Reproductive biology and ecology of selected rare and endangered Oxalis L. (Oxalidaceae) plant species

About twenty-five percent of all southern African Oxalis species are rare/endangered and highly localized, making them especially vulnerable to extinction through inbreeding, low genetic variation, disrupted biological interactions and stochastic events; all consequences of small population sizes. Moreover, Oxalis displays tristyly, which is a rare and specialized sexual system that includes a strong self-incompatibility component between three floral morphs to promote out-crossing within populations. As tristyly requires the availability of plants with different floral morphs as well as effective pollinators for seed production, this breeding system can affect small populations when fully expressed. Factors that may have an effect on rarity in Oxalis were investigated by focusing on the expression of tristyly, levels of natural seed production, clonality and the ecology of eight rare/highly localized Oxalis species. Field experiments revealed that the reproductive success of some Oxalis species may be hampered by tristyly, resulting in extremely low levels of natural seed production. Other species display a more relaxed expression of self-incompatibility, which in combination with the possibility of cross-pollinations provides reproductive assurance regardless of population structure and pollinator availability. Others are rare and endangered, but appear not to be negatively affected by the tristylous breeding system. Most species are limited by their highly specific habitat requirements and are particularly vulnerable to variation in rainfall patterns.