Final answer:
The shoot biomass of the target plants was high with a few mycorrhizal species and decreased with more species, reflecting the balance and complexity of fungal-plant symbiotic relationships.
Step-by-step explanation:
In the study by van der Heijden and colleagues, the shoot biomass of the target plants was found to be high when only a few mycorrhizal fungal species were present and then decrease as more were added. This phenomenon can be explained by the intricate symbiotic relationships between mycorrhizal fungi and plant roots, which are crucial for plant nutrition and growth. The fungi form extensive networks of hyphae that increase the surface area for nutrient absorption, particularly phosphorus, from the soil.
Mycorrhizae provide significant benefits not only in assisting with nutrient uptake but also as a protective physical barrier against pathogens and in enhancing the plant's defense mechanisms. A diverse mycorrhizal community can be advantageous, but there seems to be a threshold beyond which additional fungal species may compete for resources, potentially leading to a decrease in plant biomass. This underscores the complexity of mycorrhizal relationships and their impact on plant health and agricultural productivity.