Effects of a common mycorrhizal network and light on growth and community structure of understory shrubs, Piper and Psychotria, in a moist neotropical forest

The vast majority of plant species may form fungal root symbioses, arbuscular mycorrhizae (AM). AM fungi improve plant growth due to enhanced nutrient uptake, particularly phosphorus (P), in exchange for a carbohydrate source, the host's photosynthate. The overall goal of this study was to experimentally examine the interactive effects of AM and carbon resources (light) on plant growth and community structure for tropical understory shrub genera, Piper and Psychotria. Experiments were conducted in lab microcosms to investigate the shared common mycorrhizal network (CMN) and in the field to examine the responses under more natural conditions. AM may also alter water uptake and this was investigated separately in a pot experiment.; Lab microcosms demonstrated that AM and high light both increased plant growth with effects on the resultant shrub community structure. Treatment effects on community structure were generally unrelated to a species' dependence on AM fungi. Two light-demanding Piper species dominated the microcosms in terms of biomass. Potential transfer of nutrients, carbon, and/or water between carbon-rich and carbon-limited plant communities via a CMN enhanced the dominance of these light-demanding Piper species in high light, but reduced the growth of the shade-tolerant species.; Plant growth and species composition in the field was primarily affected by microhabitat. AM acted parasitically in the understory and reduced both plant growth and survival. Leaf area loss by insect herbivores ranged from ca 10 to 40%, and was higher in the understory and AM plots. Plant community composition was determined mainly by the differential growth, survival, and tolerance to herbivory by the light-demanding Piper culebranum.; AM improved root hydraulic conductance of the understory specialists, but negatively affected water uptake of the light-demanding species. This implies that AM may represent a selective force for these evergreen shrubs during the annual drought.; Together, these experiments demonstrate that AM and light interact in a complex manner to affect growth, survival, and the structuring of the understory shrub community.