The role of competition in serpentine specialization in a clade of Californian mustards

Background/Question/Methods

Rare habitats can contribute substantially to species diversity. For example, serpentine soil comprises less than 1% of California, but serpentine endemics comprise about 10% of the California flora.  Understanding specialization to rare habitats may thus contribute to understanding patterns of plant diversity. The ability to tolerate low Ca:Mg ratios and high concentrations of heavy metals in serpentine soils is necessary to explain occurrence on serpentine, but insufficient to explain why species are restricted to these substrates. Are serpentine specialists incapable of using the non-serpentine soil substrates of their relatives? Are they outcompeted on these non-serpentine soils?

The Streptanthus clade (sensu lato) contains about 45 species, with its center of diversity in California. Most species in the group occur on rocky substrates, and about a third on serpentine outcrops.  Serpentine association ranges from species that are intolerant of serpentine substrates to those that are restricted to them. There is also intraspecific variation in the degree of edaphic specialization, with 2-3 “bodenvag” species that have populations occurring both on and off serpentine.

Here, we explore the effects of substrate and competition on plant performance across eight species of Streptanthus planted in field soils of close relatives in a semi-controlled environment (lath-house).

Results/Conclusions

We find that while there are differences in the intensity of competition among species, overall, the impacts of competition (measured as biomass or RII) are lower in serpentine soil than in non-serpentine soil.  The same effect is evident across bodenvag species, and within a single bodenvag species for which replicate serpentine and non-serpentine populations allowed the comparison (S. glandulosus).

Moreover, serpentine specialists appear to be more inhibited by competition than bodenvag or non-serpentine specialists both in serpentine and non-serpentine substrates.  We observe a similar pattern across populations of the bodenvag species S. glandulosus across soils: populations that originate from serpentine seem to be more sensitive to competition.

Thus, serpentine-specialized species of Streptanthus, like many other edaphic specialists, perform as well, if not better, when grown on non-serpentine substrates. When grown in competition with other plant species in other soil substrates, edaphic specialists seem to perform more poorly than non-specialists.  Our results are in agreement with edaphic specialization predictions and previous work on competition-specialization tradeoffs. Future work focuses on trade-offs between competition and herbivore defense as part of the constraints of edaphic specialists on non-serpentine soils.