Reproductive and ecophysiological attributes of the rare Gardenia actinocarpa (Rubiaceae) compared with its common co-occurring congener, G. ovularis

Gardenia actinocarpa Puttock (rare) and G. ovularis F.M.Bailey (widespread) are closely related co-occurring shrub–small trees of rainforests of far north Queensland, Australia. Gardenia actinocarpa is endemic to alluvial lowlands along Noah–Oliver Creeks of Cape Tribulation, whereas G. ovularis spans a wider habitat and occurs on a range of substrates and altitudes. The rare G. actinocarpa is cryptically and strictly dioecious, with a longer reproductive period (9 months) and a low fecundity (1.77 fruits per female tree). In contrast, G. ovularis , although also dioecious, has a shorter reproductive phase (4 months) and produces more fruits (35.76 fruits per female tree), some of which may occur in the absence of fertilisation (apomixis). Under different regimes of photosynthetically active radiation (PAR), seed germination percentages and patterns were similar (70–90%) in the two species, although at 37% PAR G. actinocarpa has lower germination (55–68%). The rare G. actinocarpa loses seed viability very rapidly (<15% viability after 3 months) compared with G. ovularis (37%). Gardenia actinocarpa grows and survives in both closed forest understorey and in small gaps, while G. ovularis prefers light gaps (both small and large). Net photosynthetic measurements of seedlings grown at 2.5 and 37% PAR suggest that the physiological performance of G. actinocarpa is not inferior to that of G. ovularis , although the latter exhibits greater plasticity. In a density-dependent seedling growth experiment, G. actinocarpa , surprisingly, has a superior competitive ability. Thus, the rarity of G. actinocarpa may be a result of limited fecundity, reduced seed dispersal ability and lack of soil seed-banks rather than inferior vegetative and ecophysiological traits.