The simaruba clade of Bursera

The simaruba clade of Bursera • El clado simaruba de Bursera

Bursera is a Mexican group par excellence. Of the nearly 100 species in the genus, more than eighty are found in Mexico. Incredibly, more than half of these species are from just three Mexican states: Michoacán, Guerrero, and Oaxaca. In some places, as many as eight or ten species can be found in a single general area. Because of their abundance, incredible species diversity, and often spectacular shapes and colors, these trees are unmistakeable members of the tropical dry forests of Mexico. The simaruba clade of Bursera is remarkable for its incredible ecological breadth and the diversity in shapes and sizes of its members, which can be found from rainforest to very dry tropical deciduous forest. In Colombia, there is a species that is a giant rainforest tree, and in Central American there is a species that grows as a rainforest epiphyte! Below are images of the Mexican species, arranged more or less in order from driest to wettest. Our work with this group involves detailed measurements of the trunk dimensions, which is why many of the photos show us up in the trees.

Bursera longipes	This beautiful tree is found in the central Balsas Depression in central southern Mexico. It grows in very dry forest with many other Bursera species and many columnar cacti. This photo on the right was taken from atop a B. longipes while taking measurements of the branches, and shows the columnar cactus Neobuxbaumia and two other Bursera species.

Bursera grandifolia
	With large, arcing trunks that branch near the base, this species looks like an enormous rounded shrub. Bursera grandifolia does indeed have very large leaflets, which tend to be very fuzzy. The species is found in tropical dry forests of central Mexico. The greenish gray inner bark contrasts strongly with the reddish outermost bark, making this species a striking member of the dry forest flora.

Bursera instabilis	This strange species from western Mexico begins life like a normal tree, with a conventional trunk and branches that all support their own weight (left). With age, the branches begin to grow back on themselves, growing around and around within the crown of the tree. These branches rest on earlier-formed, self-supporting branches, much in the way that a liana would. One such branch removed from the tree is shown at right. It was 8 cm in diameter and 4 meters long!

Bursera arborea	This species is remarkable in the simaruba clade for being the species with the thinnest twigs. Whereas all of the other species have very thick twigs, the young shoots of B. arborea are remarkably slender. The species is found in tropical dry forest on the Pacific coast of Mexico. These photos were taken in Jalisco state.

Bursera cinerea	This species grows on the fringes of lowland dry tropical areas, at the transition to moister highland vegetation. These magnificent trees are particularly striking for the deep red color of their trunks, which are an amazing sight sprawling in deep, shady canyons, as at left, or ascending in open forest, as at right.

Bursera acuminata	Another large, B. simaruba-like tree, this species is often referred to as Terebinthus acuminatus. Like some other members of the simaruba clade, this species grows in moist canyons in the transition zone between highland pine-oak forest and lowland tropical subdeciduous forest.

Bursera attenuata	This species is similar in many respects to B. simaruba, differing in some leaf characteristics. It grows in fairly tall, moist tropical subdeciduous forests of the Pacific coast of Mexico. In some places, it occurs very close to sharp transition zones, with pines and oaks growing just above a zone of broadleaf forest.

Bursera simaruba	This is the most widespread species of Bursera, occurring from southern Florida and the Caribbean, along both coasts of Mexico to South America. The species is highly variable and is found in everything from tropical dry forest to tall rainforest. Further study will almost certainly reveal that what is now considered B. simarubais actually composed of several different species. The images here are from rainforest on the Gulf coast of Mexico.

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