It is well established that ecological change can give rise to evolutionary responses, but surprisingly little is known about the converse. There is good reason to think that evolution is an essential aspect of how ecosystems function- not only can the expression of heritable traits of a single species influence ecosystem processes, but evolution can occur on ecological timescales. Determining whether ecosystem structure and function hinge on evolutionary dynamics, however, requires disentangling the influence of heritable phenotypic change, non-heritable phenotypic plasticity, and the environment on key ecosystem properties. Further work is also necessary to track ecosystem outcomes of dynamically evolving populations to understand rates and trajectories of ecological change rather than conditions attributable to historical events.
The influence of rising atmospheric CO2 and corollaries of climate change (e.g., rising sea levels) on coastal marsh plants presents an especially compelling framework for considering whether evolution shapes ecosystems. Even small evolutionary changes in the capacity of plants to accommodate rising CO2 and climate-related stressors (e.g., salinity, inundation) could have pronounced aggregate impacts on coastal marshes, which notably exert outsized influence on the global carbon cycle. Outcomes of contemporary global environmental change are well understood for coastal marshes: conversion to open water occurs if platform elevation does not exceed sea level rise. Evolution might deter conversion of marshes to open water because heritable traits of marsh plants, such as root structure, stem density, and canopy height, can govern platform elevation. The sum of these effects can be large- vegetation can increase platform elevation by as much as 13.3 mm per year- thus even small evolutionary changes in the capacity of plants to accommodate rising CO2 and climate stressors could determine whether marsh elevation keeps pace with or exceeds sea level.
“Blue Genes” project
Members of the Blum lab have been collaborating with colleagues at the Smithsonian Environmental Research Center (Pat Megonigal), the University of Notre Dame (Jason McLachlan), Bryn Mawr College (Tom Mozdzer), and elsewhere to examine the evolutionary dimensions of coastal marsh responses to environmental change. Unlike previous studies of plant responses to elevated CO2 and other environmental variables, we have been able to reconstruct century-long records of response using living organisms resurrected from the past by exploiting a highly stratified and persistent soil-stored seed bank of a foundational sedge (Schoenoplectus americanus, formerly Scirpus olneyi) in a Chesapeake Bay marsh. Our study species has long served as a de facto model organism for research on tidal marsh responses to global environmental change. This is largely because S. americanus productivity drives carbon and nutrient cycling and by also influencing the accumulation of soil organic matter, S. americanus performance can govern the formation and fate of coastal marshes. We have completed a series of germination trials, common-garden exposure experiments, and ecosystem model simulations demonstrating that (1) population abundance and genotypic variation in S. americanus have shifted in the marsh since the turn of the 20th century; (2) shifts in genotypic variation correspond to shifts in phenotypic variation; (3) biomass production in response to salinity, inundation and CO2 availability differs among ancestral and descendant S. americanus genotypes; (4) the effects of genotypic variation on growth can be as large as those produced by nitrogen addition; and (5) that heritable variation in exposure response can alter rates of marsh surface elevational change.
We are currently working on an NSF-funded project to undertake experimental manipulations and mechanistic modeling to estimate heritable, non-heritable and environmental contributions to ecosystem attributes that can determine the fate of coastal marsh ecosystems. Taking ecological interactions into consideration, we have been undertaking a complementary set of genomic, experimental, and modeling studies to determine whether and how carbon assimilation and surface elevation shift as a result of evolutionary responses of S. americanus to salinity, inundation, and carbon availability. As current projections of coastal marsh ecosystem responses to global environmental change (i.e., rising atmospheric CO2, sea level rise) assume static relationships between climate-related stressors and plant performance, completing this work will clarify whether ecosystem outcomes of evolution have been mistakenly attributed in previous studies, which will improve models of ecosystem processes and persistence under past and future climate scenarios.
Interest in this work has already spun off several similarly minded collaborative efforts including research on eco-evolutionary dynamics of plant-soil-microbiome interactions. For example, we are currently working with Candice Lumibao (TAMUCC) to examine the stability of plant-microbiome partnerships over time, and to assess the potential influence of microbes on plant performance under conditions of environmental change. We have recently completed a common garden greenhouse experiment using plants derived from ancestral (~100 year old) and descendant (~20 year old) seeds, along with associated soil (to mimic natural microbiome conditions) from sediment cores taken from sites in the Chesapeake Bay. Our findings indicate that microbes confer greater capacity for host plants to tolerate and persist under environmental stress, but that outcomes of plant-microbe associations depend on prevailing environmental conditions as well as host genotype, provenance, and evolution.
“C-EVO” project
Members of the Blum lab have recently begun collaborating with colleagues at the Smithsonian Environmental Research Center (Melissa McCormick, Pat Megonigal), Bryn Mawr College (Tom Mozdzer), and elsewhere on an NSF-funded project to further examine the evolutionary dimensions of coastal marsh responses to global change. The “C-EVO” project examines how evolutionary responses of Phragmites australis australis to elevated CO2 and nitrogen enrichment influence the flow of carbon into and out of coastal marsh ecosystems. This project encompasses a complementary set of studies explicitly designed to bridge evolutionary biology and ecosystem ecology. Leveraging infrastructure, data and tissue archives from an ongoing long-term field-scale experiment, we are assaying in situ genetic and genomic variation alongside concurrent measures of functional trait variation and C cycling across more than a decade of exposure to elevated CO2 and nitrogen enrichment. To gain further insight, we are undertaking a complementary de novo quantitative genetics study of trait variation and exposure responses in relation to measures of C cycling. Alignment of new findings with prior work on community-level outcomes of global change will allow parameterization of a mechanistic model of C cycling in coastal marshes to further explore ecosystem outcomes of organismal evolution. We expect that the project will deliver new perspectives on the magnitude, pace, and nature of responses to global change, with the aim of identifying mechanistic and functional linkages that couple plant evolution to carbon cycling in coastal marshes. Findings from this project thus have the potential to improve our understanding of the global carbon budget, and to foster more effective coastal restoration by advancing understanding of marsh persistence.
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