Drought Stress, Chemical Warfare and Espionage

The elephant tree (Bursera microphylla) adopts a grotesque form; its swollen multi-trunked habit resembles that of a rubenesque contortionist. Spindly chaotic branches radiate from its stubby trunk, waving modest leaflets at the sun. Even the flowers are pale, drab, and inconspicuous. However, what it lacks in decadent display is recouped by its unusual survival strategies, particularly for a tree from a largely tropical family.

Leaves are expensive in the desert. Since leaf water loss is proportionate to surface area, desert environments should selectively favor leaves with the least surface area. The elephant tree splays a tinsel-like canopy that maximizes solar absorption while minimizing water loss via transpiration. But sometimes even these small investments are too costly during extended periods of extreme water stress. Rather than allowing the hot dry air to pull all the water from the plant through its leaves, the elephant tree becomes drought deciduous, dropping all of its leaves. Beneath the trunk’s white peeling bark lies the emergency photosynthetic machine that can compensate for the lack of leaves through several months of drought.

Leaves may be expensive but they’re a mandatory investment and should be defended. Most Bursera species produce an aromatic resin in their leaves that act as a herbivore deterrent. The unsuspecting insect that opts to feed on these leaves will be shot with hearty dose of pressurized sap. Of course not all insects are so easily discouraged; some beetle species (Blepharida) exploit the plant’s phytochemical defenses. After consuming the terpenoid sap, they cover themselves with their own laced feces and if this doesn’t keep the neighbors away; they will readily defecate and/or regurgitate on anyone that disturbs them!

Further investigation of the elephant tree spurs a deeper appreciation for its humble and homely habit. It deformed bonsai-like appearance becomes admirable when form is integrated with function.