Soil Fungi Alter Interactions Between the Invader Centaurea Maculosa and North American Natives

Soil microbes may affect the way exotic invasive plants interact with native neighbors. We investigated the effects of soil fungi on interactions between the invasive weed Centaurea maculosa (spotted knapweed) and six species native to the intermountain prairies of the northwestern United States. We also compared the effect of C. maculosa on the composition of the soil microbial community to that of the native species. In the field, fungicide (Benomyl) reduced AM mycorrhizal colonization of C. maculosa roots by \u3e80%. Fungicide did not significantly reduce non-AM fungi. When grown alone, the biomass of C. maculosa was not affected by the fungicide application. However, depending on the combination of native competitor and fungicide, C. maculosa biomass varied from 10-fold decreases to 1.9-fold increases. In untreated soils, C. maculosa grew larger in the presence of Festuca idahoensis or Koeleria cristata than when alone. When fungicide was applied these positive effects of Festuca and Koeleria on C. maculosa did not occur. A third native grass, Pseudoroegneria spicata, had much stronger competitive effects on C. maculosa than Festuca or Koeleria, and fungicide reduced the competitive effects of Pseudoroegneria. Fungicide increased Centaurea biomass when competing with the forb Gallardia aristata. However, fungicide did not affect the way two other forbs; Achillea millefolium and Linum lewisii, interacted with C. maculosa. Rhizosphere microbial communities in the root zones of the three native bunchgrass species differed from that of C. maculosa. However, despite the strong effects of soil fungi in field interactions and differences in microbial community composition, soil biota from different plant rhizospheres did not affect the growth of C. maculosa in the absence of native competitors in greenhouse experiments. Our results suggest that successful invasions by exotic plant species can be affected by complex and often beneficial effects of local soil microbial communities. These effects were not manifest as simple direct effects, but become apparent only when native plants, invasive plants, and soil microbial communities were interacting at the same time

Invasive Plant Suppresses the Growth of Native Tree Seedlings by Disrupting Belowground Mutualisms

The impact of exotic species on native organisms is widely acknowledged, but poorly understood. Very few studies have empirically investigated how invading plants may alter delicate ecological interactions among resident species in the invaded range. We present novel evidence that antifungal phytochemistry of the invasive plant, Alliaria petiolata, a European invader of North American forests, suppresses native plant growth by disrupting mutualistic associations between native canopy tree seedlings and belowground arbuscular mycorrhizal fungi. Our results elucidate an indirect mechanism by which invasive plants can impact native flora, and may help explain how this plant successfully invades relatively undisturbed forest habitat

Growth and Competitive Effects of Centaurea stoebe Populations in Response to Simulated Nitrogen Deposition

Increased resource availability can promote invasion by exotic plants, raising concerns over the potential effects of global increases in the deposition of nitrogen (N). It is poorly understood why increased N favors exotics over natives. Fast growth may be a general trait of good invaders and these species may have exceptional abilities to increase growth rates in response to N deposition. Additionally, invaders commonly displace locals, and thus may have inherently greater competitive abilities. The mean growth response of Centaurea stoebe to two N levels was significantly greater than that of North American (NA) species. Growth responses to N did not vary among C. stoebe populations or NA species. Without supplemental N, NA species were better competitors than C. stoebe, and C. stoebe populations varied in competitive effects. The competitive effects of C. stoebe populations increased with N whereas the competitive effects of NA species decreased, eliminating the overall competitive advantage demonstrated by NA species in soil without N added. These results suggest that simulated N deposition may enhance C. stoebe invasion through increasing its growth and relative competitive advantage, and also indicate the possibility of local adaptation in competitive effects across the introduced range of an invader

The competitive effects, as indicated by relative interaction intensity (RII = (<i>B_w</i>−<i>B_o</i>)/(<i>B_w</i>+<i>B_o</i>)), of different <i>C. stoebe</i> populations on the four North American (NA) species across three nitrogen levels.

<p>Bars show means+1 SE for each <i>C. stoebe</i> population. RII was calculated for each population in competition with a given NA species across three N levels, and <i>B_w</i> and <i>B_o</i> were the biomass of a plant of a given NA species in competition with a <i>C. stoebe</i> plant and the mean biomass of all the plants of a given NA species grown alone.</p

Effects of (A) low and (B) high nitrogen addition on biomass (means+1 SE) of five <i>Centaurea stoebe</i> populations (narrow black bars) and four North American (NA) species (narrow white bars).

<p>Wide bars are means (+1 SE) for all five <i>C. stoebe</i> populations (black) or all four NA species (white) combined. Increases in biomass = (<i>B_n</i>−<i>B_c</i>)/<i>B_c</i>×100%, where <i>B_n</i> was the biomass of a plant of each of the five <i>C. stoebe</i> populations or each of the four NA species, grown alone and subjected to low or high N deposition; <i>B_c</i> was the mean biomass of all the plants of each of the five <i>C. stoebe</i> populations or each of the four NA species, grown alone but subjected to no N addition.</p

Experimental scheme. Ark = Arkansas, BC = British Columbia, Mar = Maryland, Mon = Montana, Ver = Vermont, Ach = <i>Achillea</i>, Hel = <i>Helianthus</i>, Poa = <i>Poa</i>, and Vul = <i>Vulpia</i>.

<p>Single abbreviations (the left panel; e.g., Ark or Ach) represent one plant grown alone (i.e. no competition), and two abbreviations together (the right panel; e.g., Ark+Ach) represent two plants grown together (i.e. competition). There were 14 and eight replicates for <i>Centaurea stoebe</i> populations (i.e. Ark, BC, Mar, Mon, and Ver) and North American species (i.e. Ach, Hel, Poa, and Vul) grown alone (i.e. no competition), respectively; there were seven replicates for the pair-wise competition.</p

The mean competitive effects (means+1 SE), as indicated by relative interaction intensity (RII = (<i>B_w</i>−<i>B_o</i>)/(<i>B_w</i>+<i>B_o</i>)), of five <i>Centaurea stoebe</i> populations on four North American (NA) species (narrow black bars) and of four NA species on five <i>C. stoebe</i> populations (narrow white bars) under (A) no N addition, (B) low and (C) high N addition.

<p>Wide bars show means (+1 SE) for all five <i>C. stoebe</i> populations (black) or all four NA species (white). RII was calculated for each population under a given N level across four NA species, and <i>B_w</i> and <i>B_o</i> were the biomass of a plant of each of the four NA species in competition with a <i>C. stoebe</i> plant and the mean biomass of all the plants of each of the four NA species grown alone. RII was calculated for each NA species under a given N level across five <i>C. stoebe</i> populations, and <i>B_w</i> and <i>B_o</i> were the biomass of a plant of each of the five <i>C. stoebe</i> populations in competition with a NA plant and the mean biomass of all the plants of each of the five <i>C. stoebe</i> populations grown alone.</p