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!