Variation in tussock architecture of the invasive cordgrass Spartina densiflora along the Pacific Coast of North America

Abstract

Some introduced species spread rapidly beyond their native range and into novel habitats mediated by a high degree of phenotypic plasticity and/or rapid evolutionary responses. In this context, clonality has been described as a significant factor contributing to invasiveness. We studied the abiotic environment and the responses of different tussock architecture traits of the invasive cordgrass Spartina densiflora Brongn. (Poaceae). A common garden experiment and field studies of S. densiflora in salt marshes across a wide latitudinal gradient from California (USA) to British Columbia (Canada) provided a model system for an integrated study of the potential mechanisms underlying the response of invasive S. densiflora populations to changes in environmental conditions. Our results showed that S. densiflora is able to adjust to widely variable climate (specifically, air temperature and the duration of the growing season) and sediment conditions (specifically, texture and hypoxia) through phenotypical plastic key functional tussock traits (e.g. shoot density, height, above-and below-ground biomass allocation patterns). Root biomass increased in coarser sediments in contrast to rhizomes, which were more abundant in finer sediments. Above-ground biomass and leaf area index increased mainly with air temperature during summer, and more robust (taller and wider) shoots were associated with more oxygenated sediments. In view of our results, S. densiflora appears to be a halophyte with a high degree of phenotypic plasticity that would enable it to respond successfully to changes in the abiotic conditions of salt marshes driven by global climate change, such as increasing salinity and temperatures.

Low genetic diversity contrasts with high phenotypic variability in heptaploid Spartina densiflora populations invading the Pacific coast of North America

“…These genetically similar S. densiflora populations showed high levels of phenotypic variability when growing in dissimilar environments in the field. Most of the interpopulation differences in plant traits recorded in the field disappeared when growing in common‐garden conditions, indicating that the observed variation in natural populations was the result of phenotypic plasticity (Castillo et al., , ; Grewell et al., ). Nevertheless, the mean interpopulation phenotypic variability recorded for every plant trait in the common‐garden experiment was 37% (in comparison with the 56% recorded in the field), which pointed to another process, beyond phenotypic plasticity, that was also responsible for a significant amount of the recorded interpopulation phenotypic variation.…”

“…See Castillo et al. (, ) for a full description of sampling sites. The southernmost population analyzed was at Corte Madera Creek in San Francisco Bay Estuary (SF; 37°56′33″N, 122°30′55″W; California, USA) in a middle‐elevation salt marsh dominated by S. densiflora , which was intentionally introduced from Humboldt Bay to stabilize tidal creek banks in a wetland restoration project in the late 1970s (Faber, ).…”

“…Detailed methods and values of these plant traits for the studied populations of S. densiflora were reported previously by Castillo et al. (, ) and they are analyzed in this study using phenotypic plasticity indices and in relation to genetic analyses.…”

“…49°20′N, Canada) provides a natural model system for an integrated study of the potential mechanisms underlying the response of species traits to substantial variation in climate and other environmental variables and an opportunity to investigate how species establish and succeed in novel and contrasted environments. Previous studies in the field and in common‐garden experiments on invasive S. densiflora populations sampled along the Pacific coast of North America revealed high levels of phenotypic plasticity for some foliar and tussock traits (Castillo, Grewell, Pickart, Figueroa, & Sytsma, ; Castillo et al., ; Grewell, Castillo, Skaer‐Thomason, & Drenovsky, ). However, the levels of genetic diversity in these S. densiflora populations along the Pacific coast of North America experiencing high variability in sedimentary and climatic conditions have not been studied.…”

“…Our overall objective was to assess whether trait responses were dependent on genetic differentiation or phenotypic plasticity or a combination of both factors. We hypothesized that (i) S. densiflora would show low genetic diversity and high phenotypic plasticity in response to contrasted environments and that they would be independent from each other, based on previous observations (Castillo et al., , ; Grewell et al., ); (ii) due to the polyploid and hybrid nature of S. densiflora , at least certain foliar traits would show both high plasticity and high intrapopulation variability, as leaf (the organ of photosynthesis and transpiration) is highly sensitive to environmental conditions (Stephenson, Oliver, Burgos, & Gbur, ); and (iii) genetically based and environmental‐based interpopulation differences in response to a changing environment would increase with geographic distance.…”