WINTER GROWTH PHENOLOGY AND LEAF ORIENTATION IN PACHYPODIUM-NAMAQUANUM (APOCYNACEAE) IN THE SUCCULENT KAROO OF THE RICHTERSVELD, SOUTH-AFRICA

Pachypodium namaquanum (Nyley ex Harb.) Welw., an unusual arborescent stem succulent from the succulent karoo of the arid Richtersveld in no rth-western South Africa and adjacent Namibia, is characterized by a s triking curvature of the terminal 20-60 cm of the trunk toward the nor th. This orientation displays the single terminal whorl of drought-dec iduous leaves with their flat surface angled at a mean inclination of 55 degrees from horizontal. Inclination of 50-60 degrees was found in 65% of individuals sampled, and 85% were inclined between 45 and 65 de grees. Northward azimuth was also quite regular, but varied slightly b etween populations. The fixed leaf orientation in P. namaquanum maximi zes radiation absorption during the winter months when leaves are pres ent. Leaves normally form in early fall (April) and abscise early in s pring (October). Growing season conditions in the Richtersveld are rel atively mild, with mean maximum temperature dropping only to 21.6 degr ees C in July, the coldest month of the year. Frosts are rare. By the fixed orientation of its leaf whorl, P. namaquanum is able to maintain nearly twice the midwinter radiation absorptance that it would have w ith horizontal orientation. Over an annual cycle the angled leaves rec eive more radiation than would horizontal leaves for each of the 6 mon ths in which they are present on the plant. This increased winter irra diance is hypothesized to significantly increase net primary productio n by concentrating growth activities in winter months and allowing the species to remain dormant during the hyperarid conditions of the hot summer months. Midwinter flowering from apical buds in P. namaquanum m ay also he aided by its stem orientation. The evolution of this charac teristic pattern of winter growth phenology and nodding stem orientati on may have come about because of low but relatively regular autumn pr ecipitation and moderate winter temperatures. Slow and regular growth of P. namaquanum leads to long lifespans which may reach 300 years or more.