ISA Certified Arborist Practice Exam 2025 - Free ISA Arborist Practice Questions and Study Guide

Mycorrhizal fungi are crucial for tree health due to their ability to form symbiotic relationships with tree roots. By colonizing the root systems, these fungi extend the root's reach and significantly increase the surface area available for nutrient absorption. This enhanced nutrient uptake is particularly important for key elements like phosphorus and nitrogen, which are often present in soil in forms that trees cannot easily access.

In return for the nutrients, trees provide the fungi with carbohydrates produced through photosynthesis, creating a mutually beneficial relationship. This connection not only improves the overall nutrition of the tree but also enhances its resilience to environmental stresses, such as drought and poor soil conditions. Furthermore, mycorrhizal fungi can help protect trees against pathogens by outcompeting harmful microorganisms for space and resources.

The other options do not accurately represent the role of mycorrhizal fungi. Weakening tree roots would be detrimental to tree health, while decomposing dead organic matter is more associated with saprophytic fungi rather than mycorrhizal fungi. Competing with trees for nutrients misrepresents the symbiotic nature of their relationship, as mycorrhizal fungi work to improve nutrient access rather than detract from it.