Tropical Dendrology developed by Dr. L. R. Holdridge

Dr. Leslie R. Holdridge developed a procedure to know the plants in the field, by means of simple, fresh characteristics of non reproductive organs (leaves, twigs, bark, odors, tastes, exudations, buttresses, etc) (1).  This way, it is possible to teach how to identify plants down to family or genera level (and, in some cases, down to species level) during a normal quarterly course (2) . After the course, students "are able to know" a high percentage of trees and shrubs found in the American tropics.

First of all, we should clarify the meaning of "to know" the trees and shrubs, as it is used in the precedent paragraph.  We should distinguish two levels of knowledge:  one is defined by the terms 'determination' or 'identification',  and the other, by the terms 'predetermination' or 'preidentification'.

Determine and predetermine plants

When we speak on "determining", we refer to the process of fixing a plant to a taxum (variety, species, genus, family, etc).  In general terms, in the process of determining it is assumed that the person has certain taxonomic knowledge and that he / she uses systematic procedures, both involving flowers and fruits.  As plant determinations usually involve the use of herbarium material, floras, manuals, microscopes, etc, specialists prefer to work at the lab or the herbarium and not in the field.  This way of doing things is particularly important the first time a species of a given site is determined.  Experienced Taxonomists and Dendrologists know that once samples are deposited in an herbarium - after being correctly determined - it will be possible for them to determine some plant populations without having access to their flowers or fruits, just on the basis of sterile samples of which their collection site is known. However, determining the great majority of plants involve flowers and fruits, as stated above.

When mention is made in this article of the word 'predetermining', reference is indeed made to the following facts: most of plant individuals within genera, families and orders show macroscopic characteristics in their non reproductive organs that, in many cases, are held by all or by a high percentage of the individuals within the taxum involved.  Because of this, experienced dendrologists and taxonomists are able to "predetermine" - at a glance - many plants, indicating one family or a small groups of families, even if specialists are dealing with a sterile sample.

In some cases, they can even go this way down to genera level and / or to species level. Normally, such skill comes after long practice, during which the person realizes the presence of outstanding characteristics, and he / she notes differences and perceives peculiarities in the species, genera, families and orders.  These characteristics are, then, integrated through a global pattern in the mind of the botanist, emerging the image of the taxa and enabling him / her to predetermine the plant sample he / she is dealing with.

In many cases, both 'predeterming' and 'determining' are simultaneous processes, depending on the experience of the specialist and on the taxa involved.  Anyway, it is obvious that this type of predetermination pave the way for accurate determination.  Such ability to predetermine plants is seen also in persons having little or no previous formal taxonomic training, but who are used to handle plants at the herbarium or in the field.

An efficient methodology to teach tropical dendrology

After comparing the exuberance of tropical flora with that of Maine and Michigan where he studied, Dr. L. R. Holdridge developed a system that integrates the above discussed knowledge, and that also enables the teaching of tropical plant "predetermination", as we defined such a term in this article.  Two methodological aspects will be briefly explained:  the sequence followed during a tropical dendrology course, and the series of family and genus short descriptions.

The sequence followed to teach different subjects is very important.  For example:  at the beginning of the course, students learn those groups which are very easily recognizable and common in the surroundings, such as the families Asteraceae, Melastomataceae, Piperaceae, Myrtaceae, and the genera Inga (Mimosaceae) and Ficus (Moraceae).  As the course advances, instructors select those groups of plants which are increasingly "difficult to be recognized" and rare or less common in the surroundings.  At the end of the course they teach those species which do not fit with the normal characteristics of its group (the exceptions). 

Three characteristics are always present in the series of descriptions assembled by Dr. Holdridge to typify genera and families, namely:  leaf class (simple, compound), leaf arrangement (opposite, alternate, etc.), and the presence and types of stipules.

Combining these three type of characteristics with others such as type and color of exudates, pellucide-punctate structures, leaf blade consistence, nectaries, odors, etc, there appear short descriptions which are very easy to be memorized, and which enables students to predetermine most of the plants.  As descriptions do not cover all the elements belonging to the taxa involved, it is obvious that exceptions will be found.  Directions are given during the course to appropriately manage such situations.

What to do with that knowledge?

It is obvious that in most cases it is not enough to know the family and genus, but the student must go down to species level and sometimes even go down to subspecies or to variety levels.  To do this, he / she should go to herbaria, use floras, field manuals or -simply- ask somebody who knows the local flora.  In every case, knowing the family or the genus will help tremendously in going down to genus or species level, and in memorizing species and variety names, uses, distribution, etc.

In the other hand, during the course students gain special skills enabling them to continue making progress in species identification on their own when working anywhere in the Tropics. For instance: students draw plant samples, and prepare a personal matrix in which he / she arranges most of the taxa (families and genera) according to their oustanding characterisctics. These devices allow the student to quickly define a few taxa that finally lead them to identify the samples he / she is working with during the course practices (4). They also will be the basis for the student to prepare a personal matrix specially designed for the site where he /she will work in the future.

This and other details are given in the "Tropical Dendrology" page of this Website. Also, previous students have given testimonials about the efficiency of such teaching devices which are included in the "Instructors and References" page (there are links at the bottom of this page).

The Tropical Dendrology course offered in Costa Rica

The Tropical Dendrology course offered in Costa Rica is devoted mainly to teach "predetermination" techniques (see this term above) and is given as an intensive short course (two weelks).

The course is addressed to professionals and lay persons in biology, forestry, biodiversity, birding and ornithology, ecology, agroforestry, ethnobotany, field guiding, and other areas in the natural resource field.  Also professional taxonomists, working both at the herbarium or in the countryside, have attended the course because they found themselves well prepared to identify plants in the herbaria, but they usually failed when they tried with fresh samples in the field. If you are interested, please go to the "Tropical Dendrology" page of this Website (there is a link at the bottom of this page).

References

1. HOLDRIDGE, L.R.  1972?  Dendrología práctica de los trópicos americanos.  s.n.t.  (San José, Costa Rica, Centro Científico Tropical, 32 p.)

2. JIMENEZ-SAA, H.  1969.  Un método para facilitar el aprendizaje de la dendrología tropical.  Turrialba (Costa Rica) vol.19, no.1, pp.109-116, 7 refs.

3. JIMENEZ-SAA, H.  1993.  Costa Rica's Tropical Science Center offers new method for quick field plant identification.  Diversity (USA) 9(1-2):32-33 1 ref.

4. BEAUVAIS, JEAN-FRANCOIS. 1997. L'approche parataxonomique en dendologie néotropicale du Centre Scientifique de San José au Costa Rica: la formation du Dr. Humberto Jiménez Saa. Bulletin de la Société Botanique du Centre-Ouest. Nouvelle Série. Tome 28; pag. 170-176

At least four characteristics are normally attached to the idea of the Tropics as it is known by most of people in temperate zones: hot temperature, heavy rainfall, tall intermingle forests, abundant large and/or poisonous animals. To some extend this is right because those elements are present in some tropical regions. But not in all of tropical regions, and certainly nor in the majority of them. We will share some information on the tropics with our readers referred mainly to the Neotropics, which is a technical word used to name the tropical region in the American continent. In this sense that term distinguishes the American tropics from the tropics of Africa, Asia and Oceania.

The tropics are lands with hot temperatures but not the highest temperatures. In fact, it is in the subtropical regions where highest temperatures occur, specially during summer. Temperatures of 37 degrees Celsius (About 100 degrees Fahr.) may occur at sea level in the neotropics lasting about 2-3 hours a day, specially from around 11:30 a.m. to around 2:00 p.m. Later on, temperature normally decreases and it is rare to experience more than 28 degrees Celsius (about 84 degrees Fahr.) late in the afternoon or at night. Also, in the tropics there are places covered permanently with snow. Those are what in Spanish are called "las nieves perpetuas" (the perpetual snows), which are large regions, common in high mountains in Peru and Bolivia, and also present in smaller areas in Ecuador, Colombia, and Venezuela. The tropical perpetual snows occur normally above 5000 meters altitude. In middle lands above sea level temperature is really pleasant the year around, day and night. Visitors may experience temperatures that are as pleasant as a well calibrated air-conditioned room, having the great advantage of being outdoors. This is true, for instance in San José (Costa Rica), Medellín (Colombia), Mérida (Venezuela); it is also true in Bogotá (Colombia) and in Quito (Ecuador), even at night it is convenient to use a medium heavy coat when being in the last two cities.

The technical definition of the tropics is closely tight to the length of the day, which in fact does not change; there are close to 12 hours of sun light the year around (for instance, in Costa Rica higher possible daily averages are: 11.42 hours in December and 12.57 hours in June, as a possible maximum). Constant mean temperature the year around is another characteristic that helps us to define the tropics. Some examples for Costa Rica: in Puntarenas, a small city near the sea shore line, the average temperature is about 26 degrees Celsius (about 80 degrees Fahr.); and this does not change too much along the year. If we go to San Jose, at about 3800 feet (1.100 meters) a.s.l., the average temperature is about 22 degrees Celsius (about 72 degrees Fahr.). Around Chirripó, the highest peak of Costa Rica, at 12,000 feet, the average temperature is about 10 degrees Celsius (about 50 degrees Fahr.). But visitors should be careful: temperature could change dramatically in the same day in the mountains as in Chirripo, where the temperature can change from minus 5 degrees Celsius (about 25 degrees Fahr.) at 4:00 or 5:00 o clock in the early morning, to about 16 degrees Celsius (About 62 degrees Fahr.) at 1:00 or 2;00 o clock in the afternoon. Visitors should take a coat, if they want to spend the night in the mountains.

The most popular and exciting tropical life zones are the humid forests in the lowlands. These communities, when mature, are composed by moderately tall trees (up to 65 meters the most), with several layers, of which the more dense is the canopy. Lower layer at ground level are open, easy to walk throught. It only during the first stages of the natural succession that a forest community may be really intermingle.

However, such humid forests in the lowlands are not the unique, nor the most common life zones. The tropics are lands where rain fall varies more than it does in temperate zones. Rain falls occurring in the tropics gives rise to so different vegetation types as: deserts, thorn woodlands, dry forests, moist forests, wet forests, rain forests, to mention only some of the life zones in the tropical lowlands. The dry forests are short and open communities . In tropical mountains there are -besides dry and rain forests- montane steppes, tundras, and "páramos" where normally there are no trees because of the low temperature.

Because mean temperatures do not change, we may easily understand that in the tropics there are not the kind of seasons people know in temperate zones. However, the amount of rainfall and its distribution along the year produces tremendous impact on vegetation, in such a way that people in the tropics consider of having other kind of "seasons": the dry season and the rainy one. They use words in Spanish that often confuse people: summer ("verano", in Spanish) is the dry season and winter ("invierno", in Spanish) is the rainy season. I think that this terminology started early in the Colonial period when Spaniards and Portugueeses compare what they found in America with the rainy winter season and their not-rainy summer season, as they occur in the Mediterranean climate.

The Neotropics do not show large animals as it is common in Africa and Asia. So, visitors should prepare themselves to admire hummingbirds moving fast as tiny helicopters; quetzals and trogons staying several minutes in a perch; insects resembling dry leaves; turtles arriving by the thousands in quite beaches, orchids that developed strange strategies to favor pollination. They also could find themselves acting carefully in front of a tree that hosts stinging ants, and then moving fast to wear their raincoat because a sudden heavy rainfall; it is also quite possible that they should have to take that raincoat off after 5 minutes because rain stopped as suddenly as it started.

Apart from ants, abundant in some places but easy to avoid, people rarely will have problems with snakes and other poisonous animals. I worked permanently for more than 18 years in humid lowlands forests of the Neotropics in several Central and South American countries. My encounters with snakes that were a few meter from my body do not overpass a dozen, the majority of them being not poisonous animals. I remember only two times I was in potential danger of being attacked by one snake.