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Evolutionary and plastic changes in a native annual plant after a historic drought

Citation

Lambrecht, Susan; Gujral, Anjum; Renshaw, Lani; Rosenngreen, Lars (2021), Evolutionary and plastic changes in a native annual plant after a historic drought, Dryad, Dataset, https://doi.org/10.5061/dryad.3tx95x6c3

Abstract

Severe droughts are forecast to increase with global change. Approaches that enable the study of contemporary evolution, such as resurrection studies, are valuable for providing insights into the responses of populations to global change. In this study, we used a resurrection approach to study the evolution of the California native Leptosiphon bicolor (true babystars, Polemoniaceae) across populations differing in precipitation in response to the state’s recent prolonged drought (2011-2017). In the Mediterranean-climate region in which L. bicolor grows, this historic drought effectively shortened its growing season. We used seeds collected both before and after this drought from three populations found along a moisture availability gradient to assess contemporary evolution in a common garden greenhouse study. We coupled this with a drought experiment to examine plasticity. We found evolution toward earlier flowering after the historic drought in the wettest of the three populations, while plasticity to experimental drought was observed across all three. We also observed trade-offs associated with earlier flowering. In the driest population, plants that flowered earlier had lower intrinsic water-use efficiency than those flowering later, which was an expected pattern. Unexpectedly, earlier flowering plants had larger flowers. Two populations exhibited evolution and plasticity toward smaller flowers with drought. The third exhibited evolution toward larger flowers, but displayed no plasticity. Our results provide valuable insights into differences among native plant populations in response to drought.

Methods

These data were collected as part of a resurrection type study to examine contemporary evolution in response to drought. Seeds were collected from three field populations of Leptosiphon bicolor (Polemoniaceae) varying in moisture availability both before and after a multi-year historic drought in California. Seeds were grown together in a greenhouse in a "refresher" generation to remove maternal and storage effects, and allowed to set seed. These seeds were then planted in a greenhouse experiment in which plants were given one of two watering treatments (low- and well-watered). We monitored plants daily to record germination date and flowering date. On the first flowering date, flower size was measured and plants were collected to measure aboveground biomass and perform stable carbon isotope analyses to assess intrinsic water-use efficiency. Date of death was recorded for any plants that died before flowering

Usage Notes

All tests were analyzed using SPSS (v.25, IBM Corp., NY, USA). To test for evidence of earlier flowering time, we used a Cox proportional hazards model. Our analyses included generation (ancestral or descendant), population, watering treatment, and block (planting date), as well as all 2- and 3-way interaction terms between generation, population, and treatment. The test generates a Wald chi-square test statistic, which is a one-tailed test with alpha = 0.10. After our initial analysis indicated significant differences between populations, we ran post-hoc pairwise models to compare populations, using the Sidak-corrected alpha = 0.034 (Tripathi and Pandey 2017). Finally, we ran individual analyses of flowering time for each population. 

We used mixed models to examine how physiological and morphological traits (intrinsic WUE, flower size, and aboveground vegetative mass) varied among populations over time and with watering treatment using MIXED in SPSS. We first ran the full model including all 2- and 3-way interactions between population, generation, and treatment. We then ran subsequent models, eliminating non-significant interaction terms, one at a time, selecting the model with the lowest value of Aikike’s Information Criteria. When a significant population effect was detected, we used estimated marginal means with a Bonferroni correction to make pairwise comparisons between populations. Finally, intra-population models were run to determine the effect of generation and watering treatment within each population. 

We used pairwise Pearson correlations to examine the relationship between flowering time and the other measured traits. Correlations were analyzed within each population, generation, and treatment combination.

            We assessed survival of plants through the study using the non-parametric chi square-test of independence. We compared survival both across the generations and between the watering treatments. We followed these tests with an analysis that compared survival in the low water treatment between generations of each population.

Funding

U.S. Department of Education, Award: P031C1160097