Large buttressed tree and people for comparison. Large trees are only a small percentage of the lowland rainforest. Most trees here are relatively slender. Photo credit: Darwin Moscoso.
The structure and composition of Tropical Forest formations on the lowlands and altitudinal gradient of the Andes are mainly attributed to a combination of non-seasonal environment and changes in temperature influencing regional weather patterns.
The Amazon rainforest climate is typically hot and humid throughout the year, with an average annual temperature of 27°C (80.7°F). There is not a summer or winter season as the mean temperatures range by about 2°C throughout the year. In fact, the difference between day and night temperature (2 to 5°C) is greater than the difference between any two seasons.
It rains throughout the year. There is a rainy season (60-180 inches of rain) and a not-so rainy season (30-100 inches of rain), with rain showers occurring just about any time.
Plenty of sunlight and moisture from rainfall result in high plant productivity. Most of the lowland rainforest is tall, with most trees characterized by large crowns. Some trees are emergent and have massive trunks with buttressed roots.
View of the canopy and emerging trees of a tropical lowland Rainforest in Acre Brazil. Photo Credit: Ricardo Placido.
Tropical Forest Formations and Temperature
Cooler temperatures begin to occur at higher elevations along the east slope of the Andes. As the atmospheric pressure decreases, the air expands and cools at the adiabatic lapse rate. In humid tropical mountains, the lapse rate is roughly 0.55°C for every 100-meter increase in elevation. Accordingly, mean annual temperature at the equator drops below freezing at approximately 5000 meters, a level that coincides closely with the lower limit of permanent glaciers in the Andes.
Changes in temperature have subtle influences on regional weather and the availability of sunlight. The intense sun in tropical ecosystems warms the moisture-loaded air in the lowlands causing it to rise gently toward the foothill of the Andes. As the air rises, it cools, and eventually reaches the dew point. At this level, a flat bottomed layer of clouds hangs over the mountain slope regularly from mid-morning to late afternoon. Since tropical weather systems are often driv¬en by convection, rather than by large scale wind systems such as the jet stream, such cloud banks tend to form predictably and daily at the same elevation.
Ascending clouds from the lowlands shroud the canopy of montane forest daily in a predictable sequence. Photo Credit: Nestor Ccacya.
Cloud Forest Formations
These unobtrusive weather processes produce a sharp discontinuity in the vegetation on the east slope of the Andes. Below the cloud bank, the climate is somewhat cooler and wetter, but overall similar to that of the lowlands. In the clouds, the climate is one of almost perpetual gloom and dampness. As dense mists drift through the canopy, moisture condenses on every surface, producing a constant slow drip. The damp trunks and branches nurture mosses, ferns, and uniquely exotic appearance, frequently enhanced by the presence of bamboo and umbrella-like tree ferns.
Branches covered in lichens are typical in cloud forest formations along the Andes. Photo Credit: Alfredo Begazo
The drastic reduction in solar energy resulting from the presence of cloud banks markedly reduces plant productivity. Cloud forests consequently grow slowly and do not attain the stature of lowland forests. Although beautiful to see in the rare moments of sun shine, cloud forests tend to harbor few primates and other conspicuous mammals. However, they often do support an array of exotic birds that feed on fruit and nectar.
Plenty of fruit produced in montane forest is consumed mostly by birds as mammals at this elevations are largely absent. Andean-Cock-of-the-Rock, Photo credit: Braulio Puma.
Higher Elevation, Cloud Cover, and Solar Energy
At higher upslope, the vegetation on many mountains again changes character. The trees become stunted and gnarled, and on exposed ridges they may be no taller than a person.
Their branches show multihued lichens, and their leaves are minute, stiff, end tightly packed around upright twigs. This is the elfin forest, the final effort of woody plants to scale the heights. Perpetual chill and gloom, coupled with fully saturated air, stifle transpiration, the evaporative loss of water from leaves. Reduced transpiration retards the upward flow of nutrient containing sap, and growth slows almost to a standstill. Spindly twisted trees only a few meters tall and less than 10 centimeters in diameter can be hundreds of years old.
At elevations of approximately 3500 meters, woody plants five give way to grasslands. Pockets of dwarf forest often occur in pockets or the transition between sheltered ravines and grasslands. Photo Credit: Nestor Ccacya.
Elfin forests have long been a subject of speculation among botanists, for they afford prime examples of the Massenerhebung effect. This term refers to the downslope displacement of vegetation zones on small, isolated mountains. On most tropical ranges elfin forests are typically met at elevations above 3000 meters. Botanists have thus been puzzled at finding physiognomically similar formations at only 1000 meters on isolated peaks.
It is clearly that temperature is not critical to producing the Massenerhebung effect in elfin forest at 1000 meter as temperature is not colder but the same as locations at the same elevation. The puzzle centers on whether the lowering of vegetation zones results from the suppression of photosynthetic activity under heavy cloud cover and high humidity, or from severe nutrient deprivation induced by the unrelenting precipitation and leaching (removal in solution) of mineral elements.
At elevations of about 4000 meters plant is reduced to highly adapted plant species that grow very close to the ground.
Elevation and limit of Woody Plants
Timberline occurs at the point where net photosynthesis can no longer support the cost of maintaining a woody superstructure. In the tropics, this always lies well below the snow line, coming at around 3500 meters in the tropical Andes, although a few woody plants can be found in sheltered ravines even above 4000 meters. At 4000 meters on the Equator, temperatures hover near or below freezing every night of the year, and plant growth is barely perceptible.
References:
This summary is largely based on:
John Terborgh’s “Diversity And The Tropical Rainforest”.