(Opposite) Ramón (Brosimum alicastrum) trees growing on the ruins of Coba in Quintana Roo, Mexico.

ONE

The Ramón Tree and the Maya

Twelve kilometers southwest of Papantla (N20°26´52´´, W97°19´12´´), Veracruz, Mexico, 1979–1983

A number of Maya ruin complexes in Mexico and Guatemala were discovered by chicleros looking for forage to feed their mules.¹ The preferred forage was the leaves and twigs of a common forest tree known locally as ramón (Brosimum alicastrum),² from the Spanish verb ramónear, “to browse.” The chicleros would encounter a dense stand of ramón trees, start lopping off the branches, and eventually notice that the trees were growing on top of the stones and broken statuary of a Maya temple.

The relationship between ramón trees and Maya ruins can tell us much, given the many uses of the tree.³ The leaves are used for forage, the fruits and seeds are edible, the milky white latex is drinkable, and the wood is durable and easily worked. The dried ground seeds contain more protein than corn and have a high concentration of tryptophan, an essential amino acid that is usually deficient in diets based mainly on corn. Rural communities throughout Central America currently eat ramón seeds as a survival food, and a sizable archeological literature suggests that the seeds were a dietary staple of the Maya in pre-Columbian times. Many investigators conclude from this that the dense aggregations of ramón trees growing on ruin sites are, in essence, relict Maya orchards.

I first found out about the ramón tree when I was looking for a research topic for my Ph.D. dissertation. I had read a little bit about the Maya–ramón tree connection, but I was more interested in studying the population ecology of a tropical tree that was able to form high-density aggregations. Tropical forests are so diverse because they have a wide variety of niches and growth strategies, and all the plant species behave in subtly different ways to survive in a highly competitive environment where one species is rarely able to function so much better than all the others that it can take over. So the fact that ramón trees were, apparently, able to achieve such a notable degree of dominance—on top of Maya ruins or any­where else—was a finding of great interest to a fledgling tropical ecologist. I really wanted to figure out how the species was able to do this.

A research institute in Xalapa, Veracruz (Instituto de Investigaciones sobre Recursos Bióticos A.C.), had agreed to host me during my doctoral work, and I spent my first months in Mexico traveling around the state of Veracruz visiting different forests and counting ramón trees. I was trying to locate the perfect study site for my research. The forests in the southern part of the state contained about four to five adult ramón trees per hectare, while the ones in the far north, in drier, cooler climates along the border with the states of Tamaulipas and San Luis Potosi, had ramón populations of over one hundred adult trees per hectare. None of these sites met my needs. The densities of the ramón populations in the south were too low for my purposes, and the sites to the north, while chock-full of ramón trees, were too far away from Xalapa.

The forest I finally chose to work in was located about three hours north of Xalapa near the town of Papantla and the ruins of El Tajin, a complex of Totanac pyramids built between 600 and 1200 C.E.⁴ The forest was a beauty: huge trees with sinewy buttresses, palms and delicate ferns in the understory, flocks of parrots and the occasional toucanet, and, in terms of the ramón population, thirty to forty adult trees, numerous poles and saplings, and thousands of seedlings per hectare. The species was clearly happy growing here. This was the first tropical forest that I spent a great deal of time in, collecting and counting and thinking. The locals called the site Los Alpes because it was incredibly steep, but the climb did not bother me.

The basic focus of my work at Los Alpes was to document the birth, death, and growth rates of all the ramón trees. The procedure is called demography and is similar to what insurance companies and census bureaus do to calculate the probability that a person will die at a certain age or to estimate the size, age structure, and potential growth rate of a population. I wanted to use the tools of ecology and plant demography to see if the ramón trees would be able to maintain their dominance in the forest. Were these populations dense because they had been planted and managed, or were they dense because the life-cycle characteristics of the ramón tree were so effective in this habitat that they did not need humans to help them?

I carefully laid out a one-hectare study area in the forest. I identi­fied every tree, shrub, and herbaceous plant, measured the diameter and height of all of the trees, and gave each a little round metal tag with a unique number. There were too many seedlings to count, so I made regeneration plots in random locations throughout the forest to sample the smaller-size classes. I put stainless-steel growth bands around all the ramón trees, and around several representatives of each of the associated tree species. Growth bands, also known as dendrometer bands, are custom fit around the trunk of a tree, and kept in place by a spring. The installer marks a vertical “zero” line, and as the tree grows the upper and lower sections of the line separate to indicate diameter growth; the distance between the two lines is measured with a vernier caliper. Dendrometer bands are extremely sensitive, and once the tree has grown sufficiently into the band, the swelling and shrinking of the bark after a rainstorm is clearly visible.⁵ Finally, I built several dozen litter traps, using wooden stakes, hoops of plastic tubing, thick garbage bags, and clothespins, and set them under the crowns of selected ramón trees to catch the flowers and fruit that fell.

With these pieces in place, all I had to do was return to the site every few weeks. I needed to count the number of seedlings in my plots; note any trees that had died and record, if possible, the reason they had died; read the growth bands; and, if the ramón trees were flowering or fruiting, change the garbage bags in the litter traps and carry them back to Xalapa so I could sift through the contents and count the flowers and fruit. Although tedious, the work was straightforward. If done correctly, by the end of one year, I would have a reasonable estimate of the birthrate, the death rate, and the growth rate for all the trees in the population. But I admit that going up and down the steep slopes with field equipment and garbage bags full of fruit did not get any easier over the course of a year.

One evening, my truck broke down (it needed a new generator), and I had to take a bus back to Xalapa. As it happened, I had just emptied all my litter traps, and I needed to carry the contents back with me. I never knew what I was going to find when I stuck my hand down in those traps. Snakes seemed particularly drawn to the moist, dark space, and I encountered black scorpions a couple of times as well. What was guaranteed, however, was that all the flowers, fruit, and leaves, and everything else that had fallen into the trap, would have turned into a wet, stinking mess with maggots crawling through it. I usually threw the garbage bags into the back of the truck and picked through them wearing latex gloves when I got back to Xalapa. But this time, I had to carry the smelly mass of plant parts on a crowded public bus.

It was almost dark by the time I caught my bus, and it was packed—señoras with long braids wearing embroidered blouses, gunny sacks full of beans and corn, a couple of wooden crates with pigs, chickens flailing around, and a group of little kids. Every seat was filled. I got some curious looks as I stepped in with my black garbage bags, but I slowly made my way to the back of the bus and found a place to stand. The ride took four hours, and after about thirty minutes the bus was filled with the pungent odor of rotting fruit. Everyone was very tolerant of the stench and I was even offered a seat near the end of the trip.

I continued to monitor flower and fruit production by the ramón trees at Los Alpes for two years and discovered some interesting things. For example, a ramón tree can be female, hermaphroditic, or male.⁶ All the small trees were female, most of the intermediate-size trees were hermaphrodites, and all the large trees were male. I did not understand why this should be until the second year of fruit collection. One of the female trees I had monitored the year before, which had produced only female flowers and fruit, started producing male flowers. Similarly, one of the hermaphroditic trees, which had produced male and female flowers and a few fruits the year before, produced only male flowers and no fruit; functionally, it had become a male tree. Changing sex is not common in tropical trees, so far as we know, but the ramón trees were exhibiting what is called “sequential hermaphroditism”—they can modify their sexual expression if necessary.

Based on the results of the growth-band studies, I deduced that the benefit of changing sex from female to hermaphroditic to male is to allow a tree to grow faster and outcompete its neighbors. Female trees are forced to mature tens of thousands of protein-rich fruits, while male trees immediately go back to growing after they release their pollen. If the canopy conditions above a female tree remain favorable, the tree will continue to produce fruit. If, however, light levels drop too low or the competition from neighboring trees becomes too intense, the tree will start producing male flowers. In terms of its reproductive biology, the tree is now hermaphroditic. It will still produce a few fruits, but its growth rate will be significantly higher than that of a female tree. If canopy conditions or competition levels continue to be restrictive, the tree will stop producing female flowers altogether. I would guess that this type of plastic sexual expression is more common in tropical trees than most ecologists realize.

Another curious aspect of the reproductive dynamics of ramón trees is the role frugivorous bats play in seed dispersal. A number of animals, including humans, eat ramón fruit in large quantities. I have watched flocks of parrots feed for hours in the crowns of fruit-laden ramón trees, and a number of other types of birds, as well as monkeys, squirrels, and raccoons, also relish it. But these animals eat the fruit, and the seeds are either destroyed in the process or fall to the ground directly under the crown of the tree. Because of the excessive competition with siblings—other seeds produced by the same tree—this is not an ideal place for a seed to be deposited. Frugivorous bats, on the other hand, will fly to a fruiting tree, select one fruit, hold it carefully with their feet, and then fly away to a roost, where they will carefully eat the fruit and then drop the seed intact, a textbook example of successful seed dispersal.

Large colonies of bats live in the inner chambers and vaults of Maya temples, and the floors and limestone ledges of these ruins are usually littered with ramón seeds. I have seen extensive accumulations of ramón seeds and seedlings around the ruins of Palenque and Bonampak in Chiapas, and under bat roosts in mango plantations in Veracruz, where the nearest ramón tree was more than five kilometers away. I have a hunch that the commonly observed aggregation of ramón trees around Mayan ruins is at least partly the result of the inherent competitive ability of the species coupled with a continual input of bat-dispersed seeds.⁷

Of course, accepting the hypothesis that the clustering of ramón trees on ruins is the result of bat dispersal and normal ecological processes does not negate the possibility that the tree was used—or even managed—by the Maya. It is hard to believe that such an abundant, protein-rich food source would not be exploited in some manner. The issue here appears to be one of the extent or magnitude of use. Were ramón seeds casually collected from the forest only in times of famine, or was some form of deliberate cultivation, silviculture, or selection practiced?

I wondered whether evidence of intense use by the pre-Columbian Maya might still be detectable in the ramón populations growing in the forests around some of the major population centers, like Tikal, one of the civilization’s largest archaeological sites and urban centers.⁸ Much of the work on the historical use of ramón by the Maya has focused on Tikal, and I thought it might be productive to compare these data to the ecological information that I was collecting at Los Alpes. What I found was striking.

The ramón trees at Los Alpes—and in most of the other places where the species grows—produce fruit once a year, with peak fruiting occurring at the onset of the rainy season. This makes biological sense. Flowers are pollinated during the dry season, and the seeds have a moist substrate for germination and early growth. Ramón trees are pollinated by wind, a poor carrier of pollen compared to bees, beetles, or bats, especially when it is raining, and this form of abiotic pollination is rare among tropical trees. By contrast, reports from Tikal suggest that the ramón trees there bear fruit twice a year or that fruiting is largely continuous throughout the year, with three periods of peak fruit abundance. It is worth noting that the environmental conditions at Tikal—elevation, temperature, precipitation, substrate—are virtually identical to those at Los Alpes.

The reproductive behavior, or breeding system, of the ramón trees at Tikal is also different from those at Los Alpes. In contrast to the complicated sequential hermaphroditism and facultative sex change that I found at Los Alpes, the ramón trees at Tikal are consistently described as having both male and female flowers, and all the trees are capable of producing fruit.⁹

What we seem to be presented with at Tikal is a dense population of ramón trees in which fruit is produced in large quantities at frequent intervals by every adult tree. The curious feature is that similar forests of the species in other regions do not exhibit this behavior. One explanation might be that the regeneration of the species is severely limited for some reason at Tikal, and this large reproductive output is required to maintain the population. Another explanation, which I find more convincing, is that the high level of fruit production exceeds the regeneration needs of the population, and the atypical reproductive dynamics of the ramón trees at Tikal are manifestations of relict genotypes selected for by the continued use and management of the tree. Excessive collection of the seeds during times of famine would not produce such a marked effect. What appears to be suggested here is the conscious mixing of genotypes with the objective of producing an abundant, year-round supply of seeds.

At this point in my career, I was intrigued by questions of theoretical ecology and niche theory, and spent a lot of time collecting data to either prove or refute current theories about the population dynamics of tropical trees.¹⁰ I spent more time obsessing over what my data might tell me than reflecting on the motives and expertise of the people who had, in all probability, subtly groomed the floristic composition of the “undisturbed” piece of tropical forest in which I was doing my doctoral research. But my focus was starting to change.

The insight that the Maya had deliberately managed their forests during pre-Columbian times to increase the density and yield of ramón trees inevitably gave rise to the question of whether indigenous groups of tropical forest dwellers were doing the same thing today. If so, how were they going about this? If not, could some of the skills that I had learned as a forester and an ecologist be applied within a community context to facilitate this type of forest management? My fieldwork after the ramón study became more focused on practical applications. I began to put together projects that were collaborative efforts with local communities aimed at solving a problem rather than proving a theory. The nature of the stories I heard, and gradually became a part of, was also starting to change.