Ecosystem consequences of species evolution in response to global environmental change
Many global change models and experiments aim to determine how increasing atmospheric CO2 will alter carbon sequestration and competitive dynamics of important plant communities such as eastern North American pine forests and coastal marshes. However, few models consider that plant responses to global environmental change may vary by geography and genotype. We are collaborating with colleagues at the US EPA, University of Notre Dame and the South Florida Water Management District on a series of studies that aim to demonstrate whether physiological responses to global environmental change reflect genotypic differences among individual plants and genetic variation among plant populations. This research is designed to determine whether increased salinity and atmospheric CO2 act as selective agents on Schoenoplectus americanus (a common Atlantic and Gulf coast upland marsh sedge), and whether responses to selection influence soil accumulation rates and carbon cycling in coastal marshes.
Common garden and phytotron experiments have demonstrated that S. americanus responses to elevated salinity and CO2 vary by genotype and geography (Saunders et al., submitted; McLachlan et al., in prep). Comparisons across multifactorial treatments found that salinity tolerance and growth responses to CO2 have a genetic basis. Comparisons among genotypes representative of three distinct geographic locations (Texas, Maryland and New Jersey) indicated that variation in growth and senescence is attributable to geography, suggesting that plant responses to global environmental change likely reflect local adaptations (Saunders et al., submitted; McLachlan et al., in prep). Further study of S. americanus grown from seeds recovered from two 210Pb and 137Cs dated sediment cores taken from a Chesapeake Bay marsh at the Smithsonian Environmental Research Center demonstrated that the genotypic composition of this population has shifted since the onset of the industrial revolution in eastern North America (Saunders et al., submitted). Recently completed phytotron experiments showed that extant and historic genotypes differ in response to elevated salinity and atmospheric CO2, which suggests that genotypic changes over time reflect the rise of favored genotypes (e.g. those capable of increasing net carbon gain and greater allocation of surplus carbon to the acquisition of limiting resources) in the population since the late 19th century. Comparison of variation in salinity tolerance over time to historical rainfall records and paleosalinity reconstructions for Chesapeake Bay suggest that individuals with higher salinity tolerance became more prevalent in the population during periods of decreasing rainfall and that among-cohort variation in salinity tolerance tracks multi-decadal trends in rainfall. Because salinity tolerance in S. americanus is tightly coupled to primary productivity, these findings support the hypothesis that microevolutionary responses can influence ecosystem processes under changing environmental conditions.
Ongoing studies of hybridization between Schoenoplectus americanus and S. pungens in Chesapeake Bay are examining whether introgression enhances adaptive potential to global environmental change (Knapke et al., in review). This work is contributing to additional studies examining whether forecasts of future ecosystem and climate conditions are sensitive to changes in the genetic composition of ecologically dominant species and possible feedbacks between ecological and evolutionary responses to environmental change (Blum et al., in prep; McLachlan et al., in prep).
Overcoming founding effects: Admixture and hybridization in salt marsh cordgrasses
The loss of genetic variation due to genetic drift in small founding populations is thought to limit the success of introduced species. If so, then multiple introductions or large founding populations are essential for biological invasions to proceed. Yet small founding populations of non-native species can persist and become invasive following hybridization with native congeners. Admixture following secondary contact between previously allopatric populations can also lead to invasion success. We are working with colleagues at UC Davis on Spartina cordgrass invasions of Pacific estuaries to examine how hybridization and admixture can overcome founding effects. Native to the Atlantic and Gulf coasts of North America, S. alterniflora was intentionally introduced to San Francisco Bay (California) and accidentally introduced to Willapa Bay (Washington). It quickly hybridized with the endemic California cordgrass (S. foliosa) in San Francisco Bay, where hybrids have since displaced nearly all of the invasive S. alterniflora, and are quickly overtaking S. foliosa. Willapa Bay occurs to the north of the distributional limit of S. foliosa and all cordgrass herbivores. Without competition or natural enemies, S. alterniflora is expected to spread across Willapa Bay. Notably, over 50 years passed before the species began spreading within the estuary. Considering that historical records show that S. alterniflora was serially introduced into Willapa Bay via oyster shipments from New York and Chesapeake Bay, heterosis resulting from admixture is one possible explanation for the delayed onset of spread in Willapa Bay.
To test this hypothesis, and because little is known about the extent to which native and non-native S. alterniflora populations differ from one another, we completed a study to compare genetic diversity and genotypic variation in native and non-native S. alterniflora (Blum et al., 2007). Low levels of gene flow and geographic patterns of genetic variation were found among native S. alterniflora sampled from the Atlantic and Gulf coasts of North America. For example, mid-Atlantic S. alterniflora are differentiated from S. alterniflora in southern Atlantic and New England coastal marshes. Comparisons of genotypic composition and genetic variation among native and non-native populations found evidence of low diversity in introduced S. alterniflora. However, these comparisons substantiated prior studies demonstrating reciprocal interspecific hybridization in San Francisco Bay, and found that Willapa Bay S. alterniflora are genetically divergent from putative sources as a result of admixture among genets originating from allopatric native populations (Blum et al., 2007; Bando, submitted). Interspecific hybrid cordgrasses in San Francisco Bay are known to exhibit heterosis, whereas additional work will be necessary to demonstrate whether admixed genotypes exhibit heterosis. These findings suggest that forecasting models intended to predict successful invasions should consider factors that can increase fitness even when genetic diversity is low in newly founded populations. Follow-on studies demonstrating the formation of allopolyploid hybrids resulting from other post-introduction hybridization events in California (Ayres et al., 2008) and Spain (Castillo et al., in prep) support this inference.
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•Blum, M.J., McLachlan, J.S., Saunders, C.J., Hamilton, R., Herrick, J.D. Genetic variation within Schoenoplectus americanus across multiple spatial scales. (in review)
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•Blum, M.J., McLachlan, J.S., Saunders, C.J., Herrick, J.D. 2005. Characterization of microsatellite loci in Schoenoplectus americanus across multiple spatial scales. Molecular Ecology Notes 5: 661-663.
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•Saunders, C.J. Blum, M.J., McLachlan, J.S., Craft, C., Herrick, J.D. Evolutionary responses to global environmental change: Inferences from a coastal marsh sedge population resurrected from a century long seed bank. (in review)
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•McLachlan, J.S., Saunders, C.J. Blum, M.J., Herrick, J.D. Ecosystem consequences of adaptation to rising atmospheric CO2. (in preparation)
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•Blum, M.J., Knapke, E., McLachlan, J.S., Snider, S.B., Saunders, C.J. Hybridization between Schoenoplectus sedges across Chesapeake Bay marshes. (in review)
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•Ayres, D.R., Grotkopp, E., Zaremba, K., Sloop, C.M., Blum, M.J., Bailey, J., Anttila, C., Strong, D.R. 2008. Hybridization between invasive Spartina densiflora (Poaceae) and native S. foliosa in San Francisco Bay, California, USA. American Journal of Botany 95(6): 713-719.
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•Blum, M.J., Bando, J., Katz, M., Strong, D.R. 2007. Geographic structure, genetic diversity and source tracking of Spartina alterniflora. Journal of Biogeography 34(12): 2055–2069.
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•Blum, M.J., Ayres, D.R., Sloop, C.M., Strong, D.R. 2004. Characterization of microsatellite loci in Spartina alterniflora. Molecular Ecology Notes 4(1): 39-42.
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•Sloop, C.M., McGray, H.G., Blum, M.J., Strong, D.R. 2005. Characterization of 24 additional microsatellite loci in Spartina species (Poaceae). Conservation Genetics 6: 661-663.
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•Castillo, J.M., Ayres, D.R., Leira-Doce, P., Bailey, J., Blum, M.J., Strong, D.R., Figueroa, E. The production of highly competitive hybrids between exotic Spartina densiflora and native S. maritima in the Iberian Peninsula. (in review)
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