Abronia: IV. Anthocarp Dispersibility and its Ecological Implications for Nine Species of Abronia

Abstract

Field observations, morphological data and experimental findings are combined with information about the distribution and habitats of each species. Adaptive changes toward increased or decreased anthocarp dispersal by wind or water are postulated for 9 Abronia species. Anthocarp and seed weights, and anthocarp dimensions were used to calculate dispersal potentials which are contrasted with the actual movement of the anthocarps tested under artificial wind conditions. Anthocarp dispersibility, survival after dispersal, and selective impact of seed loss through dispersal are discussed. Seed germination, seedling salt tolerance, and reproductive capacity are discussed as being relevant to an interpretation of the evolution of dispersal mechanisms in Abronia species. Because Abronia species seem adapted to intermittent, unstable, nonsaline, pioneer habitats, often adjacent to saline or otherwise ecologically closed environments, it is hypothesized that the anthocarps and seeds of most of the species of Abronia examined evolved in such a way that windblown or sea-drifted dispersal units are moved across ecologically unsuitable habitats into pockets of suitable habitats where the seedlings may grow. Extremes in anthocarp modifications are represented by A. maritima, A. crux-maltae, and A. alpina. The combination of percent seed weight per total dispersal unit weight, surface area in mm2 per 0.001 g, actual distance moved under artificial wind conditions, and flotation time anthocarp characteristics suggests that: in A. maritima, from coastal foredune habitats, anthocarps evolved toward flotation; in A. crux-maltae, from Carson Desert dunes, anthocarps evolved toward wind dispersal and in A. alpina, from sandy borders of alpine meadows of the southern Sierra Nevada, anthocarps evolved toward reduced dispersibility. The remaining species include A. latifolia, A. turbinata, A. umbellata, A. pogonantha, A. nana covillei and A. villosa.