It seems the prevailing winds have led me to the south of Mexico away from my usual autumn home on the placid, green coast of Veracruz to the dry, turbulent expanses of the southern edge of the Isthmus of Tehuantepec. The area where I live here in the southeastern Oaxaca is known as La Ventosa. Literally meaning something akin to ‘the wind tunnel,’ the name offers a clue as to the character of this strange place.
La Ventosa is located at the mouth of a narrow gap between the Sierra Madre Oriental and the highlands of Chiapas. The mountains collect and channel the wind from further north until it finally reaches this gap and is violently pushed through. Every breeze becomes transformed into a gale of extreme force by the time it exits this passageway.
The mountains also cast a vast rain shadow across the whole Pacific slope, so for much of the year the highlands take the moisture from their air and leave the coast sun-scorched and dry. The resulting climate is not inviting. In light of the brutal winds, the extreme heat, the flat expanses of cattle-torn pasture land, stolid sorghum, withered trees, and razor-sharp scrubland it has been a struggle to explain to others my rational for having abandoned the verdant white-sand paradise of Central Veracruz in favor of the Isthmus.
The chronic melancholy I spend my life running from aside, the reason lies in the withered trees. Although in the lowlands they have been pushed to the very edge of existence, merely marking the borders between cow pastures, these trees are a vestige of something greater. A short sojourn upwards to where the ground is not so easily cultivated reveals what they were once a part of.
The habitat found on the slopes is an ecosystem that I have become somewhat obsessed with recently. I became acquainted with it two winters ago while working on the Chiapan side of this region, and I would like to take this opportunity to briefly introduce you to this habitat as well. Uninformatively and non-descriptively referred to as tropical dry forest, it is the poorly studied, inconspicuous, and underappreciated cousin of the rainforests. Just as vaguely as it’s name, it’s defined as any frost-free tropical area whose annual rainfall and evaporation lies somewhere between desert and moist forest (Holdridge.) Although this definition includes everything between here and baobabs, these dry forests of the pacific slope of Mexico and Central America has a distinct character to it.
It is an ugly, angry, forbidding woodland with teeth and spines designed to stand up to extremes of wind, rain, and drought. Even cactus, although a few grow here with their spined arms raised above the canopy, aren’t able to deal with the bipolar nature of this place. Because this is a type of seasonal woodland, some of the trees are reminiscent of a northern deciduous forest. I’ve noticed quite a few that seem superficially familiar, resembling sycamore, ash, or locust. Others species, however, are totally foreign to me, and might be described as not only unfamiliar, but the arboreal equivalent of an undead porcupine. Spikes grow up trunks, across branches, and amongst leaves in various combinations and permutations in many of the local species. The one thing all the trees seem to have in common is a gnarled, weathered look.
I’ve also noticed that the dominant plants are of the sort that I would usually associated with disturbance: various members of Fabacea, the compound-leafed bean family know for their ability to produce their own nitrogen with the help of their rhizoidal symbiots, and the thorn-laden acacias known here as huisache are among the most common. Studies on a somewhat similar dry forest ecosystem in Bolivia show that this type of habitat is able to regenerate much faster than most other habitat types (Kennard,) and I suspect this tenacity is a trait shared by the Mexican dry forests as well.
For now, as the torrential autumn rains mix with the dry air, the trees are green and vibrant. The pallet isn’t made up of the soothing deep bluish greens of a northern deciduous forest, but a bright lemony green. The branches are so densely laden with citreoline leaves that walking into the forest is like entering a darkened room. However, the canopy is fleeting, and the moisture in the leaves will be carried off soon enough.
As the seasons wear on and the precipitation ceases, the forest is reduced to a leafless mass of tangled, twisted gray. The rain will begin to cease in October, and by the end of December the trees will be as bare as those back home in the north eastern US. The undergrowth will finally get its season to thrive, and the only green that will remain above will be the feathers of parrots and the few pale bromeliads that incomprehensibly manage living with such a seasonal scarcity of water. Still, it’s not as monochrome as it might seem, since it is during the dry season when these trees choose to flower, so spangles and pink and yellow accent the otherwise dreary scene.
Why is it that I am so interested in this ecosystem? Flowers aside, it’s abjectly ugly by the most people’s aesthetic. The heat and the wind are maddening, and in such a disturbed habitat, the peace and solitude of wilderness is lacking. Even more so, I do not possess the kind of understanding and comfort within the dry forests that I do with the deciduous forests of the Midwest or the mixed conifers of California. However, it might be the unfamiliarity itself that draws my attention. Incredibly little is know about this habitat type, and so much about this area is totally unique. There are so many plants and animals here that are reliant on this habitat that it’s staggering.
To illustrate this point with what I know (birds,) only requires a quick look at the range maps of the avifauna of this region. Many birds, including the red-breasted chat, the orange-breasted bunting, the cinnamon hummingbird, the lesser ground cuckoo, and the lilac crowned parakeet exist solely within a thin line along the once dry-forest dominated pacific slope of the Sierra Madre Occidental. There are also two species, the Sumichrast sparrow and Rosita’s bunting that live exclusively in the tiny fleck of space where I currently live in the southern Isthmus of Tehuantepec with the sparrow occupying the lowlands, the bunting the hillsides.
The Sumichrast sparrow is a dull bird with a longish buffy-colored tail, an often hidden reddish wrist, a reddish cap, and black striped face. Where it does live, there are many. Several pairs can be heard singing their quick, bubbly duet from a single point. The Rosita’s bunting is much more striking, but much harder to find. The adult male is brilliant blue with a pink stomach and a little white eye-ring.
What about these birds is so specialized, and what ties them to such a miniscule stretch of habitat? I can only guess. It seems likely that the sparrow has a predilection for the relatively wetter dry forest conditions that the lowlands here offer in the gap between mountains. The bunting however, is more of mystery. Perhaps it is a specialist relying on a particular plant for food or nesting materials. Perhaps it has to do with local insects or soil types.
For now, only the bird knows. In fact, next to nothing has been published about the birds of the Isthumus since the biologist Sumichrast and his wife Rosita, for whom these birds were named, came to Oaxaca to make observations and collect specimens in the 1880’s. Hopefully, those of us working here now can rectify this problem.
Either of these species could easily be destroyed with a handful of ill-placed cattle pastures. The Rosita’s buntings’ remaining habitat here is so narrow that highways define their range. One side of the road remains agreeable to them, and the other side hasn’t had a record of this species for decades. If birds, the ultimate dispersers, show this level of dependence on the dry forest ecosystem, imagine the level of endemism that mammals, herptofauna, insects, and especially plants must have.
For all this uniqueness, this habitat is entirely underappreciated and is incredibly endangered. Historically about half of Central America was once tropical dry forest according to the article Ecology of Tropical Dry Forest by Murphy and Lugo, and presumably Southern Mexico was so as well. All that remains now is estimated to be about one tenth of a percent (Gillespie.) It is unfortunately far too easy to slash, burn, and replace with cows, and most people don’t even realize there’s a forest here to destroy. I’ve heard many locals refer to the area as a desert. Even biologists often cannot tell the difference between the native habitat and second growth scrub, but the birds can obviously tell the difference. There is so much we don’t know about the dry forests, and it is quickly disappearing. It might be ugly, but it possesses a unique ecology that deserves protection as much as any other habitat. As a biologist, that’s what I value – uniqueness and life. So, as long as we’re working to save the rainforest… perhaps we can save the dry forests, too?
References:
Many thanks to Amy McAndrews and Jorge Montejo for their insights on the ecology and birds of the Isthumus.
Gillespie, G.W., and Grijalva A., and C. N. Farris. Diversity, composition, and structure of tropical dry forests in Central America. Plant Ecology, Vol. 147, No. 1 (2000), pp. 37-47.
Holdridge, L. R. Life Zone Ecology. Tropical Science Center, San Jose, Costa Rica. (1967) pp 207.
Howell, S, and S. Webb. A Guide to the Birds of Mexico and Northern Central America Oxford University Press. (1995)
Kennard, D. K. Forest Succession in a Tropical Dry Forest: Patterns of Development across a 50- Year Chronosequence in Lowland Bolivia. Journal of Tropical Ecology, Vol. 18, No. 1 (Jan., 2002), pp. 53-66