| Elephants and Incense
Now that many of you have finally become familiar with the term "caudiciform" to describe those succulent plants with swollen bases, I have a new term to confuse you. It was coined to provide a general identity for those succulents whose extremely thickened trunks and main branches make them resemble squat, miniature trees. You can now think of these as "pachycauls." As a group of otherwise unrelated plants, pachycauls can be distinguished from more typical stem succulents by their generally woody stems and branches and their typically thin, seasonally deciduous leaves. In contrast to true caudiciforms, their succulent parts derive from embryonic stems rather than the hypocotyl (that part of the embryonic plant below the cotyledons and above the roots). As with succulence in general, the term is less than perfectly precise, but it was invented to describe plants for which no other descriptive term was really applicable.
The bursera family ( Burseraceae) occurs in the semi-tropics of North and South America and Africa. Generally small to medium-sized trees, most often from seasonally dry or semi-arid regions, a number of species have adapted to desert environments. The approximately 100 species of Bursera proper range from southernmost Arizona and California through Mexico, east to the Caribbean regions and southward to northwest South America. If you've ever traveled to the Florida Keys you may have seen the Gumbo-limbo tree, Bursera simaruba, open branched to sixty feet tall, with soft, light wood, pale, peeling bark and pinnately divided leaves. This habit typifies the larger members of the genus, a number of which have further evolved the ability to photosynthesize through their stems, and, in addition, have modified their soft woody tissues to store water. As we follow these succulents-in-the-making into ever more arid environments, these incipient characteristics come to the fore, with the species diminishing in overall size even as the portion of their stems used to store water increases. Finally, in several fairly widely separated areas of northern and western Mexico, a few species have adapted to extremely dry climates and become true succulents, of a "pachycaulous" nature.
The exigencies of their environments encourages dwarfed, twisted and--to our eyes--picturesque growth in these species, giving them the look of natural bonsais. B. fagaroides, widely distributed in scattered pockets from Jalisco state north, has light golden, peeling bark, a vaguely cigar-shaped trunk and leaves with up to fifteen leaflets. In keeping with the other more succulent species, under conditions of greater aridity and exposure to harsh conditions its trunk will proportionately thicken and bend over, and the plants will grow more horizontally than upright. B. hindsiana, from Sonora state and Baja California, has reddish bark, smaller leaves with fewer leaflets, and typically grows as a large, loosely branched shrub. B. microphylla, from the same area northward into southwest Arizona and California's Anza-BorregoState Park, has undergone the most modifications for desert existence, and its tiny, distinctly dark green, mullti-segmented leaves drop off readily under drought conditions, only to reappear swiftly at the first hint of rain. A typical B. microphylla resembles B. hindsiana in habit, a multi-branched shrub up to eight feet tall with a thickened main stem. Under the right conditions, though, both these species take on a greatly modified aspect, and B. microphylla, in particular, can turn into one of the most spectacular of all succulent plants. These "thick-footed" plants are one of the two types of Baja California succulents sometimes called elephant trees.
One a few hills in central Baja, ancient specimens of B. microphylla grow horizontally along the ground, their sparse main limbs stretching out like skeletons, with swollen trunks a foot thick and less than six inches high. In contrast, their leaf-bearing branches are nothing more than a few pencil-thick twigs a couple of inches long. These plants live in areas of wind-swept fog desert, almost devoid of regular rainfall, and I can only guess at their age, certainly many centuries.
In addition to their other features, burseras and the other genera in the family have highly aromatic sap (called copal in Mexico), and used locally in ceremonies for ages past. My own memory, of walking through these ancient, sprawling "trees," six feet and more from branch tip to branch tip and never more than six or seven inches tall, with every brushing contact of my shoe or pants cuff with a leaf or branch instantly sending the fragrance of piney-incense up to me, is quite unforgettable.
The members of the African genus Commiphora mimic Bursera remarkably. Commiphora can be medium-sized trees, always with spongy, moisture retaining wood, and they actually make up a major component of elephant fodder during times of drought. In the driest regions, in places such as southern Somalia and South Africa and Namibia, Commiphora too has evolved dwarf species with dramatically swollen, low growing trunks, and such species as C. dulcis and C. saxicola, from the southern end of the continent, parallel the spectacularly "bonsai-ed" form of the best burseras to an amazing degree. Other Commiphora, such as the more widely distributed C. africana, or the Somalian C. holtziana (with extraordinary, presumably photosynthesizing, blue-green bark underneath its peeling, golden outer bark) resemble the small tree/large shrub with thickened trunk of the more typical bursera forms. Madagascar, rich with pachycaulous plants, has it Commiphora too, both taller growing ones and species such as C. simplicifolia, subject to bonsai-like horizontal growth habit in the right conditions.
Other genera in the Burseraceae include Lannea, from Ethiopia and Somalia, consisting of small, swollen-trunked trees. Boswellia species, from Yemen and the island of Socotra, are less dramatically thickened, but their aromatic sap has been valued for millennia as frankincense (myrrh comes from a species of Commiphora).
As is the case with many of these types of plants, growing them is easier than finding them. They all need fast draining soil, although not as sparse as some arid-growing succulents, and very bright light. Watering depends on when the plants grow--they signal by dropping their leaves at the start of dormancy and then putting out new leaves as their growing season begins. For most of them that will be in spring, but species such as Bursera microphylla and B. hindsiana, from Baja California's winter rain and fog zone, start their growth in September or October and begin to go dormant by late March or April. When dormant the plants shouldn't be kept bone dry; some water every three or four weeks will be plenty, and once a week should be adequate during their growing seasons.
Some of these rare plants are almost non-existent in cultivation, but these days a few specialist growers are raising a surprisingly large number of varieties of them. They generally grow fairly quickly as seedlings, but will need some pruning to maintain an attractive, compacted shape unless you can expose them to constant wind, intense sun, and very little rain (and wait and wait...).
The Garden has a fine collection of burseras and some very interesting examples of other members of the family.Next month I'll write about the other elephant tree of Baja California and other odd pachycauls.
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