Les fuilles mortes se remassent à la pelle

Tu vois, je n?ai pas oublié?.

Lyrics Jacques Prévert/Song Joseph Kosma

You more likely know this song as Autumn Leaves with English words by Johnny Mercer. ?The autumn leaves drift past my window, the falling leaves of green and gold?.? It speaks of the beginning of winter as an extended analogy for depression related to a lost love. But the words en français are more appropriate for our picture. The literal translation is ?The dead leaves are picked up by (the) shovel(full)?.

The dead leaves/les fuilles mortes

As you can see, my small patch of lawn is covered by fallen leaves in a picture otherwise redolent of spring. Falling leaves in most of Costa Rica are a sign of spring, or rather the onset of the ?dry? season, and later are also a sign of the onset of the wet season. How strange after years living in Michigan. Falling leaves mean we are about to see a break-out of the year?s largest concentration of pollinators; bees, butterflies, flies, beetles and migrant hummingbirds! Which brings me to the question: why do trees lose their leaves? Leaves are expensive to build in terms of energy use. Why would trees toss them away?

Leaves are something of a high energy factory. They trap photons of light to drive a chemical reaction which consists of reduction of organic carbon. Don?t let the complicated words worry you. If you burn a piece of coal or a pile of sugar, you are oxidizing the carbon to create carbon monoxide or carbon dioxide. You get heat because this process releases energy. Reduction is just the reverse process. It turns carbon dioxide into, eventually, sugar. The reduction process absorbs energy rather than releasing energy. This is the plant equivalent of an arc furnace, using the sun as its energy source. Over time, a human factory may have external damage from fire, lightning, storms, rust, broken windows and such: for a leaf, the damage comes from insects, falling branches, and the growth of epifolia, among others. An epifolium is the leaf equivalent of an epiphyte. The equipment within a factory will wear down and wear out due to the nature of its use. Analogously, the materials within the chloroplast undergo damage from UV radiation, heat, and the chemicals that perform the work of the factory. Photons make free radicals that damage the equipment. For this reason, the maximum useful lifetime of any leaf is around four years. You can repair the structure and equipment of a factory for a while; however, eventually it is more efficient to build a new factory. A plant must eliminate inefficient leaves and build new leaves for the same reason. It will otherwise lose the competition for resources to more efficient plants.

Epifolia, including fungi, lichen, and moss.

Coffee plant branch with older leaves toward top (3+ years old) and younger leaves toward bottom (recent) illustrating progression of leaf damage.

Reasons for the timing of leaf loss vary by ecosystem. Temperate climates have four seasons. These usually include either one that is blisteringly hot or one that is uncomfortably to shockingly cold. Sometimes they have both. I leave it to someone smarter to justify the term temperate, meaning mild, for these types of climates. Temperate trees come in two general types, deciduous and evergreen. Deciduous trees lose their leaves (mostly) in the fall. Evergreen trees keep their leaves year-around. They will shed inner leaves whenever they get shaded out by growth of the tree. They also discard leaves whenever the cost of maintaining the leaves exceeds their contribution in production of sugars. This may or may not occur on some regular seasonal schedule. 

Winter scene from Kleinstuck Preserve in Kalamazoo Michigan. Note how ice coating multiple crabapple fruits (center back) weigh down the branches compared to the trees with bare branches (all of photo periphery).

Deciduous trees lose their leaves in fall to conserve water. Photosynthesis requires water. Water is also needed to cool the trees through transpiration. This is similar to us cooling ourselves by sweating. In the winter, water freezes. If transpiration continued after the water froze, the tree would dry out and die. So deciduous trees in temperate climates shed their leaves to avoid dying. Deciduous trees also shed leaves to prevent damage to the tree from wind and ice. If you have seen a very early autumn or very late spring ice storm, you will remember the leaves covered with ice. This markedly increases the weight of the branches and causes them to break. Some deciduous trees will keep some of their leaves. This is especially true of small, young trees. Larger trees may retain leaves on the ends of branches, especially lower branches. This is thought to protect delicate limb and leaf buds by deterring browsing by herbivores. These leaves are dead, but the tree does not completely seal the leaf stem. The extra support keeps the leaves from falling. 

The leaves of deciduous trees are said to turn colors to yellow, orange, red or even purple. In truth, these colors are always present in the leaf. The green color of chlorophyll predominantly masks the other colors. The other colors determine the shade of green that the leaf will display. The tree removes as much useful material from the leaf as is feasible before cutting off supplies to the leaf (termed abscission). The underlying colors from pigments, carotenoids and anthocyanins, is then revealed. The chemical decomposition of chlorophyll allows the plant to recycle precious nitrogen for other uses. The protein in the leaves is also largely recycled to make storage proteins for seeds. 

Fall foliage in a field in central Michigan.

Evergreen trees in temperate climates have small, thick, or scale-like leaves with fewer stomata. Stomata are the valves that let out water during transpiration. They are usually located on the under surface of the leaf. Reduced leaf surface areas and the reduced stomata allow the trees to avoid losing their investment in their leaves by shedding them in winter. The disadvantage is slower growth. The thicker leaves require more chlorophyll per leaf and the reduced stomata mean that transpiration is the rate limiting step in sugar formation. An evergreen tree usually must grow for years in the shade of deciduous trees before reaching canopy height. This is why many forests slowly transition from deciduous to evergreen forests over time. The ability to grow in low light levels and at slower rates also explains why forests in the far north and far south tend to be entirely evergreen forests. When leaves are finally discarded because of shading, which can happen in any season, the chlorophyll and protein are also broken down and repurposed.

Evergreen forest in Yellowstone National Park, USA. Temperature is 40 degrees below zero. At this temperature, Celsius and Fahrenheit readings are identical

In a dry tropical forest, such as the forests of Palo Verde in Guanacaste, most trees are truly deciduous. They lose their leaves for the entire dry season because it is truly a dry season. The water is not tied up because it is frozen: there simply is no water to be had. Transpiration in this setting would dry out and kill the tree. The tree must therefore stop photosynthesis and sacrifice its leaves early in the dry season. If you visit the beaches of the Guanacaste coast during the dry season, you will see multiple cacti beneath leafless trees all along the brown hillsides. The cacti have reduced their leaves to needles to reduce transpiration. The entire plant is green because it contains chlorophyll throughout. Every part of the plant contributes to photosynthesis. As in a temperate forest, the evergreen plants are growth limited because their growth is limited by their rate of transpiration. During the wet season, these cacti are predominantly hidden by the leaves of deciduous trees. Although the ecology is very different, the mechanisms of water conservation are the same in temperate and dry tropical forests. Only the timing of leaf fall is different.

Walk through a tropical moist forest, cloud forest or rain forest: you will always find a layer of fallen leaves on the ground. Water is not a problem here. The problem in these forests is access to light. This becomes the rate limiting step in growth. Most understory plants are evergreen. They depend on dappled light. A given leaf may have only a few seconds or minutes during the day with sufficient light for photosynthesis. Defective or inefficient leaves must be replaced promptly. Therefore, understory plants replace leaves continuously. New leaves are usually created without chlorophyll. This is to prevent loss of an essential commodity in case the new, tender leaves are eaten by browsers such as monkeys. For this reason, bright red, orange and purple leaves in the tropics tend to be newborn leaves instead of dying leaves. These colors may also advertise the presence of compounds in the leaves that are noxious to browsers.

Red leaves newly unfurled on Cas plant, Nuevo Arenal. Note that the one fully green leaf already is burdened by the egg case of a Lantern Fly, the so-called Peanut-headed bug.

Canopy trees have a different problem. Access to light in the canopy is best for the tallest. This creates an intense competition for growth when growth conditions are optimal. Inches count. In the rainy season, clouds obscure a large amount of sunlight. In the dry season, sunlight is much more constant and intense. Canopy trees will therefore replace their leaves during the rainy season, when the transition costs them less productivity. The leaves must be mature and efficient when sunlight is maximum to allow a canopy tree to remain in the race for light. Some trees will simultaneously lose and replace leaves in this period, changing little in appearance. Others will use leaf drop as an opportunity to achieve other purposes. They may drop all leaves at once to maximize visibility of their fruit to birds that spread their seeds. They may drop them during flowering to increase their visibility to pollinators in the great green sea of the canopy or to increase the efficiency of wind pollination. For this and similar reasons, some canopy trees will drop all of their leaves in the early spring and replace them almost immediately. This type of tree is called semi-deciduous.

Plot of rainfall, radiation and leaf area for trees in the Amazon Basin, based on data from Terra MODIS sensor on a NASA satellite. See earthobservatory.nasa.gov 31 May, 2007 for details of the study.

Nuevo Arenal, while politically part of Guanacaste, is ecologically part of Alehuela. The forests are predominantly tropical wet forests. As you increase in elevation, they become cloud forest. As one slides down from the top toward the Caribbean coast, one encounters some true rain forest. In these areas, falling leaves in early spring are quite common. This brings me to the tree that is covering my small lawn with leaves. It is a canopy tree with bright yellow flowers and small oval seeds embedded in a several-centimeter diameter, oval, paper-like structure called a samara. The leaves fall, the flowers bloom, and the seeds are formed and dropped in around 3 weeks, at or near the start of the dry season. The tree does the same at the beginning of the wet season, presumably because this provides advantages to its seeds. The flowering is so intense that the entire tree looks and sounds like one giant beehive. The extent of flowering and seed formation varies extremely. Why? That is a story for another random walk.

Sangrillo or Bloodwood tree (Pterocarpus officinalis) in my back yard at the time of maximum leaf fall, 17 March, 2023. The central green is predominantly from epiphytes. Some leaves on central and lower branches have yellowed but not yet fallen.  

The same tree from a slightly different angle on 23 March, 2023. Note the rapid replacement of leaves, which usually requires only about 10 to 12 days for completion.