From Plant Press, Vol. 23, No. 1, January 2020.
By Betsy Collins
I recently spent two unforgettable weeks collecting in the tropical dry forests of Barranquilla, Santa Marta, and the Cauca River valley in Colombia for my doctoral research on the phylogeography of the dry forest specialist tree, Bursera graveolens (palo santo, holy wood). Other than its deliciously scented essential oils and funky fruit, what makes palo santo in particular so interesting to study?
Left: Essential oil dripping from a cut in the bark of a Bursera graveolens tree in Piura, Peru. Right: Palo santo fruit are dehiscent drupes. Fruits are green when unripe, and then turn red and the capsule dehisces, exposing a red pseudaril covering the black pyrene. (photo by Betsy Collins)
Palo santo as currently described has a wide range, from Mexico, Central America (except Belize and Panama), Colombia, Venezuela, Ecuador (including the Galapagos), and Peru, although I hypothesize that it consists of multiple, distinct species. This widespread distribution is unique amongst dry forest specialist trees, which makes palo santo an ideal candidate for studying the evolutionary processes that formed the tropical dry forest biome, and whether these processes differ between Mesoamerican and South American dry forest.
One of my goals was to sample (almost) all the major tropical dry forest patches that palo santo occurs in, which is no small logistical feat!
After struggling for months trying to get collection permits for Colombia and being warned that exporting specimens from Colombia was pretty much impossible, I was close to giving up hope of ever collecting in Colombia. And then Morgan Gostel, previously a postdoctoral fellow at the Smithsonian, now a research botanist at the Botanical Research Institute of Texas (BRIT), introduced me to Alejandra Vasco, his colleague at BRIT. Vasco very kindly offered to make me a research associate at BRIT, which allowed me to collect and export under their permits and agreements with the herbarium at the Universidad de Antioquia (HUA) in Medellin.
I started my Colombian field expedition on the Caribbean coast near Barranquilla. My field team in Barranquilla consisted of Priscilla Saab, an intrepid and gregarious ecologist, and Lewis Morfe, a taxi driver-turned-field assistant. Marcela Celis, a professor at the Universidad del Norte in Barranquilla, connected me to them, in addition to allowing me to work out of the university’s herbarium.
In an attempt to beat the relentless heat in Barranquilla, the three of us met up at 5:00 am the day after I arrived in Colombia. Our destination that day was a private nature reserve about an hour from Barranquilla called Reserva Palomar, where there had been several documented field observations and herbarium specimens collected of Bursera graveolens. In Latin America much of the original extent of dry forest has been converted to agriculture and human settlements. Having witnessed extensive habitat loss and conversion in Peru and Mexico, I thought I was prepared for what I might see in Colombia. However, I was shocked by the extent of habitat loss along the Caribbean coast and the deforestation in the Cauca River valley in Antioquia. The Reserva Palomar (and especially the Santa Marta area, further east along the coast), were bright spots in an otherwise depressing (from a human impact aspect) collection trip.
After arriving at Reserva Palomar we hiked a few kilometers uphill to a farmhouse (casa de finca), where we were going to meet up with the farm workers who had agreed to help us locate palo santo. The hike to the farmhouse turned out to be magical - we spotted a flock of wild macaws! And we even had cloud cover the entire day, which made the temperature much more manageable.
Once we arrived at the farmhouse, I showed Manuel, one of the farmworkers, pictures of palo santo and he quickly recognized it as caraña. He agreed to take us to the few trees he knew of at the reserve – about a 4 km hike each way from the farmhouse.
We made collections from two palo santo trees at Palomar, both on the sites of former farm houses. Manuel told us that it is very common to plant a palo santo tree at your house (people use it to treat colds and arthritis), so that is probably why these two trees were in Palomar. It seems palo santo is not naturally found in the dry forest in Palomar, and that likely all the field observations and the other collection in Palomar (that was a few more kilometers away) were planted trees. I didn’t know it at the time, but we would not find any naturally growing palo santo trees in Atlántico Department, either because all the natural populations have been extirpated (pretty likely) or because all previous collections had been from planted trees (I think less likely although not out of the question). I found this very interesting, and definitely not what I expected based on everything I had read, including the herbarium labels. I didn’t find natural populations of palo santo until I traveled east several hours to Santa Marta and then flew south to the Cauca River valley near Medellin.
When we arrived back at the farmhouse we heard of another palo santo tree in a yard in a neighboring town called Loma de Arena.
On our way to the town of Loma de Arena in Bolivar, we hit a little snag. Our car got stuck in the mud. We tried everything - we even tied a horse to the car (that did not go well). Finally, several more people came along, we yelled for their help, and we were all able to push it out.
I really wish I could have filmed the look on this family's face when we all showed up at their door in Loma de Arena asking if we could cut some leaves from their caraña tree (a mixture of amusement and bewilderment). The whole family pulled up plastic chairs in the backyard to watch the show of these crazy biologists cutting leaves from their tree.
Left: Palo santo tree in the backyard of a family in Loma de Arena. Right: Priscilla Saab holding the new collections from the backyard tree. (photos by Betsy Collins)
The family told us the tree had been planted via cutting about 40 years ago. The cutting had been taken from a tree in the town of Clemencia in Bolivar. Maybe my genetic work can provide a clue about where these trees may have originally come from.
In addition to traveling to Colombia, this year I also had the privilege of collecting palo santo, with funding from the National Geographic Society, in northwestern Peru and Mexico (Veracruz and Oaxaca). I will be heading to Ecuador in February for my last collection trip.
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