Arbuscular fungi have a major role in directing the functioning of terrestrial ecosystems yet little is known about their biogeographical distribution. The Baas-Becking hypothesis ('everything is everywhere, but, the environment selects') was tested by investigating the distribution of arbuscular mycorrhizal fungi (AMF) at the landscape scale and the influence of environmental factors and geographical distance in determining community composition. AMF communities in Trifolium repens and Lolium perenne roots were assessed in 40 geographically dispersed sites in Ireland representing different land uses and soil types. Field sampling and laboratory bioassays were used, with AMF communities characterised using 18S rRNA terminal-restriction fragment length polymorphism. Landscape-scale distribution of AMF was driven by the local environment. AMF community composition was influenced by abiotic variables (pH, rainfall and soil type), but not land use or geographical distance. Trifolium repens and L. perenne supported contrasting communities of AMF, and the communities colonising each plant species were consistent across pasture habitats and over distance. Furthermore, L. perenne AMF communities grouped by soil type within pasture habitats. This is the largest and most comprehensive study that has investigated the landscapescale distribution of AMF. Our findings support the Baas-Becking hypothesis at the landscape scale and demonstrate the strong influence the local environment has on determining AMF community composition.

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Four shrub species of the Australian Proteaceae (Hakea sericea, H. gibbosa, H. suaveolens and H. salicifolia) were introduced to South African fynbos shrublands between 1840 and 1860. H. sericea is highly invasive, H. gibbosa and H. suaveolens are moderately invasive and H. salicifolia is not invasive. The allocation of reproductive energy, germinability, the ability to survive fires and to germinate in burnt and unburnt areas, and the nutrient content of seeds were assessed for the four species. The information was used to investigate whether the success of H. sericea relative to the other three species could be explained by the superior expression of any trait. The most important trait which separates H. sericea from the other species is its ability to produce a large seed bank in its adopted environment in the absence of seed predators. Seed production in H. sericea shrubs with an above-ground dry mass of 8 kg is four times greater than H. gibbosa and more than 16 times that of H. suaveolens. Although H. salicifolia also produces a large seed bank, its seeds are unable to survive fires due to inadequate insulation by the small follicles. The results are compared to dispersal and seed bank data for indigenous South African Proteaceae, which have low dispersal and suffer high pre-dispersal seed predation. We suggest that potential invasives in the fynbos can be identified as species that have: (i) a potentially high seed production that is limited by specialized predators; (ii) an ability to disperse over long distances; and (iii) are pre-adapted to frequent fires and low soil nutrients. The data also support the current strategy of combatting H. sericea using specialized insect seed predators.

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An assessment of the effects of arbuscular mycorrhizal (AM) infection on photosynthesis, carbon (C) allocation, translocation and biomass production of cucumber, grown in sand culture, was made using a previously determined phosphorus (P) supply (0·13 mol m -3 P) which had a significant impact on AM infection. Separation of a direct effect of AM infection from an indirect one due to an enhanced leaf P status was achieved using a comparable non-mycorrhizal treatment (NAM + P) supplemented with extra P (0·19 mol m -3 P). Total leaf P concentration, specific leaf mass, photosynthetic capacity, and incorporation of 14 C into non-structural carbohydrate pools were dependent on leaf age. Both maximum and ambient photosynthetic rates were significantly higher in the youngest fully expanded leaves from AM and NAM + P plants which also had the higher leaf P concentrations. There were no differences in the total concentrations of starch, sucrose, raffinose or stachyose in young or old leaves among AM, non-mycorrhizal (NAM) and NAM + P treatments. However, younger leaves of NAM plants showed a shift in 14 C-partitioning from stachyose and raffinose synthesis to starch accumulation. Determination of ADP-glucose pyrophosphorylase (AGPase), sucrose synthase and sucrose phosphate synthase enzyme activities revealed that only AGPase activity was correlated with the increased incorporation rate of 14 C into starch in young leaves of NAM plants. Although there were significant AM-specific effects on C translocation to the root system, AM plants had similar rate of photosynthesis to NAM + P plants. These results suggest that the increase in photosynthetic rate in leaves of AM-infected cucumber was due to an increased P status, rather than a consequence of a mycorrhizal 'sink' for assimilates.