Solidago sempervirens

Abstract: Understanding the processes that maintain species diversity patterns is vital for conserving and managing communities. In this paper, we examine the effect of plant species interactions on the maintenance of plant species number across intertidal zones characterized by different soil conditions in a New England salt marsh community. In this system, plant species number is low in the high intertidal and lower middle intertidal, and high in the upper middle intertidal. To understand the causes of this unimodal pattern of change in species diversity, we experimentally tested for the influence of competition, facilitation, and physical factors across this physical gradient. We established plots with and without plant neighbors, transplanted plant species into these plots, and measured mortality, leaf area, and flower number for a single growing season. We found that competition plays a more important role in the high intertidal habitat by decreasing leaf area and flower production and causing 100% mortality of one of the species tested. In contrast, physical factors are critical in the lower middle intertidal, causing 100% mortality of three of the four species tested, with or without neighbor interactions. In the upper middle intertidal, direct positive interactions were important to the higher species number. Three of the four species had 100% mortality without neighbors but minimal mortality or leaf area loss with neighbors. These positive effects were due to one particular facilitator species, Juncus gerardi L., which ameliorates the soil conditions that develop in its absence thus allowing other species to co-occur. Without Juncus, our data predict that plant species number would not be as high in the upper middle intertidal. We show that the rise in species number in the upper middle intertidal is dependent on three co-occurring conditions: the absence of a competitive dominant, less harsh physical conditions than the lower middle intertidal, and the presence of a facilitator species.

Abstract: Summary 1 Determining the mechanisms by which invasive species exclude natives is critical for conserving and restoring native populations in impacted habitats. In recent decades the grass Phragmites australis has been aggressively invading coastal marshes of North America, with monocultures often replacing diverse assemblages of plants. 2 Our objective was to quantify how P. australis modifies the abiotic (soil and light conditions) and biotic (litter and shoots) environment and to determine the mechanisms by which it excludes two common forbs, the annual chenopod Atriplex patula var. hastata and the perennial aster Solidago sempervirens, from the highest tidal elevations of a brackish marsh in southern New England, USA. 3 In a 3-year field experiment we added seeds of both forb species to stands of P. australis, where we manipulated shoots and litter in an orthogonal design, and to uninvaded marsh areas dominated by the rush Juncus gerardi, where we manipulated the shoots of the marsh vegetation. In general, seedling establishment and the number of plants surviving until the end of the growing season were substantially greater in areas not invaded by P. australis, and both shoots and litter limited the abundance of forbs within stands. 4 Forbs surviving within stands of P. australis grew larger and produced more seeds than those in uninvaded areas, indicating that changes to the soil resulting from invasion do not preclude the survival of established forbs. This was confirmed by a glasshouse study where the performance of forbs in soil collected from within stands of P. australis was better than in soil from areas dominated by J. gerardi. 5 Similar to many invasive grasses in terrestrial communities, P. australis excludes native forbs through competition, modifying the biotic environment of the marsh at both the ground (litter) and above-ground (shoots) levels. Our results suggest that successful invaders, such as P. australis, are likely to be the ones that can engineer habitats in multiple ways and limit populations of native species across several critical stages of their life history.

Abstract: Biogeographic theory argues that consumer–prey interactions are more intense, and prey defenses better developed, at lower latitudes. Along the Atlantic Coast of the United States, low-latitude salt marsh plants are less palatable than high-latitude conspecifics. To test the hypothesis that latitudinal variation in palatability would occur in the absence of geographically different environmental cues (i.e., that differences in palatability are constitutive rather than induced by climate or herbivore damage), we grew high- and low-latitude individuals of three species of salt marsh plants from seeds (Solidago sempervirens) or rhizome cuttings (Distichlis spicata and Spartina alterniflora) in a common-garden greenhouse environment, and compared their palatability to herbivores over time. We also quantified leaf toughness and nitrogen content over time in order to help explain results of feeding assays. High-latitude plants were always more palatable to herbivores than low-latitude conspecifics. Latitudinal ...