Reproductive biomass allocation in the dioecious perennial Acanthosicyos horrida

In a Namib population of the dioecious perennial Acanthosicyos horrida Welw. ex Hook. f. in which fruit production is declining, we investigated the effects of plant gender, plant size, and subpopulation elevation (a proxy of water availability) on a plant's biomass allocation to sexual reproduction. While males invested more heavily in buds and flowers than females, females allocated 10 times more total biomass to reproduction per m³ of vegetation than males during our survey period. Also, smaller plants, particularly females, allocated more biomass to reproduction per m³ of vegetation than larger plants. This result suggests that plant fecundity per m³ of vegetation decreases with plant size. Aging of the population could thus be a cause of the reduced fruit production; however, we found that the population was skewed towards the smallest size classes. On the other hand, the fact that plants in the subpopulation at higher elevation above the river were only six percent as productive as plants in the subpopulation at low elevation suggests that the depth of the water table may be critical to fruit production and that a reduced groundwater level caused by nearby urbanisation could have long term effects on harvests.