Marilyn Marler

MYCORRHIZAE INDIRECTLY ENHANCE COMPETITIVE EFFECTS OF AN INVASIVE FORB ON A NATIVE BUNCHGRASS

Mycorrhizae are important mediators of plant competition, but little is known about the role of mycorrhizae in the intense competitive effects that exotic plants can have on native species. In the greenhouse, we tested the effect of arbuscular mycorrhizal (AM) fungi on interspecific competition between Cerztaur-ea maculosu and Fest~ccu idahoensis, on intraspecific competition between individuals of both species, and the growth of C. rnac~closawith either inorganic or organic phosphorus. Mycorrhizae had no direct effect on either species, but mycorrhizae increased C. maculosas negative effect on F. iduhoe~zsis. When competing with C. rnac-ulosa, nonmycorrhizal F. idahoensis were 171% larger than they were when mycorrhizae were present. In a second experiment, C. maculosa grown with larger F. idahoe~zsis were 66% larger, in the presence of AM fungi, than when AM fungi were absent. Centaurea maculosu biomass was not affected by AM fungi, in either phosphorus treatment, in the absence of F. idahoensis. Root: shoot ratios differed between phosphorus treatments, but this difference seemed to be a result of slower growth in the organic phosphorus treatment. Our results were unusual in that the direct effects of my- corrhizae on both species were weak, but the indirect effect of AM fungi on the interactions between C. muc~tlosa and F. idahoerzsis was strong. Our results suggest that AM fungi strongly enhance the ability of C. ~naculosa to invade native grasslands of western North America. Key ~.ord.~: nrb~isclllcir rrrycoi-rhieal (AM) ,fuizgi; Centaurea maculosa; conzperition; exotic plants; Festuca idahoensis; interi7lor~tanr, gra.sslunris; iiz~,a.sion; mycorrhizae. ~~~~~~~~i~~~ between pairs of speciesare often me- diated by other species (Kareiva 1994, Miller 1994). Well documented indirect interactions include those be- tween sea otters, kelp and sea urchins 19781, starfish, molluscs and algae (Paine 1966),parasitic and autotrophic plants (Pennings and Callaway 1996), plants and soil microflora, (van der Putten et al, 1993. Bever 1994) and plants and mycorrhizal fungi (Grime

Soil fungal pathogens and the relationship between plant diversity and productivity

One robust result from many small-scale experiments has been that plant community productivity often increases with increasing plant diversity. Most frequently, resource-based or competitive interactions are thought to drive this positive diversity-productivity relationship. Here, we ask whether suppression of plant productivity by soil fungal pathogens might also drive a positive diversity-productivity relationship. We created plant assemblages that varied in diversity and crossed this with a ± soil fungicide treatment. In control (non-fungicide treated) assemblages there was a strong positive relationship between plant diversity and above-ground plant biomass. However, in fungicide-treated assemblages this relationship disappeared. This occurred because fungicide increased plant production by an average of 141% at the lower ends of diversity but boosted production by an average of only 33% at the higher ends of diversity, essentially flattening the diversity-productivity curve. These results suggest that soil pathogens might be a heretofore unappreciated driver of diversity-productivity relationships.

Native plant diversity resists invasion at both low and high resource levels.

Human modification of the environment is causing both loss of species and changes in resource availability. While studies have examined how species loss at the local level can influence invasion resistance, interactions between species loss and other components of environmental change remain poorly studied. In particular, the manner in which native diversity interacts with resource availability to influence invasion resistance is not well understood. We created experimental plant assemblages that varied in native species (1-16 species) and/or functional richness (defined by rooting morphology and phenology; one to five functional groups). We crossed these diversity treatments with resource (water) addition to determine their interactive effects on invasion resistance to spotted knapweed (Centaurea maculosa), a potent exotic invader in the intermountain West of the United States. We also determined how native diversity and resource addition influenced plant-available soil nitrogen, soil moisture, and light. Assemblages with lower species and functional diversity were more heavily invaded than assemblages with greater species and functional diversity. In uninvaded assemblages, experimental addition of water increased soil moisture and plant-available nitrogen and decreased light availability. The availability of these resources generally declined with increasing native plant diversity. Although water addition increased susceptibility to invasion, it did not fundamentally change the negative relationship between diversity and invasibility. Thus, native diversity provided strong invasion resistance even under high resource availability. These results suggest that the effects of local diversity can remain robust despite enhanced resource levels that are predicted under scenarios of global change.

Mycorrhizae transfer carbon from a native grass to an invasive weed: evidence from stable isotopes and physiology

Invasive exotic weeds pose one of the earths most pressing environmental problems. Although many invaders completely eliminate native plant species from some communities, ecologists know little about the mechanisms by which these exotics competitively exclude other species. Mycorrhizal fungi radically alter competitive interactions between plants within natural communities, and a recent study has shown that arbuscular mycorrhizal (AM) fungi provide a substantial competitive advantage to spotted knapweed, Centaurea maculosa, a noxious perennial plant that has spread throughout much of the native prairie in the northwestern U.S. Here we present evidence that this advantage is potentially due to mycorrhizally mediated transfer of carbon from a native bunchgrass, Festuca idahoensis, to Centaurea. Centaurea maculosa, Festuca idahoensis (Idaho fescue, C3), and Bouteloua gracilis (blue gramma, C4) were grown in the greenhouse either alone or with Centaurea in an incomplete factorial design with and without AM fungi. Centaurea biomass was 87–168% greater in all treatments when mycorrhizae were present in the soil (P < 0.0001). However, Centaurea biomass was significantly higher in the treatment with both mycorrhizae and Festuca present together than in any other treatment combination (P < 0.0001). This high biomass was attained even though Centaurea photosynthetic rates were 14% lower when grown with Festuca and mycorrhizae together than when grown with Festuca without mycorrhizae. Neither biomass nor photosynthetic rates of Centaurea were affected by competition with the C4 grass Bouteloua either with or without mycorrhizae. The stable isotope signature of Centaurea leaves grown with Festuca and mycorrhizae was more similar to that of Festuca, than when Centaurea was grown alone with mycorrhizae (P = 0.06), or with Festuca but without mycorrhizae (P = 0.09). This suggests that carbon was transferred from Festuca to the invasive weed. We estimated that carbon transferred from Festuca by mycorrhizae contributed up to 15% of the aboveground carbon in Centaurea plants. Our results indicate that carbon parasitism via AM soil fungi may be an important mechanism by which invasive plants out compete their neighbors, but that this interaction is highly species-specific.

Effects of Native Species Diversity and Resource Additions on Invader Impact

Theory and empirical work have demonstrated that diverse communities can inhibit invasion. Yet, it is unclear how diversity influences invader impact, how impact varies among exotics, and what the relative importance of diversity is versus extrinsic factors that themselves can influence invasion. To address these issues, we established plant assemblages that varied in native species and functional richness and crossed this gradient in diversity with resource (water) addition. Identical assemblages were either uninvaded or invaded with one of three exotic forbs: spotted knapweed (Centaurea maculosa), dalmatian toadflax (Linaria dalmatica), or sulfur cinquefoil (Potentilla recta). To determine impacts, we measured the effects of exotics on native biomass and, for spotted knapweed, on soil moisture and nitrogen levels. Assemblages with high species richness were less invaded and less impacted than less diverse assemblages. Impact scaled with exotic biomass; spotted knapweed had the largest impact on native biomass compared with the other exotics. Although invasion depressed native biomass, the net result was to increase total community yield. Water addition increased invasibility (for knapweed only) but had no effect on invader impact. Together, these results suggest that diversity inhibits invasion and reduces impact more than resource additions facilitate invasion or impact.

AN INVADER DIFFERENTIALLY AFFECTS LEAF PHYSIOLOGY OF TWO NATIVES ACROSS A GRADIENT IN DIVERSITY

Little is known about how exotics influence the ecophysiology of co-occurring native plants or how invader impact on plant physiology may be mediated by community diversity or resource levels. We measured the effect of the widespread invasive forb spotted knapweed (Centaurea maculosa) on leaf traits (leaf dry matter content, specific leaf area, leaf nitrogen percentage, leaf C:N ratios, and delta13C as a proxy for water use efficiency) of two co-occurring native perennial grassland species, Monarda fistulosa (bee balm) and Koeleria macrantha (Junegrass). The impact of spotted knapweed was assessed across plots that varied in functional diversity and that either experienced ambient rainfall or received supplemental water. Impact was determined by comparing leaf traits between identical knapweed-invaded and noninvaded assemblages. Virtually all M. fistulosa leaf traits were affected by spotted knapweed. Knapweed impact, however, did not scale with its abundance; the impact of knapweed on M. fistulosa was similar across heavily invaded low-diversity assemblages and lightly invaded high-diversity assemblages. In uninvaded assemblages, M. fistulosa delta13C, leaf nitrogen, and C:N ratios were unaffected by native functional group richness, whereas leaf dry matter content significantly increased and specific leaf area significantly decreased across the diversity gradient. The effects of spotted knapweed on K. macrantha were weak; instead native functional group richness strongly affected K. macrantha leaf C:N ratio, delta13C, and specific leaf area, but not leaf dry matter content. Leaf traits for both species changed in response to spotted knapweed or functional richness, and in a manner that may promote slower biomass accumulation and efficient conservation of resources. Taken together, our results show that an invader can alter native plant physiology, but that these effects are not a simple function of how many invaders exist in the community.