Why wait? Trait and habitat correlates of variation in germination speed among Kalahari annuals

Abstract

The longer soil has to stay moist to allow germination the more likely that seedlings experience favourable moisture conditions. Since theory predicts that fitness variance-reducing traits will be negatively correlated, we tested the hypothesis that time to germination is negatively correlated with the ability of radicle growth to keep up with the drying front. We measured time to germination and root elongation rate (RER) in 14 Kalahari annuals. We controlled for habitat (canopy association and sand content), germinability, median base water potential for germination (psi(50)), seed mass and seed shape as a persistence surrogate. For species and phylogenetically independent contrasts (PICs), we did not find a relationship between time to germination and RER. However, we found a negative relationship of time to germination with RER for PICs when controlling for sand content and psi(50). Seed shape increased with time to germination which can be explained by reduced opportunities for germination in slow-germinating species that select for persistence. We found a positive relationship between time to germination and psi(50), suggesting a continuum of risky to cautious germination. psi(50) was not correlated with RER suggesting that variation in psi(50) reflects different drought-adapted traits. Probably the relationship of time to germination with RER is not mediated by seed mass, which was not correlated with time to germination as found by others, though RER was positively correlated with seed mass. Instead of the seed size-seed number trade-off, a trade-off with resource capture may explain variation in RER: more root hairs or rootlets may increase resource capturing surface while reducing RER. For habitat, we found a (positive) relationship with time to germination only for canopy association. This may be explained by maximization of resource capture at the expense of RER being favoured by the higher nutrient and water availability under canopies. Future studies should clarify which trade-offs govern variation in time to germination, focussing on a possible resource capture-RER trade-off.

Seed mass and summer drought survival in a Mediterranean-climate ecosystem

“…On the other hand, small-seeded species are generally better able to take advantage of episodic rainfall due to high seed production (e.g. Rees 1995), rapid germination (Jurado and Westoby 1992b) and perhaps predictable germination (Kos and Poschlod 2010), but in unfavourable years may be restricted to limited microsites (Venable and Brown 1988;Coomes et al 2002) or remain dormant in the seed bank (Venable and Brown 1988;Jurado and Westoby 1992b).…”

Differences in the timing of germination and reproduction relate to growth physiology and population dynamics of Sonoran Desert winter annuals

“…Species differ in the time required for wetted nondormant seeds to germinate, and longer germination times may be an adaptation to prevent germination under unfavorable conditions, especially in arid regions ( Kos and Poschlod, 2010 ). As a result, species differ in their timing of germination, which may infl uence their synchrony with favorable growing conditions, their exposure to competition and predation, and community structure ( Forbis, 2010 ;Kos and Poschlod, 2010 ). Likewise, the timing of growth and reproduction differs among species, refl ecting unique physiological adaptations for resource utilization under particular environmental conditions ( Pavon and Briones, 2001 ).…”

Relationship of kinds of seed dormancy with habitat and life history in theSouthernKalahari flora

“…That environmental variables explain less of the variation in regenerative traits than plant traits has been found for seed mass (Moles et al. ) and germination behaviour (Kos & Poschlod , ). The predominance of plant traits as compared to environmental gradients in explaining variation in kinds of dormancy is likely to reflect different co‐existing plant ecological strategies.…”