| Elephants, Incense and the Bursera Family
By now most of us are familiar with the term “caudiciform,” applied to succulent plants with greatly swollen bases. Another, somewhat newer and less well known term, “pachycaul,” was devised to describe a further variant growth form, that of those succulent plants whose extremely thickened trunks and main branches make them resemble squat, extremely thickened trees, sometimes in miniature, sometimes quite large. As a group of otherwise unrelated plants, pachycauls can be distinguished from more typical stem succulents by their generally rather woody stems and branches and their typically thin, seasonally deciduous leaves. Pachycauls can further be differentiated from true caudiciforms by the morphology of their succulent parts. In a pachycaul, the succulent main trunk or trunks derive from embryonic stems rather than, as is the case with a true caudiciform, the hypocotyl, that part of the embryonic plant below the cotyledons and above the roots. As with succulents in general, the term is less than perfectly precise, but it was invented to describe plants for which no other descriptive term seemed really applicable.
There are a number of popular succulents that have evolved a pachycaulous life style. A number of these are in the bursera family (Burseraceae), which includes about a dozen and a half genera and as many as 500 species, native to semi-tropical regions in both the Americas and parts of Africa, Arabia, and even farther east. Generally small to medium-sized trees, frequently with highly aromatic sap and leaves subdivided into sets of leaflets, a number of species from several genera in Burseraceae have adapted to extremely arid environments. The approximately 100 species of Bursera itself range from southern-most Arizona and California, through Mexico (home to around eighty of the species), east to the Caribbean region and south into South America, although some species from Brazil may be better placed in other genera. For the sake of convenience, I’ll divide the genus into three pretty arbitrarily arranged groups, based on size and degree of succulence and without any real botanical significance. The thin, peeling bark that is a striking attribute of many burseras can occur in species with all of these growth forms.
The largest burseras live in tropical areas, including some true rain-forest denizens. If you’ve ever traveled to the Florida Keys, you may have seen the Gumbo-limbo tree, Bursera simaruba, open branched and up to fifty or sixty feet tall, with soft, light wood, pale, peeling bark, and pinnately divided leaves. Similar, even larger growing species live in wetter areas, where their smooth, often reddish bark makes them look a little like madrone trees, although with an entirely different leaf structure. This habit typifies the larger members of the genus. A number of these have evolved trunks, typically of a dark brownish-green to almost blue-green hue, that photosynthesize beneath their paper-like, peeling outer bark layer, and, in addition, have modified their soft woody tissues for water storage. As we follow these succulents-in-the-making into increasingly 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.
Burseras with this second type of growth habit occur in areas either simply with less total rainfall, or drastic differences between wet and dry seasons, a climate type that characterizes much of mainland Mexico. These plants may form a significant component of a low, scrub forest, or they may grow on the fringes of a somewhat taller forest, where they coexist with cacti ranging from opuntias and large columnar plants down to small species such as Mammillaria longimamma. This form of Bursera, which includes species such as B. galeottiana, to twenty-five feet tall with a foot thick trunk and reddish brown bark, or B. bolivarii, with a blue-gray stem that shows under the bark and glaucous foliage as well, often develop somewhat swollen main stems, and when small, look like good subjects for natural bonsai. As they grow taller (typically up to twenty-five or thirty feet), however, they “slim down,” their diameters not increasing in proportion to their height. Nonetheless, some of these plants, very rarely cultivated, would probably respond well to careful pruning and a regime of reduced watering.
Last but not least (in terms of horticultural desirability), in several widely separated parts of Mexico a small number of burseras have adapted the “third” growth habit and become true succulent pachycauls. These species specialize in arid habitats, where they often take the form of a multi-stemmed or heavily branching woody shrub, up to six or even eight feet tall, and almost as wide. In the most extreme cases, the exigencies of their environments encourage dwarfed, twisted and—to our eyes—picturesque growth forms in these species, giving them the look of natural bonsais. Burserafagaroides, widely distributed in scattered pockets from Jalisco state as far south as Oaxaca, has light golden, peeling bark, a vaguely cigar-shaped trunk and leaves with from three to five or more pairs of leaflets. I’ve seen it growing in the company of true desert cacti such as Astrophytum myriostigma in ground composed largely of limestone chunks, and under these harsh, dry conditions its trunks will thicken and the plants grow more horizontally than upright.
Bursera hindsiana, from Baja California and parts of neighboring Sonora, has reddish, non-peeling bark, smaller leaves with fewer segments and typically grows as a loosely branched shrub. Burseramicrophylla, also from Baja, but occasionally extending north into southwest Arizona and southern California deserts such as in Anza-Borrego State Park, has undergone the most modification for desert existence, and its tiny, distinctly dark green, multi-segmented leaves drop off readily under drought conditions while the plant photosynthesizes from beneath a thin layer of peeling outer bark. In form, a typical B. microphylla resembles a typical B. hindsiana, a good sized multi-branched shrub with a thickened main stem. Under the right circumstances, though, both these species take on a greatly modified aspect, and B. microphylla, in particular, can become quite surreal looking. These “thick-footed” plants are one of the two types of Baja California succulents known as elephant trees.On a few hills in central coastal Baja, ancient specimens of Bursera microphylla grow horizontally, their main limbs stretched along the ground like a skeleton, with swollen trunks close to 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 ages, certainly many centuries I would imagine. B. hindsiana, though less common in these situations, shows up as well, with its less obviously succulent stems bleached and sprawling, and only a few leaves here and there to distinguish them from the skeleton of some forlorn quadruped. There are several rarely seen Bursera species from mainland Mexico, with similar habits, such as B. diversifolia, from as far south as Chiapas, and B. morelensis, from somewhat farther north, or the simple-leafed B. schlechtendalii, that would make rewarding plants to cultivate if we could ever find them.
In addition to their other features, these burseras are so filled with dense, highly aromatic sap (called copal in Mexico and used for ceremonies since ancient times), that the slightest touch will release a cloud of pine-scented incense into the air. My memories (dating from what perhaps was the last official UCBG field trip) of walking past these ancient, sprawling “trees,” over six feet branch tip to branch tip and never more than six or seven inches tall, with every inadvertent contact I made instantly wafting the fragrance of piney-incense up to me, still remain vivid after almost twenty years.
In the innocent days before CITES (or before its sudden enforcement, several years later in the late seventies...) fantastic collected burseras from Baja California would show up for sale from time to time. Fortunately, the largest ones were too big to get through the Ag inspections, and, since they lived in areas of almost total isolation, I presume (and hope) they’re still there today.
Across the ocean to Africa brings us to the central habitat of the genus Commiphora , closely related to and very similar to Bursera . There may be as many as 200 species of Commiphora , and they live in Madagascar , Arabia , western India , and possibly southern Brazil as well as the African mainland. Many commiphoras are smallish trees, and they can be quite abundant. A mixed acacia/commiphora scrub dominates much of semi-arid east Africa, where the spongy, moisture retaining commiphora tree trunks make up a considerable portion of the diets of elephants during drought.
In the drier regions, in places such as southern Somalia and South Africa and Namibia, Commiphoa has evolved dwarf species with dramatically swollen, low growing stems. Under appropriate conditions, Namibian and South African species such as C. dulcis, C. cervifolia, and C. saxicola mimic the spectacularly “bonsai-ed” form of the best burseras. Other commiphoras, such as the more widely distributed C. africana, or the Somalian C. holtziana (with a photosynthesizing blue-green epidermis underneath its peeling, golden outer bark) resemble the small tree/large shrub with thickened trunk medium-sized burseras.
Madagascar, rich with pachycaulous plants, has its commiphoras too, both taller growing ones and species such as C. simplicifolia, subject to horizontal growth forms in the right conditions, while the uncommon C. gileadensis, from Yemen, has the typical small commiphora look of a short, thick upright trunk that merges into a long tap root below, and above, quickly divides into several succulent, twisting, almost horizontal main branches which then put out numerous short, non-succulent, upright secondary branches.
Other genera in the Burseraceae include the rarely seen Lannea, from Ethiopia and Somalia, consisting of small, swollen-trunked trees. Plants of Boswellia, from Yemen and the island of Socotra and points east, are less dramatically thickened, but their aromatic sap has been valued for millennia as the source of frankincense (myrrh comes from a commiphora species). In his writing on Socotra, Jon Lavranos has commented on the dwarf, rock growing Boswellia nana, a miniature plant that would probably make a fine succulent bonsai, again, if one could ever find any available.
As is the case with many of these types of plants, cultivation isn’t very difficult. They all need fast draining soil, although not as lean as the most 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 putting out new ones as their growing seasons begin. 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 area, 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; water every three or four weeks will be plenty, and once a week should be adequate during their growing seasons. Some burseras regularly withstand light frost in their environments, and if one has surplus plants it might be worth trying to grow them outdoors, with the warning that an exceptionally cold or wet winter will probably finish them off. The other succulent members of the family mostly are more tropical growing, and best kept protected from cold.
Some of these rare plants are almost non-existent in cultivation, but specialist growers are raising a surprisingly large number of varieties of them. They generally grow fairly quickly as seedlings, but it takes a lot of patience and some careful pruning to convert them from twiggy baby trees to fantastic succulent pachycauls. Plants will root from cuttings, and these rooted branches will develop a bit of thickness over the years, but if you have enough time, seed grown plants are the best. Even as tiny seedlings the most arid growing species have a tendency to grow into odd shapes that with time and perseverance will result in a nice looking specimen. Just remember that in cultivation the goal is to produce a situation equivalent to a habitat defined by constant wind, intense sun, and very little rain in order to end up with a compact, horizontally oriented, piece of living sculpture.
The Garden has a good collection of Bursera, including some very rarely seen plants, and some nice Commiphora specimens as well. Occasionally we have seedlings available for sale.
|
