Canopy architecture and PAR absorption of Euphorbia cooperi in the Matobo Hills, Zimbabwe

Tree euphorbias share many traits with arborescent cacti in having tall and multi-branched forms of growth which have selective value for maximizing photosynthetic surface area and water storage. The candelabra growth form of Euphorbia cooperi, and likely other Euphorbia species with similar canopy architecture in southern Africa, allows these plants to add photosynthetic surface area as they grow and increase in size while at the same time minimizing problems of transpirational water loss. This increase in branch surface area occurs with age and increase in size and numbers of branches at the same time that the amount of potential storage volume to support water loss from this expanded surface area is also increasing. Key to this ability is the morphological trait of a heavily suberized trunk that increases with age, providing added capacity for water storage without adding to surface area subject to transpirational water loss. This trait and the associated canopy architecture allows the development of increasing surface area with minimal self-shading as new branches develop so long as older branches are shed. Beyond the canopy architecture, the cross-sectional morphology of branches in E. cooperi with concave faces has selective value in balancing increased surface area with maintenance of adequate storage volume. Simulated patterns of branch morphology with a square cross-sectional morphology show a “safe” result in producing little change in S:V ratio as plants mature, but at the expense of limiting photosynthetic surface area. At the other extreme, a simulated cross-sectional morphology with deep concave flutes, as seen in young plants, would significantly increase photosynthetic surface area, but at a potential cost of increased sensitivity to drought stress.