppose have less stomata and therefore loose less H2O.

“As you can see, the leaves are turning brown and soon they will all rain down on us and the tree will bring forth new green leaves, as if it were spring. In spring it will do the same.”

The Wikipedia article refers to the tree as Jatobá. Thanks, again, John, for your contribution and your interest.

Another project that has been swallowing a lot of time is the installation of a weather station and the putting together of a web site to publish the data. The installation went smoothly, but right now I’m waiting for an adapter that will let me plug the radio receiver into the USB port on my computer. While waiting, I’ve been fiddling with the web site, so I hope that by the time the part arrives, I’ll be able to get the weather data online very quickly.

So, on this last day, may I wish you a prosperous and peaceful New Year, and may we all be just as productive as we’d like to be!

The leaves were striking as well – large and glossy on the top side, somewhat folded, with 3 strong veins running lengthwise as well as smaller veins running horizontally across the leaves.

But with so many plants to learn, so many things to do, it was some time before I got around to looking closely at this particular interesting tree.

Black Mouth Family

Early on, though, I learned that those strong longitudinal veins are a good indicator of the Melastomataceae family, whose leaves are

• opposite
• simple
• with alternating pairs decussate (at right angles along the stem),

and whose leaves have

• 1-4 pairs of longitudinal veins.

Melastomata comes from mela = black and stomata = mouth, so this is the “black mouth” family. You get a black mouth when you eat the ripe berries! Which, by the way, are quite good, as the birds well know. Two-thirds of the plants in this family are in the New World tropics (wikipedia). In Tree Atlas of Panama, 62 species are listed from the Melastomataceae family, 31 of them in the genus Miconia.

Gentry makes identification within the large Melastomataceae family a little easier by first separating those genera that have capsular fruits from those with berry fruits. Obviously, our plant has berries. Of the various features that sort out other groups, the two that applies to this tree are

• a flower cluster in the form of a panicle (loose and branching, as in oats) at the ends of the stems

• medium to small flowers with broad obtuse petals (the petals are at a greater than 90-degree angle).

These flowers are small, certainly, with the petals folded back at a great angle. Note that both the stamens (pollen-bearing male parts) and the petals are on the rim of the flower, which is a characteristic of the entire family, not just of the genus. The pistil (the female part) emerges from the center of the flower.

The Miconia Genus

These characters – the berries, the panicle flower cluster and the small flowers with petals at a great angle – place our tree in the genus Miconia, one of the largest neotropical plant genera.

The genus Miconia was named for Francisco Mino (or Miño?), 16th century Spanish physician and botanist (botanary). I haven’t been able to find anything on the internet about this botanist – a Google search turns up a contemporary business man in Facebook and Linkedin, which is fun but not to the point.

Mule’s Ear Tree

Two trees from the Melastomataceae family, both in the Miconia genus, are listed in Carrasquilla’s Trees and Shrubs of Panama:

• Miconia argentea, sometimes called dos caras, or two faces for the brown undersides of the leaves, and
• Miconia impetiolaris, also sometimes called dos caras but also called oreja de mula, or mule’s ear.

This tree is M. impetiolaris – the mule’s ear -, but since the common names refer to the leaves, let’s take a closer look at them.

They are certainly long enough – 30 cm in the sample below – to call a mule’s ear.

And the underside is certainly brown enough to give the leaves a “two-faced” appearance.

Further, the scientific name is based on a feature of the leaves. The petiole is the leaf stalk that attaches the leaf to the stem. If we look closely at the attachment site we see the end of the vein but no leaf stalk.

To drive the point home, here are leaves of four other Miconia plants compared with a leaf from M. impetiolaris on the right. The petioles in all four are pretty clear, and when the leaves are turned over to show their undersides, even the shortest petioles are distinct. (Click on either image for a larger view – and while you’re at it, take a moment to appreciate those incredible longitudinal veins!)

The main vein of the leaf of M. impetiolaris, then, attaches directly to the stem. There is no obvious leaf stalk, or petiole. When we take the species name and break it down we find im- meaning “less” and -petiolaris meaning “remarkable leaf stalk” (botanary).

So the scientific name, Miconia impetiolaris, means “the Miconia without a remarkable leaf stalk.”

A Neotropical Native

The mule’s ear Miconia is native to Central America and parts of South America.

Because of the spectacular leaves, flowers, and berries, this tree would make a fine ornamental. One of our neighbors has done so, letting it grow in a row with carefully tended other native trees. The Miconia impetiolaris is circled and that’s a pruned native Clusia to the right. From the flowers and berries you’ve seen here, you can only imagine what a nice spectacle this tree makes when it is either in flower or in fruit.

I love it when native plants are incorporated so nicely into a beautiful setting.

If you want to go immediately to posts featuring a specific plant family, click on the family name in the following list. Otherwise, use the Description, Identification, and Example notes as review – these notes are very simple and would be difficult if not misleading to use as a standalone identification tool.

You can download a simple plant profile to take notes in the field. A key to nine of these families, using vegetative characteristics only, can be downloaded here. For a list of books that are useful in plant identification, please click on the References tab.

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!