Phenological asynchrony between sexes of Restionaceae can explain culm δ13C differences

Abstract

AbstractDecade‐old observations of sex‐specific differences in culm δ13C for the diverse and ecologically important Restionaceae of the Cape Florisitic Region (CFR) have yet to be explained mechanistically. In the dioecious, wind‐pollinated Cannomois congesta (Mast.), we found that sexual differences in culm δ13C were not explained by intrinsic differences in water use efficiency (WUE), but rather by differences in growth phenology. When growing together, instantaneous WUE (A/g) was equal between sexes. However, we noted strong phenological asynchrony between the sexes where males began to grow new culms nearly 2 months earlier than females. This initial period of growth in males corresponded to hotter, drier conditions and temporarily higher instantaneous WUE, thus driving less negative δ13C signals in male culms. We suspect that phenological differences might explain previously reported sexual differences in culm δ13C in the Restionaceae family. Despite earlier culm emergence in males, reduced growth in males prior to flowering, presumably due to costs associated with reproduction, resulted in both sexes flowering at the same height. We discuss the potential selective advantage of earlier growth in males as it relates to inflorescence heights and pollination efficiency.