Flowering phenology of South African Oxalis-possible indicator of climate change?
Oxalis is a large geophytic genus that has diversified extensively in the winter rainfall region of the Cape Flora, South Africa. Patterns of flowering within Oxalis were investigated at both a regional scale (focusing on timing of flowering of Oxalis species in the Cape Region) and a local scale in a single habitat, the J.S. Marais Park, Stellenbosch, over 3 years (1999, 2003 and 2004). We found the active growth period of Oxalis to coincide with the peak rainfall period in the Cape Region, the start of flowering dependent on both the onset of the first significant rains and a drop in average daily temperatures. Both at a regional and local scale endospermous (dormant seed) species displayed an extended flowering season, while exendospermous (non-dormant seed) species displayed flowering peaks early in the rainy season. This correlates well with seedling strategies, in that dormant seeds of endospermous species are less affected by the dry summer months, while seeds of exendospermous species lack dormancy, and must thus germinate and establish seedlings well-before the onset of the dry summer months. Oxalis species in the local study displayed sequential replacement of flowering onset over the growing season, although there was an overlap in peak flowering times. The flowering sensitivity to alterations in temperature and delayed onset of winter rains suggests that specifically exendospermous species of Oxalis may indicate changes in climate. We hypothesize that global warming will influence the relative proportions of exendospermous vs. endospermous species flowering at local and regional scales in the Cape Region of South Africa. © 2005 Elsevier B.V. All rights reserved.