Facilitation May Buffer Competitive Effects: Indirect and Diffuse Interactions Among Salt Marsh Plants

Direct interactions among plant species may be highly modified by indirect or diffuse effects within a multispecies community. We investigated the direct and diffuse effects of two salt marsh perennials, Monanthechloe littoralis and Arthrocnemum subterminale, on winter annuals and the perennial herb Limonium californicum in a salt marsh in central California. In permanent plots, Monanthechloe had expanded substantially in the upper marsh over the past 13 yr, while Arthrocnemum and all annual species had decreased. These dynamics suggest that Monanthechloe may directly outcompete most other species in the upper marsh. In contrast, Arthrocnemum is known to facilitate some annual species. In our field experiments, Monanthechloe strongly suppressed Arthrocnemum, all four common annual species, and Limonium in direct interactions. In contrast, Arthrocnemum directly facilitated the winter annuals Parapholis incurva and Lasthenia glabrata, competed with Spergularia marina, and did not have a significant effect on Limonium. However, when the combined effects of Monanthechloe and Arthrocnemum were tested, Arthrocnemum ameliorated the negative effect of Monanthechloe on all four species. Although isolated Arthrocnemum competed with Spergularia and had no direct effects on Limonium, Arthrocnemum in the presence of Monanthechloe facilitated both species. We hypothesize that Arthrocnemum buffered the strong competitive effects of Monanthechloe on Lasthenia and Parapholis via direct positive effects and benefited Spergularia and Limonium through its competitive effect on Monanthechloe. These findings add to the growing body of literature emphasizing the importance of diversity and interdependence in the functioning of plant communities

The Advantages of Clonal Integration Under Different Ecological Conditions: a Community-Wide Test

The connectedness of clonal plants has been shown to promote survival and growth in a variety of single-species, single-factor studies, but experiments comparing the relative advantages of clonality across multiple habitats and species are rare, raising the concern that generalizations about the benefits of clonality might be biased by the particular species or habitat studied. We studied the importance of clonal integration in southeastern USA salt marsh plants, using all six of the common clonal species in the community, by following the success of intact and severed clonal fragments invading three habitat treatments. Clonal integration was most important for growth of clonal fragments invading hypersaline salt pans, likely because parent clones supplied salt-stressed fragments with water; of moderate importance for fragments invading the neighbors-clipped treatment, likely because parent clones supplied fragments with resources enabling rapid exploitation of unused patches; and least important when neighbors were present, consistent with suggestions that size-based asymmetrical competition is relatively unimportant in clonal plants. Our results indicate that the importance of clonal integration can differ between habitats and species within a community. We encourage explicit consideration of these potential sources of variability to best understand the importance of clonal integration in the field

Impact of a Parasitic Plant on the Structure and Dynamics of Salt Marsh Vegetation

We investigated the effect of a native parasitic plant, Cuscuta salina, on the structure and dynamics of the plant community in a California salt marsh. Cuscuta was common in the middle marsh zones. The abundance of Cuscuta was positively correlated with the abundance of Limonium californicum at two sampling scales (0.25— and ≈\approx40—m2 quadrats). Sampling at the scale of individual plants indicated that the dominant plant in the marsh, Salicornia virginica, was preferred by Cuscuta as a host over Arthrocnemum subterminale, Limonium californicum, and Frankenia salina. This result was confirmed with host—choice experiments in the field. Based on spatial correlations and host—choice experiments, we hypothesized that Cuscuta indirectly facilitated Limonium and Frankenia, increasing plant diversity and initiating vegetation cycles. This hypothesis was supported by sampling patches with different histories of Cuscuta infection. Patches with recent heavy Cuscuta infection had reduced Salicornia biomass and increased Limonium and Frankenia biomass relative to controls. This effect was most pronounced at higher elevations and in larger, older patches. The hypothesis was also supported by sampling permanent quadrats repeatedly over time. The probability that Cuscuta would invade a quadrat increased with increasing cover of Salicornia. Quadrats containing Cuscuta increased in Limonium and Frankenia cover between censuses relative to quadrats lacking Cuscuta. Our results indicate that parasitic plants can have strong effects on the structure and dynamics of natural vegetation assemblages. However, these effects are mediated by physical and biological gradients across the landscape

Effects of mangrove cover on coastal erosion during a hurricane in Texas, USA

We tested the hypothesis that mangroves provide better coastal protection than salt marsh vegetation using 10 1,008-m2 plots in which we manipulated mangrove cover from 0 to 100%. Hurricane Harvey passed over the plots in 2017. Data from erosion stakes indicated up to 26 cm of vertical and 970 cm of horizontal erosion over 70 months in the plot with 0% mangrove cover, but relatively little erosion in other plots. The hurricane did not increase erosion, and erosion decreased after the hurricane passed. Data from drone images indicated 196 m2 of erosion in the 0% mangrove plot, relatively little erosion in other plots, and little ongoing erosion after the hurricane. Transects through the plots indicated that the levee (near the front of the plot) and the bank (the front edge of the plot) retreated up to 9 m as a continuous function of decreasing mangrove cover. Soil strength was greater in areas vegetated with mangroves than in areas vegetated by marsh plants, or nonvegetated areas, and increased as a function of plot-level mangrove cover. Mangroves prevented erosion better than marsh plants did, but this service was nonlinear, with low mangrove cover providing most of the benefits

Buried hurricane legacies: increased nutrient limitation and decreased root biomass in coastal wetlands

Plant identity and cover in coastal wetlands is changing in worldwide, and many subtropical salt marshes dominated by low-stature herbaceous species are becoming woody mangroves. Yet, how changes affect coastal soil biogeochemical processes and belowground biomass before and after storms is uncertain. We experimentally manipulated the percent mangrove cover (Avicennia germinans) in 3 × 3 m cells embedded in 10 plots (24 × 42 m) comprising a gradient of marsh (e.g., Spartina alterniflora, Batis maritima) and mangrove cover in Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017, providing a unique opportunity to test how plant composition mitigates hurricane effects on surface sediment accretion, soil chemistry (carbon, C; nitrogen, N; phosphorus, P; and sulfur, S), and root biomass. Data were collected before (2013 and 2016), one-month after (2017), and one-year after (2018) Hurricane Harvey crossed the area, allowing us to measure stocks before and after the hurricane. The accretion depth was higher in fringe compared with interior cells of plots, more variable in cells dominated by marsh than mangrove, and declined with increasing plot-scale mangrove cover. The concentrations of P and δ34S in storm-driven accreted surface sediments, and the concentrations of N, P, S, and δ34S in underlying soils (0–30 cm), decreased post-hurricane, whereas the C concentrations in both compartments were unchanged. Root biomass in both marsh and mangrove cells was reduced by 80% in 2017 compared with previous dates and remained reduced in 2018. Post-hurricane loss of root biomass in plots correlated with enhanced nutrient limitation. Total sulfide accumulation as indicated by δ34S, increased nutrient limitation, and decreased root biomass of both marshes and mangroves after hurricanes may affect ecosystem function and increase vulnerability in coastal wetlands to subsequent disturbances. Understanding how changes in plant composition in coastal ecosystems affects responses to hurricane disturbances is needed to assess coastal vulnerability

Effects of mangrove cover on coastal erosion during a hurricane in Texas, USA

We tested the hypothesis that mangroves provide better coastal protection than salt marsh vegetation using ten 1,008 m2 plots in which we manipulated mangrove cover from 0 to 100 percent. Hurricane Harvey passed over the plots in 2017. Data from erosion stakes indicated up to 26 cm of vertical and 970 cm of horizontal erosion over 70 months in the plot with 0 percent mangrove cover, but relatively little erosion in other plots. The hurricane did not increase erosion, and erosion decreased after the hurricane passed. Data from drone images indicated 196 m2 of erosion in the 0 % mangrove plot, relatively little erosion in other plots, and little ongoing erosion after the hurricane. Transects through the plots indicated that the levee (near the front of the plot) and the bank (the front edge of the plot) retreated up to 9 m as a continuous function of decreasing mangrove cover. Soil strength was greater in areas vegetated with mangroves than in areas vegetated by marsh plants, or nonvegetated areas, and increased as a function of plot-level mangrove cover. Mangroves prevented erosion better than marsh plants did, but this service was non-linear, with low mangrove cover providing most of the benefits.Accepted manuscrip

Contrasting plant adaptation strategies to latitude in the native and invasive range of Spartina alterniflora.

Biological invasions offer model systems of contemporary evolution. We examined trait differences and evolution across geographic clines among continents of the intertidal grass Spartina alterniflora within its invasive and native ranges. We sampled vegetative and reproductive traits in the field at 20 sites over 20° latitude in China (invasive range) and 28 sites over 17° in the US (native range). We grew both Chinese and US plants in a greenhouse common garden for three years. Chinese plants were ~ 15% taller, ~10% denser, and set up to four times more seed than US plants in both the field and common garden. The common-garden experiments showed a striking genetic cline of seven-fold greater seed-set at higher latitudes in the introduced but not the native range. In contrast, there was a slight genetic cline in some vegetative traits in the native but not the introduced range. Our results are consistent with others showing that introduced plants can evolve rapidly in the new range. S. alterniflora has evolved different trait clines in the native and introduced range, showing the importance of phenotypic plasticity and genetic control of change during the invasion process

Marine Ecoregion and Deepwater Horizon Oil Spill Affect Recruitment and Population Structure of a Salt Marsh Snail

Marine species with planktonic larvae often have high spatial and temporal variation in recruitment that leads to subsequent variation in the ecology of benthic adults. Using a combination of published and unpublished data, we compared the population structure of the salt marsh snail, Littoraria irrorata, between the South Atlantic Bight and the Gulf Coast of the United States to infer geographic differences in recruitment and to test the hypothesis that the Deepwater Horizon oil spill led to widespread recruitment failure of L. irrorata in Louisiana in 2010. Size-frequency distributions in both ecoregions were bimodal, with troughs in the distributions consistent with a transition from sub-adults to adults at ~13 mm in shell length as reported in the literature; however, adult snails reached larger sizes in the Gulf Coast. The ratio of sub-adults to adults was 1.5–2 times greater in the South Atlantic Bight than the Gulf Coast, consistent with higher recruitment rates in the South Atlantic Bight. Higher recruitment rates in the South Atlantic Bight could contribute to higher snail densities and reduced adult growth in this region. The ratio of sub-adults to adults in Louisiana was lower in 2011 than in previous years, and began to recover in 2012–2014, consistent with widespread recruitment failure in 2010, when large expanses of spilled oil were present in coastal waters. Our results reveal an important difference in the ecology of a key salt marsh invertebrate between the two ecoregions, and also suggest that the Deepwater Horizon oil spill may have caused widespread recruitment failure in this species and perhaps others with similar planktonic larval stages

Disturbance and Recovery of Salt Marsh Arthropod Communities following BP Deepwater Horizon Oil Spill

Oil spills represent a major environmental threat to coastal wetlands, which provide a variety of critical ecosystem services to humanity. The U.S. Gulf of Mexico is a hub of oil and gas exploration activities that historically have impacted intertidal habitats such as salt marsh. Following the BP Deepwater Horizon oil spill, we sampled the terrestrial arthropod community and marine invertebrates found in stands of Spartina alterniflora, the most abundant plant in coastal salt marshes. Sampling occurred in 2010 as oil was washing ashore and a year later in 2011. In 2010, intertidal crabs and terrestrial arthropods (insects and spiders) were suppressed by oil exposure even in seemingly unaffected stands of plants; however, Littoraria snails were unaffected. One year later, crab and arthropods had largely recovered. Our work is the first attempt that we know of assessing vulnerability of the salt marsh arthropod community to oil exposure, and it suggests that arthropods are both quite vulnerable to oil exposure and quite resilient, able to recover from exposure within a year if host plants remain healthy