Concepción Sigüenza’s research while affiliated with University of California, Riverside and other places

Sand dune systems are among the best studied of primary successional sites and have attracted the attention of plant ecologists for over a century (Cowles 1899). Surprisingly, the traditional explanation of dune succession overlooks the critical contribution of mutualistic fungi that facilitate the invasion of barren areas. In fact, many of the dominant, dune-building plants appear to be incapable of growing in the dune environment if their roots are not associated with arbuscular mycorrhizal (AM) fungi, the topic of this chapter. The roots of dune species, like the vast majority of vascular plants, form symbiotic associations with fungi in the order Glomales (arbuscular mycorrhizas). The fungal mycelia provide mineral nutrients in exchange for carbon compounds of the host plant (Smith and Read 1997).

Anthropogenic nitrogen (N) deposition causes shifts in vegetation types as well as species composition of arbuscular mycorrhizal (AM) fungi and other soil microorganisms. A greenhouse experiment was done to determine whether there are feedbacks between N-altered soil inoculum and growth of a dominant native shrub and an invasive grass species in southern California. The region is experiencing large-scale loss of Artemisia californica shrublands and replacement by invasive annual grasses under N deposition. Artemisia californica and Bromus madritensis ssp. rubens were grown with soil inoculum from experimental plots in a low N deposition site that had (1) N-fertilized and (2) unfertilized soil used for inoculum, as well as (3) high-N soil inoculum from a site exposed to atmospheric N deposition for four decades. All treatments plus a nonmycorrhizal control were given two levels of N fertilizer solution. A. californica biomass was reduced by each of the three inocula compared to uninoculated controls under at least one of the two N fertilizer solutions. The␣inoculum from the N-deposition site caused the greatest growth depressions. By contrast, B.␣madritensis biomass increased with each of the three inocula under at least one, or both, of the N solutions. The different growth responses of the two plant species may be related to the types of AM fungal colonization. B. madritensis was mainly colonized by a fine mycorrhizal endophyte, while A. californica had primarily coarse endophytes. Furthermore, A. californica had a high level of septate, nonmycorrhizal root endophytes, while B. madritensis overall had low levels of these endophytes. The negative biomass response of A. californica seedlings to high N-deposition inoculum may in part explain its decline; a microbially-mediated negative feedback may occur in this system that causes poor␣seedling growth and establishment of A.␣californica in sites subject to N deposition and B. madritensis invasion.

Both anthropogenic nitrogen deposition and exotic annual grass invasion are occurring in the coastal sage scrub vegetation (CSS) of southern California. A study was carried out to determine the effects of these changes on soil microbial communities. Soils were sampled under exotic grasses and the native shrub Artemisia californica along an urban-to-rural N deposition gradient, and in N-fertilized plots in a rural site with low deposition. Arbuscular mycorrhizal (AM) colonization of A. californica was highest in low-N soils, but annual grasses (mainly Bromus madritensis) were colonized primarily by a fine endophyte that showed no pattern of response to soil N level. In addition, annual grasses generally had higher colonization by nonmycorrhizal, primarily septate, fungi in high N soils, while nonmycorrhizal colonization of A. californica was low in all soils. Spore density declined in the rhizosphere of A. californica with elevated N, but not of B. madritensis. Fatty acid methyl esters (FAME) were extracted from soil and their profiles were used to describe the soil microbial communities. Principal components analysis of FAME profiles showed a significant but weak relationship with levels of soil N. At the low N-deposition site that received N fertilizer, host plant was more important than soil N in determining FAME profiles. The most abundant fatty acids were the biomarkers for AM fungi. The different microbial communities of the two species, especially the predominance of fine endophyte and nonmycorrhizal fungi in roots of B. madritensis, warrant further research on functional responses to understand how these microorganisms may be involved in the invasion of native shrubland by exotic grasses.

Root-knot nematode-susceptible melons (Cantaloupe) were grown in pots with varying levels of Meloidogyne incognita and were compared to susceptible melons that were grafted onto Cucumis metuliferus or Cucurbita moschata rootstocks. In addition, the effect of using melons as transplants in nematode-infested soil was compared to direct seeding of melons in nematode-infested soil. There were no differences in shoot or root weight, or severity of root galling between transplanted and direct-seeded non-grafted susceptible melon in nematode-infested soil. Susceptible melon grafted on C. moschata rootstocks had lower root gall ratings and, at high nematode densities, higher shoot weights than non-grafted susceptible melons. However, final nematode levels were not lower on the grafted than on the non-grafted plants, and it was therefore concluded that grafting susceptible melon on to C. moschata rootstock made the plants tolerant, but not resistant, to the nematodes. Grafting susceptible melons on C. metuliferus rootstocks also reduced levels of root galling, prevented shoot weight losses, and resulted in significantly lower nematode levels at harvest. Thus, C. metuliferus may be used as a rootstock for melon to prevent both growth reduction and a strong nematode buildup in M. incognita-infested soil.

 Populations of arbuscular mycorrhizal fungi were estimated from spores associated with seven plant species in coastal dunes at El Socorro, near Ensenada, Baja California, during six months in 1992. The seasonal patterns of percent root colonization were also described in the same species during the wet season (January–March) and the dry season (April–July). Comparisons were made between the pioneer species (Abronia maritima) in the mobile dunes and six species (Abronia umbellata, Atriplex julacea, Camissonia californica, Haplopappus venetus, Helianthus niveus and Lotus spp.) in the fixed dunes. Mycorrhizal colonization in Abronia maritima was slight (<1%) and we observed few spores (<1/g soil). All of the species in the fixed dune formed mycorrhizae with up to 80% colonization in early summer, and no more than 4 spores/g soil by late summer. The highest percentages of total colonization and abundance of spores did not coincide temporally for any of the seven species, but the percentages were higher in summer than in spring. Arbuscules were more abundant when the soil was moist, and vesicles more abundant when it was dry.