Germination of desert annuals to shrub facilitation is species specific but not ecotypic
Aims Positive plant-interactions can promote higher species density of beneficiary species in deserts. However, there is limited evidence examining the trait sets of seeds from beneficiary species. In this study, we examined the hypothesis that shrubs (benefactors) influence the germination of desert annuals (beneficiaries) and promote ecotypic differentiation by generating distinct microsites through abiotic stress amelioration. The following predictions were tested using growth chamber and field-collected seeds: i) seed mass and viability will be greater and less variable for seeds collected from within shrub understory relative to seeds from open microsites, ii) germination of seeds from shrub and open microsites will be greatest under home (source) conditions, iii) seeds from the shrub will adaptively accelerate their germination rate when germinated in simulated home (source) microsites relative to their simulated away (reciprocal) microsite.
Methods Seeds and their associated maternal plants were collected from four annual species found within a shrub understory (Larrea tridentata) and open microsites in the Mojave Desert of California (35.30ᵒ N, 117.26ᵒ W, 793 m.a.s.l.), and then reciprocally germinated in growth chambers simulating both microclimatic conditions. Cumulative germination and germination rate was measured every 4-5 days for 42 days.
Important findings There was no significant difference in the mean or coefficient of variation for seed mass and viability between the shrub and open microsites. The source of the seeds did not significantly impact the cumulative germination and there was no accelerated rate of germination within species which suggests that ecotypic differentiation is not occurring. Cumulative germination was significantly higher within the shrub-simulated microhabitat for three out of the four species examined. Cumulative germination and germination rate was significantly different between species. These results support that shrubs influence the germination of desert annuals, and the outcome of this interaction is species specific, but shrub microsites do not alter the more conserved seed biology traits. Lack of gene flow, short dispersal distances, or source-sink dynamics can reduce trait differentiation and should be tested in future studies.
